SERUM-FREE MEDIUM AND SUITABLE CULTURE METHOD FOR THE CULTURE OF BLOOD CELLS, INCLUDING HUMAN HEMATOPOIETIC STEM CELLS

Abstract

The present invention describes a serum-free medium and a suitable culture method for growing human cells. The present invention provides a method for culturing human cells, the method comprising contacting human cells with polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block.

Description

SERUM-FREE MEDIUM AND SUITABLE CULTURE METHOD FOR THE CULTURE OF BLOOD CELLS, INCLUDING STEM CELLS HUMAN HEMATOPOIETIC Field of Invention The present invention describes a serum-free medium composition (especially albumin-free) and a suitable culture method for growing blood cells such as hematopoietic stem cells. The present invention provides a method for culturing blood cells such as human hematopoietic stem cells. This method may include contacting blood cells, such as human hematopoietic stem cells, with polyethylene glycol modified with a moiety having a polyvinylcaprolactam block and a polyvinyl acetate block attached. Background of the Invention The use of a chemically defined medium for the proliferation of hematopoietic stem cells has been studied. For mouse hematopoietic stem cells, it has been shown that they can be cultured in a chemically defined medium (Non-patent literature 1). List of appointments Non-patent literature Non-patent literature 1: Wilkinson et al., Nature, 571: 117-121, 2019 cl / οηη / ζζηζ / Ε / γίΛΐ Ref. 334267 Summary of the Invention The present invention provides a culture method and a medium composition (e.g., an albumin-free, serum-free or cytokine-free medium such as an albumin-free, cytokine-free and serum-free medium)) suitable for the culture of blood cells such as human hematopoietic stem cells. The present inventors have found and reported that mouse hematopoietic stem cells (sometimes referred to as KSL cells due to their method of isolation) can be cultured in large quantities for an extended period by adding polyvinyl alcohol (PVA) to an albumin-free, serum-free medium (Wilkinson et al., Nature, 571: 117-121, 2019). However, no method for expanding human hematopoietic stem cells in an albumin- and serum-free medium has been established. The present inventors have recently discovered that human hematopoietic stem cells exhibit marked proliferation in the presence of polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block, even in an albumin- and serum-free medium, while maintaining their stem cell characteristics.Furthermore, it has also been found that hematopoietic stem cell proliferation can be maintained even under cytokine-free conditions. In addition to hematopoietic stem cells, various blood cells, including erythroblasts, megakaryocytes, and leukocytes (e.g., T cells, B cells, macrophages, neutrophils), have also been found to proliferate under the same conditions. The invention can provide the following points of the invention. 1. A method for culturing human hematopoietic stem cells or human blood cells, comprising: cultivating human hematopoietic stem cells or human blood cells in a culture medium, wherein the culture medium is an albumin-free medium containing polyvinyl alcohol (especially a serum albumin-free medium), and comprises: (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist, such that the culture enables the increase or maintenance of the number of human hematopoietic stem cells or human blood cells. 2. The method according to point 1, where the increased number of human hematopoietic stem cells or human blood cells is obtained. 3. The method according to point 1 or 2, wherein the culture medium comprises a PI3K activator and is free of stem cell factor (SCF). 4. The method according to any of points 1 to 3, wherein the culture medium comprises a PI3K activator and a TPO receptor agonist. 5. The method according to point 4, wherein the culture medium comprises neither SCF nor TPO. 5A. The method according to point 4 or 5, wherein the culture medium is a cytokine-free medium. 6. The method in accordance with any of points 1 to 5, wherein the culture medium further comprises 4-N-[2benzyl-7-(2-methyltetrazol-5-yl)-9H-pyrimid[4,5-b]indol-4-yl]cyclohexane-1,4-diamine (UM171). 7. The method according to point 6, where the crop has a period of 7 days or more. 8. An albumin-free composition comprising human hematopoietic stem cells or human blood cells, polyvinyl alcohol and (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. 9. The composition according to point 8, wherein the composition comprises a PI3K activator and a TPO receptor agonist. 10. The composition according to point 8 or 9, further comprising 4-N-[2-benzyl-7-(2-methyltetrazol-5-yl)-9Hpyrimid[4,5-b]indol-4-yl]cyclohexane-1,4-diamine (UM171). 11. Human hematopoietic stem cells or human blood cells obtained by the method in accordance with any of points 1 to 7. 12. An albumin-free culture medium containing polyvinyl alcohol for human hematopoietic stem cells or human blood cells, comprising: (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; ocl / οηη / ζζηζ / E / γίΛΐ (3) a PI3K activator and a TPO receptor agonist. {1} A method for culturing or producing human hematopoietic stem cells or human blood cells, comprising: cultivate human hematopoietic stem cells or human blood cells in a culture medium, wherein the culture medium comprises polyvinyl alcohol and (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist, such that the culture enables the number of human hematopoietic stem cells or human blood cells to be increased or maintained. {2} The method according to point {1}, wherein the culture medium is a serum-free medium. {3} The method according to point {1}, wherein the culture medium is a chemically defined medium. {4} The method in accordance with any of points {1} to {3}, wherein the culture medium is substantially free of albumin. {5} The method in accordance with any of points {1} to {4}, whereby the increased number of human hematopoietic stem cells or human blood cells is obtained. {6} The method according to any of points {1} to {5}, wherein the culture medium comprises a PI3K activator and is free of stem cell factor (SCF). {7} The method according to any of points {1} to {6}, wherein the culture medium comprises a PI3K activator and a TPO receptor agonist. {8} The method according to point {7}, wherein the culture medium comprises neither SCF nor TPO. {8A} The method according to point {7} or {8}, wherein the culture medium is a cytokine-free medium. {9} The method according to any of points {1} to {8}, wherein the culture medium further comprises 4-N-{2benzyl-7-(2-methyltetrazol-5-yl)-9H-pyrimid{ 4,5-b}indol-4-yl}cyclohexane-1,4-diamine (UM171). {10} The method according to point {9}, where the crop has a period of 7 days or more. {11} A composition comprising human hematopoietic stem cells or human blood cells, polyvinyl alcohol as an alternative to albumin, and (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. {12} The composition according to point {11}, wherein the composition comprises a PI3K activator and a TPO receptor agonist. {13} The composition according to point {11} or {12}, further comprising 4-N-{2-benzyl-7-(2-methyltetrazol-5-yl)9H-pyrimid{4,5-b}indol-4-yl}cyclohexane-1,4-diamine (UM171). {14} Human hematopoietic stem cells or human blood cells obtained by the method in accordance with any of points {1} to {10}. {15} A culture medium containing polyvinyl alcohol as an alternative to albumin for human hematopoietic stem cells or human blood cells, comprising: (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. {16} The culture medium in accordance with point {15}, which is a serum-free medium. {17} The culture medium in accordance with point {15}, which is a chemically defined medium. {18} The culture medium in accordance with any of points {15} to {17}, which is free of albumin. {19} The method according to any of {1} to {8}, wherein the human cells are human hematopoietic stem cells. {20} The composition according to any of {11} to {13}, wherein the human cells are human hematopoietic stem cells. {21} The method according to any of {1} to {8}, wherein the human cells are human blood cells. {22} The composition according to any of {11} to {13}, wherein human cells are human blood cells. {23} A culture medium comprising human cells obtained by the method in accordance with any of points {1} to {8}, {19} and {21}. {24} The culture medium in accordance with point {23}, which is an albumin-free, cytokine-free and serum-free medium. The invention may also provide the following points of the invention. [IB] A method for culturing human cells, comprising culturing human cells in a culture medium, wherein the culture medium comprises polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. [2B] The method according to point [IB], where the increased number of human cells is obtained. [3B] The method according to point [IB] or [2B], wherein the human cells are human hematopoietic stem cells or human blood cells. [4B] A culture medium composition for growing human cells, comprising polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. [5B] A composition comprising human cells and polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. [6B] The composition according to point [4B] or [5B], wherein the human cells are human hematopoietic stem cells or human blood cells. [7B] Human cells obtained by the method in accordance with any of points [IB] to [3B]. [8B] The method according to point [3B], wherein the human cells are human hematopoietic stem cells. [9B] Composition according to point [6B], wherein human cells are human hematopoietic stem cells. [10B] The method according to point [3B], wherein the human cells are human blood cells. [11B] The composition according to point [6B], wherein human cells are human blood cells. [12B] A culture medium containing human cells obtained by the method in accordance with any of points [IB] to [3B]. [13B] The culture medium in accordance with point [12B], which is an albumin-free, cytokine-free and serum-free medium. [14B] The method according to any of points [IB] to [3B], [8B] and [10B], the composition according to any of points [4B] to [6B], [9B] and [11B], or the culture medium according to point

[12] or

[13] , wherein the polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block is polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. [1C] A method for culturing human cells, comprising culturing human cells in a culture medium, wherein the culture medium is a serum albumin-free medium and comprises an additive, the additive being selected from the group consisting of polyvinyl alcohol and modified polyalkylene glycol, and further comprising (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist, such that the culture enables the increase or maintenance of the number of human hematopoietic stem cells or human blood cells.

[20] The method according to point

[10] , wherein the human cells are cells selected from the group consisting of human hematopoietic stem cells and human blood cells.

[30] The method according to point

[10] , wherein the human cells are blood cells other than hematopoietic stem cells. cl / onn / zznz / E / YiAi [4C] The method according to point [3C], wherein the additive is polyvinyl alcohol. [5C] The method according to point [1C] or [2C], wherein the additive is modified polyalkylene glycol. [6C] The method according to point [5C], wherein the modified polyalkylene glycol is polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. [7C] The method according to point [6C], wherein the modified polyalkylene glycol is polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. Brief Description of the Figures Figure 1 shows the results of culture, in the presence of 10 ng / mL of mouse or human stem cell factor (SCF) and 100 ng / mL of thrombopoietin (TPO), mouse hematopoietic stem cells (mouse KSL cells) or human hematopoietic stem cells (CD34+CD38- cells) in an albumin-free, serum-free medium containing polyvinyl alcohol (PVA). Figure 2 shows graphs indicating the degree of phosphorylation of each signaling factor downstream of SCF and TPO in human or mouse hematopoietic stem cells in the presence of 10 ng / ml stem cell factor (SCF) and 100 ng / ml thrombopoietin (TPO), in the absence of albumin and in the presence of PVA. The symbol m represents mouse hematopoietic stem cells and h represents human hematopoietic stem cells. Figure 3 shows the results of culturing, in the absence of albumin and in the presence of PVA, human hematopoietic stem cells in a medium containing 10 ng / mL of human stem cell factor (SCF) and 100 ng / mL of human thrombopoietin in the presence of an AKT II Activator (AKTa) or PI3K activator (Pl3Ka). Figure 4 shows the growth rate of total cells or CD34+ cells on day 7 after human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA and 100 ng / mL of human thrombopoietin (TPO). Figure 4 compares conditions containing SCF with SCF-free conditions. Figure 4 indicates that no statistically significant difference was found in the number of total cells or CD34+ cells on day 7 of culture between the two conditions. Figure 5 shows the time course of the number of human hematopoietic stem cells cultured in the presence of albumin or PVA while TPO was replaced by each TPO receptor agonist. The symbol Buti represents butizamide, Elt represents eltrombopag, and Ava represents avatrombopag. Figure 6 shows the growth rate of total cells or CD34+ cells on day 7 when human hematopoietic stem cells were cultured in the presence of albumin or PVA while TPO was replaced by each TPO receptor agonist. Figure 7 shows the total number of cells, the number of CD34+ cells, or the number of GEmM colonies on day 7 when human hematopoietic stem cells were cultured in a medium containing a PI3K activator and either TPO or butyzamide (Buti) in the absence of albumin and in the presence of PVA. Colony types were identified under a microscope after each colony was collected, a cytospin sample was prepared, and the sample was stained with Giemsa. G indicates granulocytes, E indicates erythroblasts, m indicates macrophages, and M indicates megakaryocytes. Figure 8 shows the growth rate of total cells or CD34+ cells on day 7 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator, a TPO receptor agonist, or a combination of them. Figure 9 shows the growth rate of each cell population in a culture obtained on day 7 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator and a TPO receptor agonist. Figure 10 shows the time evolution of the total cell number or the number of CD34+ cells on days 7 and 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in an SCF- or TPO-free culture medium but containing a PI3K activator and a TPO receptor agonist. Figure 11 shows the synchronized cell results obtained on day 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator and a TPO receptor agonist. The left panel shows the flow cytometry results using CD34 and CD38 as markers, and the right panel shows the flow cytometry results using CD41a and CD42b as markers. One image in Figure 11 is a light micrograph of the resulting culture. Figure 12 shows the results of the colony assay using cells obtained on day 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator and a TPO receptor agonist. Colony types were identified under a microscope after each colony was collected, a cytospin sample was prepared, and the sample was stained with Giemsa. G indicates granulocytes, E indicates erythroblasts, m indicates macrophages, and M indicates a colony containing megakaryocytes. Figure 13 shows the growth rate of total cells or CD34+ cells on day 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator; a TPO receptor agonist; and SR1 and / or UM171. Figure 14 shows the growth rate of total cells or CD34+ cells or the number of CD41+ cells on day 14 when the respective cells were cultured under conditions 1 to 3. Figure 15 shows the flow cytometry results using cells in culture on day 14 when the respective cells were cultured under conditions 1 to 3. The upper panels show the results using CD34 (abscissa) and CD38 (ordinate), and the lower panels show the results using CD41a (abscissa) and CD42b (ordinate). Figure 16 shows graphs indicating the grafting of human hematopoietic stem cells into mouse peripheral blood 12 weeks after the human hematopoietic stem cells were cultured under the respective conditions 1 to 3 and then transplanted on day 7 into each mouse. Figure 17 shows the total number of cells, the percentage of CD34+ cells, the percentage of viable cells, or the number of CD34+ cells in a culture obtained after umbilical cord blood monocytes were separated and cultured for 14 days under conditions of presence or absence of UM171 in a culture medium without albumin, SCF, or TPO but containing PVA, a PI3K activator, and a TPO receptor agonist. Figure 18 shows the results of in vitro culture experiments of mouse hematopoietic stem cells in the absence of albumin. In each mouse hematopoietic stem cell culture medium, the indicated component was included at a final concentration of 0.1%. More specifically, each culture medium containing, at a final concentration of 0.1%, Kolliphor (registered trademark) P188 bio (188 bio), Kolliphor (registered trademark) P 188 Geismar (188 Geismar) from BASF, Soluplus (Sol+), Kolliphor (registered trademark) P 407 Geismar (407 Geismar), Kollidon (registered trademark) 30 Origin USA (30 USA), Kollidon (registered trademark) 17 PF (17 PF), Kollidon (registered trademark) 25 (25), Kollidon (registered trademark) 90F (90F), or Kollidon (registered trademark) 12PF (12PF) was used. Figure 19 shows the results of the cl / on / zzz / E / gίΛΐ in vitro colony formation assay using mouse hematopoietic stem cells in a serum-free and albumin-free medium in the presence of PVA or polymer A. Figure 20 shows the percentage (%) of mouse hematopoietic stem cells positive for the stem cell marker CD201 grown in an albumin-free, serum-free medium in the presence of PVA or polymer A. Figure 21 shows a scheme (left) in which mouse hematopoietic stem cells cultured in an albumin-free serum medium in the presence of PVA or polymer A were transplanted into an irradiated mouse, and the percentage of peripheral stem cells in the blood 4 weeks after transplantation. The percentage of chimerism indicates the proportion of cells derived from transplanted hematopoietic stem cells in all blood cells. Figure 22 indicates the percentage of CD34-positive cells in mouse hematopoietic stem cells grown in an albumin-free serum medium in the presence of PVA or polymer A. Figure 23 indicates a change in the number of mouse hematopoietic stem cells cultured in a serum-free, albumin-free medium in the presence of PVA or polymer A. Figure 24 shows graphs indicating the degree of phosphorylation of each signaling factor downstream of SCF and TPO in human or mouse hematopoietic stem cells in the presence of 10 ng / ml of stem cell factor (SCF) and 100 ng / ml of thrombopoietin (TPO), in the absence of albumin and in the presence of PVA. The symbol m represents mouse hematopoietic stem cells and h represents human hematopoietic stem cells. Figure 25 shows the results of culturing human hematopoietic stem cells in the absence of albumin and in the presence of PVA in a medium containing 10 ng / mL of human stem cell factor (SCF) and 100 ng / mL of human thrombopoietin in the presence of AKT Activator II (AKTa) or PI3K activator (Pl3Ka). Figure 26 shows the growth rate of total cells or CD34+ cells on day 7 after culturing human hematopoietic stem cells in the absence of albumin and in the presence of PVA and 100 ng / ml of human thrombopoietin (TPO). Figure 9 compares the conditions containing SCF with those without SCF. Figure 4 indicates that no statistically significant difference was found in the number of total cells or CD34+ cells on day 7 of culture between the two conditions. Figure 27 shows the time course of the number of human hematopoietic stem cells cultured in the presence of albumin or PVA while TPO was replaced by each TPO receptor agonist. The symbol Buti represents butizamide, Elt represents eltrombopag, and Ava represents avatrombopag. Figure 28 shows the growth rate of total cells or CD34+ cells on day 7 when human hematopoietic stem cells were cultured in the presence of albumin or PVA while TPO was replaced by each TPO receptor agonist. Figure 29 shows the total number of cells, the number of CD34+ cells, or the number of GEmM colonies on day 7 when human hematopoietic stem cells were cultured in a medium containing a PI3K activator and either TPO or butyzamide (Buti) in the absence of albumin and in the presence of PVA. Colony types were identified under a microscope after each colony was collected, a cytospin sample was prepared, and the sample was stained with Giemsa. G indicates granulocytes, E indicates erythroblasts, m indicates macrophages, and M indicates megakaryocytes. Figure 30 shows the growth rate of total cells or CD34+ cells on day 7 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator, a TPO receptor agonist, or a combination of them. Figure 31 shows the growth rate of each cell population in a culture obtained on day 7 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator and a TPO receptor agonist. Figure 32 shows the time course of the total cell number or the number of CD34+ cells on days 7 and 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator and a TPO receptor agonist. Figure 33 shows the results of selected cells obtained on day 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator and a TPO receptor agonist. The left panel shows the flow cytometry results using CD34 and CD38 as markers, and the right panel shows the flow cytometry results using CD41a and CD42b as markers. A photograph in Figure 16 is a light micrograph of the resulting culture. Figure 34 shows the results of the cell colony assay obtained on day 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator and a TPO receptor agonist. Colony types were identified under a microscope after each colony was collected, a cytospin sample was prepared, and the sample was stained with Giemsa. G indicates granulocytes, E indicates erythroblasts, m indicates macrophages, and M indicates a colony containing megakaryocytes. Figure 35 shows the growth rate of total cells or CD34+ cells on day 14 when human hematopoietic stem cells were cultured in the absence of albumin and in the presence of PVA in a culture medium without SCF or TPO but containing a PI3K activator; a TPO receptor agonist; and SR1 and / or UM171. Figure 36 shows the growth rate of total cells or CD34+ cells or the number of CD41+ cells on day 14 when the respective cells were cultured under conditions 1 to 3. Figure 37 shows the flow cytometry results using cells in culture on day 14 when the respective cells were cultured under conditions 1 to 3. The upper panels show the results using CD34 (abscissa) and CD38 (ordinate), and the lower panels show the results using CD41a (abscissa) and CD42b (ordinate). Figure 38 shows graphs indicating the grafting of human hematopoietic stem cells into mouse peripheral blood 12 weeks after the human hematopoietic stem cells were cultured under the respective conditions 1 to 3 and then transplanted on day 7 into each mouse. Figure 39 shows the total cell number, the percentage of CD34+ cells, the percentage of viable cells, or the number of CD34+ cells in a culture obtained after umbilical cord blood monocytes were separated and cultured for 14 days under conditions in the presence or absence of UM171 in a culture medium without albumin, SCF, or TPO but containing PVA, a PI3K activator, and a TPO receptor agonist. Figure 40 shows the growth of human T cells in an albumin-free, cytokine-free medium in the presence of PVA or Soluplus (trademark). Figures 41A and 41B show the results of human hematopoietic stem cell differentiation experiments in an albumin-free medium in the presence of PVA or Soluplus (trademark). Hematopoietic cells have been shown to proliferate and differentiate (in particular, proliferate) into a wide range of blood cell lineages. Figure 42 shows the growth potential of chronic myeloid leukemia (CML) cells in an albumin-free, cytokine-free medium in the presence of PVA or Soluplus (registered trademark). In the graphs, IM+ signifies the presence of imatinib and IM- signifies the absence of imatinib. Detailed Description of the Invention As used herein, hematopoietic stem cells refer to stem cells capable of differentiating into blood cells. Hematopoietic stem cells can be obtained from bone marrow, umbilical cord blood, the placenta, or peripheral blood. Human hematopoietic stem cells are CD34-positive. In humans, hematopoietic stem cells are known to be abundant in the CD34-positive and CD38-negative cell fractions. Therefore, human hematopoietic stem cells can be both CD34-positive and CD38-negative. Hematopoietic stem cells can be obtained by ex vivo differentiation of pluripotent stem cells such as ES cells or iPS cells. As used herein, blood cells are hematopoietic stem cells or cells differentiated from hematopoietic stem cells. Blood cells are broadly classified into hematopoietic stem cells, hematopoietic progenitors, and hematopoietic cells according to their stage of differentiation. Hematopoietic stem cells differentiate from hematopoietic progenitors into hematopoietic cells. More specifically, hematopoietic stem cells can differentiate from lymphoblasts into lymphocytes (e.g., T cells, B cells, NK cells). Hematopoietic stem cells can also differentiate from hematopoietic progenitors and monoblasts into monocytes. Additionally, hematopoietic stem cells can differentiate from hematopoietic progenitors, myeloblasts, promyelocytes, myelocytes, postmyelocytes, and rod-shaped nucleated cells into neutrophils.Hematopoietic stem cells can differentiate from hematopoietic progenitors and myeloblasts into granulocytes, such as eosinophils or basophils, or macrophages. Hematopoietic stem cells can also differentiate from hematopoietic progenitors, proerythroblasts, basophilic erythroblasts, polychromatic erythroblasts, and orthochromatic erythroblasts into erythrocytes. Hematopoietic stem cells can also produce platelets after differentiating from hematopoietic progenitors, promegakaryocytes, and megakaryocytes. All cells that can differentiate from hematopoietic stem cells are blood cells. Examples of blood cells include cancerous and non-cancerous cells. Examples of cancerous cells include chronic myeloid leukemia (CML) cells or stem cells. CML. As used herein, the term ex vivo means outside the body. As used herein, the term ex vivo is used in contrast to in vivo (inside the body) and means that cells present within the body are removed from the body from within. Culture can be performed ex vivo. As used herein, positive means that the cell is found to express a molecule identified by the immediately preceding term. As used herein, positive is sometimes indicated simply as +. As used in this document, polyvinyl alcohol (PVA) means a vinyl alcohol polymer. Polyvinyl alcohol can be obtained by saponification of polyvinyl acetate, which is obtained by polymerizing a vinyl acetate monomer. The weight-average molecular weight (MW) of polyvinyl alcohol can be, for example, from 1 kDa to 20 kDa, from 3 kDa to 17 kDa, from 5 kDa to 15 kDa, or from 7 kDa to 13 kDa. In the case of saponification of polyvinyl acetate to obtain PVA by the above method, the saponification rate can be 80% or more, 85% or more, 90% or more, 95% or more, or 99% or more. As used in this document, albumin is a protein known as a plasma component. Albumin is said to make up 60% of plasma proteins and is abundantly present in blood, serum, and plasma. Albumin is thought to be responsible for maintaining the osmotic pressure of blood in vivo and for transporting biological substances such as fatty acids and hormones by binding to them. Serum albumin is also known to be important in the maintenance and culture of hematopoietic stem cells. Human serum albumin (hereafter sometimes referred to as HSA) is human serum albumin, which may be, for example, a protein having the amino acid sequence registered as GenBank accession number AAN17825.1 or human serum albumin having the corresponding amino acid sequence. As used here, the agonist refers to a substance that can activate a protein such as a receptor or an enzyme. As used herein, PI3K stands for phosphatidylinositol 3-kinase. PI3K is an enzyme that can phosphorylate inositol phospholipids, a component of cells. Phosphorylated phosphatidylinositol 3,4,5-trisphosphate (PIP3) mediates the phosphorylation of Akt (also known as protein kinase B) and transduces downstream signaling. As used herein, PI3K activator means a tyrosine kinase receptor agonist and a substance that can activate PI3K. The PI3K activator may be selective for PI3K. As used in this document, TPO stands for thrombopoietin. TPO is a protein responsible for the differentiation of hematopoietic stem cells into megakaryocytes. TPO is known to be formally involved in the maintenance of hematopoietic stem cells. Human thrombopoietin is, for example, a protein with the amino acid sequence registered as GenBank accession number AAB33390.1, or thrombopoietin with the corresponding amino acid sequence. As used herein, a TPO receptor agonist means a substance, other than TPO, that can activate a TPO receptor. Examples of TPO receptor agonists include any TPO variant other than TPO, any peptide, or any compound that can activate a TPO receptor. As used herein, the term "compound" encompasses organic compounds. A TPO receptor agonist may be selective for a particular TPO receptor. As used in this document, stem cell factor (SCF) is a hematopoietic cell growth factor that acts in the early stages of hematopoiesis. Human stem cell factor is, for example, a protein with the amino acid sequence registered as GenBank accession number AAA85450.1, or SCF with the corresponding amino acid sequence. As used in this document, culture means incubating cells under conditions suitable for their growth or maintenance. Incubation can be carried out, in the case of human cells, under an atmosphere preferably at 37°C and 5% CO2. If culture involves proliferation, it is understood that culture involves the production of expanded cells. As used here, culture can be carried out in a serum-free medium. A chemically defined culture medium (or a completely synthetic medium) is a serum-free medium. As used here, culture medium means a medium used to grow cells. Culture medium can be prepared by adding essential components to a basic medium. Essential components may include a pH modifier, a sugar source such as glucose, antibiotics (e.g., penicillin and streptomycin), and essential amino acids such as glutamine, as well as culture additives such as insulin, transferrin, selenium (e.g., sodium selenite), and / or ethanolamine. As used here, the term "free" means that a material is not contained in concentrations above the limit of detection or is not contained at all. For example, a serum-free or albumin-free medium is serum-free, albumin-free, and free of serum or albumin. The term "albumin-free" means that albumin is not contained in concentrations equal to or greater than the limit of detection or is not contained at all. The term "cytokine-free" means that no cytokines are contained in concentrations equal to or greater than the limit of detection or are not contained at all. The medium may be albumin-free. The medium may be cytokine-free. The medium may be both albumin-free and cytokine-free. As used herein, cytotoxicity means the characteristic of having the effect of decreasing the number of cells or killing cells during culture. As used herein, non-cytotoxic refers to the characteristic of not decreasing the number of cells during culture. An increase in the concentration of a substance may cause cytotoxicity. In this case, even if the concentration is reduced to a level that does not cause cytotoxicity, the expected effect of the substance may still occur. This case may be defined as non-cytotoxic. As used herein, a medium free of any cytotoxic agent or agent having cytotoxicity means that the medium does not contain any cytotoxic agent or agent having cytotoxicity in sufficient quantities to cause cytotoxicity.Therefore, when an agent is cytotoxic at high concentrations and is used at concentrations that do not cause cytotoxicity, the agent is neither a cytotoxic agent nor an agent that has cytotoxicity. The present inventors have found and reported that mouse hematopoietic stem cells (sometimes referred to as KSL cells due to their method of isolation) can be cultured in large quantities for an extended period by adding polyvinyl alcohol (PVA) to an albumin-free, serum-free medium (Wilkinson et al., Nature, 571: 117-121, 2019). However, no method for expanding human hematopoietic stem cells in an albumin-free, serum-free medium has been established. Now, the present inventors have discovered that human hematopoietic stem cells can proliferate remarkably while maintaining their stem cell properties in the presence of polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block.The present inventors have also found that in human hematopoietic stem cells cultured in the presence of PVA, SCF, and TPO, even in an albumin- and serum-free medium, the activation of several signaling pathways, including the PI3K and Akt pathways, is weak. The present inventors have further discovered that human hematopoietic stem cells and human blood cells can proliferate upon the addition of a PI3K activator in the presence of PVA, SCF, and TPO, even in a serum- and albumin-free medium. The present inventors have further found that a PI3K activator can completely replace SCF in the presence of PVA, even in a serum- and albumin-free medium. In addition, the present inventors have found that TPO can be replaced by a TPO receptor agonist in the presence of PVA and a PI3K activator, even in an albumin- and serum-free medium.Furthermore, the present inventors have discovered that human hematopoietic stem cells or human blood cells can proliferate favorably in the presence of polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block, a PI3K activator, and a TPO receptor agonist, even in an albumin-free, serum-free medium. In such cases, it is not necessary to add SCF and TPO to the medium. Moreover, the present inventors have found that human hematopoietic stem cells can be maintained and expanded for an extended period by adding UM171 to the medium. The present invention provides a method for culturing human cells, comprising: cultivating human cells in a culture medium, wherein the culture medium is a serum albumin-free medium and comprises an additive, the additive being selected from the group consisting of polyvinyl alcohol and modified polyalkylene glycol, and further comprising (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist, such that the culture enables the increase or maintenance of the number of human hematopoietic stem cells or human blood cells. Human cells can be selected from the group consisting of human hematopoietic stem cells and human blood cells. In addition, human cells can also be blood cells other than hematopoietic stem cells. Blood cells can be selected from the group consisting of hematopoietic progenitors and hematopoietic cells. Blood cells can be one or more cells selected from the group consisting of lymphoblasts, lymphocytes (e.g., T cells, B cells, NK cells, and NKT cells), CD3-positive cells, myeloblasts, promyelocytes, myelocytes, postmyelocytes, rod-shaped nucleated cells, and neutrophils, granulocytes (e.g., eosinophils and basophils), macrophages, proerythroblasts, basophilic erythroblasts, polychromatic erythroblasts, orthochromatic erythroblasts, erythrocytes, as well as promegakaryocytes and megakaryocytes.Human cells can be, for example, one or more cells selected from the group consisting of chronic myeloid leukemia (CML) cells and CML stem cells. In one embodiment, the additive may be an additive that can replace albumin. The additive is, in one embodiment, modified polyalkylene glycol. In one embodiment, the modified polyalkylene glycol may be, for example, modified polyethylene glycol. The modified polyalkylene glycol or the modified polyethylene glycol may preferably be modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. In one particular form, the additive may be polyvinyl alcohol. According to the invention, modified PVA or polyalkylene glycol can be added. This reduces the amount of albumin, such as serum albumin, added to a culture medium, preferably leaving the culture medium albumin-free. In one embodiment, the culture medium can be a serum-free medium, for example, a serum-free and albumin-free medium. According to the present invention, a PI3K activator can replace SCF. Meanwhile, in the invention, a TPO receptor agonist can replace TPO. In this way, the amount of SCF or TPO added to the culture medium can be reduced, and preferably, the culture medium can be made cytokine-free. In one embodiment, the culture medium can be a serum-free medium, for example, a serum-free and cytokine-free medium. In one formulation, modified PVA or polyalkylene glycol can be added to a culture medium. This can reduce the amount of albumin, such as serum albumin, added to the culture medium. Additionally, a PI3K activator and a TPO receptor agonist can be added to the culture medium. This can reduce the amounts of SCF and TPO added to the culture medium. In one formulation, the culture medium can be albumin-free, cytokine-free, and serum-free. Another embodiment of the invention provides a method for culturing human hematopoietic stem cells or human blood cells, comprising: cultivating human hematopoietic stem cells or human blood cells in a culture medium, wherein the culture medium comprises polyvinyl alcohol, does not comprise albumin, and comprises: (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. Another embodiment of the invention provides a method for culturing human hematopoietic stem cells or human blood cells in a culture medium, comprising: to bring human hematopoietic stem cells or human blood cells into contact with polyvinyl alcohol in the culture medium and to bring human hematopoietic stem cells or human blood cells into contact with: (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist TPO. In this modality, the culture allows the number of human hematopoietic stem cells or human blood cells to increase or be maintained. In the invention, the PI3K activator used is not cytotoxic to human hematopoietic stem cells or human blood cells in the absence of albumin and in the presence of PVA. In other words, the PI3K activator used in the invention is not cytotoxic. Furthermore, the TPO receptor agonists used in the invention are not cytotoxic to human hematopoietic stem cells or human blood cells in the absence of albumin and in the presence of PVA. Whether or not the PI3K activators are non-cytotoxic can be verified, if appropriate, by a human hematopoietic stem cell or human blood cell culture test performed by those skilled in the art. Herein, the human hematopoietic stem cell or human blood cell culture test can be implemented using IMDM medium containing 1% insulin-transferrin-selenium, 1% penicillin-streptomycin-glutamine, 100 ng / ml of TPO, and 0.1% PVA. Whether or not TPO receptor agonists are non-cytotoxic can be verified, if appropriate, using a human hematopoietic stem cell culture test or human blood cells, by experts in the technique.Here, the human hematopoietic stem cell or human blood cell culture test can be implemented using IMDM medium containing 1% insulin-transferrinselenium, 1% penicillin-streptomycin-glutamine, 10 ng / ml SCF and 0.1% PVA. In a certain embodiment of the invention, the culture medium used in the culture method of the invention may be a culture medium for human hematopoietic stem cells or human blood cells, the medium comprising: (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. In a certain embodiment of the invention, the culture medium used in the culture method of the invention may be a culture medium for human hematopoietic stem cells or human blood cells, the medium comprising: polyvinyl alcohol or modified polyalkylene glycol and (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. In a certain embodiment of the invention, the culture medium used in the culture method of the invention may be a culture medium for human hematopoietic stem cells or human blood cells, the medium is free of albumin (especially serum albumin), and the medium comprises: (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. In a certain embodiment of the invention, the culture medium used in the culture method of the invention may be a culture medium for human hematopoietic stem cells or human blood cells, the medium comprising: polyvinyl alcohol, but without albumin, and (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. The culture medium of the invention may contain an effective amount of non-cytotoxic PI3K activator and an effective amount of PVA for expanding human hematopoietic stem cells or human blood cells in the absence of albumin. The culture medium of the invention may further contain SCF and / or TPO. The culture medium of the invention may contain an effective amount of non-cytotoxic TPO receptor agonist instead of TPO. The culture medium of the invention may contain an effective amount of non-cytotoxic PI3K activator, an effective amount of PVA, an effective amount of TPO and / or a non-cytotoxic TPO receptor agonist, and UM171. According to the invention, the human hematopoietic stem cell culture method can be a method for producing megakaryocyte lineage cells from human hematopoietic stem cells. In this embodiment, the culture medium used in the culture method can be a culture medium containing a PI3K activator and a TPO receptor agonist, wherein the PI3K activator is 740Y-R and the TPO receptor agonist is butyzamide. In this particular formulation, the culture medium may be free of UM171. In a certain embodiment of the invention, the culture method of the invention may comprise culturing in the presence of 4-N-[2-benzyl-7-(2-methyltetrazol-5-yl)-9H-pyrimid[4,5-b]indol-4-yl]cyclohexane-1,4-diamine (UM171). In a certain embodiment of the invention, the culture medium used in the culture method of the invention may further comprise UM171. This may facilitate the maintenance of human hematopoietic stem cell characteristics or inhibit differentiation into cells of the megakaryocyte lineage (e.g., megakaryocyte progenitors and megakaryocytes). In a certain embodiment of the invention, the culture method of the invention does not comprise culturing in the presence of 4-N-[2-benzyl-7-(2-methyltetrazol-5-yl)-9H-pyrimid[4,5b]indol-4-yl]cyclohexane-1,4-diamine (UM171). In a certain embodiment of the invention, the culture medium used in the culture method of the invention further does not comprise UM171. This facilitates the differentiation of human hematopoietic stem cells into cells of the megakaryocyte lineage (e.g., megakaryocyte progenitors and megakaryocytes). Another embodiment of the invention provides a method for culturing human hematopoietic stem cells or human blood cells, comprising: cultivate human hematopoietic stem cells or human blood cells in a culture medium, wherein the culture medium comprises polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. In this method, the medium for human hematopoietic stem cells or human blood cells may be albumin-free. In this method, the culture medium may further comprise: (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. In this modality, the culture allows the number of human hematopoietic stem cells or human blood cells to be increased or maintained. Another embodiment of the invention provides a method for culturing human hematopoietic stem cells or human blood cells in a culture medium, comprising: Contacting human hematopoietic stem cells or human blood cells in a culture medium with polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. In this method, the medium for the human hematopoietic stem cells or human blood cells may be albumin-free. This method further comprises: to bring into contact, in the culture medium, human hematopoietic stem cells or human blood cells with: (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. In this modality, the culture allows the number of human hematopoietic stem cells or human blood cells to increase or be maintained. The present inventors have described that some compounds, including several TPO receptor agonists, can be cytotoxic to human hematopoietic stem cells or human blood cells in the absence of albumin and in the presence of PVA. In the invention, the PI3K activator of the present document is not cytotoxic to human hematopoietic stem cells or human blood cells in the absence of albumin and in the presence of PVA. Furthermore, the TPO receptor agonist used herein is not cytotoxic to human hematopoietic stem cells or human blood cells in the absence of albumin and in the presence of PVA. Whether or not the PI3K activator is cytotoxic can be verified, if appropriate, by a human hematopoietic stem cell or human blood cell culture assay performed by those skilled in the art. Herein, the human hematopoietic stem cell or human blood cell culture assay can be implemented using IMDM medium containing 1% insulin-trans-selenium, 1% penicillin-streptomycin-glutamine, 100 ng / ml TPO, and 0.1% PVA. Whether or not the TPO receptor agonist is non-cytotoxic can be verified, if appropriate, using a human hematopoietic stem cell culture test or human blood cells, by experts in the technique.Here, the human hematopoietic stem cell or human blood cell culture test can be implemented using IMDM medium containing 1% insulin-transferrinselenium, 1% penicillin-streptomycin-glutamine, 10 ng / ml SCF and 0.1% PVA. 6 In a certain embodiment of the invention, the PI3K activator can be a peptide represented by the amino acid sequence RQIKIWFQNRRMKWKKSDGGYMDMS, wherein Y can be phosphorylated (also called 740Y-P). In a certain embodiment of the invention, the TPO receptor agonist may be 3-[4-[[[4-[2-methoxy-3-(1-tert-butyl-2-oxapentan-l-yl)phenyl]thiazol-2-yl]amino]carbonyl]-2,6-dichlorophenyl]-2-methylpropenoic acid (hereinafter sometimes referred to as butyzamide). In a certain embodiment of the invention, the PI3K activator is 740Y-P and the TPO receptor agonist is butyzamide. In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention may be a serum-free medium. In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention may be a chemically defined medium. In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention is an albumin-free medium, a cytokine-free medium, or an albumin-free and cytokine-free medium (preferably a serum-free medium and more preferably a chemically defined medium). In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention may be a serum-free medium (preferably a chemically defined medium) that is an albumin-free medium.In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention may be a serum-free medium (preferably a chemically defined medium) that is cytokine-free. In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention may be a serum-free medium (preferably a chemically defined medium) that is albumin-free and cytokine-free. In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention may contain a PI3K activator and a TPO receptor agonist. In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention is a serum-free medium (preferably a chemically defined medium) that is albumin-free and cytokine-free and contains polyvinyl alcohol, a PI3K activator, and a TPO receptor agonist.In a certain embodiment of the invention, the culture medium used in the culture method of the invention is a serum-free medium (preferably a chemically defined medium) containing polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. In a certain embodiment of the invention, the culture medium used in the culture method of the invention is a serum-free medium (preferably a chemically defined medium) that may contain: polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block; a PI3K activator instead of SCF; and a TPO receptor agonist instead of TPO.In a certain embodiment of the invention, the culture medium used in the culture method of the invention is an albumin-free, cytokine-free, serum-free medium (preferably a chemically defined medium) that may contain: polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block; a PI3K activator; and a TPO receptor agonist. In a certain embodiment of the invention, the culture medium used in the culture method of the invention is a serum-free medium (preferably a chemically defined medium) that may contain: polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block; a PI3K activator instead of SCF; a TPO receptor agonist instead of TPO; and 4-N-[2-benzyl-7-(2-methyltetrazol-5-yl)9H-pyrimido[4,5-b]indol-4-yl]cyclohexane-1,4-diamine (UM171).In a certain embodiment of the invention, the culture medium used in the culture method of the invention is an albumin-free, cytokine-free, serum-free medium (preferably a chemically defined medium) that may contain: polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block; a PI3K activator; a TPO receptor agonist; and UM171. As used herein, the expression "the culture medium contains B instead of A" means that the culture medium does not include A, but the culture medium contains B. In a certain embodiment of the invention, the medium for culturing human hematopoietic stem cells or human blood cells comprises polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block and (1) a PI3K activator and at least one compound selected from the group consisting of TPO and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist. In this embodiment, the culture medium may be albumin-free. The above medium for culturing human hematopoietic stem cells or human blood cells may further comprise UM171. The above medium for culturing human hematopoietic stem cells or human blood cells may be used in the culture method of the invention. The medium for culturing human hematopoietic stem cells or human blood cells of the invention may be used to expand human hematopoietic stem cells or human blood cells. The medium for culturing human hematopoietic stem cells or human blood cells of the invention may be used to maintain the stem cell status of human hematopoietic stem cells. The medium for culturing human hematopoietic stem cells or human blood cells of the invention may be used to expand human hematopoietic stem cells while maintaining their stem cell status. A culture medium suitable for growing human hematopoietic stem cells or human blood cells may be used, if appropriate. Examples of basal media that can be used include Sclone SF-3 medium, F12 medium, StemSpan (Stem Cell Technologies), STEMa (STEM ALPHA), StemPro-34 serum-free medium (Gibco Invitrogen), StemPro MSC serum-free medium (Invitrogen), HSC-CFU medium (Miltenyl Biotech), S-Clone serum-free medium (SF-02, SF-03, CM-B, SF-B) (Sanko Junyaku), HPGM medium (Sanko Junyaku), AIM V medium (Invitrogen), Marrow MAX bone marrow medium (Invitrogen), KnockOut DMEM / F-12 medium (Invitrogen), Stemline hematopoietic stem cell growth medium (Sigma), SYN serum-free medium (SYN H, SYN B) (AbCys SA), SPE IV medium (AbCys SA), and MyeloCult medium. (StemCell Technologies), HPG serum-free medium (Lonza), UltraCULTURE medium (Lonza), Opti-MEM medium (Gibco Invitrogen and others), MEM medium (Gibco Invitrogen and others),MEMa (Gibco Invitrogen and others), DMEM medium (Gibco Invitrogen and others), IMDM medium (Gibco Invitrogen and others), PRMI1640 medium (Gibco Invitrogen and others), Ham F-12 medium (Gibco and others), or RD medium. The culture medium includes the basal medium. The culture medium may contain one or more or all of the following, for example, insulin, transferrin (apo), sodium selenite, and / or ethanolamine. The culture medium may contain HEPES, sodium pyruvate, vitamins, amino acids, heparin, heparan sulfate, and / or chondroitin sulfate. The culture medium may contain antibiotics (e.g., penicillin and streptomycin). The culture medium may contain glutamine. The culture medium may contain, for example, insulin, transferrin (apo), sodium selenite, ethanolamine, and antibiotics, and may also contain HEPES. The culture medium of the invention may comprise polyalkylene glycol (e.g., polyethylene glycol) modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. In all embodiments of the invention, the polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block can be represented by the following chemical formula: where n is any natural number from 5 to 50, m is any natural number from 10 to 100 and 1 is any natural number from 10 to 200. In a certain modality, n can be any natural number from 5 to 20 or from 10 to 20, m can be any natural number from 20 to 50 or from 30 to 40, and 1 can be any natural number from 30 to 100 or from 50 to 60. The modified polyalkylene glycol can be a compound in which the ethylene glycol unit is an alkylene glycol unit. The alkylene can be a C1-C1 alkylene, for example, C1-C5, and preferably C2 or C3. The culture medium of the invention is designed to expand human hematopoietic stem cells or human hematopoietic cells in the presence of polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block, and may comprise an effective amount of non-cytotoxic PI3K activator and an effective amount of polyalkylene glycol (e.g., polyethylene glycol) modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block. Herein, the culture medium may be albumin-free. The culture medium of the invention may further comprise SCF and / or TPO. The culture medium of the invention may contain an effective amount of non-cytotoxic TPO receptor agonist instead of TPO.The culture medium of the invention may contain an effective amount of non-cytotoxic PI3K activator, an effective amount of PVA, an effective amount of TPO and / or non-cytotoxic TPO receptor agonist, and UM171. According to the invention, the human hematopoietic stem cell culture method can be a method for producing megakaryocyte lineage cells from human hematopoietic stem cells. In this embodiment, the culture medium used in the culture method can be a culture medium containing a PI3K activator and a TPO receptor agonist, wherein the PI3K activator is 740Y-P and the TPO receptor agonist is butyzamide. In this particular embodiment, the culture medium can be free of UM171. In a certain embodiment of the invention, the cultivation method of the invention may further comprise cultivation in the presence of UM171. In a certain embodiment of the invention, the culture medium used in the cultivation method of the invention may further comprise UM171. The human hematopoietic stem cells or human blood cells of the invention can be cultured under conditions suitable for growing human hematopoietic stem cells or human blood cells. In the invention, the method may further comprise isolating the human hematopoietic stem cells or human blood cells from a culture. The human hematopoietic stem cells can be isolated by any method known to those skilled in the art, such as flow cytometry using hematopoietic stem cell markers. The human hematopoietic stem cells can be obtained as human hematopoietic stem cells or can be further differentiated into other cells prior to use. When the human hematopoietic stem cells are differentiated into other cells, the cells to be differentiated can be cultured under conditions suitable for the differentiation of these other cells.Human blood cells can also be isolated by any method known to experts in the technique, such as flow cytometry using markers for human blood cells. The invention provides human hematopoietic stem cells or human blood cells obtained by the culture method of the invention. The human hematopoietic stem cells obtained by the culture method of the invention can be purified using CD34 and preferably CD38 as markers. The human hematopoietic stem cells obtained by the culture method of the invention may have a better uptake rate after transplantation in a recipient than human hematopoietic stem cells obtained by conventional methods. Therefore, the invention provides human hematopoietic stem cells obtained by the culture method of the invention that have a better uptake rate after transplantation in a recipient than before culture. In the culture method of the invention, the culture can be carried out in the presence of fibronectin. The culture method of the invention can be carried out under conditions that allow contact between the hematopoietic stem cells and the fibronectin. The culture is preferably carried out in the culture method of the invention, for example, by coating the interior (e.g., the bottom) of the culture material with fibronectin. The albumin-free medium used in the culture method of the invention contains less than 0.1% (w / v), 0.05% (w / v), 0.01% (w / v), 0.005% (w / v), 0.001% (w / v), 0.0005% (w / v) or 0.0001% (w / v) of serum albumin, or is completely albumin-free. The culture medium used in the culture method of the invention may contain recombinant TPO. The recombinant TPO may be, for example, recombinant mammalian TPO or recombinant human TPO. In a certain embodiment of the invention, the concentration of TPO is preferably from 20 to 200 ng / mL, more preferably from 30 to 150 ng / mL, and even more preferably from 40 to 150 ng / mL, and may be, for example, 100 ng / mL. The culture medium used in the culture method of the invention may further contain recombinant SCF. The recombinant SCF may be, for example, recombinant mammalian SCF or recombinant human SCF. In a particular embodiment of the invention, the SCF concentration is preferably from 1 to 200 ng / mL, more preferably from 1 to 150 ng / mL, even more preferably from 1 to 100 ng / mL (e.g., from 1 to 50 ng / mL), even more preferably from 1 to 30 ng / mL, and even more preferably from 1 to 20 ng / mL (e.g., from 5 to 15 ng / mL). The culture medium used in the culture method of the invention may contain recombinant TPO and recombinant SCF. In this embodiment, the concentration of TPO is preferably 20 to 200 ng / mL, more preferably 30 to 150 ng / mL, and even more preferably 40 to 150 ng / mL (e.g., 100 ng / mL), and the concentration of SCF is preferably 1 to 200 ng / mL, more preferably 1 to 150 ng / mL, even more preferably 1 to 100 ng / mL (e.g., 1 to 50 ng / mL), even more preferably 1 to 30 ng / mL, and even more preferably 1 to 20 ng / mL (e.g., 5 to 15 ng / mL). In a preferred embodiment, the medium used in the culture method of the invention may contain from 40 to 150 ng / mL of recombinant TPO and from 1 to 50 ng / mL of recombinant SCF.In a preferred embodiment, the medium used in the culture method of the invention may have a higher concentration of TPO than the concentration of SCF, and the concentration of TPO may be, for example, any of the concentrations in the above concentration range, and may be 2, 3, 4, 5, 6, 7, 8, 9 or 10 times or more. The culture medium used in the culture method of the invention may further contain FLT3L (in particular, human FLT3L). FLT3 is a cell surface receptor (also called CD135) known as FMS-related tyrosine kinase 3. FLT3 is expressed on the surface of, for example, every hematopoietic stem cell. FLT3L is a ligand for FLT3 and participates in the normal development of hematopoietic stem cells and hematopoietic progenitors. The culture medium used in the culture method of the invention may also contain, in addition to FLT3L, IL-3 and / or GM-CSF. Interleukin-3 (IL-3) is a hematopoietic growth factor and plays an important role in the proliferation and survival of myeloid progenitor cells. GM-CSF is a granulocyte-monocyte colony-stimulating factor and a cytokine that promotes differentiation into hematopoietic stem cells.GM-CSF, acting in conjunction with IL-3 and IL-5, can induce hematopoietic stem cells to differentiate into myeloid progenitor cells. For example, GM-CSF can induce hematopoietic stem cells to differentiate into, for instance, erythroid-forming units (BFU-E), granulocyte-monocyte colony-forming units (CFU-GM), eosinophil colony-forming units (CFU-Eo), and basophil colony-forming units (CFU-Ba). GM-CSF is responsible for the differentiation of CFU-GM into neutrophils and monocytes, and of CFU-Eo into eosinophils. CFU-Ba can be differentiated into eosinophils by IL-3 or IL-5. Therefore, the culture medium used in the culture method of the invention may contain FLT3L and may also contain IL-3 and / or GM-CSF.Furthermore, the culture method of the invention can be implemented under conditions suitable for the proliferation and / or differentiation of each blood cell. Such culture conditions are well known as culture conditions for a medium containing albumin, and are equally applicable to the culture method of the invention using a medium with reduced or no albumin. The culture medium used in the culture method of the invention may contain a PI3K activator instead of SCF and / or a TPO receptor agonist instead of TPO. The culture medium used in the culture method of the invention may contain a PI3K activator instead of SCF and a TPO receptor agonist instead of TPO. The culture medium used in the culture method of the invention may be a cytokine-free medium containing a PI3K activator and a TPO receptor agonist. The culture medium used in the culture method of the invention may contain a PI3K activator instead of SCF and a TPO receptor agonist instead of TPO, and may also contain UM171. The culture medium used in the culture method of the invention may be a cytokine-free medium containing a PI3K activator, a TPO receptor agonist, and UM171. The method for culturing human hematopoietic stem cells according to the invention may further comprise causing the hematopoietic stem cells to proliferate under conditions sufficient for the maintenance and / or proliferation of the hematopoietic stem cells. In this embodiment, for example, the hematopoietic stem cells are caused to expand 10, 50, 100, 200, 300, 400, or 500 times or more from the beginning of the culture. Sufficient conditions for the maintenance and / or proliferation of human hematopoietic stem cells may be, for example, those under which they are cultured in the aforementioned culture medium. Preferably, these conditions may include, for example, the presence of fibronectin, or conditions in which the human hematopoietic stem cells are in contact with fibronectin. The method for culturing human blood cells according to the invention may further comprise causing the human blood cells to proliferate under conditions sufficient for the maintenance and / or proliferation of human blood cells. In this embodiment, for example, hematopoietic stem cells are caused to expand 10, 50, 100, 200, 300, 400, or 500 times or more from the beginning of the culture. The culture method of the invention may further comprise collecting expanded human hematopoietic stem cells or human blood cells from the culture medium. Collected human hematopoietic stem cells or human blood cells can be further enriched or isolated. Human hematopoietic stem cells or human blood cells can be enriched or isolated using one or more cell surface markers. Examples of cell surface marker(s) that can be used for the enrichment or isolation of human hematopoietic stem cells include CD34 and / or CD38. Human hematopoietic stem cells or human blood cells can also be enriched or isolated using a cell sorter. Another embodiment of the invention provides a method for producing human hematopoietic stem cells or human blood cells ex vivo, comprising: the culture method of the invention. The production method of the invention makes it possible to produce functional human hematopoietic stem cells or human blood cells. As used herein, the term functional means that a hematopoietic system can be restored in a human individual (recipient) in whom human hematopoietic stem cells have been transplanted. Examples Reference Example 1: Testing hematopoietic stem cell growth using albumin-free culture medium In this reference example, the growth of mouse hematopoietic stem cells (KSL) and human hematopoietic stem cells (CD34+CD38) was analyzed using an albumin-free culture medium. The culture medium was a serum-free, albumin-free medium containing polyvinyl alcohol (PVA) as described in Wilkinson et al., Nature, 571: 117-121, 2019. Specifically, for culturing mouse hematopoietic stem cells, the culture medium was F12 medium containing 1% insulin-transferrin-selenium-ethanolamine (ITSX), 10 mM HEPES, 1% penicillin-streptomycin-glutamine, 100 ng / ml mouse thrombopoietin (mTPO), and 10 ng / ml mouse stem cell factor (mSCF). The above medium contained PVA at a final concentration of 0.1%. For culturing human hematopoietic stem cells, the culture medium was IMDM medium containing 1% insulin-transferrin-selenium-ethanolamine (ITSX), 25 mM HEPES, 1% penicillin-streptomycin-glutamine, 100 ng / ml human thrombopoietin, and 10 ng / ml human stem cell factor (hSCF). The previous medium contained PVA at a final concentration of 0.1%. In all media used in the following examples, IMDM medium (hereafter also referred to as stock medium) contains 1% insulin-transferrin-selenium-ethanolamine (ITSX), 25 mM HEPES, 1% penicillin-streptomycin-glutamine, and PVA was used unless otherwise noted. Therefore, from now on, we will focus on the components added to the stock medium and describe the medium. For example, since the above medium contains TPO and SCF in addition to the stock medium, the above medium is referred to as medium TPO+SCF for convenience. When the concentration of each factor is also expressed, it can be written as SCF10+TPO100 or abbreviated as S10+T100. Mouse bone marrow KSL cells were used as mouse hematopoietic stem cells. Specifically, mouse bone marrow cells were isolated from the tibia, femur, and pelvis and stained with an APC-c-KIT antibody. C-KIT+ cells were enriched using anti-APC magnetic beads and an LS column (Miltenyi Biotec). The c-KIT-enriched cells were then stained with a lineage antibody cocktail (biotinylated CD4, CD8, CD45R, TER119, LY-6G / LY-6C, and CD127) before being stained with an anti-CD34 antibody, an anti-cKIT antibody, an anti-SCAl antibody, and streptavidin-APC / eFluor 780 for 90 min. Next, cell populations were purified with an Aria II FACS (BD) by direct sorting into wells containing a medium while using propidium iodide as a death staining marker. The human hematopoietic stem cells used were CD34+CD38~ cells from human bone marrow. Specifically, commercially available CD34-positive human bone marrow cells (Lonza 2C-101) were acquired. The results are shown in Figure 1. According to Wilkinson et al. (2019), mouse hematopoietic stem cells failed to maintain long-term proliferation in an albumin- and serum-free medium, even in the presence of SCF and TPO. However, Wilkinson et al. (2019) demonstrated that the addition of PVA to the medium allows mouse hematopoietic stem cells to proliferate for an extended period in an albumin- and serum-free medium. As shown in Figure 1, mouse hematopoietic stem cells proliferated well in a serum-, albumin-, PVA-containing medium, but human hematopoietic stem cells did not proliferate well in this medium. Reference example 2: Differences in signaling between human and mouse hematopoietic stem cells Human and mouse hematopoietic stem cell signaling pathways were analyzed in the presence of SCF and TPO. Phosphorylation immunostaining was used to analyze the degree of phosphorylation of each downstream SCF and TPO signaling factor in human or mouse hematopoietic stem cells in the presence of 10 ng / ml stem cell factor (SCF) and 100 ng / ml thrombopoietin (TPO) in the absence of albumin and in the presence of PVA. First, the amount of phosphorylation of each factor downstream of SCF and TPO signaling was detected using an antibody specific to each form of factor phosphorylation. Specifically, antiphosphorylation antibodies against representative signaling molecules such as Akt, PI3K, and Stat5 were used and reacted with cytokine-stimulated cells. The fluorescence intensity of each reacting antibody was then quantitatively analyzed by fluorescence microscopy. As shown in Figure 2, among the seven factors examined, several were found to have different phosphorylation states between human and mouse hematopoietic stem cells. Specifically, for PI3K and Akt, the phosphorylated forms of PI3K and Akt were highly required in mouse hematopoietic stem cells 24 h after the addition of SCF and TPO, whereas human hematopoietic stem cells had few or no phosphorylated forms, or lower levels of phosphorylated forms than those found in mouse hematopoietic stem cells. Human hematopoietic stem cells were then cultured in a culture medium supplemented with either 0.3 μM AKTa (Sigma-Aldrich, product No. 123871) or 20 μM Pl3Ka. Cells were counted after 3, 5, and 7 days of incubation, and the relative cell count at each time point was calculated, while the cell count on the first day of incubation was set to 1. In the following examples, 740Y-P (supplier name: Tocris, product No. 1983) was used as Pl3Ka. As shown in Figure 3, human hematopoietic stem cells exhibited marked proliferation in the presence of Pl3Ka. In contrast, no proliferative effect of AKTa on human hematopoietic stem cells was observed under the conditions of this experiment. Human hematopoietic stem cells were then cultured to examine whether PI3Ka could completely replace SCF. Human hematopoietic stem cells were cultured for 7 days in the aforementioned human hematopoietic stem cell culture medium with 20 μM of PI3Ka (S10+PI3Ka20+T100) or in the aforementioned culture medium with 20 μM of PI3Ka but without SCF (PI3Ka20+T100). The total number of cells and the number of isolated CD34+ cells obtained were then determined by cell sorting using an anti-CD34 antibody. The results are shown in Figure 4. As shown in Figure 4, when PI3Ka was added, there was no significant difference in the total number of cells or CD34+ cells with or without SCF. Human hematopoietic stem cells were then cultured to examine whether TPO could be replaced by a TPO receptor agonist. Examples of known TPO receptor agonists include butizamide, eltrombopag, or avatrombopag. Human hematopoietic stem cells were cultured in a medium containing 0.1 μM butizamide, 3 μg / ml eltrombopag, or 3 μM avatrombopag in the presence of 0.1% recombinant human serum albumin (Albumin Biosciences) or in the presence of PVA in the aforementioned culture medium without TPO. In this experiment, Mpl32D cells were used as the human hematopoietic stem cells. The results are shown in Figure 5. As shown in Figure 5, in the presence of albumin, butizamide, eltrombopag or avatrombopag was able to promote the proliferation of human hematopoietic stem cells in the absence of TPO.Conversely, in the presence of PVA (in the absence of albumin), butizamide produced a marked cell proliferation effect in human hematopoietic stem cells in the absence of TPO, whereas the effect was small for avatrombopag and almost zero for eltrombopag. In addition, purchased human bone marrow CD34+ cells (supplier: Lonza, product No. 2C-101) or CD34+ cells isolated using fresh umbilical cord blood microbeads were used for the culture experiments. Experiments were performed in 24-well plates at 0.2 to 1.0 x 10⁵ cells per well, with TPO removed from the human hematopoietic stem cell culture medium and replaced with one of the above TPO receptor agonists (hereafter sometimes referred to as prior TPO). The cell count on day seven of culture was determined relative to the cell count on day one of culture. The results are shown in Figure 6. As shown in Figure 6, human hematopoietic stem cells exhibited significant cell proliferation only in the presence of butizamide in the absence of albumin and in the presence of PVA.In the absence of albumin and in the presence of PVA, the presence of avatrombopag or eltrombopag caused cell death in hematopoietic stem cells. However, in the in vivo setting, these TPO receptor agonists are safe compounds used to treat surgically treated patients with cirrhosis or patients with aplastic anemia. The results of this example indicate that some TPO receptor agonists can be cytotoxic to human hematopoietic stem cells in the absence of albumin and in the presence of PVA. This also suggests that some TPO receptor agonists are cytotoxic to human hematopoietic stem cells, and that the use of any agonist that is not cytotoxic to human hematopoietic stem cells is sufficient for culturing human hematopoietic stem cells. Next, it was examined whether SCF and TPO could be replaced by Pl3Ka and a TPO receptor agonist in the absence of albumin and in the presence of PVA. In the following examples, butizamide was used as the TPO receptor agonist. In addition, purchased human bone marrow CD34+ cells (Supplier: Lonza, Product No. 20-101) or CD34+ cells isolated using fresh umbilical cord blood microbeads, as described above, were used for the culture experiments. First, 0.2 to 1.0 x 105 cells were dispensed per well of a 24-well plate, and cultured in a medium with a composition in which SCF was replaced by 20 μM of PI3Ka (20pMPI3Ka+TP0100) and in a medium with a composition in which SCF was replaced by 20 μM of Pl3Ka and TPO was replaced by 0.1 μM of butyzamide (20μM of Pl3Ka+0.1pM of TPO above), respectively.After 7 days, the total number of cells and the number of CD34+ cells were determined by flow cytometry. Subsequently, the growth rate was determined and compared to that at the beginning of incubation. The results are shown in Figure 7. Figure 7 clearly demonstrates that butizamide can completely replace TPO. In addition, CD34+ cells were sorted before and after incubation using a cell sorter, and 100 cells were plated onto Methocult H4415 for the colony assay. After two weeks, each colony was selected under a microscope, a sample was prepared by cytocentrifugation, and the sample was stained with Giemsa. The colony type was then identified under a microscope. GEmM colonies were counted per 50 CD34+ cells, and the rate of increase in the number of colonies was determined compared to before culture.It has then been found that butizamide can completely replace TPO with respect to GEmM colony-forming potential. Human hematopoietic stem cells were then cultured in a medium containing 20 μM PI3Ka and / or 0.1 μM TPO (anterior) relative to a culture medium without SCF or TPO. On day 7 of culture, the total number of cells and the number of included CD34+ cells were counted by flow cytometry, and the growth rate relative to baseline was determined. As shown in Figure 8, PI3Ka alone or butyzamide alone did not cause an increase in cell count, but the presence of both PI3Ka and TPO (anterior) resulted in a marked increase in cell count. Furthermore, the next step was to determine which cell population was most likely to proliferate in a medium where SCF and TPO were replaced by Pl3Ka and previous TPO. Freshly donated cord blood was used to isolate CD34+ cells using microbeads as previously described, and the cells were used for culture experiments. Next, the cells were fractionated using a cell sorter with an anti-CD34-PE-Cy7 antibody (supplier name BD Biosciences, product no. 348791), an anti-CD38-V450 antibody (supplier name BD Biosciences, product no. 646851), an anti-CD133-PE antibody (supplier name Miltenyi Biotec, product no. 130-080-801), an anti-CD45RA-APC antibody (supplier name BioLegend, product no. 304112), and an anti-CD49f-PE antibody (supplier name BioLegend, product no. 313611).For each of the CD34+ fractions, CD34+CD38→CD133+ fraction, or CD34+CD38→CD45RA→CD49f+ fraction, whose molecules have been reported as purification markers for human hematopoietic stem cells derived from human umbilical cord blood, the total cell count and the number of included CD34+ cells were counted by flow cytometry on day 7 after the start of culture, and the growth rate was determined relative to that at the start of culture. The results are shown in Figure 9. As shown in Figure 9, significant cell proliferation was observed in all cell fractions, but the CD34+CD38→CD133+ fraction and the CD34+CD38→CD45RA→CD49f+ fraction, especially the CD34+CD38→CD45RA→CD49f+ fraction, exhibited marked cell growth. Reference Example 3: Experiment for the long-term culture of human hematopoietic stem cells Human hematopoietic stem cells were cultured in the previous human hematopoietic stem cell culture medium, which contained 20 μM of Pl3Ka and 0.1 μM of TPO instead of SCF and TPO. Total cell count and CD34+ cell count were determined as previously described on days 7 and 14 after the start of culture. The results are shown in Figure 10. As shown in Figure 10, the total cell count increased with the number of days of incubation, while the CD34+ cell count decreased on day 14 compared to day 7. When an optical microscope was used to observe the cultured cells, giant cells were detected on day 14. To test whether these giant cells were megakaryocytes or megakaryocyte progenitors, the cells were fractionated by flow cytometry using an anti-CD41a-FITC antibody (supplier: BD Pharmingen, product No. 555466) and an anti-CD42b antibody (supplier: BD Pharmingen, product No. 555473). As shown in Figure 11, the results showed that most cells at 14 days after the start of culture were CD41a+CD42b+ cells. However, as shown in Figure 11, CD34+CD38+ cells continued to proliferate under these conditions. In addition, CD34+ cells were obtained from the culture on day 14 after the start of the culture, and 100 cells were seeded on Methocult H4415 for the colony assay. After 2 weeks, colonies were collected, and samples were prepared by cytocentrifugation. The samples were then subjected to Giemsa staining, and the colony types were determined under a microscope. The results are shown in Figure 12. Regarding the colony types, G indicates granulocytes, E indicates erythroblasts, m indicates macrophages, and M indicates colonies containing megakaryocytes. As shown in Figure 12, most of the cell colonies contained megakaryocytes. Reference Example 4: Further research into suitable conditions for long-term culture of human hematopoietic stem cells Long-term cultivation of human hematopoietic stem cells was attempted in the presence of compounds capable of growing human hematopoietic stem cells. The above culture medium for human hematopoietic stem cells was prepared, containing either 20 μM Pl3Ka and 0.1 μM TPO instead of SCF and TPO (20 μM Pl3Ka + 0.1 pM TPO), or containing SR-1 (500 nM) and / or UM171 (35 nM) (+SR-1, +UM171, or +SR-1 +UM171). Purchased human bone marrow CD34+ cells (Supplier: Lonza, Product No. 2C-101) or CD34+ cells isolated using fresh umbilical cord blood microbeads as described above were used for the culture experiments. First, 0.2 to 1.0 x 10⁵ cells were dispensed per well from a 24-well plate and the CD34+ cells were cultured in the prepared medium. The total cell count and CD34+ cell count were determined as described above on day 14 after the start of culture, and the growth rate was determined relative to that at the start of culture. The results are shown in Figure 13.As shown in Figure 13, the total cell count and the CD34+ cell count increased in the culture medium that also contained UM171, and the growth rate of CD34+ cells was significantly higher in the medium containing UM171 than in the medium without UM171. In contrast, SR-1 caused cell death under these experimental conditions. In addition, the growth rate of the total cell count or the CD34+ cell count was determined by culturing CD34+ cells under three conditions: condition 1: cultured for 14 days in the previous human hematopoietic stem cell culture medium, which contained 20 μM Pl3Ka and 0.1 μM TPO instead of SCF and TPO (20 μM Pl3Ka + 0.1 μM TPO); condition 2: on day 7 or later, cultured in a medium without butizamide (Day 7-without Buti); and condition 3: cultured for 14 days in a medium (20 μM Pl3Ka + 0.1 μM TPO) that also contains UM171 (+UM171). The CD41+ cell count was determined using a flow cytometer. The results are shown in Figure 14. As shown in Figure 14, the CD34+ cell count increased and the CD41+ cell count decreased in condition 2, in which the cells were cultured in the medium without butizamide on day 7 or after the start of culture, compared to condition 1.As shown in the Figure. 14, in condition 3, in which the cells were cultured in the medium that also contained UM171, the growth rate of CD34+ cells increased markedly and the CD41+ cell count decreased markedly compared to condition 1 or 2. Therefore, in serum-free medium conditions in the presence of PVA, PI3Ka and TPO, UM171 was found to cause human hematopoietic stem cells to proliferate and inhibit their differentiation into megakaryocyte progenitors and megakaryocytes (see Figure 15). Reference Example 5: Experiment for CD34+ cell transplantation after culture Human CD34+ cells cultured for 7 days under each of conditions 1 to 3 were transplanted into irradiated NOG mice to test cell engraftment. Specifically, 1 × 10⁴ human CD34+ cells cultured into each NOG mouse irradiated with 1.5 Gy of gamma rays were transplanted. Twelve weeks post-transplant, peripheral blood was collected from each NOG mouse, and the cellular components of the peripheral blood were analyzed using a flow cytometer. The results are shown in Figure 16. As shown in Figure 16, under conditions 1 and 2, HSC (hematopoietic stem cell) engraftment rates increased from 14.9% and 11.1%, respectively, when pre-cultured CD34+ cells were engrafted to 66.6% and 54.9%, respectively, when post-cultured CD34+ cells were engrafted. In condition 3, the HSC grafting rate is shown in Figure 17.As shown in Figure 17, under all conditions, the total cell count of mononuclear cells obtained from human umbilical cord blood decreased with incubation. On the other hand, in condition 3, the percentage of CD34+ cells in the total cell count was significantly higher than in condition 1. Cell viability was also significantly higher in condition 3 than in condition 1. Furthermore, the CD34+ cell count showed a marked increase in condition 3, while the cell count remained relatively stable in condition 1, with no increase observed in the total cell count. Example 1: Testing hematopoietic stem cell growth using albumin-free culture medium In this example, the growth of mouse hematopoietic stem cells (KSL) and human hematopoietic stem cells (CD34+CD38) was analyzed using an albumin-free culture medium. The culture medium was a serum-free, albumin-free medium, as described in Wilkinson et al., Nature, 571: 117-121, 2019, containing each type of polymer instead of polyvinyl alcohol (PVA). Specifically, for mouse hematopoietic stem cell culture, the culture medium was Ham's F12 containing 1% insulin-transferrin-selenium-ethanolamine (ITSX), 10 mM HEPES, 1% penicillin-streptomycin-glutamine, 100 ng / ml mouse thrombopoietin (mTPO), and 10 ng / ml mouse stem cell factor (mSCF). For the culture of human hematopoietic stem cells, the culture medium was IMDM medium containing 1% insulin-transferrin-selenium-ethanolamine (ITSX), 25 mM HEPES, 1% penicillin-streptomycin-glutamine, 100 ng / ml human thrombopoietin (hTPO), and 10 ng / ml human stem cell factor (hSCF). In the following examples, unless otherwise noted, the above culture medium was used to culture hematopoietic stem cells. The polymer added to the medium was the following: polyvinylpyrrolidone K12, povidone K17, povidone, polyoxyethylene polyoxypropylene glycol, a polyvinylcaprolactam-polyvinyl acetate-polyethylene glycol graft copolymer (hereafter referred to as polymer A). More specifically, each culture medium was prepared containing, at a final concentration of 0.1%, BASF's Kolliphor (registered trademark) P188 bio (188 bio), Kolliphor (registered trademark) P 188 Geismar (188 Geismar), Soluplus (Sol+), Kolliphor (registered trademark) P 407 Geismar (407 Geismar), Kollidon (registered trademark) 30 Origin USA (30 USA), Kollidon (registered trademark) 17 PF (17 PF), Kollidon (registered trademark) 25 (25), Kollidon (registered trademark) 90F (90F), or Kollidon (registered trademark) 12PF (12PF). A KSL cell fraction of mouse bone marrow CD34 CD150+ cells was used as mouse hematopoietic stem cells. Specifically, mouse bone marrow cells were isolated from the tibia, femur, and pelvis and stained with an APC-c-KIT antibody. The c-KIT+ cells were enriched using anti-APC magnetic beads and an LS column (Miltenyi Biotec). The c-KIT-enriched cells were then stained with a lineage antibody cocktail (biotylated CD4, CD8, CD45R, TER119, LY-6G / LY-6C, and CD127) before being stained with an anti-CD34 antibody, an anti-cKIT antibody, an anti-SCAl antibody, and streptavidin-APC / eFluor 780 for 90 min. Next, cell populations were purified with an Aria II FACS (BD) by direct sorting into wells containing a medium while using propidium iodide as a death staining marker. The human hematopoietic stem cells used were CD34+CD38~ cells from human bone marrow. Specifically, commercially available CD34-positive human bone marrow cells (Lonza 2C-101) were acquired. Mouse hematopoietic stem cells were cultured in each medium for one week and the number of cells was counted after culture. The results are shown in Figure 18. As shown in Figure 18, mouse hematopoietic stem cells proliferated well in the albumin-free serum medium containing PVA, but not in the presence of any of the other compounds tested instead of PVA, and only in the presence of compound A. Soluplus (registered trademark) is a compound with the following structure: cl / οηη / ζζηζ / Ε / γίΛΐ where n is 13, m is 30 and 1 is 57. Note that, according to Wilkinson et al., 2019, mouse hematopoietic stem cells failed to maintain long-term proliferation in an albumin-free, serum-free medium, even in the presence of SCF and TPO. However, Wilkinson et al., 2019, show in Figure 2b that the addition of PVA to the medium allows mouse hematopoietic stem cells to proliferate for an extended period in an albumin-free, serum-free medium. This has shown that mouse hematopoietic stem cells proliferate well in the presence of PVA or polymer A, even in an albumin-free and serum-free medium. Mouse hematopoietic stem cells were dispensed at a rate of 50 cells / well and cultured in the presence of 0.1% PVA or polymer A and SCF and TPO. The results are shown in Figure 19. Figure 19 shows that mouse hematopoietic stem cells formed colonies well in the presence of PVA or polymer A, even in an albumin-free serum-free medium. This indicates that hematopoietic stem cells have mitotic potential in the presence of PVA or polymer A. The percentage of CD201-positive cells among the proliferated cells was assessed using an alkaline phosphatase-conjugated mouse anti-CD201 antibody (eBiol560). As shown in Figure 20, mouse hematopoietic stem cells cultured in the presence of polymer A had a higher percentage of CD201-positive cells than those cultured in the presence of PVA. CD201 is a well-known marker of hematopoietic stem cells, and the high percentage of CD201-positive cells suggests that stem cell lineage was better maintained after proliferation. In addition, mouse hematopoietic stem cells were cultured in the presence of polymer A or PVA at a final concentration of 0.1% for 1 week and then transplanted into irradiated mice. Four weeks post-transplant, peripheral blood from each irradiated mouse was analyzed to determine the chimerism rate of blood cells derived from the transplanted hematopoietic stem cells. The results showed that hematopoietic stem cells cultured in the presence of polymer A exhibited a chimerism rate (contribution rate) comparable to that of hematopoietic stem cells cultured in PVA. This indicates that the bone marrow regeneration potential of hematopoietic stem cells cultured in the presence of polymer A was equivalent to that of hematopoietic stem cells cultured in the presence of PVA. Example 2: Differences in signaling between human and mouse hematopoietic stem cells Human and mouse hematopoietic stem cell signaling pathways were analyzed in the presence of SCF and TPO. Phosphorylation immunostaining was used to analyze the degree of phosphorylation of each downstream SCF and TPO signaling factor in human or mouse hematopoietic stem cells in the presence of 10 ng / ml stem cell factor (SCF) and 100 ng / ml thrombopoietin (TPO), in the absence of albumin and in the presence of PVA. First, the amount of phosphorylation of each factor downstream of SCF and TPO signaling was detected using an antibody specific to each phosphorylation form of the factor. Specifically, antiphosphorylation antibodies against representative signaling molecules such as Akt, PI3K, and Stat5 were reacted with cytokine-stimulated cells, and the fluorescence intensity of each reacting antibody was quantitatively analyzed by fluorescence microscopy. As shown in Figure 24, among the seven factors examined, several were found to have different phosphorylation states between mouse and human hematopoietic stem cells. Specifically, for PI3K and Akt, the phosphorylated forms of PI3K and Akt were highly required in mouse hematopoietic stem cells 24 h after the addition of SCF and TPO, whereas human hematopoietic stem cells had few or no phosphorylated forms, or lower levels than those found in mouse hematopoietic stem cells. Human hematopoietic stem cells were then cultured in a culture medium supplemented with either 0.3 μM of AKTa (Sigma-Aldrich, Product No. 123871) or 20 μM of Pl3Ka. Cells were counted after 3, 5, and 7 days of incubation, and the relative cell count at each time point was calculated, while the cell count on the first day of incubation was set to 1. In the following examples, 740Y-P (Supplier Name: Tocris, Product No. 1983) was used as Pl3Ka. As shown in Figure 25, human hematopoietic stem cells exhibited marked proliferation in the presence of Pl3Ka. In contrast, no proliferative effect of AKTa on human hematopoietic stem cells was observed under the conditions of this experiment. Human hematopoietic stem cells were then cultured to examine whether Pl3Ka could completely replace SCF. Human hematopoietic stem cells were cultured for 7 days in the aforementioned human hematopoietic stem cell culture medium with 20 μM of Pl3Ka (SI0 + PI3Ka20+T100) or in the aforementioned culture medium with 20 μM of Pl3Ka but without SCF (Pl3Ka20+T100). The total number of cells and the number of isolated CD34+ cells obtained were then determined by cell sorting using an anti-CD34 antibody. The results are shown in Figure 26. As shown in Figure 26, when Pl3Ka was added, there was no significant difference in the total number of cells or CD34+ cells with or without SCF. Human hematopoietic stem cells were then cultured to examine whether TPO could be replaced by a TPO receptor agonist. Examples of known TPO receptor agonists include butizamide, eltrombopag, or avatrombopag. Human hematopoietic stem cells were cultured in a medium containing 0.1 μM butizamide, 3 μg / ml eltrombopag, or 3 μM avatrombopag in the presence of 0.1% recombinant human serum albumin (Albumin Biosciences) or in the presence of PVA in the aforementioned culture medium without TPO. In this experiment, Mpl32D cells were used as the human hematopoietic stem cells. The results are shown in Figure 27. As shown in Figure 27, in the presence of albumin, butizamide, eltrombopag or avatrombopag was able to promote the proliferation of human hematopoietic stem cells in the absence of TPO.Conversely, in the presence of PVA (in the absence of albumin), butizamide produced a marked cell proliferation effect in human hematopoietic stem cells in the absence of TPO, whereas the effect was small for avatrombopag and almost zero for eltrombopag. In addition, purchased human bone marrow CD34+ cells (supplier: Lonza, Product No. 2C-101) or CD34+ cells isolated with fresh umbilical cord blood microbeads were used for the culture experiments. Experiments were performed in 24-well plates with 0.2 to 1.0 x 10⁵ cells per well, with TPO removed from the human hematopoietic stem cell culture medium and replaced with one of the above TPO receptor agonists (hereafter sometimes referred to as prior TPO). The cell count on day seven of culture was determined relative to the cell count on day one of culture. The results are shown in Figure 28. As shown in Figure 28, human hematopoietic stem cells exhibited significant cell proliferation only in the presence of butizamide in the absence of albumin and in the presence of PVA.In the absence of albumin and in the presence of PVA, the presence of avatrombopag or eltrombopag caused cell death in hematopoietic stem cells. However, in the in vivo setting, these TPO receptor agonists are safe compounds used to treat surgically treated patients with cirrhosis or patients with aplastic anemia. The results of this example indicate that some TPO receptor agonists can be cytotoxic to human hematopoietic stem cells in the absence of albumin and in the presence of PVA. This also suggests that some TPO receptor agonists are cytotoxic to human hematopoietic stem cells, and that the use of any agonist that is not cytotoxic to human hematopoietic stem cells is sufficient for culturing human hematopoietic stem cells. Next, it was examined whether SCF and TPO could be replaced by PI3KA and a TPO receptor agonist in the absence of albumin and in the presence of PVA. In the following examples, butizamide was used as the TPO receptor agonist. Additionally, purchased human bone marrow CD34+ cells (supplier: Lonza, product No. 20-101) or CD34+ cells isolated using microbeads of fresh cord blood, as described above, were used for culture experiments. First, 0.2 to 1.0 × 10⁵ cells were dispensed per well of a 24-well plate and cultured in a medium with a composition in which SCF was replaced by 20 μM of PI3KA (20 μM PI3KA + TPO100) and in a medium with a composition in which SCF was replaced by 20 μM PI3KA and TPO was replaced by 0.1 μM butyzamide (20 μM PI3KA + 0.1 μM TPOAGE), respectively. After 7 days, the total number of cells was determined, and the number of CD34+ cells was determined by flow cytometry.Subsequently, the growth rate was determined and compared to that of incubation. The results shown in Fig. 29 clearly demonstrate that butizamide can completely replace TPO. Furthermore, CD34+ cells were sorted before and after incubation using a cell sorter, and 100 cells were seeded on Methocult H4415 for the colony assay. After two weeks, each colony was collected under a microscope, a cytospin sample was prepared, and the sample was Giemsa-stained. The colony type was then identified under a microscope. GEMM colonies were counted per 50 CD34+ cells, and the rate of increase in the number of colonies was calculated compared to before incubation. It was then found that butizamide can completely replace TPO with respect to GEMM colony formation potential. Next, human hematopoietic stem cells were cultured in a medium containing 20 μM of Pl3Ka and / or 0.1 μM of TPO were added to a culture medium without SCF or TPO. On culture day 7, the total number of cells and the number of included CD34+ cells were counted by flow cytometry, and the growth rate relative to the start of culture was determined. As shown in Figure 30, Pl3Ka alone or butizamide alone did not cause an increase in cell count, but the presence of both Pl3Ka and TPO resulted in a marked increase in cell count. Furthermore, the next step was to determine which cell population was most likely to proliferate in a medium where SCF and TPO were replaced by Pl3Ka and previous TPO. Freshly donated cord blood was used to isolate CD34+ cells using microbeads as previously described, and the cells were used for culture experiments. The cells were then fractionated using a cell sorter with an anti-CD34-PE-Cy7 antibody (supplier name BD Biosciences, product no. 348791), an anti-CD38-V450 antibody (supplier name BD Biosciences, product no. 646851), an anti-CD133-PE antibody (supplier name Miltenyi Biotec, product no. 130-080-801), an anti-CD45RA-APC antibody (supplier name BioLegend, product no. 304112), and an anti-CD49f-PE antibody (supplier name BioLegend, product no. 313611). For each of the fractions, the CD34+cl / onοηη / ζζηζ / E / γίΛΐ fraction, the CD34+CD38 fraction, and the CD133+o fraction were obtained. CD34+CD38“CD45RA~CD49f+, molecules that have been reported as purification markers for human hematopoietic stem cells derived from human umbilical cord blood, were included in the total cell count and the number of included CD34+ cells. These counts were performed by flow cytometry on day 7 after the start of culture, and the growth rate was determined relative to that at the start of culture. The results are shown in Figure 31. As shown in Figure 31, significant cell proliferation was observed in all cell fractions, but the CD34+CD38~CD133+ and CD34+CD38~CD45RA-CD49f+ fractions, especially the CD34+CD38“CD45RA“CD49f+ fraction, exhibited marked cell growth. Example 3: Experiment for long-term culture of human hematopoietic stem cells Human hematopoietic stem cells were cultured in the previous human hematopoietic stem cell culture medium, which contained 20 μM of Pl3Ka and 0.1 μM of TPO instead of SCF and TPO. Total cell count and CD34+ cell count were determined as previously described on days 7 and 14 after the start of culture. The results are shown in Figure 32. As shown in Figure 32, the total cell count increased with the number of days of incubation, while the CD34+ cell count decreased on day 14 compared to day 7. When a light microscope was used to observe the cultured cells, giant cells were detected on day 14. To test whether these giant cells were megakaryocytes or megakaryocyte progenitors, the cells were fractionated by flow cytometry using an anti-CD41a-FITC antibody (supplier: BD Pharmingen, product no. 555466) and an anti-CD42b antibody (supplier: BD Pharmingen, product no. 555473). As shown in Figure 33, the results showed that most cells at day 14 of the start of culture were CD41a+CD42b+ cells. However, as shown in Figure 33, CD34+CD38~ cells continued to proliferate under these conditions. In addition, CD34+ cells were obtained from the culture on day 14 after the start of the culture, and 100 cells were seeded onto Methocult H4415 for the colony assay. After 2 weeks, colonies were collected, and samples were prepared by cytocentrifugation. The samples were then subjected to Giemsa staining, and colony types were determined under a microscope. The results are shown in Figure 34. Regarding colony types, G indicates granulocytes, E indicates erythroblasts, m indicates macrophages, and M indicates colonies containing megakaryocytes. As shown in Figure 34, the majority of cell colonies contained megakaryocytes. Example 4: Further investigation of suitable conditions for long-term culture of human hematopoietic stem cells Long-term cultivation of human hematopoietic stem cells was attempted in the presence of compounds capable of growing human hematopoietic stem cells.The aforementioned human hematopoietic stem cell culture medium was prepared, containing either 20 μM Pl3Ka and 0.1 μM TPO instead of SCF and TPO (20 μM Pl3Ka + 0.1 pM TPO), or containing SR-1 (500 nM) and / or UM171 (35 nM) (+SR-1, +UM171, or +SR-1 +UM171). Additionally, purchased human bone marrow CD34+ cells (Supplier: Lonza, Product No. 20-101) or CD34+ cells isolated using fresh umbilical cord blood microbeads, as described above, were used for the culture experiments. First, 0.2 to 1.0 x 10⁵ cells were dispensed per well from a 24-well plate and the CD34+ cells were cultured in the prepared medium. The total cell count and CD34+ cell count were determined as described above on day 14 after the start of culture, and the growth rate was determined relative to that at the start of culture. The results are shown in Figure 35.As shown in Figure 35, the total cell count and the CD34+ cell count increased in the culture medium that also contained UM171, and the growth rate of CD34+ cells was significantly higher in the medium containing UM171 than in the medium without UM171. In contrast, SR-1 caused cell death under these experimental conditions. In addition, the growth rate of the total cell count or the CD34+ cell count was determined by culturing CD34+ cells under three conditions: condition 1: cultured for 14 days in the previous human hematopoietic stem cell culture medium, which contained 20 μM Pl3Ka and 0.1 μM TPO instead of SCF and TPO (20 μM Pl3Ka + 0.1 μM TPO); condition 2: on day 7 or later, cultured in a medium without butizamide (Day 7-without Buti); and condition 3: cultured for 14 days in a medium (20 μM Pl3Ka + 0.1 μM TPO) that also contains UM171 (+UM171). The CD41+ cell count was determined using a flow cytometer. The results are shown in Figure 36. As shown in Figure 36, the CD34+ cell count increased and the CD41+ cell count decreased in condition 2, in which the cells were cultured in the medium without butizamide on day 7 or after the start of culture, compared to condition 1.As shown in Figure 36, in condition 3, in which the cells were cultured in the medium which also contained UM171, the growth rate of the CD34+ cells increased markedly and the cell count. CD41+ decreased markedly compared to condition 1 or 2. Therefore, under serum-free medium conditions in the presence of PVA, Pl3Ka and TPO, UM171 was found to cause human hematopoietic stem cells to proliferate and inhibit their differentiation into megakaryocyte progenitors and megakaryocytes (see Figure 37). Example 5: Experiment for transplantation of CD34+ cells after culture Human CD34+ cells cultured for 7 days under conditions 1 to 3 were transplanted into irradiated NOG mice to test cell engraftment. Specifically, 1 × 10⁴ human CD34+ cells cultured into each NOG mouse irradiated with 1.5 Gy of gamma rays were transplanted. Twelve weeks post-transplant, peripheral blood was collected from each NOG mouse, and the cellular components of the peripheral blood were analyzed using a flow cytometer. The results are shown in Figure 38. As shown in Figure 38, under conditions 1 and 2, HSC (hematopoietic stem cell) engraftment rates increased from 14.9% and 11.1%, respectively, when pre-cultured CD34+ cells were engrafted to 66.6% and 54.9%, respectively, when post-cultured CD34+ cells were engrafted. In condition 3, the HSC engraftment rate increased from 17%, when precultured CD34+ cells were engrafted, to 67%, when postcultured CD34+ cells were engrafted. These results have shown that the PI3K activator and the TPO receptor agonist can induce favorable proliferation of human hematopoietic stem cells under serum-free medium conditions in the absence of albumin and in the presence of PVA, and that the proliferated human CD34+ cells can maintain their HSC characteristics and the engraftment rate after transplantation to other individuals can be improved. Furthermore, human hematopoietic stem cells tend to proliferate as CD34+ cells, while some of them differentiate into megakaryocytes when cultured under these conditions for an extended period, thus producing megakaryocytes. The addition of UM171 to the cells increased the growth rate of the CD34+ cells and inhibited their differentiation into megakaryocytes. Human mononuclear cells were then isolated from fresh human umbilical cord blood. Under conditions 1 and 3, the obtained cells (5 × 10⁵ cells) were cultured, and the total cell count, CD34+ cell count, percentage of CD34+ cells in the total cell count, and cell viability were determined on days 7 and 14 after the start of culture. The results are shown in Figure 39. As shown in Figure 39, under all conditions, the total cell count of mononuclear cells obtained from human umbilical cord blood decreased with incubation. On the other hand, under condition 3, the percentage of CD34+ cells in the total cell count was significantly higher than under condition 1. Cell viability was also significantly higher under condition 3 than under condition 1.Furthermore, the CD34+ cell count showed a marked increase in condition 3, while the cell count remained favorable in condition 1, while no increase in cell count was observed. Example 6: Human hematopoietic stem cell culture experiment in the presence of polymer A The examples above have shown that human hematopoietic stem cells, unlike mouse hematopoietic stem cells, require activation of the PI3K pathway, and that TPO can be replaced by butyzamide, a TPO receptor agonist. It has also been found that human hematopoietic stem cells can be cultured in the absence of UM171 when not cultured for an extended period, but that differentiation into megakaryocyte lineages can be inhibited by culturing human hematopoietic stem cells in the presence of UM171 for an extended period. In the experiments in this Example, human hematopoietic stem cells were cultured in the presence of polymer A, a PI3K activator, and a receptor agonist of TPO and UM171 were used, while PVA was replaced with polymer A in the previous experiments. As a control experiment, human hematopoietic stem cells were cultured in the presence of PVA, a PI3K activator, a TPO receptor agonist, and UM171. PI3Ka was used as the PI3K activator and butizamide as the TPO receptor agonist, and the concentrations were as in the previous examples. The results are shown in Figure 22. As shown in Figure 22, human hematopoietic stem cells cultured in the presence of polymer A instead of PVA had a higher percentage of CD34-positive cells. Furthermore, the percentage of CD34-positive cells was higher in the presence of polymer A than in the presence of PVA. This suggests that human hematopoietic stem cells proliferate well in the presence of polymer A and under conditions that activate the PI3K pathway (and in the absence of SCF). The results also suggest that human hematopoietic stem cells can proliferate well even when TPO is replaced by a TPO receptor agonist under these conditions. Human hematopoietic stem cells have also been found to proliferate well in the presence of UM171. Furthermore, changes in cell count were observed over time. In this experiment, human hematopoietic stem cells were cultured for 3 weeks using cl / onn / zznz / E / YiAi, a culture medium supplemented with a T-cell expansion kit (Miltenyi Biotec) and human IL-2 (Peprotech). The results are shown in Figure 23. As shown in Figure 23, human hematopoietic stem cells proliferated equally in the presence of PVA or in the presence of polymer A. Example 7: To culture human CD3-positive cells Human CD3-positive cells were stimulated with anti-CD28 and anti-CD3 antibodies using a Human T Cell Activation / Expansion Kit (Miltenyi Biotec) under standard conditions and cultured in an albumin- and cytokine-free basal medium (IMDM medium containing 1% ITSX and 1% penicillin) containing either 0.1% PVA or 0.1% Soluplus (registered trademark) for 6 weeks. The initial cell count was set at 1000 cells. The results are shown in Figure 40. As shown in Figure 40, CD3-positive cells (primarily T cells) were shown to proliferate well in the presence of Soluplus or PVA. This demonstrates that an albumin-free culture medium can be used for the maintenance and proliferation of human T cells. Example 8: Differentiation of human CD34-positive cells into hematopoietic cells Human CD34-positive cells (hematopoietic stem cells) were cultured in an albumin-free basal medium containing PVA or Soluplus (registered trademark). The initial cell count was set at 1000 cells. Hematopoietic stem cells were differentiated into hematopoietic cells by adding a cytokine cocktail to the medium. Culture was maintained for 10 days. The results are shown in Figures 41A and 41B. As shown in Figure 41A, the number of CD34-positive cells cultured in the medium increased favorably in either the presence of Soluplus or PVA. As shown in Figure 41B, the cells in the culture medium obtained by culturing in the presence of Soluplus contained megakaryocytes, erythroblasts, neutrophils, and macrophages. The results demonstrated that almost all cells of the blood cell lineages were obtained and that each of them proliferated, indicating that human blood cells can expand, maintain themselves, and differentiate well in the presence of an additive such as PVA or Soluplus, even in an albumin-free environment. Example 9: To culture chronic myeloid leukemia (CML) cells Bone marrow cell samples from patients with CML (chronic myeloid leukemia) were cultured for one week under albumin-free and cytokine-free conditions. The culture medium used was a basal medium containing 0.1% PVA, 20 μM Pl3Ka, and 0.1 μM TPOanterior. Culture was performed in the presence or absence of an inhibitor (imatinib (IM)) of Bcr-Abl, the gene responsible for CML. The results are shown in Figure 42. As shown in Figure 42, the results demonstrated 0.5% cell proliferation in the absence of IM compared to the total cell count before culture. Specifically, a more than 30-fold expansion / maintenance was observed in CD34-positive cells, a fraction of leukemia stem cells, representing a more than 6-fold expansion / maintenance of the absolute cell count. Conversely, no significant cell expansion was observed in the presence of IM, indicating that the cells proliferating in IM+ were leukemia stem cells from CML. It is hereby stated that, as of this date, the best method known to the applicant for putting the aforementioned invention into practice is the one that is clear from the present description of the invention.

Claims

CLAIMS Having described the invention as above, the following claims are claimed as property:

1. A method for culturing human cells, characterized in that it comprises culturing human cells in a culture medium, wherein the culture medium is a serum albumin-free medium and comprises an additive, the additive being selected from the group consisting of polyvinyl alcohol and modified polyalkylene glycol, and further comprising (1) a phosphatidylinositol 3-kinase (PI3K) activator and at least one compound selected from the group consisting of thrombopoietin (TPO) and TPO receptor agonists; (2) a TPO receptor agonist and at least one compound selected from the group consisting of stem cell factor (SCF) and PI3K activators; or (3) a PI3K activator and a TPO receptor agonist, such that the culture enables the increase or maintenance of the number of human hematopoietic stem cells or human blood cells.

2. The method according to claim 1, characterized in that the human cells are cells selected from the group consisting of human hematopoietic stem cells and human blood cells.

3. The method according to claim 1, characterized in that the human cells are blood cells other than hematopoietic stem cells.

4. The method according to claim 3, characterized in that the additive is polyvinyl alcohol.

5. The method according to claim 1 or 2, characterized in that the additive is modified polyalkylene glycol. 6.The method according to claim 5, characterized in that the modified polyalkylene glycol is polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block.

7. The method according to claim 6, characterized in that the modified polyalkylene glycol is polyethylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block.

8. A method for culturing human cells, characterized in that it comprises culturing human cells in a culture medium, wherein the culture medium comprises polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block.

9. The method according to claim 8, characterized in that the increased number of human cells is obtained. 10.The method according to claim 8 or 9, characterized in that the human cells are human hematopoietic stem cells.

11. A culture medium composition for the cultivation of human cells, characterized in that it comprises polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block.

12. A composition, characterized in that it comprises human cells and polyalkylene glycol modified with a copolymer containing a polyvinylcaprolactam block and a polyvinyl acetate block.

13. The composition according to claim 11 or 12, characterized in that the human cells are human hematopoietic stem cells.

14. Human cells, characterized in that they are obtained by the method according to any one of claims 8 to 10.