Folic acid sandwich method antibody mAb18 or antigen binding fragment thereof, and preparation method and application thereof

Abstract

The invention belongs to the technical field of immunodetection, and discloses a folic acid sandwich method antibody mAb18 or an antigen binding fragment thereof, and a preparation method and application thereof. The folic acid sandwich antibody mAb18 or the antigen binding fragment thereof comprises a light chain variable region VL and a heavy chain variable region VH, the light chain variable region VL comprises complementary determining regions LCDR1, LCDR2 and LCDR3, and the heavy chain variable region VH comprises complementary determining regions HCDR1, HCDR2 and HCDR3. When the folic acid sandwich method antibody mAb18 is used for detecting a folic acid standard antigen, the detection sensitivity is lower than 0.2 ng / mL, a clinical sample is detected through a magnetochemiluminescence method, the correlation with R clinical comparison is good in the sample range of 0-20 ng / mL, and the folic acid sandwich method antibody mAb18 has important significance in the field of diagnosis and treatment of birth defects, cardiovascular diseases, tumors and neurodegenerative diseases.

Description

Technical Field

[0001] This application relates to the field of immunoassay technology, specifically to a folic acid sandwich antibody mAb18 or its antigen-binding fragment, its preparation method, and its application. Background Technology

[0002] Folic acid (vitamin B9), a water-soluble B vitamin, is not only a key coenzyme in cell metabolism and proliferation, but also considered an essential nutrient for human growth, development, and health maintenance due to its central role in nucleic acid and amino acid synthesis. With ongoing research, the clinical significance of folic acid has far exceeded the traditional scope of nutrition. Its functions in preventing neural tube defects in fetuses, reducing the risk of cardiovascular and cerebrovascular diseases, assisting in tumor prevention and treatment, and delaying neurodegenerative diseases have made it a highly regarded micronutrient in modern precision medicine. Folic acid deficiency can lead to clinical consequences such as megaloblastic anemia, immune dysfunction, and fetal developmental abnormalities; however, excessive supplementation also poses potential risks. Therefore, establishing accurate and efficient folic acid detection methods is of great significance for guiding clinical nutritional interventions and achieving early screening and monitoring of related diseases.

[0003] Currently, folic acid detection technologies mainly fall into two categories: one is the gold standard method based on mass spectrometry, which, while possessing high accuracy and specificity, relies on expensive instruments, complex sample pretreatment procedures, and specialized operators, making it difficult to meet the needs of large-scale, time-sensitive clinical testing; the other is immunological detection methods, which, due to their ease of operation, speed, and automation, have become the standard choice for clinical laboratories. However, due to the small molecular weight and weak immunogenicity of folic acid, traditional immunoassays often employ competitive methods. These methods have inherent technical limitations: generally low sensitivity, specificity easily affected by sample matrix, insufficient stability of test results, and complex process development, making precise quantification difficult, thus restricting their application in low-concentration sample testing and high-end clinical diagnostics.

[0004] Therefore, existing immunoassay techniques cannot fully meet the clinical demand for highly sensitive and specific quantitative analysis of folic acid. To overcome this technical bottleneck, it is urgent to break through the technical barriers that make it difficult to detect small molecules using sandwich methods, and to develop anti-folate monoclonal antibodies with high affinity and high specificity, laying the foundation for establishing a dual-antibody sandwich detection system. Summary of the Invention

[0005] This application provides folic acid sandwich antibody mAb18 or its antigen-binding fragment, which has high specificity and affinity, as well as its preparation method and applications. Antibody mAb18 or its antigen-binding fragment can successfully enable rapid detection of folic acid in immunoassay kits prepared by double-antibody sandwich ELISA or chemiluminescence methods, which is of great significance in the diagnosis and treatment of birth defects, cardiovascular diseases, tumors, and neurodegenerative diseases.

[0006] In one aspect, this application provides a folic acid sandwich antibody mAb18 or its antigen-binding fragment, comprising a light chain variable region VL and a heavy chain variable region VH. The light chain variable region VL includes LCDR1, LCDR2, and LCDR3. The amino acid sequence of LCDR1 is the sequence shown in SEQ ID NO.1 or a variant with one or two amino acid substitutions, deletions, or additions compared to the sequence shown in SEQ ID NO.1. The amino acid sequence of LCDR2 is ETS (SEQ ID NO.2) or a variant having one amino acid substitution, deletion, or addition compared to ETS. The amino acid sequence of LCDR3 is the sequence shown in SEQ ID NO.3 or a variant with one or two amino acid substitutions, deletions, or additions compared to the sequence shown in SEQ ID NO.3; The heavy chain variable region VH includes HCDR1, HCDR2, and HCDR3. The amino acid sequence of HCDR1 is the sequence shown in SEQ ID NO.4 or a variant with one or two amino acid substitutions, deletions, or additions compared to the sequence shown in SEQ ID NO.4. The amino acid sequence of HCDR2 is the sequence shown in SEQ ID NO.5 or a variant with one or two amino acid substitutions, deletions, or additions compared to the sequence shown in SEQ ID NO.5. The amino acid sequence of HCDR3 is the sequence shown in SEQ ID NO.6 or a variant with one or two amino acid substitutions, deletions or additions compared to the sequence shown in SEQ ID NO.6; Furthermore, the folic acid sandwich antibody mAb18 or its antigen-binding fragment can specifically bind to folic acid.

[0007] Further, the light chain variable region VL comprises an amino acid sequence having more than 95% homology to the amino acid sequence shown in SEQ ID NO.7, wherein any one or more amino acids have been substituted, deleted, and / or added, and / or terminally modified; the heavy chain variable region VH comprises an amino acid sequence having more than 95% homology to the amino acid sequence shown in SEQ ID NO.8, wherein any one or more amino acids have been substituted, deleted, and / or added, and / or terminally modified.

[0008] Furthermore, amino acid sequences with 95%, 96%, 97%, 98%, or 99% identity.

[0009] Furthermore, the light chain variable region VL includes the amino acid sequence shown in SEQ ID NO.7, and the heavy chain variable region VH includes the amino acid sequence shown in SEQ ID NO.8.

[0010] Furthermore, the antigen-binding fragment is one of F(ab')2, Fab', Fab, Fv, scFv, dsFv, and bispecific antibodies.

[0011] Secondly, this application also provides a biomaterial selected from polynucleotides, carriers, or cells. The polynucleotide encodes the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in the first aspect; The vector carries the polynucleotide; The cell carries the polynucleotide, or contains the carrier, or is capable of expressing the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in the first aspect.

[0012] Furthermore, the cells are eukaryotic cells, preferably mammalian cells, more preferably 293 cells or CHO cells.

[0013] Thirdly, this application also provides a method for preparing the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in the first aspect, comprising culturing cells as described in the second aspect, wherein the cells are prepared by transforming the cells with a polynucleotide encoding a folic acid sandwich antibody mAb18 or its antigen-binding fragment, the polynucleotide comprising a heavy chain expression plasmid and a light chain expression plasmid, and the transformation comprising co-transforming the heavy chain expression plasmid and the light chain expression plasmid into the cells.

[0014] Fourthly, this application also provides the application of the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in the first aspect, or the biological material as described in the second aspect, wherein the application is selected from one or more of the following: 1) Folic acid testing for purposes other than disease diagnosis and treatment; 2) Prepare immunoassay products for detecting folic acid; 3) Used for purifying folic acid; 4) Prepare products for purifying folic acid.

[0015] In this application, the application is not for diagnostic or therapeutic purposes. Specifically, it can be used to assess the folic acid nutritional status of specific regions and populations (such as pregnant women and the elderly) to provide data support for public health policies; it can be used for antibody detection to rapidly screen engineered strains or cell lines that produce high levels of folic acid or its intermediate metabolites (such as dihydrofolate and tetrahydrofolate); it can also be used in microbial fermentation monitoring to monitor the concentration of folic acid in the fermentation broth in real time and optimize fermentation conditions during the production of folic acid using probiotics or yeast; it can also be used to study the metabolic, transport, and regulatory mechanisms of folic acid in organisms, as well as its interactions with other nutrients or biomolecules.

[0016] Fifthly, this application also provides a folic acid immunoassay product, which comprises the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in the first aspect, or the biological material as described in the second aspect.

[0017] Furthermore, the folic acid immunoassay product is a reagent or kit for detection using enzyme-linked immunosorbent assay (ELISA), Western blot, immunohistochemical staining (IHC), flow cytometry (FCM), immunoprecipitation, and magnetic microparticle chemiluminescence immunoassay.

[0018] Compared with the prior art, this application provides a folic acid-specific antibody mAb18 or its antigen-binding fragment, which exhibits both high specificity and high affinity when binding folic acid, with an affinity constant (Ka) reaching 9 × 10⁻⁶. 8 L / mol. The antibody is suitable for preparing immunoassay kits for folic acid detection, especially showing excellent performance in double-antibody sandwich ELISA and chemiluminescence detection systems. Experiments have demonstrated that the folic acid-specific antibody mAb18 of this application, based on a double-antibody sandwich chemiluminescence method, exhibits a detection sensitivity better than 0.15 ng / mL for folic acid standard antigens. Analysis of clinical samples using a magnetochemiluminescence platform showed good correlation with the reference method (R method) within a concentration range of 1–20 ng / mL. This antibody and related detection methods have significant clinical application prospects in the auxiliary diagnosis and treatment monitoring of birth defect screening, cardiovascular diseases, tumors, and neurodegenerative diseases. Attached Figure Description

[0019] Figure 1 This is an electrophoresis diagram of the full-length amplification products of the mAb18 heavy and light chains, where M is the DNA molecular weight marker.

[0020] Figure 2 The mAb18 sandwich ELISA method was used to detect cross proteins and target antigen samples. The vertical axis represents the detected OD value.

[0021] Figure 3 The standard curve for folic acid standard antigen was determined using mAb18 magnetic microparticle chemiluminescence immunoassay. The x-axis represents the concentration of folic acid standard antigen (ng / mL), and the y-axis represents the detected luminescence value. The R² of the standard curve was 0.9993, and the linear detection range was 0-20 ng / mL. The formula for calculating the sample concentration was derived as: y = 2540.1x² - 4653.5x + 11969.

[0022] Figure 4 The mAb18 magnetic microparticle chemiluminescence method was used to detect folic acid in clinical samples. The x-axis represents the concentration of free folic acid in the clinical samples (ng / mL), and the y-axis represents the detected luminescence value. The standard curve had an R² of 0.9646, and the linear detection range was 0-20 ng / mL. The formula for calculating the sample concentration was derived as: y = 52044x - 77598. Detailed Implementation

[0023] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Experimental methods in the following embodiments that do not specify specific conditions are generally performed under conventional conditions, conditions described in a laboratory manual, or conditions recommended by the manufacturer.

[0024] Example 1: Preparation of rabbit monoclonal antibody using the folic acid sandwich method

[0025] 1) Preparation of immunogens The folic acid complex was prepared with an immunogen purity of over 90%, meeting the purity requirements for preparing monoclonal antibodies.

[0026] 2) Animal immunization The folic acid complex prepared above was emulsified with complete Freund's adjuvant at a 1:1 volume ratio and administered subcutaneously to approximately 2 kg New Zealand white rabbits at a dose of 800 μg / rabbit. A second immunization was performed two weeks later, emulsified with incomplete Freund's adjuvant at a 1:1 volume ratio, at a dose of 400 μg / rabbit. Tail blood was collected after both immunizations and serum titers were determined using a serially diluted ELISA method. The OD450 at an ELISA titer of 128,000 was considered to be greater than 1.0. Based on the results, it was determined whether to collect PBMCs or continue immunization. Rabbits with the highest antibody titers were selected for PBMC collection.

[0027] 3) PBMC isolation, specific B cell sorting, and clonal recombination The rabbit was placed supine on the operating table. The fur around the heart was trimmed, and the skin was disinfected with alcohol. The area with the most prominent heartbeat was selected and punctured with a 50mL syringe. Blood flowed into the syringe immediately after the needle entered the heart. The needle was quickly withdrawn after obtaining the required amount of blood. The whole blood in the syringe was transferred into a sterile 50mL tube and mixed with an equal amount of PBS. The mixture was then slowly added dropwise to the lymphocyte separation medium. The mixture was centrifuged at 400×g for 30 minutes at room temperature. After centrifugation, the liquid surface separated into four layers from top to bottom: a yellow plasma layer, a white thin film layer (i.e., a mononuclear cell layer), a separation medium layer, and a red blood cell layer. The mononuclear cell layer was carefully aspirated and washed with PBS to remove platelets and lymphocyte separation medium, thus obtaining rabbit PBMCs.

[0028] Antigen-specific B cells were further sorted from rabbit PBMCs and cultured. Positive clones were selected from the B cell supernatant using antigen-coated ELISA plates. Cells from positive clones were collected, lysed, and RNA was extracted and reverse transcribed into cDNA. The full-length light and heavy chain sequences of naturally paired rabbit monoclonal antibodies were amplified from the cDNA of the corresponding positive clones. Rabbit monoclonal antibody expression vectors were constructed using clonal recombination methods, and the sequences were confirmed by sequencing. The results of the amplified full-length PCR products are shown below. Figure 1 .

[0029] 4) Preparation and purification of monoclonal antibodies To obtain multiple rabbit monoclonal antibodies that recognize folic acid molecules, the heavy and light chain genes of rabbit monoclonal antibodies were loaded into an expression vector. The plasmid was transfected into KEK293 cells, and after 120-144 hours, the culture supernatant contained recombinant rabbit monoclonal antibodies recognizing folic acid molecules. The cell suspension was collected, the supernatant was obtained by centrifugation, and the antibody was purified by affinity chromatography. The concentration of the purified monoclonal antibody was determined by the BCA method, and then aliquoted, lyophilized, and named rabbit monoclonal antibody mAb18.

[0030] Example 2: Identification of rabbit monoclonal antibodies using the folic acid sandwich method.

[0031] 1) Specificity identification of rabbit monoclonal antibodies Indirect ELSA was used for detection. The ELISA plate was coated with cross-linked protein and folic acid complex antigen at a concentration of 1 μg / mL and incubated overnight at 4°C. The plate was then blocked with PBST containing 1% BSA. 4 The purified rabbit monoclonal antibody was diluted 10 times and reacted at 37°C for 50 min. The plate was washed 3 times with PBST, HRP-goat anti-rabbit IgG secondary antibody was added, and the plate was reacted at 37°C for 50 min. The plate was washed 5 times with PBST, TMB was added for color development for 10 min, stop solution was added, and the A450 was measured by microplate reader.

[0032] Figure 2The results showed that the cross protein reacted negatively with the rabbit monoclonal antibody mAb18, with OD450 values ​​less than 0.1; the folic acid complex reacted positively with the mAb18 antibody, with OD values ​​much higher than those of the cross protein, indicating that the folic acid sandwich method rabbit monoclonal antibody of this application specifically recognizes the folic acid complex.

[0033] 2) Determination of affinity constant of rabbit monoclonal antibody Affinity constant (Ka) was determined using a non-competitive ELISA method.

[0034] Coating: Dilute the antigen with carbonate buffer to concentrations of 1, 0.5, 0.1, and 0.05 μg / mL, add 100 μL / well to a 96-well microplate for coating, and incubate at 4°C for 24 h.

[0035] Blocking: Wash the plate 4 times with PBST, add BSA solution at 200 μL / well, and incubate at 37°C for 2 h.

[0036] Add monoclonal antibody: Wash the plate 4 times with PBST, serially dilute the rabbit monoclonal antibody with carbonate buffer starting at 100 μg / mL, add 100 μL to each well, and incubate at 37°C for 2 h.

[0037] Add enzyme-labeled secondary antibody: Wash the plate 4 times with PBST, add 100 μL of HRP enzyme-labeled goat anti-rabbit Ig secondary antibody diluted 1:10000 to each well, and incubate at 37℃ for 30 min.

[0038] Color development and termination: Wash the plate 4 times with PBST, add 100 μL of substrate color development solution to each well, and react at 37℃ in the dark for 15 min; add 50 μL of 1.0 mol / L H2SO4 stop solution to each well to terminate the reaction.

[0039] Detection: The absorbance value at a wavelength of 450 nm (A450nm) was measured.

[0040] Plotting the logarithm of antibody concentration on the x-axis and OD value on the y-axis, an S-shaped curve was obtained. The calculated affinity constant Ka for the folic acid sandwich monoclonal antibody mAb18 was 9 × 10⁻⁶. 8 L / mol.

[0041] 3) Sandwich antibody pairing To select the optimal combination of coating and detection antibodies, folic acid-binding protein was coated onto an ELISA plate and incubated overnight at 4°C. The next day, the plate was removed, washed once with PBST, blocked with 1% BSA solution at 37°C for 2 hours, and washed three times with PBST. 100 μl of folic acid (20 ng / mL) was added to each well, and the plate was incubated at 37°C for 1 hour. After incubation, the plate was removed, washed three times with PBST, and HRP-labeled rabbit monoclonal antibody mAb18 was added as the detection antibody, incubated at 37°C for 1 hour. The plate was washed five times with PBST, TMB substrate was added, and the plate was incubated at 37°C for 10 minutes. After incubation, stop solution was added, and the OD450 reading was measured using an ELISA reader. Based on the OD values ​​of the samples and the background value of the negative control, the most ideal antibody pair was selected. The pairing screening results are shown in Table 1.

[0042] Table 1. Results of antibody pairing assay

[0043] Therefore, the antibody mAb18 involved in this application is optimal for sandwich assay.

[0044] Example 3: Analysis of the gene and amino acid sequence of the variable region of a monoclonal antibody

[0045] Using the recombinant plasmid of the antibody as a DNA template, sequencing primers for the light chain variable region and heavy chain variable region were designed based on the vector sequences at the 5' ends of the light and heavy chains on the template. Sequencing was performed using an ABI 3730 sequencer. The nucleotide sequences of the light and heavy chain variable regions of the rabbit monoclonal antibody were obtained by sequencing.

[0046] Using the internet and the IMGT / V-QUEST analysis software at http: / / www.imgt.org, the nucleotide sequences of the light chain variable region and the heavy chain variable region were analyzed. The amino acid sequence of the light chain variable region of rabbit monoclonal antibody mAb18 is shown in SEQ ID NO.7, and the amino acid sequence of the heavy chain variable region is shown in SEQ ID NO.8.

[0047] The VL is 113 amino acids in length. The number of amino acids in the four domains of its FR are 26, 17, 36 and 11, respectively. The number of amino acids in the three domains of LCDR are 8, 3 and 12, respectively. The regions of LCDR1, LCDR2 and LCDR3 are 27aa-34aa, 52aa-54aa and 91aa-102aa, respectively. Their amino acid sequences are QSVYKNND (SEQ ID NO.1), ETS (SEQ ID NO.2) and AGGYSDSSDFYA (SEQ ID NO.3), respectively.

[0048] The VH is 122 amino acids in length. The number of amino acids in the four domains of its FR are 25, 16, 40 and 11, respectively. The number of amino acids in the three domains of HCDR are 8, 7 and 15, respectively. HCDR1, HCDR2 and HCDR3 are 26aa-33aa, 50aa-56aa and 97aa-111aa, respectively. Their amino acid sequences are GFSLSSYA (SEQ ID NO.4), IYARGTT (SEQ ID NO.5) and ARDEYLDIDYWTFNL (SEQ ID NO.6), respectively.

[0049] Example 4: Folic acid sandwich method rabbit monoclonal antibody used for magnetic microparticle chemiluminescent immunoassay. 1. Detection Principle and Method A magnetic microparticle chemiluminescence immunoassay technique based on the double-antibody sandwich method was employed. Biotin-labeled folic acid-binding protein was immobilized with SA magnetic beads, and ALP was conjugated to a rabbit monoclonal antibody using the folic acid sandwich method. Simultaneously, folic acid, ALP substrate, and corresponding buffer components were placed on a Cosma fully automated magnetic microparticle chemiluminescence analyzer, and the instrument program was set for detection. A positive result was interpreted as a signal-to-noise ratio (SNR) greater than 2.0. The magnitude of the luminescence value reflects the amount of bound enzyme-labeled antibody and is directly proportional to the folic acid concentration in the sample. A standard curve was plotted based on the measured luminescence values ​​of the standards, as shown below. Figure 3 The folic acid concentration in the sample to be tested can be obtained from the standard curve.

[0050] 2. Composition of the magnetic particle chemiluminescence detection kit for folic acid detection 1) SA magnetic beads bound to biotin-folic acid binding protein: Take 50 μl of magnetic beads into a 0.5 mL centrifuge tube, place it on a magnetic rack, and remove the supernatant after 1 min; wash the magnetic beads 3 times with 0.5 mL of antibody dilution buffer; add a certain amount of biotin-labeled folic acid binding protein and mix at room temperature for 60 min; after magnetic separation, resuspend in magnetic preservation buffer at a working concentration of 0.5 mg / mL.

[0051] 2) mAb18 conjugation with ALP: First, the 2-IT antibody mAb18 is reduced; then, an ALP-SMCC intermediate is formed; finally, ALP-SMCC is conjugated with the reducing antibody. After conjugation, the ALP is diluted to the working concentration using ALP storage buffer.

[0052] 3) Washing buffer: is standard pH 7.4 PBST containing 0.05% Proclin 300, prepared as a 20-fold concentrate.

[0053] 4) Chemiluminescent colorimetric solution: purchased from Aivid Biotechnology.

[0054] 5) Sample diluent: PBST containing 1% BSA and 0.05% Proclin 300, filtered for sterilization.

[0055] 6) Standard: Folic acid (small molecule), diluted to 5 μg / mL with PBS containing 1% BSA, 5% sucrose, 10% glycerol and 0.05% Proclin 300, filtered for sterilization and aseptically dispensed.

[0056] 3. Testing of folic acid in clinical samples Clinical samples with different folic acid concentrations were processed. Using folic acid-binding protein as the coating antibody and mAb18 antibody as the detection antibody, the above-described detection method was used to detect clinical samples of different concentrations. The results are shown below. Figure 4 .

[0057] Based on the results, the rabbit monoclonal antibody described in this application, used in a magnetic microparticle chemiluminescent immunoassay reagent, showed good correlation with clinical results within a sample range of 0-20 ng / mL.

[0058] In summary, when the folic acid sandwich rabbit monoclonal antibody mAb18 of this application is applied to immunoassay kits prepared by double-antibody sandwich ELISA or chemiluminescence methods, the detection sensitivity of folic acid standard antigen on the double-antibody sandwich chemiluminescence platform is less than 0.2 ng / mL. When detecting clinical samples by magnetochemiluminescence, the clinical composite rate is >0.96 in the sample range of 0-20 ng / mL, which is significantly higher than that of traditional competitive detection methods. Moreover, the process is simple and breaks through the limitations of traditional competitive methods.

[0059] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.

Claims

1. A folic acid sandwich antibody mAb18 or its antigen-binding fragment, characterized in that, Includes the light chain variable region VL and the heavy chain variable region VH. The light chain variable region VL includes LCDR1, LCDR2, and LCDR3. The amino acid sequence of LCDR1 is the sequence shown in SEQ ID NO.1 or a variant with one or two amino acid substitutions, deletions, or additions compared to the sequence shown in SEQ ID NO.

1. The amino acid sequence of LCDR2 is ETS or a variant of ETS with one amino acid substitution, deletion, or addition. The amino acid sequence of LCDR3 is the sequence shown in SEQ ID NO.3 or a variant with one or two amino acid substitutions, deletions or additions compared to the sequence shown in SEQ ID NO.3; The heavy chain variable region VH includes HCDR1, HCDR2, and HCDR3. The amino acid sequence of HCDR1 is the sequence shown in SEQ ID NO.4 or a variant with one or two amino acid substitutions, deletions, or additions compared to the sequence shown in SEQ ID NO.

4. The amino acid sequence of HCDR2 is the sequence shown in SEQ ID NO.5 or a variant with one or two amino acid substitutions, deletions, or additions compared to the sequence shown in SEQ ID NO.

5. The amino acid sequence of HCDR3 is the sequence shown in SEQ ID NO.6 or a variant with one or two amino acid substitutions, deletions or additions compared to the sequence shown in SEQ ID NO.6; Furthermore, the folic acid sandwich antibody mAb18 or its antigen-binding fragment can specifically bind to folic acid.

2. The folic acid sandwich antibody mAb18 or its antigen-binding fragment according to claim 1, characterized in that, The light chain variable region VL comprises an amino acid sequence having more than 95% homology to the amino acid sequence shown in SEQ ID NO.7, obtained by substitution, deletion, and / or addition of one or more amino acids and / or terminal modification of any one or more amino acids; the heavy chain variable region VH comprises an amino acid sequence having more than 95% homology to the amino acid sequence shown in SEQ ID NO.8, obtained by substitution, deletion, and / or addition of one or more amino acids and / or terminal modification of any one or more amino acids.

3. The folic acid sandwich antibody mAb18 or its antigen-binding fragment according to claim 2, characterized in that, The light chain variable region VL includes the amino acid sequence shown in SEQ ID NO.7, and the heavy chain variable region VH includes the amino acid sequence shown in SEQ ID NO.

8.

4. The folic acid sandwich antibody mAb18 or its antigen-binding fragment according to claim 1, characterized in that, The antigen-binding fragment is one of F(ab')2, Fab', Fab, Fv, scFv, dsFv, and bispecific antibodies.

5. A biomaterial, characterized in that, The biomaterial is selected from polynucleotides, carriers, or cells. The polynucleotide encodes the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in any one of claims 1-4; The vector carries the polynucleotide; The cell carries the polynucleotide, or contains the carrier, or is capable of expressing the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in any one of claims 1-4.

6. The method for preparing folic acid sandwich antibody mAb18 or its antigen-binding fragment according to any one of claims 1-4, characterized in that, The method includes culturing cells as described in claim 5, wherein the cells are prepared by transforming cells with a polynucleotide encoding a folic acid sandwich antibody mAb18 or an antigen-binding fragment thereof, the polynucleotide including a heavy chain expression plasmid and a light chain expression plasmid, and the transformation includes co-transforming the heavy chain expression plasmid and the light chain expression plasmid into the cells.

7. The application of the folic acid sandwich antibody mAb18 or its antigen-binding fragment according to any one of claims 1-4, or the biomaterial according to claim 5, characterized in that, The application is selected from one or more of the following. 1) Folic acid testing for purposes other than disease diagnosis and treatment; 2) Prepare immunoassay products for detecting folic acid; 3) Used for purifying folic acid; 4) Prepare products for purifying folic acid.

8. A folic acid immunoassay product, characterized in that, The immunoassay product comprises the folic acid sandwich antibody mAb18 or its antigen-binding fragment as described in any one of claims 1-4, or the biological material as described in claim 5.

9. The folic acid immunoassay product according to claim 8, characterized in that, The folic acid immunoassay products are reagents or kits for detection using enzyme-linked immunosorbent assay (ELISA), Western blot, immunohistochemical staining (IHC), flow cytometry (FCM), immunoprecipitation, and magnetic microparticle chemiluminescence immunoassay.

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