Reporter cell

Abstract

The invention relates to a method of assessing binding of an antibody or Fc fusion protein to its target antigen, comprising: (i) contacting the antibody / Fc fusion protein with a reporter T cell which

Description

Field of Invention This disclosure concerns a method of assessing binding of an antibody or Fc fusion protein to its target antigen. This disclosure also concerns a reporter T cell, a kit, and a composition for assessing binding of an antibody or Fc fusion protein to a target antigen. Background to the Invention Monoclonal antibodies (mAbs) are an important class of drug that mediate targeted drug therapies in a wide array of diseases including cancer and autoimmune conditions. mAbs are particularly attractive for disease therapeutics due to (i) the high specificity and affinity that can be achieved; (ii) their diversity, allowing for a wide range of targets, cell types and applications; and (iii) the ability to further engineer and refine these drugs, facilitating an even greater range of functional activities. From a safety perspective, it is essential to fully characterise the functional activity of any mAb before use, in order to ensure clinical efficacy. The functional activity of a mAb refers to its ability to produce a biological effect, and this is mediated by its two functional fragments: the fragment antigen-binding region (Fab region) and the fragment crystallisable region (Fc region). The Fab region is the antigen-binding domain of the mAb and contains the variable regions that allow the mAb to bind specifically to different epitopes on antigens. The Fab region therefore dictates the target that the mAb binds to. Meanwhile, the Fc region of the mAb has no antigen-binding activity and is instead responsible for the interaction of the mAb with effector molecules (e.g. complement system proteins) and binding to Fc receptors on cells, allowing the mAb to activate the immune system and induce an immune response. The Fc region therefore dictates the type of immune response elicited by the mAb. Functional assays to determine the activity of the mAb can therefore be based on either the Fab region or the Fc region. Fab-based functional assays mainly involve determining the direct effect of mAb antigen binding on the antigen-expressing cell itself, e.g., measuring target cell proliferation or apoptosis upon binding of the mAb to antigens expressed on the target cell. Fc-based assays, meanwhile, mainly measure the activation of immune effector cells in response to the interaction of mAb Fc regions with Fc receptors on effector cells or to complement proteins, e.g., measuring the activation of phagocytosis by macrophages upon binding of mAb Fc regions to Fc receptors on macrophages; or measuring the release of cytotoxic granules by natural killer (NK) cells upon binding of mAb Fc regions to Fc receptors on NK cells. Existing functional assays have issues with sensitivity and are not optimised for determining the functional activity of antibodies, for example in applications for extending shelf life of the antibodies. Enabling the preparation or procurement of ready to use biopharmaceuticals and in particular for monoclonal antibody (mAb), mAb fragment products or fusion proteins, is driven by both patient safety and potential cost savings from more efficient product use. However, for most ready to use biopharmaceuticals the shelf life assigned within the SmPC is often 24 to 48 hours, requiring that extended shelf life must be applied to these products to enable more efficient procurement and use by healthcare providers. Within the United Kingdom, extended shelf-lives can be applied to antibody therapeutics prepared under a Section 10 exemption or a Specials Licence, where robust data exists to support them. Here, the National Health Service (NHS) Guidance document 'A Standard Protocol for Deriving and Assessment of Stability; Part 2: Aseptic Preparations (Biopharmaceuticals) incorporating addendum on antibody drug conjugates. Edition 5, June 2021 j serves to define the criteria necessary for a robust study of stability, towards the application of an extended shelf-life to the clinical product. Procurement of aseptically compounded biopharmaceuticals should only be considered where either the shelf life assigned is within the SmPC or where the stability study and any additional data (e.g. end of shelf-life testing) has been assessed as suitable in line with requirements of this NHS guidance document. Assessment of biological properties constitutes an essential step in establishing an understanding of the stability profile under specific conditions. The technique should be relevant to the specific biological activity that enables the product to achieve its defined biological effect. These assays measure biochemical or physiological response at the cellular level. A cell-based assay needs to show that the molecule will have the defined biological activity for its function, hence each assay is likely to be specific for one particular aspect of biological activity and two or more cell-based assays may be necessary to assess products with complex mechanisms of action, for example a monoclonal antibody may engage two effector functions. There exists a need to provide improved assays to determine the functional activity of antibodies and other Fc-based molecules, particularly for applications of extending shelf-life in line with requirements of the NHS guidance document. Summary of the Invention Described herein is a reporter cell-based method based on the interaction between antibodies and chimeric receptors. The reporter cell assesses the ability of antibodies to bind to and activate chimeric receptors by measuring downstream reporter gene induction following binding of the antibody to chimeric receptors expressed on the reporter cells. The method is also able to assess the ability of Fc fusion proteins to bind to and activate Fc receptors in a similar manner. This invention therefore provides a convenient, robust and efficient method of evaluating the functional activity and consequently the clinical efficacy of therapeutic antibodies and Fc fusion proteins. According to a first aspect, there is provided a method of assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the method comprises: (i) contacting the antibody or Fc fusion protein with a reporter T cell, wherein the reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain, and wherein the reporter T cell comprises a reporter gene responsive to activation of the chimeric receptor, wherein: (a) the extracellular domain is an Fc receptor extracellular domain, and wherein the method further comprises contacting the antibody or Fc fusion protein with a secondary cell expressing the target antigen on its surface; or (b) the extracellular domain is a PD-1 extracellular domain, and wherein the method optionally further comprises contacting the antibody or Fc fusion protein with a secondary cell expressing the target antigen on its surface; and (ii) detecting activation of the reporter gene. Advantageously, the method can be used to assess the functional activity of the antibody or Fc fusion protein, and can distinguish between non-degraded and degraded forms of the antibody or Fc fusion protein. This is particularly useful as a readout of antibody stability and may therefore be used as a stability assay to assign extended shelf life to the antibody or Fc fusion protein. The expression "method of assessing binding of an antibody or Fc fusion protein to its target antigen", as used herein, refers to the assessment of the activated signalling brought about by the binding of the antibody to its target antigen or the Fc fusion protein to its binding partner. The method may also be referred to as a method of assessing the functional activity of an antibody or Fc fusion protein. In some embodiments, the method is a method of assessing binding of an antibody to a target antigen. The term "antibody", as used herein, refers to a protein, or polypeptide sequences derived from an immunoglobulin molecule, which specifically binds to an antigen. Antibodies can be intact immunoglobulins of polyclonal or monoclonal origin, or functional fragments thereof, and can be derived from natural or from recombinant sources. When a functional antibody fragment is used, the functional fragment must be appropriate for engagement with the chimeric receptor extracellular domain, i.e., the functional fragment must comprise the Fc region for engagement with an Fc receptor extracellular domain, whereas the functional fragment must comprise the Fab region for engagement with a PD-1 extracellular domain. In some embodiments, the method is a method of assessing binding of a monoclonal antibody to a target antigen. In some embodiments, the method is a method of assessing binding of an Fc fusion protein to its binding partner. The term "Fc fusion protein", as used herein, refers to a recombinant protein comprising an Fc domain of an immunoglobulin covalently linked to a peptide or protein of interest. The binding partner of the Fc fusion protein is a molecule which specifically binds to the peptide or protein of interest. The step of detecting activation of the reporter gene can be done in any suitable manner after step (i). In some embodiments, activation of the reporter gene is detected by providing a substrate to the reporter T cell and measuring the conversion of the substrate into another product, for example through luminescence, absorbance or fluorescence detection methods. In some embodiments, the expression of the reporter protein encoded by the reporter gene may be quantified, for example through luminescence, absorbance or fluorescence detection methods. Typically, activation of the reporter gene is detected by a reporter gene assay. In a particular embodiment, the reporter gene encodes luciferase and activation of the reporter gene results in expression of luciferase. Activation of the reporter gene in this embodiment is detected by providing a luciferase substrate and detecting the luminescence signal of the luciferase reaction. In some embodiments, the method further comprises: (iii) determining that the antibody or Fc fusion protein is functional if the detected activation of the reporter gene is at least 80% of a reference value and no more than 120% of a reference value. The skilled person will recognise that the readout from the method is a detected value of reporter gene activation, which can be used as a substitute for functional activity of the antibody or Fc fusion protein. The reporter gene activation detected in the method may be compared against a reference value to determine whether the reporter gene activation, and thus functional activity, of the antibody of Fc fusion protein is within an acceptable range. The reference value may be selected in any suitable manner, as would be known to the skilled person. Typically, the reference value is the detected activation of the reporter gene when the method is carried out on a control antibody / Fc fusion protein (i.e., having the same structure and binding specificity as the product being tested) known to have maximal functional activity. For example, a control antibody may be an antibody that has not been subjected to storage and / or conditions that may result in a loss of functional activity. The reference value may be obtained during the method (e.g., by running parallel tests with a control antibody / Fc fusion protein) or may be pre-determined (i.e., a historical reference value). The skilled person will appreciate that the reference value may be a single data point, or be represented by a range. In some embodiments, the antibody or Fc fusion protein is determined as functional if the detected activation of the reporter gene is at least 85% of a reference value and no more than 115% of a reference value. In some embodiments, the antibody or Fc fusion protein is determined as functional if the detected activation of the reporter gene is at least 90% of a reference value and no more than 110% of a reference value. In some embodiments, the antibody or Fc fusion protein is determined as functional if the detected activation of the reporter gene is at least 95% of a reference value and no more than 105% of a reference value. In some embodiments, the antibody or Fc fusion protein has an assigned shelf life, wherein the method further comprises: (iv) extending the assigned shelf-life of the functional antibody or Fc fusion protein. The skilled person will appreciate that biologies, such as antibodies and Fc fusion proteins, are provided with a shelf life to indicate how long the product can be stored before an unacceptable loss in stability or function is reached. Typically, a shelf life is provided for a product after reconstitution. In other embodiments, a shelf life may be given for a packaged and unopened product. In certain embodiments, a first shelf life is provided for storage of the reconstituted product at 4°C and a second shelf life is provided for storage of the reconstituted product at ambient temperature. Typically, the shelf life is provided in the summary of product characteristics (SmPC). The skilled person will appreciate that the assigned shelf life of a particular biologic or batch of biologies can be reviewed and extended, such that a product that is nearing the end of its shelf life can be assessed for functional activity and the shelf life can be extended if the product has an acceptable level of functional activity. The methods of the present disclosure are suitable and intended for these shelf life reviews. In some embodiments, the assigned shelf-life of the functional antibody or Fc fusion protein is extended by up to 1 month. In some embodiments, the assigned shelf-life of the functional antibody or Fc fusion protein is extended by up to 2 months. In some embodiments, the assigned shelf-life of the functional antibody or Fc fusion protein is extended by up to 3 months. In some embodiments, the assigned shelf-life of the functional antibody or Fc fusion protein is extended by up to 4 months. In some embodiments, the assigned shelf-life of the functional antibody or Fc fusion protein is extended by up to 5 months. In some embodiments, the assigned shelf-life of the functional antibody or Fc fusion protein is extended by up to 6 months. The target antigen is the molecule that the antibody is targeted to. Similarly, the binding partner is the molecule that specifically binds to the peptide or protein of interest of the Fc fusion protein. The skilled person will appreciate that the invention is not limited by any particular target antigen or binding partner. In some embodiments, the target antigen or binding partner is an antigen or binding partner that is differentially expressed on a diseased or abnormal cell compared to a healthy cell. In some embodiments, the target antigen or binding partner is a tumour-associated antigen (i.e., an antigen overexpressed on cancer cells and expressed at lower levels on healthy cells) or a tumour-specific antigen (i.e., an antigen expressed by cancer cells and not by healthy cells). In some embodiments, the target antigen or binding partner is CD19, CD38, CD20 or HER-2. In some embodiments, the target antigen or binding partner is CD20 or HER-2. The reporter T cell may be any suitable T cell provided the T cell can be engineered to stably express the chimeric receptor and express the reporter gene. In some embodiments, the reporter T cell is a cell-line cell. In some embodiments, the reporter T cell is an immortalised cell-line cell. In some embodiments, the reporter T cell is a Jurkat cell. The skilled person will recognise that the reporter T cell is a platform that can be used to evaluate the binding of an antibody or Fc fusion protein, and the reporter T cell is not itself a therapeutic product (such as a chimeric antigen receptor (CAR.) T cell). In some embodiments, the reporter T cell does not express the target antigen or binding partner. As an example, the reporter T cell may not express a tumour-associated antigen or a tumour-specific antigen. The reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain selected from an Fc receptor extracellular domain or a PD-1 extracellular domain. The expression "chimeric receptor", as used herein, refers to an engineered receptor having at least two amino acid sequences that are not naturally linked in nature. The amino acid sequences normally exist in separate proteins that are brought together in the chimeric receptor. The term "extracellular domain", as used herein, refers to the part of the chimeric receptor that is located outside of the cell membrane and is capable of binding to an antibody or Fc fusion protein. The extracellular domain is selected from an Fc receptor extracellular domain or a PD-1 extracellular domain. In some embodiments, the extracellular domain is an Fc receptor extracellular domain. In some embodiments, the extracellular domain is an Fey receptor extracellular domain. In some embodiments, the extracellular domain is an FcyRIIIa extracellular domain. In some embodiments, the extracellular domain is a PD-1 extracellular domain. The extracellular domain may comprise one or more additional sequences, such as a spacer sequence. The term "Fc receptor extracellular domain" refers to the entire extracellular cellular domain of the Fc receptor, or a functional fragment thereof that retains the ability to bind to the Fc region of an antibody. Similarly, term "PD-1 extracellular domain" refers to the entire extracellular cellular domain of PD-1, or a functional fragment thereof that retains the ability to bind to a PD-1 ligand. In some embodiments, the extracellular domain comprises the amino acid sequence as defined in SEQ ID NO: 3 or 8. In some embodiments, the extracellular domain comprises the amino acid sequence as defined in SEQ ID NO: 3. In some embodiments, the extracellular domain comprises the amino acid sequence as defined in SEQ ID NO: 8. In some embodiments, the chimeric receptor does not comprise a singlechain variable fragment. The term "transmembrane domain", as used herein, refers to the portion of the chimeric receptor that extends across the cell membrane and anchors the chimeric receptor to the cell membrane. The transmembrane domain can be any suitable transmembrane domain provided it connects the extracellular and cytoplasmic portions of the chimeric receptor. Typically, the transmembrane domain connects the extracellular domain to the intracellular signalling domain. However, the skilled person will appreciate that a spacer may be present between the extracellular domain and the transmembrane domain, and / or a spacer may be present between the transmembrane domain and the intracellular signalling domain. In some embodiments, the transmembrane domain is naturally associated with one or more of the domains in the chimeric receptor, for example the intracellular signalling domain. In some embodiments, the transmembrane domain comprises a CD28 transmembrane domain. The term "intracellular signalling domain", as used herein, refers to the part of the chimeric receptor that is located inside of the cell membrane and is capable of transducing an effector signal. While usually the entire intracellular signalling domain can be employed, in many cases it is not necessary to use the entire chain. To the extent that a truncated portion of the intracellular signalling domain is used, such truncated portion may be used in place of the intact chain as long as it transduces the effector signal. The term intracellular signalling domain is thus meant to include any truncated portion of the intracellular signalling domain sufficient to transduce the effector signal. In some embodiments, the intracellular signalling domain comprises a CD3^-chain and one or more additional co-stimulatory domains, most often derived from CD28. In some embodiments, the intracellular signalling domain comprises a CD28-CD3£ intracellular signalling domain. In some embodiments, the chimeric receptor comprises the sequences as defined in SEQ ID NOs: 4 and 5. Without wishing to be bound by theory, the combination of signalling domains from multiple receptors is thought to improve the strength of signalling and thus enhance the sensitivity of the method. In some embodiments, the chimeric receptor comprises an amino acid sequence as defined in SEQ ID NOs: 6 or 9. In some embodiments, the chimeric receptor comprises the amino acid sequence as defined in SEQ ID NO: 6. In some embodiments, the chimeric receptor comprises the amino acid sequence as defined in SEQ ID NO: 9. In some embodiments, the chimeric receptor comprises or consists of an amino acid sequence having at least 85% identity to SEQ ID NOs: 6 or 9 and is capable of binding to an antibody or Fc fusion protein. In some embodiments, the chimeric receptor comprises or consists of an amino acid sequence having at least 90% identity to SEQ ID NOs: 6 or 9 and is capable of binding to an antibody or Fc fusion protein. In some embodiments, the chimeric receptor comprises or consists of an amino acid sequence having at least 95% identity to SEQ ID NOs: 6 or 9 and is capable of binding to an antibody or Fc fusion protein. In some embodiments, the chimeric receptor comprises or consists of an amino acid sequence having at least 98% identity to SEQ ID NOs: 6 or 9 and is capable of binding to an antibody or Fc fusion protein. In some embodiments, the chimeric receptor is encoded by a nucleotide sequence as defined in SEQ ID NO: 7 or 10, or a codon-optimised version thereof. In some embodiments, the chimeric receptor is encoded by a nucleotide sequence having at least 80% sequence identity to SEQ ID NO: 7 or 10. In some embodiments, the chimeric receptor is encoded by a nucleotide sequence having at least 90% sequence identity to SEQ ID NO: 7 or 10. In some embodiments, the chimeric receptor is encoded by a nucleotide sequence having at least 95% sequence identity to SEQ ID NO: 7 or 10. The reporter T cell comprises a reporter gene responsive to activation of the chimeric receptor. The term "reporter gene", as used herein, refers to a nucleic acid molecule comprising a nucleotide sequence encoding a reporter protein operably linked to a promoter that is responsive to activation of the chimeric receptor. As such, activation of the chimeric receptor activates the promoter which drives the expression of the reporter gene. The reporter protein can then be assayed, for example by detecting enzymatic activity or spectrophotometric characteristics, or indirectly using antibody-based assays. Various reporter genes are known to and may be implemented by the skilled person. See, for example, Serganova, I. and Blasberg, R.G., Molecular Imaging with Reporter Genes: Has Its Promise Been Delivered?, Journal of Nuclear Medicine Dec 2019, 60 (12) 1665-1681; DOI: 10.2967 / jnumed. 118.220004. In some embodiments, the reporter gene encodes a reporter protein selected from a fluorescent protein, a luminescent protein, a chemiluminescent protein, or an enzyme. In some embodiments, the reporter gene encodes luciferase. In some embodiments, the reporter gene comprises or consists of a nucleotide sequence as defined in SEQ ID NO:2. In some embodiments, the reporter gene comprises or consists of a nucleotide sequence having at least 80% identity to SEQ ID NO: 2 and encoding a functional luciferase protein. In some embodiments, the reporter gene comprises or consists of a nucleotide sequence having at least 85% identity to SEQ ID NO: 2 and encoding a functional luciferase protein. In some embodiments, the reporter gene comprises or consists of a nucleotide sequence having at least 90% identity to SEQ ID NO: 2 and encoding a functional luciferase protein. In some embodiments, the reporter gene comprises or consists of a nucleotide sequence having at least 95% identity to SEQ ID NO: 2 and encoding a functional luciferase protein. In some embodiments, the reporter gene comprises or consists of a nucleotide sequence having at least 98% identity to SEQ ID NO: 2 and encoding a functional luciferase protein. In some embodiments, the reporter gene comprises or consists of a nucleotide sequence having at least 99% identity to SEQ ID NO: 2 and encoding a functional luciferase protein. In some embodiments, the luciferase is any one or more of GLuc, NanoLuc (NLuc), MLuc7, HtLuc, LoLuc, PaLucI, PaLuc2, MpLucI, McLucI, MaLucI, MoLucI, MoLuc2, MLuc39, PsLucI, LocLucl-3, HtLuc2 Renilla, TurboLucl6 (TLuc) or homologs or orthologs thereof. The skilled person will appreciate that the invention is not restricted by the particular reporter gene exemplified in the Examples. Instead, it is envisaged that any suitable reporter gene can be used. The promoter may be any suitable promoter that is responsive to activation of the chimeric receptor. In some embodiments, the promoter is an IL-2 promoter. In some embodiments, the promoter is a minimal IL-2 promoter. In some embodiments, the promoter comprises or consists of a nucleotide sequence as defined in SEQ ID NO: 1. In some embodiments, the promoter comprises or consists of a nucleotide sequence having at least 80% identity to SEQ ID NO: 1. In some embodiments, the promoter comprises or consists of a nucleotide sequence having at least 85% identity to SEQ ID NO: 1. In some embodiments, the promoter comprises or consists of a nucleotide sequence having at least 90% identity to SEQ ID NO: 1. In some embodiments, the promoter comprises or consists of a nucleotide sequence having at least 95% identity to SEQ ID NO: 1. In some embodiments, the promoter comprises or consists of a nucleotide sequence having at least 98% identity to SEQ ID NO: 1. In some embodiments, the promoter comprises or consists of a nucleotide sequence having at least 99% identity to SEQ ID NO: 1. In embodiments of the method where the chimeric receptor comprises an Fc receptor extracellular domain, the antibody or Fc fusion protein is contacted with a secondary cell expressing the target antigen / binding partner on its surface. The secondary cell may be any suitable cell that provides (1) the antigen / binding partner of the antibody or Fc fusion protein being tested on the surface of the secondary cell, such that the antibody or Fc fusion protein specifically binds to its antigen / binding partner and provides an activation signal to the secondary cell; and (2) co-receptor binding to the reporter T cell. The skilled person will recognise that the antigen or binding partner present on the surface of the secondary cell must correspond to the antibody or Fc fusion protein being assessed. For example, if the antibody being assessed is an anti-CD20 antibody, the antigen present on the secondary cell will be CD20. As a further example, if the Fc fusion protein being assessed is a soluble tumour necrosis factor (TNF) receptor:Fc fusion protein, the binding partner present on the secondary cell will be a molecule which specifically binds to TNF receptor, such as TNF. The skilled person is able to select the appropriate secondary cell for use in the method based on the antibody or Fc fusion protein being assessed. In some embodiments, the secondary cell is a cell line cell. In some embodiments, the secondary cell is an immortalised cell line cell. In some embodiments, the secondary cell is a Raji cell, a SKBR3 cell, or a Daudi cell. In embodiments of the method where the chimeric receptor comprises a PD-1 extracellular domain, optionally the antibody or Fc fusion protein is contacted with a secondary cell expressing the target antigen / binding partner on its surface. The inventors have demonstrated that co-receptor binding via the secondary cell is not essential for the method to operate when PD-1 is used as the extracellular domain. In particular embodiments, the antibody or Fc fusion protein is contacted with a secondary cell expressing the target antigen / binding partner on its surface. The secondary cell may be any suitable secondary cell as discussed above. According to a second aspect, there is provided a reporter T cell for assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain selected from an Fc receptor extracellular domain or a PD-1 extracellular domain, and wherein the reporter T cell comprises a reporter gene responsive to activation of the chimeric receptor. As described above, the reporter T cell is particularly useful to assess the functional activity / binding of an antibody or Fc fusion protein to its antigen / binding partner. The description of the reporter T cell in relation to the first aspect is equally applicable to this aspect. For example, the reporter gene may encode luciferase. As a further example, the extracellular domain may be an FcyRIIIa extracellular domain. In some embodiments, the reporter T cell is for assessing binding of an antibody, such as a monoclonal antibody. According to a third aspect, there is provided a kit for assessing binding of an antibody or Fc fusion protein to a target antigen, wherein the kit comprises the reporter T cell as described herein and a secondary cell expressing the target antigen on its surface. The description of the reporter T cell in relation to the first aspect is equally applicable to this aspect. For example, the reporter gene may encode luciferase. As a further example, the extracellular domain of the chimeric receptor may be an FcyRIIIa extracellular domain. The secondary cell may be any suitable secondary cell as discussed above in relation to the first aspect. In some embodiments, the secondary cell is an immortalised cell line cell. In some embodiments, the secondary cell is a Raji cell, a SKBR3 cell, or a Daudi cell. The kit may be for assessing binding of an antibody, such as a monoclonal antibody. In some embodiments, the kit further comprises suitable assay components for assessing activation of the reporter gene. For example, in some embodiments, the kit further comprises luciferase assay components for assessing activation of the luciferase reporter gene. According to a fourth aspect, there is provided a composition for assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the composition comprises: the reporter T cell as described herein; and a secondary cell expressing on its surface the target antigen of the antibody or Fc fusion protein. The description of the reporter T cell in relation to the first aspect is equally applicable to this aspect. For example, the reporter gene may encode luciferase. As a further example, the extracellular domain of the chimeric receptor may be an FcyRIIIa extracellular domain. The secondary cell may be any suitable secondary cell as discussed above in relation to the first aspect. In some embodiments, the secondary cell is an immortalised cell line cell. In some embodiments, the secondary cell is a Raji cell, a SKBR3 cell, or a Daudi cell. The composition may be for assessing binding of an antibody, such as a monoclonal antibody. In some embodiments, the composition further comprises an antibody or Fc fusion protein (i.e., the antibody or Fc fusion protein to be tested and which binds to the target antigen expressed on the surface of the secondary cell). In some embodiments, the composition further comprises an antibody, such as a monoclonal antibody (i.e., the monoclonal antibody to be tested and which binds to the target antigen expressed on the surface of the secondary cell). According to a fifth aspect, there is provided a method of assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the method comprises: (i) contacting the antibody or Fc fusion protein with (1) a secondary cell expressing the target antigen on its surface and (2) a reporter T cell, wherein the reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain selected from an Fey receptor extracellular domain or a PD-1 extracellular domain, and wherein the reporter T cell comprises a luciferase reporter gene responsive to activation of the chimeric receptor; and (ii) detecting activation of the luciferase reporter gene. According to a sixth aspect, there is provided a method of assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the method comprises: (i) contacting the antibody or Fc fusion protein with (1) a secondary cell expressing the target antigen on its surface and (2) a reporter T cell, wherein the reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain selected from an FcyRIIIa extracellular domain or a PD-1 extracellular domain, and wherein the reporter T cell comprises a luciferase reporter gene responsive to activation of the chimeric receptor; and (ii) detecting activation of the luciferase reporter gene. According to a seventh aspect, there is provided a method of assessing binding of a monoclonal antibody to its target antigen, wherein the method comprises: (i) contacting the monoclonal antibody with (1) a secondary cell expressing the target antigen on its surface and (2) a reporter T cell, wherein the reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain selected from an FcyRIIIa extracellular domain or a PD-1 extracellular domain, and wherein the reporter T cell comprises a luciferase reporter gene responsive to activation of the chimeric receptor; and (ii) detecting activation of the luciferase reporter gene. The description above in relation to the first to fourth aspects is equally applicable to each of the fifth to seventh aspects. In the description above, the term "sequence identity" is used to refer to the similarity of two sequences. For the purpose of this invention, it is defined here that in order to determine the percent identity of two sequences, the sequences are aligned for optimal comparison purposes (e.g., gaps can be introduced in the sequence of a first sequence for optimal alignment with a second amino or nucleic acid sequence). The nucleotide / amino acid residues at each position are then compared. When a position in the first sequence is occupied by the same amino acid or nucleotide residue as the corresponding position in the second sequence, then the molecules are identical at that position. The percent identity between the two sequences is a function of the number of identical positions shared by the sequences (i.e., % identity = number of identical positions / total number of positions (i.e. overlapping positions) x 100). Generally, the two sequences are the same length. A sequence comparison is typically carried out over the entire length of the two sequences being compared. The skilled person will be aware of the fact that several different computer programs are available to determine the identity between two sequences. For instance, a comparison of sequences and determination of percent identity between two sequences can be accomplished using a mathematical algorithm. In a preferred embodiment, the percent identity between two nucleic acid sequences is determined using the sequence alignment software Clone Manager 9 (Sci-Ed software - www.scied.com) using global DNA alignment; parameters: both strands; scoring matrix: linear (mismatch 2, OpenGap 4, ExtGap 1). Alternatively, the percent identity between two amino acid or nucleic acid sequences can be determined using the Needleman and Wunsch (1970) algorithm which has been incorporated into the GAP program in the Accelrys GCG software package (available at http: / / www.accelrys.com / products / gcg / ), using either a Blosum 62 matrix or a PAM250 matrix, and a gap weight of 16, 14, 12, 10, 8, 6, or 4 and a length weight of 1, 2, 3, 4, 5, or 6. A further method to assess the percent identity between two amino acid or nucleic acid sequences can be to use the BLAST sequence comparison tool available on the National Center for Biotechnology Information (NCBI) website (www.blast.ncbi.nlm.nih.gov), for example using BLASTn for nucleotide sequences or BLASTp for amino acid sequences using the default parameters. A skilled person will appreciate that all aspects of the invention, whether they relate to, for example, the method, the reporter T cell, the kit, or the composition, are equally applicable to all other aspects of the invention. In particular, features of the reporter T cell, for example, may have been described in greater detail than in other aspects of the invention, for example, the kit. However, the skilled person will appreciate where more detailed information has been given for a particular aspect of the invention, this information is generally equally applicable to other aspects of the invention. All patent and literature references cited in the present specification are hereby incorporated by reference in their entirety. Brief Description of the Drawings The invention will now be described in detail, by way of example only, with reference to the figures. Figure 1: IL2-Luciferase reporter gene. (A) The reporter gene consists of the minimal IL-2 promoter region fused to the luciferase gene. (B) Vector map of the IL2-Luciferase lentiviral expression vector Figure 2: FcyRIIIa chimeric receptor. (A, B) The chimeric receptor comprises of the human FcyRIIIa extracellular domain (ECD, red), which is responsible for binding to the Fc regions of antibodies. This is followed by the murine CD28 transmembrane domain, and the human CD28 intracellular domain (purple) fused to the human CD3^ intracellular domain (green), both of which contain multiple signalling motifs important for signal transduction. (C) Vector map of the FcyRIIIa chimeric receptor lentiviral expression vector. Figure 3: Chemical stimulation induces activation of the IL-2 promoter in Jurkat-Luciferase cells. Treatment of Jurkat-Luciferase cells with PMA, ionomycin and human CD28 antibody resulted in a 9-fold induction of luminescence signal in Jurkat-Luciferase cells. No induction of luminescence signal was detected in Jurkat-Luciferase cells in the absence of PMA, ionomycin and human CD28 antibody. This indicates that the minimal IL-2 promoter can be successfully activated by chemical stimulation resulting in expression of luciferase. Figure 4: Expression of the FcyRIIIa chimeric receptor in Jurkat cells. The expression of the chimeric receptor was detected on the surface of Jurkat FcyRIIIa-Luciferase cells using a fluorescent-conjugated antibody against FcyRIIIa (AlexaFluor 488 anti-human CD16) and flow cytometry. All Jurkat FcyRIIIa-Luciferase cells expressed the FcyRIIIa chimeric receptor (green). The presence of the chimeric receptor was not detected on control Jurkat-Luciferase cells (grey). Figure 5: Diagrammatic representation of Jurkat FcyRIIIa-Luciferase stimulation assay. The Jurkat FcyRIIIa-Luciferase reporter cell line can be stimulated by the monoclonal antibody Obinutuzumab. First, Obinutuzumab, a CD20-binding antibody, is incubated with the CD20-expressing Raji cell line, allowing for the binding of Obinutuzumab to CD20 expressed on the surface of Raji cells. Jurkat FcyRIIIa-Luciferase cells are then added and mixed with the Obinutuzumab-coated Raji cells. The Fc regions of the CD20-bound Obinutuzumab then binds to the FcyRIIIa extracellular domain (ECD) of the chimeric receptor, resulting in signal transduction across the cell membrane of the Jurkat reporter cells to the CD28 and CD3^ intracellular tails. This then activates signalling cascades via the signalling motifs on the CD28 and CD3^ intracellular domains (ICD) and leads to the activation of the minimal IL-2 promoter and finally, induction of luciferase reporter gene expression. Figure 6: (A) Obinutuzumab induces IL-2 activity in the Jurkat FcyRIIIa-Luciferase reporter cells. In the presence of 10 pg / mL of the anti-CD20 antibody Obinutuzumab and CD20-expressing Raji cells, there was a 10-fold induction of luminescence signal in the Jurkat FcyRIIIa-Luciferase reporter cells. No induction of luminescence signal was detected in Jurkat FcyRIIIa-Luciferase cells in the absence of Obinutuzumab. Additionally, there was no induction of luminescence in Jurkat-Luciferase control cells (cells containing the IL2-Luciferase reporter gene but without the chimeric receptor) in the presence or absence of Obinutuzumab. This indicates that activation of luciferase expression by the minimal IL-2 promoter was induced by the activation of the FcyRIIIa chimeric receptor upon binding to Obinutuzumab. (B) The Jurkat FcyRIIIa-Luciferase cells were treated with 10 pg / mL of Obinutuzumab alone (in the absence of CD20-expressing Raji cells). In the absence of Raji cells, there was a less than 2-fold induction of luminescence signal in Jurkat FcyRIIIa-Luciferase cells when treated with Obinutuzumab. This demonstrates that in addition to an anti-CD20 antibody, CD20-expressing Raji cells are also required to produce a strong induction of the luciferase reporter gene in the Jurkat FcyRIIIa-Luciferase reporter cells. Figure 7: Rituximab produces a 7-fold induction of luminescence signal in the Jurkat FcyRIIIa-Luciferase reporter cells. In the presence of 10 pg / mL of the anti-CD20 antibody Rituximab and CD20-expressing Raji cells, there was a 7-fold induction of luminescence signal in the Jurkat FcyRIIIa-Luciferase reporter cells. No induction of luminescence signal was detected in Jurkat FcyRIIIa-Luciferase cells in the absence of Rituximab. Additionally, there was no induction of luminescence in Jurkat-Luciferase control cells (cells containing the IL2-Luciferase reporter gene but without the chimeric receptor) in the presence or absence of Rituximab. This indicates that activation of luciferase expression by the minimal IL-2 promoter was induced by the activation of the FcyRIIIa chimeric receptor upon binding to Rituximab and demonstrates that the Jurkat FcyRIIIa-Luciferase reporter cell line can be used to determine the activity of different CD20-binding monoclonal antibodies. Figure 8: Trastuzumab can produce a 4-fold induction of luminescence signal in the Jurkat FcyRIIIa-Luciferase reporter cells. In the presence of 10 pg / mL of the anti-HER2 antibody Trastuzumab and HER2-expressing SKBR3 cells, there was a 4-fold induction of luminescence signal in the Jurkat FcyRIIIa-Luciferase reporter cells. No induction of luminescence signal was detected in Jurkat FcyRIIIa-Luciferase cells in the absence of Trastuzumab. Additionally, there was no induction of luminescence in Jurkat-Luciferase control cells (cells containing the IL2-Luciferase reporter gene but without the chimeric receptor) in the presence or absence of Trastuzumab. This indicates that activation of luciferase expression by the minimal IL-2 promoter was induced by the activation of the FcyRIIIa chimeric receptor upon binding to Trastuzumab and demonstrates that the Jurkat FcyRIIIa-Luciferase reporter cell line can be used to determine the activity of monoclonal antibodies with different target specificities and using different target cells. Figure 9: The Jurkat FcyRIIIa-Luciferase reporter cell line can be used to distinguish between non-degraded and degraded monoclonal antibodies. Samples of Obinutuzumab and Rituximab were degraded by exposure to 10 J / cm2 of UV-B light. (A) In the presence of 0.01 pg / mL UV-B degraded Obinutuzumab, there was a 1.3-fold decrease in the fold induction of luminescence signal compared to its non-degraded matched control in Jurkat FcyRIIIa-Luciferase reporter cells. (B) In the presence of 0.1 pg / mL UV-B degraded Rituximab, there was a 2-fold decrease in the fold induction of luminescence signal compared to its nondegraded matched control in Jurkat FcyRIIIa-Luciferase reporter cells. This demonstrates that the Jurkat FcyRIIIa-Luciferase reporter cell line can detect a reduction in the activity of UV-B degraded monoclonal antibodies and can therefore be used to distinguish between non-degraded and degraded antibodies. Figure 10: The chimeric receptor composed of the PD-1 extracellular domain joined to the CD28 / CD3 intracellular domain does enable that receptor to propagate intracellular signals that promote Luciferase reporter gene expression. Open bars represent reporter cells not expressing the PD-1 chimeric receptor, filled bars represent cells expressing the PD-1 chimeric receptor. Numbers denote fold induction of luciferase over basal expression. Detailed Description of the Invention The inventors have developed a reporter T cell and an associated method for assessing the binding of an antibody to its target antigen or Fc fusion protein to its binding partner, and can thus be used as a readout of functional activity of the antibody or Fc fusion protein. The reporter T cell expresses a chimeric receptor and a reporter gene, which is capable of reporting on the functional activity of the antibody / Fc fusion protein. The following non-limiting examples demonstrate that the reporter T cell based method can detect the functional activity of antibodies and Fc fusion proteins having different target specificities and using different secondary cells, and can be used to distinguish between nondegraded and degraded products. Accordingly, the method is suitable for assessing the stability of these biologies and can therefore be used as a shelf life extension assay. Example 1 A luciferase reporter gene fused to the minimal IL-2 promoter was first introduced into Jurkat T cells using lentiviral transduction. This was followed by a second lentiviral transduction of the Jurkat T cells to introduce the chimeric Fc receptor, which is expressed on the surface of the cell. The receptor specifically comprises the extracellular domain of human FcyRIIIa fused to the transmembrane and intracellular domain of CD28, followed by the intracellular domain of human CD3^. The extracellular domain of FcyRIIIa binds to the conserved Fc regions of IgG antibodies. In the cell, binding of the Fc regions of IgG antibodies to FcyRIIIa initiates signal transduction pathways leading to antibody-mediated phagocytosis or antibody-dependent cell-mediated cytotoxicity. Meanwhile, the intracellular chains of CD28 and CD3^ both contain multiple signaling motifs that are critical for activating downstream signaling pathways that include the activation of IL-2 promoter in the cell. Obinutuzumab is a monoclonal antibody that binds to CD20 on the cell surface. In theory, when incubated with a CD20-expressing cell line, (termed Raji), Obinutuzumab will bind to CD20 on the cell surface of the Raji cells. The extracellular FcyRIIIa domain of the chimeric receptor on the cell surface of the Jurkat reporter cell line then binds to the Fc regions of Raji-bound Obinutuzumab, leading to signal transduction across the cell membrane of the reporter Jurkat T cells. This induces the activation of signaling cascades via the CD28 and CD3£ intracellular tails that ultimately results in the induction of luciferase reporter gene expression due to activation of the inserted minimal IL-2 promoter. The induction of luciferase expression by the activation of the chimeric receptor is measured via a luciferase assay and is a direct indication of the activity of the drug of interest (Obinutuzumab). In the reporter assay, Obinutuzumab was incubated with Raji cells and allowed to bind to CD20. The Obinutuzumab-coated Raji cells were then incubated with the Jurkat reporter cell line expressing the FcyRIIIa chimeric receptor and luciferase reporter gene. Strong induction of luciferase activity was detected via a luciferase assay, indicating that the reporter cell line could successfully measure the activity of the Obinutuzumab used. Furthermore, to extend the studies, induction of luciferase activity via the chimeric receptor was also achieved using the CD20-binding drug Rituximab (along with CD20-expressing Raji cells), as well as the HER2-binding drug Trastuzumab (along with HER2-expressing SKBR3 cells), indicating that this reporter cell line can determine the activity of monoclonal antibodies with different targeting specificities. Additionally, when incubated with versions of the same drugs that had been exposed to 10 J / cm2 UV-B, the induction of luciferase in the reporter Jurkat cell line was reduced, indicating that the reporter cells can detect a decrease in the activity of the mAbs of interest due to degradation. This reporter cell line is therefore an important and convenient tool for measuring and evaluating the functional activity of monoclonal antibodies of interest and can additionally be used to detect possible degradation due to extended storage. Vector Design IL2-Luciferase reporter gene The reporter gene consists of the minimal IL-2 promoter sequence fused with the luciferase gene sequence as shown in Figure 1(A). The minimal IL-2 promoter DNA sequence and luciferase DNA sequence were encoded in a viral vector containing a puromycin drug resistance cassette according to Figure 1(B). The corresponding lentivirus was then produced in ready-to-use aliquots at >108 TU / mL. Chimeric Receptor The chimeric receptor includes the extracellular domain (ECD - red) of human FcyRIIIa (amino acids 17-208) fused to the transmembrane domain (TMD - purple) of murine CD28 (amino acids 151-176), followed by the intracellular domain (ICD) of human CD28 (amino acids 179-220), and the ICD (green) of human CD3^ (amino acids 52-164), as shown in Figure 2(A-B). This chimeric receptor was placed under the control of the human EFla promoter to maintain high level transgene expression. The DNA sequence for the chimeric receptor was inserted into a viral vector, along with an mCherry tag and a blasticidin drug resistance cassette linked by P2A and T2A peptides, according to Figure 2(C). Lentiviral transduction of Jurkat T cells with IL2-Luciferase lentivirus Jurkat T cells were first transduced via spinoculation with the lentiviral vector containing the IL2-Luciferase reporter gene construct. The Jurkat T cells, at a cell density of 1 million cells / well, were combined with polybrene at a final concentration of 8 pg / mL. The lentivirus was then added directly to the cells at a multiplicity of infection (MOI) of 1. The cell / polybrene / virus mixture was seeded onto a 24-well plate and the plate was subjected to centrifugation at 800xg for 2 hours at 30°C. The supernatant was then removed carefully from the well without disturbing the cells and the cells resuspended in fresh media. After a recovery period of 48-72 hours, puromycin resistant cells were selected for by culturing the transduced cells in 0.5 pg / mL puromycin for 7-14 days. These cells are now referred to as Jurkat-Luciferase cells. Expression of Luciferase in Jurkat-Luciferase cells The expression of the inserted luciferase gene in Jurkat-Luciferase cells was determined by chemical stimulation of the cells with phorbol-myristate-acetate (PMA), ionomycin and human CD28 antibody. Treatment of T cells with a combination of PMA, ionomycin and human CD28 antibody leads to the activation of a series of intracellular signalling pathways, resulting ultimately in cytokine production via activation of the promoter regions of many cytokines, including IL-2. Stimulation of the Jurkat-Luciferase cells with PMA, ionomycin and human CD28 antibody would thus lead to activation of the inserted minimal IL-2 promoter and result in activation of luciferase expression. The Jurkat-Luciferase cells were seeded at a density of 0.5 million cells / mL in a total volume of 500 pL per well in a 24-well plate. The cells were treated with 50 ng / mL PMA (phorbol 12 myristate 13 acetate), 1 pg / mL ionomycin and 5 pg / mL human CD28 antibody by adding each reagent directly into the wells. As a negative control, cells were also left untreated with no reagents added. The cells were then stimulated overnight at 37°C. The cells were transferred from the wells of the 24-well plate into Eppendorf tubes and subjected to centrifugation at 400xg for 5 minutes. 300 pL of supernatant was removed and discarded and the cell pellets resuspended in the remaining 200 pL of media. 50 pL of the cell suspension was then transferred into a white opaque 96-well plate. 50 pL / well of Steady-Gio® Luciferase Assay Reagent (Promega) was added directly into the wells and incubated for 5 minutes at room temperature. Luminescence was then measured using a CLARIOstar microplate reader. In the absence of PMA, ionomycin and human CD28 antibody (untreated controls), only background levels of luminescence were detected. When treated with PMA, ionomycin and human CD28 antibody, there was a 9-fold induction of luminescence (Figure 3). This indicates that the luciferase gene was successfully introduced into Jurkat cells and expression of the luciferase gene could be successfully induced via chemical stimulation of the inserted minimal IL-2 promoter. The Jurkat-Luciferase cells therefore now contain a functional luciferase gene controlled by a functional IL-2 promoter. Lentiviral transduction of Jurkat-Luciferase cells with FcyRIIIa chimeric receptor lentivirus Once the Jurkat-Luciferase cells were growing normally in culture, they were subjected to a second round of lentiviral transduction using lentivirus containing the FcyRIIIa chimeric receptor and the same lentiviral transduction method described above. This time, blasticidin resistant cells were selected for by culturing the transduced cells in 2 pg / mL blasticidin for 7-14 days. These cells are now referred to as Jurkat FcyRIIIa-Luciferase cells. Expression of FcyRIIIa chimeric receptor in Jurkat FcyRIIIa-Luciferase cells The expression of the FcyRIIIa chimeric receptor in the Jurkat FcyRIIIa-Luciferase cells was determined using a fluorescent-conjugated antibody against FcyRIIIa: AlexaFluor 488 anti-human CD16. The cells were collected at a density of 1 million cells / mL and incubated with AlexaFluor 488 anti-human CD16 (at a 1:10 dilution) for 1 hour at room temperature. As a negative control, Jurkat-Luciferase cells (without the chimeric receptor) were also incubated with the same AlexaFluor 488 anti-human CD16 antibody. Fluorescent signal was detected using a Sony ID7000 Spectral Cell analyser in all Jurkat FcyRIIIa-Luciferase cells, indicating that the cells were all successfully expressing the FcyRIIIa chimeric receptor (Figure 4). No fluorescent signal was detected on Jurkat-Luciferase cells, indicating the absence of the chimeric receptor in those cells. Stimulation of Jurkat FcyRIIIa-Luciferase with monoclonal antibodies The Jurkat FcyRIIIa-Luciferase reporter cell line was first stimulated with the anti-CD20 monoclonal antibody Obinutuzumab. This was performed in the presence of Raji cells, which highly express CD20 on their cell surface. The binding of Obinutuzumab to CD20 on the Raji cell surface positions the Fc regions of Obinutuzumab and facilitates the binding of the Fc regions to the FcyRIIIa extracellular domain of the chimeric receptor on Jurkat FcyRIIIa-Luciferase cells. This then leads to signal transmission across the cell membrane to the CD28 and CD3^ intracellular tails of the chimeric receptor, which propagates signalling cascades leading to the induction of the minimal IL-2 promoter, resulting in the expression of luciferase (Figure 5). Luciferase expression can then be determined via a luciferase assay, which measures the amount of luminescence signal produced. Raji cells were collected at a density of 800000 cells / mL and incubated with 20 pg / mL Obinutuzumab. As a negative control, Raji cells were also prepared in the absence of Obinutuzumab. The Raji cells were incubated for 20 minutes on ice and then 50 pL / well of the cell suspension was seeded into the wells of a white opaque 96-well plate at a density of 40000 cells / well. 50 pL / well of Jurkat FcyRIIIa-Luciferase cells were then added into the same wells at a density of 200000 cells / well. The final concentration of Obinutuzumab in each well was 10 pg / mL. As a negative control, Jurkat-Luciferase cells (cells containing the luciferase reporter gene fused to the minimal IL-2 promoter but without the chimeric receptor) were also added to the Raji-containing wells, in the absence and presence of Obinutuzumab. The cells were mixed and then incubated for 5 hours at 37°C. 100 pL / well of Steady-Gio® Luciferase Assay Reagent (Promega) was added directly into the wells and incubated for 5 minutes at room temperature. Luminescence was then measured using a CLARIOstar microplate reader. In the absence of Obinutuzumab, only background levels of luminescence were detected in wells containing the Jurkat FcyRIIIa-Luciferase cells. In the presence of 10 pg / mL Obinutuzumab, there was a 10-fold induction of luminescence signal in the wells containing Jurkat FcyRIIIa-Luciferase cells (Figure 6). Luminescence signal was not detected in Jurkat-Luciferase cells that did not contain the chimeric receptor. To test the application of the system with other mAbs, the Jurkat FcyRIIIa-Luciferase reporter cells were also incubated with 10 pg / mL of the anti-CD20 monoclonal antibody, Rituximab, in the presence of Raji cells. The same conditions were used as described above. In the presence of 10 pg / mL of Rituximab, there was a 7-fold induction of luminescence signal in the wells containing Jurkat FcyRIIIa-Luciferase cells. Luminescence was not detected in wells containing Jurkat-Luciferase cells (Figure 7). This indicated that this reporter cell line was able to detect the activity of different CD20-binding monoclonal antibodies. The induction of luminescence signal in cells treated with Rituximab was lower than that of cells treated with Obinutuzumab. Given the Fc regions of Obinutuzumab have been engineered to possess enhanced affinity for FcyRIIIa receptors, it is therefore expected to have increased activity towards the chimeric receptor compared to Rituximab. The results in Figures 6 and 7 clearly show that the Jurkat FcyRIIIa-Luciferase reporter cell line can detect the enhanced activity of Obinutuzumab and can therefore be used to detect differences in the activity of different monoclonal antibodies. Next, the inventors tested the ability of the Jurkat FcyRIIIa-Luciferase cells to detect the activity of the anti-HER2 monoclonal antibody, Trastuzumab (which acts through HER2), this time in the presence of SKBR3 cells. SKBR3 is a cell line expressing HER2. In this case, SKBR3 cells were seeded into the white opaque 96-well plate at a density of 40000 cells / well. The next day, the media was removed and 50 pL / well of media containing 20 pg / mL Trastuzumab was added onto the cells. The cells were incubated for 20 minutes at room temperature. 50 pL / well of the Jurkat FcyRIIIa-Luciferase cells were then added into the same wells at a concentration of 200000 cells / well. The final concentration of Trastuzumab in each well was 10 pg / mL. Luminescence signal was detected after 5 hours as described above. In the presence of 10 pg / mL of Trastuzumab, there was a 4-fold induction of luminescence signal in the wells containing Jurkat FcyRIIIa-Luciferase cells. Luminescence was not detected in wells containing Jurkat-Luciferase cells (Figure 8). This indicates that the reporter cell line can detect the activity of monoclonal antibodies with different target specificities and using different target cells. This invention can therefore be used to measure the activities of a range of different monoclonal antibodies with differing binding specificities. Stimulation of Jurkat FcyRIIIa-Luciferase with degraded monoclonal antibodies The Jurkat FcyRIIIa-Luciferase cells were then tested to determine whether the Jurkat FcyRIIIa-Luciferase reporter cells were capable of detecting differences in the functional activity of monoclonal antibodies degraded using UV-B light. Samples of Obinutuzumab and Rituximab were exposed to 10 J / cm2 of UV-B light in order to induce degradation of the antibody via oxidation. The degraded antibodies, along with non-degraded matched control samples, were then incubated with Raji cells at a concentration of 0.01 pg / mL for Obinutuzumab and 0.1 pg / mL for Rituximab. The lower concentrations of monoclonal antibodies were used to avoid over-saturation of the receptors on the Raji cells. The antibodies were incubated for 20 minutes on ice as before and then excess antibody was washed off before the addition of Jurkat FcyRIIIa-Luciferase cells. Luminescence was measured after 5 hours using the same conditions as described above. Degradation of Obinutuzumab with 10 J / cm2 of UV-B light caused only a slight decrease in fold induction of luciferase when compared with its matched controls, as shown in Figure 9(A). A larger decrease in fold induction of luciferase was observed in cells treated with degraded Rituximab (Figure 9(B)). This may be due to the Fc regions of Obinutuzumab, which have been engineered to have stronger binding affinity to FcyRIIIa receptors, and therefore could continue to strongly bind the chimeric receptor despite degradation with UV-B light. However, with Rituximab, which has not been engineered for stronger Fc binding, a clear difference in fold induction of luciferase can be seen between UV-B degraded Rituximab and its matched controls. This demonstrated that the Jurkat FcyRIIIa-Luciferase reporter cell line can be used to show a clear reduction in the functional activity of degraded monoclonal antibodies and can therefore be used to distinguish between nondegraded and degraded monoclonal antibodies. Example 2 Combining the extracellular domain of PD-1 with the intracellular domain described below converts inhibitory intracellular signalling to signals that promote reporter expression when the receptor is bound by PD-1 targeting antibodies. Functional activity of PD-1 targeting antibodies can then be measured directly as an increase in reporter gene expression. Vector Design IL2-Luciferase reporter gene The IL2-Luciferase reporter gene was designed, encoded in a viral vector and the corresponding lentivirus produced as per Example 1. PD-1 Chimeric Receptor The chimeric receptor included the extracellular domain of human PD-1 (amino acids 25-170) fused to the transmembrane domain of murine CD28 (amino acids 151-176), followed by the intracellular domain (ICD) of human CD28 (amino acids 179-220), and the ICD of human CD3^ (amino acids 52-164). This chimeric receptor was placed under the control of the human EFla promoter to maintain high level transgene expression. The DNA sequence for the PD-1 chimeric receptor was inserted into a viral vector, along with an mCherry tag and a blasticidin drug resistance cassette linked by P2A and T2A peptides. Lentiviral transduction of Jurkat-Luciferase cells with PD-1 chimeric receptor lentivirus Jurkat-Luciferase cells were produced according to Example 1 and were subjected to a second round of lentiviral transduction using lentivirus containing the PD-1 chimeric receptor to provide Jurkat PD-l-Luciferase cells. Expression of PD-1 chimeric receptor in Jurkat PD-l-Luciferase cells The expression of the PD-1 chimeric receptor in the Jurkat PD-l-Luciferase cells was determined using a fluorescent-conjugated antibody against PD-1. The cells were collected at a density of 1 million cells / mL and incubated with the fluorescent-conjugated antibody (at an appropriate dilution) for 1 hour at room temperature. As a negative control, Jurkat-Luciferase cells (without the chimeric receptor) were also incubated with the same fluorescent-conjugated antibody. Fluorescent signal was detected using a Sony ID7000 Spectral Cell analyser in all Jurkat PD-l-Luciferase cells, to indicate that the cells were all successfully expressing the PD-1 chimeric receptor. Stimulation of Jurkat PD-l-Luciferase with monoclonal antibodies The Jurkat PD-l-Luciferase reporter cell line was stimulated with the anti-PD-1 monoclonal antibodies nivolumab or pembrolizumab. Binding of the antibodies leads to signal transmission across the cell membrane to the CD28 and CD3^ intracellular tails of the chimeric receptor, which propagates signalling cascades leading to the induction of the minimal IL-2 promoter, resulting in the expression of luciferase (Figure 10). Luciferase expression was determined via a luciferase assay, which measures the amount of luminescence signal produced. Jurkat PD-l-Luciferase cells, or Jurkat-luciferase cells not expressing the PD-1 chimera, were added into assay plate wells at a density of 200,000 cells / well. The antibodies nivolumab or pembrolizumab were then added to the well at concentrations of 0 pg / mL (control), 0.1 pg / mL or 1 pg / mL. The cells were then mixed and incubated for 5 hours at 37°C. 100 pL / well of Steady-Gio® Luciferase Assay Reagent (Promega) was added directly into the wells and incubated for 5 minutes at room temperature. There was no induction of luciferase expression in the cells not expressing the PD-1 chimera. In the cells expressing the PD-1 chimera there was an induction of an 8 to 9 fold increase over basal expression of luciferase. In assay wells coated with CHO cells this induction was enhanced by roughly 50%. Sequences SEQ ID NO: 1 (Minimal IL-2 promoter DNA sequence) CAAGCTCAGATCCAAGCTTGTCGACAGATCTATCACCCTGTGTGCAATTAGCTCATTGTG TAGATAAGAAGGTAAAACCATCTTGAAACAGGAAACCAATATCCTTCCTGTCTAATCAAC AAATCTAAAAGATTTATTCTTTTCATCTATCTCCTCTTGCGTTTGTCCACCACAACAGGCT GCTTACAGGTTCAGGATGGTTTTGACAAAGAGAACATTTTCATGAGTTACTTTTGTGTCTC CACCCCAAAGAGGAAAATTTGTTTCATACAGAAGGCGTTCATTGTATGAATTAAAACTGC CACCTAAGTGTGGGCTAACCCGACCAAGAGGGATTTCACCTAAATCCATTCAGTCAGTGT ATGGGGGTTTAAAGAAATTCCAGAGAGTCATCAGAAGAGGAAAAACAAAGGTAATGCTT TTTGCCACACAGGTAGACTCTTTGAAAATATGTGTAATATGTAAAACATCGTGACACCCC CATATTATTTTTCCAGCATTAACAGTATAAATTGCCTCCCATGCTGAAGAGCTGCCTATCA CCCTTGCTAAGCTTGGATCTGAGCTTGGCATTCCGGTACTGTTGGTAAA SEQ ID NO: 2 (Luciferase DNA sequence) ATGGAAGACGCCAAAAACATAAAGAAAGGCCCGGCGCCATTCTATCCGCTGGAAGATGG AACCGCTGGAGAGCAACTGCATAAGGCTATGAAGAGATACGCCCTGGTTCCTGGAACAA TTGCTTTTACAGATGCACATATCGAGGTGGACATCACTTACGCTGAGTACTTCGAAATGT CCGTTCGGTTGGCAGAAGCTATGAAACGATATGGGCTGAATACAAATCACAGAATCGTC GTATGCAGTGAAAACTCTCTTCAATTCTTTATGCCGGTGTTGGGCGCGTTATTTATCGGA GTTGCAGTTGCGCCCGCGAACGACATTTATAATGAACGTGAATTGCTCAACAGTATGGGC ATTTCGCAGCCTACCGTGGTGTTCGTTTCCAAAAAGGGGTTGCAAAAAATTTTGAACGTG CAAAAAAAGCTCCCAATCATCCAAAAAATTATTATCATGGATTCTAAAACGGATTACCAGG GATTTCAGTCGATGTACACGTTCGTCACATCTCATCTACCTCCCGGTTTTAATGAATACGA TTTTGTGCCAGAGTCCTTCGATCAGGACAAGACAATTGCACTGATCATGAACTCCTCTGG ATCTACTGGTCTGCCTAAAGGTGTCGCTCTGCCTCATAGAACTGCCTGCGTGAGATTCTC GCATGCCAGAGATCCTATTTTTGGCAATCAAATCATTCCGGATACTGCGATTTTAAGTGTT GTTCCATTCCATCACGGTTTTGGAATGTTTACTACACTCGGATATTTGATATGTGGATTTC GAGTCGTCTTAATGTATAGATTTGAAGAAGAGCTGTTTCTGAGGAGCCTTCAGGATTACA AGATTCAAAGTGCGCTGCTGGTGCCAACCCTATTCTCCTTCTTCGCCAAAAGCACTCTGA TTGACAAATACGATTTATCTAATTTACACGAAATTGCTTCTGGTGGCGCTCCCCTCTCTAA GGAAGTCGGGGAAGCGGTTGCCAAGAGGTTCCATCTGCCAGGTATCAGGCAAGGATAT GGGCTCACTGAGACTACATCAGCTATTCTGATTACACCCGAGGGGGATGATAAACCGGG CGCGGTCGGTAAAGTTGTTCCATTTTTTGAAGCGAAGGTTGTGGATCTGGATACCGGGA AAACGCTGGGCGTTAATCAAAGAGGCGAACTGTGTGTGAGAGGTCCTATGATTATGTCC GGTTATGTAAACAATCCGGAAGCGACCAACGCCTTGATTGACAAGGATGGATGGCTACA TTCTGGAGACATAGCTTACTGGGACGAAGACGAACACTTCTTCATCGTTGACCGCCTGAA GTCTCTGATTAAGTACAAAGGCTATCAGGTGGCTCCCGCTGAATTGGAATCCATCTTGCT CCAACACCCCAACATCTTCGACGCAGGTGTCGCAGGTCTTCCCGACGATGACGCCGGTG AACTTCCCGCCGCCGTTGTTGTTTTGGAGCACGGAAAGACGATGACGGAAAAAGAGATC GTGGATTACGTCGCCAGTCAAGTAACAACCGCGAAAAAGTTGCGCGGAGGAGTTGTGTT TGTGGACGAAGTACCGAAAGGTCTTACCGGAAAACTCGACGCAAGAAAAATCAGAGAGA TCCTCATAAAGGCCAAGAAGGGCGGAAAGATTGCCGTGTAA SEQ ID NO: 3 (FcyRIIIa extracellular domain amino acid sequence) MWQLLLPTALLLLVSAGMRTEDLPKAVVFLEPQWYRVLEKDSVTLKCQGAYSPEDNSTQWF HNESLISSQASSYFIDAATVDDSGEYRCQTNLSTLSDPVQLEVHIGWLLLQAPRWVFKEEDP IHLRCHSWKNTALHKVTYLQNGKGRKYFHHNSDFYIPKATLKDSGSYFCRGLFGSKNVSSE TVNITITQGLAVSTISSFFPPGYQ SEQ ID NO: 4 (CD28 transmembrane and intracellular domain amino acid sequence) FWALVVVAGVLFCYGLLVTVALCVIWVRSKRSRLLHSDYMNMTPRRPGPTRKHYQPYAPPR DFAAYRS SEQ ID NO: 5 (CD3^ intracellular domain amino acid sequence) RVKFSRSADAPAYQQGQNQLYNELNLGRREEYDVLDKRRGRDPEMGGKPQRRKNPQEGLY NELQKDKMAEAYSEIGMKGERRRGKGHDGLYQGLSTATKDTYDALHMQALPPR SEQ ID NO: 6 (FcYRIIIa-CD28-CD3^ chimeric receptor amino acid sequence) MWQLLLPTALLLLVSAGMRTEDLPKAVVFLEPQWYRVLEKDSVTLKCQGAYSPEDNSTQWF HNESLISSQASSYFIDAATVDDSGEYRCQTNLSTLSDPVQLEVHIGWLLLQAPRWVFKEEDP IHLRCHSWKNTALHKVTYLQNGKGRKYFHHNSDFYIPKATLKDSGSYFCRGLFGSKNVSSE TVNITITQGLAVSTISSFFPPGYQFWALVVVAGVLFCYGLLVTVALCVIWVRSKRSRLLHSDY MNMTPRRPGPTRKHYQPYAPPRDFAAYRSRVKFSRSADAPAYQQGQNQLYNELNLGRREEY DVLDKRRGRDPEMGGKPQRRKNPQEGLYNELQKDKMAEAYSEIGMKGERRRGKGHDGLY QGLSTATKDTYDALHMQALPPR SEQ ID NO: 7 (FcYRIIIa-CD28-CD3^ chimeric receptor DNA sequence) ATGTGGCAACTGCTGCTGCCAACCGCCCTGCTGCTGCTGGTGTCCGCCGGAATGAGAAC AGAAGACCTGCCAAAGGCCGTGGTGTTCCTGGAGCCACAGTGGTACCGCGTGCTGGAA AAAGACAGCGTCACCTTGAAGTGCCAGGGAGCCTATTCCCCCGAGGACAACAGCACACA GTGGTTTCACAACGAGTCCCTGATCAGCAGCCAGGCCTCCTCCTACTTTATCGACGCAGC CACCGTGGACGACAGTGGGGAGTATAGATGCCAGACTAACCTGAGCACCCTGTCCGACC CCGTGCAGCTGGAGGTCCACATCGGGTGGCTCCTGCTGCAGGCCCCAAGATGGGTGTTT AAGGAAGAAGATCCTATTCACCTGAGGTGCCATAGCTGGAAGAATACCGCCCTGCACAA GGTGACCTATCTGCAGAATGGCAAGGGACGGAAATACTTCCACCACAACAGCGATTTTTA CATCCCTAAGGCCACACTGAAAGATAGCGGGAGCTACTTTTGCAGGGGCCTGTTTGGCA GCAAAAACGTGTCTAGCGAGACCGTGAACATTACAATTACTCAGGGCCTGGCCGTGAGC ACTATCTCTAGCTTCTTCCCCCCCGGCTACCAGTTCTGGGCCCTGGTGGTGGTGGCCGG AGTGCTGTTCTGCTATGGACTCCTGGTGACCGTGGCCCTGTGCGTGATCTGGGTGCGCT CCAAGAGGTCTAGACTGCTGCACAGCGACTACATGAATATGACCCCAAGGCGCCCAGGC CCTACAAGGAAGCACTATCAGCCATACGCCCCCCCAAGGGATTTTGCTGCCTACCGGTC CCGGGTGAAGTTCTCCAGAAGCGCCGATGCCCCTGCCTATCAGCAGGGTCAGAACCAGC TGTATAATGAACTGAACCTGGGCCGGCGGGAAGAGTACGACGTGCTGGACAAGCGGCG CGGAAGAGACCCCGAGATGGGCGGAAAGCCCCAGAGAAGGAAGAATCCTCAGGAGGG GCTGTATAACGAACTGCAGAAAGATAAGATGGCCGAGGCCTACAGCGAAATCGGCATGA AAGGCGAGCGCAGGAGAGGCAAGGGCCACGATGGCCTGTACCAGGGCCTCAGCACCGC CACCAAAGACACCTATGACGCGCTGCACATGCAGGCCCTGCCCCCACGC SEQ ID NO: 8 (PD-1 extracellular domain amino acid sequence) MQIPQAPWPVVWAVLQLGWRPGWFLDSPDRPWNPPTFSPALLVVTEGDNATFTCSFSNTS ESFVLNWYRMSPSNQTDKLAAFPEDRSQPGQDCRFRVTQLPNGRDFHMSVVRARRNDSG TYLCGAISLAPKAQIKESLRAELRVTERRAEVPTAHPSPSPRPAGQFQTLV SEQ ID NO: 9 (PD- 1-CD28-CD3^ chimeric receptor amino acid sequence) MQIPQAPWPVVWAVLQLGWRPGWFLDSPDRPWNPPTFSPALLVVTEGDNATFTCSFSNTS ESFVLNWYRMSPSNQTDKLAAFPEDRSQPGQDCRFRVTQLPNGRDFHMSVVRARRNDSG TYLCGAISLAPKAQIKESLRAELRVTERRAEVPTAHPSPSPRPAGQFQTLVHVA / .WV'AGVT.F CYGLLVTVAL CVYWVRSKRSRLLHSDYMNMTPRRPGPTRKHYQPYAPPRDFAAYRSRVKFS RSADAPAYQQGQNQLYNELNLGRREEYDVLDKRRGRDPEMGGKPQRRKNPQEGLYNELQK DKMAEAYSEIGMKGERRRGKGHDGLYQGLSTATKDTYDALHMQALPPR SEQ ID NO: 10 (PD-1-CD28-CD3^ chimeric receptor DNA sequence) ATGCAGATCCCCCAGGCCCCCTGGCCCGTGGTGTGGGCCGTGCTGCAGCTGGGCTGGA GGCCCGGCTGGTTCCTGGACAGCCCCGACAGGCCCTGGAACCCCCCCACCTTCAGCCCC GCCCTGCTGGTGGTGACCGAGGGCGACAACGCCACCTTCACCTGCAGCTTCAGCAACAC CAGCGAGAGCTTCGTGCTGAACTGGTACAGGATGAGCCCCAGCAACCAGACCGACAAG CTGGCCGCCnCCCCGAGGACAGGAGCCAGCCCGGCCAGGACTGCAGGTTCAGGGTGA CCCAGCIGCCCAACGGCAGGGAC i 1 CCACAIGAGCG i GG ] GAGGGCCAGGAGGAACGA CAGCGGCACCTACCTGTGCGGCGCCATCAGCCTGGCCCCCAAGGCCCAGATCAAGGAG AGCG1GAGGGCCGAGC f GAGGG f GACCGAGAGGAGGGCCGAGGT GCCCACCGCCCAC CCCAGCCCCAGCCCCAGGCCCGCCGGCCAGnCCAGACCCTGGTGTTCTGGGCCCTGG TGGTGGTGGCCGGCGTGCTGnCTGCTACGGCCTGCTGGTGACCGTGGCCCTGTGCGT GATC] GGG f GAGGAGCAAGAGGAGCAGGC 3 GC i GCACAGGGACIACAIGAACA 3 GACC CCCAGGAGGCCCGGCCCCACCAGGAAGCACTACCAGCCCTACGCCCCCCCCAGGGACT i CGCCGCC ] ACAGGAGCAGGG ] GAAG f i CAGCAGGAGCGCGGACGCCCCCGCC 3 ACCA GCAGGGCCAGAACCAGG ] GIACAACGAGC i GAACCT GGGCAGGAGGGAGGAG1ACGAC G i GC J GGACAAGAGGAGGGGCAGGGACCCGGAGATGGGCGGCAAGCCGCAGAGGAGG AAGAACCCCCAGGAGGGCCTGTACAACGAGCTGCAGAAGGACAAGATGGCCGAGGCCT ACAGCGAGATCGGCATGAAGGGCGAGAGGAGGAGGGGCAAGGGCCACGACGGCCTGT ACCAGGGCCTGAGCACCGCCACCAAGGACACCTACGACGCCCTGCACATGCAGGCCCT GCCCCCCAGG Claims 1. A method of assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the method comprises: (i) contacting the antibody or Fc fusion protein with a reporter T cell, wherein the reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain, and wherein the reporter T cell comprises a reporter gene responsive to activation of the chimeric receptor, wherein: (a) the extracellular domain is an Fc receptor extracellular domain, and wherein the method further comprises contacting the antibody or Fc fusion protein with a secondary cell expressing the target antigen on its surface; or (b) the extracellular domain is a PD-1 extracellular domain, and wherein the method optionally further comprises contacting the antibody or Fc fusion protein with a secondary cell expressing the target antigen on its surface; and (ii) detecting activation of the reporter gene. 2. The method according to claim 1, further comprising: (iii) determining that the antibody or Fc fusion protein is functional if the detected activation of the reporter gene is at least 80% of a reference value and no more than 120% of a reference value. 3. The method according to claim 2, wherein the antibody or Fc fusion protein has an assigned shelf life, wherein the method further comprises: (iv) extending the assigned shelf-life of the functional antibody or Fc fusion protein. 4. The method according to any one of claims 1-3, wherein activation of the reporter gene is detected by a reporter gene assay, optionally wherein the reporter gene assay is a luciferase assay. 5. The method according to any preceding claim, wherein the secondary cell is an immortalised cell line cell, optionally wherein the secondary cell is a Raji cell, a SKBR3 cell, or a Daudi cell. 6. A reporter T cell for assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the reporter T cell expresses a chimeric receptor comprising an intracellular signalling domain, a transmembrane domain and an extracellular domain selected from an Fc receptor extracellular domain or a PD-1 extracellular domain, and wherein the reporter T cell comprises a reporter gene responsive to activation of the chimeric receptor. 7. The method according to any one of claims 1-5 or the reporter T cell according to claim 6, wherein the extracellular domain is an Fc receptor extracellular domain. 8. The method according to any one of claims 1-5 or the reporter T cell according to claim 6 or claim 7, wherein the extracellular domain is an Fey receptor extracellular domain. 9. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-8, wherein the extracellular domain is an FcyRIIIa extracellular domain. 10. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-9, wherein the reporter gene comprises a nucleic acid molecule comprising a nucleotide sequence encoding a reporter protein operably linked to a promoter that is responsive to activation of the chimeric receptor, wherein the promoter is a minimal IL-2 promoter. 11. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-10, wherein the reporter T cell is an immortalised cell-line cell. 12. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-11, wherein the reporter T cell is a Jurkat cell. 13. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-12, wherein the reporter gene encodes a reporter protein selected from a fluorescent protein, a luminescent protein, a chemiluminescent protein, or an enzyme. 14. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-13, wherein the reporter gene encodes luciferase. 15. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-14, wherein the transmembrane domain comprises a CD28 transmembrane domain. 16. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-15, wherein the intracellular signalling domain comprises a CD28-CD3£ intracellular signalling domain. 17. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-16, wherein the target antigen is CD38, CD19, CD20 or HER-2. 18. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-17, wherein the reporter T cell does not express the target antigen. 19. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-18, wherein the chimeric receptor does not comprise a single-chain variable fragment. 20. The method according to any one of claims 1-5 or the reporter T cell according to any one of claims 6-19, wherein the chimeric receptor comprises an amino acid sequence set forth in SEQ ID NO: 6. 21. A kit for assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the kit comprises: the reporter T cell according to any one of claims 6-20; and a secondary cell expressing the target antigen on its surface. 22. The kit according to claim 21, wherein the secondary cell is an immortalised cell line cell, optionally wherein the secondary cell is a Raji cell, a SKBR3 cell, or a Daudi cell. 23. A composition for assessing binding of an antibody or Fc fusion protein to its target antigen, wherein the composition comprises: the reporter T cell according to any one of claims 6-20; and a secondary cell expressing on its surface the target antigen of the antibody or Fc fusion protein. 24. The composition of claim 23, further comprising an antibody or Fc fusion protein.

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