Technology
Adoptive T cell therapy
Adoptive therapy is the process of taking blood cells from a patient, manipulating them in some way to improve their therapeutic potential when outside the body (i.e. ex-vivo), and returning them to the same patient.
Adaptimmune focuses on adoptive T cell therapy because T-lymphocytes or T cells are the key immune cells responsible for recognising and removing infected or cancerous cells within the body. T cells patrol the body, using their T cell receptors (TCRs) to scan the surface of other cells, looking for foreign fragments of cell-derived proteins (peptide antigens) that are presented on the cell surface by proteins called Human Leukocyte Antigens (HLA). A passing T cell that recognises a cancer or viral antigen through its TCR will be activated and go on to mount an immune response against the target cell. However, the human T cell repertoire is selected to strike a delicate balance: it needs the capacity for recognising all alien antigen-derived peptides whilst avoiding reaction to self-derived peptides, both classes being presented by the same MHC molecules. Consequently, T cell receptor binding (TCR) to peptide-MHC, which forms the basis for this differentiation, is characterised by extremely low affinities.
TCRs specific for cancer antigen-derived peptides generally appear to be of particularly low affinity, probably because most of these peptides are derived from, or are closely related to, self antigens. Because initiation of T cell activation is governed by the avidity generated by TCR – peptide-MHC interaction, and cancer cells are selected to present very low antigen levels, most T cells specific for cancer antigens fail to efficiently recognise tumour cells. Similarly, viral peptide antigens may mutate and avoid recognition after the initial wave of infection (as in the case for HIV) reducing the ability of effector memory T cells to control the ongoing infection.
The vast majority of T cells, therefore, will not recognise the foreign antigen being presented, and sometimes an antigen will never be recognised adequately, leading to little or no immune response. The therapeutic rationale behind adoptive T cell therapy therefore is to artificially boost the numbers of T cells able to recognise the desired target antigen.
This strategy has provided some notable clinical results but it has become clear that the strength of the therapeutic response depends crucially on the quality of the TCR used. The ability of a TCR to activate the T cell depends largely on the strength of its binding to the target antigen; however, most naturally occurring TCRs are relatively weak and therefore not ideal for use in adoptive T cell therapy.
Adaptimmune’s approach is to artificially introduce a gene encoding a new TCR of known antigen specificity and optimised affinity into this expanded population of patients’ T cells to enable them to more effectively recognise and kill the desired target cancer or infected cell.
Adaptimmune’s Technology – Engineering improved TCRs
Adaptimmune’s key advantage in this field is the exclusive use of a pipeline of TCRs from Immunocore Ltd. which have improved binding to their disease antigens. By fine-tuning the affinity of these TCRs, Adaptimmune is able to establish the optimum binding characteristics for transfected TCRs that allows them to stimulate the maximum possible T cell response whilst maintaining absolute specificity for the target antigen, thus maximising the possibility of clinical success.
In their natural form, TCRs are only expressed as membrane-anchored proteins which undergo a selection process such that only those T cells which carry TCRs with very low affinities for antigen survive. To generate T cells with enhanced ability to recognise cancer antigens, it is therefore necessary to engineer TCRs with higher antigen-binding affinities. This involves 1) expression of TCRs as soluble proteins, enabling accurate affinity measurements and analysis of mutants and 2), an in vitro molecular evolution and selection system for identifying higher affinity variants using phage display. These aspects are explained in greater detail below:
1) TCRs have been the subject of many approaches to make soluble versions, but for many years it was notoriously difficult to identify a design that produced stable proteins and was applicable to more than one or two selected TCRs. In 2003, Adaptimmune’s founders described a novel approach that involves mutation of a residue in each of the TCR α and β chain constant regions, respectively1. TCR polypeptides carrying these mutations and expressed in E. coli can be readily refolded as stable, soluble proteins. This approach has proven generally applicable to human and, to a lesser extent, murine TCRs and has enabled both structural and biochemical studies of many TCR proteins using Biacore and other approaches2-6.
2) By far the most powerful approach for molecular evolution of proteins involves phage display and selection from large, diversified mutant libraries. Adaptimmune’s forerunner Avidex, first described this approach for TCRs in 20057,8 and has since used the methodology for isolating high affinity variants of more than eight different TCRs. The specificities of these high affinity TCRs have been validated through antigen binding studies, using both recombinant antigens and cell-based methodologies9,10.
REFERENCES
1. Boulter et al. 2003, Protein Engineering 16; 9: 707-711.
2. Laugel et al. 2005, The Journal of Biological Chemistry 280; 3: 1882-1892.
3. Gadola et al. 2006, Journal of Experimental Medicine 203: 699-710.
4. Cole et al. 2007, The Journal of Immunology 178: 5727-5734.
5. Sami et al. 2007, Protein Engineering, Design & Selection 20; 8: 397-403.
6. Cole et al. 2008, Molecular Immunology 45: 2700-2709.
7. Li et al. 2005, Nature Biotechnology 23; 3: 349-354.
8. Dunn et al. 2006, Protein Science 15: 710-721.
9. Purbhoo et al. 2006, The Journal of Immunology 176: 7308-7316.
10. Purbhoo et al. 2007, Molecular Cancer Therapeutics 6: 2081-2091.
T Cell Killing Movie – 1 Mb.
Non-cancer specific T-cells (small blue cells) redirected to kill red cancer cells whilst ignoring green innocent bystander cells.
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