Generation of a HuTCR mouse platform-derived MAGE-A1-directed high-affinity TCR with superior potency versus human-derived TCRs.

Abstract

e14515 Background: As cancer-testis antigens are self-antigens, T cells expressing high-affinity TCRs against such antigens are suppressed via negative thymic selection. Therefore, patient- or donor-derived TCRs are typically of low affinity and result in a reduced antitumor effect. Using our proprietary HuTCR platform, which consists of mouse lines carrying the full human TCR α/β loci in combination with common human HLA alleles, we have isolated high-affinity TCRs specific for the cancer-testis antigen MAGE-A1 and compared them to human-derived MAGE-A1-specific TCRs that are currently reported to be in clinical development. Furthermore, we validated MAGE-A1 as a potential cancer therapy target by using immunohistochemistry to evaluate expression in several major tumor types and healthy tissue. Methods: Using scRNAseq, TCRs were isolated from HuTCR mice. Human-derived MAGE-A1-specific TCR sequences were obtained from publicly available databases. All TCRs were expressed in primary human T cells as verified using peptide-MHC-multimer staining. Functional avidity of the TCRs was analyzed by coculture with T2 target cells loaded with titrated amounts of epitope peptides and measuring cytokine concentration by ELISA. Reactivity of TCRs to endogenously processed MAGE-A1 protein was assessed by co-culture with a panel of tumor cell lines varying in MAGE-A1 and/or MHC-class-I expression. MAGE-A1 expression on protein level was evaluated by immunohistochemistry. Results: Immunization of HuTCR mice with the antigen resulted in robust CD8+ T cell responses and several TCR clonotypes were identified by scRNAseq, with the majority of clonotypes being specific to the MAGE-A1-derived peptide KVLEYVIKV and TCR affinities ranging from 0.3 nM to 3 nM. By comparison, human-derived TCRs exhibited generally lower functional avidity from 3 nM to 60 nM. In addition, HuTCR-mouse-derived TCRs were more sensitive in recognition of tumor cell lines expressing low MAGE-A1 and/or HLA-A2. Immunohistochemical analysis of MAGE-A1 expression in healthy tissues demonstrated highly selective expression of MAGE-A1 in testis, only. Screening for expression confirmed that a significant proportion of several major cancer types expresses MAGE-A1 as reported by various other groups [reviewed in Curr Opin Cell Biol. 2015 December; 37: 1–8]. Conclusions: The HuTCR mouse platform allows for the generation of high-affinity MAGE-A1-specific TCRs with increased anti-tumor efficacy as compared to human-derived TCRs against the same cancer antigen. In addition, it was confirmed that MAGE-A1 has a highly selective expression pattern in healthy tissues (testis, only), but shows distinct expression in several major human tumor types.