Clinical observations suggest that responses to
cancer immunotherapy are correlated with intra-tumoral
T cell receptor (TCR) clonality,
tumor mutation burden (TMB) and host HLA genotype, highlighting the importance of host T cell recognition of
tumor antigens. However, the dynamic interplay between T cell activation state and changes in TCR repertoire in driving the identification of potential immunodominant
antigen(s) remains largely unexplored. Here, we performed single-cell
RNA-sequencing on CD8+
tumor-infiltrating T cells (TILs) using the murine
colorectal tumor model MC38 to identify unique TCR sequences and validate their
tumor reactivity. We found that the majority of clonally expanded TILs are
tumor-reactive and their TCR repertoire is unique amongst individual MC38
tumor-bearing mice. Our query identified that multiple expanded TCR clones recognized the retroviral
epitope p15E as an immunodominant
antigen. In addition, we found that the endogenous retroviral genome encoding for p15E is highly expressed in MC38
tumors, but not in normal tissues, due to epigenetic derepression. Further, we demonstrated that the p15E-specific TILs exhibit an activated phenotype and an increase in frequency upon treatment with anti-41BB and anti-PD-1 combination
immunotherapy. Importantly, we showed that although p15E-specific TILs are not required to mount a primary anti-
tumor response, they contributed to the development of strong immune memory. Overall our results revealed that endogenous retroviral
antigens expressed by
tumor cells may represent an important and underappreciated category of
tumor antigens that could be readily targeted in the clinic.