Mapping of immunopeptidome of cancer immunotherapy with oncolytic virus and checkpoint blockade

The elucidation of therapy-induced changes to the class I major histocompatibility complex (MHC-I)-bound tumor antigens is crucial for understanding immune-mediated tumor eradication and identifying potential targets for peptide vaccines to enhance the efficacy of immunotherapies. Here, we investigated how oncolytic reovirus therapy with and without immune checkpoint blockade (ICB) alters the tumor peptide-MHC repertoire. Using mass spectrometry analysis of immunoaffinity purified MHC peptides, we first showed that changes to the MHC immunopeptidome following reovirus treatment is cancer type-dependent, where a murine fibrosarcoma model displayed quantitative and qualitative variance in differentially expressed peptides (DEPs) as compared to those identified in a murine ovarian cancer model. We then determined that the combination therapy of reovirus and ICB in the fibrosarcoma model resulted in higher numbers of DEPs relative to either monotherapy alone. Most importantly, we identified reovirus and ICB-induced MHC peptides that are biologically active in stimulating interferon-gamma response in cognate CD8+ T cells, which likely contribute to cancer immunoediting. These findings highlight the importance of therapy-induced changes to the MHC immunopeptidome in shaping the underlying anti-tumor immune responses during reovirus and ICB combination therapy.