Tapasin inhibits MHC I-presentation of a low affinity peptidome which elicits a unique population of highly effective CD8 T cells, in multiple mouse cancers

Neoepitopes are generated from tumor-specific mutations that make tumors targets for CD8 and CD4 T cells. We and others have reported that a significant proportion of neoepitopes which elicit CD8 T cell mediated tumor control in vivo (such as Ccdc85cMUT in the BALB/c Meth A sarcoma and PLK1MUT in the C57BL/6 MC38-FABF colon carcinoma) bind MHC I with low to extremely low affinities (IC 50 of 500- 50,000 nM) (Srivastava PK 2024 J. Clinical Investigation). The protein Tapasin facilitates the loading the MHCI with peptides that bind MHCI with high affinity. This high affinity immunopeptidome is a key to defense against acute viral infections. We observe that paradoxically, the Tapasin-sculpted immunopeptidome becomes a barrier in control of cancers. Genetic ablation of Tapasin in cancer cells (Meth A fibrosarcoma and 4T1 triple negative breast cancer of BALB/c mice and MC38-FABF colon carcinoma of C57BL/6 mice) leads to highly immunogenic cancer variants that fail to form tumors at challenge doses at which Tapasin-proficient parental tumors readily form tumors. The immune response elicited by Tapasin-deficient tumors is fully active even against the Tapasin-proficient parental tumors. The immunopeptidome of Tapasin-deficient tumors is dominated by low affinity pMHC I complexes. Mass spectrometry analysis of the MHC I presented peptidome shows 760 peptides presented by Tapasin-proficient cells and 1175 peptides by Tapasin-deficient cells. Of this, 500 peptides are shared between the parental and Tapasin-deficient cells. The unique peptidome of Tapasin-deficient cells shows a greater diversity of low affinity peptides (~700 peptide with predicted IC 50 >2,000 nM in Tapasin deficient cells as compared to ~250 such peptides in Tapasin proficient cells). The low affinity immunopeptidome engages a pool of CD8 T cells with profound anti-cancer activity mediating robust tumor control of Tapasin-proficient parental tumors in adoptive transfer assays. The CD8 T cell repertoire elicited by Tapasin deficient tumors is highly cytotoxic and less exhausted as compared to the CD8 T cells elicited by Tapasin-proficient parental tumors. Lower expression of Tapasin in human cancers is significantly associated with better survival. These observations suggest that ablation of Tapasin can be used for immunotherapy of human cancers.