Minimal tolerance to a tumor antigen encoded by a cancer-germline gene.

Central tolerance toward tissue-restricted Ags is considered to rely on ectopic expression in the thymus, which was also observed for tumor Ags encoded by cancer-germline genes. It is unknown whether endogenous expression shapes the T cell repertoire against the latter Ags and explains their weak immunogenicity. We addressed this question using mouse cancer-germline gene P1A, which encodes antigenic peptide P1A(35-43) presented by H-2L(d). We made P1A-knockout (P1A-KO) mice and asked whether their anti-P1A(35-43) immune responses were stronger than those of wild-type mice and whether P1A-KO mice responded to other P1A epitopes, against which wild-type mice were tolerized. We observed that both types of mice mounted similar P1A(35-43)-specific CD8 T cell responses, although the frequency of P1A(35-43)-specific CD8 T cells generated in response to P1A-expressing tumors was slightly higher in P1A-KO mice. This higher reactivity allowed naive P1A-KO mice to reject spontaneously P1A-expressing tumors, which progressed in wild-type mice. TCR-Vß usage of P1A(35-43)-specific CD8 cells was slightly modified in P1A-KO mice. Peptide P1A(35-43) remained the only P1A epitope recognized by CD8 T cells in both types of mice, which also displayed similar thymic selection of a transgenic TCR recognizing P1A(35-43). These results indicate the existence of a minimal tolerance to an Ag encoded by a cancer-germline gene and suggest that its endogenous expression only slightly affects diversification of the T cell repertoire against this Ag.

Resistance to cancer immunotherapy mediated by apoptosis of tumor-infiltrating lymphocytes.

Despite impressive clinical success, cancer immunotherapy based on immune checkpoint blockade remains ineffective in many patients due to tumoral resistance. Here we use the autochthonous TiRP melanoma model, which recapitulates the tumoral resistance signature observed in human melanomas. TiRP tumors resist immunotherapy based on checkpoint blockade, cancer vaccines or adoptive T-cell therapy. TiRP tumors recruit and activate tumor-specific CD8+ T cells, but these cells then undergo apoptosis. This does not occur with isogenic transplanted tumors, which are rejected after adoptive T-cell therapy. Apoptosis of tumor-infiltrating lymphocytes can be prevented by interrupting the Fas/Fas-ligand axis, and is triggered by polymorphonuclear-myeloid-derived suppressor cells, which express high levels of Fas-ligand and are enriched in TiRP tumors. Blocking Fas-ligand increases the anti-tumor efficacy of adoptive T-cell therapy in TiRP tumors, and increases the efficacy of checkpoint blockade in transplanted tumors. Therefore, tumor-infiltrating lymphocytes apoptosis is a relevant mechanism of immunotherapy resistance, which could be blocked by interfering with the Fas/Fas-ligand pathway.