Deletion of MyD88 in T Cells Improves Antitumor Activity in Melanoma

Cytotoxic CD8⁺ T cells are central to the antitumor immune response by releasing cytotoxic granules that kill tumor cells. They are activated by antigen-presenting cells, which become activated by DAMPs (damage associated molecular patterns) through MyD88. However, the suppressive tumor microenvironment promotes T-cell tolerance to tumor antigens, in part by enhancing the activity of immune checkpoint molecules that prevent CD8⁺ T-cell activation and cytotoxicity. MyD88 limits CD4⁺ T-cell activation during cardiac adaptation to stress. A similar mechanism is hypothesized to exist in CD8⁺ T cells that could be modulated to improve antitumor immunity. Herein, adoptive transfer of MyD88^-/- CD8⁺ T cells in melanoma-bearing T-cell-deficient mice resulted in slower tumor growth, greater intratumoral T-cell accumulation, and higher melanoma cell death compared with transfer of wild-type CD8⁺ T cells. These findings were also observed in T-cell-specific MyD88^-/- mice compared with wild-type littermates implanted with melanoma. Mechanistically, deletion of MyD88 enhanced CD8⁺ T-cell activation and survival, and T-cell receptor induced degranulation of cytotoxic molecules, overall improving the killing of melanoma cells. This enhanced cytotoxicity was retained in mice bearing tumors expressing the specific antigen for which cytotoxic T-cells were restricted. This study's results demonstrate a conserved mechanism for MyD88 in modulating CD8⁺ T-cell activation and represent a novel target in improving cancer immunotherapy.