CD8 T cells licensed with immune checkpoint blockade kill murine tumors lacking MHC-I

Abstract

Abstract MHC-I downregulation is a well described mechanism of tumor immune escape. Thus, targeting tumors with low or no MHC-I expression remains a significant challenge for T cell-based immunotherapies, including immune checkpoint blockade (ICB). We previously demonstrated that the combination of PD-1 blockade and 4-1BB agonism has marked efficacy against intracranial murine CT2A glioma in a CD8 T cell-dependent manner. Surprisingly, this combination therapy remained effective in a β2 microglobulin knockout CT2A line overexpressing TRP2 (CT2A-TRP2-β2mKO). These tumors lack MHC-I but retain the Trp2 rejection antigen. Remarkably, the persisting efficacy remained dependent on CD8 T cells, but independent of NK cells, CD4 T cells, and B cells. Furthermore, the efficacy was demonstrated to be antigen-specific in vivo, as adoptively transferred Trp2-specific T cells prolonged survival of mice bearing CT2A-TRP2-β2mKO tumors, but not those bearing CT2A-β2mKO, which lacked the TRP2 antigen. To analyze the mechanism driving antigen-dependent killing of tumor cells lacking MHC-I, we performed a series of in vitro cytotoxicity experiments using Trp2-specific T cells in the presence or absence of Trp2 antigen-loaded bone marrow-derived macrophages (BMDM). These studies demonstrated that in the absence of BMDM, Trp2-specific T cells efficiently killed MHC-I-expressing CT2A-Trp2, but failed to kill MHC-I deficient CT2A-Trp2-B2mKO tumors. However, killing was restored in the presence of Trp2 loaded BMDMs; the latter of which possessed little to no cytotoxic effect alone. These findings suggest a novel role for myeloid populations in mediating the influence of ICB on T cell function and challenge the traditional model of T cell tumor killing.