The NKG2D/NKG2DL Pathway in CD8+ T Cell-Mediated Anti-Tumor Immunity and Immunotherapy

Abstract

The long-accepted paradigm for adaptive anti-tumor cellular immunity relies on antigen-specific tumor targeting by activated CD8+ T cells. CD8+ T cell cytotoxicity, in turn, is classically believed to depend upon T cell receptor (TCR) recognition of tumor antigens presented exclusively in the context of cell surface major histocompatibility complex I (MHC-I) molecules. Mutations leading to decreased or absent MHC-I expression are therefore purported to constitute a common mechanism by which tumors can evade T cell responses, rendering them immunologically “cold”.Recent pre-clinical and clinical studies, however, have demonstrated somewhat mixed roles for MHC-I in dictating responses to cancer immune-based platforms, such as immune checkpoint blockade (ICB). These mixed findings highlight the need for further investigation into the role of tumor MHC-I, as well as revisiting traditional notions of anti-tumor immunity. Here we sought to better evaluate the impact of tumor MHC-I expression on response to ICB. We engineered murine glioma and melanoma lines to lack cell surface MHC-I by knocking out beta-2 microglobulin (b2m), a critical component of MHC-I. Surprisingly, the efficacy of ICB was maintained in vivo, despite the absence of MHC-I on tumor cells. Furthermore, survival in treated mice remained dependent on the presence of CD8+ T cells and was independent of NK cells or CD4+ T cells. This cytotoxicity is both antigen- and MHC-agnostic and, instead, is mediated by T cell NKG2D engagement of NKG2D ligands on tumor cells. Subsequent tumor cell kill depends on prior TCR activation (albeit even by irrelevant antigen), revealing that adaptive priming can beget subsequent innate killing. This cytotoxicity mechanism is active in vivo in mice, as well as in vitro in human cells, and is required for killing of MHC-negative tumor cells even in tumors with heterogenous MHC expression. In these studies, we demonstrate that CD8+ T cell-dependent immunotherapies can indeed remain effective against tumors, even when uniformly lacking MHC-I. These findings challenge the long-advanced notion that downregulation of MHC-I is a viable means of tumor immune escape, and instead identify the NKG2D/NKG2DL axis as a therapeutic target for enhancing T cell-dependent anti-tumor immunity against MHC loss variants.