MHC class II on melanoma cells regulates the anti-tumor T cell response

In human melanoma, tumor cell-specific MHC class II expression is associated with improved response to programmed death 1 and programmed death-ligand 1 blockade; yet the direct effect of melanoma cell-specific MHC class II expression on anti-melanoma T cell responses remains largely unknown. In the clinically relevant Yale University Mouse Melanoma Exposed to Radiation (YUMMER) cell lines, MHC class II expression was IFN-γ-inducible in a subset of cells. Transduction of melanoma cells with CIITA resulted in uniformly high, constitutive expression of MHC class II. YUMMER.G cells transduced with CIITA vector had increased in vivo tumor growth in wild-type mice compared to melanoma cells transduced with empty vector. YUMMER.G cells transduced with empty or CIITA vectors had the same in vivo tumor growth in RAG-/- mice, indicating that the difference in tumor growth between CIITA- and empty-transduced cells was dependent on an intact adaptive immune system. CIITA-transduced YUMMER.G tumors exhibited an increased frequency and infiltration depth of CD4+ T cells, an increased frequency of T regulatory cells, and a decreased frequency and infiltration depth of CD8+ T cells, compared to empty-transduced tumors. Antibody depletion revealed that CIITA- and empty-transduced YUMMER.G tumors were constrained by CD4+ and CD8+ T cells, but only tumor growth of CIITA-transduced cells was decreased by CD25 blockade, which primarily blocks T regulatory cell function. Overall, this work reveals a potential pro-tumorigenic role of melanoma cell-specific overexpression of MHC class II.