UVB-induced tumor heterogeneity and mutational load, dictate the anti-melanoma immune response

Little is known regarding the relationship between the mutational load, intra-tumor heterogeneity (ITH) and the reactivity of tumor infiltrating lymphocytes (TILs). Here, we explored the single and combined effects of mutation load and ITH on tumor rejection by the immune system by establishing a melanoma mouse model and inducing in it UVB-derived mutations that increase both the ITH and the mutational load. These mutations give rise to highly aggressive tumors, due to decreased cytotoxic activity of infiltrating TILs. In contrast, single cell-derived clones of irradiated and non-irradiated cells that had reduced ITH compared to their parental cells were massively rejected, accompanied by increased TIL reactivity, increased infiltration into the tumor core, altered TCR repertoire and the detection of HLA bound neopeptides. Finally, our analysis of the mutation burden and ITH of immunotherapy treated and non-treated melanoma patient data validates our mouse data and demonstrates the significant contribution of ITH to patient survival but the little effect of mutational load. These findings emphasize the importance of ITH in patient matching to checkpoint blockade immunotherapies.