Mismatch repair deficiency is not sufficient to elicit tumor immunogenicity

Abstract

DNA mismatch repair deficiency (MMRd) is associated with high tumor mutational burden (TMB), increased T cell infiltration, and remarkable responsiveness to immune checkpoint blockade (ICB) therapy1. Nevertheless, about half of MMRd tumors do not respond to ICB for unclear reasons. While cell line transplant models of MMRd have reinforced the importance of TMB in immune response2,3, critical questions remain regarding the role of immunosurveillance in the evolution of MMRd tumors induced in vivo. Here, we developed autochthonous mouse models of lung and colon cancer with ablation of MMR via in vivo CRISPR/Cas9 targeting. Surprisingly, MMRd in these models did not increase T cell infiltration or response to ICB. Mechanistically, we showed this lack of immunogenicity to be driven by profound intratumoral heterogeneity. Studies in immune deficient animals further demonstrated that immunosurveillance in MMRd tumors has no impact on TMB but shapes the clonal architecture of neoantigens by exacerbating heterogeneity. These results provide important context for understanding immune evasion in cancers with high TMB and have major implications for therapies aimed at increasing TMB.