Genetic and pharmacological modulation of DNA mismatch repair heterogeneous tumors promotes immune surveillance

Patients affected by colorectal cancer (CRC) with DNA mismatch repair deficiency (MMRd), often respond to therapies based on immune checkpoint blockade, while those with mismatch repair proficient (MMRp) tumors do not. Interestingly, a subset of MMRp CRCs contains variable fractions of MMRd cells, but it is unknown how their presence impacts immune surveillance. We asked whether modulation of the MMRd fraction in MMR heterogeneous tumors acted as an 'endogenous cancer vaccine' by promoting immune surveillance. To test this hypothesis, we used isogenic MMRp and MMRd mouse CRC cells generated by genetic inactivation of Mlh1, a key component of the MMR machinery. Populations of MMRp/MMRd cells mixed at different ratios were injected in immunocompetent mice. Tumor rejection was observed when at least 50% of cells were MMRd, while a growth delay was already evident when as low as 20% of MMRd cells were present. To enrich pharmacologically the MMRd fraction, MMRp/MMRd tumors were treated with the antimetabolite 6-Thioguanine (6TG), which led to immune surveillance and tumor rejection. These results suggest that it is possible to increase immunogenicity of MMR heterogeneous tumors through an endogenous vaccination approach based on genetic or pharmacological modulation the DNA mismatch repair, which edits the tumor microenvironment.