Regulatory T cells in the tumor microenvironment display a unique chromatin accessibility profile

Regulatory T cells (Tregs) are a suppressive CD4+ T cell population that limit the antitumor immune response. In this study, we analyzed the chromatin accessibility of Tregs in the murine tumor microenvironment (TME) to identify tumor-specific accessible peaks and if these are altered over time in the tumor microenvironment, with or without anti-PD-1 immunotherapy. We found that despite little change in chromatin accessibility of Tregs in the tumor over time, Tregs have a distinct chromatin accessibility signature in the TME compared with Tregs in the periphery. This distinct tumor Treg chromatin accessibility profile highlights reduced accessibility at loci important for an CD4+ conventional T cell (CD4+ Foxp3-) effector phenotype. Analysis of chromatin accessibility in Tregs from B16 and MC38 tumor models indicated that Tregs from skin-resident tumors are most similar to naïve skin resident Tregs but still bear key differences attributable to the TME. We also found that Tregs do not alter their transcriptome or chromatin accessibility following immunotherapy. We conclude that although chromatin accessibility in Tregs is somewhat similar to their tissue residency, the TME may drive a unique chromatin accessibility profile. Treg chromatin accessibility in the tumor appears remarkably stable and unaltered by tumor type, over time, or following immunotherapy.