Distinct Mechanisms of Mismatch-Repair Deficiency Delineate Two Modes of Response to Anti–PD-1 Immunotherapy in Endometrial Carcinoma

Responses to immune checkpoint blockade (ICB) are variable among mismatch repair-deficient (MMRd) cancers. We completed a phase 2 clinical trial of the PD-1 inhibitor pembrolizumab in 24 patients with MMRd endometrial cancer (NCT02899793). Patients with mutational MMRd tumors (6 patients) had higher response rates and longer survival than those with epigenetic MMRd tumors (18 patients). Mutation burden was higher in tumors with mutational MMRd compared to epigenetic MMRd; however, within each category of MMRd, mutation burden was not associated with ICB response. Notably, JAK1 mutations did not confer resistance to pembrolizumab. Longitudinal single-cell RNA-seq of circulating immune cells revealed contrasting modes of anti-tumor immunity against mutational and epigenetic MMRd cancers. Whereas effector CD8+ T cell responses correlated with regression of mutational MMRd tumors, highly active CD16+ NK cells were associated with ICB-responsive epigenetic MMRd tumors. These data highlight the interplay between tumor-intrinsic and extrinsic factors that influence ICB response. Overall design: Longitudinal single cell RNA-seq and TCR-seq of PBMCs from endometrial cancer patients undergoing PD-1 immunotherapy