Association of Polymerase e–Mutated and Microsatellite-Instable Endometrial Cancers With Neoantigen Load, Number of Tumor-Infiltrating Lymphocytes, and Expression of PD-1 and PD-L1

IMPORTANCE: Immune checkpoint inhibitor therapy has shown benefit in various cancers, but their potential in endometrial cancer (EC) is unknown. OBSERVATIONS: Prediction of neoantigen load was performed using sequencing data from the Cancer Genome Atlas data set. Evaluation of tumor-infiltrating lymphocytes (TILs) and PD-1 and PD-L1 expression was performed in 63 patients with EC referred to our institution. The predicted median (range) neoantigen load (predicted neoepitopes per sample) was proportional to the mutational load: highest in ultramutated polymerase e (POLE) tumors (8342 [628-20 440]), less in hypermutated MSI (541 [146-8063]; P < .001), and lowest in microsatellite-stable tumors (70.5 [7-1877]; P < .001). The POLE and MSI ECs exhibited higher numbers of CD3+ (44.5 vs 21.8; P = .001) and CD8+ (32.8 vs 13.5; P < .001) TILs compared with microsatellite-stable tumors. PD-1 was overexpressed in TILs (81% vs 28%; P < .001) and peritumoral lymphocytes (90% vs 28%; P < .001) of POLE and MSI tumors. PD-L1 expression was infrequently noted in tumor cells but was common in intraepithelial immune cells and more frequent in POLE and MSI tumors (39% vs 13%; P = .02). CONCLUSIONS AND RELEVANCE: Polymerase e-mutated and MSI ECs are associated with high neoantigen loads and number of TILs, which is counterbalanced by overexpression of PD-1 and PD-L1. Polymerase e-mutated and MSI EC tumors may be excellent candidates for PD-1-targeted immunotherapies.