Identification of a Subset of Microsatellite-Stable Endometrial Carcinoma with High PD-L1 and CD8+ Lymphocytes

Immune checkpoint blockade has emerged as an effective therapeutic strategy for patients with advanced cancer. Identification of biomarkers associated with treatment efficacy will help to select patients more likely to respond to this approach. High levels of microsatellite instability, tumor expression of PD-L1, high tumor mutation burden, and increased tumor infiltrating lymphocytes have all been associated with response to checkpoint inhibitor blockade. The purpose of this study was to determine if a subset of microsatellite stable endometrioid endometrial carcinomas have higher immune cell infiltrates and/or expression of PD-L1. PD-L1 expression and characterization of immune cell infiltrates were analyzed in 132 microsatellite stable, FIGO grade 2 endometrioid carcinomas. PD-L1 was positive in 48% (63/132) of the tumors. Tumor cell expression of PD-L1 was significantly associated with lymphatic/vascular invasion and deep myometrial invasion. PD-L1 expression was especially prominent at the invasive front and in foci of tumor-associated squamous metaplasia. Twenty-one cases (16% of the total) with more diffuse and/or especially strong PD-L1 expression were identified. This PD-L1 high subset was associated with significantly higher numbers of tumor-associated CD3+ and CD8+ lymphocytes. Only one tumor in the PD-L1 high subset harbored a POLE mutation. PTEN immunohistochemical loss, a common event in endometrioid-type endometrial carcinoma and associated with local immune suppression in melanoma, was not associated with PD-L1 expression or lymphocyte/macrophage infiltration of the tumor. These results suggest that a subset of microsatellite-stable endometrial cancers has higher expression of PD-L1 and increased tumor-associated CD3+ and CD8+ lymphocytes, characteristics more commonly associated with endometrial cancers with high levels of microsatellite instability. These results suggest that screening strategies to select only microsatellite instability-high advanced endometrial cancers for checkpoint inhibitor therapy might exclude patients who could potentially benefit from this therapeutic approach.