TIM-3 in endometrial carcinomas: an immunotherapeutic target expressed by mismatch repair-deficient and intact cancers.

The checkpoint molecule TIM-3 is a target for emerging immunotherapies and has been identified on a variety of malignancies. Mismatch repair-deficient endometrial carcinomas have demonstrated durable responses to other checkpoint inhibitors due to high neoantigen loads and robust tumor-associated immune responses. However, little is known about TIM-3 expression in this tumor type. Tumor-associated immune and tumoral expression of TIM-3 were evaluated by immunohistochemistry on 75 endometrial carcinomas [25 MLH1 promoter hypermethylated (MLH1-hypermethylated), 25 non-hypermethylated mismatch repair-deficient, and 25 mismatch repair-intact]. All cases showed at least focal immune staining, but moderate and robust immune cell expression were more often observed in mismatch repair-deficient vs intact cases [66 vs 12%, P = 0.00002]. While the majority (77%) of endometrial cancers showed ≥1% tumoral TIM-3 expression, the MLH1-hypermethylated subset was more likely to demonstrate >5% tumoral staining when compared to both mismatch repair-intact and non-methylated mismatch repair-deficient cancers [64 vs. 28% and 32%, respectively; P = 0.02 and P = 0.05]. Within the non-methylated mismatch repair-deficient subset, high-level expression was most often associated with MSH6 loss. Across mismatch repair subgroups, tumoral TIM-3 expression was more common among intermediate and high-grade vs. low-grade tumors using both the 1% (P = 0.02) and 5% expression cut-offs (P = 0.02). In conclusion, tumoral TIM-3 expression is common in both mismatch repair-intact and deficient endometrial cancers, with particularly high levels of expression identified in the setting of MLH1-hypermethylation, MSH6 loss, and intermediate to high histologic grade. Although focal immune cell expression was seen in all tumors, robust expression was significantly more common in the context of mismatch repair deficiency. These data support a potential role for checkpoint inhibitors targeting TIM-3 in a subset of endometrial cancers, including some mismatch repair-intact tumors which are not currently considered immunotherapy candidates.