Immune checkpoint expression, microsatellite instability, and mutational burden: Identifying immune biomarker...

Increasing grade of endometrioid endometrial cancer (EEC) is associated with aggressive behavior and poor prognosis. The traditional classification system is limited in its ability to guide treatment planning and prognostication. We identify distinct immune biomarker phenotypes using known markers of immunogenicity to identify patients who may benefit from immune therapy (IT).

621 tumors were analyzed by multiplatform profiling. NextGen sequencing was performed on 592 genes. Tumor mutational burden (TMB) was defined as high (H) ≥10mutations/megabase. Microsatellite Instability (MSI) by NGS was ≥46 loci. PD-L1 positivity was ≥2+, >5% by IHC. Chi-square tests were used.

Overall, MSI-H was found in 33% of EECs, most frequent in grade 3 (G3), followed by grade 2 (G2) and grade 1 (G1) tumors (G3: 37%, G2: 32%, G1: 22%, p = 0.007). TMB-H was identified in 25% of EECs. TMB-H was most common in G3, followed by G2 and G1 tumors (G3: 34%, G2: 23%, G1: 13%, p = 0.006). Overall, PD-L1 expression was found in 5.5% of EECs. G3 EECs had the most frequent PD-L1 expression, followed by G2 and G1 tumors (G3: 12%, G2: 3.0%, G1: 0.9%, p < 0.0001). We identified POLE mutations in 4.5% (28/618). All POLE mutated tumors harbored TMB-H phenotypes but MSI-H and PD-L1 were only present in 10.7% and 14.8% of tumors respectively, suggesting upregulation of T-cell immune response in only a fraction of POLE mutated EECs. Triple negative (TN) biomarker phenotype (ER-/PR-/Her2-) was evaluated as a potential surrogate marker of tumor immunogenicity. We identified TN phenotype in 4% of G1 EEC compared with 9% in G2 and 33% in G3, suggesting loss of hormone expression and possible greater immunogenicity with increasing tumor grade.

High grade tumors appear to be more immunogenic than low grade tumors and may preferentially benefit from IT.