Loss of Major Histocompatibility Complex Class I, CD8⁺Tumor-infiltrating Lymphocytes, and PD-L1 Expression in Ovarian Clear Cell Carcinoma

Ovarian clear cell carcinoma (OCCC), a chemoresistant ovarian cancer, shows a modest response to anti-programmed death-1/programmed death ligand-1 (PD-1/PD-L1) therapies. The effects of anti-PD-1/PD-L1 therapies rely on cytotoxic T-cell response, which is triggered by antigen presentation mediated by major histocompatibility complex (MHC) class I. The loss of MHC class I with simultaneous PD-L1 expression has been noted in several cancer types; however, these findings and their prognostic value have rarely been evaluated in OCCC. We collected data from 76 patients with OCCC for clinicopathologic analysis. Loss of MHC class I expression was seen in 44.7% of the cases including 39.3% to 47.4% of the PD-L1⁺cases and was associated with fewer CD8⁺tumor-infiltrating lymphocytes (TILs). PD-L1 positivity was associated with a higher number of CD8⁺TILs. Cox proportional hazard models showed that high (=50/mm²) CD8⁺TILs was associated with shorter disease-specific survival (hazard ratio [HR]=3.447, 95% confidence interval [CI]: 1.222-9.720, P=0.019) and overall survival (HR=3.053, 95% CI: 1.105-8.43, P=0.031). PD-L1 positivity using Combined Positive Score was associated with shorter progression-free survival (HR=3.246, 95% CI: 1.435-7.339, P=0.005), disease-specific survival (HR=4.124, 95% CI: 1.403-12.116, P=0.010), and overall survival (HR=4.489, 95% CI: 1.553-12.972, P=0.006). Loss of MHC class I may contribute to immune evasion and resistance to anti-PD-1/PD-L1 therapies in OCCC, and CD8⁺TILs and PD-L1 positivity using Combined Positive Score may have a negative prognostic value.