Clinicopathologic and Prognostic Implications of Programmed Death Ligand 1 Expression in Thymoma.

BACKGROUND: Programmed death ligand 1 (PD-L1) has been reported to be expressed in various malignancies and is considered to be a prognostic factor and an immunotherapeutic target. The aim of this study was to characterize PD-L1 expression in thymoma and determine statistical associations between this expression and clinical features.

METHODS: We reviewed formalin-fixed, paraffin-embedded tissue specimens from 82 thymoma cases accumulated at Kurume University, the majority of which achieved surgical complete resection. Expression of PD-L1 was evaluated by immunohistochemistry. Statistical associations between PD-L1 expression and clinicopathologic features were evaluated by using χ(2) test and Fisher's exact test. Disease-free survival and overall survival curves were established by the Kaplan-Meier method and compared using a log-rank test. Predictive factors for disease-free survival after complete resection were analyzed by using a Cox proportional hazards model in univariate and multivariate analysis.

RESULTS: Overall, 44 thymoma cases (54%) revealed high PD-L1 expression. High PD-L1 expression was statistically associated with Masaoka stage III/IV disease (p = 0.043) and World Health Organization type B2 or B3 thymoma (p = 0.044). Disease-free survival after complete resection in high PD-L1 expression was significantly worse than that in low PD-L1 expression (p = 0.021), although there was no significant difference in overall survival (p = 0.957). Multivariate analysis also revealed high PD-L1 expression as an independent risk factor for recurrence (p = 0.008).

CONCLUSIONS: Characterization of PD-L1 expression in thymoma should enable more effective clinical approaches, including prognostic stratification of patients and potential use of anti-PD-L1 antibody immunotherapy.