Prognostic Value of PD-L1 Immunohistochemical Marker in Gastric Carcinoma and Its Correlation with HER2 Status.

Objective: Programmed death-ligand 1 (PD-L1) and human epidermal growth factor receptor 2 (HER2) are currently considered as prognostic markers and therapeutic targets in many human cancers. This study aims to evaluate immunohistochemical (IHC) expression of PD-L1 in gastric cancer (GC) and explore its prognostic role in terms of association with HER2 expression, different clinico-pathological variables, in particular density and cluster designation (CD)8 positivity in tumor infiltrating lymphocytes (TILs) and with patients' disease-free and overall survival (DFS, OS).

Methods: This retrospective cohort study included 111 diagnosed primary GC patients who underwent surgical resection at the Gastrointestinal Surgery Center (GISC), Faculty of Medicine, Mansoura University, Egypt. After demographic, clinicopathological and survival data collection, histopathological evaluation was done for GC typing, staging and assessment of the histopathological prognostic parameters. IHC was performed for PD-L1, HER2 and CD8. PDL-1 was scored using the Combined Positive Score (CPS).

Results: PD-L1 was expressed in 43.2% of GCs at a CPS cut-off value ≥ 1. PDL-1 positivity was significantly associated with high TILs and CD8+ TILs (p=0.008, 0.016 respectively), indicating its contribution to tumor microenvironment along with the TILs. Multivariate analysis spotted PD-L1 positivity as an independent prognostic predictor for shorter OS in GC (p=0.013), with a tendency toward shorter DFS. Only 9.9% GCs were HER2 positive (score +3) with no significant association with PD-L1.

Conclusion: PDL-1 is a promising prognostic and therapeutic target in GC that may direct the selection of patients for immunotherapy and checkpoint-blockade (pembrolizumab) therapy.