The contribution of programmed cell death ligand-1 (PD-L1) immune checkpoint molecule toward progression of non-small cell lung cancer (NSCLC) has not yet been elucidated, in part, because of lack of a standardised method to evaluate PD-L1 expression. In this study, we developed a novel method for the evaluation of PD-L1 expression on NSCLC cells and examined its correlation with clinicopathological characteristics.

After immunohistochemical examination of PD-L1 expression for surgically resected pulmonary adenocarcinomas (n=106), based on the findings that PD-L1 are consistently expressed on alveolar macrophages, PD-L1 staining intensity of tumour cells was classified into four levels relative to PD-L1 staining intensity in alveolar macrophages; PD-L1 expression scores (range, 0–300) were semiquantitatively assessed. An analysis of statistical association between PD-L1 expression score and clinicopathological characteristics was performed.

Almost all of the alveolar macrophages in the specimens were moderately to strongly stained with PD-L1, serving as an internal positive control in the immunohistochemistry of PD-L1. PD-L1 expression score (median, 52.3) was significantly higher in tumours with G2/3 differentiation than in those with G1 (p=0.022) and higher in those with lymphatic invasion than in those without invasion (p=0.032). Postoperative relapse-free survival was significantly shorter in patients with a high PD-L1 expression score than in those with low PD-L1 expression score (p=0.035). Smoking habits, histological subtype, and epidermal growth factor receptor mutation status were not associated with PD-L1 expression score.

Given the heterogeneous distribution of PD-L1 expression in pulmonary adenocarcinoma cells, the scoring of PD-L1 expression on tumour cells relative to that in alveolar macrophages appears to be a valid indicator of PD-L1 status of patients with pulmonary adenocarcinomas, demonstrating a significant correlation with several factors associated with tumour progression.