Purpose: Knowledge regarding programmed death-ligand 1 (PD-L1) expression in lung cancer is limited. We aim to clarify PD-L1-positive expression in non-small-cell lung cancer (NSCLC), including adenocarcinoma subtypes.

Methods: In all, 90 NSCLC specimens containing various adenocarcinoma subtypes, in addition to squamous cell carcinoma and large-cell carcinoma were selected. PD-L1 was immunohistochemically stained by murine monoclonal antibody clone 22C3.

Results: When PD-L1-positive expression was defined by tumor proportion score (TPS) ≥1%, the positive cases were 0/11 in adenocarcinoma in situ, 0/12 in minimally invasive adenocarcinoma, 1/10 in lepidic predominant adenocarcinoma, 1/13 in papillary predominant adenocarcinoma, 8/14 in acinar predominant adenocarcinoma, 6/11 in solid predominant adenocarcinoma, 0/3 in micropapillary predominant adenocarcinoma, 0/4 in invasive mucinous adenocarcinoma, 4/9 in squamous cell carcinoma, and 2/3 in large-cell carcinoma. PD-L1 positivity was higher in males, smokers, advanced pathologic stages, positive vessel invasion, and positive lymphatic invasion. Postoperative survival analysis revealed that PD-L1-positive expression was a significantly worse prognostic factor in univariate analysis for recurrence-free survival (RFS).

Conclusion: PD-L1-positive tumors were frequent in acinar predominant adenocarcinoma and solid predominant adenocarcinoma than other adenocarcinoma subtypes. PD-L1 expression seemed to increase according to pathologic tumor progression, suggesting a worse postoperative prognosis in NSCLC patients.