Background: Programmed death-ligand 1 (PD-L1) expression on lung cancer cells is a prognostic marker and a predictive biomarker for response to immunotherapy. However, previous clinical trials have suggested that other programmed cell death 1 ligands, including programmed death-ligand 2 (PD-L2), might have clinical impacts. This study aimed to analyze the prognostic significance of PD-L2 expression in lung adenocarcinoma patients. Methods: The study included 433 patients who underwent surgical resection for lung adenocarcinoma between 2003 and 2012 at Kyushu University Hospital. Both PD-L1 and PD-L2 expression were evaluated by immunohistochemistry. The cutoff value for PD-L2 positivity was set at 1% according to a time-dependent receiver operating characteristic curve for 5-year survival. Results: Of the 433 patients, 306 (70.7%) were positive for PD-L2. No significant association between PD-L1 and PD-L2 expression was observed (P = 0.094). The multivariate analysis showed that the independent predictors of PD-L2 positivity were never-smoker status (P = 0.002), poor differentiation grade (P = 0.008), and advanced stage (P = 0.048). The PD-L2-positive patients had significantly shorter disease-free survival (DFS) (P = 0.018) and overall survival (OS) (P = 0.016). Both PD-L1 and PD-L2 positivity were independent predictors of OS (P < 0.001 and P = 0.027, respectively). In the subgroup analysis of the PD-L1-negative patients, PD-L2 positivity was significantly associated with shorter DFS (P = 0.018). Conclusions: The study demonstrated that the clinical characteristics of patients with PD-L1 expression may differ from those of patients with PD-L2 expression, and that both might contribute to poor prognoses. The potential role of PD-L2 expression as a predictive biomarker for response to immunotherapy should be investigated in future studies.
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