Background: The prevalence of programmed death-ligand 1 (PD-L1) and PD-L2 expression on tumor cells and tumor-infiltrating immune cells in primary central nervous system lymphoma (PCNSL) remains unclear. In the present study, we analyzed needle biopsy and craniotomy specimens of patients with PCNSL to compare the PD-L1 and PD-L2 levels in the tumor and surrounding (peritumoral) tissue. We also assessed the correlation between biological factors and the prognostic significance of PD-L1 and PD-L2 expression.
Methods: We retrospectively analyzed the cases of 70 patients histologically diagnosed with PCNSL (diffuse large B-cell lymphoma). Immunohistochemistry for CD20, CD68, PD-L1, and PD-L2 was performed. In cases with specimens taken by craniotomy, the percentages of PD-L1- and PD-L2-positive macrophages were evaluated in both tumor and peritumoral tissue. The Kaplan-Meier method with log-rank test and Cox proportional hazard model were used for survival analysis.
Results: The tumor cells expressed little or no PD-L1 and PD-L2, but macrophages expressed PD-L1 and PD-L2 in most of the patients. The median percentage of PD-L2-positive cells was significantly higher among peritumoral macrophages (32.5%; 95% CI: 0-94.6) than intratumoral macrophages (27.5%; 95% CI: 0-81.1, p = 0.0014). There was a significant correlation between the percentages of PD-L2-positive intratumoral macrophages and PD-L2-positive peritumoral macrophages (p = 0.0429), with very low coefficient correlation (ρ = 0.098535). PD-L1 expression on macrophages was significantly associated with biological factors (intratumoral macrophages: better KPS, p = 0.0008; better MSKCC score, p = 0.0103; peritumoral macrophages: low proportion of LDH elevation, p = 0.0064) and longer OS (for intratumoral macrophages: high PD-L1 = 60 months, 95% CI = 30-132.6; low PD-L1 = 24 months, 95% CI = 11-48; p = 0.032; for peritumoral macrophages: high PD-L1 = 60 months, 95% CI = 30.7-NR; low PD-L1 = 14 months, 95% CI = 3-26). PD-L1 expression on peritumoral macrophages was strongly predictive of a favorable outcome (HR = 0.30, 95% CI = 0.12-0.77, p = 0.0129).
Conclusions: Macrophages in intratumoral and peritumoral tissue expressed PD-L1 and PD-L2 at a higher rate than tumor cells. PD-L1 expression, especially on peritumoral macrophages, seems to be an important prognostic factor in PCNSL. Future comprehensive analysis of checkpoint molecules in the tumor microenvironment, including the peritumoral tissue, is warranted.
Keywords: Macrophage; PD-L1; PD-L2; Primary central nervous system lymphoma; Tumor microenvironment.