Immune Checkpoint Molecule Expression, 9p24.1 Gene Alterations, and Tumor Microenvironment of Primary Central Nervous System Lymphomas and Their Clinical Relevance.

Primary central nervous system large B-cell lymphoma (PCNS-LBCL) is a rare, aggressive lymphoma that affects immune-privileged sites. Immune checkpoint molecules have been implicated in its aggressive biology, and promising results have emerged from immune checkpoint inhibitor therapy in relapsed/refractory cases. This study evaluates the tumor microenvironment (TME), immune checkpoint molecule expression, and the relationship with 9p24.1 gene region alterations in a large cohort of PCNS-LBCL, with detailed quantitative analyses. Tissue microarrays were constructed with 57 PCNS-LBCL cases and 45 systemic non-germinal center diffuse large B-cell lymphoma (DLBCL) controls. Immunostaining for CD3, CD8, CD68, CD163, PD-1, PD-L1, PD-L2, EBER in situ hybridization (ISH), and FISH for PD-L1/PD-L2 copy number alterations and translocations were performed. Digital image analysis was used for quantitative evaluations, which were compared with clinical and pathologic parameters. PCNS-LBCL showed significantly lower T-cell and histiocyte presence in the TME compared with nodal DLBCL ( P <0.001), independent of preoperative steroid therapy. Cytotoxic T-cell ratio was higher in PCNS-LBCL ( P <0.001). PD-1, PD-L1, and PD-L2 expressions in the TME of PCNS-LBCL were 89%, 96%, and 90%, respectively, and were positively correlated with TME density. Tumor cell expressions of PD-L1 and PD-L2 were 31% and 34%, respectively. FISH alterations in the 9p24.1 region were infrequent and did not consistently correlate with protein expression in either PCNS-LBCL or DLBCL. Higher CD8+ T-cell and CD68+ histiocyte counts were associated with better survival in PCNS-LBCL. Lower TME density and high expression of PD-1/PD-L1/PD-L2 in PCNS-LBCL reflect the unique CNS microanatomy and may contribute to poorer prognosis. These findings support the potential benefit of immune checkpoint inhibitors in treating PCNS-LBCL, aligning with ongoing clinical trials and current literature.