Discrepancies in PD-L1 expression, lymphocyte infiltration, and tumor mutational burden in non-small cell lung cancer and matched brain metastases.

Background: Differences in the immune microenvironment and responses to immunotherapy may exist between primary non-small cell lung cancer (NSCLC) and brain metastases (BMs). This study aimed to investigate discrepancies in programmed death-ligand 1 (PD-L1) expression, tumor-infiltrating lymphocytes (TILs), tertiary lymphoid structures (TLS), and tumor mutational burden (TMB) between matched BMs and primary tumors (PTs) in NSCLC.

Methods: Twenty-six pairs of surgically resected BMs and corresponding PTs from NSCLC patients were collected. PD-L1 expression and TILs, including CD8, CD3, CD4, CD20, CD68, and CD21, were analyzed using immunohistochemistry (IHC) and quantitatively assessed through digital image analysis. Whole-exome sequencing (WES) was performed to investigate genomic discrepancies and variations in TMB.

Results: The density of PD-L1⁺ cells did not differ significantly between matched PTs and BMs (P>0.99). However, BMs exhibited a higher tumor proportion score (TPS) compared to PTs (mean TPS: 31.92% vs. 25.96%, P=0.049), with moderate agreement in categorized TPS (κ=0.653). Analysis of TILs revealed a significant reduction in CD3⁺ T cells (P<0.001), CD8⁺ cytotoxic T cells (P<0.001), CD20⁺ B cells (P<0.001), and CD68⁺ macrophages (P=0.02) in BMs compared to PTs. BMs also exhibited a loss of TLS, with no presence of mature TLS marked by CD21 expression. The number of non-synonymous mutations was generally higher in BMs than in PTs, with only 34.69% of mutations shared between paired PTs and BMs. TMB was slightly increased in BMs (mean TMB: 34.2 mutations/Mb in BMs vs. 26.8 mutations/Mb in PTs; P=0.30). Additionally, the log-rank test indicated that a higher density of CD20⁺ B cells in BMs was significantly associated with better overall survival (P=0.007).

Conclusions: Compared to primary NSCLC tumors, matched BMs show an increase in TPS of PD-L1 expression and TMB, but a significant reduction in TILs and loss of mature TLS, suggesting an immune-suppressive microenvironment in BMs. The infiltration of CD20⁺ B cells may serve as a potential prognostic biomarker in NSCLC with BMs.