Correlation of PD-L1 expression with tertiary lymphoid structure maturity and immune cell spatial distribution in colorectal cancer.

Abstract

e15582 Background: PD-L1 expression is a key biomarker for predicting immune checkpoint inhibitor (ICI) efficacy but shows significant interpatient heterogeneity in colorectal cancer (CRC), suggesting the complex tumor immune microenvironment (TIME) may affect its predictive value. Mature tertiary lymphoid structure (TLS) formation is associated with favorable prognosis and enhanced ICI response. However, the relationship between PD-L1 expression, TLS maturity, and spatial localization of different immune cell subsets remains unclear. Methods: We included 474 CRC patients whose TIME was profiled using multiplex immunohistochemistry (mIHC) and an AI-based image analysis system (Timepro platform). Spearman's rank correlation analyzed PD-L1 metrics [tumor proportion score (TPS) and combined positive score (CPS)] with the density/proportion of TLSs and immune cells in tumor parenchyma and stroma. P-values were corrected for multiple testing. Results: PD-L1 expression showed no significant correlation with TLS density or proportion, suggesting TLS maturity may be independent of PD-L1 and serve as a complementary biomarker for ICI response, particularly in PD-L1-low/negative patients. PD-L1 TPS was weakly negatively correlated with stromal CD8+ T-cell proportion ( ρ = -0.188, p < 0.01), density/proportion of CD68+CD163+ M2 macrophages in both parenchyma and stroma ( ρ range: -0.179 to -0.201, p < 0.01), and density/proportion of CD68+CD163- M1 macrophages in parenchyma ( ρ = -0.196, -0.200, p < 0.01). A weak negative correlation was also observed with stromal FoxP3+ T-cell proportion ( ρ = -0.174, p < 0.05). In contrast, PD-L1 CPS was weakly positively correlated with density/proportion of CD8+ T cells ( ρ range: 0.174-0.192, p < 0.05), M2 macrophages (ρ range: 0.210-0.251, p < 0.001) in both parenchyma and stroma, M1 macrophage ( ρ = 0.228, 0.223, p < 0.001) in parenchyma, and FoxP3+ T-cell proportion ( ρ = 0.172, p < 0.05) in stroma. These findings indicate that PD-L1 CPS positively correlates with broad immune cell infiltration in CRC, potentially better identifying patients with an immunologically active yet PD-L1-mediated adaptive resistance phenotype. Tumor-associated macrophages (TAMs) may be key regulators of PD-L1 expression in the CRC TIME. The strong correlation between PD-L1 and TAMs supports future exploration of combined TAM-targeted and PD-L1 inhibitor therapies to reverse immunosuppression and enhance efficacy. Conclusions: TLS is a PD-L1-independent biomarker in CRC. The opposing correlations of PD-L1 TPS and CPS with immune infiltration underscore the importance of scoring methodology. PD-L1 CPS reflects an immune-active, adaptive-resistant microenvironment. Combined assessment of TLS, PD-L1 CPS, and spatial immune architecture may improve immunotherapy stratification.