Abstract
BACKGROUND: The CD274 (PD-L1)/PDCD1 (PD-1) immune checkpoint interaction may promote cancer progression, but the expression patterns and prognostic significance of PD-L1 and PD-1 in the colorectal cancer microenvironment are inadequately characterised. METHODS: We used a custom 9-plex immunohistochemistry assay to quantify the expression patterns of PD-L1 and PD-1 in macrophages, T cells, and tumour cells in 910 colorectal cancer patients. We evaluated cancer-specific mortality according to immune cell subset densities using multivariable Cox regression models. RESULTS: Compared to PD-L1(-) macrophages, PD-L1(+) macrophages were more likely M1-polarised than M2-polarised and located closer to tumour cells. PD-L1(+) macrophage density in the invasive margin associated with longer cancer-specific survival [P(trend) = 0.0004, HR for the highest vs. lowest quartile, 0.52; 95% CI: 0.34-0.78]. T cell densities associated with longer cancer-specific survival regardless of PD-1 expression (P(trend) < 0.005 for both PD-1(+) and PD-1(-) subsets). Higher densities of PD-1(+) T cell/PD-L1(+) macrophage clusters associated with longer cancer-specific survival (P(trend) < 0.005). CONCLUSIONS: PD-L1(+) macrophages show distinct polarisation profiles (more M1-like), spatial features (greater co-localisation with tumour cells and PD-1(+) T cells), and associations with favourable clinical outcome. Our comprehensive multimarker assessment could enhance the understanding of immune checkpoints in the tumour microenvironment and promote the development of improved immunotherapies.