| Summary: | Background: BRAF-mutated colorectal cancers (BRAF-MT CRCs) are known to have poor prognoses. BRAF-MT CRC was reported to be possibly related to the immune-activated phenotype. Objectives: This study aimed to investigate the association between the immune microenvironment and prognosis of BRAF-MT CRC. Methods: We evaluated clinical outcomes and investigated the immune profile of the BRAF-MT CRC tumors using the multiplex immunohistochemistry of immune-related markers: cytokeratin, programmed death ligand-1 (PD-L1), programmed cell death protein-1 (PD-1), and a cluster of differentiation 8 (CD8). Results: Out of 2313 tumors, 123 were BRAF-MT tumors. Among them, 86 tumors with available tissue were included. Out of 86 patients, 75 patients were non-good responders (GR), whereas 11 patients were GR. Median progression-free survival after first-line chemotherapy (4.6 vs. 12.4 months, <i>p</i> = 0.008) and overall survival (11.8 vs. 45.0 months) were longer in the GR group (<i>p</i> < 0.001). Median CD8+ T cell (254.29 vs. 656.0, <i>p</i> = 0.092), PD-L1+ tumor cell (0.95 vs.15.56, <i>p</i> = 0.050), PD-L1+ stromal cell (3.17 vs. 72.38, <i>p</i> = 0.025), PD-L1+ tumor and stromal cell (5.08 vs. 74.92, <i>p</i> = 0.032), and PD-1+ stromal cell (45.08 vs. 325.40, <i>p</i> = 0.046) counts were greater in the GR group. Conclusion: The clinical outcomes of unselected patients with BRAF-MT CRC were generally similar to those in previous studies. Based on the immune profile analysis, higher PD-L1 expression and CD8-positive cell infiltration were observed in BRAF-MT tumors with a good prognosis.
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