[miR-199a-3p targets PD-L1 to regulate immune evasion in microsatellite stable colorectal cancer]

Objective Immune checkpoint inhibitors show limited efficacy in microsatellite-stable (MSS) colorectal cancer (CRC). This study aims to elucidate the molecular mechanisms underlying immune evasion in MSS CRC. Prior miRNA profiling of MSS CRC tissues from anti-programmed death 1 (PD-1) monoclonal antibody responders and non-responders identified miR-199a-3p as a candidate molecule. This study further validates its clinical relevance and investigates its mechanistic role. Methods Quantitative real-time PCR was performed to assess gene expression levels. Dual-luciferase reporter assays were conducted to evaluate the targeting relationship between miR-199a-3p and programmed death-ligand 1 (PD-L1). A co-culture system of MSS CRC cells and peripheral blood mononuclear cells (PBMC) was established to analyze the proportion of CD8⁺ T cell and cytokine secretion. Cell proliferation was assessed using Cell Counting Kit-8 (CCK-8) assays and colony formation assays. Cell migration was evaluated by wound healing assays, and apoptosis was measured using Annexin V-FITC/7-AAD staining. Results miR-199a-3p directly targeted PD-L1 and downregulated its expression in MSS SW620 cells. Overexpression of miR-199a-3p relieved the inhibition of CD8⁺ T cells, enhanced their cell cytotoxic activity and alleviated immunosuppression in the tumor microenvironment. Furthermore, miR-199a-3p overexpression inhibited cell proliferation and migration while promoting apoptosis in MSS SW620 cells. In contrast, inhibition of miR-199a-3p exerted opposite effects. Conclusion miR-199a-3p mediates immune evasion in MSS CRC by targeting PD-L1. It may serve as a potential noninvasive diagnostic biomarker and therapeutic target, providing a theoretical basis for promoting novel immunotherapeutic strategies.