Gastric cancer cells-derived exosomal miR-151a-5p induces an immunosuppressive microenvironment through promoting LAG3+TAMs infiltration

The incidence and mortality of gastric cancer (GC) are still in the forefront worldwide. Up to now, the population benefiting from immunotherapy is still very limited. Tumor associated macrophages (TAM) play an important role in immune response and immune microenvironment remodeling. This study aimed to clarify the altered TAM subsets in GC microenvironment and elucidate the internal molecular mechanism. Sample collection and Cytometry by Time-of-Flight (CyTOF) were used to identify TAM subpopulations with differential infiltration in the GC microenvironment, followed by multiplex immunofluorescence and flow cytometry to validate the immunosuppressive function. Exosomes were isolated and characterized by transmission electron microscopy, nanoparticle tracking analysis and western blot for subsequent high-throughput RNA sequencing. The key regulatory role of LAG3+ TAMs in the progression of gastric cancer was explored by Chromatin Immunoprecipitation, dual luciferase reporter, cytokine array and subcutaneous xenograft models. The infiltration of LAG3+ TAMs was elevated in GC and correlated with poorer prognosis. LAG3+ TAMs significantly suppressed the effector function and cytotoxic activity of CD8+ T cells. Mechanistically, GC cells-derived exosomes delivered miR-151a-5p to macrophages, inducing LAG3 expression via DUSP8/MAPK-dependent activation of ELK1 transcription. Reciprocally, LAG3+ TAMs released CXCL8, which bound CXCR2 on GC cells, amplifying exosomal miR-151a-5p secretion and reinforcing the immunosuppressive loop. LAG3⁺ TAMs are expanded in GC through a tumor-macrophage feedback circuit driven by exosomal miR-151a-5p, resulting in CD8⁺ T-cell dysfunction and immune resistance. Targeting this axis may enhance GC immunotherapy efficacy.