B-Cells Derived EVs Shape Melanoma Response to Immune Checkpoint Therapy

In recent years, the understanding of the immune tumor microenvironment has evolved to include B cells as active participants rather than mere bystanders. Retrospective studies investigating the presence of B cells and B cells-associated signatures within tumors have revealed a significant correlation with therapeutic outcomes. Although conflicting data exist regarding whether B cells act as pro-tumor or anti-tumor agents, it is becoming increasingly clear that understanding B cells as a heterogeneous population, shaped by tumor microenvironment cues, is essential. This study explores the crucial roles played by B cells and their extracellular vesicles (EVs) in shaping responses to immune checkpoint blockade (ICB) therapy. We have independently verified the significant enrichment of B cells in ICB therapy responders in comparison to non-responder in tumors of melanoma patients. B cell depletion experiments in an in vitro tumor-killing assay demonstrate a clear impairment of T cell-mediated anti-tumor activity. To investigate the clinical relevance, EVs were isolated from melanoma patient tumors, and fractioned into subpopulations, including B cell-derived EVs. MiRNA profiling of CD19+ EVs identifies miR-99a-5p as a top candidate, significantly upregulated in responder EVs. In downstream assays, we observed a B cell-dependent phenotype of miR-99a-5p, where its silencing impaired T cell-mediated anti-tumor cytotoxicity in an in vitro tumor-killing assay. Mechanistically, miR-99a-5p appears to regulate B cell cell-cycle, favoring non-homologous end joining DNA repair pathway, and promoting class-switch recombination. Collectively, data reported herein emphasizes the indispensable role of B cells and their derived EVs in shaping cancer immunotherapy outcome.