Munc13-4 mediates tumor immune evasion by regulating the sorting and secretion of PD-L1 via exosomes

Tumor-derived extracellular vesicles primarily carry PD-L1 via exosomes, which interact with PD-1 receptors on T cells, impacting immune responses in the tumor microenvironment and beyond, leading to a more extensive immunosuppressive landscape. However, the mechanisms governing exosomal PD-L1 sorting and secretion remain elusive. In this study, we identified Munc13-4 as a crucial regulator of exosomal PD-L1 sorting and secretion. Deletion of Munc13-4 in breast tumors enhances T cell-mediated anti-tumor immunity, suppresses tumor growth, and improves the efficacy of immune checkpoint inhibitors. Our results illustrate how Munc13-4 collaborates with HRS, Rab27, and SNAREs to facilitate PD-L1 sorting and secretion via exosomes. The cryo-EM structure of the Munc13-4-Rab27a complex provide new insights into its role in exosome secretion. Importantly, we discovered that Munc13-4 has a novel role in sorting PD-L1 onto exosomes, which relies on the formation of a ternary complex with PD-L1 and HRS. In addition, IFN{gamma} stimulation modifies Munc13-4 and HRS, establishing a dynamic regulatory mechanism that enables tumor cells to adapt to immune pressure by modulating PD-L1 sorting. Using a specially designed peptide to disrupt the Munc13-4-PD-L1 interaction and impede PD-L1 sorting significantly enhances anti-tumor immunity and slows tumor growth in vivo. These results highlight the potential of targeting the Munc13-4-PD-L1 axis to suppress tumor immune evasion.