PAK4 exacerbates melanoma immunosuppression by promoting exosome release.

Melanoma remains a lethal malignancy with frequent resistance to immune checkpoint blockade therapy. Clinical evidence associates elevated PAK4 expression with poor response to anti-PD-1 treatment in melanoma patients. While tumor-derived exosomes are recognized as key immunosuppressive mediators, the role of PAK4 in regulating exosome secretion to induce immunosuppression remains incompletely understood. Here, we demonstrate that PAK4-mediated phosphorylation of Annexin-A2 (ANXA2) at Ser26 promotes its nuclear-to-cytoplasmic translocation, facilitating the formation of S100A10-ANXA2 heterotetramers that drive multivesicular body docking to the plasma membrane. Furthermore, PAK4 stabilizes synaptotagmin-1 by suppressing Trim21-mediated ubiquitin degradation, enabling SNARE complex assembly for membrane fusion. PAK4-driven exosomes exhibit potent immunosuppressive activity, substantially reducing CD8 + T cell numbers and impairing their function in mouse models. Notably, melanoma cells with high PAK4 expression secrete PD-L1-enriched exosomes that promote systemic immune evasion. In patient-derived xenograft models, pharmacological inhibition of PAK4 attenuates tumor progression and reduces circulating exosomal PD-L1 levels. Our study unveils a dual mechanism through which PAK4 drives exosome-dependent immunosuppression, providing a rationale for combining PAK4 pathway inhibitors with immune checkpoint blockade and offering translational strategies to overcome melanoma immunotherapy resistance.