Melittin enhances PD-L1 blockade in prostate cancer by inhibiting M2 macrophage polarization and recruitment

Objective: The immunosuppressive tumor microenvironment (TME) limits the efficacy of immunotherapy in prostate cancer treatment. This study aims to investigate how melittin remodels the TME in prostate cancer and enhances the response mechanism to anti-PD-L1 therapy. Methods: The anti-tumor efficacy of melittin was evaluated in a C57BL/6J mouse prostate cancer model. An in vitro co-culture model of prostate cancer cells (PC-3, 22RV1) and THP-1-derived macrophages was established. RNA sequencing, flow cytometry, qPCR, Western blot, ELISA, Transwell migration assays, and immunohistochemistry were employed to explore the anti-tumor mechanisms of melittin. The synergistic effect of combining melittin with an anti-PD-L1 antibody was assessed in vivo. Results: Melittin significantly inhibited tumor growth in vivo. It remodeled the TME by increasing CD8⁺ T cell infiltration and reducing macrophage numbers. Furthermore, melittin specifically inhibited M2 macrophage polarization, downregulating the expression of CD206, CD163, and TGF-β, an effect potentially mediated through inhibition of the JAK-STAT pathway. Additionally, melittin reduced macrophage recruitment by downregulating tumor-derived CCL2. Combination therapy using melittin and anti-PD-L1 antibody demonstrated synergistic anti-tumor effects, significantly reducing tumor volume and weight, decreasing intratumoral expression of Ki67 and PD-L1, and enhancing CD8⁺ T cell infiltration. Conclusion: Melittin reverses the immunosuppressive TME in prostate cancer by simultaneously inhibiting JAK-STAT-mediated M2 polarization and CCL2-dependent macrophage recruitment. This reprogramming of the TME, combined with PD-L1 blockade, offers a novel combination immunotherapy strategy for prostate cancer.