Manganese improves anti-PD-L1 immunotherapy via eliciting type I interferon signaling in melanoma

The immune checkpoint inhibitor therapy represented by blocking programmed cell death protein 1/ programmed cell death-ligand 1 (PD-1/PD-L1) has made significant progress in melanoma treatment. However, the response rate and therapeutic effect of immunotherapy alone are still not ideal for melanoma. In this study, we aimed to evaluate the defects of treating anti-PD-L1 alone and the therapeutic effect and molecular mechanism of combined therapy with anti-PD-L1 and MnCl₂. We detected the changes of immune cell populations after anti-PD-L1 treatment in melanoma xenograft mouse model. Further, we evaluated the regulatory effect of MnCl₂ on dendritic cells (DCs) maturation in vitro. Next, we tested the therapeutic effect and regulatory effect on the tumor microenvironment with anti-PD-L1 and MnCl₂ via combining treatment with anti-PD-L1 and MnCl₂. Anti-PD-L1 therapy has a certain tumor suppressive function, but the effect is not ideal. The results of flow cytometry showed that the number of CD4⁺ T cells and CD8⁺ T cells significantly increased after anti-PD-L1 treatment. However, the number of DCs remained basically unchanged after anti-PD-L1 treatment. In vitro, we confirmed that MnCl₂ significantly promoted DCs maturation vis activating cGAS-STING signaling pathway. The combination of anti-PD-L1 and MnCl₂ displayed the best tumor suppression effect in melanoma xenograft mouse model. In tumor microenvironment, the infiltration of T cells and the maturation of DCs were significantly promoted, demonstrating a strong anti-tumor immune response. In summary, we conclude that combining anti-PD-L1 with MnCl₂ is a promising therapeutic strategy for melanoma.