Photodynamic therapy with zinc phthalocyanine suppresses immune checkpoints in human melanoma cells

Melanoma remains a major public health concern due to rising incidence and frequent resistance to immune checkpoint inhibitors. Photodynamic therapy (PDT) has emerged as a promising complementary strategy that exerts direct cytotoxic effects while simultaneously reshaping the tumor microenvironment to enhance immune recognition. This study aimed to examine the effects of PDT on immune checkpoint expression and immunogenic cell death markers in melanoma cells. We performed in silico analysis using TCGA-SKCM and GTEx databases via GEPIA platform to identify dysregulated immune checkpoint molecules in melanoma. We then evaluated zinc phthalocyanine-based PDT (ZnPc-PDT) effects on cell viability and immune checkpoint expression in three human melanoma cell lines (A375, A2058, SK-MEL-3). Cytotoxicity was assessed by MTT assay, while qRT-PCR measured changes in PD-L1, CD47, B7-H3, galectin-9, and HLA-G, HMGB1, and Hsp70 expression post-treatment. In silico analysis identified PD-L1, CD47, B7-H3, galectin-9, and HLA-G as critical immune evasion regulators. ZnPc-PDT significantly reduced these checkpoints in A375 and A2058 cells (p < 0.0001 for PD-L1 and CD47; p < 0.01–0.001 for others). SK-MEL-3 showed limited response, with significant downregulation only for CD47 and HLA-G (p < 0.01). Importantly, ZnPc-PDT induced substantial upregulation of immunogenic cell death markers HMGB1 and Hsp70 across all cell lines, with the most pronounced increases observed in A2058 cells. ZnPc-PDT demonstrated potent phototoxic effects across all lines, with IC₅₀ values of 0.121, 0.109, and 0.032 µg/ml for A375, A2058, and SK-MEL-3, respectively. ZnPc-PDT acts as a multi-dimensional therapeutic strategy by simultaneously inducing tumor cell death and modulating immune checkpoint expression. Pronounced effects on PD-L1 and CD47 highlight potential synergy with current immunotherapies by converting melanoma cells into endogenous vaccines that can stimulate anti-tumor immune responses. Although in vivo validation is needed, our results support ZnPc-PDT as a complementary approach for treating immune-resistant melanomas and pave the way for future combination therapies.