Upregulation of PD-L1 via HMGB1-Activated IRF3 and NF-kB Contributes to UV Radiation-Induced Immune Suppression

Solar ultraviolet radiation (UVR) suppresses skin immunity, which facilitates initiation of skin lesions and establishment of tumors by promoting immune evasion. It is unclear whether immune checkpoints are involved in the modulation of skin immunity by UVR. Here, we report that UVR exposure significantly increased expression of immune checkpoint molecule PD-L1 in melanoma cells. The damage-associated molecular patterns molecule HMGB1 was secreted by melanocytes and keratinocytes upon UVR, which subsequently activated the receptor for advanced glycation endproducts (RAGE) receptor to promote NF-kB-and IRF3-dependent transcription of PDL1 in melanocytes. UVR exposure significantly reduced the susceptibility of melanoma cells to CD8 thorn T-cell-dependent cytotoxicity, which was mitigated by inhibiting the HMGB1/ TBK1/IRF3/NF-kB cascade or by blocking the PD-1/PD-L1 checkpoint. Taken together, our findings demonstrate that UVR-induced upregulation of PD-L1 contributes to immune suppression in the skin microenvironment, which may promote immune evasion of oncogenic cells and drive melanoma initiation and progression. Significance: These findings identify PD-L1 as a critical component of UV-induced immune suppression in the skin, which facilitates immunoevasion of oncogenic melanocytes and development of melanoma.