Intrinsic Photosensitivity Enhances Motility of T Lymphocytes

Sunlight has important biological effects in human skin. Ultraviolet (UV) light striking the epidermis catalyzes the synthesis of Vitamin D and triggers melanin production. Although a causative element in skin cancers, sunlight is also associated with positive health outcomes including reduced incidences of autoimmune diseases and cancers. The mechanisms, however, by which light affects immune function remain unclear. Here we describe direct photon sensing in human and mouse T lymphocytes, a cell-type highly abundant in skin. Blue light irradiation at low doses (<300 mJ cm^-2) triggers synthesis of hydrogen peroxide (H₂O₂) in T cells revealed by the genetically encoded reporter HyPerRed. In turn, H₂O₂ activates a Src kinase/phospholipase C-γ1 (PLC-γ1) signaling pathway and Ca²⁺ mobilization. Pharmacologic inhibition or genetic disruption of Lck kinase, PLC-γ1 or the T cell receptor complex inhibits light-evoked Ca²⁺ transients. Notably, both light and H₂O₂ enhance T-cell motility in a Lck-dependent manner. Thus, T lymphocytes possess intrinsic photosensitivity and this property may enhance their motility in skin.