CD69 and neoplasm: In addition, CD103+ tumor-infiltrating CD8+ T cells can self-regulate their CD103 expression by producing TGF-β.28 Furthermore, ultraviolet radiation-induced extracellular ATP release by keratinocytes activates skin resident T cells and upregulates CD69.29 Extracellular ATP also activates the purinergic receptor P2R×7, which is required for the generation and functionality of long-lived CD103+ TRMs in tissues.30