TCR signaling via NFATc1 constrains IL-15-induced NK-like activation of human memory CD8+ T cells

Here we investigated the regulatory mechanisms of TCR-independent bystander activation and NK-like cytotoxicity of human CCR7- memory CD8+ T cells. We found that TCR signals suppressed characteristic features of IL-15-induced CD8+ T-cell activation, including upregulated NKG2D expression and genes related to NK cytotoxicity and IFN response. Moreover, ionomycin suppressed IL-15-induced bystander activation and NK-like cytotoxicity, indicating that Ca2+-calcineurin signaling is responsible for TCR-mediated suppression of IL-15-induced bystander activation. NFATc1 suppressed IL-15-induced bystander activation via binding to AP-1 that is necessary for the IL-15-induced upregulation of NK cytotoxicity-related genes. Consistently, calcineurin inhibitors enhanced IL-15-induced NKG2D expression under concurrent TCR stimulation. Additionally, we defined genes upregulated by IL-15 and downregulated by concurrent TCR signals as an IL-15-induced bystander activation gene set, which was upregulated in bystander CD8+ T cells from patients with hepatitis A virus infection. Our findings open avenues for investigating bystander CD8+ T-cell activation and its regulation in pathological conditions.