Ca²⁺-calcineurin axis-controlled NFAT nuclear translocation is crucial for optimal T cell immunity in an early vertebrate

Calcium ion (Ca²⁺) is a widespread and primitive second messenger that regulates physiological cell functions in almost all life beings. Ca²⁺ influx-induced NFAT activation is essential for T cell function and adaptive immunity. However, whether and how Ca²⁺ signaling modulates T cell immunity in early vertebrates, especially in nontetrapods, remains largely unknown. To address these questions, a Nile tilapia (Oreochromis niloticus) model was employed to investigate the regulation of ancestral T cell immunity by Ca²⁺-NFAT signaling in jawed fish. In Nile tilapia, an evolutionarily conserved Ca²⁺-NFAT signaling pathway is involved in the primary adaptive immune response during Streptococcus agalactiae infection. Meanwhile, T cell signals trigger several events along the Ca²⁺-NFAT axis in this early vertebrate, including Ca²⁺ influx, calcineurin activation, and NFAT nuclear import. More critically, suppression of Ca²⁺-NFAT signaling by the calcineurin inhibitor cyclosporine A impairs primordial T cell activation, clonal expansion, and infection clearance. Mechanistically, Nile tilapia NFAT interacts with several other transcription factors for potent gene expression, and T cells in this nontetrapod employ Cabin1 and DYRK1A to regulate NFAT nuclear import and export, respectively. To the best of our knowledge, this study is the first to demonstrate the regulatory mechanism of Ca²⁺-NFAT signaling on T cell immunity in a nontetrapod species. We suggest that modulation of T cell immunity by Ca²⁺-NFAT signaling is a primitive strategy that already existed prior to the divergence of bony fish from the tetrapod lineage. The findings of this study provide valuable perspectives for understanding the evolution of adaptive immune system.