Full T-cell activation and function in teleosts require collaboration of first and co-stimulatory signals.

Mammalian T-cell responses require synergism between the first signal and co-stimulatory signal. However, whether and how dual signaling regulates the T-cell response in early vertebrates remains unknown. In the present study, we discovered that the Nile tilapia ( ) encodes key components of the LAT signalosome, namely, LAT, ITK, GRB2, VAV1, SLP-76, GADS, and PLC-γ1. These components are evolutionarily conserved, and CD3ε mAb-induced T-cell activation markedly increased their expression. Additionally, at least ITK, GRB2, and VAV1 were found to interact with LAT for signalosome formation. Downstream of the first signal, the NF-κB, MAPK/ERK, and PI3K-AKT pathways were activated upon CD3ε mAb stimulation. Furthermore, treatment of lymphocytes with CD28 mAbs triggered the AKT-mTORC1 pathway downstream of the co-stimulatory signal. Combined CD3ε and CD28 mAb stimulation enhanced ERK1/2 and S6 phosphorylation and elevated NFAT1, c-Fos, IL-2, CD122, and CD44 expression, thereby signifying T-cell activation. Moreover, rather than relying on the first or co-stimulatory signal alone, both signals were required for T-cell proliferation. Full T-cell activation was accompanied by marked apoptosis and cytotoxic responses. These findings suggest that tilapia relies on dual signaling to maintain an optimal T-cell response, providing a novel perspective for understanding the evolution of the adaptive immune system.