Genomic organization of the TCRα locus and essential roles of αβ T cells in antibacterial immunity in Nile tilapia.

T cells utilize diverse T cell receptors (TCRs) to recognize antigenic peptides and mediate adaptive immunity. However, the organization and function of αβ T cells in early vertebrates remain poorly understood. Here, we systematically characterized the TCRα locus and αβ T cell responses in Nile tilapia (Oreochromis niloticus) during Edwardsiella piscicida infection. Genomic analysis revealed that the TCRα locus in Nile tilapia exhibits a distinctive Vα(61)-Jα(82)-Cα(1)-Vα(65) structure with conserved synteny in the flanking regions, while the constant region retains essential residues for TCR/CD3 complex assembly. We further generated monoclonal antibody against tilapia TCRα and TCRβ and confirmed their coexpression by the majority of CD3ε+ T cells, validating these cells as bona fide αβ T cells. Functional assays showed that CD3ε/CD28 monoclonal antibody stimulation induced robust αβ T cell activation, as evidenced by enhanced phosphorylation of S6, NF-κB, and ERK1/2, together with vigorous cellular proliferation. In vivo, E. piscicida infection triggered a pronounced expansion of αβ T cells, indicating their active involvement in antibacterial immunity. Importantly, depletion of αβ T cells severely impaired pathogen clearance and significantly increased host mortality. Together, these findings elucidate the indispensable role of αβ T cells in the antibacterial immunity of teleosts and provide critical insights into the functional mechanisms of adaptive immunity in early vertebrates.