Bacterial infection triggers CD3ε+ T cell expansion and interaction with macrophages in grass carp (Ctenopharyngodon idella).

While T cells are well established as key players in mammalian adaptive immunity, their functional roles in teleosts remain poorly understood. Here, a monoclonal antibody against grass carp (Ctenopharyngodon idella) CD3ε was generated, and characterized by Western blotting, flow cytometry, and immunofluorescence. In healthy fish, CD3ε+ T cells accounted for 5.1 %, 4.0 %, and 2.6 % of splenic, head kidney, and peripheral blood lymphocytes, respectively. Upon Aeromonas hydrophila infection, CD3ε+ T cell populations elevated significantly in immune tissues such as spleen, head kidney, and posterior intestine, accompanied by upregulated transcription of T cell markers (Cd3ε, Tcr-β, and Lck). Strikingly, immunofluorescence analysis revealed that CD3ε+ T cells and MCSFR+ macrophages were located in the same region in the spleen tissue of A. hydrophila-infected fish, suggesting functional crosstalk between T cells and macrophages. Our findings provide the first evidence of T cell-macrophage interaction in teleost antibacterial defense, offering insights into the adaptive immune responses in fish.