Characterization of CD8+ T Cells in large yellow croaker Larimichthys crocea and their dynamics in response to Cryptocaryon irritans infection.

CD8+ T cells are critical cytotoxic effectors involved in pathogen clearance, including defense against parasitic infections in aquaculture species. In this study, we developed a monoclonal antibody specifically targeting the CD8α chain, enabling the identification of CD8+ T cells in large yellow croaker Larimichthys crocea and providing a valuable tool for studying cytotoxic T cell responses in teleosts. Croaker CD8+ T cells were more abundant in mucosal than systemic lymphoid organs, suggesting their preferential localization to sites of pathogen entry. Using this tool, we investigated CD8+ T cell responses to Cryptocaryon irritans infection. During acute infection, CD8+ T cells significantly proliferated in the spleen, head kidney (HK), and gills, accompanied by upregulation of cytotoxic genes including granzyme a, granzyme b, and perforin 1. Notably, a previously unrecognized depletion of CD8+ T cells was observed in the spleen and HK, despite the total numbers of lymphocytes remaining unchanged. Our findings demonstrate an indispensable role of CD8+ T cells as cytotoxic effectors involved in the defense against parasitic infection, and suggest their potential as valuable immunological biomarkers for disease monitoring or as targets for vaccine-based interventions in aquaculture.