Edwardsiella tarda is an intracellular pathogen that causes edwardsiellosis in fish. Although cell-mediated immunity and innate immunity play a major role in protection against intracellular bacterial infection in mammals, their importance in protecting fish against E. tarda infection remain unclear. In this study, we examined cell-mediated and humoral immune responses in ginbuna crucian carp (Carassius auratus langsdorfii) after E. tarda infection. Innate immunity was observed to be the principal immune system for eliminating the majority of E. tarda, while a proportion of the bacteria might be resistant to its bactericidal activity. Bacterial clearance in kidney and spleen was also observed following higher cytotoxic activities of cytotoxic T lymphocytes (CTLs) and increased numbers of CD8α(+) cells, suggesting that CTLs might contribute to the elimination of E. tarda-infected cells with specific cytotoxicity. On the other hand, E. tarda-specific antibody titers did not increase until after bacterial clearance, indicating that induction of humoral immunity would be too late to provide protection against infection. Overall, these data suggest that both cell-mediated immunity and innate immunity may play important roles in the protection against intracellular bacterial infection, as they do in mammals. Our study would also contribute toward the understanding of immune responses that provide protection against other intracellular pathogens.
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