Although a previous study using ginbuna crucian carp suggested that cell-mediated immunity can be induced by the oral administration of inactivated viruses, which are exogenous antigens, there is no direct evidence that CD8 + cytotoxic T cells (CTLs) in teleost fish are generated by vaccination with exogenous antigens. In the present study, we investigated whether antigen-specific CD8 + CTLs in ginbuna crucian carp can be elicited by intestinal immunization with an exogenous antigen without any adjuvant. The IFNγ-1 and T-bet mRNA expressions were up-regulated in intestinal leukocytes following the administration of formalin-inactivated crucian hematopoietic necrosis virus (FI-CHNV), whereas the down-regulation of these genes was observed in kidney leukocytes. Furthermore, an increase in the percentage of proliferating CD8 + cells was detected in the posterior portion of the hindgut, suggesting that the virus-specific CTLs are locally generated in this site. In addition, cell-mediated cytotoxicity against CHNV-infected syngeneic cells and the in vivo inhibition of viral replication were induced by immunization with FI-CHNV. Unexpectedly, intraperitoneal immunization with FI-CHNV induced a type I helper T cell (Th1)-response in the intestine, but not in the kidney; however, its effect was slightly lower than that reported after intestinal immunization. These findings suggest that the posterior portion of the intestine is an important site for generating virus-specific CTLs by vaccination with the inactivated vaccine.
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