Induction of virus-specific cell-mediated cytotoxic responses of isogeneic ginbuna crucian carp, after oral immunization with inactivated virus

Clonal ginbuna crucian carp (Carassius auratus langsdorfii) were immunized orally with inactivated crucian carp hematopoietic necrosis virus (CHNV) and the cytotoxic activities of peripheral blood leukocytes against CHNV-infected syngeneic target cells were investigated. Although a single oral administration of the vaccine did not prime measurable cytotoxic responses to CHNV-infected targets, detectable lytic activities were observed after a booster oral administration. The vaccine-induced cytotoxic cells were poorly responded against both eel virus from America (EVA)-infected syngeneic cells and CHNV-infected allogeneic target cells, suggesting that the responses were viral antigen-specific and restricted to the major histocompatibility complex (MHC). Oral immunization with the vaccine also induced neutralizing antibody responses. Orally immunized fish were able to rapidly eliminate viruses. Although elevated cytotoxic activities and antibody responses were observed in orally immunized fish following viral infection, the rapid elimination of virus appeared to be associated with elevated cytotoxic responses. These results show that orally administered inactivated viruses can evoke antiviral cellular immune responses in fish.