We report the in vivo role of specific cytotoxic cells in protecting fish from acute viral infections. Specifically, we found that (1) there is an inverse relationship between cytotoxic activities and viral load and (2) adoptive transfer of immune leukocytes prevented viral infection. Crucian carp hematopoietic necrosis virus (CHNV), which has a virulence to ginbuna crucian carp, was recently found and identified in Japan. Specific cell-mediated cytotoxicity of ginbuna leukocytes against CHNV-infected syngeneic cells was induced as a result of intraperitoneal inoculation with CHNV. This cytotoxicity was not induced against either virus-infected allogeneic cells or eel rhabidovirus- (EVA) infected syngeneic cells. In these respects, the cytotoxic activity was similar to that of mammalian cytotoxic T-lymphocyte (CTL) activity. Viral titers of tissues from infected fish were remarkably reduced 8 days after infections, when specific cytotoxic activity reached a peak. This result suggested that specific cytotoxic cells were responsible for the early control of CHNV replication. On the other hand, CHNV-specific antibody was greatly increased when the virus was eliminated by cytotoxic activities. The effectiveness of the virus-specific cytotoxicity was evaluated using adoptive cell transfer. Recipients that received leukocytes from immune syngeneic donors escaped CHNV infection. These findings suggest that virus-specific cytotoxic cells have a role in controlling viral infections in a fish.
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