Antibodies to infectious salmon anaemia virus (ISAV) augment virus entry in macrophage-like fish cell lines

Infectious salmon anaemia virus (ISAV), a member of the family Orthomyxoviridae, is an important fish pathogen in marine aquaculture in the Northern Hemisphere. The pathogenesis of ISAV infection at the cellular level is not well studied. We investigated three permissive cell lines for ISAV, (SHK-1, TO and CHSE-214) as indicator systems for virus neutralization (VN) with anti-ISAV polyclonal antibodies of rabbit or fish (Atlantic salmon) origin. It was shown that the rabbit antiserum showed higher VN antibody titres in the CHSE-214 cell line than in macrophage-like fish cell lines SHK-1 and TO. With the fish antiserum, VN was observed only on CHSE-214 cells. The input virus used in the CHSE-214 cell system (i.e., 100TCID50) was found to be 1000 and 2000 times more than that used in the SHK-1 and TO cell systems, respectively, indicating that the true VN antibody titres of the antisera used were higher than the titres obtained on CHSE-214 cells. Neutralization of infectious pancreatic necrosis virus (IPNV) by rabbit anti-IPNV polyclonal serum in both CHSE-214 and TO cells demonstrated that effective virus neutralization is possible in TO cells. The VN antibody titre of rabbit anti-ISAV serum was increased 48-fold in TO cells when the assay was performed in presence of staphylococcal Protein A, which binds to the Fc moiety of immunoglobulins. Antibody-facilitated entry of ISAV in TO cells was confirmed using fluoresceine isothiocyanate (FITC)-labelled ISAV and rabbit anti-ISAV serum. These findings indicate that antibodies to ISAV augment the virus infection of macrophage-like fish cells. In conclusion, this study has demonstrated, for the first time, possible Fc receptor-mediated infection of macrophage-like fish cell lines by a fish virus, ISAV..