In vitro neutralization of viral hemorrhagic septicemia virus by plasma from immunized zebrafish

We studied humoral long-term adaptive viral neutralization responses in zebrafish (Danio rerio), an increasingly useful vertebrate model for viral diseases actually limited by the absence of standardized anti-zebrafish immunoglobulin M (IgM) antibodies. We established an alternative method, similar to those used in other fish, to achieve a first estimation of zebrafish anti-viral antibody-like responses. We used the viral hemorrhagic septicemia virus (VHSV) model because, although protection after this non-natural infection was demonstrated in cold-acclimatized zebrafish, little is known about their induced anti-VHSV antibody-like responses. Therefore, we first optimized a micro-neutralization method based on immunostaining VHSV-infected fish cell monolayers to detect zebrafish neutralizing activity in plasma samples in one day. We then used the method to measure the specific anti-VHSV neutralization in plasma obtained from individual zebrafish under various VHSV challenges or immunization protocols. The neutralizing activity was inhibited by protein A-sepharose and rabbit anti-zebrafish IgM antibodies, suggesting the implication of IgM zebrafish antibodies in such responses. To our knowledge, this is the first report to demonstrate detectable and significant VHSV neutralization titers in zebrafish surviving VHSV infections. This micro-method might be useful, not only for the follow-up of infection/vaccine development in the zebrafish/VHSV model in particular, but also for similar work involving other in vitro neutralizable zebrafish pathogens. This technique might also further the development of alternative ELISA assay methods to measure specific immunoglobulins in zebrafish.