Identification of VP3 as an important neutralising epitope from DRT strain, a Korean isolate of infectious pancreatic necrosis virus (IPNV)

Infectious pancreatic necrosis virus (IPNV) has long been known to have at least four distinct viral proteins, with the smallest one being a broken piece of VP3. We have previously reported a Korean isolate of IPNV (Parket al., 1989), referred to as DRT, with serotype similar to VR-299. DRT has the typical pattern of viral proteins of IPNV; VP1 (96 kDa), VP2 (53 kDa), VP3 (32 kDa), and VP4 (30 kDa). On the basis that VP2 is responsible for the determination of different serotypes of IPNV and all different serotypes so far known share the same host specificity, it was assumed that there must be an important molecule(s) other than VP2 for the infection of host cells. We developed antisera either against whole viral particles or against each of the purified viral proteins, and tested if any antiserum had a neutralising activity during the infection. Immunisation with whole viral particles induced neutralising activity on day 5 after injection, detected by virus neutralisation assay and ELISA, and the neutralising activity increased abruptly thereafter. It was found from this antiserum that VP2 was the first epitope against which antibody was developed. Antibodies against VP3 /VP4 were detected much later. While the activity against VP2 remained constant for at least a month, the amount of antibodies specific to VP3 /VP4 increased in proportion to the titre of virus neutralising activity. Antiserum against VP2 of DRT, raised in mice, showed extremely low levels of neutralising effect. On the other hand, the mouse antiserum raised against VP3 /VP4 had a high level of virus neutralising activity. Thus, it appears that VP3 /VP4 are major immunogens of IPNV-DRT and that these viral polypeptides contain neutralising epitopes.