Variation in populations of aquatic birnaviruses was investigated by propagating several virus strains in cell cultures in the presence of neutralizing monoclonal antibodies (MAbs) to inhibit replication of the parental antigenic type in the virus population and to isolate MAb neutralizationresistant viruses representing one or more subpopulations of viruses. Selected MAb neutralization-resistant viruses were plaque purified and characterized in regard to antigenic and biological properties. In addition to the MAb used for selection, some variants failed to react in immunoassays with one or more other MAbs that reacted with the parental virus type. Other viruses, although completely resistant to neutralization, reacted with the selecting MAb. Several viruses selected for resistance to neutralization with a given MAb reacted in immunobinding and neutralization assays with one or more MAbs that did not react with the parental virus type. Also, several of the selected viruses replicated to higher titers and produced largersized plaques in susceptible cell cultures. Three MAb-resistant viruses were also found to be in vitro host range variants; two were no longer capable of infecting fat head minnow (FHM) cell cultures, whereas one, in contrast to the parental virus population, replicated to high titers in FHM cells. Comparison of the molecular weights of virion proteins of the MAb-selected viruses and the parental virus type revealed little or no alterations of virion proteins VP1 and VP3. However, differences in size were demonstrated between VP2 proteins of all MAb-selected viruses. In some cases, VP2 and VP2 precursor proteins of the MAb-resistant viruses were larger whereas in other cases they were smaller in size.
Infectious pancreatic necrosis virus, Antigenic, Neutralization-resistant variants
