Influenza viruses of the H1N1 subtype have been continually circulating in pigs in the U.S.A. for at least 50 years. To examine the level of antigenic variation in these swine viruses, a panel of 60 monoclonal antibodies (MAbs) to the haemagglutinin (HA) of recent swine isolates was prepared. Evaluation of neutralization escape mutants selected with these MAbs defined four antigenic sites on the HA, two of which overlap. Swine viruses isolated over 24 years in an enzootic area in Wisconsin were examined by ELISA and haemagglutination inhibition (HI) with these MAbs and the results indicated that the antigenic sites defined by these MAbs were highly conserved in these viruses. In comparing recent H1N1 viruses from pigs, turkeys, ducks and humans, changes in the antigenic sites were detected on the basis of HI reactivity. However, results of ELISA with these viruses clearly showed that the antigenic sites were still present on almost all H1N1 viruses of swine origin; thus, altered reactivity of these viruses in HI tests with MAbs was not a reflection of changes in the antigenic sites defined by the MAbs. It seems likely that the variation detected in these viruses occurs by a mechanism other than immune selection.
Animals Antibodies, Monoclonal