Antigenic variation in the hemagglutinin-neuraminidase protein of human parainfluenza type 3 virus.

Sixteen monoclonal antibodies directed to the hemagglutinin-neuraminidase (HN) protein of a 1957 isolate of parainfluenza type 3 virus (PIV3) were produced and used to examine antigenic variation in clinical strains. Analysis of hemagglutination-inhibition reactivity patterns of antigenic variants selected in vitro in the presence of monoclonal antibodies indicated that there were a minimum of six distinct epitopes detectable on the HN molecule. Competitive-binding assays indicated that these epitopes were located in two topologically nonoverlapping antigenic sites. An additional four epitopes were detected when 37 PIV3 clinical strains isolated over a period of 26 years in three geographic regions were tested for reactivity with the antibodies. Of the 10 unique epitopes defined by our monoclonal antibodies, 5 did not undergo detectable antigenic variation in any of the 37 strains examined. These results were expected since PIV3 viruses have been characterized as being antigenically monotypic. In contrast, antigenic variation was detected in the remaining five epitopes. This variation was not characterized by the accumulation of antigenic alterations with time (as for influenza A viruses), but appeared to represent genetic heterogeneity within the PIV3 population.