Delineation of four antigenic sites on a paramyxovirus glycoprotein via which monoclonal antibodies mediate distinct antiviral activities.

Six hybridoma antibodies specific for the hemagglutinin-neuraminidase (HN) molecule of the parainfluenza type 1 virus strain 6/94 were used to demonstrate the existence of four distinct antigenic sites on the HN molecule. Three of the sites (A, B-C, D) are topologically nonoverlapping, because antibodies to these sites bind noncompetitively to the HN molecule. Two sites (B, C) are operationally nonoverlapping, because mutations in site B do not detectably modify the antigenic site C. Although antibodies to each site had similar potencies (activity per microgram of antibody) in hemagglutination inhibition tests, antibodies to sites A and C or D differed approximately 100-fold in their potency to neutralize virus. Also, the antibody to site A strongly inhibited viral neuraminidase activity, whereas antibodies to sites C and D (and to a lesser extent to site B) enhanced the neuraminidase activity. Lastly, only antibodies to sites C and D formed precipitates in Ouchterlony double diffusion against detergent-disrupted virus. Because all six anti-HN antibodies are of IgG isotype and exhibited similar avidity for HN, these findings suggest that the ability of anti-HN antibodies to interact with the viral protein and to alter viral functions is largely dependent on their fine specificity.