ANTIBODY-DEPENDENT ENHANCEMENT OF RESPIRATORY SYNCYTIAL VIRUS-INFECTION BY SERA FROM YOUNG INFANTS

Respiratory syncytial virus (RSV) convalescent-phase sera and control sera from both infants (< 6 months) and older individuals (1.5 to 90 y ears) were assayed for RSV-specific antibodies by neutralization, in v itro enhancing activity, and immunoprecipitation. Enhancement of RSV i nfection in U937 cells was demonstrated with convalescent-phase sera a nd was shown to be dependent on Fc receptors by blocking with human im munoglobulin G (P < 0.01). Convalescent-phase sera from infants enhanc ed infection at concentrations closer to physiological ones (10(-1) to 10(-3) dilutions of serum), while convalescent-phase sera from older individuals enhanced infection only at much lower concentrations (10(- 3) to 10(-6) dilutions of serums; P < 0.01). To our knowledge, this is the first report of RSV-enhancing antibody activity in the sera of in fants. The observed enhancing activity and the low neutralizing antibo dy levels are confined mostly to convalescent-phase sera from infants aged 0 to 6 months, suggesting that these factors may contribute to th e increased severity of RSV disease frequently encountered in young in fants.