Antibody-dependent enhancement, a possible mechanism in augmented pulmonary disease of respiratory syncytial virus in the Bonnet monkey model.

Bonnet monkeys develop an enhanced disease after immunization with the formalin-inactivated (FI) respiratory syncytial virus (RSV) vaccine that is characterized by increased viral replication in perivascular sites of the lung. These sites contain many mononuclear cells, which are known to be permissive for RSV replication. To test the hypothesis that FI-RSV vaccine stimulates the production of enhancing antibodies that serve to increase the replication of RSV in macrophages, in vitro studies were done. Antibody-dependent enhancement was observed in animals immunized with FI-RSV but not in control animals with primary and tertiary infections or those immunized with FI-Vero cell culture. In the presence of serum samples from animals immunized with FI-RSV, an increased number of U937 cells was infected. The enhancement index correlated positively with the pathologic scores of the FI-RSV-vaccinated monkeys.