Quantitative aspects of passive immunity to respiratory syncytial virus infection in infant cotton rats

Abstract

The amount of passively acquired serum respiratory syncytial virus (RSV)-neutralizing antibodies required to protect the respiratory tract of cotton rats against infection was studied. Infant cotton rats were inoculated i.p. with various dilutions of a single pool of sera derived from cotton rats convalescent from RSV infection. After 24 h, these animals were inoculated with RSV intranasally. Virus replication in the respiratory tract was suppressed in cotton rats which had a serum neutralizing antibody titer of .gtoreq. 1:100. Resistance was greater in the lungs than in the nose. Complete or almost complete resistance in the lungs was observed in cotton rats with a serum neutralizing antibody titer of .gtoreq. 1:380. The level of serum RSV-neutralizing antibodies required to confer significant resistance to infection in the cotton rat was similar to the level of maternally derived serum antibodies possessed by human infants < 2 mo. of age, who as a group exhibit relative resistance to RSV disease compared with infants 2-6 mo. of age.