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The degree of antigenic relatedness between human respiratory syncytial virus (RSV) subgroups A and B was estimated from antibody responses induced in cotton rats by respiratory tract infection with RSV. Glycoprotein-specific enzyme-linked immunosorbent assays of antibody responses induced by RSV infection demonstrated that the F glycoproteins of subgroups A and B were antigenically closely related (relatedness, R approximately 50%), whereas the G glycoproteins were only distantly related (R approximately 5%). Intermediate levels of antigenic relatedness (R approximately 25%) were seen in neutralizing antibodies from cotton rats infected with RSV of the two subgroups. Immunity against the F glycoprotein of subgroup A, induced by vaccinia-A2-F, conferred a high level of protection which was of comparable magnitude against challenge by RSV of either subgroup. In comparison, immunity against the G glycoprotein of subgroup A, induced by vaccinia-A2-G, conferred less complete, but significant, protection. Importantly, in vaccinia-A2-G-immunized animals, suppression of homologous challenge virus replication was significantly greater (13-fold) than that observed for the heterologous virus.

Cotton rats previously inoculated with Formalin-inactivated respiratory syncytial virus (RSV) were challenged intranasally with live RSV to induce an enhancement of RSV disease similar to that observed after the administration of Formalin-inactivated RSV vaccine to human infants 20 years ago. Within 24 h after infection with RSV, cotton rats developed pulmonary lesions that reached a maximum by day 4. Histologically, the lesions resembled an experimental pulmonary Arthus reaction. An action of Formalin on RSV appears to be responsible for this effect, because live virus or virus heated in the absence of Formalin did not induce enhanced immunopathology. Selected epitopes on the fusion (F) or attachment (G) or both RSV surface glycoproteins that are involved in inducing neutralizing antibodies were modified to reduce or ablate their antigenicity. However, other epitopes on the F or G or both glycoproteins were not ablated by Formalin, because cotton rats inoculated parenterally with a Formalin-inactivated virus developed a high level of F and G antibodies measurable by an enzyme-linked immunosorbent assay. At this time, the effect of Formalin on RSV cannot be localized to either the F or G glycoprotein of RSV.

Serum samples from 31 children who experienced two or three infections with respiratory syncytial virus (RSV) in the first four years of life were tested in an enzyme-linked immunosorbent assay to examine the immunoglobulin G (IgG) subclass responses to the RSV F and G surface glycoproteins associated with primary infection and reinfection. We sought to determine whether the greater degree of glycosylation of the G glycoprotein was reflected in an IgG subclass immune response more like that to a polysaccharide antigen than to a protein antigen. We found that the IgG1/IgG2 ratio of postinfection antibody titers to F was fourfold higher than that to the G glycoprotein after RSV infections 1, 2, and 3. The IgG2 response to the heavily glycosylated G glycoprotein differed from that to a polysaccharide antigen in that the IgG1/IgG2 ratio remained constant with age, whereas the response to a polysaccharide antigen decreased as the IgG2 response increased with age. We also noted that antibody responses to both surface glycoproteins in the IgG1 and IgG2 subclasses reached their maximum levels after RSV infection 2.

The serum antibody response of infants and children immunized with Formalin-inactivated respiratory syncytial virus (RSV) vaccine 20 years ago was determined by using an enzyme-linked immunosorbent assay specific for the RSV fusion (F) and large (G) glycoproteins and a neutralization assay. Twenty-one young infants (2 to 6 months of age) developed a high titer of antibodies to the F glycoprotein but had a poor response to the G glycoprotein. Fifteen older individuals (7 to 40 months of age) developed titers of F and G antibodies comparable to those in children who were infected with RSV. However, both immunized infants and children developed a lower level of neutralizing antibodies than did individuals of comparable age with natural RSV infections. Thus, the treatment of RSV with Formalin appears to have altered the epitopes of the F or G glycoproteins or both that stimulate neutralizing antibodies, with the result that the immune response consisted largely of "nonfunctional" (i.e., nonneutralizing) antibodies. Subsequent natural infection of the vaccinees with wild-type RSV resulted in enhanced pulmonary disease. Despite this potentiation of illness, the infected vaccinees developed relatively poor G, F, and neutralizing antibody responses. Any or all of three factors may have contributed to the enhancement of disease in the RSV-infected vaccinees. First, nonfunctional antibodies induced by the inactivated RSV vaccine may have participated in a pulmonary Arthus reaction during RSV infection. Second, the poor antibody response of infants to the G glycoprotein present in the Formalin-inactivated vaccine may have been inadequate to provide effective resistance to subsequent wild-type virus infection. Third, the relatively reduced neutralizing antibody response of the infant vaccinees to wild-type RSV infection may have contributed to their enhanced disease by delaying the clearance of virus from their lungs.

An enzyme-linked immunosorbent assay (ELISA) with immunoaffinity-purified fusion (F) or attachment (G) glycoprotein was used to measure the serum and secretory immune responses of 18 infants and children, 4 to 21 months of age, who underwent primary infection with respiratory syncytial virus (RSV). Most of the 10 older individuals (9 to 21 months of age) developed moderate levels of serum and nasal-wash immunoglobin A (IgA) and IgG F and G antibodies. These individuals developed a moderate level of serum or nasal-wash antibodies that neutralized virus infectivity. One of the eight younger individuals (4 to 8 months of age) failed to develop an F antibody response, while three failed to develop a G antibody response. The most notable difference in the responses of the two age groups involved the titer in convalescent sera of G, F, and neutralizing antibodies which were 8- to 10-fold lower in younger individuals. Most of the younger infants failed to develop a rise in serum or nasal-wash neutralizing antibody. It is possible that the presence of maternally derived antibody in the younger infants suppressed the immune response to RSV infection, and that this accounted, in part, for the low level of postinfection antibody titer in this group. This low level and the irregular response of the infants less than 8 months of age may contribute to the severity of their initial infection and may also be responsible, in part, for their failure to develop effective resistance to subsequent reinfection by RSV.

Six monoclonal antibodies directed against respiratory syncytial virus proteins were produced. Each was characterized by immunoprecipitation and indirect immunofluorescence. One was directed against the nucleocapsid protein. NP 44, two were directed against a 37,000-dalton protein, two were directed against the major envelope glycoprotein, GP 90, and one was directed against the 70,000-dalton envelope protein, VP 70. Indirect immunofluorescence stain patterns of infected HEp-2 cells defined GP 90 and VP 70 as viral proteins expressed on the cell surface, whereas NP 44 and the 37,000-dalton protein were detected as intracytoplasmic inclusions. One of the anti-GP 90 antibodies neutralized virus only in the presence of complement but did not inhibit cell-cell fusion. The anti-VP 70 antibody neutralized virus without complement and inhibited cell-cell fusion of previously infected HEp-2 cells, thus identifying VP 70 as the fusion protein.

The serum antibody response of 50 infants and children infected with respiratory syncytial virus (RSV) was determined by a glycoprotein-specific enzyme-linked immunosorbent assay, and the effects of age and preexisting antibody titer at the time of RSV infection on response to the G and F glycoproteins of RSV were examined. The immune response to the G and F glycoproteins was assessed with anti-human immunoglobulin A to permit measurement of the response of young infants in the presence of maternally derived immunoglobulin G. The findings suggested that age primarily affects the response to the F glycoprotein and that preexisting antibody titer affects the response to the G glycoprotein.