Evaluation of the antibody specificities of human convalescent-phase sera against the attachment (G) protein of human respiratory syncytial virus: influence of strain variation and carbohydrate side chains

The C-terminal third of the attachment protein (G) of several human respiratory syncytial virus isolates was obtained as either a glycosylated protease-resistant fragment of the purified protein or a nonglycosylated GST fusion protein expressed in bacteria. The reactivity of human convalescent-phase sera with both forms of the protein segment was evaluated in immunoblots. While all serum samples reacted with the mature intact protein of the different isolates, only certain samples reacted with the nonglycosylated C-terminal segment of some viral isolates. The number of human serum samples reacting with the glycosylated C-terminal fragment was even more limited. These results highlight the heterogeneity of the human antibody response against epitopes located in the C-terminal hypervariable region of the G molecule and the influence of carbohydrate side chains for expression of these epitopes. We also have analysed the specificities of human sera by competitive enzyme-linked immunosorbent assay with murine monoclonal antibodies (MAbs). Most human serum samples inhibited virus binding of MAbs that recognised conserved or group-specific epitopes of the G protein, while only a limited fraction of those samples inhibited binding of MAbs that recognised strain-specific epitopes. These results are discussed in terms of the antibody repertoire induced after human respiratory syncytial virus infection and the relevance of escape mechanisms to preexisting antibodies for the evolution of this virus.