Avidity

Abstract: The characterization of the cross-reactive, or heterologous, neutralizing antibody responses developed during human immunodeficiency virus type 1 (HIV-1) infection and the identification of factors associated with their generation are relevant to the development of an HIV vaccine. We report that in healthy HIV-positive, antiretroviral-naive subjects, the breadth of plasma heterologous neutralizing antibody responses correlates with the time since infection, plasma viremia levels, and the binding avidity of anti-Env antibodies. Anti-CD4-binding site antibodies are responsible for the exceptionally broad cross-neutralizing antibody responses recorded only in rare plasma samples. However, in most cases examined, antibodies to the variable regions and to the CD4-binding site of Env modestly contributed in defining the overall breadth of these responses. Plasmas with broad cross-neutralizing antibody responses were identified that targeted the gp120 subunit, but their precise epitopes mapped outside the variable regions and the CD4-binding site. Finally, although several plasmas were identified with cross-neutralizing antibody responses that were not directed against gp120, only one plasma with a moderate breadth of heterologous neutralizing antibody responses contained cross-reactive neutralizing antibodies against the 4E10 epitope, which is within the gp41 transmembrane subunit. Overall, our study indicates that more than one pathway leads to the development of broad cross-reactive neutralizing antibodies during HIV infection and that the virus continuously escapes their action.

Abstract: Two monoclonal antibodies to mouse Ia antigens were produced by fusion of xenoimmune rat spleen cells with the NSI myeloma. These monoclonal antibodies detect polymorphic determinants present on B cells and activated T lymphocytes from mice carrying the H-2b, H-2d, H-2k, H-2r, and H-2q haplotypes but not from mice carrying the H-2s or H-2r haplotypes. Antigenic site number determinations showed the positive haplotypes can be divided into 2 groups. Mice bearing the H-2b, H-2d, and H-2q haplotypes express a high number--40,000 to 80,000--of antigenic sites per B lymphocyte, and monoclonal antibody plus complement can lyse B cells from these mice. In contrast, mice bearing the H-2k and H-2r haplotypes express a low number of antigenic sites--about 5000 per cell. Spleen cells from mice carrying the latter haplotypes are not lysed with monoclonal antibody and complement. Genetic mapping demonstrated that high and low expression map to the I-A and I-E subregions, respectively. The monoclonal antibodies detect an Ia specificity on I-Ab, I-Ad, I-Ed, and I-Ek molecules. These observations were confirmed using several different experimental approaches, i.e., cytotoxicity, fluorescent staining, competitive inhibition of monoclonal antibody binding, and 2-dimensional gel electrophoresis of immunoprecipitates. The avidity for A alpha b A beta b and E alpha k E beta k is 5 to 7 x 10(-9) M-1. The antigenic determinant is heat labile, which suggests that it is not carbohydrate. The results imply that Ia antigens encoded by distinct subregions share sequence homology, which may be a consequence of ancestral gene duplication.

Abstract: Rapid antigenic evolution in the influenza A virus hemagglutinin precludes effective vaccination with existing vaccines. To understand this phenomenon, we passaged virus in mice immunized with influenza vaccine. Neutralizing antibodies selected mutants with single-amino acid hemagglutinin substitutions that increased virus binding to cell surface glycan receptors. Passaging these high-avidity binding mutants in naive mice, but not immune mice, selected for additional hemagglutinin substitutions that decreased cellular receptor binding avidity. Analyzing a panel of monoclonal antibody hemagglutinin escape mutants revealed a positive correlation between receptor binding avidity and escape from polyclonal antibodies. We propose that in response to variation in neutralizing antibody pressure between individuals, influenza A virus evolves by adjusting receptor binding avidity via amino acid substitutions throughout the hemagglutinin globular domain, many of which simultaneously alter antigenicity.