Deficient synthesis of class-switched, HIVneutralizing antibodies to the CD4 binding site and correction by electrophilic gp120 immunogen

Objective: HIV is vulnerable to antibodies that recognize a linear CD4 binding site epitope of gp120 (C^LIN), but inducing C^LIN-directed antibody synthesis by traditional vaccine principles is difficult. We wished to understand the basis for deficient CLIN- directed antibody synthesis and validate correction of the deficiency by an electrophilic gp120 analog (E-gp120) immunogen that binds B-cell receptors covalently. Methods: Serum antibody responses to a C^LIN peptide and full-length gp120 epitopes induced by HIV infection in humans and immunization of mice with gp120 or E-gp120 were monitored. HIV neutralization by monoclonal and variable domain-swapped antibodies was determined from tissue culture and humanized mouse infection assays. Results: We describe deficient C^LIN-directed IgG but not IgMantibodies in HIV-infected patients and mice immunized with gp120 accompanied by robust synthesis of IgGs to the immunodominant gp120 epitopes. Immunization with the E-gp120 corrected the deficient C^LIN-directed IgG synthesis without overall increased immunogenicity of the C^LIN or other gp120 epitopes. E-gp120-induced monoclonal IgGs neutralized diverse HIV strains heterologous to the immunogen. A C^LIN-directed IgG neutralized HIV more potently compared to its larger IgM counterpart containing the same variable domains, suggesting obstructed access to HIV surface-expressed C^LIN. An E-gp120-induced IgG suppressed HIV infection in humanized mice, validating the tissue culture neutralizing activity. Conclusion: A C^LIN-selective physiological defect of IgM→IgG class-switch recombination (CSR) or restricted post-CSR B-cell development limits the functional utility of the humoral immune response to gp120. The E-gp120 immunogen is useful to bypass the restriction and induce broadly neutralizing C^LIN-directed IgGs (see Supplemental Video.