Dissecting the neutralizing antibody specificities of broadly neutralizing sera from human immunodeficiency virus type 1-infected donors

Attempts to elicit broadly neutralizing antibody responses by human immunodeficiency virus type 1 (HIV-1) vaccine antigens have been met with limited success. To better understand the requirements for cross-neutralization of HIV-1, we have characterized the neutralizing antibody specificities present in the sera of three asymptomatic individuals exhibiting broad neutralization. Two individuals were infected with clade B viruses and the third with a clade A virus. The broadly neutralizing activity could be exclusively assigned to the protein A-reactive immunoglobulin G (IgG) fraction of all three donor sera. Neutralization inhibition assays performed with a panel of linear peptides corresponding to the third hypervariable (V3) loop of gp120 failed to inhibit serum neutralization of a panel of HIV-1 viruses. The sera also failed to neutralize chimeric simian immunodeficiency virus (SIV) and HIV-2 viruses displaying highly conserved gp41-neutralizing epitopes, suggesting that antibodies directed against these epitopes likely do not account for the broad neutralizing activity observed. Polyclonal IgG was fractionated on recombinant monomeric clade B gp120, and the neutralization capacities of the gp120-depleted samples were compared to that of the original polyclonal IgG. We found that the gp120-binding antibody population mediated neutralization of some isolates, but not all. Overall, the data suggest that broad neutralization results from more than one specificity in the sera but that the number of these specificities is likely small. The most likely epitope recognized by the monomeric gp120 binding neutralizing fraction is the CD4 binding site, although other epitopes, such as the glycan shield, cannot be excluded.

Production of a Short Recombinant C4V3 HIV-1 Immunogen That Induces Strong Anti-HIV Responses by Systemic and Mucosal Routes Without the Need of Adjuvants

Synthetic peptides have been shown to evoke neutralizing and cytotoxic protective anti-HIV responses in mice and other animal models. Recent data support that C4V3 peptides can induce anti- V3 antibodies that neutralize primary isolates. Critical to the success of peptide-based vaccines is the development of strategies to augment their immunogenicity while reducing their large-scale production costs. Therefore, finding efficient and economical alternatives for the production of epitopic vaccines could have an impact on researches using such immunogens. Herein, we report the recombinant production and immunological characterization of a short polypeptide which carries the three relevant epitopes contained in a C4V3 peptide. This polypeptide, named rC4V3, was efficiently produced in E. coli, yielding more than 75 mg per culture liter. No major difficulties were found in the recovery, refolding and purification of this peptide; the latter facilitated by C-terminal inclusion of a histidine tag. The immunogenicity of this protein was studied by administering it intramuscularly or intranasally to mice and it demonstrated to be a strong elicitor of anti-HIV antibodies at systemic and mucosal compartments. Remarkably, such responses were attained with rC4V3 even without the need of adjuvants. We can conclude that this protein might be a promising tool for studies using epitope-based vaccine designs.