Serum IgA of HIV-exposed uninfected individuals inhibit HIV through recognition of a region within the α-helix of gp41

Background: HIV-specific IgA is present in HIV-exposed uninfected individuals (EU) and neutralizes primary strains of HIV-1 in vitro. Objectives: To analyse the antigenic correlates of HIV-1 neutralization using HIV epitopes and IgA from EU and HIV-seropositive individuals. Methods: Sera from six heterosexual couples discordant for HIV serostatus, six agematched HIV-infected subjects and six healthy controls (HC; as negative controls) were analysed. IgA binding on HIV Env recombinant proteins was assayed. Serum IgA was affinity purified on specific Env peptides and tested in HIV neutralization using resting and activated peripheral blood mononuclear cells as target. Monoclonal antibody 2F5 was used as neutralizing positive control. BALB/c mice were immunized with specific gp41 peptide and anti-sera were tested in syncytia formation and in HIV viral replication. Results: IgA of EU exclusively bound an epitope within gp41; this epitope was restricted to residues 582-588 (QARILAV) and corresponded to the leucine zip motif in the α-helical region. IgA of HIV-positive patients recognized epitopes expressed both in gp120 and gp41; these epitopes were in the N-terminal portion of the extramembrane region. Additionally, IgA of EU and antisera of QARILAV-immunized Balb/C mice blocked syncytia formation and viral replication. The dose-dependent neutralization behaviour of specific QARILAV-purified IgA was very similar to that obtained with monoclonal antibody 2F5. Conclusion: These results have important implications for the development of vaccines and therapeutical strategies against HIV infection.