Background:  Hepatitis B Surface antigen (HBsAg) particles have been used as a very effective human vaccine to prevent hepatitis B infection. Recently HBsAg has been used to present neutralization epitopes by expressing them as fusion proteins either to the N-terminus for malarial anitgens or to the C-terminus for HIV-1 gp120 antigens. The gp41 membrane proximal region (MPR) of the HIV-1 envelope glycoprotein is recognized by the 2 broadly cross-reactive neutralizing antibodies 2F5 and 4E10.

Methods:  We have fused selected MPR variant sequences, in combination with partial and complete regions of the HIV-1 transmembrane (TM), to the C-terminus of HBsAg and generated HBsAg-MPR particles. Increasing length of the MPR had a negative effect on the particle production. The monoclonal antibody 2F5 bound to the HBsAg-MPR and HBsAg-MPR-5 residue TM particles with apparent nanomolar affinity. The binding of 4E10 to these particles could also be observed, although at a lower relative affinity. We could compete-out the binding of 2F5 antibody to HBsAg-MPR particle using a 16-mer peptide containing the complete 2F5 epitope. We characterized the ability of the particles to bind antibodies from selected human sera.

Results:  We observed that IgG from several HIV-1 sera possessing a broad neutralizing activity efficiently bound to the HBsAg particles with the MPR, but not to particles without the MPR. Selected non-neutralizing HIV human sera, HIV IgG and 1 broadly neutralizing HIV sera neutralizer did not display detectable binding to the HBsAg-MPR particles. The human sera that bound well to HBsAg-MPR particles could not be competed-out by the 16-mer MPR peptide, suggesting presence of other gp41-specific like antibodies in these human sera, perhaps against the 4E10 epitope. We also showed that neutralization ability of 2F5 could be partially adsorbed using the HBsAg-MPR particles.

Conclusions:  In addition to using this novel platform to detect gp41-directed binding antibodies, we are also evaluating the HBsAg-MPR particles as immunogens to elicit neutralizing antibodies specific for the HIV gp41 MPR.

Keywords: HIV vaccine; Membrane Proximal region; HbsAg