BACKGROUND:HIV-1 infection of target cells is mediated via the binding of the viral envelope protein, gp120, to the cell surface receptor CD4. This interaction leads to conformational rearrangements in gp120 forming or revealing CD4 induced (CD4i) epitopes which are critical for the subsequent recognition of the co-receptor required for viral entry. The CD4-bound state of gp120 has been considered a potential immunogen for HIV-1 vaccine development. Here we report on an alternative means to induce gp120 into the CD4i conformation.RESULTS:Combinatorial phage display peptide libraries were screened against HIV-1 gp120 and short (14aa) peptides were selected that bind the viral envelope and allosterically induce the CD4i conformation. The lead peptide was subsequently systematically optimized for higher affinity as well as more efficient inductive activity. The peptide:gp120 complex was scrutinized with a panel of neutralizing anti-gp120 monoclonal antibodies and CD4 itself, illustrating that peptide binding does not interfere with or obscure the CD4 binding site.CONCLUSIONS:Two surfaces of gp120 are considered targets for the development of cross neutralizing antibodies against HIV-1; the CD4 binding site and CD4i epitopes. By implementing novel peptides that allosterically induce the CD4i epitopes we have generated a viral envelope that presents both of these surfaces simultaneously.

Binding of CD4 locks gp120 into a unique conformation, enabling chemokine-receptor recognition necessary for viral entry. Stabilized gp120 in the CD4 induced conformation (CD4i) has been generated via a single chain CD4-gp120 recombinant molecule which has been demonstrated to be a preferred vaccine modality in macaques (DeVico et al 2007 PNAS 104:17477). These results indicate that CD4-gp120 complex reveals epitopes able to elicit neutralizing antibodies; however, this probably is at the expense of occlusion of the CD4 binding-site (BS) epitopes on gp120 - which are also regarded as highly desirable vaccine targets. We have isolated peptides that bind gp120 and in doing so induce the CD4i conformation - as demonstrated by binding of defining mAbs such as 17b, 19e and CG10. Moreover, the peptide-gp120 complex continues to bind CD4, illustrating that this site remains accessible. Furthermore, the peptide-gp120 complex efficiently binds a panel of CD4-BS defining mAbs, such as b12, b6 and M14 thus illustrating that the CD4i conformation is compatible with simultaneous presentation of the CD4-BS epitopes. We therefore propose that the peptide-gp120 complex represents a novel vaccine candidate that benefits from “both worlds”; a CD4i stabilized conformation that maintains accessible neutralizing epitopes associated with the CD4-BS of gp120.

Meeting abstracts. A s ingle PDF containing all abstracts in this Supplement is available here http://ww w.biomedcentral. com/content/pdf/ 1742-4690-3-S1-i nfo.pdf