A structure-based approach to a synthetic vaccine for HIV-1

The generation of neutralizing antibodies by peptide immunization is depend ent on achieving conformational compatibility between antibodies and native protein. Consequently, approaches are needed for developing conformational mimics of protein neutralization sites. We replace putative main-chain hyd rogen bonds (NH --> O=CRNH) with a hydrazone link (N-N=CH-CH2CH2) and scan constrained peptides for fit with neutralizing monoclonal antibodies (MAbs) . To explore this approach, a V3 MAb 58.2 that potently neutralizes T-cell lab-adapted HIV-1(MN) was used to identify a cyclic peptide, [JHIGPGR-(Aib) F(D-Ala)GZ]G-NH2 (loop 5), that binds with > 1000-fold higher affinity than the unconstrained peptide. NMR structural studies suggested that loop 5 st abilized p-turns at GPGR and R(Aib)F(D-Ala) in aqueous solvent implying con siderable conformational mimicry of a Fab 58.2 bound V3 peptide determined by X-ray crystallography [Stanfield, R. L. et al. (1999) Structure 142, 131 -142]. Rabbit polyclonal antibodies (PAbs) generated to loop 5 but not to t he corresponding uncyclized peptide bound the HIV-1(MN) envelope glycoprote in, gp120. When individual rabbit antisera were scanned with linear and cyc lic peptides, further animal-to-animal differences in antibody populations were characterized. Loop 5 PAbs that most closely mimicked MAb 58.2 neutral ized HIV-1(MN) with similar potency. These results demonstrate the remarkab le effect that conformation can have on peptide affinity and immunogenicity and identify an approach that can be used to achieve these results. The im plications for synthetic vaccine and HIV-1 vaccine research are discussed.