Search

A successful prophylactic human immunodeficiency virus type 1 (HIV-1) vaccine must elicit an immune response that will prevent establishment of the persistent viral infection. The only response shown to be effective in this regard is virus-neutralizing antibody directed against the viral gp120 hypervariable V3-loop region. Conjugate immunogens, containing cyclic peptides representing the V3 determinant covalently bound to a carrier protein, were capable of eliciting virus-neutralizing antibodies. The consistency of the response was related to peptide size. The smaller cyclic peptides, expressing relatively conserved sequences from the V3-loop apex, were poor inducers of neutralizing activity. In contrast, the largest cyclic peptides mediated neutralizing responses that were similar to those observed and previously reported for intact gp120 immunogens. A cyclic synthetic peptide expressing most of the prototypic HIV-1 MN variant V3 determinant warrants further study as a potentially effective vaccine immunogen.

Branched undecapeptides with sequences related to the virus glycoprotein V3 domain sequences of the MN and IIIB variants of HIV-1 were synthesized and cyclized with a peptide (amide) closure to cyclic decapeptides. Two-dimensional NMR studies allowed protons for the MN variant-related cycle (L-697,250) to be assigned. Molecular modelling with distance geometry methods permitted a conformation to be identified which showed good agreement with ROESY and 2D NMR study data. A molecular dynamics simulation showed that the highly conserved loop tip sequence (Gly-Pro-Gly-Arg) was in a conventional beta-turn less than 50% of the time. For evaluation of immunogenicity and antibody characterization studies, covalent carrier conjugates were prepared. 3-Maleimidopropionylation of the Nle amino group of the cyclic peptides gave an electrophilic tether which captured a thiol group from a thiolated carrier protein, OMPC (outer membrane protein complex of Neisseria meningitidis). Through the use of a novel co-conjugation procedure, soluble immunogen-carrier molecules were prepared which had suitable physical properties for use as a vaccine. These V3-loop-based vaccines could elicit neutralizing antibody, but not consistently in all animals. Characterization of sera showed that responses were broadly virus neutralizing.