CONSTRUCTION OF AN HIV-1 PEPTIDE VACCINE CONTAINING A MULTIDETERMINANT HELPER PEPTIDE LINKED TO A V3 LOOP PEPTIDE-18 INDUCING STRONG NEUTRALIZING ANTIBODY-RESPONSES IN MICE OF MULTIPLE MHC HAPLOTYPES AFTER 2 IMMUNIZATIONS

Peptide constructs comprised of multideterminant Th peptides from the envelope glycoprotein of HIV previously identified to induce prolifera tive responses in four different haplotypes of mice and IL-2 responses in 52 to 73% of HIV positive, Ag-responsive patients, were colinearly synthesized with the peptide 18 of the V3 loop of HIV-1 gp 160, corre sponding to the principal neutralizing determinant of HIV-IIIB. The se gments containing clusters of overlapping Th epitopes were called clus ter peptides. Cognate help for peptide 18 antibody was elicited after a single immunization in all strains of mice that had previously respo nded to a T cell epitope encompassed by the cluster peptides. Animals boosted with cluster peptide-peptide 18 constructs 36 to 52 wk later d isplayed secondary antibody responses. Cluster peptide 3-peptide 18 in duced antibody that neutralized homologous virus in one strain of mice although strong peptide 18 antibody responses were detected in all fo ur strains of mice. The most promising construct, cluster peptide 6-pe ptide 18, induced neutralizing antibody in all strains of mice tested, and in two strains the level of neutralizing antibody achieved was co mparable to levels adequate for protection from homologous viral chall enge in chimpanzees. After a single boost, antibody titers for 90% neu tralization in the range of 1/1000 to 1/16,000 were achieved. These ne utralizing titers against the homologous viral strain, after just two immunizations, are at least four- to eight fold higher than the highes t titered other polyclonal V3-specific immune sera we have ever observ ed in our laboratories. We also asked why some sera neutralized and ot hers with similar ELISA titers did not. No correlation was found betwe en neutralization and isotype or affinity for peptide or gp120. We cou ld not account for neutralization by antibodies to the helper sites. S ubstitutions made in the central loop region of peptide 18, amino acid residues PGRAF, dramatically reduced binding of both neutralizing and nonneutralizing sera although some fine specificity differences betwe en neutralizing and nonneutralizing sera were noted. These results hav e implications for the design of synthetic peptide vaccines for HIV.