Antibody recognition of amino acid divergence within an HIV-1 neutralization epitope

Antibodies elicited by HIV-1 strains, and which neutralize such strains in vitro, bind to synthetic peptides of 5-8 amino acids in length. These amino acids, although variable, have a fixed location between two cysteines in the carboxyl terminus of the HIV-1 external envelope. Nine peptides of 9 amino acids corresponding to the gp120 domains of European and American (LAV-1, NY5, CDC4, SF2), Haitian (RF) and African (ELI, MAL, Z3, Z6) HIV-1 strains, were synthesized using LAV-1 and RF neutralization epitopes as models. Serum of chimpanzees infected with LAV-1, HTLV-IIIB or HTLV-IIIRF reacted predominantly with the homologous peptide, although cross-reactivity with heterologous peptides occurred: 8 out of 11 human sera with HTLV-IIIB-neutralizing activity bound the LAV-1/HTLV-IIIB peptide, and 6 out of 7 sera with HTLV-IIIRF-neutralizing activity bound the RF peptide. African sera reacted most frequently with the Z3 peptide (78%) while only 35% (p = 0.0001) of European and 20% (p less than 0.0001) of American sera recognized it. Recognition patterns of children from the USA and Europe were different. Although multiple reactivities were observed, blocking experiments favoured cross-reactivity as the explanation. Based on the antibody profiles of nonapeptide recognition, peptides LAV-1, RF and SF2 were clustered, as were NY5 and CDC4, and so were Z6, MAL and ELI. This antigenic relatedness of HIV-1 strains could not entirely be explained by the physico-chemical characteristics of the nonamers per se. Resemblance was observed with the clustering of HIV-1 strains based on the divergence of the nucleotide sequence of entire HIV-1 envelopes. This implies a role of peripheral envelope residues, in the context of infectious particles or infected cells, in determining the specificity of antibodies reactive to the V3 domain. Therefore, the neutralization domain in this variable region may be considered part of a conformational structure involving several envelope regions which appear distinct from each other in the primary sequence