Epitope contiguous chains and antibody recognition in HIV-1 synthetic peptide antigens

Five peptides derived from human immuno deficiency virus (HIV-1) gp41 transmembrane protein have been synthesized: M9 (610-618), M12 (598-609), M15 (600-614), M21 (584-604) and M23 (587-609). These sequences partially overlap in the region vicinal to the immunodominant epitope CSGKLIC, between two cysteine residues 603-609 and three of them (M12, M15 and M23) include this complete heptapeptide. M23, the longer peptide, includes an hydrophilic chain in addition to the heptapeptide loop. The purpose of this work was to determine the influence of contiguous chains to the heptapeptide loop on antibody recognition in fluid and solid phases, and dissociation constants (KD) of each sequence with human anti-HIV-1 antibodies. Two peptides, M13 and M23, overlapped on this loop, were found to be more reactive. Antigen-antibody dissociation constants were determined for both peptides by competition enzyme-linked immunosorbent assay, using each peptide alternatively as the solid phase-immobilized antigen. In addition to the influence of solid-phase antigen on calculated dissociation constants (a phenomenon described by Seligman, 1994), the inhibitory effect of M15 in liquid phase on antibody binding to solid phase M23 was higher than exerted by M23 in solution over antibody binding to M15 on solid phase. On the basis of peptide sequence and predicted antigenicity, this behavior appeared to be contradictory. It is assured that the possible origin of this phenomenon is due to unfavorable conformation of the longer peptide. Even though synthetic peptides mimic mainly sequential epitopes, conformational preferences in fluid or solid phase play an important role in epitope functionality. In particular, addition of residues to known immunodominant sequences may not always amplify antibody recognition if conformation provokes steric hindrance in the native epitope.