Peptides isolated from random peptide libraries on phage elicit a neutralizing anti-HIV-1 response: analysis of immunological mimicry.

Peptides binding to a murine, human immunodeficiency virus type 1 (HIV-1) neutralizing monoclonal antibody (F58/H3) were isolated from two random peptide libraries expressed on the surface of phage. The antibody was originally elicited by immunization with HIV-1 envelope protein gp120LAI, and has previously been shown to interact with the -I-GPGRA- motif of the V3 loop. The peptide libraries consisted of nine or 15 random amino acid residues flanked by two cysteines, and fused to the amino terminal end of the cpIII protein on the filamentous phage. Selection of specific peptides was carried out in three rounds, with decreasing antibody concentration. An expected peptide motif -GPGRA-, a similar segment, -GPAR-, and two unrelated motifs -FRLLG- and -WRM/ALG- were selected. Binding of antibody was tested both to synthetic peptides in solution, and the corresponding peptide on phage. The GPXR motifs bound in both formats, while the FRLLG bound antibody only when present on the phage The reactivity of peptides on phage was highly dependent on an intact disulphide bond between the cysteines flanking the peptide. The molecular mimicry of the found motifs was tested by immunizing mice and rabbits with conjugated synthetic peptides or peptide on phage. In mice, peptide-specific antisera were raised, but no reactivity to the whole protein (gp120) was detected. In rabbits, however, this was accomplished with the -GPGRA- containing peptide when present on phage. In addition, this antisera precipitated virus particles, and neutralized HIV-1SF2 virus in vitro.