Carrier-dependent specificity of antibodies to a conserved peptide determinant of gp120

Amino acid residues 421-436 constitute a comparatively conserved determinant of gp120 that participates in the binding of host cell CD4 receptors by HIV-1. We compared the immunogenicity of synthetic Cys-gp120 (421-436) conjugated to KLH via the N terminal Cys residue (KLH-I) and gp120 (421-436) extended at its N terminus with a 15 residue tetanus toxoid T cell epitope (T-I) in non-autoimmune mice (BALB/cstrain) and Fas-defective autoimmune mice (MRL/lpr strain). Both immunogens elicited high titer Abs detected as the binding to gp120 (421-436) conjugated to bovine serum albumin (BSA-I) immobilized in ELISA plates. Abs from KLH-I immunized mice displayed binding to full-length gp120 but the Abs from T-I immunized mice did not. Proteins unrelated in sequence to gp120 did not bind the Abs. Soluble I and T-I failed to compete with immobilized BSA-I for binding to anti-KLH-I Abs, whereas these peptides inhibited anti-T-I Ab binding by BSA-I (rank potency order: BSA-I > T-I >> I). These results indicate the influence of the carrier protein on the specificity of Abs to synthetic I. Low level BSA-I and gp120 binding Abs were detected in sera from non-immunized MRL/lpr mice. Similar Ab binding titers and specificity profiles were evident in MRL/lpr and BALB/c mice following immunization with KLH-I and T-I, indicating that pre-existing immunity to gp120 in the former strain does not influence the magnitude or specificity of the Ab response.