Identification of Regions of HIV-1 p24 Reactive with Sera Which Give "Indeterminate" Results in Electrophoretic Immunoblots with the Help of Long Synthetic Peptides

We analyzed nine sera from persons unlikely to be HIV infected which had an IgG reactivity directed against HIV-1 p24, and in two cases also to its precursor p55, but to no other HIV proteins, nor to proteins of the H9 host cell, in electrophoretic immunoblots (EIB). These sera are also referred to as having an indeterminate HIV EIB pattern or as HIV antibody false positive sera. Seven of nine sera reacted with longer (61-77 amino acids) and none with shorter (17-25 amino acids) p24-derived peptides in enzyme immunoassays (EIAs). This is compatible with a conformational (discontinuous) nature of the epitopes involved in many false positive HIV-1 p24 antibody reactions. Four sera reacted with an N-terminal, one with an internal, and two with a C-terminal fragment. Each of the seven sera thus only reacted with one of the long p24 peptides. The specificity and singularity of the reaction was further demonstrated by competition and/or absorption experiments with synthetic peptides. In contrast, 18 of 20 confirmed HIV-1⁺ sera with p24 reactivity in EIB reacted with at least one and often several of the longer peptides, most frequently the C-terminal one. Thus, the distribution of peptide reactivity of true HIV-1 antibody-positive sera was different from that of the falsely reactive sera. According to two of several explanations, these antibodies may have arisen because of (1) molecular mimicry by chance or by functional selection, (2) immunization by activation, noninfectious exposure, or infection involving non-HIV endogenous or exogenous retroviral antigens. The latter gains some support from our finding of antibody reactions with capsid proteins of the simian viruses, simian sarcoma-associated virus (SSAV), and Mason-Pfizer monkey retrovirus in some of the p24 ± p55 reactive sera.