Human immunodeficiency virus (HIV) Tat-reactive antibodies present in normal HIV-negative sera and depleted in HIV-positive sera. Identification of the epitope.

We have detected, in sera of normal human immunodeficiency virus (HIV)-free subjects, IgM antibodies reactive with the Tat protein of HIV in significant titers and at very high frequency, and, in HIV-positive sera, progressively lower titers as HIV pathogenesis ensues. Epitope analysis indicates that the Tat-reactive antibodies of both HIV-negative and HIV-positive sera are homologous, suggesting, therefore, that their decline in HIV-positive sera may represent attrition of a host defense factor. The identified epitope displays minimal homology with that previously defined for another set of IgM antibodies shown to be present in normal sera, deficient in HIV-positive sera, and postulated to be natural antibodies. We propose that the Tat-reactive antibodies, as well, are a set of natural antibodies and that the normal humoral immune system includes a repertoire of antibodies, nonimmunogenic in origin, that contribute to immune homeostasis and, consequently, to host resistance to HIV pathogenesis.