Epitopes for natural antibodies of human immunodeficiency virus (HIV)-negative (normal) and HIV-positive sera are coincident with two key functional sequences of HIV Tat protein.

We have previously shown that IgM antibodies that react with human immunodeficiency virus (HIV) Tat, a regulatory protein essential for viral replication, are present in sera of all normal, HIV-negative individuals and deficient in sera of HIV-positive individuals at progressively greater frequency as diagnosis of AIDS nears. That IgM was designated as a set of natural antibodies, a repertoire of the normal humoral immune system believed to provide early defense against infectious invaders. In the prior study, by means of a series of synthetic peptides representing the amino acid sequence of HIV-1 Tat, one epitope for the IgM natural antibodies was defined within the cysteine-rich domain, shown in cell transfection studies to participate in Tat function. In this study we have defined another epitope, within the basic domain, with which the natural antibodies react. The specific sequence and amino acid residues required for that epitope are coincident with those required for the role of Tat in viral replication. The IgM antibodies reactive with the two epitopes of Tat make up two distinct sets, which, together, account for the total Tat reactivity of both HIV-negative and HIV-positive sera. The striking coincidence of the two epitopes with the two functional sequences of Tat suggests a potential role of those natural antibodies in control of HIV pathogenesis. By inference from the extensive evidence for the presence of extracellular Tat in cultures of HIV-infected cells, Tat may be expected to be present in the circulating plasma of infected people. We propose, therefore, that the Tat-reactive natural antibodies, documented in these studies to be present in the circulating plasma in the pre-AIDS stages of HIV infection, may inhibit cell entry of plasma-borne Tat and thereby curtail HIV propagation. Thus, those natural antibodies may be a host factor for delay in HIV pathogenetic progression.