Circulating natural IgM antibodies and their corresponding human cord blood cell–derived Mabs specifically combat the Tat protein of HIV

Objective IgM antibodies reactive with each of two specifically defined sequences of HIV Tat protein have been identified in sera from both HIV+ and normal (HIV−) humans. This study was designed to confirm that those antibodies are innate immune factors capable of restriction of specific mechanisms of HIV pathogenicity attributed to the Tat protein. Materials and Methods Antibody-secreting hybridomas were generated from human cord blood cells and processed for monoclonality. Those Mabs reactive with each of the sequences of Tat with which the circulating antibodies are reactive were isolated and their heavy and light chains identified and DNA sequenced. Pools of IgM isolated from blood of normal humans, chimpanzees, rhesus macaques, and mice and the isolated Tat reactive Mabs were tested for capacity to inhibit Tat-induced human T-cell apoptosis. Results Human and chimpanzee IgM pools, as well as the human cord blood cell–derived Mabs, showed a definite capacity to inhibit the Tat-induced apoptosis, while the IgM pools of rhesus macaques or of mice did not. Conclusion These studies establish that the circulating IgM of normal humans include innate antibodies capable of restriction of HIV Tat-induced pathogenesis. That capacity is shared by chimpanzee IgM but not by IgM of other primates or of mice. The identification of those human circulating antibodies as innate is confirmed by the display of similar epitopic identity and apoptosis inhibition capacity by Mabs from human cord blood cell hybridomas. Thus, the arsenal of human cord blood cell hybridomas provides a resource by which, specifically, the potential therapeutic role of the identified HIV Tat-reactive Mabs and, broadly, the fundamental role of innate antibodies in infection control may be explored.