Tat toxoid as a component of a preventive vaccine in seronegative subjects

Because administration of Tat protein, the HIV-1 toxin that induces immunosuppression and apoptosis, may be deleterious to the host immune system, a chemically inactivated but nonetheless immunogenic Tat preparation, Tat toxoid, was used to immunize seronegative individuals against Tat. In an open, controlled, phase I clinical trial, Tat toxoid turned out to be safe, well tolerated, and able tO trigger a specific immune reaction. In particular, a threefold to more than 10-fold increase of circulating antibodies directed against the native Tat was observed after immunization in all of 5 immunized study subjects, together with a positive reaction to delayed-type hypersensitivity (DTH) skin test with Tat toxoid in vivo and increased lymphoproliferative response to native Tat in vitro. Persistent (≥ year) high levels of circulating anti-Tat antibodies could prevent the Tat- induced immune suppression and, following HIV-1 exposure, allow the anti- HIV-1 cellular immune response, with its early release of protective β- chemokines, to occur leading to an increase of host resistance, that is, protection.