SPID#: 19 This study's main objective was to develop information on allogeneic and xenogeneic thymic transplantation in SIV immunosuppressed rhesus macaques. This included comparisons between 1) placing the thymic transplant in the omentum or muscle, with or without adding the recipient's own stem cells (CD34 purified) in culture pre-transplant; 2) treating some of the transplanted animals with zidovudine or PMPA; 3) using anti-thymocyte globulin as the only immunosuppressive drug around the time of transplant; 4) detecting engraftment by biopsy or by MRI; 5) determining immune responses by newly developed flow cytometric and other methods. So far, engraftment has been obtained for 2 to 4 months when a) anti-viral therapy was used; b) the omentum was the site of transplant; c) allogeneic or human thymic xenograft was employed. Most supportive of the potential benefit of the transplant was the autopsy findings in a severely immunosuppressed monkey in which the mediastinal thymus was markedly lymphopenic, while the omental thymic transplant was populated by lymphocytes (identified as being of monkey origin). No evidence of graft-versus-host disease or host-versus-graft rejection was demonstrated. MRI was capable of identifying thymic transplant in vivo. No definite evidence of immune reconstitution was obtained, although several new approaches for evaluating the immune response have been developed for future studies. This model has already provided important information on the correlative attempts using thymic transplantation for immune reconstitution of children and adults with advanced AIDS.