Induction of chimerism in rhesus macaques through stem cell transplant and costimulation blockade-based immunosuppression

Immunologic approaches to composite tissue allograft

This article discusses the immunologic principles and the most promising immunologic approaches for composite tissue allograft tolerance. We have previously reviewed some of the pharmacologic approaches for composite tissue allo-transplantation. In this review, we will summarize the range of options that may address the challenge of transplantation in reconstructive surgery.

Frontiers in nephrology: immune tolerance to allografts in humans

Vascularized allografts are rejected unless some indefinite modification to the recipient's immune system is made. This modification is typically achieved through the long-term administration of immunosuppressive drugs. Patients thus trade their end-stage organ failure for dependence on daily drug therapy and the accompanying chronic condition of immunodeficiency. However, it is clear from studies in experimental animals that rejection can be prevented through the use of several therapeutic approaches, including donor hematopoietic cell infusion, chimerism, T cell depletion, and/or co-stimulation blockade. Successfully treated animals avoid rejection beyond the period of therapy without a phenotype of chronic immunosuppression and are thus considered to be tolerant of their grafts. Although intriguing, this success in animals has yet to be reproducibly translated to the clinic, and human transplant recipients remain tethered to immunosuppressive drugs with rare exceptions. This article provides an overview of the existing, largely anecdotal, clinical experience with organ allograft tolerance. It reviews the various approaches that are being applied in pilot human trials and suggests avenues for future clinical investigation.

Recent Progress in Tolerance Induction through Mixed Chimerism

Organ transplant recipients require life-long treatment with immunosuppressive drugs. Currently available immunosuppression is associated with substantial morbidity and mortality, and is ineffective in inhibiting chronic rejection and graft loss. Therefore, a permanent state of donor-specific tolerance remains a primary goal for transplantation research. The induction of mixed hematopoietic chimerism is an attractive concept in this regard. Hematopoietic chimerism modulates the immunologic repertoire by extending the mechanisms of self-tolerance to donor-specific allotolerance. Despite recent progress in developing nontoxic bone marrow transplantation protocols for rodents, translation to large animals has remained difficult. Here, we outline the concept of tolerance via mixed chimerism, and review recent progress and remaining challenges in bringing this approach to the clinical setting.

Characterization, cryopreservation, and ablation of spermatogonial stem cells in adult rhesus macaques

Spermatogonial stem cells (SSCs) are at the foundation of mammalian spermatogenesis. Whereas rare A(single) spermatogonia comprise the rodent SSC pool, primate spermatogenesis arises from more abundant A(dark) and A(pale) spermatogonia, and the identity of the stem cell is subject to debate. The fundamental differences between these models highlight the need to investigate the biology of primate SSCs, which have greater relevance to human physiology. The alkylating chemotherapeutic agent, busulfan, ablates spermatogenesis in rodents and causes infertility in humans. We treated adult rhesus macaques with busulfan to gain insights about its effects on SSCs and spermatogenesis. Busulfan treatment caused acute declines in testis volume and sperm counts, indicating a disruption of spermatogenesis. One year following high-dose busulfan treatment, sperm counts remained undetectable, and testes were depleted of germ cells. Similar to rodents, rhesus spermatogonia expressed markers of germ cells (VASA, DAZL) and stem/progenitor spermatogonia (PLZF and GFRalpha1), and cells expressing these markers were depleted following high-dose busulfan treatment. Furthermore, fresh or cryopreserved germ cells from normal rhesus testes produced colonies of spermatogonia, which persisted as chains on the basement membrane of mouse seminiferous tubules in the primate to nude mouse xenotransplant assay. In contrast, testis cells from animals that received high-dose busulfan produced no colonies. These studies provide basic information about rhesus SSC activity and the impact of busulfan on the stem cell pool. In addition, the germ cell-depleted testis model will enable autologous/homologous transplantation to study stem cell/niche interactions in nonhuman primate testes.