Fadi Issa

Ex vivo-expanded human regulatory T cells prevent the rejection of skin allografts in a humanized mouse model.

Background. Composite tissue transplantation effectively reconstructs the most complex defects, but its use is limited because of harmful immunosuppression and the high susceptibility of skin to rejection. Development of tolerance is an ideal solution, and protocols using regulatory T cells (Tregs) to achieve this have been promising in experimental animal models. The aim of this study was to investigate the ability of human Tregs to regulate immune responses to a human skin allograft in vivo. Methods. We isolated and expanded naturally occurring CD127loCD25+CD4+ human Tregs from peripheral blood mononuclear cells (PBMCs) and examined their phenotype and suppressive activity in vitro. Using a clinically relevant chimeric humanized mouse system, we transplanted mice with human skin grafts followed by allogeneic populations of PBMCs with or without Tregs derived from the same PBMC donor. Results. Ex vivo–expanded Tregs maintain the appropriate Treg markers and retain suppressive activity against allostimulated and polyclonally stimulated autologous PBMCs in vitro. Mice receiving allogeneic PBMCs alone consistently reject human skin grafts, whereas those also receiving Tregs display stable long-term human skin transplant survival along with a reduction in the CD8+ human cellular graft infiltrate. Conclusions. We show for the first time the unique ability of human Tregs to prevent the rejection of a skin allograft in vivo, highlighting the therapeutic potential of these cells clinically.

Role of T cells in graft rejection and transplantation tolerance

Transplantation is the most effective treatment for end-stage organ failure, but organ survival is limited by immune rejection and the side effects of immunosuppressive regimens. T cells are central to the process of transplant rejection through allorecognition of foreign antigens leading to their activation, and the orchestration of an effector response that results in organ damage. Long-term transplant acceptance in the absence of immunosuppressive therapy remains the ultimate goal in the field of transplantation and many studies are exploring potential therapies. One promising cellular therapy is the use of regulatory T cells to induce a state of donor-specific tolerance to the transplant. This article first discusses the role of T cells in transplant rejection, with a focus on the mechanisms of allorecognition and the alloresponse. This is followed by a detailed review of the current progress in the field of regulatory T-cell therapy in transplantation and the translation of this therapy to the clinical setting.

CD4+ regulatory T cells in solid organ transplantation

Purpose of reviewSolid organ transplantation is the most effective treatment for end-stage organ failure, but the long-term outcomes remain suboptimal. CD4+ regulatory T cells (Tregs) are emerging as a potential therapy to facilitate long-term allograft survival. This review provides a general overview of the biology of CD4+ Tregs and then goes on to discuss the most relevant and recent experimental and clinical evidence for their therapeutic use in solid organ transplantation. Recent findingsThere have been major advances in our understanding of Tregs, including improvements in methods for their isolation and expansion. Experimental models are providing very important data on the in-vitro and in-vivo behavior of Tregs in transplantation, while recent clinical trials of Treg cellular therapy in graft-versus-host disease are offering a valuable insight into the efficacy of Treg adoptive cellular therapy. SummaryData in favor of Treg cellular therapy in transplantation are mounting, and we predict that their use in clinical trials is on the horizon.

Translating tolerogenic therapies to the clinic - where do we stand?

Manipulation of the immune system to prevent the development of a specific immune response is an ideal strategy to improve outcomes after transplantation. A number of experimental techniques exploiting central and peripheral tolerance mechanisms have demonstrated success, leading to the first early phase clinical trials for tolerance induction. The first major strategy centers on the facilitation of donor-cell mixed chimerism in the transplant recipient with the use of bone marrow or hematopoietic stem cell transplantation. The second strategy, utilizing peripheral regulatory mechanisms, focuses on cellular therapy with regulatory T cells. This review examines the key studies and novel research directions in the field of immunological tolerance.

Multiple unit pooled umbilical cord blood is a viable source of therapeutic regulatory T cells.

Background Regulatory T cells (Treg) are potentially a useful therapeutic option for the treatment of immunopathological conditions including graft-versus-host disease. Umbilical cord blood (UCB) offers certain advantages over adult peripheral blood (APB) as a source of Treg for cellular therapy but yields far fewer Treg per unit. Pooling of Treg from multiple donors may overcome this challenge. Methods In this study, we assessed the in vitro and in vivo efficacy of multiple donor pooled UCB or APB-derived Treg. Results In vitro, pooled freshly isolated UCB-derived Treg were as suppressive as APB-derived Treg. However, in a mouse model of human skin allodestruction, pooled UCB-derived Treg were more potent at suppressing alloresponses and prolonging skin survival compared with pooled APB-derived Treg. Improved survival of UCB Treg in an in vivo cell survival assay and their lower expression of human leukocyte antigen-ABC suggested that lower immunogenicity may account for their superior efficacy in vivo. Conclusion Multiple-unit UCB is therefore a viable source of human Treg for cellular therapy, and pooling of Treg from multiple donors offers a useful strategy for achieving required therapeutic doses.

Ischemia-reperfusion injury accelerates human antibody-mediated transplant vasculopathy.

Background The pathogenesis of transplant vasculopathy (TV) is a multifactorial process. We hypothesized that ischemia-reperfusion injury and antibody-mediated damage contribute to the development of TV. Methods Human vessels were procured from nine separate donors undergoing cardiac surgery and stored in saline solution on ice until transplantation. BALB/c Rag2-/-IL-2R&ggr;-/- mice were transplanted with a human vessel graft on day 0. Purified anti–human leukocyte antigen class I antibody (W6/32), isotype control antibody, or saline was injected into recipient mice weekly until day 42, at which point the degree of intimal expansion (IE) of vessels was assessed by histologic analysis. Results We found that a prolonged cold ischemia time (6–12 hr) alone did not induce IE. In mice that received antibody where vessels were transplanted within 6 hr of procurement, no IE was observed. By contrast, in vessels exposed to more than 6 hr cold ischemia, both W6/32 antibody (30.4%±6.9%) and isotype control antibody (39.5%±6.0%) promoted significant IE (P<0.05 vs. saline [12.4%±1.7%]). Importantly, the isotype control antibody did not cross-react with human tissue. Interestingly, the number of mouse Fc-receptor–positive cells was significantly increased in human vessels exposed to more than 6 hr cold ischemia but only in the presence of antibody (P<0.05). Conclusions Antibody, regardless of its specificity, may promote IE in human vessels that are injured through cold ischemia via interaction with Fc-receptor–positive cells. This highlights the importance of controlling the degree of cold ischemia in clinical transplantation in an effort to reduce the risk of TV development.

The potential role for regulatory T-cell therapy in vascularized composite allograft transplantation

Purpose of reviewVascularized composite allograft (VCA) transplantation restores defects to a degree not possible by conventional techniques. However, it is limited by the need for long-term immunosuppression and high rates of acute rejection directed against skin. There is therefore a need for a therapy that may shift the risk-benefit ratio in favour of VCA transplantation. Regulatory T cells (Tregs) are a subset of T cells with potent immunoregulatory properties and the potential to promote immunosuppression-free allograft survival. In this review, we consider the evidence for Treg therapy in VCA transplantation. Recent findingsCD4+ Tregs are the best-studied immunoregulatory cell type, and a large amount of experimental and clinical data is emerging to endorse their use in VCA transplantation. Data from animal and humanized models are particularly encouraging and demonstrate the potent efficacy of Treg at preventing skin allograft rejection. Moreover, central tolerance induction techniques in VCA transplantation models are demonstrating a dependence on Tregs for graft survival. SummaryAn improvement in outcomes after VCA transplantation has the potential to revolutionize the field. Several effective therapeutic strategies have demonstrated great promise experimentally, and there is now a need to assess their safety and efficacy in a clinical setting.

Novel biomarkers and functional assays to monitor cell-therapy-induced tolerance in organ transplantation.

Purpose of reviewCell-based immunotherapy offers a novel approach to minimize the need for immunosuppressive drugs and to promote a state of immunological tolerance to a transplanted organ. We review the most promising biomarkers and functional assays able to identify patients tolerant to their graft. Such a signature of tolerance is essential in the assessment of the efficacy with which trials of cellular therapies promote immunoregulation and minimize graft rejection. Recent findingsA multitude of novel cellular therapies have entered early-phase clinical trials in solid-organ transplant patients. Recent multicentre collaborations have enabled the determination of distinct tolerance profiles for both liver and kidney transplant recipients. These have been shown to be highly predictive of tolerance in certain settings and show utility in identifying patients in whom immunosuppressive drugs can be weaned or discontinued. SummaryIn order to become a viable treatment option in solid-organ transplantation, the latest large, multicentre clinical trials of cellular therapies must utilize, validate and discover the biomarkers with the capacity to reliably identify a signature of immune tolerance.

Regulatory T cells for tolerance

Regulatory T cells (Tregs) are critical mediators of immune homeostasis and hold significant promise in the quest for transplantation tolerance. Progress has now reached a critical threshold as techniques for production of clinical therapies are optimised and Phase I/II clinical trials are in full swing. Initial safety and efficacy data are being reported, with trials assessing a number of different strategies for the introduction of Treg therapy. It is now more crucial than ever to elucidate further the function and behaviour of Tregsin vivoand ensure safe delivery. This review will discuss the current state of the art and future directions in Treg therapy.

Selective blockade of CD28 on human T cells facilitates regulation of alloimmune responses

T cells are central to the detrimental alloresponses that develop in autoimmunity and transplantation, with CD28 costimulatory signals being key to T cell activation and proliferation. CTLA4-Ig molecules that bind CD80/86 and inhibit CD28 costimulation offer an alternative immunosuppressive treatment, free from some of the chronic toxicities associated with calcineurin inhibition. However, CD80/86 blockade by CTLA4-Ig also results in the loss of coinhibitory CTLA4 signals that are critical to the regulation of T cell activation. Here, we show that a nonactivating monovalent anti-CD28 that spares CTLA4 signaling is an effective immunosuppressant in a clinically relevant humanized mouse transplant model. We demonstrate that selective CD28 blockade prolongs human skin allograft survival through a mechanism that includes a reduction in the cellular graft infiltrate. Critically, selective CD28 blockade promotes Treg function in vivo and synergizes with adoptive Treg therapy to promote transplant survival. In contrast to CTLA4-Ig treatment, selective CD28 blockade promotes regulation of alloimmune responses and facilitates Treg-based cellular therapy.