Clinical tolerance in allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (HSCT) has been a curative therapeutic option for a wide range of immune hematologic malignant and non-malignant disorders including genetic diseases and inborn errors. Once in the host, allogeneic transplanted cells have not only to ensure myeloid repopulation and immunological reconstitution but also to acquire tolerance to host human leukocyte antigens via central or peripheral mechanisms. Peripheral tolerance after allogeneic HSCT depends on several regulatory mechanisms aimed at blocking alloimmune reactivity while preserving immune responses to pathogens and tumor antigens. Patients transplanted with HSCT represent an ideal model system in humans to identify and characterize the key cellular and molecular players underlying these mechanisms. The knowledge gained from these studies has allowed the development of novel therapeutic strategies aimed at inducing long-term peripheral tolerance, which can be applicable not only in allogeneic HSCT but also in autoimmune diseases and solid-organ transplantation. In the present review, we describe Type 1 regulatory T cells, initially discovered and characterized in chimeric patients transplanted with human leukocyte antigen-mismatched HSCT, and how their presence correlates to tolerance induction and maintenance. Furthermore, we summarize different cell therapy approaches with regulatory T cells, designed to facilitate tolerance induction, minimizing pharmaceutical interventions.

Evaluation of alloreactivity in kidney transplant recipients treated with antithymocyte globulin versus IL-2 receptor blocker

Induction therapy is used in kidney transplantation to inhibit the activation of donor-reactive T cells which are detrimental to transplant outcomes. The choice of induction therapy is decided based on perceived immunological risk rather than by direct measurement of donor T-cell reactivity. We hypothesized that immune cellular alloreactivity pretransplantation can be quantified and that blocking versus depleting therapies have differential effects on the level of donor and third-party cellular alloreactivity. We studied 31 kidney transplant recipients treated with either antithymocyte globulin (ATG) or an IL-2 receptor blocker. We tested pre- and posttransplant peripheral blood cells by flow cytometry to characterize T-cell populations and by IFN-γ ELISPOT assays to assess the level of cellular alloreactivity. CD8(+) T cells were more resistant to depletion by ATG than CD4(+) T cells. Posttransplantation, frequencies of donor-reactive T cells were markedly decreased in the ATG-treated group but not in the IL-2 receptor blocker group, whereas the frequencies of third-party alloreactivity remained nearly equivalent. In conclusion, when ATG is used, marked and prolonged donor hyporesponsiveness with minimal effects on nondonor responses is observed. In contrast, induction with the IL-2 receptor blocker is less effective at diminishing donor T-cell reactivity.

Rationale for using belatacept in combination with sirolimus

Kidney transplantation can be used to replace failing native kidneys; however, it requires long-term immunosuppression, and immunological tolerance for this is not yet achievable. The cornerstone of immunosuppression is based on calcineurin inhibitors, which are nephrotoxic. Therefore, new drugs are being developed that provide efficacious immunosuppression and almost no renal toxicity. The first family of drugs that have these properties are mammalian target of rapamycin inhibitors: these include sirolimus and everolimus. These two drugs, besides their immunosuppressive properties, also have beneficial effects regarding cytomegalovirus (CMV) infection, which is a very common posttransplantation complication. In phase III trials, belatacept, a costimulatory blocker, has also been shown to provide a good immunosuppressive effect and also gives a significantly better cardiovascular profile than cyclosporine-based immunosuppression. However, belatacept can potentially increase infections such as CMV. Thus, herein, we describe the rationale for combining belatacept with sirolimus for kidney transplant patients.

Expression of CD39 by human peripheral blood CD4+ CD25+ T cells denotes a regulatory memory phenotype

We have shown that CD39 and CD73 are coexpressed on the surface of murine CD4+ Foxp3+ regulatory T cells (Treg) and generate extracellular adenosine, contributing to Treg immunosuppressive activity. We now describe that CD39, independently of CD73, is expressed by a subset of blood-derived human CD4+ CD25+ CD127lo Treg, defined by robust expression of Foxp3. A further distinct population of CD4+ CD39+ T lymphocytes can be identified, which do not express CD25 and FoxP3 and exhibit the memory effector cellular phenotype. Differential expression of CD25 and CD39 on circulating CD4+ T cells distinguishes between Treg and pathogenic cellular populations that secrete proinflammatory cytokines such as IFNγ and IL-17. These latter cell populations are increased, with a concomitant decrease in the CD4+ CD25+ CD39+ Tregs, in the peripheral blood of patients with renal allograft rejection. We conclude that the ectonucleotidase CD39 is a useful and dynamic lymphocytes surface marker that can be used to identify different peripheral blood T cell-populations to allow tracking of these in health and disease, as in renal allograft rejection.

Potential T regulatory cell therapy in transplantation: how far have we come and how far can we go?

Summary Graft survival has been lately improved by the introduction of efficient immunosuppressive drugs. However, late graft loss caused by chronic rejection and the side effects of long-term immunosuppression remain major obstacles for successful transplantation. Operational tolerance, which is defined by the lack of acute and chronic rejection and indefinite graft survival with normal graft function in the absence of continuous immunosuppression, represents an attractive alternative. Nevertheless, tolerance after allogeneic transplantation is commonly considered the 'mission impossible' for both immunologists and clinicians. One of the mechanisms involved in tolerance is the suppression of graft-specific alloreactive T cells, which largely mediate graft rejection, by regulatory T cells (Tregs) or by soluble factors produced by Treg cells. With this review, I will make an effort to collect and describe the significant studies performed in transplanted patients, and not in animal models or in in vitro systems, with the attempt to: (i) understand how tolerance is achieved, (ii) define whether and how Treg cells influence transplant tolerance, (iii) describe the first clinical trials with Treg cells in humans (i.e. how far have we come) and (iv) predict the future of Treg cell-based therapy in humans (i.e. how far can we go).