Transplantation tolerance induced by mixed chimerism

IL-10 modified mRNA monotherapy prolongs survival after composite facial allografting through the induction of mixed chimerism

Vascularized composite allotransplantation has great potential in face transplantation by supporting functional restoration following tissue grafting. However, the need for lifelong administration of immunosuppressive drugs still limits its wide use. Modified mRNA (modRNA) technology provides an efficient and safe method to directly produce protein in vivo. Nevertheless, the use of IL-10 modRNA-based protein replacement, which exhibits anti-inflammatory properties, has not been shown to prolong composite facial allograft survival. In this study, IL-10 modRNA was demonstrated to produce functional IL-10 protein in vitro, which inhibited pro-inflammatory cytokines and in vivo formation of an anti-inflammatory environments. We found that without any immunosuppression, C57BL/6J mice with fully major histocompatibility complex (MHC)-mismatched facial allografts and local injection of IL-10 modRNA had a significantly prolonged survival rate. Decreased lymphocyte infiltration and pro-inflammatory T helper 1 subsets and increased anti-inflammatory regulatory T cells (Tregs) were seen in IL-10 modRNA-treated mice. Moreover, IL-10 modRNA induced multilineage chimerism, especially the development of donor Treg chimerism, which protected allografts from destruction because of recipient alloimmunity. These results support the use of monotherapy based on immunomodulatory IL-10 cytokines encoded by modRNA, which inhibit acute rejection and prolong allograft survival through the induction of donor Treg chimerism.

Tolerance and chimerism and allogeneic bone marrow/stem cell transplantation in liver transplantation

The liver has particular tolerogenic properties that allow its spontaneous acceptance in some animal species. Liver structure is considered to favor a tolerogenic environment. The peripheral tolerance mechanisms also play a role in spontaneous tolerance to liver graft. In a clinical setting, the main challenge nowadays facing liver transplantation is minimization of immunosuppression with the goal of donor-specific tolerance. Mechanisms involved in tolerance to transplanted organs are complex and partly unknown. A significant mechanism in tolerance induction is chimerism. Chimerism can be induced through transplantation of allogeneic donor bone marrow/stem cells under appropriate host conditioning. This review focuses on the tolerance mechanisms in liver transplantation and highlights the role of chimerism and allogeneic bone marrow/stem cell transplantation in tolerance development.

Role of Toll-Like Receptor–Driven Innate Immunity in Thoracic Organ Transplantation

Innate immunity represents the first line of defense against microbial invasion. Recent studies have determined that a group of germline-encoded receptors, termed Toll-like receptors (TLRs), are critical for recognizing foreign motifs on microbial organisms and initiating innate responses. An exciting area of research has recently linked activation of TLRs on antigen-presenting cells (APCs) to effective antigen presentation and activation of naive T cells. Most studies have shown that TLR-dependent immune function leads to T-helper 1 (TH1) immunity, although evidence also supports that TH2 immune responses may be initiated by TLR signaling in certain contexts. In either case, innate immune signaling via TLRs leads to a productive adaptive immune response. In contrast to studies in purely infectious models, emerging data from experimental and clinical studies have provided evidence that TLR immune function is important in acute allograft rejection. Specifically, MyD88, an important TLR signal adaptor, was found to be critical for the rejection of minor-mismatched skin allografts, and important for alloimmune priming and TH1 immunity against fully allogeneic skin grafts. Furthermore, a clinical study has shown that recipients with TLR 4 polymorphisms associated with endotoxin hyporesponsiveness manifest reduce acute lung allograft rejection. Collectively, these studies demonstrate that innate immunity is important for alloimmunity. Future therapeutic modalities that target innate rather than adaptive immune mechanisms represent a promising avenue for future studies in thoracic organ transplantation.

Current trends in heart transplantation

With the introduction of cyclosporin A in the early 1980s, heart transplantation was transformed from an experimental procedure into a successful therapeutic option for patients with end-stage heart disease. Since then, constant progress has extended the benefits of the procedure to an increasing number of patients. Despite all this progress, heart transplantation is not an option that can be offered to the vast majority of the world population, in particular to the over 5 billion inhabitants of underdeveloped or developing countries in the three most populated continents, namely, Asia, Africa and South America. While the North American continent and Europe account for only 17% of the world population, they donate and receive over 95% of the heart transplants performed worldwide. In addition, the number of transplant candidates continues to exceed the number of available donors, and the donor shortage is not expected to improve. Opportunistic infections, rejection, malignancy and graft coronary artery disease continue to plague heart transplantation and remain the Achilles heel of the procedure. With the beginning of the new millennium, new perspectives are arising in heart transplantation. Strategies addressing donor-specific tolerance and the development of selective immunosuppressive therapies are on the horizon and could improve the quality of life after transplantation and also prolong survival.

Myocyte growth in the failing heart

Adult ventricular myocytes can undergo mitotic division, resulting in an increase in the aggregate number of cells in the heart. The improvement in the methodological approach to the analysis of tissue sections by immunostaining and confocal microscopy has defeated the dogma that myocyte regeneration cannot occur in the adult heart. Most importantly, primitive and progenitor cells have been identified in the human heart. These cells express telomerase and have the capability of undergoing lineage commitment and rapid cell division, expanding significantly the contracting ventricular myocardium. These cell populations possess all the molecular components regulating the entry and progression through the cell cycle, karyokinesis, and cytokinesis. The recognition that myocyte hypertrophy and regeneration, as well as myocyte necrosis and apoptosis, occur in cardiac diseases has contributed to enhancing our understanding of the plasticity of the human heart.

Immunosuppression for cardiac transplantation—the past, present and future

The field of cardiac transplant immunosuppression is rapidly developing and has evolved over the past 35 years. Anecdote, experience and registry based practice is giving way to an increasing bounty of well designed, randomized controlled trials which will guide future therapy. Current therapy is based on triple therapy with corticosteroids, a calcineurin inhibitor and an antimetabolite, but these regimens may be replaced by substitution or addition of newer antiproliferative agents. The true nemesis is coronary graft vasculopathy, which affects 50% of patients at 5 years and until recently had very few preventive therapeutic options. Renal toxicity remains among the most challenging adverse effects of immunosuppression to be overcome.

Cardiac transplantation for hypoplastic left heart syndrome

Future improvements can be expected in cardiac transplantation in children. We continue to advance our understanding of the immune system, and to develop more specific immunosuppressive agents. Ultimately, the future for recipients may be improved by strategies such as induction therapy or donor-derived chimeric destined transfusions, designed to enhance the tolerance of the host to a human leukocyte antigen incompatible graft. Improvements in tolerance of the host would allow for reduction or elimination of many, if not all, of the immunosuppressive agents, and for longevity extending well into the adulthood. Survival, particularly for infants, has improved dramatically in the last decade. The most recent results from the registry of the International Society of Heart and Lung Transplantation/United Network for Organ Sharing show that recipients less than one year old at transplantation, who survive the first year, have greater than a 95% survival to four years (Fig. 1). As late outcomes continue to improve, transplantation will provide a better quality and duration of life for infants with hypoplastic left heart syndrome. It is possible, nonetheless, that some infants will require retransplantation, since the half life of a transplanted heart in children has been about 12 years. The alternative is conventional surgery with multiple palliative operations, and the need for later transplantation as end-stage cardiac function is reached. Efforts to increase potential donors and donor utilization can be supported by innovative schemes, such as ABO incompatible transplants. Additional efforts are made more urgent when the current data indicate excellent outcomes after transplantation, but a high mortality while waiting for transplantation.

Pediatric heart transplantation

Heart transplantation is now a treatment option with good outcome for infants and children with end-stage heart failure or complex, inoperable congenital cardiac defects. One-year and 5-year actuarial survival rates are high, approximately 75% and 65%, respectively, with overall patient survival half-life greater than 10 years. To date, survival has been improving as a result of reducing early mortality. Further reductions in late mortality, in part because of graft coronary artery disease and rejection, will allow achievement of the goal of decades-long survival. Quality of life in surviving children, as judged by activity, is usually "normal." Somatic growth is usually at the low normal range but linear growth can be reduced. Of infant recipients, 85% evaluated at 6 years of age or older were in an age-appropriate grade level. Long-term management of childhood heart recipients requires the collaboration of transplant physicians, given the increasing number of immunosuppressive agents and the balance between rejection and infection. Currently, recipients are maintained on immunosuppressive medications that target calcineurin (eg, cyclosporine, tacrolimus), lymphocyte proliferation (eg, azathioprine, mycophenolate mofetil [MMF], sirolimus) and, in some instances antiinflammatory corticosteroids. Emerging evidence now suggests a favorable immunologic opportunity for transplantation in childhood and, conversely, a higher mortality rate in children who have had prior cardiac surgery. Further studies are needed to define age-dependent factors that are likely to play a role in graft survival and possible graft-specific tolerance (eg, optimal conditions for tolerance induction and how immunosuppressive regimens should be changed with maturation of the immune system). As late outcomes continue to improve, the need for donor organs likely will increase, as transplantation affords a better quality and duration of life for children with complex congenital heart disease, otherwise facing a future of multiple palliative operations and chronic heart failure.