Innate immunity in transplant tolerance and rejection

Long-term heart transplant survival by targeting the ionotropic purinergic receptor P2X7

BACKGROUND

Heart transplantation is a lifesaving procedure for patients with end-stage heart failure. Despite much effort and advances in the field, current immunosuppressive regimens are still associated with poor long-term cardiac allograft outcomes, and with the development of complications, including infections and malignancies, as well. The development of a novel, short-term, and effective immunomodulatory protocol will thus be an important achievement. The purine ATP, released during cell damage/activation, is sensed by the ionotropic purinergic receptor P2X7 (P2X7R) on lymphocytes and regulates T-cell activation. Novel clinical-grade P2X7R inhibitors are available, rendering the targeting of P2X7R a potential therapy in cardiac transplantation.

METHODS AND RESULTS

We analyzed P2X7R expression in patients and mice and P2X7R targeting in murine recipients in the context of cardiac transplantation. Our data demonstrate that P2X7R is specifically upregulated in graft-infiltrating lymphocytes in cardiac-transplanted humans and mice. Short-term P2X7R targeting with periodate-oxidized ATP promotes long-term cardiac transplant survival in 80% of murine recipients of a fully mismatched allograft. Long-term survival of cardiac transplants was associated with reduced T-cell activation, T-helper cell 1/T-helper cell 17 differentiation, and inhibition of STAT3 phosphorylation in T cells, thus leading to a reduced transplant infiltrate and coronaropathy. In vitro genetic upregulation of the P2X7R pathway was also shown to stimulate T-helper cell 1/T-helper cell 17 cell generation. Finally, P2X7R targeting halted the progression of coronaropathy in a murine model of chronic rejection as well.

CONCLUSIONS

P2X7R targeting is a novel clinically relevant strategy to prolong cardiac transplant survival.

Transplant immunology for non-immunologist

Transplantation is the treatment of choice for end-stage kidney, heart, lung, and liver disease. Short-term outcomes in solid-organ transplantation are excellent, but long-term outcomes remain suboptimal. Advances in immune suppression and human leukocyte antigen matching techniques have reduced the acute rejection rate to <10%. Chronic allograft injury remains problematic and is in part immune-mediated. This injury is orchestrated by a complex adaptive and innate immune system that has evolved to protect the organism from infection, but, in the context of transplantation, could result in allograft rejection. Such chronic injury is partially mediated by anti-human leukocyte antigen antibodies. Severe rejections have largely been avoided by the development of tissue-typing techniques and crossmatch testing, which are discussed in detail. Further advances in the understanding of T- and B-cell immunology have led to the development of new immunomodulatory therapies directed at prolonging allograft survival, including those that decrease antibody production as well as those that remove antibodies from circulation. Further application of these immunomodulatory therapies has allowed expansion of the donor pool in some cases by permitting ABO-incompatible transplantation and transplantation in patients with preformed antibodies. Although vast improvements have been made in allograft survival, patients must remain on lifetime immunosuppression. Withdrawal of immunosuppression almost always ultimately leads to allograft rejection. The ultimate dream of transplant biologists is the induction of tolerance, where immune function remains intact but the allograft is not rejected in the face of withdrawn immunosuppression. This, however, has remained a significant challenge in human studies.

Altered tryptophan metabolism as a paradigm for good and bad aspects of immune privilege in chronic inflammatory diseases

The term "immune privilege" was coined to describe weak immunogenicity (hypo-immunity) that manifests in some transplant settings. We extended this concept to encompass hypo-immunity that manifests at local sites of inflammation relevant to clinical diseases. Here, we focus on emerging evidence that enhanced tryptophan catabolism is a key metabolic process that promotes and sustains induced immune privilege, and discuss the implications for exploiting this knowledge to improve treatments for hypo-immune and hyper-immune syndromes using strategies to manipulate tryptophan metabolism.

Innate immunity and resistance to tolerogenesis in allotransplantation

The development of immunosuppressive drugs to control adaptive immune responses has led to the success of transplantation as a therapy for end-stage organ failure. However, these agents are largely ineffective in suppressing components of the innate immune system. This distinction has gained in clinical significance as mounting evidence now indicates that innate immune responses play important roles in the acute and chronic rejection of whole organ allografts. For instance, whereas clinical interest in natural killer (NK) cells was once largely confined to the field of bone marrow transplantation, recent findings suggest that these cells can also participate in the acute rejection of cardiac allografts and prevent tolerance induction. Stimulation of Toll-like receptors (TLRs), another important component of innate immunity, by endogenous ligands released in response to ischemia/reperfusion is now known to cause an inflammatory milieu favorable to graft rejection and abrogation of tolerance. Emerging data suggest that activation of complement is linked to acute rejection and interferes with tolerance. In summary, the conventional wisdom that the innate immune system is of little importance in whole organ transplantation is no longer tenable. The addition of strategies that target TLRs, NK cells, complement, and other components of the innate immune system will be necessary to eventually achieve long-term tolerance to human allograft recipients.

Preconditioning donor liver with Nodosin perfusion lessens rat ischemia reperfusion injury via heme oxygenase-1 upregulation

BACKGROUND AND AIM

Ischemia reperfusion injury (IRI) remains a major cause of graft injury, dysfunction and even failure post-transplantation. Heme oxygenase 1 (HO-1) has been found to be an attractive target for anti-inflammatory therapies and a potential candidate responsible for cell injury. The objective of this study was to investigate whether preconditioning the donor liver with Nodosin perfusion upregulates HO-1 and then lessens IRI in rat models.

METHODS

Wistar rats were divided into four groups: experimental group, control group, positive control group and negative control group in which the donor liver was preconditioned with Nodosin, lactated ringer's solution, cobalt protoporphyrin and zinc protoporphyrin perfusion, respectively. We measured HO-1 expression and enzyme activity in rat livers of each group ex vivo at 0, 1 and 2 h after perfusion. At 1 h after perfusion, donor livers of Wistar rats were transplanted into Sprague-Dawley rats orthotopically. Serum transaminase levels, degree of cell apoptosis and Suzuki's score were used to assess ischemia/reperfusion injury in recipients at 24 h after transplantation.

RESULTS

Ex vivo, donor liver preconditioning with Nodosin perfusion induced HO-1 expression and enzyme activity significantly, compared with the control group (P < 0.05). In vivo, serum transaminase levels, cell apoptosis degree and Suzuki's score of representative recipients in the Nodosin group were lower than that in the control group (P < 0.05). Preconditioning with Nodosin perfusion induced HO-1 protein mainly in Kupffer cells.

CONCLUSIONS

This study suggests that preconditioning with Nodosin perfusion provides a potential protective effect through inducing HO-1 expression to attenuate ischemia/reperfusion injury in liver transplantation.

Contagious cancer: lessons from the devil and the dog

Cancer is generally defined as uncontrollable growth of cells caused by genetic aberrations and/or environmental factors. Yet contagious cancers also occur. The recent emergence of a contagious cancer in Tasmanian devils has reignited interest in transmissible cancers. Two naturally occurring transmissible cancers are known: devil facial tumour disease and canine transmissible venereal tumour. Both cancers evolved once and have then been transmitted from one individual to another as clonal cell lines. The dog cancer is ancient; having evolved more than 6,000 years ago, while the devil disease was first seen in 1996. In this review I will compare and contrast the two diseases focusing on the life histories of the clonal cell lines, their evolutionary trajectories and the mechanisms by which they have achieved immune tolerance. A greater understanding of these contagious cancers will provide unique insights into the role of the immune system in shaping tumour evolution and may uncover novel approaches for treating human cancer.

Tolerance and lymphoid organ structure and function

This issue of Frontiers in Immunologic Tolerance explores barriers to tolerance from a variety of views of cells, molecules, and processes of the immune system. Our laboratory has spent over a decade focused on the migration of the cells of the immune system, and dissecting the signals that determine how and where effector and suppressive regulatory T cells traffic from one site to another in order to reject or protect allografts. These studies have led us to a greater appreciation of the anatomic structure of the immune system, and the realization that the path taken by lymphocytes during the course of the immune response to implanted organs determines the final outcome. In particular, the structures, microanatomic domains, and the cells and molecules that lymphocytes encounter during their transit through blood, tissues, lymphatics, and secondary lymphoid organs are powerful determinants for whether tolerance is achieved. Thus, the understanding of complex cellular and molecular processes of tolerance will not come from “96-well plate immunology,” but from an integrated understanding of the temporal and spatial changes that occur during the response to the allograft. The study of the precise positioning and movement of cells in lymphoid organs has been difficult since it is hard to visualize cells within their three-dimensional setting; instead techniques have tended to be dominated by two-dimensional renderings, although advanced confocal and two-photon systems are changing this view. It is difficult to precisely modify key molecules and events in lymphoid organs, so that existing knockouts, transgenics, inhibitors, and activators have global and pleiotropic effects, rather than precise anatomically restricted influences. Lastly, there are no well-defined postal codes or tracking systems for leukocytes, so that while we can usually track cells from point A to point B, it is exponentially more difficult or even impossible to track them to point C and beyond. We believe this represents one of the fundamental barriers to understanding the immune system and devising therapeutic approaches that take into account anatomy and structure as major controlling principles of tolerance.