Second-set rejection of mouse liver allografts is dependent on radiation-sensitive nonparenchymal cells of graft bone marrow origin

Generation of Powerful Human Tolerogenic Dendritic Cells by Lentiviral-Mediated IL-10 Gene Transfer

The prominent role of dendritic cells (DC) in promoting tolerance and the development of methods to generate clinical grade products allowed the clinical application of tolerogenic DC (tolDC)-based therapies for controlling unwanted immune responses. We established an efficient method to generate tolerogenic human DC, producing supra-physiological levels of IL-10, by genetically engineering monocyte-derived DC with a bidirectional Lentiviral Vector (bdLV) encoding for IL-10 and a marker gene. DC^IL−10 are mature DC, modulate T cell responses, promote T regulatory cells, and are phenotypically and functionally stable upon stimulation. Adoptive transfer of human DC^IL−10 in a humanized mouse model dampens allogeneic T cell recall responses, while murine DC^IL−10 delays acute graft-vs.-host disease in mice. Our report outlines an efficient method to transduce human myeloid cells with large-size LV and shows that stable over-expression of IL-10 generates an effective cell product for future clinical applications in the contest of allogeneic transplantation.

Trained immunity in organ transplantation

Consistent induction of donor-specific unresponsiveness in the absence of continuous immunosuppressive therapy and toxic effects remains a difficult task in clinical organ transplantation. Transplant immunologists have developed numerous experimental treatments that target antigen-presentation (signal 1), costimulation (signal 2), and cytokine production (signal 3) to establish transplantation tolerance. While promising results have been obtained using therapeutic approaches that predominantly target the adaptive immune response, the long-term graft survival rates remain suboptimal. This suggests the existence of unrecognized allograft rejection mechanisms that contribute to organ failure. We postulate that trained immunity stimulatory pathways are critical to the immune response that mediates graft loss. Trained immunity is a recently discovered functional program of the innate immune system, which is characterized by nonpermanent epigenetic and metabolic reprogramming of macrophages. Since trained macrophages upregulate costimulatory molecules (signal 2) and produce pro-inflammatory cytokines (signal 3), they contribute to potent graft reactive immune responses and organ transplant rejection. In this review, we summarize the detrimental effects of trained immunity in the context of organ transplantation and describe pathways that induce macrophage training associated with graft rejection.

Role of myeloid regulatory cells (MRCs) in maintaining tissue homeostasis and promoting tolerance in autoimmunity, inflammatory disease and transplantation

Myeloid cells play a pivotal role in regulating innate and adaptive immune responses. In inflammation, autoimmunity, and after transplantation, myeloid cells have contrasting roles: on the one hand they initiate the immune response, promoting activation and expansion of effector T-cells, and on the other, they counter-regulate inflammation, maintain tissue homeostasis, and promote tolerance. The latter activities are mediated by several myeloid cells including polymorphonuclear neutrophils, macrophages, myeloid-derived suppressor cells, and dendritic cells. Since these cells have been associated with immune suppression and tolerance, they will be further referred to as myeloid regulatory cells (MRCs). In recent years, MRCs have emerged as a therapeutic target or have been regarded as a potential cellular therapeutic product for tolerance induction. However, several open questions must be addressed to enable the therapeutic application of MRCs including: how do they function at the site of inflammation, how to best target these cells to modulate their activities, and how to isolate or to generate pure populations for adoptive cell therapies. In this review, we will give an overview of the current knowledge on MRCs in inflammation, autoimmunity, and transplantation. We will discuss current strategies to target MRCs and to exploit their tolerogenic potential as a cell-based therapy.

PPARα is down-regulated following liver transplantation in mice

BACKGROUND & AIMS

Graft dysfunction is one of the major complications after liver transplantation, but its precise mechanism remains unclear. Since steatotic liver grafts are susceptible to post-transplant dysfunction, and peroxisome proliferator-activated receptor (PPAR) α plays an important role in the maintenance of hepatic lipid homeostasis, we examined the role of PPARα in liver transplantation.

METHODS

Livers were harvested from Sv/129 wild-type (Ppara(+/+)) mice and PPARα-null (Ppara(-/-)) mice and transplanted orthotopically into syngeneic Ppara(+/+) mice.

RESULTS

Hepatocellular damage was unexpectedly milder in transplanted Ppara(-/-) livers compared with Ppara(+/+) ones. This was likely due to decreased lipid peroxides in the Ppara(-/-) livers, as revealed by the lower levels of fatty acid oxidation (FAO) enzymes, which are major sources of reactive oxygen species. Hepatic PPARα and its target genes, such as FAO enzymes and pyruvate dehydrogenase kinase 4, were strongly down-regulated after transplantation, which was associated with increases in hepatic tumor necrosis factor-α expression and nuclear factor-κB activity. Inhibiting post-transplant PPARα down-regulation by clofibrate treatment markedly augmented oxidative stress and hepatocellular injury.

CONCLUSIONS

Down-regulation of PPARα seemed to be an adaptive response to metabolic alterations following liver transplantation. These results provide novel information to the understanding of the pathogenesis of early post-transplant events.

Steatotic liver transplantation in the mouse: a model of primary nonfunction

BACKGROUND

The number of potential donor organs deemed suboptimal for transplantation because of hepatic steatosis is rising as the obesity rate increases. However, no mouse transplant model has been described within the framework of hepatic steatosis. We describe the development of and our initial experience with a steatotic mouse orthotopic liver transplant model using the ob/ob mouse. This model is technically achievable and functionally mimics primary nonfunction.

MATERIALS AND METHODS

Adapting techniques of a nonarterialized murine transplant model, C57BL6 ob/ob mice aged 5-7 weeks (26-35 g) and lean controls served as liver donors and recipients. Orthotopic liver transplantation (OLT) was performed using a two-cuff technique at the infrahepatic cava and portal vein. The suprahepatic cava was anastomosed end to end, and the bile duct was stented. The hepatic artery was not reconstructed.

RESULTS

Lean-to-lean OLT was performed with 70% (n = 10) long-term survival. ob/ob-to-age-matched lean recipients had 0% (n = 10) survival because of size discrepancy. ob/ob livers were transplanted to size-matched lean recipients (>3 months old) with short-term survival of 30% (n = 10). These mice survived the operation, awakened, but expired within 24 h. Serum transaminases revealed a significantly higher injury profile in the recipients of the steatotic livers, and histology showed massive centrilobular coagulative necrosis with hemorrhage, the overall picture being that of primary nonfunction.

CONCLUSIONS

This novel use of the ob/ob mouse for OLT provides us with a model for steatotic transplantation with primary nonfunction as the end point and may help to better understand the response of the steatotic liver to the insult of transplantation.

Prevention and restoration of second-set liver allograft rejection in presensitized mice: the role of "passenger" leukocytes, donor major histocompatibility complex antigens, and host cytotoxic effector mechanisms

BACKGROUND

The aim was to determine whether sublethal donor total body irradiation (TBI) might be as effective as lethal TBI in preventing mouse second-set liver allograft rejection, and to evaluate the role of passenger leukocytes, donor major histocompatibility complex (MHC) antigens, and host effector mechanisms in the response to livers from sublethally irradiated donors.

METHODS

B10 (H2b) donors received various doses of TBI at different times before their livers were transplanted orthotopically into normal or donor skin-presensitized C3H (H2k) recipients. The influence of irradiation on graft non-parenchymal cells (NPC) was determined by monoclonal antibody staining, and flow cytometric analysis. Hematopoietic cells within the grafts were reconstituted by intravenous infusion of syngeneic or third-party bone marrow cells. Allograft survival was determined in recipients that received no treatment, or that were given spleen cells from either normal B10 donors, or MHC class I - or class II-deficient mice syngeneic with the donors. Cytotoxic activity of graft-infiltrating cells and host spleen cells, and complement-dependent cytotoxic alloantibody titers were determined by isotype release assays.

RESULTS

The protective effect of donor TBI was observed both at lethal (9.5 Gy) and sublethal doses (5 and 3 Gy; graft median survival time: >100 days). Extended delay in liver transplantation, allowing hematopoietic recovery and graft reconstitution eliminated the effect. Liver NPC were reduced about 80% within 24 hr of 3 Gy TBI, with a selective reduction in the incidence of B cells. The NPC-depleted livers underwent accelerated rejection when donor (but not third-party) spleen cells (5 x 10(7) were administered systemically to the recipient immediately after graft revascularization. Spleen cells from MHC class I-deficient (but not MHC class II-deficient) mice failed to fully restore accelerated rejection of TBI liver grafts. Freshly isolated graft NPC, or spleen cells from TBI liver recipients, harvested 4 days after transplantation, exhibited lower, donor-specific cytotoxic activity than cells from mice given normal livers. Recipients of TBI livers also showed much lower serum complement-dependent cytotoxic alloantibody titers.

CONCLUSIONS

By substantially depleting "passenger leukocytes," sublethal donor TBI undermines anti-donor cell-mediated and humoral immune reactivity and inhibits second-set liver allograft rejection in presensitized recipients. The interval between irradiation and transplantation is important in conferring resistance to rejection. Expression of MHC class I on donor leukocyte infusions is important for overcoming resistance to second-set rejection induced by donor irradiation.

Bone marrow chimerism and transplantation tolerance

Engraftment of allogeneic or xenogeneic pluripotent hematopoietic stem cells into nonmyeloablated but immunodepleted (preconditioned) recipients can produce a state of immunological tolerance to donor and host. Host and donor hematopoietic cells entering the thymus ensure deletion of both donor- and host-reactive thymocytes. Additional mechanisms are involved in tolerance induced in recipients that are not immunodepleted. Grafting of donor thymic tissue to thymectomized recipients is an alternative approach for inducing central T cell tolerance without the requirement for engraftment of donor hematopoietic stem cells. During the past year, advances have been made in understanding both the requirements for preconditioning and the mechanisms of tolerance induction in the above transplantation models.

Rejection of spontaneously accepted rat liver allografts with recipientinterleukin-2 treatment or donor irradiation

While MHC incompatible DA (RTl(a)) to Lewis (RT1(1), LEW) rat liver allografts are acutely rejected, the reciprocal LEW to DA liver grafts are spontaneously accepted. The mechanism of this acceptance remains unclear. We evaluated the effect of donor treatment with total body irradiation (TBI) or gadolinium chloride (GdCl3), and recipient treatment with exogenous IL-2 after transplantation on the survival of the spontaneously accepted liver grafts. Male LEW and DA rats were used as donors and recipients for orthotopic liver allo- or iso-graft transplants. The LEW liver donor was treated by TBI (10 gray) 7 days before transplantation, or LEW donor Kupffer cell phagocytosis was blocked with GdCl3 (7 mg/kg) on days -2 and -1 pretransplant. In an attempt to reverse LEW liver graft acceptance, 180,000 units human IL-2 (hIL-2) were administered daily IP to the DA liver recipients from days 1 to 7 after liver grafting. While untreated LEW recipients rejected DA liver grafts within 13 days, DA recipients accepted LEW livers indefinitely (>302 days). In contrast, irradiation of the LEW liver donor prevented the spontaneous acceptance by DA recipients, and resulted in acute rejection of the liver grafts in 9-20 days. However, spontaneous graft tolerance was restored by parking the irradiated LEW donor liver in naive LEW rats for 48 hr before retransplantation to DA recipients (>50 days). When LEW donors were treated with GdCl3, which is known to block Kupffer cell phagocytosis and antigen processing, the spontaneous acceptance of the LEW liver grafts by DA recipients was unaffected. However, when exogenous rhIL-2 was given daily, LEW liver allografts were rejected by the DA recipients. The resulting liver failure correlated with a progressive increase in serum bilirubin and the development of a predominantly lymphocytic portal tract infiltration, bile duct epithelial damage, and portal vein endothelitis, which is consistent with acute allograft rejection. LEW and DA recipients of liver isografts developed no toxicity and survived indefinitely (>100 days) when treated with the same dose of IL-2. These results indicate that spontaneous rat liver allograft acceptance is associated with the presence of radiosensitive cells in the donor liver that may interact with recipient T cells to inhibit (Th1) production of IL-2.

Clinical trials and projected future of liver xenotransplantation

The trial and error of the pioneering xenotransplant trials over the past three decades has defined the limitation of the species used. Success was tantalizingly close with the chimpanzee, baboon, and other primates. The use of more disparate species has been frustrated by the xenoantibody barrier. Future attempts at clinical xenotransplantation will be hampered by the consideration of the species of animals and the nature of the organs to be transplanted. On one hand, primate donors have the advantage of genetic similarity (and therefore potential compatibility) and less risk of immunologic loss. On the other hand, pig donors are more easily raised, are not sentient animals, and may be less likely to harbor transmissible disease. It is recognized that the success of xenotransplantation may very with different organs. Because it is relatively resistant to antibody-mediated rejection, the liver is the organ for which there is the greatest chance of long-term success. Consideration of using xenotransplants on a temporary basis, or as a "bridge" to permanent human transplantation, may allow clinical trials utilizing hearts or kidney xenografts. Issues on metabolic compatibility and infection risks cannot be accurately determined until routine success in clinical xenotransplantation occurs. Based on a limited experience, the conventional approaches to allotransplantation are unlikely to be successful in xenotransplantation. The avoidance of immediate xenograft destruction by hyperacute rejection, achieved using transgenic animals bearing human complement regulatory proteins or modulating the antigenic target on the donor organ, is the first step to successful xenotransplantation. The ability to achieve tolerance by establishing a state of bone marrow chimerism is the key to overcoming the long-term immunologic insults and avoiding the necessarily high doses of nonspecific immunosuppression that would otherwise be required and associated with a high risk of infections complications. Xenotransplantation faces criticism that is strongly reminiscent of that leveled against human-to-human transplantation during the late 1960s and early 1970s. Yet with persistence, the field of human-to-human transplantation has proved highly successful. This success was the result of a stepwise increase in our understanding of the biology of rejection, improvements in drug management, and experience. It is possible that xenotransplantation may not be universally successful until further technologic advances occur; yet cautions exploration of xenotransplantation appears warranted to identify those areas that require further study.