Evidence that apoptosis of activated T cells occurs in spontaneous tolerance of liver allografts and is blocked by manipulations which break tolerance

Transplant Tolerance, Not Only Clonal Deletion

The quest to understand how allogeneic transplanted tissue is not rejected and how tolerance is induced led to fundamental concepts in immunology. First, we review the research that led to the Clonal Deletion theory in the late 1950s that has since dominated the field of immunology and transplantation. At that time many basic mechanisms of immune response were unknown, including the role of lymphocytes and T cells in rejection. These original observations are reassessed by considering T regulatory cells that are produced by thymus of neonates to prevent autoimmunity. Second, we review "operational tolerance" induced in adult rodents and larger animals such as pigs. This can occur spontaneously especially with liver allografts, but also can develop after short courses of a variety of rejection inhibiting therapies. Over time these animals develop alloantigen specific tolerance to the graft but retain the capacity to reject third-party grafts. These animals have a "split tolerance" as peripheral lymphocytes from these animals respond to donor alloantigen in graft versus host assays and in mixed lymphocyte cultures, indicating there is no clonal deletion. Investigation of this phenomenon excludes many mechanisms, including anti-donor antibody blocking rejection as well as anti-idiotypic responses mediated by antibody or T cells. This split tolerance is transferred to a second immune-depleted host by T cells that retain the capacity to effect rejection of third-party grafts by the same host. Third, we review research on alloantigen specific inhibitory T cells that led to the first identification of the CD4+CD25+T regulatory cell. The key role of T cell derived cytokines, other than IL-2, in promoting survival and expansion of antigen specific T regulatory cells that mediate transplant tolerance is reviewed. The precise methods for inducing and diagnosing operational tolerance remain to be defined, but antigen specific T regulatory cells are key mediators.

Understanding, predicting and achieving liver transplant tolerance: from bench to bedside

In the past 40 years, liver transplantation has evolved from a high-risk procedure to one that offers high success rates for reversal of liver dysfunction and excellent patient and graft survival. The liver is the most tolerogenic of transplanted organs; indeed, immunosuppressive therapy can be completely withdrawn without rejection of the graft in carefully selected, stable long-term liver recipients. However, in other recipients, chronic allograft injury, late graft failure and the adverse effects of anti-rejection therapy remain important obstacles to improved success. The liver has a unique composition of parenchymal and immune cells that regulate innate and adaptive immunity and that can promote antigen-specific tolerance. Although the mechanisms underlying liver transplant tolerance are not well understood, important insights have been gained into how the local microenvironment, hepatic immune cells and specific molecular pathways can promote donor-specific tolerance. These insights provide a basis for the identification of potential clinical biomarkers that might correlate with tolerance or rejection and for the development of novel therapeutic targets. Innovative approaches aimed at promoting immunosuppressive drug minimization or withdrawal include the adoptive transfer of donor-derived or recipient-derived regulatory immune cells to promote liver transplant tolerance. In this Review, we summarize and discuss these developments and their implications for liver transplantation.

The Immunological Basis of Liver Allograft Rejection

Liver allograft rejection remains a significant cause of morbidity and graft failure in liver transplant recipients. Rejection is caused by the recognition of non-self donor alloantigens by recipient T-cells. Antigen recognition results in proliferation and activation of T-cells in lymphoid tissue before migration to the allograft. Activated T-cells have a variety of effector mechanisms including direct T-cell mediated damage to bile ducts, endothelium and hepatocytes and indirect effects through cytokine production and recruitment of tissue-destructive inflammatory cells. These effects explain the histological appearances of typical acute T-cell mediated rejection. In addition, donor specific antibodies, most typically against HLA antigens, may give rise to antibody-mediated rejection causing damage to the allograft primarily through endothelial injury. However, as an immune-privileged site there are several mechanisms in the liver capable of overcoming rejection and promoting tolerance to the graft, particularly in the context of recruitment of regulatory T-cells and promotors of an immunosuppressive environment. Indeed, around 20% of transplant recipients can be successfully weaned from immunosuppression. Hence, the host immunological response to the liver allograft is best regarded as a balance between rejection-promoting and tolerance-promoting factors. Understanding this balance provides insight into potential mechanisms for novel anti-rejection therapies.

The Role of Diverse Liver Cells in Liver Transplantation Tolerance

Liver transplantation is the ideal treatment approach for a variety of end-stage liver diseases. However, life-long, systemic immunosuppressive treatment after transplantation is required to prevent rejection and graft loss, which is associated with severe side effects, although liver allograft is considered more tolerogenic. Therefore, understanding the mechanism underlying the unique immunologically privileged liver organ is valuable for transplantation management and autoimmune disease treatment. The unique hepatic acinus anatomy and a complex cellular network constitute the immunosuppressive hepatic microenvironment, which are responsible for the tolerogenic properties of the liver. The hepatic microenvironment contains a variety of hepatic-resident immobile non-professional antigen-presenting cells, including hepatocytes, liver sinusoidal endothelial cells, Kupffer cells, and hepatic stellate cells, that are insufficient to optimally prime T cells locally and lead to the removal of alloreactive T cells due to the low expression of major histocompatibility complex (MHC) molecules, costimulatory molecules and proinflammatory cytokines but a rather high expression of coinhibitory molecules and anti-inflammatory cytokines. Hepatic dendritic cells (DCs) are generally immature and less immunogenic than splenic DCs and are also ineffective in priming naïve allogeneic T cells via the direct recognition pathway in recipient secondary lymphoid organs. Although natural killer cells and natural killer T cells are reportedly associated with liver tolerance, their roles in liver transplantation are multifaceted and need to be further clarified. Under these circumstances, T cells are prone to clonal deletion, clonal anergy and exhaustion, eventually leading to tolerance. Other proposed liver tolerance mechanisms, such as soluble donor MHC class I molecules, passenger leukocytes theory and a high-load antigen effect, have also been addressed. We herein comprehensively review the current evidence implicating the tolerogenic properties of diverse liver cells in liver transplantation tolerance.

Transplant Tolerance Induction: Insights From the Liver

A comparison of pre-clinical transplant models and of solid organs transplanted in routine clinical practice demonstrates that the liver is most amenable to the development of immunological tolerance. This phenomenon arises in the absence of stringent conditioning regimens that accompany published tolerizing protocols for other organs, particularly the kidney. The unique immunologic properties of the liver have assisted our understanding of the alloimmune response and how it can be manipulated to improve graft function and survival. This review will address important findings following liver transplantation in both animals and humans, and how these have driven the understanding and development of therapeutic immunosuppressive options. We will discuss the liver's unique system of immune and non-immune cells that regulate immunity, yet maintain effective responses to pathogens, as well as mechanisms of liver transplant tolerance in pre-clinical models and humans, including current immunosuppressive drug withdrawal trials and biomarkers of tolerance. In addition, we will address innovative therapeutic strategies, including mesenchymal stem cell, regulatory T cell, and regulatory dendritic cell therapy to promote liver allograft tolerance or minimization of immunosuppression in the clinic.

Role of programmed death ligand 1 and Kupffer cell in immune regulation after orthotopic liver transplantation in rats

INTRODUCTION

Role of programmed death ligand 1 (PD-L1) and Kupffer cells (KCs) in liver transplantation immune regulation was unclear.

METHODS

Lewis and Brown-Norway (BN) rats were assigned to LEW-BN group (Lewis-to-BN liver transplantation) and BN-BN group (BN-to-BN). Receipts were sacrificed for histology and assessment of cytokines and PD-L1 production. Effect of PD-L1 and KCs on T cells (TCs) was monitored by co-culture of ³H-Thymidine TCs. KCs transfected with PD-L1-shRNA interference plasmids were co-cultured with TCs, PD-L1 expression and cytokines production were measured respectively.

RESULTS

Recipients in BN-BN group survived a long time while acute rejection was found in LEW-BN group. ELISA showed plasma levels of IL-2, IFN-γ and TNF-α in BN-BN group were significantly lower and levels of IL-10 were significantly higher than that in LEW-BN group on day 7 after transplantation (P<0.05). PD-L1 expression of KCs in BN-BN group was significantly higher than that in the LEW-BN group (P<0.05). Proliferation rate of TCs in KCs+TCs group was significantly lower and its apoptosis rate was significantly higher than that in TCs group (P<0.05). IL-2, TNF-α and INF-γ levels were remarkably higher and IL-10 levels were lower in KCs+TCs group than that in TCs group (P<0.05). Levels of IL-2, IFN-γ and TNF-α in transfection group were significantly higher and that of IL-10 was notably lower than that in the un-transfected group (P<0.05).

CONCLUSION

KCs with high expression of PD-L1 could significantly suppress the proliferation and function of TCs. Silencing the expression of PD-L1 in KCs in vivo could restore the function of TCs.

Can immunosuppression be stopped after liver transplantation?

Liver transplantation has improved dramatically over the past three decades, mainly as a result of advances in surgical techniques and management of post-transplant complications. The focus has now turned towards rescuing additional organs in the face of scarce organ supply, or prevention of long-term toxicity associated with immunosuppression. The liver appears to be privileged in terms of immune tolerance, with a low incidence of antibody-mediated rejection, which is in sharp contrast to other solid organ transplants, such as kidney, lung, and heart transplants. However, tolerogenic processes remain poorly understood, and strategies for complete drug withdrawal should be selected carefully to avoid graft rejection. In this Review, we summarise the current understanding of liver-specific immune responses and provide an outlook on future approaches.

The CD8 T-cell response during tolerance induction in liver transplantation

Both experimental and clinical studies have shown that the liver possesses unique tolerogenic properties. Liver allografts can be spontaneously accepted across complete major histocompatibility mismatch in some animal models. In addition, some liver transplant patients can be successfully withdrawn from immunosuppressive medications, developing ‘operational tolerance'. Multiple mechanisms have been shown to be involved in inducing and maintaining alloimmune tolerance associated with liver transplantation. Here, we focus on CD8 T-cell tolerance in this setting. We first discuss how alloreactive cytotoxic T-cell responses are generated against allografts, before reviewing how the liver parenchyma, donor passenger leucocytes and the host immune system function together to attenuate alloreactive CD8 T-cell responses to promote the long-term survival of liver transplants.

Functional Immune Anatomy of the Liver-As an Allograft

The liver is an immunoregulatory organ in which a tolerogenic microenvironment mitigates the relative "strength" of local immune responses. Paradoxically, necro-inflammatory diseases create the need for most liver transplants. Treatment of hepatitis B virus, hepatitis C virus, and acute T cell-mediated rejection have redirected focus on long-term allograft structural integrity. Understanding of insults should enable decades of morbidity-free survival after liver replacement because of these tolerogenic properties. Studies of long-term survivors show low-grade chronic inflammatory, fibrotic, and microvascular lesions, likely related to some combination of environment insults (i.e. abnormal physiology), donor-specific antibodies, and T cell-mediated immunity. The resultant conundrum is familiar in transplantation: adequate immunosuppression produces chronic toxicities, while lightened immunosuppression leads to sensitization, immunological injury, and structural deterioration. The "balance" is more favorable for liver than other solid organ allografts. This occurs because of unique hepatic immune physiology and provides unintended benefits for allografts by modulating various afferent and efferent limbs of allogenic immune responses. This review is intended to provide a better understanding of liver immune microanatomy and physiology and thereby (a) the potential structural consequences of low-level, including allo-antibody-mediated injury; and (b) how liver allografts modulate immune reactions. Special attention is given to the microvasculature and hepatic mononuclear phagocytic system.

From immunosuppression to tolerance

The past three decades have seen liver transplantation becoming a major therapeutic approach in the management of end-stage liver diseases. This is due to the dramatic improvement in survival after liver transplantation as a consequence of the improvement of surgical and anaesthetic techniques, of post-transplant medico-surgical management and of prevention of disease recurrence and other post-transplant complications. Improved use of post-transplant immunosuppression to prevent acute and chronic rejection is a major factor in these improved results. The liver has been shown to be more tolerogenic than other organs, and matching of donor and recipients is mainly limited to ABO blood group compatibility. However, long-term immunosuppression is required to avoid severe acute and chronic rejection and graft loss. With the current immunosuppression protocols, the risk of acute rejection requiring additional therapy is 10-40% and the risk of chronic rejection is below 5%. However, the development of histological lesions in the graft in long-term survivors suggest atypical forms of graft rejection may develop as a consequence of under-immunosuppression. The backbone of immunosuppression remains calcineurin inhibitors (CNI) mostly in association with steroids in the short-term and mycophenolate mofetil or mTOR inhibitors (everolimus). The occurrence of post-transplant complications related to the immunosuppressive therapy has led to the development of new protocols aimed at protecting renal function and preventing the development of de novo cancer and of dysmetabolic syndrome. However, there is no new class of immunosuppressive drugs in the pipeline able to replace current protocols in the near future. The aim of a full immune tolerance of the graft is rarely achieved since only 20% of selected patients can be weaned successfully off immunosuppression. In the future, immunosuppression will probably be more case oriented aiming to protect the graft from rejection and at reducing the risk of disease recurrence and complications related to immunosuppressive therapy. Such approaches will include strategies aiming to promote stable long-term immunological tolerance of the liver graft.

CD39 deficiency in murine liver allografts promotes inflammatory injury and immune-mediated rejection

Adenosine triphosphate (ATP), an essential metabolic energy source, is released following cell apoptosis or necrosis. It acts as a damage-associated molecule pattern to stimulate innate immune cells. The ectonucleotidase CD39 regulates immune activation by hydrolysis of extracellular ATP. We have shown previously that CD39 expression by donor livers helps protect syngeneic grafts with extended (24 hr) cold preservation time from ischemia reperfusion injury. Given its immune regulatory properties, we hypothesized that CD39 expression in donor livers might modulate transplant tolerance that occurs following mouse allogeneic liver transplantation (LTx). Livers from C57BL/6 (B6) wild-type (WT) or CD39 KO mice were transplanted into normal C3H recipients with minimal (approximately 1 hr) cold ischemia. Serum alanine aminotransferase levels at day 4 post LTx were significantly higher in animals given CD39KO compared with WT livers. Moreover, IFN-γ production by liver-infiltrating CD8⁺ T cells at day 4 was significantly higher in CD39KO than in WT grafts. Furthermore, splenic T cells from CD39KO liver recipients exhibited greater proliferative responses to donor alloantigens than those from mice given WT grafts. By contrast, there was a concomitant significant reduction in the frequency of regulatory T cells (Treg) in CD39KO than in WT livers. Whereas WT liver allografts survived > 100 days, no CD39KO grafts survived beyond 40 days (median survival time [MST]: WT: >100 days vs CD39KO: 8 days; p<0.01). In addition, soluble CD39 administration significantly prolonged CD39KO liver allograft survival (MST: 27.5 days). These novel data suggest that CD39 expression in liver allografts modulates tissue injury, inflammation, anti-donor effector T cell responses and Treg infiltration and can suppress transplant rejection.

Differential migration of passenger leukocytes and rapid deletion of naive alloreactive CD8 T cells after mouse liver transplantation

Donor passenger leukocytes (PLs) from transplanted livers migrate to recipient lymphoid tissues, where they are thought to induce the deletion of donor-specific T cells and tolerance. Difficulties in tracking alloreactive T cells and PLs in rats and in performing this complex surgery in mice have limited progress in identifying the contribution of PL subsets and sites and the kinetics of T cell deletion. Here we developed a mouse liver transplant model in which PLs, recipient cells, and a reporter population of transgenic CD8 T cells specific for the graft could be easily distinguished and quantified in allografts and recipient organs by flow cytometry. All PL subsets circulated rapidly via the blood as soon as 1.5 hours after transplantation. By 24 hours, PLs were distributed differently in the lymph nodes and spleen, whereas donor natural killer and natural killer T cells remained in the liver and blood. Reporter T cells were activated in both liver and lymphoid tissues, but their numbers dramatically decreased within the first 48 hours. These results provide the first unequivocal demonstration of the differential recirculation of liver PL subsets after transplantation, and show that alloreactive CD8 T cells are deleted more rapidly than initially reported. This model will be useful for dissecting early events leading to the spontaneous acceptance of liver transplants.

Splenectomy increases the survival time of heart allograft via developing immune tolerance

Background

The spleen is an active lymphoid organ. The effect of splenectomy on the immune response remains unclear. This study investigated whether splenectomy can induce immune tolerance and has a beneficial role in cardiac allograft.

Methods

Wistar rats were used for heart donors. The Sprague–Dawley (SD) rats designated as the recipients of heart transplantation (HT) were randomly assigned into four groups: sham, splenectomy, HT, splenectomy + HT. The survival of transplanted hearts was assessed by daily checking of abdominal palpation. At various time points after transplantation, the transplanted hearts were collected and histologically examined; the level of CD₄⁺CD₂₅⁺ T regulatory lymphocytes (Tregs) and rate of lymphocyte apoptosis (annexin-v⁺ PI⁺ cells) in the blood were analyzed by using flow cytometric method.

Results

1) Splenectomy significantly prolonged the mean survival time of heart allografts (7 ± 1.1 days and 27 ± 1.5 days for HT and splenectomy + HT, respectively; n = 12-14/group, HT vs. splenectomy + HT, p < 0.001); 2) Splenectomy delayed pathological changes (inflammatory cell infiltration, myocardial damage) of the transplanted hearts in splenectomy + HT rats; 3) The level of CD₄⁺CD₂₅⁺ Tregs in the blood of splenectomized rats was significantly increased within 7 days (2.4 ± 0.5%, 4.9 ± 1.3% and 5.3 ± 1.0% for sham, splenectomy and splenectomy + HT, respectively; n = 15/group, sham vs. splenectomy or splenectomy + HT, p < 0.05) after splenectomy surgery and gradually decreased to baseline level; 4) Splenectomy increased the rate of lymphocyte apoptosis (day 7: 0.3 ± 0.05%, 3.9 ± 0.9% and 4.1 ± 0.9% for sham, splenectomy and splenectomy + HT, respectively; n = 15/group, sham vs. splenectomy or splenectomy + HT, p < 0.05) in a pattern similar to the change of the CD₄⁺CD₂₅⁺ Tregs in the blood.

Conclusions

Splenectomy inhibits the development of pathology and prolongs the survival time of cardiac allograft. The responsible mechanism is associated with induction of immune tolerance via elevating CD₄⁺CD₂₅⁺ Tregs and increasing lymphocyte apoptosis.

Galectin-1 prolongs survival of mouse liver allografts from Flt3L-pretreated donors

Liver allografts are spontaneously accepted across MHC barriers in mice. The mechanisms underlying this phenomenon remain poorly understood. Galectin-1, an endogenous lectin expressed in lymphoid organs, plays a vital role in maintaining central and peripheral tolerance. This study was to investigate the role of galectin-1 in spontaneous tolerance of liver allografts in mice, and to evaluate the therapeutic effects of galectin-1 on liver allograft rejection induced by donor Flt3L pretreatment. Blockade of the galectin-1 pathway via neutralizing antigalectin-1 mAb did not affect survival of the liver allografts from B6 donors into C3H recipients. Administration of rGal-1 significantly prolonged survival of liver allografts from Flt3L-pretreated donors and ameliorated Flt3L-triggered liver allograft rejection. This effect was associated with increased apoptosis of T cells in both allografts and spleens, decreased frequencies of Th1 and Th17 cells, decreased expression of Th1-associated cytokines (IL-12, IL-2 and IFN-γ), Th17-associated cytokines (IL-23 and IL-17) and granzyme B, in parallel with selectively increased IL-10 expression in liver allografts. In vitro, galectin-1 inhibited Flt3L-differentiated DC-mediated proliferation of allo-CD4(+) T cells and production of IFN-γ and IL-17. These data provide new evidence of the potential regulatory effects of galectin-1 in alloimmune responses in a murine model of liver transplantation.

Two immunogenic passenger dendritic cell subsets in the rat liver have distinct trafficking patterns and radiosensitivities

The aim of this study was to investigate the trafficking patterns, radiation sensitivities, and functions of conventional dendritic cell (DC) subsets in the rat liver in an allotransplantation setting. We examined DCs in the liver, hepatic lymph, and graft tissues and recipient secondary lymphoid organs after liver transplantation from rats treated or untreated by sublethal irradiation. We identified two distinct immunogenic DC subsets. One was a previously reported population that underwent blood-borne migration to the recipient's secondary lymphoid organs, inducing systemic CD8(+) T-cell responses; these DCs are a radiosensitive class II major histocompatibility complex (MHCII)(+) CD103(+) CD172a(+) CD11b(-) CD86(+) subset. Another was a relatively radioresistant MHCII(+) CD103(+) CD172a(+) CD11b(+) CD86(+) subset that steadily appeared in the hepatic lymph. After transplantation, the second subset migrated to the parathymic lymph nodes (LNs), regional peritoneal cavity nodes, or persisted in the graft. Irradiation completely eliminated the migration and immunogenicity of the first subset, but only partly suppressed the migration of the second subset and the CD8(+) T-cell response in the parathymic LNs. The grafts were acutely rejected, and intragraft CD8(+) T-cell and FoxP3(+) regulatory T-cell responses were unchanged. The radioresistant second subset up-regulated CD25 and had high allostimulating activity in the mixed leukocyte reaction, suggesting that this subset induced CD8(+) T-cell responses in the parathymic LNs and in the graft by the direct allorecognition pathway, leading to the rejection.

CONCLUSION

Conventional rat liver DCs contain at least two distinct immunogenic passenger subsets: a radiosensitive blood-borne migrant and a relatively radioresistant lymph-borne migrant. LNs draining the peritoneal cavity should be recognized as a major site of the intrahost T-cell response by the lymph-borne migrant. This study provides key insights into liver graft rejection and highlights the clinical implications of immunogenic DC subsets.

Expression of common gamma chain signalling cytokines and their receptors distinguishes rejection from tolerance in a rat organ transplant model

BACKGROUND

Signalling through the cytokine common γ chain (γc) is crucial for survival of activated T cells. In its absence, severe combined immunodeficiency ensues and transplanted tissues are not rejected.

METHODS

To determine whether differences in the availability of γc signalling cytokines correlate with rejection or acceptance, we examined expression of all γc signalling components in organs transplanted between PVG donors and DA recipients. In this combination hearts or kidneys are rejected in <10 days while livers survive >100 days. Expression of the γc cytokines IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 and their receptors γc, IL-2Rα, IL-2Rβ/IL-15Rβ, IL-4Rα, IL-7Rα, IL-9Rα, IL-15Rα and IL-21Rα was determined by real-time PCR pre-transplant and on days 3, 5 and 7 after transplantation.

RESULTS

Most increased after transplantation, although there were significantly lower levels of IL-2, IL-2Rα, IL-4 and IL-15Rα in tolerant livers compared to rejecting hearts or kidneys. IL-9 was only expressed in normal kidneys and decreased during rejection. IL-15 was constitutively expressed and did not change after transplantation. IL-21 and IL-21R increased in all transplanted organs to a similar extent. IL-7Rα in liver was considerably increased compared with heart or kidney, consistent with its known inverse relationship to global levels of γc signalling.

CONCLUSIONS

In transplanted livers, acceptance is associated with low levels of all γc cytokines or receptors except IL-21. This is consistent with "dilution" of γc cytokines from a finite clone size of alloreactive T cells in livers, which are ten times larger than kidneys or hearts.

Immunosuppression induction with rabbit anti-thymocyte globulin with or without rituximab in 1000 liver transplant patients with long-term follow-up

Rabbit anti-thymocyte globulin (rATG)-based immunosuppression induction is being increasingly used in liver transplantation (LT) in conjunction with steroid-free protocols to delay the initiation of calcineurin inhibitors. This study reports a single-center comparison of transplant outcomes and complications in 3 immunosuppression eras. Data were obtained retrospectively from a center research database, and the analysis included LT patients from 2001 to 2008. The immunosuppression consisted of rATG induction in 3 doses (6 mg/kg in all): (1) the first dose was administered perioperatively [the rabbit anti-thymocyte globulin in the operating room (rATG-OR) era]; (2) the first dose was delayed until 48 hours after transplantation [the rabbit anti-thymocyte globulin after a delay (rATG-D) era]; or (3) the first dose was delayed until 48 hours after transplantation, and a single dose of rituximab was added 72 hours after transplantation [the rabbit anti-thymocyte globulin after a delay plus rituximab (rATG-D-Ritux) era]. The initial maintenance immunosuppression was tacrolimus monotherapy, which was started on postoperative day 2. There were 166 patients (16%) in the rATG-OR era, 259 patients (26%) in the rATG-D era, and 588 patients (58%) in the rATG-D-Ritux era (1013 patients in all). Demographically, the latter eras were characterized by higher recipient and donor ages; greater percentages of liver-kidney transplants, hepatocellular carcinoma (HCC), donation after cardiac death (DCD), and imported organs; and shorter graft ischemia times. There were no significant differences between the 3 immunosuppression groups in unadjusted patient survival 3 and 5 years after transplantation (80% and 75% for the rATG-OR era, 75% and 67% for the rATG-D era, and 79% and 71% for the rATG-D-Ritux era, P = 0.15). The 5-year survival rates for patients with hepatitis C virus (HCV) and HCC were 65% and 68%, respectively. The factors included in the Cox regression model for patient death included the Model for End-Stage Liver Disease score [hazard ratio (HR) = 1.03, P = 0.001], HCV (HR = 1.28, P = 0.04), donor age (HR = 1.01, P = 0.001), recipient age (HR = 1.01, P = 0.05), and DCD (HR = 1.55, P = 0.11). rATG-based induction immunosuppression can be safely used in adult LT recipients with excellent survival and low rejection rates and without increases in immunosuppression-related side effects.

High expression of Fas ligand on cord blood dendritic cells: a possible immunoregulatory mechanism after cord blood transplantation

BACKGROUND

Allogeneic cord blood transplantation is associated with less severe graft-versus-host disease (GVHD). Dendritic cells (DCs), as the most potent antigen-presenting cells of the immune system, play a central role in the development of GVHD. Because apoptosis induction is one of the known mechanisms that DCs use to regulate T-cell responses, we studied the immunostimulatory and apoptosis induction capacities of cord blood dendritic cells (CBDCs) and peripheral blood dendritic cells (PBDCs) to evaluate the mechanisms underlying the lower incidence of GVHD after cord blood transplantation. Presence of apoptosis-related markers Fas, Fas ligand (FasL), and CD40 and costimulatory molecules, along with the proportion of myeloid and lymphoid DCs subsets, were also measured on CBDCs and PBDCs.

METHODS

Fresh CBDCs and PBDCs were isolated from cord and peripheral mononuclear cells as lineage-negative cells by using monoclonal antibodies against CD3, CD11b, CD14, CD16, CD19, CD56, CD34, and CD66b. DCs were cocultured with allogeneic T cells, and the effect of CBDCs and PBDCs on T-cell apoptosis and proliferation were determined through flow cytometric analysis and 3H-thymidine incorporation.

RESULTS

Our findings showed that CBDCs markedly augment apoptosis of CD3+ T-cells. FasL expression on CBDCs was significantly higher than on PBDCs. However, there was no difference between Fas expression on CBDCs and PBDCs. Moreover, CBDCs were poor stimulators of allogenic T cells in mixed leukocyte reaction compared with adult peripheral blood DCs. They also displayed decreased expression of HLA-DR and CD86 molecules. The ratio of lymphoid DCs (CD11c-, CD123+) to myeloid DCs (CD11c+, CD123-) was also significantly higher in CBDCs compared with PBDCs.

CONCLUSIONS

It seems that less severe GVHD after cord blood transplantation is due not only to a higher degree of immaturity of CBDCs, but also to delivery of apoptotic signals to the host T cells that recognize allo-MHC molecules on CBDCs in the early phase of immune response.

Tolerance in liver transplantation

Operational tolerance (OT) in liver transplant patients occurs much more frequently than OT of other transplanted organs; however the rate of OT varies considerably with the centre and patient population. Rates of OT range from 15% of the total liver transplant (LTX) patient population down to less than 5%. This review examines the reports of liver OT and compares the factors that could contribute to this variation. Multiple factors were examined, including the time from transplantation when weaning of immunosuppression (IS) was commenced, the rapidity of weaning, the contribution of maintenance and induction IS and the patient population transplanted. The approaches that might be used to increase the likelihood of OT are discussed and the approaches to monitoring OT in LTX patients are reviewed.

Spontaneous acceptance of mouse kidney allografts is associated with increased Foxp3 expression and differences in the B and T cell compartments

Spontaneous acceptance of organ allografts can identify novel mechanisms of drug-free transplantation tolerance. Spontaneous acceptance occurs in both mouse kidney transplants and rat liver transplants however the early immune processes of mouse kidney acceptance have not been studied. Acceptance of C57BL/6 strain kidney allografts in fully MHC-incompatible B10.BR recipients was compared with rejection (REJ) of heart allografts in the same strain combination. Graft infiltrate and antibody deposition were examined by immunohistochemical staining. Expression of mRNA was measured by quantitative real-time PCR. Apoptosis was examined by TUNEL staining. The majority of kidney allografts were accepted long-term and induced tolerance (TOL) of donor-strain skin grafts, showing that acceptance was not due to immune ignorance. There was an extensive infiltrate of T cells in the TOL kidney that exceeded the level in REJ hearts but subsequently declined. The main differences were deposition of IgG2a antibody in REJ that was absent in TOL, more B cells infiltrating TOL kidneys and a progressive increase in the ratio of CD8:CD4 cells during rejection. There was also significantly greater Foxp3 mRNA expression in TOL. Kidneys from RAG-/- donors were accepted, showing that donor lymphocytes were not necessary for acceptance. Neutralising antibodies to TGF-β administered from day 0 to day 6 did not prevent TOL. On the basis of cytokine expression and apoptosis there was no evidence for immune deviation or deletion as mechanisms of acceptance. In accord with the findings of spontaneous acceptance of liver allografts in rats, the main difference between mouse kidney TOL and heart REJ was in the B cell compartment. The major difference to rat liver allograft acceptance was that apoptosis of infiltrate did not appear to play a role. Instead, increased Foxp3 expression in TOL kidneys implies that regulatory T cells might be important.

Liver grafts contain a unique subset of natural killer cells that are transferred into the recipient after liver transplantation

In contrast to other solid organ transplantations, liver grafts have tolerogenic properties. Animal models indicate that donor leukocytes transferred into the recipient after liver transplantation (LTX) play a relevant role in this tolerogenic phenomenon. However, the specific donor cell types involved in modulation of the recipient alloresponse are not yet defined. We hypothesized that this unique property of liver grafts may be related to their high content of organ-specific natural killer (NK) and CD56(+) T cells. Here, we show that a high proportion of hepatic NK cells that detach from human liver grafts during pretransplant perfusion belong to the CD56bright subset, and are in an activated state (CD69(+)). Liver NK cells contained perforin and granzymes, exerted stronger cytotoxicity against K562 target cells when compared with blood NK cells, and secreted interferon-gamma, but no interleukin-10 or T helper 2 cytokines, upon stimulation with monokines. Interestingly, whereas the CD56bright subset is classically considered as noncytolytic, liver CD56bright NK cells showed a high content of cytolytic molecules and degranulated in response to K562 cells. After LTX, but not after renal transplantation, significant numbers of donor CD56dim NK and CD56(+) T cells were detected in the recipient circulation for approximately 2 weeks. In conclusion, during clinical LTX, activated and highly cytotoxic NK cells of donor origin are transferred into the recipient, and a subset of them mixes with the recirculating recipient NK cell pool. The unique properties of the transferred hepatic NK cells may enable them to play a role in regulating the immunological response of the recipient against the graft and therefore contribute to liver tolerogenicity.

Induced tolerance to rat liver allografts involves the apoptosis of intragraft T cells and the generation of CD4(+)CD25(+)FoxP3(+) T regulatory cells

Posttransplant total lymphoid irradiation is a nonmyeloablative regimen that has been extensively studied in rodent models for the induction of tolerance to bone marrow and solid organ allografts. Previous studies of experimental models and clinical transplantation have used total lymphoid irradiation in combination with anti-lymphocyte-depleting reagents and donor cell infusion to promote graft acceptance. In a rat model of orthotopic liver transplantation, we demonstrated that total lymphoid irradiation alone induced long-term graft survival. Apoptotic T cells were detected in markedly higher numbers in the livers of the total lymphoid irradiation-treated group in comparison with the control group of liver allograft recipients. Intragraft CD4(+)CD25(+)FoxP3(+) cells were increased in the total lymphoid irradiation group in the first week post-transplant and remained elevated in the graft and in the spleen. Importantly, the adoptive transfer of splenocytes from recipients that received posttransplant total lymphoid irradiation prolonged the survival of donor heart grafts, but not third-party heart grafts, whereas the depletion of CD4(+)CD25(+) cells from transferred splenocytes abrogated this prolongation. We conclude that posttransplant total lymphoid irradiation significantly increases the apoptosis of T cells in the liver graft and allows the accumulation of CD4(+)CD25(+)FoxP3(+) T regulatory cells, which facilitate the generation of donor-specific tolerance.

Donor IL-4-treatment induces alternatively activated liver macrophages and IDO-expressing NK cells and promotes rat liver allograft acceptance

Most approaches to transplant tolerance involve treatment of the recipient to prevent rejection. This study investigates donor treatment with IL-4 for its effect on subsequent rat liver allograft survival. Rat orthotopic liver transplants were performed in rejecting (PVG donor to Lewis recipient) or spontaneously tolerant (PVG to DA) strain combinations. Donors were untreated or injected intraperitoneally with IL-4 (30,000U/day) for 5days. Tissue infiltrates and gene expression were examined by immunohistochemistry and real-time quantitative PCR. IL-4 induced a marked leukocyte infiltrate in donor livers prior to transplant. Macrophages comprised the major population, although B cells, T cells and natural killer (NK) cells also increased. IL-4-induced liver macrophages had an alternatively activated phenotype with increased expression of mannose receptor but not inducible nitric oxide synthase (NOS2). IL-4 also induced IDO and IFN-gamma expression by NK cells. Donor IL-4-treatment converted rejection to acceptance in the majority of Lewis recipients (median survival time >96days) and did not prevent acceptance in DA recipients. Acceptance in Lewis recipients was associated with increased donor cell migration to recipient spleens and increased splenic IL-2, IFN-gamma and IDO expression 24h after transplantation. Donor IL-4-treatment increased leukocytes in the donor liver including potentially immunosuppressive populations of alternatively activated macrophages and IDO-expressing NK cells. Donor treatment led to long-term acceptance of most livers in association with early immune activation in recipient lymphoid tissues.

CpG oligodeoxynucleotide triggers the liver inflammatory reaction and abrogates spontaneous tolerance

Liver allografts are spontaneously accepted in the liver transplantation mouse model; however, the basis for this tolerance and the conditions that abrogate spontaneous tolerance to liver allografts are incompletely understood. We examined the role of CpG oligodeoxynucleotide (ODN) in triggering the liver inflammatory reaction and allograft rejection. Bioluminescence imaging quantified the activation of nuclear transcriptional factor kappaB (NF-kappaB) at different time points post-transplantation. Intrahepatic lymphocyte subsets were analyzed by immunofluorescence assay and flow cytometry. The results showed that liver allografts survived for more than 100 days without a requirement for any immunosuppressive therapy. Donor-matched cardiac allografts were permanently accepted, whereas third-party cardiac grafts were rejected with delayed kinetics; this confirmed donor-specific tolerance. NF-kappaB activation in the liver allografts was transiently increased on day 1 and diminished by day 4; in comparison, it was elevated up to 10 days post-transplantation in the cardiac allografts. When CpG ODN was administered at a high dose (50 microg per mouse x 1) to the recipients on day 7 post-transplantation, it induced an acute liver inflammatory reaction with elevated NF-kappaB activation in both allogeneic and syngeneic liver grafts. Multiple doses of CpG ODN (10 microg per mouse x 3) elicited acute rejection of the liver allografts with significant T cell infiltration in the liver allografts, reduced T regulatory cells, and enhanced interferon gamma-producing cells in the intrahepatic infiltrating lymphocytes. These data demonstrate that CpG ODN initiates an inflammatory reaction and abrogates spontaneous tolerance in the liver transplantation mouse model. Liver Transpl 15:915-923, 2009. (c) 2009 AASLD.

Immunologic role of Kupffer cells in liver transplantation

Orthotopic liver transplantation (OLT) is an effective treatment for the end-stage liver diseases. Rejection reaction of graft remains a major cause of post-transplantation liver dysfunction and even failure. Immunologic role of Kupffer cells in liver transplantation is frequently ignored. Many investigations demonstrated that Kupffer cells activate T cells through direct antigen presentation, and aggravate transplantation rejection reaction. At the same time, Kupffer cells may induce apoptosis of T cells by FasL and evoke transplantation tolerance. This review discusses the immunologic role of Kupffer cells in liver transplantation.

Poor allostimulatory function of liver plasmacytoid DC is associated with pro-apoptotic activity, dependent on regulatory T cells

BACKGROUND/AIMS

The liver is comparatively rich in plasmacytoid (p) dendritic cells (DC), - innate immune effector cells that are also thought to play key roles in the induction and regulation of adaptive immunity.

METHODS

Liver and spleen pDC were purified from fms-like tyrosine kinase ligand-treated control or lipopolysaccharide-injected C57BL/10 mice. Flow cytometric and molecular biologic assays were used to characterize their function and interaction with naturally occurring regulatory T cells (Treg).

RESULTS

While IL-10 production was greater for freshly isolated liver compared with splenic pDC, the former produced less bioactive IL-12p70. Moreover, liver pDC expressed a low Delta4/Jagged1 Notch ligand ratio, skewed towards T helper 2 cell differentiation/cytokine production, and promoted allogeneic CD4(+)T cell apoptosis. T cell proliferation in response to liver pDC was, however, enhanced by blocking IL-10 function at the initiation of cultures. In the absence of naturally occurring CD4(+)CD25(+) regulatory T cells, similar levels of T cell proliferation were induced by liver and spleen pDC and the pro-apoptotic activity of liver pDC was reversed.

CONCLUSIONS

The inferior T cell allostimulatory activity of in vivo-stimulated liver pDC may depend on the presence and function of Treg, a property that may contribute to inherent liver tolerogenicity.

Exhaustive differentiation of alloreactive CD8+ T cells: critical for determination of graft acceptance or rejection

BACKGROUND

The precise role that CD8+ T cells play in the rejection and acceptance of different types of allograft is unclear and has been shown to vary between donor-recipient combinations.

METHODS

The response of adoptively transferred CD8+ T cells reactive to the donor alloantigen H2Kb was examined after transplantation of H2Kb liver, kidney, and heart grafts in mice.

RESULTS

After transfer of 6 x 10(6) alloreactive CD8+ T cells to T-cell depleted syngeneic mice spontaneous long-term acceptance of liver grafts was observed, whereas kidney and heart grafts were acutely rejected. Within 5 days of liver transplantation, we found that the entire H2Kb-reactive T-cell pool was stimulated to proliferate and differentiate into memory or effector cells that were detectable within lymphoid tissues as well as the liver graft itself. However, despite the generation of effector or memory T cells, liver allografts were accepted, which correlated with the exhaustion or deletion of such cells. In contrast, although activation and proliferation of H2Kb-reactive CD8+ T cells was observed after transplantation of heart or kidney grafts, unactivated, H2Kb-reactive CD8+ T cells were still present in the spleen even long term. Interestingly, differences in the effector function of liver and kidney graft infiltrating donor-reactive CD8+ T cells were not detected after adoptive transfer into immunodeficient mice, despite a reduction in Th1-type cytokines within liver grafts.

CONCLUSIONS

The rapid and extensive initial activation and differentiation of donor-reactive CD8+ T cells that occurs after liver transplantation leads to clonal exhaustion or deletion of the alloreactive CD8+ T-cell repertoire resulting in spontaneous tolerance induction.

Blocking indoleamine dioxygenase activity early after rat liver transplantation prevents long-term survival but does not cause acute rejection

In a well-characterized rat model of liver transplantation, Piebald Virol Glaxo strain livers are accepted long term in fully mismatched Dark Agouti recipients (tolerance; TOL), but rejected in Lewis recipients (rejection; REJ). Spontaneous tolerance induction is associated with increased interferon-gamma expression, and we examined the role of the interferon-gamma-inducible immunomodulatory enzyme indoleamine dioxygenase (IDO) in this model. On day 3 after transplantation, IDO expression in the spleen of TOL recipients was significantly greater than in REJ. The B-cell population accounted for this early IDO increase. Intragraft expression of IDO increased to the same extent in both TOL and REJ. IDO inhibition for 7 days after transplantation reduced survival, but did not cause acute rejection of the liver in the TOL model. In conclusion, the differential IDO expression by B lymphocytes in the spleen of TOL recipients is not critical for preventing acute rejection.

Role of Kupffer cells in the induction of tolerance of orthotopic liver transplantation in rats

Because the role of Kupffer cells (KCs) in liver transplantation (LT) tolerance is not well understood, we investigated their role in liver allograft acceptance in rats. Male Sprague-Dawley rats were randomly assigned to either an LT group or a transplantation group pretreated with GdCl(3) (Gd group). The rats were postoperatively sacrificed at indicated times for histology and assessment of KC function, nuclear factor kappa B (NF-kappaB) activity, and cytokine production. KCs and T cells (TCs) were isolated from allografts to assess Fas/Fas ligand (FasL) expression. Cytotoxicity of KCs against TCs was monitored by coculturing of (3)H-thymidine TCs with KCs at various effector-to-target ratios. The results were as follows. First, grafts were spontaneously accepted in the LT group with evident apoptosis of TCs; however, inhibition of KCs by pretreatment with GdCl(3) decreased TC apoptosis and shortened the survival of allografts. Second, KCs in the LT group had increased levels of FasL messenger RNA and protein with respect to that in the Gd group. Third, by in vitro cocultivation assays, KCs induced TC apoptosis though elevated expression of FasL, and this process could be blocked by anti-FasL antibody. Fourth, there was a positive correlation between activation of NF-kappaB and FasL expression in KCs and interleukin-4 production in the LT group, and the activation of NF-kappaB was inhibited by pretreatment with GdCl(3). In conclusion, KC-induced depletion of TCs via the Fas/FasL pathway might play a critical role in LT tolerance. However, the tolerance is abrogated by suppression of FasL and IL-4 expression via inhibition of NF-kappaB activity by GdCl(3).

Dendritic cells, the liver, and transplantation

Interstitial liver dendritic cells (DCs) exhibit phenotypic diversity and functional plasticity. They play important roles in both innate and adaptive immunity. Their comparatively low inherent T cell stimulatory ability and the outcome of their interactions with CD4(+) and CD8(+) T cells, as well as with natural killer (NK) T cells and NK cells within the liver, may contribute to regulation of hepatic inflammatory responses and liver allograft outcome. Liver DCs migrate in the steady state and after liver transplantation to secondary lymphoid tissues, where the outcome of their interaction with antigen-specific T cells determines the balance between tolerance and immunity. Systemic and local environmental factors that are modulated by ischemia-reperfusion injury, liver regeneration, microbial infection, and malignancy influence hepatic DC migration, maturation, and function. Current research in DC biology is providing new insights into the role of these important antigen-presenting cells in the complex events that affect liver transplant outcome.

FasL expression in hepatic antigen-presenting cells and phagocytosis of apoptotic T cells by FasL+ Kupffer cells are indicators of rejection activity in human liver allografts

Fas-Fas ligand (FasL) interaction and apoptosis are important in the mechanism of allograft rejection. However, the interaction between donor and recipient cells, specifically focusing on antigen-presenting cells (APCs), under various conditions is poorly understood in human liver allografts. FasL expression on APCs, its association with apoptosis, and the origin of apoptotic lymphocytes in human liver allografts were assessed by immunohistochemistry and in situ hybridization. We found increased expression of FasL on Kupffer cells (KCs) and endothelium in acute cellular rejection (n = 20) and to lesser extent in chronic rejection (n = 6) and septic cholangitis (n = 5) compared with stable grafts and normal controls. In addition, the graft specificity of infiltrating T cells was confirmed by polymerase chain reaction examination of T-cell receptor-gamma loci. T-cell apoptosis occurred at a higher rate in acute cellular rejection than in chronic rejection or septic cholangitis. The number of apoptotic bodies derived from recipient lymphocytes correlated with the severity of rejection and was reversed by treatment. FasL(+) KCs phagocytosed CD4(+) interferon-gamma(+) T cells, rather than CD4(+) interleukin-4(+) T cells, suggesting a role of KCs in regulating CD4(+) T-cell subset differentiation. In conclusion, our data suggest that FasL expression on APCs and phagocytosis of apoptotic T cells by FasL(+) KCs are indicators of rejection activity in human liver allografts.

Tolerogenic dendritic cells and the quest for transplant tolerance

In recent years, there has been a shift from the perception of dendritic cells (DCs) solely as inducers of immune reactivity to the view that these cells are crucial regulators of immunity, which includes their ability to induce and maintain tolerance. Advances in our understanding of the phenotypical and functional plasticity of DCs, and in our ability to manipulate their development and maturation in vitro and in vivo, has provided a basis for the therapeutic harnessing of their inherent tolerogenicity. In this Review, we integrate the available information on the role of DCs in the induction of tolerance, with a focus on transplantation.

Biological therapy in the management of recent-onset Crohn's disease: why, when and how?

Crohn's disease is a chronic inflammatory bowel disease that may involve any part of the gastrointestinal tract. Conventional therapy consists of corticosteroids, azathioprine or methotrexate, but the clinical management of Crohn's disease is significantly hampered by adverse effects. With the introduction of biological agents (such as infliximab), the goals of therapy have advanced, including induction of remission with bowel healing as well as reduction in the rate of complications, surgeries and mortality. Current therapy for moderate to severe Crohn's disease is based on 'step-up' algorithms, which initiate treatment with corticosteroids followed by immunomodulatory agents, and defer therapy with biological agents until patients become refractory to conventional therapeutics. Recently, it has been shown that induction therapy with infliximab and azathioprine in recent-onset Crohn's disease (i.e. 'top-down' approach) is superior to current step-up algorithms to induce clinical remission. The underlying molecular mechanisms responsible for these differences in clinical outcome remain to be defined. Experimental studies have demonstrated that corticosteroids are able to induce impaired apoptosis of immune cells, including T cells and dendritic cells, resulting in loss of tolerance and subsequent autoimmunity. Further research will have to determine whether corticosteroid therapy augments the mechanism of loss of tolerance in Crohn's disease, which could complicate future clinical management.

CD25+CD4+ Regulatory T Cells Develop in Mice Not Only During Spontaneous Acceptance of Liver Allografts but Also After Acute Allograft Rejection

BACKGROUND

Liver grafts transplanted across a major histocompatibility barrier are accepted spontaneously and induce donor specific tolerance in some species. Here, we investigated whether liver allograft acceptance is characterized by, and depends upon, the presence of donor reactive CD25CD4 regulatory T cells.

METHODS

CD25 and CD25CD4 T cells, isolated from CBA. Ca (H2) recipients of C57BL/10 (B10; H2) liver and heart allografts 10 days after transplantation, were transferred into CBA. Rag1 mice to investigate their influence on skin allograft rejection mediated by CD45RBCD4 effector T Cells.

RESULTS

Fully allogeneic B10 liver allografts were spontaneously accepted by naive CBA.Ca recipient mice, whereas B10 cardiac allografts were acutely rejected (mean survival time=7 days). Strikingly, however, CD25CD4 T cells isolated from both liver and cardiac allograft recipients were able to prevent skin allograft rejection in this adoptive transfer model. Interestingly, CD25CD4 T cells isolated from liver graft recipients also showed suppressive potency upon adoptive transfer. Furthermore, depletion of CD25CD4 T cells in primary liver allograft recipients did not prevent the acceptance of a secondary donor-specific skin graft.

CONCLUSIONS

Our data provide evidence that the presence of CD25CD4 regulatory T cells is not a unique feature of allograft acceptance and is more likely the result of sustained exposure to donor alloantigens in vivo.

Monitoring tolerance after human liver transplantation

The validation of reliable, non-invasive immunological assays evaluating anti-donor responsiveness in allograft recipients would provide a clinically relevant tool for the early detection of ongoing rejection process as well as for the identification of operational tolerance in the long term. A sequential approach towards immunological monitoring of allografts is proposed in this review: (i) investigations exploring the initial donor-recipient alloresponses, including the analysis of the cytokine network; (ii) investigations regarding graft acceptance and operational tolerance in long-term transplant patients, consisting in the analysis of regulatory T cells and of circulating precursors of dendritic cells, in the measurement of T cell alloreactivity as well as in the study of T cell receptor repertoires. Beside the conventional in vivo and in vitro immunological techniques, the potential applications of molecular imaging in transplantation also deserve further exploration, with particular respect to allograft immune monitoring. Enforced collaboration between transplant clinicians and immunologists will be required to develop the translational research protocols required for the development of immunological monitoring, within an international multicentric network.

Gene array analysis of a rat model of liver transplant tolerance identifies increased complement C3 and the STAT-1/IRF-1 pathway during tolerance induction

This study aimed to define the molecular mechanism during induction of spontaneous liver transplant tolerance using microarrays and to focus on molecular pathways associated with tolerance by meta-analysis with published studies. The differences in the early immune response between PVG to DA liver transplant recipients that are spontaneously tolerant (TOL) and PVG to Lewis liver transplants that reject (REJ) were examined. Spleens from TOL and REJ on days 1 and 3 were compared by 2 color microarray. Forty-six of 199 genes differentially expressed between TOL and REJ had an immunological function. More immune genes were increased in TOL vs. REJ on day 1, including STAT-1, IRF-1 and complement C3. Differential expression of selected genes was confirmed by quantitative RT-PCR. The results were compared to two published high-throughput studies of rat liver transplant tolerance and showed that C3 was increased in all three models, while STAT-1 and IRF-1 were increased in two models. The early increases in immune genes in TOL confirmed previous reports of an active early immune response in TOL. In conclusion, the increase in STAT-1, IRF-1 and complement component C3 in several models of liver transplant tolerance suggests that the STAT-1/IRF-1 apoptotic pathway and C3 may be involved in the tolerogenic mechanism.

The immunological monitoring of alloreactive responses in liver transplant recipients: a review

The aim of this work is to review the current knowledge in the field of immunological monitoring of allogenic responsiveness in clinical liver transplantation. When compared to other solid-organ transplants, liver allografts are considered as immunologically privileged, and, accordingly, constitute a favorable setting to develop experimental as well as clinical strategies for minimization of immunosuppression and even induction of operational tolerance. The validation of simple, reliable, noninvasive assays exploring antidonor alloreactivity will constitute a crucial step toward implementing such approaches in the clinic. In contrast to research in rodents claiming the development of donor-specific tolerance in case of graft survivals of over 100 days without immunosuppression, it is impractical to confirm tolerance induction in this way in humans. Promising candidate assays include the detection of post-transplant immune deviation, of circulating precursors of dendritic cells subtypes, and of regulatory T cells. A conceptual framework for the development of tolerance assays in clinical liver transplantation is also proposed.

Study of Fas (CD95) and FasL (CD178) polymorphisms in liver transplant recipients

The Fas receptor is capable of transducing apoptotic cell death upon interaction with their ligand (FasL). Recent studies suggest that the Fas/FasL system is involved both in graft rejection and in transplantation tolerance. In this study, we analyzed the effect of Fas and FasL polymorphisms in liver allograft outcome. Fas and FasL polymorphisms were analyzed in 151 primary liver graft recipients. The Fas (-670 A/G) and the FasL (IVS2nt -124 A/G and IVS3nt 169 T/delT) polymorphisms were analyzed by polymerase chain reaction-restriction fragment length polymorphism. Fas -1377 G/A polymorphism was determined by allele-specific amplification. Fas and FasL polymorphisms were not associated with acute and chronic rejection in liver transplant. In contrast, those recipients bearing the AA -670 Fas genotype showed significantly lower graft survival rate (S = 40%) than those bearing the GA genotype (S = 63.1%). These differences were detected from the first year post-transplant. Multivariate analysis confirmed that the AA genotype increased the risk of liver graft loss. This work suggests for the first time a possible harmful effect of Fas -670 AA genotype on liver graft survival, whereas the Fas and FasL polymorphisms are not associated with acute or chronic rejection in liver graft recipients.

The presence of donor-reactive CD4 T cells in early-phase liver-induced tolerance in rats: analysis using donor passenger leukocytes from the recipient

In previous studies, we showed that the recipient spleen 12 h after liver transplantation included highly immunogenic donor passenger leukocytes (PLs) in the non-rejector rat strain combination (Lewis: LEW-->Wistar Furth: WF). Furthermore, we noted that giving anti-CD4 mAb to WF recipients at the time of liver grafting prolonged the liver damage due to primary rejection reaction, suggesting that CD4 T-cell regulation but not the deletional mechanism played a dominant role during the induction phase of tolerance in this combination. We proposed that in vitro CD4 T-cell response to the donor PLs from the recipient lymphoid organ may be observed in the early phase of tolerance induction. We initially observed donor-specific acceptance by 10 days after liver transplantation from the result of secondary heart transplantation, and showed that donor (LEW) PLs in recipient WF spleen 12 h after grafting were able to prime cytotoxic CD8 T cells in vivo. We then isolated functional donor PLs from the recipient WF spleen and established in vitro proliferation assay. When the enriched donor PLs were used as stimulator cells in the proliferation assay, a powerful alloimmunostimulatory ability was observed. Moreover, a potent response of recipient splenocytes to the PLs was observed on postoperative day 14. From these observations, we confirmed that clonal deletion of donor-reactive CD4 T cells did not participate in early phase of tolerance induction in this setting.

Early biological and immune response to semi-identical liver or kidney allograft in miniature swine

In inbred miniature swine, semi-identical liver allograft recipients survive up to 3 months without immunosuppression, whereas similarly mismatched kidney allografts are uniformly rejected within 2 weeks. The early biological and immunological events were assessed in this unique model. SLA(d/d) pigs (MGH, Harvard Medical School, Boston, MA, USA) received liver or kidney allograft from heterozygous SLA(c/d) miniature swine. Survival, graft function, histology, intragraft cytokines, peripheral lymphocyte and platelet count, plasma cortisol level and cellular/humoral anti-donor immune response were assessed. Kidney allografts were uniformly rejected within 2 weeks, whereas liver allografts survived for up to 87 days. After both liver and kidney transplantation, the peripheral lymphocyte count decreased during the first week concomitantly to a significant elevation of plasma cortisol level. Early decrease of peripheral platelet count was observed after liver but not renal transplantation. Up-regulation of transforming growth factor beta1 (TGF-beta1) and interferon-gamma (IFN-gamma) was observed during the first postoperative week in semi-identical liver allografts and IFN-gamma as well as IL-10 in kidney allografts. In liver recipients, labelled autologous lymphocytes accumulated in the liver graft and native spleen, whereas after renal allograft, lymphocytes accumulated in the native spleen and liver but never in the kidney allograft. Specific cellular anti-donor unresponsiveness was observed from the first post-transplant day in both liver and kidney recipients, while the humoral anti-donor response remained intact. In semi-identical liver allograft, recipient rejection is milder and slower than in similarly matched kidney allograft. The intragraft up-regulation of TGF-beta1 in semi-identical liver allograft might be one mediator to explain the modulation of rejection after liver transplant. The rapid, nonspecific accumulation of recipient lymphocytes in the liver allograft but not in kidney allograft might also play a role in the different survival time in this model.

CTLA4 engagement is required for induction of murine liver transplant spontaneous tolerance

Liver transplantation in mice is accepted spontaneously in all strain combinations. The mechanisms remain largely undefined. We hypothesize that signaling via the B7-CTLA4 receptor pathway is required for induction of liver transplant tolerance. Liver transplantation was performed from B10 (H2(b)) to C3H (H2(k)) mice. The recipients received anti-mouse CTLA4 mAb 0.25 mg i.p. every other day post-operatively. Liver grafts in anti-CTLA4 mAb treated recipients were acutely rejected. The allo-specific proliferative responses, anti-donor CTL and NK cell activities of GIC and SC and the serum levels of IFN-gamma and IL-2 from anti-CTLA4 mAb treated recipients were elevated significantly in comparison to the control mice. The frequency of IFN-gamma and IL-2 producing cells were markedly increased also in the anti-CTLA4 treated recipients. The immunohistology of liver grafts from anti-CTLA4 mAb treated mice showed extensively increased lymphocyte infiltration in the portal and general parenchymal areas, and expanded T-cell area in the spleen, with a reduction in the frequency of apoptotic cells observed by TUNEL staining compared with control mice. Thus CTLA4 signaling is critical for murine liver transplant tolerance induction. CTLA4 blockade promotes donor specific T-cell activation, cytotoxicity and Th1 polarization; protects alloreactive T cells from apoptotic death and induces liver allograft acute rejection.

The response of human T lymphocytes against alloantigens following Fas-mediated activation-induced cell death

We examined the response of T lymphocytes activated with specific alloantigens following Fas-mediated apoptosis; using a mixed lymphocyte culture (MLC) system. Cells obtained from an MLC after 6 or 7 days of culture were incubated for are additional 24 hours in the presence or absence of the agonistic monoclonal antibody (MoAb), 7C11, or the antagonistic MoAb, ZB4. We assessed DNA fragmentation/specific cytotoxiy of the MoAb-treated cells. Cells harvested after 4 days of culture were sensitive to apoptosis induced by 7C11 with maximum DNA fragmentation observed on day 6. ZB4 slightly inhibited apoptosis of the cells compared with controls. The simultaneous addition of recombinant interleukin-2 (rIL-2) with the MoAbs significantly inhibited DNA fragmentation in control and ZB4-treated cells, but had little effect on the 7C11-treated cells. Control and ZB4-treated MLC cells showed cytotoxic activities against specific target cells, namely >10%. In contrast, the 7C11-treated cells showed <5% cytotoxicity. Although the addition of rIL-2 increased specific percentage cytotoxicity of control and ZB4-treated cells, it had little effect on the specific cytotoxic activity of the 7C11-treated MLC cells. These results suggest that specific cytotoxic T lymphocytes may be eliminated via apoptosis mediated by the Fas/Fas ligand system.

Comparative analysis of the fate of donor dendritic cells and B cells and their influence on alloreactive T cell responses under tacrolimus immunosuppression

We have shown that tacrolimus (TAC)-induced liver allograft acceptance is associated with migration and persistence of donor B cells and dendritic cells (DC). To clarify whether these MHC class II+ leukocytes have favorable roles in inducing tolerance, we analyzed recipient T cell reactions after allogeneic B or DC infusion. LEW rat B cells localized exclusively in BN host B cell follicles without any direct contact with host T cells. While few donor DC migrated to T cell areas and marginal zones, they were captured by host APC, suggesting that allogeneic MHC class II+ cells may induce immune reactions via the indirect pathway. Although DC-infused non-immunosuppressed recipients showed enhanced ex vivo anti-donor responses, persistent in vitro donor-specific hyporeactivity was seen equally with donor DC or B cell infusion under TAC. The results indicate that donor MHC class II+ APC are capable of regulating recipient immune reactions under TAC. Possible involvement of the indirect pathway of allorecognition is discussed.

Corticosteroid avoidance in pediatric renal transplantation: can it be achieved?

Corticosteroids have been a cornerstone therapy in renal transplantation, which is the treatment modality of choice for adult and pediatric end-stage renal disease. Their use is associated with significant morbidity, notably cardiovascular, endocrine, and bone complications, body disfiguration, and almost universal growth retardation in children. While newer immunosuppressants have reduced the incidence of these adverse effects, they continue to pose significant post-transplant challenges. There are various strategies that can be used to avoid these adverse effects including the use of an alternative corticosteroid such as deflazacort, minimization of corticosteroid dosage, corticosteroid withdrawal after a period of early use, and more recently complete corticosteroid avoidance. Recent randomized studies have demonstrated significant improvement in growth parameters, lipid profile, and in the amount of bone loss in patients treated with deflazacort, an oxazoline analog of prednisone, compared with methylprednisone.Corticosteroid minimization has been associated with an increased rate of acute rejection. While augmentation with newer immunosuppressants has helped reduce the incidence of acute rejection, significant improvements in growth have not been demonstrated. Alternate-day corticosteroid therapy has been shown to have a beneficial effect on growth but regimen compliance has limited its widespread applicability. Studies of corticosteroid withdrawal have met with varied success. Early corticosteroid withdrawal has been associated with rejection rates ranging from 10% to 81% and late corticosteroid withdrawal, from 13% to 68.8%, with acute rejection episodes occurring as late as 4 years after corticosteroid withdrawal. The rates of clinical acute rejection have been unacceptably high, and corticosteroid withdrawal is thus used very sparingly in adults and even less so in children. Complete corticosteroid avoidance as reported by an initial study has been associated with a 23% incidence of acute rejection and 'catch-up' growth post-transplantation in 14 pediatric recipients, as measured by the change in height standard deviation scores post-transplantation. A second renal transplant study, in adults, demonstrated similar rejection rates of 25% with improvement in post-transplant hypertension and lipid profiles. A more recent pediatric study using a novel extended daclizumab induction protocol demonstrated an 8% incidence of clinical acute rejection with significant improvements in graft function, hypertension, and growth, without an increased incidence of infectious complications. Renal transplantation with a corticosteroid-free protocol may offer significant advantages in the incidence of acute rejection, graft function, growth, blood pressure, lipidemia, and body appearance and appears to be well tolerated when used with a variety of current induction protocols to replace early corticosteroid use. This protocol may also be applicable to other areas of solid organ transplantation in all age groups.

Combined donor leucocyte administration and immunosuppressive drug treatment for survival of rat heart allografts

BACKGROUND

Donor leucocytes (DL) play an important role in rat liver transplant tolerance and their postoperative administration can convert rejection to tolerance. They appear to induce early activation, altered patterns of infiltration and death of recipient alloreactive T cells. The ability of immunosuppressive drugs to combine with DL administration was examined in a rat heart transplant model.

METHODS

Immediately after PVG to DA heterotopic heart transplantation, 6 x 10(7) spleen DL were injected. Cyclosporine A (CsA), 1.5 mg/kg/day, or methotrexate (MTX), 0.1 or 0.2 mg/kg/day, were given from day (d) 0 to d4 (early) or from d3 to d7 (delayed). Castanospermine (CAST) was administered from d0 to d7 at 100 or 300 mg/kg/day. In a separate experiment, transplanted hearts and recipient spleens were collected from treatment groups for analysis of infiltrate and cytokine mRNA expression.

RESULTS

Delayed treatment with CsA or early treatment with MTX but not CAST combined with DL to result in prolonged graft survival. Recipients treated with DL and delayed CsA had a reduced level of intra-graft interleukin (IL)-2, interferon (IFN)-gamma, IL-4, and IL-4R mRNA expression and reduced infiltrate compared to DL alone. Early MTX plus DL led to almost complete inhibition of all markers of inflammation during treatment followed by a rapid increase after cessation. In combination with DL, CsA was more effective than MTX for induction of donor-specific tolerance at the dose and administration regimens tested.

CONCLUSIONS

Delayed CsA or early MTX combine with DL to prolong heart allograft survival. Early and extensive inhibition of rejection by MTX was less effective than delayed and partial inhibition of the response by CsA for induction of transplant tolerance.

Tolerogenic protocols for intestinal transplantation

The intestine has long been considered as a "forbidden" organ to transplant [Ann. Surg. 216 (1992) 223-33]. This is due to the particularly challenging nature of the immunological conflict that an intestinal graft may cause: a particularly vigorous rejection response, in addition to the capacity to mount a graft-versus-host disease (GVHD) [Transplantation 37 (1984) 429]. Currently, the short-term success of intestinal transplantation (Itx) depends upon the chronic delivery of profound immunosuppression but this causes infection, malignancies--in particular posttransplant lymphoproliferative disorder (PTLD)--and direct drug toxicity. For these reasons, the results of Itx remain inferior to those of other solid organ transplants in the middle and in the long term (Intestinal Transplant Registry: www.small-bowel-transplant.org). Improved results and wider application of Itx requires the development of protocols that would facilitate acceptance of the new intestine thereby allowing to reduce the need for immunosuppression with its attending complications. Relevant experimental data and the recent evolution in the clinical strategies used to promote acceptance of intestinal grafts are reviewed.