Spontaneous acceptance of liver transplants in rodents: evidence that liver leucocytes induce recipient T-cell death by neglect

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.

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.

Kupffer Cells Promote the Differentiation of Adult Liver Hematopoietic Stem and Progenitor Cells into Lymphocytes via ICAM-1 and LFA-1 Interaction

It has been reported that the adult liver contains hematopoietic stem and progenitor cells (HSPCs), which are associated with long-term hematopoietic reconstitution activity. Hepatic hematopoiesis plays an important role in the generation of cells involved in liver diseases. However, how the progenitors differentiate into functional myeloid cells and lymphocytes in the liver microenvironment remains unknown. In the present study, HSPC transplantation experiments were used to confirm that adult murine liver HSPCs differentiate into both myeloid cells and lymphocytes (preferentially T cells) compared with bone marrow HSPCs. Using a coculture system comprised of kupffer cells and HSPCs, we found that kupffer cells promote adult liver HSPCs to primarily generate T cells and B cells. We then demonstrated that kupffer cells can also promote HSPC expansion. A blockade of intercellular cell adhesion molecule-1 (ICAM-1) in a liver HSPC and kupffer cell coculture system impaired the adhesion, expansion, and differentiation of HSPCs. These results suggest a critical role of kupffer cells in the maintenance and promotion of adult mouse liver hematopoiesis. These findings provide important insight into understanding liver extramedullary hematopoiesis and its significance, particularly under the state of some liver diseases, such as hepatitis, nonalcoholic fatty liver disease (NAFLD), and hepatocellular carcinoma (HCC).

Dynamics of virus-specific T cell immunity in pediatric liver transplant recipients

Immunosuppression following solid organ transplantation (SOT) has a deleterious effect on cellular immunity leading to frequent and prolonged viral infections. To better understand the relationship between posttransplant immunosuppression and circulating virus-specific T cells, we prospectively monitored the frequency and function of T cells directed to a range of latent (CMV, EBV, HHV6, BK) and lytic (AdV) viruses in 16 children undergoing liver transplantation for up to 1 year posttransplant. Following transplant, there was an immediate decline in circulating virus-specific T cells, which recovered posttransplant, coincident with the introduction and subsequent routine tapering of immunosuppression. Furthermore, 12 of 14 infections/reactivations that occurred posttransplant were successfully controlled with immunosuppression reduction (and/or antiviral use) and in all cases we detected a temporal increase in the circulating frequency of virus-specific T cells directed against the infecting virus, which was absent in 2 cases where infections remained uncontrolled by the end of follow-up. Our study illustrates the dynamic changes in virus-specific T cells that occur in children following liver transplantation, driven both by active viral replication and modulation of immunosuppression.

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.

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.

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

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

Gene therapy for tolerance: high-level expression of donor major histocompatibility complex in the liver overcomes naive and memory alloresponses to skin grafts

BACKGROUND

The liver has long been recognized as having tolerogenic properties. We investigated whether recombinant adenoassociated virus (rAAV)-mediated expression of donor major histocompatibility complex in recipient livers could induce tolerance to donor-strain grafts.

METHODS

Naive B10.BR (H-2) or B10.BR recipients primed with a H-2K-expressing (K) skin graft were injected with rAAV-expressing H-2K (rAAV-K) to induce K expression on hepatocytes 7 days before challenge with a K skin graft. K-specific responses were measured by interferon (IFN)-γ ELISpot and flow cytometric assessment of directly H-2K reactive cells. Fully allogeneic grafts from C57BL/6 (H-2) donors were transplanted onto longstanding B10.BR recipients of K skin to test for linked epitope suppression.

RESULTS

rAAV-K-treated B10.BR mice accepted K skin grafts with increased median survival time (MST) more than 169 days compared to uninoculated (MST=18.5 days) and rAAV-K-treated controls (MST=19 days). rAAV-K-treated B10.BR animals primed with K skin grafts also accepted secondary K skin grafts in the long term (MST>100 days) compared to accelerated rejection in primed, uninoculated mice (MST=12 days). Treatments did not induce liver pathology, assessed by serum alanine aminotransferase levels and histology. IFN-γ ELISpot analysis of splenocytes from rAAV-K-treated mice indicated reduced responses to donor K antigen, but protection was not extended to fully allogeneic C57BL/6 skin or heart grafts, even in recipients that had accepted K skin grafts in the long term.

CONCLUSIONS

High-level expression of donor major histocompatibility complex in recipient livers promotes tolerance to skin allografts, even in animals primed to produce a memory response. This provides proof of concept for an approach using liver-targeted gene delivery for tolerance induction to donor antigen.

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.

Regulatory T-cell activation among patients who displayed operational tolerance following intra-portal administration of donor-specific antigens in living donor liver transplantation

Immunologic tolerance is the goal for all transplant surgeons. We have reported that repeated donor-specific antigen transfusion (DST) via the portal vein allowed rapid reduction of immunosuppressants with decreased acute cellular rejection episodes among living donor liver transplantations (LDLT). Moreover, we demonstrated that intraportal DST induced macrochimerism of donor type CD56+ T cells in the liver graft. We examined the impact of FoxP3+CD4+CD25+ T cells in recipients who acquired almost tolerance after LDLT with intraportal DST. We defined the amount of immunosuppressants administered less than one time per week as "almost tolerance" after LDLT, which occurred among 14% of DST patients after adult-to-adult LDLT. Two patients (4%) have gotten been we used from immunosuppressants more than 2 years after LDLT 4 years prior. We examined the impact of FoxP3+CD4+CD25+ T cells both in recipients with almost daily immunosuppressants and those who acquired almost tolerance. The proportion of FoxP3+/CD4+CD25+ T cells in the almost tolerance group was significantly higher than that in the almost daily immunosuppressant group (P<.05). The increased proportion of FoxP3+/CD4+CD25+ T cells significantly correlated with time after LRLT (y=0.0964x+42.02, R2=0.8854). Repeated intraportal DST may be a goot tool to induce immunologic tolerance after LDLT. Both donor type CD56+ T cells and FoxP3+/CD4+CD25+ T cells may act as important regulatory cells for tolerance. The period after LDLT is important for acquiring immunologic tolerance.

Selective expansion of allogeneic regulatory T cells by hepatic stellate cells: role of endotoxin and implications for allograft tolerance

Hepatic stellate cells (HSCs) may play an important role in hepatic immune regulation by producing numerous cytokines/chemokines and expressing Ag-presenting and T cell coregulatory molecules. Due to disruption of the endothelial barrier during cold-ischemic storage and reperfusion of liver grafts, HSCs can interact directly with cells of the immune system. Endotoxin (LPS), levels of which increase in liver diseases and transplantation, stimulates the synthesis of many mediators by HSCs. We hypothesized that LPS-stimulated HSCs might promote hepatic tolerogenicity by influencing naturally occurring immunosuppressive CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs). Following their portal venous infusion, allogeneic CD4(+) T cells, including Tregs, were found closely associated with HSCs, and this association increased in LPS-treated livers. In vitro, both unstimulated and LPS-stimulated HSCs upregulated Fas (CD95) expression on conventional CD4(+) T cells and induced their apoptosis in a Fas/Fas ligand-dependent manner. By contrast, HSCs induced Treg proliferation, which required cell-cell contact and was MHC class II-dependent. This effect was augmented when HSCs were pretreated with LPS. LPS increased the expression of MHC class II, CD80, and CD86 and stimulated the production of IL-1α, IL-1β, IL-6, IL-10 and TNF-α by HSCs. Interestingly, production of IL-1α, IL-1β, IL-6, and TNF-α was strongly inhibited, but that of IL-10 enhanced in LPS-pretreated HSC/Treg cocultures. Adoptively transferred allogeneic HSCs migrated to the secondary lymphoid tissues and induced Treg expansion in lymph nodes. These data implicate endotoxin-stimulated HSCs as important immune regulators in liver transplantation by inducing selective expansion of tolerance-promoting Tregs and reducing inflammation and alloimmunity.

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.

Liver transplantation: Issues for the next 20 years

The growing numbers of potential transplant recipients on waiting lists is increasingly disproportionate to the supply of cadaveric donor organs. The hope for the next 20 years is that supply will satisfy demand. This requires both a reduction in indications for the procedure and an increase in the transplants performed. A multi-pronged approach is needed to increase cadaveric organ donation, generating enthusiasm for donation among both the general public and hospital staff. Accurate assessment of marginal grafts with stringent criteria known to predict graft function will diminish wastage of organs. Methods of rehabilitating marginal grafts during extracorporeal perfusion will increase organ availability. Supply of non-heart beating donors can be greatly expanded and protocols developed with ethical consent to optimize their initial function despite warm ischemia. Splitting livers that fulfill selection criteria, thus providing for two recipients, should be universally applied with acceptable incentives to those units who do not directly benefit. A proportion of recipients, though not those transplanted for autoimmune disease, will be spared the side-effects of immunosuppression thanks to immune tolerance. Protocols for close monitoring of those patients for rejection during treatment withdrawal must be carefully observed. In addition to gene therapy, it is highly likely that hepatocyte transplantation will replace orthotopic grafting in patients without cirrhosis, especially for inherited metabolic diseases. It is much more difficult to envisage that heterologous stem cell transplantation or xenotransplantation will have clinical impact in the next 20 years, although research in those areas has obvious long-term potential.

Statins induce immunosuppressive effect on heterotopic limb allografts in rat through inhibiting T cell activation and proliferation

Long-term use of immunosuppressive agents could bring many side effects. Recently, 3-Hydroxy-3-methyl-gutaryl coenzyme A reductase inhibitors (statins) have been reported to be immunomodulatory besides lowering serum cholesterol level. The aim of this study was to investigate the effects of statins on composite tissue allografts and T lymphocyte in vivo and in vitro. Rats were divided into 5 groups: syngeneic transplantation group (Lewis-Lewis); allogeneic control group (Brown Norway-Lewis, no treatment); low-dose statins group (15 mg /kg); high-dose statins group (30 mg /kg) and cyclosporin A group. In vivo, treatment of statins significantly prolonged allografts survival as compared to control group. Histological findings further supported these clinical results and demonstrated less extent of rejection. Immunohistochemical analysis showed that there was a remarkably reduced T cells infiltration in statins groups. Moreover, the serum levels of IL-2 and IFN-gamma were decreased after statins therapy, while these in control group increased significantly. Meanwhile, transcriptional activities of IL-2 and IFN-gamma were also dramatically down-regulated after statins treatment. In vitro, mixed lymphocyte reaction assay was performed and the results revealed lymphocyte proliferation was inhibited by statins in a dose-dependent manner. Furthermore, administration of statins exhibited inhibitory effects on CD3/CD28 mediated T cell activation and proliferation. Besides, the results demonstrated that statins significantly down-regulated mRNA expression and suppress cytokine production of IL-2 and IFN-gamma in vitro. In conclusion, our data demonstrated that application of statins could induce immunosuppressive effect and prolong allografts survival through inhibiting activation and proliferation of T cell and reducing production of IL-2 and IFN-gamma.

Hematopoietic stem cell engraftment and seeding permits multi-lymphoid chimerism in vascularized bone marrow transplants

Vascularized bone marrow transplantation (VBMT) across a MHC barrier under a 7-day alphabeta-TCR mAb and CsA protocol facilitated multiple hematolymphoid chimerism via trafficking of the immature (CD90) bone marrow cells (BMC) between donor and recipient compartments. Early engraftment of donor BMC [BN(RT1(n))] into the recipient BM compartment [LEW(RT1(l))] was achieved at 1 week posttransplant and this was associated with active hematopoiesis within allografted bone and correlated with high chimerism in the hematolymphoid organs. Two-way trafficking between donor and recipient BM compartments was confirmed by the presence of recipient MHC class I cells (RT1(l)) within the allografted bone up to 3 weeks posttransplant. At 10 weeks posttransplant, decline of BMC viability in allografted bone corresponded with bone fibrosis and lack of hematopoiesis. In contrast, active hematopoiesis was present in the recipient bone as evidenced by the presence of donor-specific immature (CD90/RT1(n)) cells, which correlated with chimerism maintenance. Clonogenic activity of donor-origin cells (RT1(n)) engrafted into the host BM compartment was confirmed by colony-forming units (CFU) assay. These results confirm that hematolymphoid chimerism is developed early post-VBMT by T-cell lineage and despite allografted bone fibrosis chimerism maintenance is supported by B-cell linage and active hematopoiesis of donor-origin cells in the host BM compartment.

Activation of innate immunity (NK/IFN-gamma) in rat allogeneic liver transplantation: contribution to liver injury and suppression of hepatocyte proliferation

Liver transplantation is presently the only curative treatment for patients with end-stage liver disease. However, the mechanisms underlying liver injury and hepatocyte proliferation posttransplantation remain obscure. In this investigation, liver injury and hepatocyte proliferation in syngeneic and allogeneic animal models were compared. Male Lewis and Dark Agouti (DA) rats were subjected to orthotopic liver transplantation (OLT). Rat OLT was performed in syngeneic (Lewis-Lewis) and allogeneic (Lewis-DA or DA-Lewis) animal models. Allogeneic liver grafts exhibited greater injury and cellular apoptosis than syngeneic grafts but less hepatocyte proliferation after OLT. Expression of IFN-gamma mRNA and activation of the downstream signal transducer and activator of transcription 1 (STAT1) and genes (interferon regulatory factor-1 and cyclin-dependent kinase inhibitor p21(CDKN1A)) were also greater in the allogeneic grafts compared with the syngeneic grafts. In contrast, STAT3 activation was lower in the allogeneic grafts. Furthermore, in the allogeneic grafts, depletion of natural killer (NK) cells decreased IFN-gamma/STAT1 activation but enhanced hepatocyte proliferation. These findings suggest that, compared with syngeneic transplantation, innate immunity (NK/IFN-gamma) is activated after allogeneic transplantation, which likely contributes to liver injury and inhibits hepatocyte proliferation.

Donor Leukocytes Combined With Delayed Immunosupressive Drug Therapy Prolong Limb Allograft Survival

Donor leukocytes administered at the time of transplantation may prolong organ allograft survival. Delayed administration of calcineurin inhibitors, such as FK506 or cyclosporine, may enhance their efficacy. Herein the effectiveness of this strategy to promote limb transplant survival was investigated in the strong histocompatibility barrier of Brown-Norway donor to Lewis recipients. Donor leukocytes (6 x 10(7) intravenously) were injected on the day of transplantation followed on day 1 to 14 with mycophenolate mofetil (MMF; 15 mg/kg/d) and prednisone, (0.5 mg/kg/d) which were then tapered by 20% each week and stopped at week 7. Administration of of FK506 (2 mg/kg/d) was started on day 4 and continued for 8 weeks, then tapered for 4 weeks to a maintenance dose of 0.8 mg/kg/d, which was continued for 12 weeks (group A; n = 8). A control group (n = 8) underwent identical treatment save for donor leukocyte injection but rather commencement of FK506 on day 1. Rejection was common during FK506 tapering in both groups. However group A showed a significantly later onset, a shorter period for reversal of the first rejection, and a significantly lower dosage of FK506 at the time of rejection. After the completion of immunosuppression, rejection occurred significantly later in group A than the control group with one animal surviving without immunosuppression on day 344. This is the first trial of a donor leukocyte injection combined with delayed FK506 administration in limb transplantation, which suggested that it could produce a modest but significant improvement in outcome.

The persistence of regulatory cells developing after rat spontaneous liver acceptance

BACKGROUND

We have previously reported that the spontaneous acceptance of Lewis (LEW, RT1(l)) to Dark Agouti (DA, RT1(a)) rat orthotopic liver transplant (OLT) is eliminated by donor gamma-irradiation. The acceptance of the irradiated LEW liver is also reestablished in a naïve rat after the adoptive transfer of T regulatory (T-reg) cells from a LEW to a DA liver-tolerant long-term (>60 days) survivor (LTS) into a naïve DA rat. However, little is known about the growth conditions required to maintain T-reg function. In this study, we examined the need for continued donor-specific alloantigen stimulation for the maintenance and function of T-reg cells.

METHODS

Splenocytes (SCs; 1.5 x 10(8) cells) from a LEW liver allograft-tolerant LTS DA recipient were adoptively transferred fresh or after in vitro stimulation into another naive DA rat on day 1, 4, or 7 before an irradiated (1000R) LEW liver transplant. For in vitro alloantigen stimulation, SCs from LEW to DA LTS were co-cultured with mitomycin-C (MMC)-treated naïve LEW (donor alloantigen-specific) or Brown Norway (BN) (RT1(n); third party) SCs for 72 hours. Graft rejection, as defined by death of the recipient, was confirmed histologically.

RESULTS

All LEW liver grafts were accepted spontaneously by DA recipients for more than 60 days (n=32), while all irradiated LEW livers were acutely rejected (n=9; mean survival time [MST]=12.8 +/- 4.0 days). When LTS DA SCs were adoptively transferred into a naive DA rat 1 day before OLT, all irradiated LEW grafts were accepted greater than 60 days (n=9). However, when fresh LTS DA SCs were transferred to a new naïve DA rat on 4 or 7 days before OLT, all irradiated LEW liver grafts were acutely rejected (MST=10.2 +/- 0.5 days [n=4] and MST=13.5 +/- 5.0 days [n=4], respectively). When LTS DA SCs were stimulated in vitro before adoptive transfer, irradiated LEW liver grafts after 4 days (n=5) were then accepted. In vitro culture of LTS DA SCs with MMC-treated BN SCs (third-party) for 72 hours before adoptive transfer resulted in 3 of 5 irradiated LEW livers at day 4 being accepted (n=5).

CONCLUSIONS

The maintenance of T-reg function requires continuous LEW donor-specific alloantigen stimulation.

Early Accumulation of Interferon-?? in Grafts Tolerized by Donor-Specific Blood Transfusion: Friend or Enemy?

BACKGROUND

We previously documented an early (day-2) interferon (IFN)-gamma accumulation in cardiac allografts of rats made tolerant by donor-specific blood transfusion (DSBT) but not in rejecting controls. This contrasted with the IFN-gamma peak seen later (day 5) in rejecting but not in tolerant rats.

METHODS

To further examine the role of early intragraft IFN-gamma in DSBT-induced tolerance, we studied whether IFN-gamma up-regulation correlates with the magnitude of the DSBT effect and how IFN-gamma is influenced by interventions abrogating tolerance.

RESULTS

The protective effect of DSBT depended upon the timing of administration: day-12 DSBT induced indefinite graft survival; day-6 DSBT gave a moderate, and day-0 DSBT, no graft prolongation. IFN-gamma up-regulation correlated with the DSBT effect: it was maximal after day-12 DSBT, intermediate after day-6 DSBT, and absent after day-0 DSBT. Tolerant splenocytes transferred tolerance into naive rats in a donor-specific manner, indicating that alloantigen-specific regulatory cells operate. Thymectomy prevented regulatory cells development, caused further amplification of intragraft IFN-gamma, and led to rejection, although graft survival was still prolonged.

CONCLUSIONS

Day 2 intragraft IFN-gamma correlates with the DSBT protective effect. Thymectomy abrogates DSBT-induced tolerance, prevents regulatory cell development, and paradoxically causes further accumulation of intragraft IFN-gamma. These data indicate that DSBT has a stimulatory and a (thymus-dependent) inhibitory effect on early intragraft IFN-gamma. Intragraft IFN-gamma is beneficial, providing it occurs early and remains moderate. The role of intragraft IFN-gamma in tolerance and rejection depends upon the timing and the degree of production and perhaps the type of IFN-gamma producing cells (regulatory or effector).

Revisiting liver transplant immunology: from the concept of immune engagement to the dualistic pathway paradigm

Ever since the demonstration that allografts are rejected through immune reactions of the host, clinical therapies for organ allografts have relied on immune suppression to prevent these destructive events. A growing body of clinical and experimental data suggests that allografts elicit multiple, interactive immune responses. The result is not inevitably graft rejection, and "spontaneous" acceptance of fully allogeneic liver grafts occurs in rodents without immunosuppression. A spectrum of results range from spontaneous acceptance without immunosuppression to rejection with immunosuppression. The "dualistic pathway paradigm" aims to reconcile apparently conflicting observations in liver transplantation and proposes that: (1) immune engagement between the host and the allograft is instrumental in both rejection and acceptance; (2) there exist in all mammalian species congruent interactive pathways of immune activation whereby the fate of the allograft is determined by the quantitative results of these interactions; (3) the dualistic effect of immunosuppressive drugs on pathways of immune activation, conferring the capacity for favorable or unfavorable graft outcome should be investigated in experimental models in which organ allografts are spontaneously accepted. In conclusion the design of clinical strategies based on this research may contribute to protocols resulting in allograft acceptance without chronic immunosuppression.

Increased mononuclear cell activation and apoptosis early after human liver transplantation is associated with a reduced frequency of acute rejection

Experimental models of orthotopic liver transplantation (OLT) have shown that the very early events post-OLT are critical in distinguishing immunogenic and tolerogenic reactions. In rodents, increased leukocyte apoptosis and cytokine expression have been demonstrated in tolerogenic strain combinations. Information from human OLT recipients is less abundant. The aim of this study was to determine the amount of early leukocyte activation and apoptosis following human OLT, and to correlate this with subsequent rejection status. Peripheral blood mononuclear cells (PBMC) were isolated from 76 patients undergoing OLT - on the day prior, 5 hrs after reperfusion (day 0), and 18-24 hrs post-OLT (day 1). The mean level of apoptotic PBMCs on post OLT day 1 was higher than healthy recipients (0.9% +/- 0.2 vs. 0.2% +/- 0.1, p=0.013). Apoptosis was greater in nonrejecting (NR) (1.1% +/- 0.3) compared with acutely-rejecting (R) (0.3% +/- 0.1, p=0.021) patients. On day 1, PBMC from NR patients had increased expression of IFN-gamma (p=0.006), IL-10 (p=0.016), and CD40 ligand (p=0.02) compared with R. Donor cell chimerism on day 1 did not differ between the groups indicating that this was unlikely to account for increased PBMC apoptosis in the NR group. Interestingly, the level of chimerism on day 0 was significantly higher in NR (3.8% +/- 0.6) compared with R (1.2% +/- 0.4, p=0.004) patients and there was a close correlation between chimerism on day 0 and cytokine expression on day 1. These results imply that similar mechanisms are occurring in the human liver to promote graft acceptance as in the experimental models of liver transplantation and suggest that strategies that promote liver transplant acceptance in rodents might be applicable to humans.

Influence of liver nonparenchymal cell infusion combined with cyclosporin A on rejection of rat small bowel transplantation

AIM: To investigate the effect of liver nonparenchymal cell infusion combined with cyclosporin A (CsA) on rejection of heterostrain rat small bowel transplantation.

METHODS: The liver nonparenchymal cell suspension was prepared by density gradient centrifugation method with Percoll centrifugal solution. Heterotopic small bowel transplantation was performed. Then the rats were divided into four groups. Group one: homogenic transplantation (F344/N→F344/N), group two: allotransplantation (F344/N →Wistar), group three: allotransplantation (F344/N→Wistar) + CsA, with CsA 10 mg·kg^-1·d^-1 after transplantation, group four: allotransplantation + CsA (F344/N→Wistar) + liver nonparenchymal cell infusion + CsA (F344/N→Wistar), in which recipient Wistar rats had been injected with 2 × 10⁸ F344/N liver nonparenchymal cells 20 days before transplantation, and treated with CsA after transplantation. Finally, the survival time after small bowel transplantation, gross and histopathological examination, and IL-2 levels in serum were observed.

RESULTS: The survival time after small bowel transplantation was 7.14 ± 0.33 d, 16.32 ± 0.41 d and 31.41 ± 0.74 d in group 2, 3, and 4, respectively. The survival time was significant longer (P < 0.01) in group 4. The gross and histopathological examination showed that the rejection degree in group 4 was lower than those in groups 2 and 3. Serum IL-2 level in group 4 was also lower than those in groups 2 and 3 (P < 0.01).

CONCLUSION: Liver nonparenchymal cell infusion combined with CsA can prolong the survival time of rat small bowel transplantation, and the anti-rejection effect is good.

A short course of mycophenolate immunosuppression inhibits rejection, but not tolerance, of rat liver allografts in association with inhibition of interleukin-4 and alloantibody responses

BACKGROUND

Some immunosuppressive drug therapies inhibit transplant tolerance in animal models, and we have shown that treatment of recipients with methylprednisolone, but not cyclosporine, inhibits spontaneous acceptance of liver transplants. This study investigates the effects of mycophenolate mofetil (MMF) on liver acceptance and rejection.

METHODS

Piebald Virol Glaxo rat livers were transplanted into Dark Agouti recipients, which spontaneously tolerate (TOL) the liver, or into Lewis recipients, which reject (REJ) the liver. MMF (40 mg/kg/day subcutaneously) was given for 5 days from days 0 to 4 (early) or from days 3 to 7 (late). In separate experiments, liver grafts were collected for assessment of infiltrate and of interleukin (IL)-2, IL-4, IL-10, and interferon-gamma mRNA expression.

RESULTS

TOL liver transplants had a median survival time (MST) of more than 100 days (n=6), and neither early nor late MMF treatment of TOL transplants reduced survival (MST 85 days, P=0.19 and 78 days, P=0.08, respectively). Liver failure in most of these animals was the result of biliary problems, not rejection. There were few consistent differences between treated and untreated TOL animals in infiltrate or liver cytokine expression, although there was a moderate reduction in T-cell infiltrate in MMF-treated TOL animals (P=0.003 on day 5 TOL). In contrast, REJ transplants had an MST of 13 days (n=10), and early MMF treatment led to five of six animals surviving more than 100 days (P=0.0002), whereas late treatment was much less effective, with one of six animals surviving more than 100 days. REJ livers had significantly more IL-4 mRNA expression and immunoglobulin G1 deposition in the graft than TOL livers, and this was inhibited by early, but not late, MMF treatment.

CONCLUSIONS

MMF treatment inhibited rejection but not acceptance of liver allografts. Early administration was more effective in preventing rejection and demonstrated a more marked effect on IL-4 expression and alloantibody deposition than on graft T-cell infiltrate and expression of other cytokines.

A short course of cyclosporine immunosuppression inhibits rejection but not tolerance of rat liver allografts

BACKGROUND

Orthotopic liver transplants in many animal models are spontaneously accepted without requiring immunosuppression. Liver transplant acceptance is associated with early immune activation, and immunosuppressive drugs such as methylprednisolone inhibit acceptance. We investigated whether cyclosporine (CsA) inhibits rat liver transplant acceptance. We also examined the effects of CsA on infiltration and cytokine gene expression.

METHODS

Orthotopic liver transplantation was performed in the PVG donor to Dark Agouti recipient rat strain combination, which accepts the graft (tolerance; TOL), and in the PVG-to-Lewis combination, which rejects the graft in 9 to 16 days (rejection; REJ). CsA (1.5 mg/kg per day subcutaneously) was given to recipients for 5 days, starting from the day of transplantation to day 4 or from day 3 to day 7. In a separate experiment, transplanted livers were collected at days 1, 3, 5, and 7 after transplantation and examined for infiltration by immunohistochemistry and for expression of interleukin (IL)-2, IL-4, IL-10, and interferon-gamma mRNA by quantitative reverse transcriptase-polymerase chain reaction.

RESULTS

Both early and delayed treatment with CsA significantly increased survival in the REJ strain combination, with a median survival time of 81 days and more than 100 days, respectively, compared with 13 days in the untreated group. Neither treatment affected survival of TOL animals, and all TOL groups had a median survival time of more than 100 days. Delayed treatment did not reduce survival; more animals survived for greater than 100 days after delayed treatment, although this did not reach significance ( P=0.08). T-cell infiltrate was inhibited in CsA-treated TOL animals compared with untreated animals at all times after treatment, whereas CD25 cells were only inhibited on day 3. CsA treatment of TOL grafts markedly reduced expression of IL-2, IL-4, and interferon-gamma compared with untreated recipients.

CONCLUSIONS

CsA did not significantly inhibit liver transplant acceptance and allowed some activation of T cells and CD25 expression but almost completely inhibited IL-2 and IL-4, which are required for survival of activated T cells.