Crucial Fas-Fas ligand interaction in spontaneous acceptance of hepatic allografts in mice

Supervised machine learning of outbred mouse genotypes to predict hepatic immunological tolerance of individuals

It is essential to elucidate the molecular mechanisms underlying liver transplant tolerance and rejection. In cases of mouse liver transplantation between inbred strains, immunological rejection of the allograft is reduced with spontaneous apoptosis without immunosuppressive drugs, which differs from the actual clinical result. This may be because inbred strains are genetically homogeneous and less heterogeneous than others. We exploited outbred CD1 mice, which show highly heterogeneous genotypes among individuals, to search for biomarkers related to immune responses and to construct a model for predicting the outcome of liver allografting. Of the 36 mice examined, 18 died within 3 weeks after transplantation, while the others survived for more than 6 weeks. Whole-exome sequencing of the 36 donors revealed more than 9 million variants relative to the C57BL/6 J reference. We selected 6517 single-nucleotide and indel variants and performed machine learning to determine whether or not we could predict the prognosis of each genotype. Models were built by both deep learning with a one-dimensional convolutional neural network and linear classification and evaluated by leave-one-out cross-validation. Given that one short-lived mouse died early in an accident, the models perfectly predicted the outcome of all individuals, suggesting the importance of genotype collection. In addition, linear classification models provided a list of loci potentially responsible for these responses. The present methods as well as results is likely to be applicable to liver transplantation in humans.

Cellular strategies to induce immune tolerance after liver transplantation: Clinical perspectives

Liver transplantation (LT) has become the most efficient treatment for pediatric and adult end-stage liver disease and the survival time after transplantation is becoming longer due to the development of surgical techniques and perioperative management. However, long-term side-effects of immunosuppressants, like infection, metabolic disorders and malignant tumor are gaining more attention. Immune tolerance is the status in which LT recipients no longer need to take any immunosuppressants, but the liver function and intrahepatic histology maintain normal. The approaches to achieve immune tolerance after transplantation include spontaneous, operational and induced tolerance. The first two means require no specific intervention but withdrawing immunosuppressant gradually during follow-up. No clinical factors or biomarkers so far could accurately predict who are suitable for immunosuppressant withdraw after transplantation. With the understanding to the underlying mechanisms of immune tolerance, many strategies have been developed to induce tolerance in LT recipients. Cellular strategy is one of the most promising methods for immune tolerance induction, including chimerism induced by hematopoietic stem cells and adoptive transfer of regulatory immune cells. The safety and efficacy of various cell products have been evaluated by prospective preclinical and clinical trials, while obstacles still exist before translating into clinical practice. Here, we will summarize the latest perspectives and concerns on the clinical application of cellular strategies in LT recipients.

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.

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.

Liver transplantation in the mouse: Insights into liver immunobiology, tissue injury, and allograft tolerance

The surgically demanding mouse orthotopic liver transplant model was first described in 1991. It has proved to be a powerful research tool for the investigation of liver biology, tissue injury, the regulation of alloimmunity and tolerance induction, and the pathogenesis of specific liver diseases. Liver transplantation in mice has unique advantages over transplantation of the liver in larger species, such as the rat or pig, because the mouse genome is well characterized and there is much greater availability of both genetically modified animals and research reagents. Liver transplant experiments using various transgenic or gene knockout mice have provided valuable mechanistic insights into the immunobiology and pathobiology of the liver and the regulation of graft rejection and tolerance over the past 25 years. The molecular pathways identified in the regulation of tissue injury and promotion of liver transplant tolerance provide new potential targets for therapeutic intervention to control adverse inflammatory responses/immune-mediated events in the hepatic environment and systemically. In conclusion, orthotopic liver transplantation in the mouse is a valuable model for gaining improved insights into liver biology, immunopathology, and allograft tolerance that may result in therapeutic innovation in the liver and in the treatment of other diseases.

Lessons and limits of mouse models

Seminal studies in rabbits and rodent transplantation models by Peter Medawar revealed that cellular processes, rather than humoral antibodies, are central to the acute rejection of transplanted organs, and much of basic transplantation research continues to be focused on the biology and control of these cells, which were subsequently shown to be T cells. However, the success of current immunosuppression at controlling T-cell-mediated rejection has resulted in an increasing awareness of antibody-mediated rejection in the clinic. This, in turn, has fueled an emerging interest in the biology of allospecific antibodies, the B cells that produce these antibodies, and the development of mouse models that allow their investigation. Here we summarize some of the more widely used mouse models that have been developed to study the immunobiology of alloreactivity, transplantation rejection and tolerance, and used to identify therapeutic strategies that modulate these events.

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.

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.

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).

OVA-specific CD8+ T cells do not express granzyme B during anterior chamber associated immune deviation

PURPOSE

To examine antigen (Ag)-specific CTL response during anterior chamber associated immune deviation (ACAID).

METHODS

OVA or OVA257-264 peptide was injected into the anterior chamber (AC) of C57BL/6 mice. There were 16 mice in each ACAID group induced with OVA or OVA257-264 peptide. The mice were primed by SC injection with OVA or OVA 257-264 peptide in complete Freund's adjuvant (CFA) on day 7. Ag-specific CD8+ T cells in spleens were analyzed on day 14 using Pentamer H-2K(b)-SIINFEKL(OVA257-264 peptide). IFN-gamma ELISPOT and intracellular granzyme B staining were used to characterize the CTL response. Twelve mice in each group immunized with OVA or OVA257-264 peptide in CFA served as positive controls. Twelve normal mice served as negative controls and 12 receiving injection of CFA as CFA controls for studying the influence of CFA on the Ag-specific CTL response.

RESULT

The results showed that anterior chamber inoculation of OVA or OVA257-264 peptide could induce ACAID as evidenced by an impaired DTH response. The frequency of Ag-specific CD8+ T cells in ACAID mice was not different from that in mice challenged with Ags in CFA only (positive controls). IFN-gamma production by these cells in ACAID mice was not different compared to positive controls. However, Ag-specific CD8+ T cells in ACAID mice failed to secrete granzyme B. Mice challenged only with OVA peptide and CFA also showed a granzyme B negative CD8+ T cell response. Ag-specific CTL response induced by CFA alone was similar with the negative control.

CONCLUSION

These results show that the frequency of Ag-specific CD8+ T cells is not altered during ACAID. The Ag-specific CTL response during ACAID is characterized by the absence of granzyme B expression.

Liver sinusoidal endothelial cells tolerize T cells across MHC barriers in mice

Although livers transplanted across MHC barriers in mice are normally accepted without recipient immune suppression, the underlying mechanisms remain to be clarified. To identify the cell type that contributes to induction of such a tolerance state, we established a mixed hepatic constituent cell-lymphocyte reaction (MHLR) assay. Irradiated C57BL/6 (B6) or BALB/c mouse hepatic constituent cells (HCs) and CFSE-labeled B6 splenocytes were cocultured. In allogeneic MHLR, whole HCs did not promote T cell proliferation. When liver sinusoidal endothelial cells (LSECs) were depleted from HC stimulators, allogeneic MHLR resulted in marked proliferation of reactive CD4(+) and CD8(+) T cells. To test the tolerizing capacity of the LSECs toward alloreactive T cells, B6 splenocytes that had transmigrated through monolayers of B6, BALB/c, or SJL/j LSECs were restimulated with irradiated BALB/c splenocytes. Nonresponsiveness of T cells that had transmigrated through allogeneic BALB/c LSECs and marked proliferation of T cells transmigrated through syngeneic B6 or third-party SJL/j LSECs were observed after the restimulation. Transmigration across the Fas ligand-deficient BALB/c LSECs failed to render CD4(+) T cells tolerant. Thus, we demonstrate that Fas ligand expressed on naive LSECs can impart tolerogenic potential upon alloantigen recognition via the direct pathway. This presents a novel relevant mechanism of liver allograft tolerance. In conclusion, LSECs are capable of regulating a polyclonal population of T cells with direct allospecificity, and the Fas/Fas ligand pathway is involved in such LSEC-mediated T cell regulation.

Inhibition of mouse hepatocyte apoptosis via anti-Fas ribozyme

AIM: To investigate the effects of anti-Fas ribozyme on Fas expression and apoptosis in primary cultured mouse hepatocytes.

METHODS: Mouse hepatocytes were isolated by using collagenase irrigation. A hammerhead ribozyme targeting the Fas mRNA was constructed, and transfected into mouse hepatocytes via Effectene. Then Fas expression in mouse hepatocytes was detected by RT-PCR and western blotting. After being treated with anti-Fas antibody (JO₂), hepatocytes viability was measured with MTT assay. Caspase-3 proteolytic activity was detected, and cell apoptosis was measured according to Annexin V-FITC apoptosis detection kit.

RESULTS: Fas expressed in primary mouse hepatocytes. Fas expression in hepatocytes transfected with anti-Fas ribozyme was decreased remarkably and correlated with the resistance to Fas-mediated apoptosis as determined by flow cytometry and caspase-3 proteolytic activity.

CONCLUSION: Anti-Fas ribozyme can remarkably decrease the Fas expression in mouse hepatocytes, thus inhibit Fas-mediated apoptosis in hepatocytes. It is suggested that anti-Fas ribozyme could significantly increase the resistance of transplanted hepatocytes to apoptosis and improve the survival of transplanted hepatocytes.

Hepatic allograft-derived Kupffer cells regulate T cell response in rats

In liver transplantation, the development of tolerance is associated with an increased rate of apoptosis of T lymphocytes in the portal inflammatory infiltrate and the presence of an intragraft Th2-like T cell population. Underlying mechanisms are poorly understood. Kupffer cells (KC), which reside in the hepatic sinosoids, can directly interact with circulating T lymphocytes and thus are uniquely positioned to play a role in immunomodulation. In this study, the immunoregulatory effects of KC were investigated. We show that KC can significantly suppress T cell proliferation in mixed leukocyte reaction (MLR). Furthermore, KC express functional Fas ligand (FasL) and can induce apoptosis of Fas+ cells. This process can be blocked by addition of neutralizing anti-FasL antibody. Moreover, using an allogeneic liver transplant model we have determined that 1. KC recovered from chronically accepted hepatic allografts have increased FasL messenger RNA (mRNA) and protein expression and a greater ability to induce apoptosis of alloreactive T cells compared with KC recovered from an acute rejection model; 2. KC not only induce apoptosis of T cells, but also regulate cytokine production and Th2/Th3-like cytokine (interleukin [IL]-10 / transforming growth factor [TGF]-beta) mRNA expression in allogeneic MLR in vitro; and 3. administration of KC derived from chronically accepted liver allografts significantly prolongs the survival of hepatic allografts in an acute rejection model in an alloantigen-specific manner. In conclusion, these data implicate the possible role of KC-mediated regulation of T cell response in the induction of immune tolerance in liver allografts.