DAP12 deficiency in liver allografts results in enhanced donor DC migration, augmented effector T cell responses and abrogation of transplant tolerance

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.

Characterization of the Immune Microenvironment and Identification of Biomarkers in Chronic Rhinosinusitis with Nasal Polyps Using Single-Cell RNA Sequencing and Transcriptome Analysis

Purpose

Chronic rhinosinusitis is a prevalent condition in the field of otorhinolaryngology; however, its pathogenesis remains to be elucidated. The immunological defense of the nasal mucosa is significantly influenced by dendritic cells (DCs). We identified specific biological indicators linked to DCs and explored their significance in cases of chronic rhinosinusitis with nasal polyps (CRSwNP).

Patients and Methods

We categorized cells using single-cell RNA (scRNA) sequencing, and combined transcriptome sequencing was used to identify potential candidate genes for CRSwNP. We selected three biomarkers based on two algorithms and performed enrichment and immune correlation analyses. Biomarkers were verified using training and validation sets, receiver operating characteristic curves, immunohistochemistry, and quantitative real-time reverse-transcription PCR (qRT-PCR). Variations in biomarker expression were validated using pseudotime analysis. The networks of competing transcription factor (TF)-mRNA and competing endogenous RNA (ceRNA) were established, and the protein drugs associated with these biomarkers were predicted.

Results

Both scRNA-seq and transcriptome data showed that DCs immune infiltration was higher in the CRSwNP group than in the control group. Three DC-related biomarkers (NR4A1, CLEC4G, and CD163) were identified. In CRSwNP, NR4A1 expression decreased, whereas CLEC4G and CD163 expression increased. All biomarkers were shown to be involved in immunological and metabolic pathways by enrichment analysis. These biomarkers were associated with γδ T cells, effector memory CD4 + T cells, regulatory T cells, and immature DCs. According to pseudotime analysis, NR4A1 and CD163 expression decreased from high to low, whereas CLEC4G expression remained low.

Conclusion

We screened and identified potential DC-associated biomarkers of CRSwNP progression by integrating scRNA-seq with whole transcriptome sequencing. We analyzed the biological pathways in which they were involved, explored their molecular regulatory mechanisms and related drugs, and constructed ceRNA, TF-mRNA, and biomarker-drug networks to identify new CRSwNP treatment targets, laying the groundwork for the clinical management of CRSwNP.

Bile Duct Stent and Strain Selection Influences Long-Term Survival of Mouse Orthotopic Liver Transplantation

BACKGROUND

Although mouse orthotopic liver transplantation (MOLT) is considered the gold standard in basic liver transplantation research, only a handful of transplantation research centers can establish the MOLT model reliably and reproducibly. Besides techniques and instruments, some nontechnical factors determine the outcomes of MOLT. This study aimed to investigate the influence of different bile duct stents and mouse strains on the long-term survival rate of MOLT.

METHODS

Varying donor-recipient-bile duct stent combinations were applied to groups 1-6 (G1, B6J-B6J-PP tube; G2, B6J-C3H-PP tube; G3, B6J-B6J-1.5XPE10 tube; G4, B6N-C3H-1.5XPE10 tube; G5, B10-C3H-1.5XPE10 tube; G6, B6N-C3H-1.25XPE10 tube) to value their effect on the long-term survival of MOLT. Liver transplantation was performed based on these experimental designs. The survival state was monitored for 3 months.

RESULTS

The 1-month survival rate of G1 and G2 was 14.3% and 70%, respectively. The 1-month survival rate of G3 was 80%, which had no significant difference compared with G2. Both G4 and G5 had a favorable 1-month survival rate of 100%. The 3-month survival rate of G3, G4, and G5 was 0%, 25%, and 80%, respectively. G6 had the same 1-month and 3-month survival rates as G5, which were 100% and 80%.

CONCLUSIONS

This study suggests that C3H mice were better recipient selections than B6J mice. Donor strains and stent materials are also important factors for the long-term survival of MOLT. An ideal long-term survival of MOLT could be achieved by a rational donor-recipient-stent combination.

Dendritic Cell-Mediated Regulation of Liver Ischemia-Reperfusion Injury and Liver Transplant Rejection

Liver allograft recipients are more likely to develop transplantation tolerance than those that receive other types of organ graft. Experimental studies suggest that immune cells and other non-parenchymal cells in the unique liver microenvironment play critical roles in promoting liver tolerogenicity. Of these, liver interstitial dendritic cells (DCs) are heterogeneous, innate immune cells that appear to play pivotal roles in the instigation, integration and regulation of inflammatory responses after liver transplantation. Interstitial liver DCs (recruited in situ or derived from circulating precursors) have been implicated in regulation of both ischemia/reperfusion injury (IRI) and anti-donor immunity. Thus, livers transplanted from mice constitutively lacking DCs into syngeneic, wild-type recipients, display increased tissue injury, indicating a protective role of liver-resident donor DCs against transplant IRI. Also, donor DC depletion before transplant prevents mouse spontaneous liver allograft tolerance across major histocompatibility complex (MHC) barriers. On the other hand, mouse liver graft-infiltrating host DCs that acquire donor MHC antigen via "cross-dressing", regulate anti-donor T cell reactivity in association with exhaustion of graft-infiltrating T cells and promote allograft tolerance. In an early phase clinical trial, infusion of donor-derived regulatory DCs (DCreg) before living donor liver transplantation can induce alterations in host T cell populations that may be conducive to attenuation of anti-donor immune reactivity. We discuss the role of DCs in regulation of warm and liver transplant IRI and the induction of liver allograft tolerance. We also address design of cell therapies using DCreg to reduce the immunosuppressive drug burden and promote clinical liver allograft tolerance.

Donor plasmacytoid dendritic cells modulate effector and regulatory T cell responses in mouse spontaneous liver transplant tolerance

We assessed the role of donor liver non-conventional plasmacytoid dendritic cells (pDCs) in spontaneous liver transplant tolerance in a fully MHC-mismatched (C57BL/6 (H2^b ) to C3H (H2^k )) mouse model. Compared with spleen pDCs, liver pDCs expressed higher levels of DNAX-activating protein of 12 kDa and its co-receptor, triggering receptor expressed by myeloid cells 2, and higher ratios of programed death ligand-1 (PD-L1):costimulatory CD80/CD86 in the steady state and after Toll-like receptor 9 ligation. Moreover, liver pDCs potently suppressed allogeneic CD4⁺ and CD8⁺ T cell proliferative responses. Survival of pDC-depleted livers was much poorer (median survival time: 25 days) than that of either untreated donor livers or pDC-depleted syngeneic donor livers that survived indefinitely. Numbers of forkhead box p3 (FoxP3)⁺ regulatory T cells in grafts and mesenteric lymph nodes of mice given pDC-depleted allogeneic livers were reduced significantly compared with those in recipients of untreated livers. Graft-infiltrating CD8⁺ T cells with an exhausted phenotype (programed cell death protein 1⁺ , T cell immunoglobulin and mucin domain-containing protein 3⁺ ) were also reduced in recipients of pDC-depleted livers. PD1-PD-L1 pathway blockade reversed the reduction in exhausted T cells. These novel observations link immunoregulatory functions of liver interstitial pDCs, alloreactive T cell exhaustion, and spontaneous liver transplant tolerance.

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.

Antigen presentation, autoantibody production, and therapeutic targets in autoimmune liver disease

The liver is an important immunological organ that controls systemic tolerance. The liver harbors professional and unconventional antigen-presenting cells that are crucial for tolerance induction and maintenance. Orchestrating the immune response in homeostasis depends on a healthy and well-toned immunological liver microenvironment, which is maintained by the crosstalk of liver-resident antigen-presenting cells and intrahepatic and liver-infiltrating leukocytes. In response to pathogens or autoantigens, tolerance is disrupted by unknown mechanisms. Intrahepatic parenchymal and nonparenchymal cells exhibit unique antigen-presenting properties. The presentation of microbial and endogenous lipid-, metabolite- and peptide-derived antigens from the gut via conventional and nonconventional mechanisms can educate intrahepatic immune cells and elicit effector responses or tolerance. Perturbation of this balance results in autoimmune liver diseases, such as autoimmune hepatitis, primary biliary cholangitis, and primary sclerosing cholangitis. Although the exact etiologies of these autoimmune liver diseases are unknown, it is thought that the disruption of tolerance towards self-antigens and microbial metabolites and lipids, as well as alterations in bile acid composition, may result in changes in effector cell activation and polarization and may reduce or impair protective anti-inflammatory regulatory T and B cell responses. Additionally, the canonical and noncanonical transmission of antigens and antigen:MHC complexes via trogocytosis or extracellular vesicles between different (non) immune cells in the liver may play a role in the induction of hepatic inflammation and tolerance. Here, we summarize emerging aspects of antigen presentation, autoantibody production, and the application of novel therapeutic approaches in the characterization and treatment of autoimmune liver diseases.

Identification of TYROBP and C1QB as Two Novel Key Genes With Prognostic Value in Gastric Cancer by Network Analysis

Background: Gastric cancer (GC) is the fifth most frequently diagnosed malignancy, and the third leading cause of tumor-related mortalities worldwide. Due to a high heterogeneity in GC, its treatment and prognosis are challenging, necessitating urgent identification of novel prognostic predictors for GC patients. Methods: We downloaded RNA sequence data, from the Cancer Genome Atlas and microarray data from Gene Expression Omnibus database, then identified common differentially-expressed genes (DEGs) between GC and normal gastric tissues across four datasets. We then used a combination of protein-protein interaction (PPI) network and weighted gene co-expression network analysis (WGCNA) to identify key genes with prognostic value in GC. Thereafter, we used quantitative real time polymerase chain reaction (qRT-PCR) to validate expression of the identified key genes in the Zhejiang University (ZJU) cohort. Finally, we evaluated the relationships between gene expression and immune factors, including immune cells and biomarkers of immunotherapy. Results: Among 426 common DEGs screened, 333 and 93 were upregulated and downregulated, respectively. PPI network and WGCNA successfully identified the top 30 hub genes, among which PTPRC, TYROBP, CCR1, CYBB, LCP2, and C1QB were common. Furthermore, TYROBP and C1QB were negatively associated with prognosis of GC patients, implying that they were key GC predictors. Interestingly, TYROBP and C1QB were positively correlated with predictive biomarkers for GC immunotherapy, including PD-L1 expression, CD8⁺ T cells infiltration, and EBV status. Conclusions: TYROBP and C1QB were identified as two novel key genes with prognostic value in GC by network analysis.

Strategies for Deliberate Induction of Immune Tolerance in Liver Transplantation: From Preclinical Models to Clinical Application

The liver exhibits intrinsic immune regulatory properties that maintain tolerance to endogenous and exogenous antigens, and provide protection against pathogens. Such an immune privilege contributes to susceptibility to spontaneous acceptance despite major histocompatibility complex mismatch when transplanted in animal models. Furthermore, the presence of a liver allograft can suppress the rejection of other solid tissue/organ grafts from the same donor. Despite this immune privilege of the livers, to control the undesired alloimmune responses in humans, most liver transplant recipients require long-term treatment with immune-suppressive drugs that predispose to cardiometabolic side effects and renal insufficiency. Understanding the mechanism of liver transplant tolerance and crosstalk between a variety of hepatic immune cells, such as dendritic cells, Kupffer cells, liver sinusoidas endothelial cells, hepatic stellate cells and so on, and alloreactive T cells would lead to the development of strategies for deliberate induction of more specific immune tolerance in a clinical setting. In this review article, we focus on results derived from basic studies that have attempted to elucidate the immune modulatory mechanisms of liver constituent cells and clinical trials that induced immune tolerance after liver transplantation by utilizing the immune-privilege potential of the liver.

Details determining the success in establishing a mouse orthotopic liver transplantation model

Liver transplantation (LT) is currently the only effective treatment option for end-stage liver disease. The importance of animal models in transplantation is widely recognized among researchers. Because of the well-characterized mouse genome and the greater diversity and availability of both genetically modified animals and research reagents, mouse orthotopic LT (MOLT) has become an ideal model for the investigation of liver biology, tissue injury, regulation of alloimmunity and tolerance induction, and the pathogenesis of specific liver diseases. However, due to its complicated and technically demanding procedure, the model has merely been used by only a few research groups in the world for years. For a new learner, training lasting at least a couple of months or even years is required. Most of the investigators have emphasized the importance of elaborate techniques and dedicated instruments in establishing a MOLT model, but some details are often neglected. The nontechnical details are also significant, especially for researchers who have little experience in mouse microsurgery. Here, we review and summarize the crucial technical and nontechnical details in establishing the model of MOLT based on scientific articles and our experience in six aspects: animal selection, anesthesia, perioperative management, organ procurement, back-table preparation, and implantation surgery. We aim to enable research groups to shorten the learning curve and implement the mouse LT procedure with high technical success.

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.

DNAX Activating Protein of 12 kDa/Triggering Receptor Expressed on Myeloid Cells 2 Expression by Mouse and Human Liver Dendritic Cells: Functional Implications and Regulation of Liver Ischemia-Reperfusion Injury

Liver interstitial dendritic cells (DCs) have been implicated in the control of ischemia-reperfusion injury (IRI) and host immune responses following liver transplantation. Mechanisms underlying these regulatory functions of hepatic DCs remain unclear. We have shown recently that the transmembrane immunoadaptor DNAX-activating protein of 12 kDa (DAP12) negatively regulates mouse liver DC maturation and proinflammatory and immune stimulatory functions. Here, we used PCR analysis and flow cytometry to characterize expression of DAP12 and its associated triggering receptor, triggering receptor expressed on myeloid cells 2 (TREM2), by mouse and human liver DCs and other immune cells compared with DCs in other tissues. We also examined the roles of DAP12 and TREM2 and their expression by liver DCs in the regulation of liver IRI. Injury was induced in DAP12^-/- , TREM2^-/- , or wild-type (WT) mice by 1 hour of 70% clamping and quantified following 6 hours of reperfusion. Both DAP12 and TREM2 were coexpressed at comparatively high levels by liver DCs. Mouse liver DCs lacking DAP12 or TREM2 displayed enhanced levels of nuclear factor κB and costimulatory molecule expression. Unlike normal WT liver DCs, DAP12^-/- liver DC failed to inhibit proliferative responses of activated T cells. In vivo, DAP12^-/- and TREM2^-/- mice exhibited enhanced IRI accompanied by augmented liver DC activation. Elevated alanine aminotransferase levels and tissue injury were markedly reduced by infusion of WT but not DAP12^-/- DC. Conclusion: Our data reveal a close association between DAP12 and TREM2 expression by liver DC and suggest that, by negatively regulating liver DC stimulatory function, DAP12 promotes their control of hepatic inflammatory responses; the DAP12/TREM2 signaling complex may represent a therapeutic target for control of acute liver injury/liver inflammatory disorders.

Regulatory dendritic cells: profiling, targeting, and therapeutic application

PURPOSE OF REVIEW

There is currently increased focus on improved understanding of how dendritic cell tolerogenicity is determined and maintained, and on their therapeutic potential. We review recent progress in profiling of regulatory dendritic cells (DCreg), innovative approaches to enhancing dendritic cell tolerogenicity in situ, ex-vivo generation of DCreg and initial clinical testing of these cells in organ transplantation.

RECENT FINDINGS

"Omics' studies indicate that the distinctive properties of DCreg are the result of a specific transcriptional program characterized by activation of tolerance-enhancing genes, rather than the retention of an immature state. In situ dendritic cell-directed targeting of nanovesicles bearing immune regulatory molecules can trigger in-vivo expansion of Ag-specific regulatory cells. Innovative approaches to ex-vivo modification of dendritic cells to enhance their regulatory function and capacity to migrate to secondary lymphoid organs has been described. Cross-dressing (with donor major histocompatibility complex molecules) of graft-infiltrating host dendritic cells that regulate antidonor T-cell responses has been implicated in "spontaneous' liver transplant tolerance. Clinical trials of DCreg therapy have begun in living donor renal and liver transplantation.

SUMMARY

Further definition of molecules that can be targeted to promote the function and stability of DCreg in vivo may lead to standardization of DCreg manufacturing for therapeutic application.

Maintaining T cell tolerance of alloantigens: Lessons from animal studies

Achieving host immune tolerance of allogeneic transplants represents the ultimate challenge in clinical transplantation. It has become clear that different cells and mechanisms participate in acquisition versus maintenance of allograft tolerance. Indeed, manipulations which prevent tolerance induction often fail to abrogate tolerance once it has been established. Hence, elucidation of the immunological mechanisms underlying maintenance of T cell tolerance to alloantigens is essential for the development of novel interventions that preserve a robust and long lasting state of allograft tolerance that relies on T cell deletion in addition to intra-graft suppression of inflammatory immune responses. In this review, we discuss some essential elements of the mechanisms involved in the maintenance of naturally occurring or experimentally induced allograft tolerance, including the newly described role of antigen cross-dressing mediated by extracellular vesicles.

Graft-infiltrating PD-L1hi cross-dressed dendritic cells regulate antidonor T cell responses in mouse liver transplant tolerance

Although a key role of cross-dressing has been established in immunity to viral infection and more recently in the instigation of transplant rejection, its role in tolerance is unclear. We investigated the role of intragraft dendritic cells (DCs) and cross-dressing in mouse major histocompatibility complex (MHC)-mismatched liver transplant tolerance that occurs without therapeutic immunosuppression. Although donor interstitial DCs diminished rapidly after transplantation, they were replaced in the liver by host DCs that peaked on postoperative day (POD) 7 and persisted indefinitely. Approximately 60% of these recipient DCs displayed donor MHC class I, indicating cross-dressing. By contrast, only a very minor fraction (0%-2%) of cross-dressed DCs (CD-DCs) was evident in the spleen. CD-DCs sorted from liver grafts expressed much higher levels of T cell inhibitory programed death ligand 1 (PD-L1) and high levels of interleukin-10 compared with non-CD-DCs (nCD-DCs) isolated from the graft. Concomitantly, high incidences of programed death protein 1 (PD-1)^hi T cell immunoglobulin and mucin domain containing 3 (TIM-3)⁺ exhausted graft-infiltrating CD8⁺ T cells were observed. Unlike nCD-DCs, the CD-DCs failed to stimulate proliferation of allogeneic T cells but markedly suppressed antidonor host T cell proliferation. CD-DCs were much less evident in allografts from DNAX-activating protein of 12 kDa (DAP12)^-/- donors that were rejected acutely.

CONCLUSION

These findings suggest that graft-infiltrating PD-L1^hi CD-DCs may play a key role in the regulation of alloimmunity and in the induction of liver transplant tolerance. (Hepatology 2018;67:1499-1515).

Orthotopic mouse liver transplantation to study liver biology and allograft tolerance

Orthotopic liver transplantation in the mouse is a powerful research tool that has led to important mechanistic insights into the regulation of hepatic injury, liver immunopathology, and transplant tolerance. However, it is a technically demanding surgical procedure. Setup of the orthotopic liver transplantation model comprises three main stages: surgery on the donor mouse; back-table preparation of the liver graft; and transplant of the liver into the recipient mouse. In this protocol, we describe our procedure in stepwise detail to allow efficient completion of both the donor and recipient operations. The protocol can result in consistently high technical success rates when performed by personnel experienced in the protocol. The technique can be completed in ∼2-3 h when performed by an individual who is well practiced in performing mouse transplantation in accordance with this protocol. We have achieved a perioperative survival rate close to 100%.

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.

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.