Mechanisms of Immune Tolerance in Liver Transplantation-Crosstalk Between Alloreactive T Cells and Liver Cells With Therapeutic Prospects

Liver metastasis and resistance to immunotherapy in microsatellite stable colorectal cancer. A literature review

Background

Microsatellite stable (MSS) metastatic colorectal cancer (CRC) remains predominantly managed with chemotherapy. The use of immunotherapy, whether alone or in combination with other systemic or local treatments, displays limited success, especially in the context of active liver metastases (LM). The mechanisms responsible for this resistance are not fully understood.

Methods

We conducted a comprehensive search across electronic databases such as Medline, PubMed, Google Scholar and ScienceDirect. This search targeted translational studies evaluating the liver tumour immune microenvironment and immune tolerance mechanisms in CRC with LM and prospective studies that assessed immunotherapy either as a standalone treatment or in combination with other systemic or local therapies for patients diagnosed with MSS CRC. Our primary objectives included elucidating the mechanisms of resistance originating from LM in a non-systematic literature review and presenting a summary of the outcomes observed in prospective trials utilising immune checkpoint inhibitors (ICIs), with a focus on the presence of LM.

Findings

There were 16 prospective trials evaluating immunotherapy for metastatic CRC comprising 1,713 patients. Response rates to immunotherapy inpatients with colorectal liver metastases (CRLM) varied from 0% to 23%. Overall, reduced or null responses to immunotherapy in the presence of liver metastasis in comparison to patients without liver involvement were observed.

Conclusion

Studies consistently show the resistance derived from classical ICI, both alone and in combination with other systemic treatments in patients with CRLM. The design of upcoming trials using immunotherapy should consider LM as a stratification factor or contemplate excluding patients with liver involvement.

Immune regulation and therapeutic application of T regulatory cells in liver diseases

CD4+ CD25+ FOXP3+ T regulatory cells (Tregs) are a subset of the immunomodulatory cell population that can inhibit both innate and adaptive immunity by various regulatory mechanisms. In hepatic microenvironment, proliferation, plasticity, migration, and function of Tregs are interrelated to the remaining immune cells and their secreted cytokines and chemokines. In normal conditions, Tregs protect the liver from inflammatory and auto-immune responses, while disruption of this crosstalk between Tregs and other immune cells may result in the progression of chronic liver diseases and the development of hepatic malignancy. In this review, we analyze the deviance of this protective nature of Tregs in response to chronic inflammation and its involvement in inducing liver fibrosis, cirrhosis, and hepatocellular carcinoma. We will also provide a detailed emphasis on the relevance of Tregs as an effective immunotherapeutic option for autoimmune diseases, liver transplantation, and chronic liver diseases including liver cancer.

Immunology demystified: A guide for transplant hepatologists

Liver transplantation has become standard practice for treating end-stage liver disease. The success of the procedure relies on effective immunosuppressive medications to control the host's immune response. Despite the liver's inherent capacity to foster tolerance, the early post-transplant period is marked by significant immune reactivity. To ensure favorable outcomes, it is imperative to identify and manage various rejection types, encompassing T-cell-mediated, antibody-mediated, and chronic rejection. However, the approach to prescribing immunosuppressants relies heavily on clinical judgment rather than evidence-based criteria. Given that the majority of patients will require lifelong immuno suppression as the mechanisms underlying operational tolerance are still being investigated, healthcare providers must possess an understanding of immune responses, rejection mechanisms, and the pathways targeted by immunosuppressive drugs. This knowledge enables customization of treatments and improved patient care, even though a consensus on an optimal immunosuppressive regimen remains elusive.

Liver-Targeting Nanoplatforms for the Induction of Immune Tolerance

Liver-targeting nanoparticles have emerged as a promising platform for the induction of immune tolerance by taking advantage of the liver's unique tolerogenic properties and nanoparticles' physicochemical flexibility. Such an approach provides a versatile solution to the treatment of a diversity of immunologic diseases. In this review, we begin by assessing the design parameters integral to cell-specific targeting and the tolerogenic induction of nanoplatforms engineered to target the four critical immunogenic hepatic cells, including liver sinusoidal epithelial cells (LSECs), Kupffer cells (KCs), hepatic stellate cells (HSCs), and hepatocytes. We also include an overview of multiple therapeutic strategies in which nanoparticles are being studied to treat many allergies and autoimmune disorders. Finally, we explore the challenges of using nanoparticles in this field while highlighting future avenues to expand the therapeutic utility of liver-targeting nanoparticles in autoimmune processes.

Role of the immune system in liver transplantation and its implications for therapeutic interventions

Liver transplantation (LT) stands as the gold standard for treating end-stage liver disease and hepatocellular carcinoma, yet postoperative complications continue to impact survival rates. The liver's unique immune system, governed by a microenvironment of diverse immune cells, is disrupted during processes like ischemia-reperfusion injury posttransplantation, leading to immune imbalance, inflammation, and subsequent complications. In the posttransplantation period, immune cells within the liver collaboratively foster a tolerant environment, crucial for immune tolerance and liver regeneration. While clinical trials exploring cell therapy for LT complications exist, a comprehensive summary is lacking. This review provides an insight into the intricacies of the liver's immune microenvironment, with a specific focus on macrophages and T cells as primary immune players. Delving into the immunological dynamics at different stages of LT, we explore the disruptions after LT and subsequent immune responses. Focusing on immune cell targeting for treating liver transplant complications, we provide a comprehensive summary of ongoing clinical trials in this domain, especially cell therapies. Furthermore, we offer innovative treatment strategies that leverage the opportunities and prospects identified in the therapeutic landscape. This review seeks to advance our understanding of LT immunology and steer the development of precise therapies for postoperative complications.

The Effect of Genetic HLA Matching on Liver Transplantation Outcome: A Systematic Review and Meta-Analysis

Objective

We aim to investigate the effects of genetically based HLA matching on patient and graft survival, and acute and chronic rejection after liver transplantation.

Background

Liver transplantation is a common treatment for patients with end-stage liver disease. In contrast to most other solid organ transplantations, there is no conclusive evidence supporting human leukocyte antigen (HLA) matching for liver transplantations. With emerging alternatives such as transplantation of bankable (stem) cells, HLA matching becomes feasible, which may decrease the need for immunosuppressive therapy and improve transplantation outcomes.

Methods

We systematically searched the PubMed, Embase, and Cochrane databases and performed a meta-analysis investigating the effect of genetic HLA matching on liver transplantation outcomes (acute/chronic rejection, graft failure, and mortality).

Results

We included 14 studies with 2682 patients. HLA-C mismatching significantly increased the risk of acute rejection (full mismatching: risk ratio = 1.90, 95% confidence interval = 1.08 to 3.33, P = 0.03; partial mismatching: risk ratio = 1.33, 95% confidence interval = 1.07 to 1.66, P = 0.01). We did not discern any significant effect of HLA mismatching per locus on acute rejection for HLA-A, -B, -DR, and -DQ, nor on chronic rejection, graft failure, or mortality for HLA-DR, and -DQ.

Conclusions

We found evidence that genetic HLA-C matching reduces the risk of acute rejection after liver transplantation while matching for other loci does not reduce the risk of acute rejection, chronic rejection, graft failure, or mortality.

Immunosuppression Monitoring-What Clinician Needs to Know?

The liver is well known for its immunotolerance, but rejection without immunosuppression is frequently encountered post liver transplantation, especially in humans.1 Indeed, the amount of immunosuppression required post liver transplant is less compared to other organ transplants like kidney, heart, and intestine.2 Reports of successful weaning of immunosuppression have been reported but are not practiced for fear of unwanted alloimmune response leading to rejection. Life-long immunosuppression is needed in most patients for graft survival but is associated with side effects like renal dysfunction, metabolic abnormalities, or risk of de novo malignancies. Also, the appropriate dose of immunosuppression to achieve adequate graft function and prevention of toxicities is very important. One shoe does not fit all. There are significant individual variations in response and side effect profile. Also, the level of immunosuppression varies with the underlying liver disease like autoimmune disease requires higher immunosuppression. Thus, monitoring the adequate immunosuppression with the minimization of drug toxicity is imperative post-transplant. Unfortunately, the current methods for immunosuppression monitoring rely on testing the immunosuppressive drug levels rather than the immune system activity. We have discussed the concept of alloreactivity, available methods of immunosuppression and drug monitoring and investigational methods in this review.

Nonalcoholic steatohepatitis-related hepatocellular carcinoma: pathogenesis and treatment

Nonalcoholic fatty liver disease (NAFLD), including its more severe manifestation, nonalcoholic steatohepatitis (NASH), has a global prevalence of 20-25% and is a major public health problem. Its incidence is increasing in parallel to the rise in obesity, diabetes and metabolic syndrome. Progression from NASH to NASH-related hepatocellular carcinoma (HCC) (~2% of cases per year) is influenced by many factors, including the tissue and immune microenvironment, germline mutations in PNPLA3, and the microbiome. NASH-HCC has unique molecular and immune traits compared with other aetiologies of HCC and is equally prevalent in men and women. Comorbidities associated with NASH, such as obesity and diabetes mellitus, can prevent the implementation of potentially curative therapies in certain patients; nonetheless, outcomes are similar in patients who receive treatment. NASH-HCC at the early to intermediate stages is managed with surgery and locoregional therapies, whereas advanced HCC is treated with systemic therapies, including anti-angiogenic therapies and immune-checkpoint inhibitors. In this Review, we present the latest knowledge of the pathogenic mechanisms and clinical management of NASH-HCC. We discuss data highlighting the controversy over varying responses to immune-checkpoint inhibitors according to underlying aetiology and suggest that the future of NASH-HCC management lies in improved surveillance, targeted combination therapies to overcome immune evasion, and identifying biomarkers to recognize treatment responders.

Preclinical-to-clinical innovations in stem cell therapies for liver regeneration

Acute and chronic liver diseases are the major cause of high morbidity and mortality globally. Liver transplantation is a widely used therapeutic option for liver failure. However, the shortage of availability of liver donors has encouraged research on the alternative approach to liver regeneration. Cell-based regenerative medicine is the best alternative therapy to cater to this need. To date, advanced preclinical approaches have been undertaken on stem cell differentiation and their use in liver tissue engineering for generating efficacious and promising regenerative therapies. Advancements in the bioengineering of stem cells, and organoid generation are the way forward to efficient therapies against liver injury. This review summarizes the recent approaches for stem cell therapy-based liver regeneration and their proof of concepts for clinical application, bioengineering liver organoids to alleviate the liver failure caused due to chronic liver diseases.

Transcriptomic analysis of graft liver provides insight into the immune response of rat liver transplantation

Background

As an “immune-privileged organ”, the liver has higher rates of both spontaneous tolerance and operational tolerance after being transplanted compared with other solid organs. Also, a large number of patients still need to take long-term immunosuppression regimens. Liver transplantation (LT) rejection involves varieties of pathophysiological processes and cell types, and a deeper understanding of LT immune response is urgently needed.

Methods

Homogenic and allogeneic rat LT models were established, and recipient tissue was collected on postoperative day 7. The degree of LT rejection was evaluated by liver pathological changes and liver function. Differentially expressed genes (DEGs) were detected by transcriptome sequencing and confirmed by reverse transcription-polymerase chain reaction. The functional properties of DEGs were characterized by the Gene Ontology (GO), Kyoto Encyclopedia of Genes and Genomes (KEGG), and Reactome pathway analyses. The cells infiltrating the graft and recipient spleen and peripheral blood were evaluated by immunofluorescence and flow cytometry.

Result

A total of 1,465 DEGs were screened, including 1,177 up-regulated genes and 288 down-regulated genes. GO enrichment and KEGG pathway analysis indicated that DEGs were involved in several immunobiological processes, including T cell activation, Th1, Th2 and Th17 cell differentiation, cytokine-cytokine receptor interaction and other immune processes. Reactome results showed that PD-1 signaling was enriched. Further research confirmed that mRNA expression of multiple immune cell markers increased and markers of T cell exhaustion significantly changed. Flow cytometry showed that the proportion of Treg decreased, and that of PD-1+CD4+ T cells and PD-1+CD8+ T cells increased in the allogeneic group.

Conclusion

Using an omic approach, we revealed that the development of LT rejection involved multiple immune cells, activation of various immune pathways, and specific alterations of immune checkpoints, which would benefit risk assessment in the clinic and understanding of pathogenesis regarding LT tolerance. Further clinical validations are warranted for our findings.

Immunology of simultaneous liver and kidney transplants with identification and prevention of rejection

Simultaneous liver and kidney (SLK) transplantation is considered the best treatment modality among selected patients with both chronic kidney disease (CKD) and end-stage liver disease (ESLD). Since the first SLK transplant in 1983, the number of SLK transplants has increased worldwide, and particularly in the United States since the implementation of the MELD system in 2002. SLK transplants are considered a relatively low immunological risk procedure evidenced by multiple studies displaying the immunomodulatory properties of the liver on the immune system of SLK recipients. SLK recipients demonstrate lower rates of both cellular and antibody-mediated rejection on the kidney allograft when compared to kidney transplant-alone recipients. Therefore, SLK transplants in the setting of preformed donor-specific HLA antibodies (DSA) are a common practice, at many centers. Acceptance and transplantation of SLKs are based solely on ABO compatibility without much consideration of crossmatch results or DSA levels. However, some studies suggest an increased risk for rejection for SLK recipients transplanted across high levels of pre-formed HLA DSA. Despite this, there is no consensus regarding acceptable levels of pre-formed DSA, the role of pre-transplant desensitization, splenectomy, or immunosuppressive management in this unique population. Also, the impact of post-transplant DSA monitoring on long-term outcomes is not well-studied in SLK recipients. In this article, we review recent and relevant past articles in this field with a focus on the immunological risk factors among SLK recipients, and strategies to mitigate the negative outcomes among them.

Liver mesenchymal stem cells are superior inhibitors of NK cell functions through differences in their secretome compared to other mesenchymal stem cells

Liver-resident mesenchymal stem cells (L-MSCs) are superior inhibitors of alloreactive T cell responses compared to their counterparts from bone marrow (BM-MSCs) or adipose tissue (A-MSCs), suggesting a role in liver’s overall tolerogenic microenvironment. Whether L-MSCs also impact NK cell functions differently than other MSCs is not known. We generated and characterized L-MSCs, A-MSCs and BM-MSCs from human tissues. The mass spectrometry analysis demonstrated that L-MSC secretome is uniquely different than that of A-MSC/BM-MSC, with enriched protein sets involved in IFNγ responses and signaling. When co-cultured with primary human NK cells, L-MSCs but not other MSCs, decreased surface expression of activating receptors NKp44 and NKG2D. L-MSCs also decreased IFNγ secretion by IL-2-stimulated NK cells more effectively than other MSCs. Cytolytic function of NK cells were reduced significantly when co-cultured with L-MSCs, whereas A-MSCs or BM-MSCs did not have a major impact. Mechanistic studies showed that the L-MSC-mediated reduction in NK cell cytotoxicity is not through changes in secretion of the cytotoxic proteins Perforin, Granzyme A or B, but through increased production of HLA-C1 found in L-MSC secretome that inhibits NK cells by stimulating their inhibitory receptor KIRDL2/3. L-MSCs are more potent inhibitors of NK cell functions than A-MSC or BM-MSC. Combined with their T cell inhibitory features, these results suggest L-MSCs contribute to the tolerogenic liver microenvironment and liver-induced systemic tolerance often observed after liver transplantation.

Incidence of acute rejection and patient survival in combined heart-liver transplantation

Combined heart-liver transplantation (CHLT) is indicated for patients with concomitant end-stage heart and liver disease or patients with amyloid heart disease where liver transplantation mitigates progression. Limited data suggest that the liver allograft provides immunoprotection for heart and kidney allografts in combined transplantation from the same donor. We hypothesized that CHLT reduces the incidence of acute cellular rejection (ACR) and the development of de novo donor-specific antibodies (DSAs) compared with heart-alone transplantation (HA). We conducted a retrospective analysis of 32 CHLT and 280 HA recipients in a single-center experience. The primary outcome was incidence of ACR based on protocol and for-cause myocardial biopsy. Rejection was graded by the International Society of Heart and Lung Transplantation guidelines with Grade 2R and higher considered significant. Secondary outcomes included the development of new DSAs, cardiac function, and patient and cardiac graft survival rates. Of CHLT patients, 9.7% had ACR compared with 45.3% of HA patients (p < 0.01). Mean pretransplant calculated panel reactive antibody (cPRA) levels were similar between groups (CHLT 9.4% vs. HA 9.5%; p = 0.97). Among patients who underwent testing, 26.9% of the CHLT and 16.7% of HA developed DSA (p = 0.19). Despite the difference in ACR, patient and cardiac graft survival rates were similar at 5 years (CHLT 82.1% vs. HA 80.9% [p = 0.73]; CHLT 82.1% vs. HA 80.9% [p = 0.73]). CHLT reduced the incidence of ACR in the cardiac allograft, suggesting that the liver offers immunoprotection against cellular mechanisms of rejection without significant impacts on patient and cardiac graft survival rates. CHLT did not reduce the incidence of de novo DSA, possibly portending similar long-term survival among cardiac allografts in CHLT and HA.

The Effect of Normothermic Machine Perfusion on the Immune Profile of Donor Liver

Background

Normothermic machine perfusion (NMP) allows viability assessment and potential resuscitation of donor livers prior to transplantation. The immunological effect of NMP on liver allografts is undetermined, with potential implications on allograft function, rejection outcomes and overall survival. In this study we define the changes in immune profile of human livers during NMP.

Methods

Six human livers were placed on a NMP device. Tissue and perfusate samples were obtained during cold storage prior to perfusion and at 1, 3, and 6 hours of perfusion. Flow cytometry, immunohistochemistry, and bead-based immunoassays were used to measure leukocyte composition and cytokines in the perfusate and within the liver tissue. Mean values between baseline and time points were compared by Student’s t-test.

Results

Within circulating perfusate, significantly increased frequencies of CD4 T cells, B cells and eosinophils were detectable by 1 hour of NMP and continued to increase at 6 hours of perfusion. On the other hand, NK cell frequency significantly decreased by 1 hour of NMP and remained decreased for the duration of perfusion. Within the liver tissue there was significantly increased B cell frequency but decreased neutrophils detectable at 6 hours of NMP. A transient decrease in intermediate monocyte frequency was detectable in liver tissue during the middle of the perfusion run. Overall, no significant differences were detectable in tissue resident T regulatory cells during NMP. Significantly increased levels of pro-inflammatory and anti-inflammatory cytokines were seen following initiation of NMP that continued to rise throughout duration of perfusion.

Conclusions

Time-dependent dynamic changes are seen in individual leukocyte cell-types within both perfusate and tissue compartments of donor livers during NMP. This suggests a potential role of NMP in altering the immunogenicity of donor livers prior to transplant. These data also provide insights for future work to recondition the intrinsic immune profile of donor livers during NMP prior to transplantation.

Adoptive transfer of ex vivo expanded regulatory T-cells improves immune cell engraftment and therapy-refractory chronic GvHD

Graft-versus-Host-Disease (GvHD) is still the major non-relapse, life-limiting complication following hematopoietic stem cell transplantation. Modern pharmacologic immunosuppression is often insufficient and associated with significant side effects. Novel treatment strategies now include adoptive transfer of ex vivo expanded regulatory T-cells (Tregs), but their efficacy in chronic GvHD is unknown. We treated three children suffering from severe, therapy-refractory GvHD with polyclonally expanded Tregs generated from the original stem cell donor. Third-line maintenance immunosuppression was tapered to Cyclosporin A and low-dose steroids shortly before cell transfer. Regular follow-up included assessment of the subjective and objective clinical development, safety parameters and in-depth immune monitoring. All patients showed marked clinical improvement with substantially reduced GvHD activity. Laboratory follow-up showed a significant enhancement of the immunologic engraftment including lymphocytes and dendritic cells. Monitoring the fate of Tregs by next generation sequencing demonstrated clonal expansion. In summary, adoptive transfer of Tregs was well tolerated and able to modulate an established undesired T-cell mediated allo-response. Although no signs of overimmunosuppression were detectable, treatment of patients with invasive opportunistic infections should be undertaken with caution. Further controlled studies, are necessary to confirm these encouraging effects and eventually pave the way for adoptive Treg therapy in chronic GvHD.

Immunological Determinants of Liver Transplant Outcomes Uncovered by the Rat Model

For many individuals with end-stage liver disease, the only treatment option is liver transplantation. However, liver transplant rejection is observed in 24%-80% of transplant patients and lifelong drug regimens that follow the transplant procedure lead to serious side effects. Furthermore, the pool of donor livers available for transplantation is far less than the demand. Well-characterized and physiologically relevant models of liver transplantation are crucial to a deeper understanding of the cellular processes governing the outcomes of liver transplantation and serve as a platform for testing new therapeutic strategies to enhance graft acceptance. Such a model has been found in the rat transplant model, which has an advantageous size for surgical procedures, similar postoperative immunological progression, and high genome match to the human liver. From rat liver transplant studies published in the last 5 years, it is clear that the rat model serves as a strong platform to elucidate transplant immunological mechanisms. Using the model, we have begun to uncover potential players and possible therapeutic targets to restore liver tolerance and preserve host immunocompetence. Here, we present an overview of recent literature for rat liver transplant models, with an aim to highlight the value of the models and to provide future perspectives on how these models could be further characterized to enhance the overall value of rat models to the field of liver transplantation.

Determination of therapeutic agents efficiencies of microsatellite instability high colon cancer cells in post-metastatic liver biochip modeling

Two distinct genetic mutational pathways characterized by either chromosomal instability or high-frequency microsatellite instability (MSI-H) are recognized in the pathogenesis of colorectal cancer (CRC). Recently, it has been shown that patients with primary CRC that displays MSI-H have a significant, stage-independent, multivariate survival advantage. Biological properties of CMS1 (MSI-H type) can affect therapeutic efficiencies of agents used in the treatment of CRC, and therefore become a new predictive factor of the treatment. But, the predictive impact of MSI-H status for adjuvant chemotherapy remains controversial. This study will assess whether there is any unnecessary or inappropriate use of treatment agents recommended for adjuvant therapy of stage 2 and 3 of disease and for palliative or curative treatment of liver metastatic disease in microsatellite instability high group, a molecular subtype of colon cancer. Within this scope, the efficiencies of fluorouracil- and oxaliplatin-based chemotherapeutic agents will be shown on stage 3 microsatellite instability high colon tumor cell lines first, and then a microfluidic model will be created, imitating the metastasis of colon cancer to the liver. In the microfluidic chip model, we will create in liver tissue, where the metastasis of microsatellite instability high colon cancer will be simulated; the effectiveness of chemotherapeutic agents, immunotherapy agents, and targeted agents on tumor cells as well as drug response will be assessed according to cell viability through released biomarkers from the cells. The proposed hypothesis study includes the modeling and treatment of patient-derived post-metastatic liver cancer in microfluidics which has priority at the global and our region and consequently develop personal medication.

Advances in single-cell sequencing: insights from organ transplantation

Single-cell RNA sequencing (scRNA-seq) is a comprehensive technical tool to analyze intracellular and intercellular interaction data by whole transcriptional profile analysis. Here, we describe the application in biomedical research, focusing on the immune system during organ transplantation and rejection. Unlike conventional transcriptome analysis, this method provides a full map of multiple cell populations in one specific tissue and presents a dynamic and transient unbiased method to explore the progression of allograft dysfunction, starting from the stress response to final graft failure. This promising sequencing technology remarkably improves individualized organ rejection treatment by identifying decisive cellular subgroups and cell-specific interactions.

Emerging Role of Immunotherapy for Colorectal Cancer with Liver Metastasis

Colorectal cancer (CRC) is the third most common malignant tumor in the world and the second leading cause of cancer-related deaths, with the liver as the most common site of distant metastasis. The prognosis of CRC with liver metastasis is poor, and most patients cannot undergo surgery. In addition, conventional antitumor approaches such as chemotherapy, radiotherapy, targeted therapy, and surgery result in unsatisfactory outcomes. In recent years, immunotherapy has shown good prospects in the treatment of assorted tumors by enhancing the host's antitumor immune function, and it may become a new effective treatment for liver metastasis of CRC. However, challenges remain in applying immunotherapy to CRC with liver metastasis. This review examines how the microenvironment and immunosuppressive landscape of the liver favor tumor progression. It also highlights the latest research advances in immunotherapy for colorectal liver metastasis and identifies immunotherapy as a treatment regimen with a promising future in clinical applications.

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.

A novel 3D printing PCL/GelMA scaffold containing USPIO for MRI-guided bile duct repair

Making artificial bile ducts in vitro for repairing and replacing diseased bile ducts is an important concept in tissue engineering. This study printed a tubular composite scaffold using polycaprolactone (PCL) through the current 3D printing method. It served as a matrix for the organoid cells of the bile duct to proliferation, migration, and differentiation. The PCL scaffold full of bile duct-like organ cells can achieve the effect of bionics, replacing the original bile duct to perform its proper function. In order to enrich the performance of the tubular scaffold, hydrogels were also used in this study. Applying a layer of gelatin methacryloyl (GelMA) hydrogel with an appropriate thickness on the outer layer of the PCL scaffold not only protects and supports the scaffold, but also improves the biocompatibility of the printed bile duct. In addition, ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles dispersed in GelMA served as the contrast agent to monitor the repair of the lesion site and the degradation of the bile duct in real time by magnetic resonance imaging (MRI). In this study, a tubular composite scaffold that could reconstruct bile duct function and possess a real-time MRI imaging property was constructed by 3D printing. After 13 days of the co-culture of bone marrow derived stem cells (BMSCs), the survival rate of the BMSCs was greater than 95%, and the coverage of the BMSCs was as high as 90%. At the same time, the compression modulus of the stent could reach 17.41 kPa and the Young's modulus could reach 5.03 kPa. Thus, the mechanical properties of it can meet the needs of human implantation. USPIO can achieve MRI imaging in situ and nondestructively monitor the degradation of the stent in the body. In summary, PCL/GelMA/USPIO bile duct scaffolds are beneficial to the proliferation of cells on the scaffolds and can be used to construct biologically active artificial bile ducts.