Decreased Long-Term Severe Acute Respiratory Syndrome Coronavirus 2-Specific Humoral Immunity in Liver Transplantation Recipients 12 Months After Coronavirus Disease 2019.

Long-term humoral immunity and its protective role in liver transplantation (LT) patients have not been elucidated. We performed a prospective multicenter study to assess the persistence of immunoglobulin G (IgG) antibodies in LT recipients 12 months after coronavirus disease 2019 (COVID-19). A total of 65 LT recipients were matched with 65 nontransplanted patients by a propensity score including variables with recognized impact on COVID-19. LT recipients showed a lower prevalence of anti-nucleocapsid (27.7% versus 49.2%; P = 0.02) and anti-spike IgG antibodies (88.2% versus 100.0%; P = 0.02) at 12 months. Lower index values of anti-nucleocapsid IgG antibodies were also observed in transplantation patients 1 year after COVID-19 (median, 0.49 [interquartile range, 0.15-1.40] versus 1.36 [interquartile range, 0.53-2.91]; P < 0.001). Vaccinated LT recipients showed higher antibody levels compared with unvaccinated patients (P < 0.001); antibody levels reached after vaccination were comparable to those observed in nontransplanted individuals (P = 0.70). In LT patients, a longer interval since transplantation (odds ratio, 1.10; 95% confidence interval, 1.01-1.20) was independently associated with persistence of anti-nucleocapsid IgG antibodies 1 year after infection. In conclusion, compared with nontransplanted patients, LT recipients show a lower long-term persistence of anti-severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies. However, SARS-CoV-2 vaccination after COVID-19 in LT patients achieves a significant increase in antibody levels, comparable to that of nontransplanted patients.

Emerging Role of NLRP3 Inflammasome and Pyroptosis in Liver Transplantation.

The nucleotide-binding domain leucine-rich repeat-receptor, pyrin domain-containing-3 (NLRP3) inflammasome contributes to the inflammatory response by activating caspase-1, which in turn participates in the maturation of interleukin (IL)-1ß and IL-18, which are mainly secreted via pyroptosis. Pyroptosis is a lytic type of cell death that is controlled by caspase-1 processing gasdermin D. The amino-terminal fragment of gasdermin D inserts into the plasma membrane, creating stable pores and enabling the release of several proinflammatory factors. The activation of NLRP3 inflammasome and pyroptosis has been involved in the progression of liver fibrosis and its end-stage cirrhosis, which is among the main etiologies for liver transplantation (LT). Moreover, the NLRP3 inflammasome is involved in ischemia-reperfusion injury and early inflammation and rejection after LT. In this review, we summarize the recent literature addressing the role of the NLRP3 inflammasome and pyroptosis in all stages involved in LT and argue the potential targeting of this pathway as a future therapeutic strategy to improve LT outcomes. Likewise, we also discuss the impact of graft quality influenced by donation after circulatory death and the expected role of machine perfusion technology to modify the injury response related to inflammasome activation.

Epidemiological pattern, incidence, and outcomes of COVID-19 in liver transplant patients.

BACKGROUND & AIMS: The incidence and outcomes of coronavirus disease 2019 (COVID-19) in immunocompromised patients are a matter of debate. METHODS: We performed a prospective nationwide study including a consecutive cohort of liver transplant patients with COVID-19 recruited during the Spanish outbreak from 28 February to 7 April, 2020. The primary outcome was severe COVID-19, defined as the need for mechanical ventilation, intensive care, and/or death. Age- and gender-standardised incidence and mortality ratios (SIR and SMR) were calculated using data from the Ministry of Health and the Spanish liver transplant registry. Independent predictors of severe COVID-19 among hospitalised patients were analysed using multivariate Cox regression. RESULTS: A total of 111 liver transplant patients were diagnosed with COVID-19 (SIR = 191.2 [95% CI 190.3-192.2]). The epidemiological curve and geographic distribution overlapped widely between the liver transplant and general populations. After a median follow-up of 23 days, 96 patients (86.5%) were admitted to hospital and 22 patients (19.8%) required respiratory support. A total of 12 patients were admitted to the ICU (10.8%). The mortality rate was 18%, which was lower than in the matched general population (SMR = 95.5 [95% CI 94.2-96.8]). Overall, 35 patients (31.5%) met criteria of severe COVID-19. Baseline immunosuppression containing mycophenolate was an independent predictor of severe COVID-19 (relative risk = 3.94; 95% CI 1.59-9.74; p = 0.003), particularly at doses higher than 1,000 mg/day (p = 0.003). This deleterious effect was not observed with calcineurin inhibitors or everolimus and complete immunosuppression withdrawal showed no benefit. CONCLUSIONS: Being chronically immunosuppressed, liver transplant patients have an increased risk of acquiring COVID-19 but their mortality rates are lower than the matched general population. Upon hospital admission, mycophenolate dose reduction or withdrawal could help in preventing severe COVID-19. However, complete immunosuppression withdrawal should be discouraged. LAY SUMMARY: In liver transplant patients, chronic immunosuppression increases the risk of acquiring COVID-19 but it could reduce disease severity. Complete immunosuppression withdrawal may not be justified. However, mycophenolate withdrawal or temporary conversion to calcineurin inhibitors or everolimus until disease resolution could be beneficial in hospitalised patients.

VI consensus document by the Spanish Liver Transplantation Society. / VI documento de consenso de la sociedad española de trasplante hepático (SETH).

The goal of the Spanish Liver Transplantation Society (La Sociedad Española de Trasplante Hepático) is to promote and create consensus documents about current topics in liver transplantation with a multidisciplinary approach. To this end, on October 20, 2016, the 6th Consensus Document Meeting was held, with the participation of experts from the 24 authorized Spanish liver transplantation programs. This Edition discusses the following subjects, whose summary is offered below: 1) limits of simultaneous liver-kidney transplantation; 2) limits of elective liver re-transplantation; and 3) liver transplantation after resection and hepatocellular carcinoma with factors for a poor prognosis. The consensus conclusions for each of these topics is provided below.

Differential profile of activated regulatory T cell subsets and microRNAs in tolerant liver transplant recipients.

Regulatory T cells (Tregs) play a potential role in operational tolerance in liver transplantation (LT) patients, and microRNAs (miRNAs) are known to be involved in immunological responses and tolerance. Thus, we analyzed the implication of different peripheral blood Treg subsets and miRNAs on LT tolerance in 24 tolerant (Tol) and 23 non-tolerant (non-Tol) LT recipients by cellular, genetic, and epigenetic approximation. Non-Tol patients had a lower demethylation rate of the forkhead box P3 (FOXP3) regulatory T cell-specific demethylated region (TSDR) than Tol patients that correlated with the frequency of circulating Tregs. Tol patients presented a different signature of Treg subset markers compared with non-Tol patients with increased expression of HELIOS and FOXP3 and a higher proportion of latency-associated peptide (LAP)+ Tregs and CD45RA- human leukocyte antigen D related (HLA-DR)+ activated effector-memory Tregs. The expression of miR95, miR24, miR31, miR146a, and miR155 was higher in Tol than in non-Tol patients and was positively correlated with activated Treg markers. In conclusion, these data suggest that activated effector-memory Tregs and a TSDR-demethylation state of Tregs may play a role in the complex system of regulation of LT tolerance. In addition, we describe a set of miRNAs differentially expressed in human LT Tol patients providing suggestive evidence that miRNAs are implied in the preservation of self-tolerance as mediated by Tregs. Liver Transplantation 23 933-945 2017 AASLD.

Efficacy and safety of daclatasvir-based antiviral therapy in hepatitis C virus recurrence after liver transplantation. Role of cirrhosis and genotype 3. A multicenter cohort study.

Direct-acting antiviral agents (DAA) combining daclatasvir (DCV) have reported good outcomes in the recurrence of hepatitis C virus (HCV) infection after liver transplant (LT). However, its effect on the severe recurrence and the risk of death remains controversial. We evaluated the efficacy, predictors of survival, and safety of DAC-based regimens in a large real-world cohort. A total of 331 patients received DCV-based therapy. Duration of therapy and ribavirin use were at the investigator's discretion. The primary end point was sustained virological response (SVR) at week 12. A multivariate analysis of predictive factors of mortality was performed. Intention-to-treat (ITT) and per-protocol SVR were 93.05% and 96.9%. ITT-SVR was lower in cirrhosis (n = 163) (96.4% vs. 89.6% P = 0.017); the SVR in genotype 3 (n = 91) was similar, even in advanced fibrosis (96.7% vs. 88%, P = 0.2). Ten patients (3%) experienced virological failure. Therapy was stopped in 18 patients (5.44%), and ten died during treatment. A total of 22 patients (6.6%) died. Albumin (HR = 0.376; 95% CI 0.155-0.910) and baseline MELD (HR = 1.137; 95% CI: 1.061-1.218) were predictors of death. DCV-based DAA treatment is efficacious and safe in patients with HCV infection after LT. Baseline MELD score and serum albumin are predictors of survival irrespective of viral response.

A hemodynamic, metabolic and histopathological study of a heterotopic auxiliary swine liver graft with portal vein arterialization.

BACKGROUND: Auxiliary heterotopic liver transplantation with portal vein arterialization (AHLT-PVA) is a model that has been hardly studied, despite its therapeutic potential. METHODS: Hemodynamic and biochemical characterization was carried out during graft implantation, in a pig-to-pig model (n=15 AHLT-PVA). Furthermore a histopathological study was performed to establish microscopic alterations due to PVA. RESULTS: Reperfusion of the arterialized graft produced an increase in heart rate (HR) vs. baseline (P=.004) and vs. inferior vena cava clamping phase (P=.004); and a decrease in systemic vascular resistance vs. cava clamping phase (P=.021). At the end of implantation, cardiac output remained elevated (P=.001), likewise HR remained increased vs. baseline phase (P=.002). Mean arterial pressure decreased with cava clamping, but was not affected by the reperfusion of the graft, nor the skin closure. The histopathological study at 3, 10, and 21 days post-PVA revealed that functional liver structure was maintained although it is common to find foci of perilobular necrosis on day 3 (P=.049), and perilobular connective tissue proliferation at day 10 (P=.007), vs. native liver. CONCLUSIONS: The described arterialized liver graft model minimizes the number of vascular anastomoses vs. previously described models. It is hemodynamically and metabolically well tolerated and the double arterial vascularization of the graft does not cause significant changes in liver histology.

[V Consensus Meeting of the Spanish Society for Liver Transplant on high-risk recipients, immunosupression scenarios and management of hepatocarcinoma on the transplant waiting list]. / V Reunión de Consenso de la Sociedad Española de Trasplante Hepático sobre receptores de riesgo elevado, escenarios actuales de inmunosupresión y manejo del hepatocarcinoma en espera de trasplante.

With the aim to promote the elaboration of consensus documents on state of the art topics in liver transplantation with multidisciplinary management, the Spanish Society for Liver Transplantation (SETH) organized the V Consensus Meeting with the participation of experts from all the Spanish liver transplant programs. In this edition, the following topics were revised, and we present the summary: 1. High-risk receptors; 2. Immunosuppression scenarios; and 3. Management of the patient with hepatocarcinoma in the waiting list.

V Consensus Meeting of the Spanish Society for Liver Transplant on high-risk recipients, immunosupression scenarios and management of hepatocarcinoma on the transplant waiting list.

With the aim to promote the elaboration of consensus documents on state of the art topics in liver transplantation with multidisciplinary management, the Spanish Society for Liver Transplantation (SETH) organized the V Consensus Meeting with the participation of experts from all the Spanish liver transplant programs. In this edition, the following topics were revised, and we present the summary: 1. High-risk receptors; 2. Immunosuppression scenarios; and 3. Management of the patient with hepatocarcinoma in the waiting list.

Operational tolerance research in liver transplantation: a bibliometric analysis using a new web resource.

Background: Operational tolerance in liver transplantation (OT-LT), defined as the graft survival with normal function in absence of immunosuppression, has been a field of intense research since the 1980s. Thereafter, tens of clinical trials and hundreds of articles have been published, making it challenging for researchers to assimilate all the information, more so outside of their disciplines. The aim of the present study was to analyze the research in OT-LT through a new web tool (https://tolerance.imib.es). Methods: We have developed a web resource that allowed the identification of the present trends and potential research avenues in OL-LT, an overview biomedical terms that were most often cited, including which journals published the most articles, and an advanced search engine that exploited all the information in these publications. Results: A total of 734 studies were analyzed until November 2023, with a mean of 15 articles published per year, a total sum of 3,751 impact factor points and a total of 26,542 citations. The analysis of citations allowed us to establish a ranking of the most prolific countries, authors, journals and institutions, in addition to the most influential publications in OT-LT. Likewise, keyword and co-occurrence analyses answered which themes involving OT-LT are the most popular, whereas cooperation analysis showed that principal authors in OT-LT form a network, although the lack of international cooperation, especially with regard to clinical trials, appears to be one of the main challenges. Conclusion: Despite its limitations, our web tool will allow both OT-LT expert and novel researchers to be able to draw a comprehensive picture of the past, present and future of OT-LT research.

Cell-specific Extracellular Vesicles and Their miRNA Cargo Released Into the Organ Preservation Solution During Cold Ischemia Storage as Biomarkers for Liver Transplant Outcomes.

BACKGROUND: Liver transplantation (LT) is crucial for end-stage liver disease patients, but organ shortages persist. Donation after circulatory death (DCD) aims to broaden the donor pool but presents challenges. Complications like acute rejection, hepatic artery thrombosis, and biliary issues still impact posttransplant prognosis. Biomarkers, including extracellular vesicles (EVs) and microRNAs (miRNAs), show promise in understanding and monitoring posttransplant events. This study explores the role of EVs and their miRNA cargo in LT, including their potential as diagnostic tools. METHODS: EVs from intrahepatic end-ischemic organ preservation solution (eiOPS) in 79 donated livers were detected using different techniques (nanosight tracking analysis, transmission electron microscopy, and flow cytometry). EV-derived miRNAs were identified by quantitative real time-polymerase chain reaction. Bioinformatics analysis was performed using the R platform. RESULTS: Different-sized and origin-specific EVs were found in eiOPS, with significantly higher concentrations in DCD compared with donation after brain death organs. Additionally, several EV-associated miRNAs, including let-7d-5p, miR-28-5p, miR-200a-3p, miR-200b-3p, miR-200c-3p, and miR-429, were overexpressed in DCD-derived eiOPS. These miRNAs also exhibited differential expression patterns in liver tissue biopsies. Pathway analysis revealed enrichment in signaling pathways involved in extracellular matrix organization and various cellular processes. Moreover, specific EVs and miRNAs correlated with clinical outcomes, including survival and early allograft dysfunction. A predictive model combining biomarkers and clinical variables showed promise in acute rejection detection after LT. CONCLUSIONS: These findings provide new insights into the use of EVs and miRNAs as biomarkers and their possible influence on posttransplantation outcomes, potentially contributing to improved diagnostic approaches and personalized treatment strategies in LT.

GEMA-Na and MELD 3.0 severity scores to address sex disparities for accessing liver transplantation: a nationwide retrospective cohort study.

Background: The Gender-Equity Model for liver Allocation corrected by serum sodium (GEMA-Na) and the Model for End-stage Liver Disease 3.0 (MELD 3.0) could amend sex disparities for accessing liver transplantation (LT). We aimed to assess these inequities in Spain and to compare the performance of GEMA-Na and MELD 3.0. Methods: Nationwide cohort study including adult patients listed for a first elective LT (January 2016-December 2021). The primary outcome was mortality or delisting for sickness within the first 90 days. Independent predictors of the primary outcome were evaluated using multivariate Cox's regression with adjusted relative risks (RR) and 95% confidence intervals (95% CI). The discrimination of GEMA-Na and MELD 3.0was assessed using Harrell c-statistics (Hc). Findings: The study included 6071 patients (4697 men and 1374 women). Mortality or delisting for clinical deterioration occurred in 286 patients at 90 days (4.7%). Women had reduced access to LT (83.7% vs. 85.9%; p = 0.037) and increased risk of mortality or delisting for sickness at 90 days (adjusted RR = 1.57 [95% CI 1.09-2.28]; p = 0.017). Female sex remained as an independent risk factor when using MELD or MELD-Na but lost its significance in the presence of GEMA-Na or MELD 3.0. Among patients included for reasons other than tumours (n = 3606; 59.4%), GEMA-Na had Hc = 0.753 (95% CI 0.715-0.792), which was higher than MELD 3.0 (Hc = 0.726 [95% CI 0.686-0.767; p = 0.001), showing both models adequate calibration. Interpretation: GEMA-Na and MELD 3.0 might correct sex disparities for accessing LT, but GEMA-Na provides more accurate predictions of waiting list outcomes and could be considered the standard of care for waiting list prioritization. Funding: Instituto de Salud Carlos III, Agencia Estatal de Investigación (Spain), and European Union.

Efficacy of the retreatment of hepatitis C virus infections after liver transplantation: role of an aggressive approach.

A sustained virological response (SVR) is achieved by 30% of naive liver transplantation (LT) recipients treated with pegylated interferon (PEG-IFN) and ribavirin (RBV). Almost no data are available about retreatment. The aim of this study was to assess the efficacy, tolerability, and SVR predictors of retreatment. Data were collected from 4 centers on the retreatment of prior nonresponders to standard therapy or PEG-IFN (with or without RBV) and relapsers. Seventy-nine of 301 treatment-experienced LT patients (26%), who had a median age of 59 years (range = 35-77 years) and were mostly male (72%) and infected with genotype 1 (87%), were retreated with PEG-IFN and RBV at a median of 6.9 years after LT. During the first course of therapy, 35% were treated with interferon, 49% received tacrolimus, 52% received steroids, and 49.5% were relapsers. Retreatment was started at a median of 1.9 years (range = 45 days to 8.2 years) after the end of the first course. The proportion of patients with cirrhosis increased from 10% to 37% (P < 0.001). In addition, in retreated patients, full initial RBV doses (P = 0.03), growth factors [erythropoietin (P < 0.001) and granulocyte colony-stimulating factor (P = 0.048)], and transfusions (P = 0.03) were used more frequently, and the treatment duration was longer (P = 0.03). An end-of-treatment response was achieved in 61%, whereas SVR, which was associated with improved survival, occurred in 28 (35%). The variables predicting SVR were age (P = 0.04), disease severity [fibrosis (50% with F0-F2 versus 26% with F3-4), P = 0.03; bilirubin, P = 0.006; platelet count, P = 0.03], adherence, and viral kinetics. None of the patients without an early virological response achieved SVR. There was a trend of prior relapsers achieving higher SVR rates than prior nonresponders. In conclusion, SVR, which was achieved by approximately one-third of the retreated patients, can be predicted with the same variables used for naive LT recipients (age, disease severity, adherence, and viral kinetics) and is associated with enhanced survival.

Is the Development of Ascites in Alcoholic Liver Patients Influenced by Specific KIR/HLA Gene Profiles?

Decompensated cirrhosis is the most common cause of ascites due to hemodynamic and renal alteration by continuous fluid leakage from the hepatic sinusoids and splanchnic capillaries into the interstitial space. Then, fluid leakage exceeds lymphatic return, leading to progressive fluid accumulation directly into the peritoneal cavity. Alcohol consumption is one of the main risks of developing alcoholic cirrhosis (AC), but not all AC patients develop ascites. Avoiding the development of ascites is crucial, given that it deteriorates prognosis and increases the patient mortality patient. The innate immune system plays a crucial role in cirrhosis through natural killer cells, which are abundant in the liver. The aim of this study was to analyze the KIR/HLA-C genetic profile in AC patients with and without ascites to understand this pathology and find predictive clinical susceptibility biomarkers that can help to establish risks and prevent the development of ascites in AC patients. A total of 281 AC patients with and without ascites were analyzed and compared with 319 healthy controls. Genomic DNA was extracted from peripheral blood in all groups. A PCR-SSO assay was performed for KIR/HLA genotyping analysis. A total of 16 activating and inhibitor KIR genes and their corresponding known ligands, epitopes of HLA-C, and their genotypes were analyzed. According to our analysis, C1 epitopes were statistically significantly decreased in AC patients with and without ascites. When comparing AC patients with ascites and healthy controls, a significant decrease in C1 epitope frequency was also observed. A statistically significant decrease was also found when comparing the C1C2 genotype in AC patients without ascites with controls. In conclusion, the absence of KIR2DL2 and KIR3DL1 genes may be a predisposing factor for the development of ascites in AC patients. The KIR2DS2/KIR2DL2 may could be involved in grade I ascites development, and the presence of the C1+ epitope and the homozygous C2C2 genotype may be protective genetic factors against ascites development in AC patients.

Selecting the Appropriate Downstaging and Bridging Therapies for Hepatocellular Carcinoma: What Is the Role of Transarterial Radioembolization? A Pooled Analysis.

BACKGROUND: Transarterial radioembolization in HCC for LT as downstaging/bridging has been increasing in recent years but some indication criteria are still unclear. METHODS: We conducted a systematic literature search of primary research publications conducted in PubMed, Scopus and ScienceDirect databases until November 2022. Relevant data about patient selection, HCC features and oncological outcomes after TARE for downstaging or bridging in LT were analyzed. RESULTS: A total of 14 studies were included (7 downstaging, 3 bridging and 4 mixed downstaging and bridging). The proportion of whole liver TARE was between 0 and 1.6%. Multiple TARE interventions were necessary for 16.7% up to 28% of the patients. A total of 55 of 204 patients across all included studies undergoing TARE for downstaging were finally transplanted. The only RCT included presents a higher tumor response with the downstaging rate for LT of TARE than TACE (9/32 vs. 4/34, respectively). Grade 3 or 4 adverse effects rate were detected between 15 and 30% of patients. CONCLUSIONS: TARE is a safe therapeutic option with potential advantages in its capacity to necrotize and reduce the size of the HCC for downstaging or bridging in LT.

Postoperative negative-pressure incision therapy after liver transplant (PONILITRANS study): A randomized controlled trial.

BACKGROUND: Postoperative complications of surgical incisions are frequent in liver transplantation. However, evidence justifying the use of incisional negative pressure wound therapy to improve surgical wound outcomes remains limited. METHODS: Participating patients were randomly assigned to receive incisional negative pressure wound therapy or standard surgical dressing on the closed surgical incision of the liver transplantation. The primary endpoint was surgical site infection incidence 30 days postoperatively. The secondary endpoints included surgical site events (ie, surgical site infection, dehiscence, hematoma, and seroma) and wound quality of life. RESULTS: Between December 2018 and September 2021, 108 patients (54 in the incisional negative pressure wound therapy group and 54 in the control group) were enrolled in this study. The incidence of surgical site infection at 30 days postoperatively was 7.4% in the treatment group and 13% in the control group (P = .34). The rate of surgical site events was similar in the treatment in the and control group (27.8% vs 29.6%, P = .83). In relation to wound quality of life, the mean score was 75.20 ± 7.27 in the incisional negative pressure wound therapy group and 72.82 ± 10.57 in the control group (P = .23). CONCLUSION: The prophylactic use of negative pressure wound therapy on primarily closed incisions did not significantly reduce incisional surgical site infection and surgical site event rates after liver transplantation compared with standard surgical dressings.

Using transcriptional profiling to develop a diagnostic test of operational tolerance in liver transplant recipients.

A fraction of liver transplant recipients are able to discontinue all immunosuppressive therapies without rejecting their grafts and are said to be operationally tolerant to the transplant. However, accurate identification of these recipients remains a challenge. To design a clinically applicable molecular test of operational tolerance in liver transplantation, we studied transcriptional patterns in the peripheral blood of 80 liver transplant recipients and 16 nontransplanted healthy individuals by employing oligonucleotide microarrays and quantitative real-time PCR. This resulted in the discovery and validation of several gene signatures comprising a modest number of genes capable of identifying tolerant and nontolerant recipients with high accuracy. Multiple peripheral blood lymphocyte subsets contributed to the tolerance-associated transcriptional patterns, although NK and gammadeltaTCR+ T cells exerted the predominant influence. These data suggest that transcriptional profiling of peripheral blood can be employed to identify liver transplant recipients who can discontinue immunosuppressive therapy and that innate immune cells are likely to play a major role in the maintenance of operational tolerance in liver transplantation.

[Development of immune tolerance in liver transplantation]. / Desarrollo de inmunotolerancia en el trasplante hepático.

The liver is a privileged organ and has a lower incidence of rejection than other organs. However, immunosuppressive regimens are still required to control the alloreactive T-lymphocyte response after transplantation. These treatments may lead to severe complications, such as infectious diseases, cancers, cardiovascular diseases and chronic renal insufficiency. In clinical transplantation there is increasing evidence that some liver transplant recipients who cease taking immunosuppressive (IS) drugs maintain allograft function, suggesting that tolerance is already present. This strategy is feasible in 25-33% of liver transplant recipients. A series of experimental and clinical observations indicates that liver allografts can even provide "tolerogenic" properties for other organ grafts. In the clinical setting, clinical operational tolerance (COT) is defined as the absence of acute and chronic rejection and graft survival with normal function and histology in an IS-free, fully immunocompetent host, usually as an end result of a successful attempt at IS withdrawal. The exact mechanisms involved in achieving transplant tolerance remain unknown, although animal models suggest a possible role for regulatory T cells (Treg). Recent data have demonstrated an increase in the frequency of CD4+ CD25(high) T cells and FoxP3 transcripts during IS withdrawal in operationally tolerant liver transplant recipients. The data obtained from transcriptional profiling of the peripheral blood of IS-free liver transplant recipients suggest that there is a molecular signature of tolerance that could be employed to identify tolerant liver transplant recipients and that innate immune cells are likely to play a major role in the maintenance of COT after liver transplantation.