Allospecific CD154+ T cells associate with rejection risk after pediatric liver transplantation

Enhanced Donor Antigen Presentation by B Cells Predicts Acute Cellular Rejection and Late Outcomes After Transplantation

Background

Enhanced B-cell presentation of donor alloantigen relative to presentation of HLA-mismatched reference alloantigen is associated with acute cellular rejection (ACR), when expressed as a ratio called the antigen presenting index (API) in an exploratory cohort of liver and intestine transplant (LT and IT) recipients.

Methods

To test clinical performance, we measured the API using the previously described 6-h assay in 84 LT and 54 IT recipients with median age 3.3 y (0.05-23.96). Recipients experiencing ACR within 60 d after testing were termed rejectors.

Results

We first confirmed that B-cell uptake and presentation of alloantigen induced and thus reflected the alloresponse of T-helper cells, which were incubated without and with cytochalasin and primaquine to inhibit antigen uptake and presentation, respectively. Transplant recipients included 76 males and 62 females. Rejectors were tested at median 3.6 d before diagnosis. The API was higher among rejectors compared with nonrejectors (2.2 ± 0.2 versus 0.6 ± 0.04, P value = 1.7E-09). In logistic regression and receiver-operating-characteristic analysis, API ≥1.1 achieved sensitivity, specificity, and positive and negative predictive values for predicting ACR in 99 training set samples. Corresponding metrics ranged from 80% to 88% in 32 independent posttransplant samples, and 73% to 100% in 20 independent pretransplant samples. In time-to-event analysis, API ≥1.1 predicted higher incidence of late donor-specific anti-HLA antibodies after API measurements in LT recipients (P = 0.011) and graft loss in IT recipients (P = 0.008), compared with recipients with API <1.1, respectively.

Conclusions

Enhanced donor antigen presentation by circulating B cells predicts rejection after liver or intestine transplantation as well as higher incidence of DSA and graft loss late after transplantation.

T-cell infiltrate intensity is associated with delayed response to treatment in late acute cellular rejection in pediatric liver transplant recipients

BACKGROUND

Late acute cellular rejection (ACR) is associated with donor-specific antibodies (DSA) development, chronic rejection, and allograft loss. However, accurate predictors of late ACR treatment response are lacking. ACR is primarily T-cell mediated, yet B cells and plasma cells (PC) also infiltrate the portal areas during late ACR. To test the hypothesis that the inflammatory milieu is associated with delayed response (DR) to rejection therapy, we performed a single-center retrospective case-control study of pediatric late liver ACR using multiparameter immunofluorescence for CD4, CD8, CD68, CD20, and CD138 to identify immune cell subpopulations.

METHODS

Pediatric liver transplant recipients transplanted at <17 years of age and treated for biopsy-proven late ACR between January 2014 and 2019 were stratified into rapid response (RR) and DR based on alanine aminotransferase (ALT) normalization within 30 days of diagnosis. All patients received IV methylprednisolone as an initial rejection treatment. Immunofluorescence was performed on archived formalin-fixed paraffin embedded (FFPE) liver biopsy tissue.

RESULTS

Liver biopsies from 60 episodes of late ACR in 54 patients were included in the analysis, of which 33 were DR (55%). Anti-thymocyte globulin was only required in the DR group. The frequency of liver-infiltrating CD20+ and CD8+ lymphocytes and the prevalence of autoantibodies were higher in the DR group. In univariate logistic regression analysis, serum gamma-glutamyl transpeptidase (GGT) level at diagnosis, but not ALT, Banff score or presence of DSA, predicted DR.

CONCLUSIONS

Higher serum GGT level, presence of autoantibodies, and increased CD8+ T-cell infiltration portends DR in late ACR treatment in children.

Analysis of T-cell alloantigen response via a direct pathway in kidney transplant recipients with donor-specific antibodies

Donor-specific antibodies (DSAs) are the main cause of graft loss over time. The direct pathway of alloantigen recognition is important in the pathogenesis of acute rejection. Recent studies have suggested that the direct pathway also contributes to the pathogenesis of chronic injury. Nevertheless, there are no reports on T-cell alloantigen response via the direct pathway in kidney recipients with DSAs. We analyzed the T-cell alloantigen response via the direct pathway in kidney recipients with DSAs (DSA+) or without DSAs (DSA-). A mixed lymphocyte reaction assay was implemented to assess the direct pathway response. DSA+ patients showed significantly higher CD8⁺ and CD4⁺ T cell responses to donor cells than DSA- patients. Furthermore, proliferating CD4⁺ T cells showed a marked increase in Th1 and Th17 responses in DSA+ patients than in DSA- patients. In a comparison between anti-donor and third-party responses, the anti-donor CD8⁺ and CD4⁺ T cell response was significantly lower than the anti-third-party response. In contrast, the donor-specific hyporesponsiveness was absent in DSA+ patients. Our study demonstrated that DSA+ recipients have a greater potential for developing immune responses against the donor tissues via the direct alloantigen recognition pathway. These data contribute to an understanding of DSAs pathogenicity during kidney transplantation.

Operational tolerance after intestine re-transplantation in childhood and immunological correlates. Case report and review

BACKGROUND

Operational tolerance after retransplantation of the intestine has never been reported.

PURPOSE

To two recently described intestine transplant recipients with operational tolerance, we now add a third.

METHODS

Review of case record and immunological testing to confirm donor-specific hyporesponsiveness in multiple immune cell compartments.

RESULTS

Re-transplanted with a multivisceral liver- and kidney-inclusive intestine allograft at age 12 years, this recipient self-discontinued immunosuppression 14 years after the retransplant and has been rejection free for 2 years thereafter. As in the two previous reports, immunological testing demonstrated decreased donor-specific inflammatory response of T-cytotoxic memory cells and B-cells, decreased presentation of donor antigen by B-cells and monocytes, absence of donor-specific anti-HLA antibodies, circulating FOXP3 + T-helper cells, and intact cellular and humoral immunity to cytomegalovirus and Epstein-Barr virus. Additionally, our recipient demonstrated enhanced donor-activation-induced apoptosis of alloreactive T-cytotoxic memory cells.

CONCLUSIONS

Despite variable paths to tolerance which include graft versus host disease in two previous cases, and rejection-related loss of the primary isolated intestinal allograft in our recipient, the three cases with operational tolerance are bound by common themes: a relatively large donor antigenic load transmitted during intestine transplantation, and donor-specific hyporesponsiveness. Cell-based assays suggest enhanced donor-induced apoptosis of recipient T-cells and circulating T-regulatory cells as mechanistic links between antigenic load and donor-specific hyporesponsiveness.

A network-based approach to identify expression modules underlying rejection in pediatric liver transplantation

Selecting the right immunosuppressant to ensure rejection-free outcomes poses unique challenges in pediatric liver transplant (LT) recipients. A molecular predictor can comprehensively address these challenges. Currently, there are no well-validated blood-based biomarkers for pediatric LT recipients before or after LT. Here, we discover and validate separate pre- and post-LT transcriptomic signatures of rejection. Using an integrative machine learning approach, we combine transcriptomics data with the reference high-quality human protein interactome to identify network module signatures, which underlie rejection. Unlike gene signatures, our approach is inherently multivariate and more robust to replication and captures the structure of the underlying network, encapsulating additive effects. We also identify, in an individual-specific manner, signatures that can be targeted by current anti-rejection drugs and other drugs that can be repurposed. Our approach can enable personalized adjustment of drug regimens for the dominant targetable pathways before and after LT in children.

Renal allograft surveillance with allospecific T-cytotoxic memory cells

Background

Allo-antigen-specific T-cytotoxic memory cells (TcM) which express CD40 ligand (CD154) in overnight lymphocyte co-culture are strongly associated with acute cellular rejection (ACR) seen in “for cause” biopsies for renal allograft dysfunction. Specifically, when the likelihood of rejection is increased, donor-specific allospecific TcM exceed those induced by HLA-non-identical third-party cell by 1.15-fold or greater.

Methods

The performance of allospecific TcM was evaluated retrospectively in primary renal transplant recipients (RTR) at routine clinical visits, cross-sectionally at presentation for biopsies, and serially. Performance metrics were sensitivity, specificity, positive and negative predictive values (PPV and NPV).

Results

Twenty-two primary RTR, median age 45 years (range 19–72) were tested with allospecific CD154 + TcM. Samples were obtained at the mean ± SD time interval of 806 ± 239 days after kidney transplantation. Six of 22 patients experienced biopsy proven T- Cell Mediated Rejection (TCMR). A seventh showed antibody mediated rejection (ABMR). Of these seven patients six demonstrated increased likelihood of rejection with allospecific TcM (sensitivity 83%). Ten of these 15 patients with no rejection had a negative test (specificity 67%). False positive tests were seen in five patients. Six out of 11 patients with positive tests had ACR/ABMR with a PPV of 54%, while 10 out of 11 patients with negative tests were non-rejecters with a NPV of 91%.

Conclusion

Allospecific T-cytotoxic memory cells distinguished primary RTR with quiescent allografts from those with dysfunction. With serial surveillance measures, this test system may facilitate decisions to manage immunosuppression in RTR.

Tackling Chronic Kidney Transplant Rejection: Challenges and Promises

Despite advances in post-transplant management, the long-term survival rate of kidney grafts and patients has not improved as approximately forty percent of transplants fails within ten years after transplantation. Both immunologic and non-immunologic factors contribute to late allograft loss. Chronic kidney transplant rejection (CKTR) is often clinically silent yet progressive allogeneic immune process that leads to cumulative graft injury, deterioration of graft function. Chronic active T cell mediated rejection (TCMR) and chronic active antibody-mediated rejection (ABMR) are classified as two principal subtypes of CKTR. While significant improvements have been made towards a better understanding of cellular and molecular mechanisms and diagnostic classifications of CKTR, lack of early detection, differential diagnosis and effective therapies continue to pose major challenges for long-term management. Recent development of high throughput cellular and molecular biotechnologies has allowed rapid development of new biomarkers associated with chronic renal injury, which not only provide insight into pathogenesis of chronic rejection but also allow for early detection. In parallel, several novel therapeutic strategies have emerged which may hold great promise for improvement of long-term graft and patient survival. With a brief overview of current understanding of pathogenesis, standard diagnosis and challenges in the context of CKTR, this mini-review aims to provide updates and insights into the latest development of promising novel biomarkers for diagnosis and novel therapeutic interventions to prevent and treat CKTR.

Detection of alloreactive T cells from cryopreserved human peripheral blood mononuclear cells

Precise analyses of alloreactive T cell phenotype and function can inform both the nature and intensity of adaptive responses to transplant antigens. However, alloreactive T cells are sparse and difficult to detect, particularly in cryopreserved peripheral blood mononuclear cells (PBMCs) and from hypo-responsive individuals. An assay to identify and phenotype alloreactive cells would be particularly valuable, especially for multi-center clinical trials that often store frozen samples for batch analysis. Herein we demonstrate consistent and reproducible alloreactive T cell detection in cryopreserved PBMC following a short-term mixed lymphocyte reaction (MLR). The inherent background expression levels of activation markers on responder T cells were minimized by including a resting period prior to the assay. Stimulator cells were activated before inclusion in the MLR by addition of CD40L and IL-4. The time frame and markers to identify and phenotype alloreactive T cells following stimulation were optimized using short term co-cultures. We defined subsets of CD4+ and CD8+ T cells co-expressing CD69 and either CD154 or CD137 following allostimulation as alloreactive, and further phenotyped these cells with a variety of surface markers such as PD-1, LAG-3, and TIM-3. This assay may allow for the monitoring of donor-specific T cells in transplant recipients with longitudinally collected and cryopreserved PBMCs and provide a useful tool to identify biomarkers associated with tolerance. These biomarkers may add to mechanistic insights in immune recognition of transplanted tissues and/or cells.

Identification of peripheral CD154+ T cells and HLA-DRB1 as biomarkers of acute cellular rejection in adult liver transplant recipients

Decreasing graft rejection and increasing graft and patient survival are great challenges facing liver transplantation (LT). Different T cell subsets participate in the acute cellular rejection (ACR) of the allograft. Cell-mediated immunity markers of the recipient could help to understand the mechanisms underlying acute rejection. This study aimed to analyse different surface antigens on T cells in a cohort of adult liver patients undergoing LT to determine the influence on ACR using multi-parametric flow cytometry functional assay. Thirty patients were monitored at baseline and during 1 year post-transplant. Two groups were established, with (ACR) and without (NACR) acute cellular rejection. Leukocyte, total lymphocyte, percentages of CD4+ CD154+ and CD8+ CD154+ T cells, human leukocyte antigen (HLA) mismatch between recipient-donor and their relation with ACR as well as the acute rejection frequencies were analysed. T cells were stimulated with concanavalin A (Con-A) and surface antigens were analysed by fluorescence activated cell sorter (FACS) analysis. A high percentage of CD4+ CD154+ T cells (P = 0·001) and a low percentage of CD8+ CD154+ T cells (P = 0·002) at baseline were statistically significant in ACR. A receiver operating characteristic analysis determined the cut-off values capable to stratify patients at high risk of ACR with high sensitivity and specificity for CD4+ CD154+ (P = 0·001) and CD8+ CD154+ T cells (P = 0·002). In logistic regression analysis, CD4+ CD154+ , CD8+ CD154+ and HLA mismatch were confirmed as independent risk factors to ACR. Post-transplant percentages of both T cell subsets were significantly higher in ACR, despite variations compared to pretransplant. These findings support the selection of candidates for LT based on the pretransplant percentages of CD4+ CD154+ and CD8+ CD154+ T cells in parallel with other transplant factors.

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.

Maintenance Immunosuppression in Solid Organ Transplantation: Integrating Novel Pharmacodynamic Biomarkers to Inform Calcineurin Inhibitor Dose Selection

Calcineurin inhibitors, the primary immunosuppressive therapy used to prevent alloreactivity of transplanted organs, have a narrow therapeutic index. Currently, treatment is individualized based on clinical assessment of the risk of rejection or toxicity guided by trough concentration monitoring. Advances in immune monitoring have identified potential markers that may have value in understanding calcineurin inhibitor pharmacodynamics. Integration of these markers has the potential to complement therapeutic drug monitoring. Existing pharmacokinetic-pharmacodynamic (PK-PD) data is largely limited to correlation between the biomarker and trough concentrations at single time points. Immune related gene expression currently has the most evidence supporting PK-PD integration. Novel biomarker-based approaches to pharmacodynamic monitoring including development of enhanced PK-PD models are proposed to realize the full clinical benefit.

Applicability, safety, and biological activity of regulatory T cell therapy in liver transplantation

Regulatory T cells (Tregs) are a lymphocyte subset with intrinsic immunosuppressive properties that can be expanded in large numbers ex vivo and have been shown to prevent allograft rejection and promote tolerance in animal models. To investigate the safety, applicability, and biological activity of autologous Treg adoptive transfer in humans, we conducted an open-label, dose-escalation, Phase I clinical trial in liver transplantation. Patients were enrolled while awaiting liver transplantation or 6-12 months posttransplant. Circulating Tregs were isolated from blood or leukapheresis, expanded under good manufacturing practices (GMP) conditions, and administered intravenously at either 0.5-1 million Tregs/kg or 3-4.5 million Tregs/kg. The primary endpoint was the rate of dose- limiting toxicities occurring within 4 weeks of infusion. The applicability of the clinical protocol was poor unless patient recruitment was deferred until 6-12 months posttransplant. Thus, only 3 of the 17 patients who consented while awaiting liver transplantation were dosed. In contrast, all six patients who consented 6-12 months posttransplant received the cell infusion. Treg transfer was safe, transiently increased the pool of circulating Tregs and reduced anti-donor T cell responses. Our study opens the door to employing Treg immunotherapy to facilitate the reduction or complete discontinuation of immunosuppression following liver transplantation.

CD154-expressing CMV-specific T cells associate with freedom from DNAemia and may be protective in seronegative recipients after liver or intestine transplantation

Cell-mediated immunity to CMV, if known, could improve antiviral drug therapy in at-risk children and young adults with LT and IT. Host immunity has been measured with CMV-specific T cells, which express IFNγ, but not those which express CD154, a possible substitute for IFNγ. CMV-specific CD154+ T cells and their subsets were measured with flow cytometry after stimulating PBL from recipient blood samples with an overlapping peptide mix of CMV-pp65 antigen for up to 6 hours. CMV-specific CD154+ T cells co-expressed IFNγ in PBL from three healthy adults and averaged 3.8% (95% CI 3.2%-4.4%) in 40 healthy adults. CMV-specific T cells were significantly lower in 19 CMV DNAemic LT or IT recipients, compared with 126 non-DNAemic recipients, 1.3% (95% CI 0.8-1.7) vs 4.1 (95% CI 3.6-4.6, P < .001). All T-cell subsets demonstrated similar between-group differences. In logistic regression analysis of 46 training set samples, 12 with DNAemia, all obtained between days 0 and 60 from transplant, CMV-specific T-cell frequencies ≥1.7% predicted freedom from DNAemia with NPV of 93%. Sensitivity, specificity, and PPV were 83%, 74%, and 53%, respectively. Test performance was replicated in 99 validation samples. In 32 of 46 training set samples, all from seronegative recipients, one of 19 recipients with CMV-specific T-cell frequencies ≥1.7% experienced DNAemia, compared with 8 of 13 recipients with frequencies <1.7% (P = .001). CMV-specific CD154+ T cells are associated with freedom from DNAemia after LT and IT. Among seronegative recipients, CMV-specific T cells may protect against the development of CMV DNAemia.

Characterization of T cell immunophenotypes in intestinal transplantation: A pilot study

Immunophenotyping of peripheral blood mononuclear cells has been shown to be a useful, non-invasive method of predicting acute cellular rejection (ACR) following intestinal transplantation (ITx). Our objectives were to characterize differences in the T cell immunophenotype of ITx recipients in peripheral blood samples (1) collected late versus early after ITx and (1) associated with episodes of ACR and infectious enteritis. An IRB-approved, cross-sectional study of ITx recipients was performed. Peripheral blood samples were collected during normal visits and episodes of allograft dysfunction. A total of 38 patients were included in the analysis: 31 ITx recipients (87% liver-inclusive allografts) and 7 intestinal failure control patients. Of the ITx patients, 26 patients were pediatric patients (<21 years). A total of 70 samples were analyzed from ITx recipients, including 51 during normal visits and 19 during episodes of allograft dysfunction (median of 2 samples per patient; range of 1-6 samples per patient). In the late (n = 32) versus early post-ITx (n = 19) normal samples, there was a significantly higher percentage of central memory CD4 T cells (p = .001). In the ACR (n = 5) versus infectious enteritis (n = 14) samples, there was a higher percentage of CD8 T cells expressing HLA-DR (p = .002), CD57 (p < .001), and KLRG1 (p < .001) and a higher percentage of CD4 T cells expressing CD57 (p = .03). Additional studies are needed with larger cohorts to validate these changes in the T cell immunophenotype. Further elucidating T cell immunophenotypes in ITx will lead to a better understanding of immune mechanisms of allograft dysfunction, identification of potential biomarkers in ITx, and optimized selection of immunosuppressive therapies.

Improvements in intestine transplantation

Transplantation of the intestine in children has presented significant challenges even as it has become a standard to treat nutritional failure due to short gut syndrome. These challenges have been addressed in part by significant improvements in short and long-term care. Noteworthy enhancements include reduced need for intestine transplantation, drug-sparing immunosuppressive regimens, immune monitoring, and improved surveillance and management of PTLD and non-adherence.

Donor-specific hypo-responsiveness occurs in simultaneous liver-kidney transplant recipients after the first year

Kidney allografts of patients who undergo simultaneous liver-kidney transplantation incur less immune-mediated injury, and retain better function compared to other kidney allografts. To characterize the host alloimmune responses in 28 of these patients, we measured the donor-specific alloresponsiveness and phenotypes of peripheral blood cells after the first year. These values were then compared to those of 61 similarly immunosuppressed recipients of a solitary kidney or 31 recipients of liver allografts. Four multicolor, non-overlapping flow cytometry protocols were used to assess the immunophenotypes. Mixed cell cultures with donor or third party cells were used to measure cell proliferation and interferon gamma production. Despite a significant overlap, simultaneous liver-kidney transplant recipients had a lower overall frequency of circulating CD8⁺, activated CD4⁺ and effector memory T cells, compared to solitary kidney transplant recipients. Simultaneous liver-kidney transplant recipient T cells had a significantly lower proliferative response to the donor cells compared to solitary kidney recipients (11.9 vs. 42.9%), although their response to third party cells was unaltered. The frequency of interferon gamma producing alloreactive T cells in simultaneous liver-kidney transplant recipients was significantly lower than that of solitary kidney transplant recipients. Flow cytometric analysis of the mixed cultures demonstrated that both alloreactive CD4⁺ and CD8⁺ compartments of the simultaneous liver-kidney transplant recipient circulating blood cells were smaller. Thus, the phenotypic and functional characteristics of the circulating blood cells of the simultaneous liver-kidney transplant recipients resembled those of solitary liver transplant recipients, and appear to be associated with donor-specific hypo-alloresponsiveness.

Human hepatocyte transplantation for liver disease: current status and future perspectives

Liver transplantation is the accepted treatment for patients with acute liver failure and liver-based metabolic disorders. However, donor organ shortage and lifelong need for immunosuppression are the main limitations to liver transplantation. In addition, loss of the native liver as a target organ for future gene therapy for metabolic disorders limits the futuristic treatment options, resulting in the need for alternative therapeutic strategies. A potential alternative to liver transplantation is allogeneic hepatocyte transplantation. Over the last two decades, hepatocyte transplantation has made the transition from bench to bedside. Standardized techniques have been established for isolation, culture, and cryopreservation of human hepatocytes. Clinical hepatocyte transplantation safety and short-term efficacy have been proven; however, some major hurdles-mainly concerning shortage of donor organs, low cell engraftment, and lack of a long-lasting effect-need to be overcome to widen its clinical applications. Current research is aimed at addressing these problems, with the ultimate goal of increasing hepatocyte transplantation efficacy in clinical applications.

Hepatocyte transplantation: Consider infusion before incision

Human hepatocyte transplantation is undergoing study as a bridge, or even alternative, to orthotopic liver transplantation (OLT). This technique has undergone multiple developments over the past thirty years in terms of mode of delivery, source and preparation of cell cultures, monitoring of graft function, and use of immunosuppression. Further refinements and improvements in these techniques will likely allow improved graft survival and function, granting patients higher yield from this technique and potentially significantly delaying need for OLT.

Clinical Hepatocyte Transplantation: What Is Next?

Purpose of review

Significant recent scientific developments have occurred in the field of liver repopulation and regeneration. While techniques to facilitate liver repopulation with donor hepatocytes and different cell sources have been studied extensively in the laboratory, in recent years clinical hepatocyte transplantation (HT) and liver repopulation trials have demonstrated new disease indications and also immunological challenges that will require the incorporation of a fresh look and new experimental approaches.

Recent findings

Growth advantage and regenerative stimulus are necessary to allow donor hepatocytes to proliferate. Current research efforts focus on mechanisms of donor hepatocyte expansion in response to liver injury/preconditioning. Moreover, latest clinical evidence shows that important obstacles to HT include optimizing engraftment and limited duration of effectiveness, with hepatocytes being lost to immunological rejection. We will discuss alternatives for cellular rejection monitoring, as well as new modalities to follow cellular graft function and near-to-clinical cell sources.

Summary

HT partially corrects genetic disorders for a limited period of time and has been associated with reversal of ALF. The main identified obstacles that remain to make HT a curative approach include improving engraftment rates, and methods for monitoring cellular graft function and rejection. This review aims to discuss current state-of-the-art in clinical HT and provide insights into innovative approaches taken to overcome these obstacles.

Alloimmune T cells in transplantation

Alloimmune T cells are central mediators of rejection and graft-versus-host disease in both solid organ and hematopoietic stem cell transplantation. Unique among immune responses in terms of its strength and diversity, the T cell alloresponse reflects extensive genetic polymorphisms between allogeneic donors and recipients, most prominently within the major histocompatibility complex (MHC), which encodes human leukocyte antigens (HLAs) in humans. The repertoire of alloreactive T cell clones is distinct for every donor-recipient pair and includes potentially thousands of unique HLA/peptide specificities. The extraordinary magnitude of the primary alloresponse and diversity of the T cell population mediating it have presented technical challenges to its study in humans. High-throughput T cell receptor sequencing approaches have opened up new possibilities for tackling many fundamental questions about this important immunologic phenomenon.

A New Window into the Human Alloresponse

Alloreactive T lymphocytes are the primary mediators of allograft rejection. The size and diversity of the HLA-alloreactive T cell repertoire has thus far precluded the ability to follow these T cells and thereby to understand their fate in human transplant recipients. This review summarizes the history, challenges, and recent advances in the study of alloreactive T cells. We highlight the historical development of assays to measure alloreactivity and discuss how high-throughput T cell receptor (TCR) sequencing-based assays can provide a new window into the fate of alloreactive T cells in human transplant recipients. A specific approach combining a classical in vitro assay, the mixed lymphocyte reaction, with deep T cell receptor sequencing is described as a tool to track the donor-reactive T cell repertoire for any specific HLA-mismatched donor-recipient pair. This assay can provide mechanistic insights and has potential as a noninvasive, highly specific biomarker for rejection and tolerance.

Predicting Cellular Rejection With a Cell-Based Assay: Preclinical Evaluation in Children

BACKGROUND

Allospecific CD154+T-cytotoxic memory cells (CD154+TcM) predict acute cellular rejection after liver transplantation (LTx) or intestine transplantation (ITx) in small cohorts of children and can enhance immunosuppression management, but await validation and clinical implementation.

METHODS

To establish safety and probable benefit, CD154+TcM were measured in cryopreserved samples from 214 children younger than 21 years (National Clinical Trial 1163578). Training set samples (n = 158) were tested with research-grade reagents and 122 independent validation set samples were tested with current good manufacturing practices-manufactured reagents after assay standardization and reproducibility testing. Recipient CD154+TcM induced by stimulation with donor cells were expressed as a fraction of those induced by HLA nonidentical cells in parallel cultures. The resulting immunoreactivity index (IR) if greater than 1 implies increased rejection-risk.

RESULTS

Training and validation set subjects were demographically similar. Mean coefficient of test variation was less than 10% under several conditions. Logistic regression incorporating several confounding variables identified separate pretransplant and posttransplant IR thresholds for prediction of rejection in the respective training set samples. An IR of 1.1 or greater in posttransplant training samples and IR of 1.23 or greater in pretransplant training samples predicted LTx or ITx rejection in corresponding validation set samples in the 60-day postsampling period with sensitivity, specificity, positive, and negative predictive values of 84%, 80%, 64%, and 92%, respectively (area under the receiver operator characteristic curve, 0.792), and 57%, 89%, 78%, and 74%, respectively (area under the receiver operator characteristic curve, 0.848). No adverse events were encountered due to phlebotomy.

CONCLUSIONS

Allospecific CD154+T-cytotoxic memory cells predict acute cellular rejection after LTx or ITx in children. Adjunctive use can enhance clinical outcomes.

Host conditioning and rejection monitoring in hepatocyte transplantation in humans

BACKGROUND & AIMS

Hepatocyte transplantation partially corrects genetic disorders and has been associated anecdotally with reversal of acute liver failure. Monitoring for graft function and rejection has been difficult, and has contributed to limited graft survival. Here we aimed to use preparative liver-directed radiation therapy, and continuous monitoring for possible rejection in an attempt to overcome these limitations.

METHODS

Preparative hepatic irradiation was examined in non-human primates as a strategy to improve engraftment of donor hepatocytes, and was then applied in human subjects. T cell immune monitoring was also examined in human subjects to assess adequacy of immunosuppression.

RESULTS

Porcine hepatocyte transplants engrafted and expanded to comprise up to 15% of irradiated segments in immunosuppressed monkeys preconditioned with 10Gy liver-directed irradiation. Two patients with urea cycle deficiencies had early graft loss following hepatocyte transplantation; retrospective immune monitoring suggested the need for additional immunosuppression. Preparative radiation, anti-lymphocyte induction, and frequent immune monitoring were instituted for hepatocyte transplantation in a 27year old female with classical phenylketonuria. Post-transplant liver biopsies demonstrated multiple small clusters of transplanted cells, multiple mitoses, and Ki67+ hepatocytes. Mean peripheral blood phenylalanine (PHE) level fell from pre-transplant levels of 1343±48μM (normal 30-119μM) to 854±25μM (treatment goal ≤360μM) after transplant (36% decrease; p<0.0001), despite transplantation of only half the target number of donor hepatocytes. PHE levels remained below 900μM during supervised follow-up, but graft loss occurred after follow-up became inconsistent.

CONCLUSIONS

Radiation preconditioning and serial rejection risk assessment may produce better engraftment and long-term survival of transplanted hepatocytes. Hepatocyte xenografts engraft for a period of months in non-human primates and may provide effective therapy for patients with acute liver failure.

LAY SUMMARY

Hepatocyte transplantation can potentially be used to treat genetic liver disorders but its application in clinical practice has been impeded by inefficient hepatocyte engraftment and the inability to monitor rejection of transplanted liver cells. In this study, we first show in non-human primates that pretreatment of the host liver with radiation improves the engraftment of transplanted liver cells. We then used this knowledge in a series of clinical hepatocyte transplants in patients with genetic liver disorders to show that radiation pretreatment and rejection risk monitoring are safe and, if optimized, could improve engraftment and long-term survival of transplanted hepatocytes in patients.

Moving Biomarkers toward Clinical Implementation in Kidney Transplantation

Long-term kidney transplant outcomes remain suboptimal, delineating an unmet medical need. Although current immunosuppressive therapy in kidney transplant recipients is effective, dosing is conventionally adjusted empirically on the basis of time after transplant or altered in response to detection of kidney dysfunction, histologic evidence of allograft damage, or infection. Such strategies tend to detect allograft rejection after significant injury has already occurred, fail to detect chronic subclinical inflammation that can negatively affect graft survival, and ignore specific risks and immune mechanisms that differentially contribute to allograft damage among transplant recipients. Assays and biomarkers that reliably quantify and/or predict the risk of allograft injury have the potential to overcome these deficits and thereby, aid clinicians in optimizing immunosuppressive regimens. Herein, we review the data on candidate biomarkers that we contend have the highest potential to become clinically useful surrogates in kidney transplant recipients, including functional T cell assays, urinary gene and protein assays, peripheral blood cell gene expression profiles, and allograft gene expression profiles. We identify barriers to clinical biomarker adoption in the transplant field and suggest strategies for moving biomarker-based individualization of transplant care from a research hypothesis to clinical implementation.

Barcelona Consensus on Biomarker-Based Immunosuppressive Drugs Management in Solid Organ Transplantation

With current treatment regimens, a relatively high proportion of transplant recipients experience underimmunosuppression or overimmunosuppression. Recently, several promising biomarkers have been identified for determining patient alloreactivity, which help in assessing the risk of rejection and personal response to the drug; others correlate with graft dysfunction and clinical outcome, offering a realistic opportunity for personalized immunosuppression. This consensus document aims to help tailor immunosuppression to the needs of the individual patient. It examines current knowledge on biomarkers associated with patient risk stratification and immunosuppression requirements that have been generally accepted as promising. It is based on a comprehensive review of the literature and the expert opinion of the Biomarker Working Group of the International Association of Therapeutic Drug Monitoring and Clinical Toxicology. The quality of evidence was systematically weighted, and the strength of recommendations was rated according to the GRADE system. Three types of biomarkers are discussed: (1) those associated with the risk of rejection (alloreactivity/tolerance), (2) those reflecting individual response to immunosuppressants, and (3) those associated with graft dysfunction. Analytical aspects of biomarker measurement and novel pharmacokinetic-pharmacodynamic models accessible to the transplant community are also addressed. Conventional pharmacokinetic biomarkers may be used in combination with those discussed in this article to achieve better outcomes and improve long-term graft survival. Our group of experts has made recommendations for the most appropriate analysis of a proposed panel of preliminary biomarkers, most of which are currently under clinical evaluation in ongoing multicentre clinical trials. A section of Next Steps was also included, in which the Expert Committee is committed to sharing this knowledge with the Transplant Community in the form of triennial updates.

Analytical Aspects of the Implementation of Biomarkers in Clinical Transplantation

In response to the urgent need for new reliable biomarkers to complement the guidance of the immunosuppressive therapy, a huge number of biomarker candidates to be implemented in clinical practice have been introduced to the transplant community. This includes a diverse range of molecules with very different molecular weights, chemical and physical properties, ex vivo stabilities, in vivo kinetic behaviors, and levels of similarity to other molecules, etc. In addition, a large body of different analytical techniques and assay protocols can be used to measure biomarkers. Sometimes, a complex software-based data evaluation is a prerequisite for appropriate interpretation of the results and for their reporting. Although some analytical procedures are of great value for research purposes, they may be too complex for implementation in a clinical setting. Whereas the proof of "fitness for purpose" is appropriate for validation of biomarker assays used in exploratory drug development studies, a higher level of analytical validation must be achieved and eventually advanced analytical performance might be necessary before diagnostic application in transplantation medicine. A high level of consistency of results between laboratories and between methods (if applicable) should be obtained and maintained to make biomarkers effective instruments in support of therapeutic decisions. This overview focuses on preanalytical and analytical aspects to be considered for the implementation of new biomarkers for adjusting immunosuppression in a clinical setting and highlights critical points to be addressed on the way to make them suitable as diagnostic tools. These include but are not limited to appropriate method validation, standardization, education, automation, and commercialization.

Tolerance in liver transplantation: Biomarkers and clinical relevance

Transplantation is the optimal treatment for end-stage organ failure, and modern immunosuppression has allowed important progress in short-term outcomes. However, immunosuppression poorly influences chronic rejection and elicits chronic toxicity in current clinical practice. Thus, a major goal in transplantation is to understand and induce tolerance. It is well established that human regulatory T cells expressing the transcription factor FoxP3 play important roles in the maintenance of immunological self-tolerance and immune homeostasis. The major regulatory T cell subsets and mechanisms of expansion that are critical for induction and long-term maintenance of graft tolerance and survival are being actively investigated. Likewise, other immune cells, such as dendritic cells, monocyte/macrophages or natural killer cells, have been described as part of the process known as "operational tolerance". However, translation of these results towards clinical practice needs solid tools to identify accurately and reliably patients who are going to be tolerant. In this way, a plethora of genetic and cellular biomarkers is raising and being validated worldwide in large multi-center clinical trials. Few of the studies performed so far have provided a detailed analysis of the impact of immunosuppression withdrawal on pre-existing complications derived from the long-term administration of immunosuppressive drugs and the side effects associated with them. The future of liver transplantation is aimed to develop new therapies which increase the actual low tolerant vs non-tolerant recipients ratio.

Profile of the Pleximmune blood test for transplant rejection risk prediction

The Pleximmune™ test (Plexision Inc., Pittsburgh, PA, USA) is the first cell-based test approved by the US FDA, which predicts acute cellular rejection in children with liver- or intestine transplantation. The test addresses an unmet need to improve management of immunosuppression, which incurs greater risks of opportunistic infections and Epstein-Barr virus-induced malignancy during childhood. High-dose immunosuppression and recurrent rejection after intestine transplantation also result in a 5-year graft loss rate of up to 50%. Such outcomes seem increasingly unacceptable because children can experience rejection-free survival with reduced immunosuppression. Pleximmune test sensitivity and specificity for predicting acute cellular rejection is 84% and 80% respectively in training set-validation set testing of 214 children. Among existing gold standards, the biopsy detects but cannot predict rejection. Anti-donor antibodies, which presage antibody-mediated injury, reflect late-stage allosensitization as a downstream effect of engagement between recipient and donor cells. Therefore, durable graft and patient outcomes also require accurate management of cellular immune responses in clinical practice.

Enhanced B Cell Alloantigen Presentation and Its Epigenetic Dysregulation in Liver Transplant Rejection

T cell suppression prevents acute cellular rejection but causes life-threatening infections and malignancies. Previously, liver transplant (LTx) rejection in children was associated with the single-nucleotide polymorphism (SNP) rs9296068 upstream of the HLA-DOA gene. HLA-DOA inhibits B cell presentation of antigen, a potentially novel antirejection drug target. Using archived samples from 122 white pediatric LTx patients (including 77 described previously), we confirmed the association between rs9296068 and LTx rejection (p = 0.001, odds ratio [OR] 2.55). Next-generation sequencing revealed that the putative transcription factor (CCCTC binding factor [CTCF]) binding SNP locus rs2395304, in linkage disequilibrium with rs9296068 (D' 0.578, r(2) = 0.4), is also associated with LTx rejection (p = 0.008, OR 2.34). Furthermore, LTx rejection is associated with enhanced B cell presentation of donor antigen relative to HLA-nonidentical antigen in a novel cell-based assay and with a downregulated HLA-DOA gene in a subset of these children. In lymphoblastoid B (Raji) cells, rs2395304 coimmunoprecipitates with CTCF, and CTCF knockdown with morpholino antisense oligonucleotides enhances alloantigen presentation and downregulates the HLA-DOA gene, reproducing observations made with HLA-DOA knockdown and clinical rejection. Alloantigen presentation is suppressed by inhibitors of methylation and histone deacetylation, reproducing observations made during resolution of rejection. Enhanced donor antigen presentation by B cells and its epigenetic dysregulation via the HLA-DOA gene represent novel opportunities for surveillance and treatment of transplant rejection.

High-dose immunosuppressant alters the immunological status of New Zealand white rabbits following skin transplantation

The aim of this study was to investigate the effect of an immunosuppressant on the immunological status of New Zealand white rabbits after skin grafting, and to evaluate a method for monitoring the immunological status of subjects with skin transplants. The rabbits were randomly divided into allograft rejection, autograft tolerance, nontransplant, allograft low-dose immunosuppressant and allograft high-dose immunosuppressant groups. The rabbits in the low- and high-dose immunosuppressant groups were treated with cyclosporine A intravenously 8 h prior to skin transplantation and once daily following transplantation at doses of 2 and 25 mg/kg, respectively. At 12 days after skin transplantation, the spleens of donor (female) rabbits and recipient (male) rabbits were harvested for the preparation of single-cell suspensions. The splenocytes from recipient and donor rabbits were labeled with 0.3 or 6 µM carboxy fluorescein diacetate succinimidyl ester, respectively, and a mixed cell suspension was prepared. The final preparation was intravenously injected into recipient New Zealand white rabbits. The ratio of the two fluorescently labeled cell populations in the peripheral blood was measured using flow cytometry at 1, 2, 4 and 8 h after the injection, and the cell death rate was calculated. Histological analysis was also performed on samples collected at the time of splenectomy. The cell death rates of the allograft rejection and low-dose immunosuppressant groups reached their highest levels 8 h after the injection of spleen cell suspension. Allogeneic spleen cells from donor male rabbits were almost completely removed within 8 h of injection. The cell death rate increased slowly in the nontransplant, autograft and high-dose immunosuppressant groups without specificity. This study provides a specific method for the in vivo monitoring of the immunological status of patients after skin grafting. This method can quickly and accurately detect the immunological status of recipients following the injection of a mixed splenocyte suspension, thereby indicating the strength of immune rejection by the immune systems of the recipients.

T-Cell Surface Antigens and sCD30 as Biomarkers of the Risk of Rejection in Solid Organ Transplantation

T-cell activation is a characteristic of organ rejection. T cells, located in the draining lymph nodes of the transplant recipient, are faced with non-self-molecules presented by antigen presenting cells and become activated. Activated T cells are characterized by up-regulated surface antigens, such as costimulatory molecules, adhesion molecules, chemokine receptors, and major histocompatibility complex class II molecules. Surface antigen expression can be followed by flow cytometry using monoclonal antibodies in either cell function assays using donor-specific or nonspecific stimulation of isolated cells or whole blood and without stimulation on circulating lymphocytes. Molecules such as CD30 can be proteolytically cleaved off the surface of activated cells in vivo, and the determination of the soluble protein (sCD30) in serum or plasma is performed by immunoassays. As promising biomarkers for rejection and long-term transplant outcome, CD28 (costimulatory receptor for CD80 and CD86), CD154 (CD40 ligand), and sCD30 (tumor necrosis factor receptor superfamily, member 8) have been identified. Whereas cell function assays are time-consuming laboratory-developed tests which are difficult to standardize, commercial assays are frequently available for soluble proteins. Therefore, more data from clinical trials have been published for sCD30 compared with the surface antigens on activated T cells. This short review summarizes the association between selected surface antigens and immunosuppression, and rejection in solid organ transplantation.

A novel approach to measuring cell-mediated lympholysis using quantitative flow and imaging cytometry

In this study, we established a novel isotope-free approach for the detection of cell-mediated lympholysis (CML) in MHC defined peripheral blood mononuclear cells (PBMCs) using multiparameter flow and imaging cytometry. CML is an established in vitro assay to detect the presence of cytotoxic effector T-lymphocytes precursors (CTLp). Current methods employed in the identification of CTLp in the context of transplantation are based upon the quantification of chromium ((51)Cr) released from target cells. In order to adapt the assay to flow cytometry, primary porcine PBMC targets were labeled with eFluor670 and incubated with major histocompatibility complex (MHC) mismatched effector cytotoxic lymphocytes (CTLs). With this method, we were able to detect target-specific lysis that was comparable to that observed with the (51)Cr-based assay. In addition, the use of quantitative cell imaging demonstrates the presence of accessory cells involved in the cytotoxic pathway. This innovative technique improves upon the standard (51)Cr release assay by eliminating the need for radioisotopes and provides enhanced characterization of the interactions between effector and target cells. This technique has wide applicability to numerous experimental and clinical models involved with effector-cell interactions.

Alloreactive CD154-expressing T-cell subsets with differential sensitivity to the immunosuppressant, belatacept: potential targets of novel belatacept-based regimens

Belatacept blocks CD28-mediated T-cell costimulation and prevents renal transplant rejection. Understanding T-cell subset sensitivity to belatacept may identify cellular markers for immunosuppression failure to better guide treatment selection. Here, we evaluate the belatacept sensitivity of allo-antigen-specific CD154-expressing-T-cells, whose T-cytotoxic memory (TcM) subset predicts rejection with high sensitivity after non-renal transplantation. The belatacept concentration associated with half-maximal reduction (EC50) of CD154 expression was calculated for 36 T-cell subsets defined by combinations of T-helper (Th), Tc, T-memory and CD28 receptors, following allostimulation of peripheral blood leukocytes from 20 normal healthy subjects. Subsets were ranked by median EC50, and by whether subset EC50 was correlated with and therefore could be represented by the frequency of other subsets. No single subset frequency emerged as the significant correlate of EC50 for a given subset. Most (n = 25) T-cell subsets were sensitive to belatacept. Less sensitive subsets demonstrated a memory phenotype and absence of CD28 receptor. Potential drug-resistance markers for future validation include the low frequency highly differentiated, Th-memory-CD28-negative T-cells with the highest median EC50, and the least differentiated, high-frequency Tc subset, with the most CD28-negative T-cells, the third highest median EC50, and significant correlations with frequencies of the highest number of CD28-negative and memory subsets.

Intestinal and multivisceral transplantation

Intestinal transplantation has shown exceptional growth over the past 10 years. At the end of the 1990’s, intestinal transplantation moved out of the experimental realm to become a routine practice in treating patients with severe complications related to total parenteral nutrition and intestinal failure. In the last years, several centers reported an increasing improvement in survival outcomes (about 80%), during the first 12 months after surgery, but long-term survival is still a challenge. Several advances led to clinical application of transplants. Immunosuppression involved in intestinal and multivisceral transplantation was the biggest gain for this procedure in the past decade due to tacrolimus, and new inducing drugs, mono- and polyclonal anti-lymphocyte antibodies. Despite the advancement of rigid immunosuppression protocols, rejection is still very frequent in the first 12 months, and can result in long-term graft loss. The future of intestinal transplantation and multivisceral transplantation appears promising. The major challenge is early recognition of acute rejection in order to prevent graft loss, opportunistic infections associated to complications, post-transplant lymphoproliferative disease and graft versus host disease; and consequently, improve results in the long run.

Lymphocyte surface molecules as immune activation biomarkers

Immunosuppression is mandatory after solid organ transplantation between HLA mismatched individuals. It is a lifelong therapy that needs to be closely monitored to avoid under- and over-immunosuppression. For many drugs, pharmacokinetic monitoring has been proven to be beneficial. However, the therapeutic ranges are statistically derived surrogate markers for the effects that cannot predict the individual response of single patients. Better tailored immunosuppression biomarkers are needed that indicate immune activation. T cells are critically involved in organ rejection, and the means to assess their activation state may be promising to individualize immunosuppressive therapies. Activated T cells can be monitored with flow cytometry based on surface molecules that are typically up regulated or with molecules that are cleaved off the cell surface. Among these molecules are the interleukin-2 receptor (CD25); transferrin receptor (CD71); the T cell co-stimulatory molecules CD28, CD69, and CD154 and sCD30, which is a member of the TNF-alpha family. The effect of immunosuppressive drugs on T cell activation can be recorded with indirect cell function assays or by directly monitoring activated T cells in whole blood. Soluble proteins can be measured with immunoassays. This review provides a summary of the experimental and clinical studies investigating the potential of surface molecules as a tool for immune monitoring. It critically discusses the obstacles and shortcomings from an analytical and diagnostic perspective that are currently preventing their use in multicenter trials and clinical routine monitoring of transplant patients.

Antithymocyte globulin facilitates alloreactive T-cell apoptosis by means of caspase-3: potential implications for monitoring rejection-free outcomes

BACKGROUND

Alloreactive T-cell apoptosis may explain reduced immunosuppression requirements with proapoptotic immunosuppression and among rejection-free recipients. This possibility remains unproven.

METHODS

Apoptotic (caspase-3+, cathepsin B+) and inflammatory (CD154+) T-cell subsets were evaluated before and after adding rabbit antithymocyte globulin (rATG) to mixed lymphocyte co-cultures between human leukocyte antigen-mismatched peripheral blood lymphocytes from healthy adults. In random samples from children with liver (LTx-20) and intestine (ITx-13) transplantation, apoptotic T cells were evaluated for association with rejection-free outcomes using the caspase-3 substrate, phiphilux.

RESULTS

In mixed lymphocyte co-cultures between normal human peripheral blood lymphocytes, (1) frequencies of memory (M) and naive (N) Th and Tc, which expressed activated caspase-3, were enhanced most by the combination of allostimulation and rATG, than either stimulus alone. These findings were confirmed with antibody to activated caspase-3, phiphilux, and terminal deoxynucleotide transferase-mediated dUTP nick-end labeling (TUNEL) assay; (2) frequencies of Th subsets, which expressed activated cathepsin B, were similarly increased with combined stimulation. Tc seemed resistant to cathepsin B activation; (3) with increasing rATG concentrations, proportionately more allospecific CD154+T-cytotoxic memory cells (TcM) survived than TcM, resulting in relative enrichment of allospecific CD154+TcM. In random blood samples, phiphilux+T-cell subset frequencies were higher among 14 rejection-free LTx and ITx recipients and demonstrated a greater increase with ex vivo rATG pretreatment than 19 rejectors. In logistic regression analysis, phiphilux+TcM associated best with rejection-free outcomes with a sensitivity of 57% and a specificity of 89%.

CONCLUSION

Rabbit antithymocyte globulin facilitates apoptosis of alloreactive T cells by means of caspase-3 activation, which may explain its steroid-sparing effect in pediatric liver and intestine recipients. Apoptotic susceptibility of T-cytotoxic memory cells, which resist cathepsin B activation, may distinguish rejection-free and rejection-prone liver recipients.

Monitoring T cell alloreactivity

Currently, immunosuppressive therapy in kidney transplant recipients is center-specific, protocol-driven, and adjusted according to functional or histological evaluation of the allograft and/or signs of drug toxicity or infection. As a result, a large fraction of patients receive too much or too little immunosuppression, exposing them to higher rates of infection, malignancy and drug toxicity, or increased risk of acute and chronic graft injury from rejection, respectively. The individualization of immunosuppression requires the development of assays able to reliably quantify and/or predict the magnitude of the recipient's immune response toward the allograft. As alloreactive T cells are central mediators of allograft rejection, monitoring T cell alloreactivity has become a priority for the transplant community. Among available assays, flow cytometry based phenotyping, T cell proliferation, T cell cytokine secretion, and ATP release (ImmuKnow), have been the most thoroughly tested. While numerous cross-sectional studies have found associations between the results of these assays and the presence of clinically relevant post-transplantation outcomes, data from prospective studies are still scanty, thereby preventing widespread implementation in the clinic. Future studies are required to test the hypothesis that tailoring immunosuppression on the basis of results offered by these biomarkers leads to better outcomes than current standard clinical practice.

The current state of intestine transplantation: indications, techniques, outcomes and challenges

Intestine transplantation is the least common form of organ transplantation in the United States and often deemed one of the most difficult. Patient and graft survival have historically trailed well behind other organ transplants. Over the past 5-10 years registry reports and single center series have demonstrated improvements to patient survival after intestinal transplantation that now match patient survival for those without life-threatening complications on parenteral nutrition. For various reasons including improvements in medical care of patients with intestinal failure and difficulty accessing transplant care, the actual number of intestine transplants has declined by 25% over the past 6 years. In light of the small numbers of intestine transplants, many physicians and the lay public are often unaware that this is a therapeutic option. The aim of this review is to describe the current indications, outcomes and advances in the field of intestine transplantation and to explore concerns over future access to this important and life-saving therapy.

Immune monitoring post liver transplant

Many of the causes of short and late morbidity following liver transplantation are associated with immunosuppression or immunosuppressive medications. Current care often involves close monitoring of liver biochemistry as well as therapeutic drug levels. However, the postoperative course following liver transplantation can often be associated with significant complications including infection and rejection, suggesting an inadequacy in current immune function monitoring. Many assays have been tested in the research setting to identify possible biomarkers that may be used to predict clinical events such as acute cellular rejection, and therefore allow modification of a patient’s immunosuppressive regimen prior to a clinical event. However, these generally require significant laboratory processing and have had difficulty becoming established in common clinical use outside the research setting. One assay, Cylex ImmuKnow has been food and drug administration approved but has had variable results. In this review we discuss the assays that have been used to assess monitoring of immune function after liver transplantation and consider possible future directions.

Elevation of CD4+ differentiated memory T cells is associated with acute cellular and antibody-mediated rejection after liver transplantation

BACKGROUND

It is now well known that the outcome after allogeneic transplantation, such as incidence of acute rejections, very much depends on the individual's immune reactivity status. There is also increasing evidence that the presence of preexisting memory T cells can affect antigraft immune responses.

METHODS

In a prospective study, we monitored peripheral CD4 and CD8 central memory, effector memory, and terminal differentiated effector memory (TEMRA) T cells in 55 patients who underwent deceased liver transplantation and received conventional immunosuppressive treatment with or without basiliximab induction. The primary endpoint of the study was acute allograft rejection during a 1-year follow-up period.

RESULTS

We observed significantly increased proportions of CD4 and CD8 TEMRA cells in patients before transplantation compared with healthy controls (P=0.006 and 0.009, respectively). This characteristic was independent of the underlying disease. In patients with no signs of acute rejection, we observed an immediate reduction of CD4 TEMRA cells. In contrast, patients who experienced acute cellular rejection, and especially antibody-mediated rejection, displayed persistent elevated TEMRA cells (P=0.017 and 0.027, respectively). Basiliximab induction therapy did not influence CD4 and CD8 TEMRA numbers.

CONCLUSIONS

Conventional immunosuppressive or basiliximab treatment cannot control the persistence of TEMRA T cells, which may contribute to acute cellular rejection and antibody-mediated rejection after liver transplantation. In the future, specific targeting of TEMRA cells in selected patients may prevent the occurrence of difficult to treat steroid-resistant rejections, thereby leading to improved patient outcome.

Clinical operational tolerance in liver transplantation: state-of-the-art perspective and future prospects

BACKGROUND

Liver transplantation is the definite treatment for end-stage liver diseases with satisfactory results. However, untoward effects of life-long immunosuppression prevent the development of alternative strategies to achieve better long-term outcome. Achieving clinical operational tolerance is the ultimate goal.

DATA SOURCES

A PubMed and Google Scholar search using terms: "immune tolerance", "liver transplantation", "clinical trial", "operational tolerance" and "immunosuppression withdrawal" was performed, and relevant articles published in English in the past decade were reviewed. Full-text publications relevant to the field were selected and relevant articles from reference lists were also included. Priority was given to those articles which are relevant to the review.

RESULTS

Because of the inherent tolerogenic property, around 20%-30% of liver transplantation recipients develop spontaneous operational tolerance after immunosuppression withdrawal, and the percentage may be even higher in pediatric living donor liver transplantation recipients. Several natural killer and gammadeltaT cell related markers have been identified to be associated with the tolerant state in liver transplantation patients. Despite the progress, clinical operational tolerance is still rare in liver transplantation. Reprogramming the recipient immune system by creating chimerism and regulatory cell therapies is among newer promising means to achieve clinical liver transplantation tolerance in the future.

CONCLUSION

Although clinical operational tolerance is still rare in liver transplantation recipients, ongoing basic research and collaborative clinical trials may help to decipher the mystery of transplantation tolerance and extend the potential benefits of drug withdrawal to an increasing number of patients in a more predictable fashion.

Transplantation tolerance

Tolerance has been defined as graft-specific survival in the absence of continued immunosuppression. The mechanisms of central and peripheral tolerance are discussed in this review, as well as the barriers and limitations in achieving graft-specific tolerance. The need remains for definitive laboratory assays to determine the presence of a tolerant state. Genetic biomarker analysis pre-transplant may allow for better donor: recipient matching, lessening the need for immunosuppression, while post-transplant analysis of biomarkers, certain cytokines, and regulatory leukocytes may permit minimally invasive assessment of graft function and potentially, of graft-specific tolerance.

Citrulline level is a potent indicator of acute rejection in the long term following pediatric intestinal/multivisceral transplantation

Citrulline has been advocated as a marker for acute cellular rejection (ACR) in intestinal transplantation; however, its significance as a forewarning in the long-term follow-up remains unknown. This study aimed to investigate the association between citrulline levels and the grading of ACR to establish a cutoff point that accurately predicts ACR beyond 3 months posttransplant in the pediatric patient population. During a 16-year period (1995-2011), a total of 13 499 citrulline samples were prospectively collected from 111 consecutive pediatric intestinal/multivisceral transplant recipients: 2155 were obtained concurrently with intestinal biopsies. There were 185 ACR episodes observed among 74/111 (67%) patients (median follow-up: 4.4 years). Citrulline levels were inversely proportional to the severity of ACR. Negative predictive values for any type of ACR (cutoff, 20 μmol/L) and moderate/severe ACR (cutoff, 10 μmol/L) were 95% and 99%, respectively. When patients were divided according to graft size, diagnostic accuracy using the same cutoff was identical. Similarly, subgroup analysis by the timing of citrulline measurement prior to biopsy varying from 1 to 7 days demonstrated comparable results. Citrulline is a potent indicator as a danger signal for ACR, being an exclusionary, noninvasive biomarker with excellent negative predictive values in the long term after pediatric intestinal/multivisceral transplant.

Long-term hepatic allograft acceptance based on CD40 blockade by ASKP1240 in nonhuman primates

Blockade of the CD40-CD154 costimulatory signal is an attractive strategy for immunosuppression and tolerance induction in organ transplantation. Treatment with anti-CD154 monoclonal antibodies (mAbs) results in potent immunosuppression in nonhuman primates (NHPs). Despite plans for future clinical use, further development of these treatments was halted by complications. As an alternative approach, we have been focusing on the inhibition of the counter receptor, CD40 and have shown that a novel human anti-CD40 mAb, ASKP1240, markedly prolongs renal allograft survival in NHPs, although allografts eventually underwent chronic allograft nephropathy. On the basis of our previous findings that a CD40-CD154 costimulation blockade induces tolerance to hepatic, but not cardiac, allografts in rodents, we tested here our hypothesis that a blockade of CD40 by ASKP1240 allows acceptance of hepatic allografts in NHPs. A 2-week ASKP1240 induction treatment prolonged liver allograft survival in NHPs; however, the graft function deteriorated due to chronic rejection. In contrast, a 6-month ASKP1240 maintenance monotherapy efficiently suppressed both cellular and humoral alloimmune responses and prevented rejection on the hepatic allograft. No serious side effects, including thromboembolic complications, were noted in the ASKP1240-treated monkeys. We conclude that CD40 blockade by ASKP1240 would be a desirable immunosuppressant for clinical liver transplantation.

Allospecific CD154 + T-cytotoxic memory cells as potential surrogate for rejection risk in pediatric intestine transplantation

Clinical end-points dictate large trial enrollments and exclude children with the rare intestine transplant procedure (ITx), who experience higher drug-related morbidity. We evaluate the novel rejection-risk parameter, allo-(antigen)-specific CD154 + TcMs (i) as surrogates for ACR using Prentice's criteria, (ii) for association with immunosuppression targets to determine Fleming's surrogate end-point designation, and (iii) as time-to-event end-point in a simulated comparison of alemtuzumab (NCT#01208337, n = 14) and rabbit anti-human thymocyte globulin (rATG, n = 16) among 30 children with ITx. CD154 + TcM were measured in MLR before, and at 1-60 and 61-200 days after ITx (NCT#01163578). CD154 + TcM correlate significantly with rejection severity (Spearman r = 0.685, p = 2.03E-5) and associate with biopsy-proven ITx rejection with sensitivity/specificity of 94%/84% [corrected] independent of immunosuppressant. Previously stated sensitivity of 90% is incorrect. [corrected]. The rejection-risk threshold of CD154 + TcM resolves rapidly in 200-day follow-up (46 ± 20 vs. 158 ± 59 days, p = 0.009, K-M) with alemtuzumab, which demonstrates lower 90-day ACR incidence (50% vs. 69%, p=NS, Fisher's exact), and is associated with accelerated prednisone minimization to ≤2.5 mg/day, compared with rATG (120 ± 28 vs. 180 ± 30 days, p = 0.027, K-M). As a surrogate end-point, time-to-rejection-risk resolution measured with CD154 + TcM portends 50% reduction in sample sizes in a simulated trial of alemtuzumab vs. rATG. Rejection-risk assessment with CD154 + TcM may enable informed immunosuppression minimization, and preliminary efficacy comparisons in pediatric ITx.