Current and future challenges in therapy for antibody-mediated rejection

Outcomes of untreated subclinical antibody-mediated rejection after heart transplantation

Subclinical antibody-mediated rejection (AMR) is represented by histopathological and/or immunopathological manifestations in the absence of significant cardiac allograft dysfunction. Treatment remains uncertain as there is a lack of data on asymptomatic heart transplant (HT) recipients (HTR) with a positive cardiac biopsy. We sought to determine the impact of untreated subclinical biopsy-proven AMR, regardless of circulating donor-specific antigen (DSA) expression, when diagnosed on surveillance biopsies in the first year after HT. This retrospective case control study evaluated 260 HTR between May 2004 and February 2021. These comprised 231 controls and 29 patients with untreated subclinical AMR. The mortality event rate was higher in controls (2.63 events per 100 person-years) compared to the scAMR Group (1.71 events per 100 person-years), a difference that did not reach statistical significance (hazard ratio 0.66, CI: 0.18-2.36). The combined event rate of cardiac allograft vasculopathy (CAV), graft dysfunction, or mortality was higher in the subclinical AMR group (5.60 events per 100 person-years) than in controls (3.89 events per 100 person-years) but did not reach statistical significance (hazard ratio 1.63, CI: 0.07-40.09). Our results suggest that subclinical AMR diagnosed in the first year after HT on surveillance biopsy is not associated with decreased survival. This may sway the management of subclinical AMR towards a more conservative approach in transplant-capable institutions that currently prioritize treatment, though prospective, randomized studies of such a management strategy are required.

Assessment and management of allosensitization following heart transplant in adults

Immunological injury to the allograft, specifically by antibodies to de novo donor specific human leukocyte antigen (dnDSA) and antibody mediated injury and rejection are the major limitations to graft survival after heart transplantation (HT). As such, our approach to allosensitization remains limited by the inability of contemporaneous immunoassays to unravel pathogenic potential of dnDSA. Additionally, the role of dnDSA is continuously evaluated with emerging methods to detect rejection. Moreover, the timing and frequency of dnDSA monitoring for early detection and risk mitigation as well as management of dnDSA remain challenging. A strategic approach to dnDSA employs diagnostic assays to determine relevant antibodies in conjunction with clinical presentation and injury/rejection of allograft to tailor therapeutics. In this review, we aim to outline contemporary knowledge involving detection, monitoring and management of dnDSA after HT. Subsequently, we propose a diagnostic and therapeutic approach that may mitigate morbidity and mortality while balancing adverse reactions from pharmacotherapy.

Noninvasive biomarkers for prediction and diagnosis of heart transplantation rejection

For most patients with end-stage heart failure, heart transplantation is the treatment of choice. Allograft rejection is one of the major post-transplantation complications affecting graft outcome and survival. Recent advancements in science and technology offer an opportunity to integrate genomic and other omics-based biomarkers into clinical practice, facilitating noninvasive evaluation of allograft for diagnostic and prognostic purposes. Omics, including gene expression profiling (GEP) of blood immune cell components and donor-derived cell-free DNA (dd-cfDNA) are of special interest to researchers. Several studies have investigated levels of dd-cfDNA and miroRNAs in blood as potential markers for early detection of allograft rejection. One of the achievements in the field of transcriptomics is AlloMap, GEP of peripheral blood mononuclear cells (PBMC), which can identify 11 differentially expressed genes and help with detection of moderate and severe acute cellular rejection in stable heart transplant recipients. In recent years, the utilization of GEP of PBMC for identifying differentially expressed genes to diagnose acute antibody-mediated rejection and cardiac allograft vasculopathy has yielded promising results. Advancements in the field of metabolomics and proteomics as well as their potential implications have been further discussed in this paper.

Galectin-9 is required for endometrial regenerative cells to induce long-term cardiac allograft survival in mice

Background

Endometrial regenerative cells (ERCs), a novel type of mesenchymal-like stem cells, were identified as an attractive candidate for immunoregulation and induction of cardiac allograft tolerance. However, the underlying mechanisms of ERCs in immune regulation still remain largely unclear. The present study is designed to determine whether the expression of Galectin-9 (Gal-9), a soluble tandem-repeat member of the galectin family, is crucial for ERC-based immunomodulation.

Methods

In this study, we measured Gal-9 expression on ERCs and then co-cultured Gal-9-ERCs, ERCs, and ERCs+lactose (Gal-9 blocker) with activated C57BL/6-derived splenocytes. Furthermore, we performed mouse heart transplantation between BALB/c (H-2^d) donor and C57BL/6 (H-2^b) recipient. ERCs were administrated 24 h after the surgery, either alone or in combination with rapamycin.

Results

Our data demonstrate that ERCs express Gal-9, and this expression is increased by IFN-γ stimulation in a dose-dependent manner. Moreover, both in vitro and in vivo results show that Gal-9-ERC-mediated therapy significantly suppressed Th1 and Th17 cell response, inhibited CD8⁺ T cell proliferation, abrogated B cell activation, decreased donor-specific antibody production, and enhanced the Treg population. The therapeutic effect of ERCs was further verified by their roles in prolonging cardiac allograft survival and alleviating graft pathological changes.

Conclusions

Taken together, these data indicate that Gal-9 is required for ERC-mediated immunomodulation and prevention of allograft rejection.

When to intervene for donor-specific antibody after heart transplantation

PURPOSE OF REVIEW

Posttransplant donor-specific human leukocyte antigen (HLA) antibodies (DSA) represent a complex area in heart transplantation with nonstandardized practice and paucity of clinical data to guide optimal management.

RECENT FINDINGS

De novo DSA after heart transplantation is common and associated with rejection, cardiac allograft vasculopathy, allograft failure, and mortality. Advances in methods for HLA antibody detection have enabled identification of DSA with high precision and sensitivity. The detection of HLA antibodies must, however, be interpreted within appropriate laboratory and clinical contexts; it remains unclear which DSA are associated with greatest clinical risk. Increased antibody and clinical surveillance as well as optimization of maintenance immunosuppression are required for all patients with DSA. Antibody-directed therapies are reserved for patients with allograft dysfunction or rejection. Treatment of DSA may also be considered in asymptomatic high-risk patients including those in whom DSA arise de novo posttransplant, is persistent, high titer, or complement activating. The impact of DSA reduction and removal on long-term clinical outcomes remains unknown.

SUMMARY

Despite improvements in DSA detection, identification, and characterization, best therapeutic strategies are unclear. Prospective multicenter studies are needed to develop effective standardized approaches for DSA management in heart transplantation.

Relative Frequencies of Alloantigen-Specific Helper CD4 T Cells and B Cells Determine Mode of Antibody-Mediated Allograft Rejection

Humoral alloimmunity is now recognized as a major determinant of transplant outcome. MHC glycoprotein is considered a typical T-dependent antigen, but the nature of the T cell alloresponse that underpins alloantibody generation remains poorly understood. Here, we examine how the relative frequencies of alloantigen-specific B cells and helper CD4 T cells influence the humoral alloimmune response and how this relates to antibody-mediated rejection (AMR). An MHC-mismatched murine model of cardiac AMR was developed, in which T cell help for alloantibody responses in T cell deficient (Tcrbd-/-) C57BL/6 recipients against donor H-2Kd MHC class I alloantigen was provided by adoptively transferred "TCR75" CD4 T cells that recognize processed H-2Kd allopeptide via the indirect-pathway. Transfer of large numbers (5 × 105) of TCR75 CD4 T cells was associated with rapid development of robust class-switched anti-H-2Kd humoral alloimmunity and BALB/c heart grafts were rejected promptly (MST 9 days). Grafts were not rejected in T and B cell deficient Rag2-/- recipients that were reconstituted with TCR75 CD4 T cells or in control (non-reconstituted) Tcrbd-/- recipients, suggesting that the transferred TCR75 CD4 T cells were mediating graft rejection principally by providing help for effector alloantibody responses. In support, acutely rejecting BALB/c heart grafts exhibited hallmark features of acute AMR, with widespread complement C4d deposition, whereas cellular rejection was not evident. In addition, passive transfer of immune serum from rejecting mice to Rag2-/- recipients resulted in eventual BALB/c heart allograft rejection (MST 20 days). Despite being long-lived, the alloantibody responses observed at rejection of the BALB/c heart grafts were predominantly generated by extrafollicular foci: splenic germinal center (GC) activity had not yet developed; IgG secreting cells were confined to the splenic red pulp and bridging channels; and, most convincingly, rapid graft rejection still occurred when recipients were reconstituted with similar numbers of Sh2d1a-/- TCR75 CD4 T cells that are genetically incapable of providing T follicular helper cell function for generating GC alloimmunity. Similarly, alloantibody responses generated in Tcrbd-/- recipients reconstituted with smaller number of wild-type TCR75 CD4 T cells (103), although long-lasting, did not have a discernible extrafollicular component, and grafts were rejected much more slowly (MST 50 days). By modeling antibody responses to Hen Egg Lysozyme protein, we confirm that a high ratio of antigen-specific helper T cells to B cells favors development of the extrafollicular response, whereas GC activity is favored by a relatively high ratio of B cells. In summary, a relative abundance of helper CD4 T cells favors development of strong extrafollicular alloantibody responses that mediate acute humoral rejection, without requirement for GC activity. This work is composed of two parts, of which this is Part I. Please read also Part II: Chhabra et al., 2019.

Pathological antibody-mediated rejection in pediatric heart transplant recipients: Immunologic risk factors, hemodynamic significance, and outcomes

Biopsy-diagnosed pAMR has been observed in over half of pediatric HT recipients within 6 years of transplantation. We report the incidence and outcomes of pAMR at our center. All endomyocardial biopsies for all HT recipients transplanted between 2010 and 2015 were reviewed and classified using contemporary ISHLT guidelines. Graft dysfunction was defined as a qualitative decrement in systolic function by echocardiogram or an increase of ≥3 mm Hg in atrial filling pressure by direct measurement. Among 96 patients, pAMR2 occurred in 7 (7%) over a median follow-up period of 3.1 years, while no cases of pAMR3 occurred. A history of CHD, DSA at transplant, and elevated filling pressures were associated with pAMR2. Five-sixths (83%) of patients developed new C1q+ DSA at the time of pAMR diagnosis. There was a trend toward reduced survival, with 43% of patients dying within 2.3 years of pAMR diagnosis.

Exercise-based cardiac rehabilitation in heart transplant recipients

BACKGROUND

Heart transplantation is considered to be the gold standard treatment for selected patients with end-stage heart disease when medical therapy has been unable to halt progression of the underlying pathology. Evidence suggests that aerobic exercise training may be effective in reversing the pathophysiological consequences associated with cardiac denervation and prevent immunosuppression-induced adverse effects in heart transplant recipients.

OBJECTIVES

To determine the effectiveness and safety of exercise-based rehabilitation on the mortality, hospital admissions, adverse events, exercise capacity, health-related quality of life, return to work and costs for people after heart transplantation.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) in the Cochrane Library, MEDLINE (Ovid), Embase (Ovid), CINAHL (EBSCO) and Web of Science Core Collection (Thomson Reuters) to June 2016. We also searched two clinical trials registers and handsearched the reference lists of included studies.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) of parallel group, cross-over or cluster design, which compared exercise-based interventions with (i) no exercise control (ii) a different dose of exercise training (e.g. low- versus high-intensity exercise training); or (iii) an active intervention (i.e. education, psychological intervention). The study population comprised adults aged 18 years or over who had received a heart transplant.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened all identified references for inclusion based on pre-specified inclusion criteria. Disagreements were resolved by consensus or by involving a third person. Two review authors extracted outcome data from the included trials and assessed their risk of bias. One review author extracted study characteristics from included studies and a second author checked them against the trial report for accuracy.

MAIN RESULTS

We included 10 RCTs that involved a total of 300 participants whose mean age was 54.4 years. Women accounted for fewer than 25% of all study participants. Nine trials which randomised 284 participants to receive exercise-based rehabilitation (151 participants) or no exercise (133 participants) were included in the main analysis. One cross-over RCT compared high-intensity interval training with continued moderate-intensity training in 16 participants. We reported findings for all trials at their longest follow-up (median 12 weeks).Exercise-based cardiac rehabilitation increased exercise capacity (VO_2peak) compared with no exercise control (MD 2.49 mL/kg/min, 95% CI 1.63 to 3.36; N = 284; studies = 9; moderate quality evidence). There was evidence from one trial that high-intensity interval exercise training was more effective in improving exercise capacity than continuous moderate-intensity exercise (MD 2.30 mL/kg/min, 95% CI 0.59 to 4.01; N = 16; 1 study). Four studies reported health-related quality of life (HRQoL) measured using SF-36, Profile of Quality of Life in the Chronically Ill (PLC) and the World Health Organization Quality Of Life (WHOQoL) - BREF. Due to the variation in HRQoL outcomes and methods of reporting we were unable to meta-analyse results across studies, but there was no evidence of a difference between exercise-based cardiac rehabilitation and control in 18 of 21 HRQoL domains reported, or between high and moderate intensity exercise in any of the 10 HRQoL domains reported. One adverse event was reported by one study.Exercise-based cardiac rehabilitation improves exercise capacity, but exercise was found to have no impact on health-related quality of life in the short-term (median 12 weeks follow-up), in heart transplant recipients whose health is stable.There was no evidence of statistical heterogeneity across trials for exercise capacity and no evidence of small study bias. The overall risk of bias in included studies was judged as low or unclear; more than 50% of included studies were assessed at unclear risk of bias with respect to allocation concealment, blinding of outcome assessors and declaration of conflicts of interest. Evidence quality was assessed as moderate according to GRADE criteria.

AUTHORS' CONCLUSIONS

We found moderate quality evidence suggesting that exercise-based cardiac rehabilitation improves exercise capacity, and that exercise has no impact on health-related quality of life in the short-term (median 12 weeks follow-up), in heart transplant recipients. Cardiac rehabilitation appears to be safe in this population, but long-term follow-up data are incomplete and further good quality and adequately-powered trials are needed to demonstrate the longer-term benefits of exercise on safety and impact on both clinical and patient-related outcomes, such as health-related quality of life, and healthcare costs.

Exercise-based cardiac rehabilitation in heart transplant recipients

BACKGROUND

Heart transplantation is considered to be the gold standard treatment for selected patients with end-stage heart disease when medical therapy has been unable to halt progression of the underlying pathology. Evidence suggests that aerobic exercise training may be effective in reversing the pathophysiological consequences associated with cardiac denervation and prevent immunosuppression-induced adverse effects in heart transplant recipients.

OBJECTIVES

To determine the effectiveness and safety of exercise-based rehabilitation on the mortality, hospital admissions, adverse events, exercise capacity, health-related quality of life, return to work and costs for people after heart transplantation.

SEARCH METHODS

We searched the Cochrane Central Register of Controlled Trials (CENTRAL) in the Cochrane Library, MEDLINE (Ovid), Embase (Ovid), CINAHL (EBSCO) and Web of Science Core Collection (Thomson Reuters) to June 2016. We also searched two clinical trials registers and handsearched the reference lists of included studies.

SELECTION CRITERIA

We included randomised controlled trials (RCTs) of parallel group, cross-over or cluster design, which compared exercise-based interventions with (i) no exercise control (ii) a different dose of exercise training (e.g. low- versus high-intensity exercise training); or (iii) an active intervention (i.e. education, psychological intervention). The study population comprised adults aged 18 years or over who had received a heart transplant.

DATA COLLECTION AND ANALYSIS

Two review authors independently screened all identified references for inclusion based on pre-specified inclusion criteria. Disagreements were resolved by consensus or by involving a third person. Two review authors extracted outcome data from the included trials and assessed their risk of bias. One review author extracted study characteristics from included studies and a second author checked them against the trial report for accuracy.

MAIN RESULTS

We included 10 RCTs that involved a total of 300 participants whose mean age was 54.4 years. Women accounted for fewer than 25% of all study participants. Nine trials which randomised 284 participants to receive exercise-based rehabilitation (151 participants) or no exercise (133 participants) were included in the main analysis. One cross-over RCT compared high-intensity interval training with continued moderate-intensity training in 16 participants. We reported findings for all trials at their longest follow-up (median 12 weeks).Exercise-based cardiac rehabilitation increased exercise capacity (VO2peak) compared with no exercise control (MD 2.49 mL/kg/min, 95% CI 1.63 to 3.36; N = 284; studies = 9; moderate quality evidence). There was evidence from one trial that high-intensity interval exercise training was more effective in improving exercise capacity than continuous moderate-intensity exercise (MD 2.30 mL/kg/min, 95% CI 0.59 to 4.01; N = 16; 1 study). Four studies reported health-related quality of life (HRQoL) measured using SF-36, Profile of Quality of Life in the Chronically Ill (PLC) and the World Health Organization Quality Of Life (WHOQoL) - BREF. Due to the variation in HRQoL outcomes and methods of reporting we were unable to meta-analyse results across studies, but there was no evidence of a difference between exercise-based cardiac rehabilitation and control in 18 of 21 HRQoL domains reported, or between high and moderate intensity exercise in any of the 10 HRQoL domains reported. One adverse event was reported by one study.Exercise-based cardiac rehabilitation improves exercise capacity, but exercise was found to have no impact on health-related quality of life in the short-term (median 12 weeks follow-up), in heart transplant recipients whose health is stable.There was no evidence of statistical heterogeneity across trials for exercise capacity and no evidence of small study bias. The overall risk of bias in included studies was judged as low or unclear; more than 50% of included studies were assessed at unclear risk of bias with respect to allocation concealment, blinding of outcome assessors and declaration of conflicts of interest. Evidence quality was assessed as moderate according to GRADE criteria.

AUTHORS' CONCLUSIONS

We found moderate quality evidence suggesting that exercise-based cardiac rehabilitation improves exercise capacity, and that exercise has no impact on health-related quality of life in the short-term (median 12 weeks follow-up), in heart transplant recipients. Cardiac rehabilitation appears to be safe in this population, but long-term follow-up data are incomplete and further good quality and adequately-powered trials are needed to demonstrate the longer-term benefits of exercise on safety and impact on both clinical and patient-related outcomes, such as health-related quality of life, and healthcare costs.

Significance of Anti-HLA Antibodies on Adult and Pediatric Heart Allograft Outcomes

As methods for human leukocyte antigens (HLA) antibody detection have evolved and newer solid phase assays are much more sensitive, the last 15 years has seen a renewed focus on the importance of HLA antibodies in solid organ transplant rejection. However, there is still much controversy regarding the clinical significance of antibody level as depicted by the mean fluorescence intensity of a patient’s neat serum. Emerging techniques, including those that identify antibody level and function, show promise for the detection of individuals at risk of allograft rejection, determination of the effectiveness of desensitization prior to transplant, and for monitoring treatment of rejection. Here, we review current publications regarding the relevance of donor-specific HLA antibodies (DSA) in adult and pediatric heart transplantation (HT) with graft survival, development of antibody-mediated rejection and cardiac allograft vasculopathy (CAV). The negative impact of DSA on patient and allograft survival is evident in adult and pediatric HT recipients. Many questions remain regarding the most appropriate frequency of assessment of pre- and posttransplant DSA as well as the phenotype of DSA memory vs. true de novo antibody using large multicenter adult and pediatric cohorts and state-of-the-art methodologies for DSA detection and characterization.

Antibody-Mediated Rejection in Solid Organ Transplant

Within a little more than a decade, the transplant of human organs for end-stage organ disease became a reality. The early barriers to successful long-term graft and patient survival were related to the inability to effectively control the immune system such that it would not attack the donor tissue but would still recognize and destroy invading organisms and cells. As immunosuppressive therapy has been refined and proper matching of donors and recipients has been improved, hyperacute rejection has become a rare occurrence and acute rejection has been markedly controlled. However, antibody-mediated rejection remains an important impediment to increased survival of transplanted organs. This article provides readers with a broad overview of the immune system, discusses mechanisms of transplant rejection, and details prevention, detection, and treatment of antibody-mediated rejection in solid organ transplant.

Heart Transplantation-The Pediatric Cardiac Critical Care Perspective

OBJECTIVES

Although there have been tremendous advancements in the care of severe pediatric cardiovascular disease, heart transplantation remains the standard therapy for end-stage heart disease in children. As such, these patients comprise an important and often complex subset of patients in the ICU. The purpose of this article is to review the causes and management of allograft dysfunction and the medications used in the transplant population.

DATA SOURCES

MEDLINE, PubMed, and Cochrane Database of systemic reviews.

CONCLUSIONS

Pediatric heart transplant recipients represent a complex group of patients that frequently require critical care. Their immunosuppressive medications, while being vital to maintenance of allograft function, are associated with significant short- and long-term complications. Graft dysfunction can occur from a variety of etiologies at different times following transplantation and remains a major limitation to long-term posttransplant survival.

The Complement System and Antibody-Mediated Transplant Rejection

Complement activation is an important cause of tissue injury in patients with Ab-mediated rejection (AMR) of transplanted organs. Complement activation triggers a strong inflammatory response, and it also generates tissue-bound and soluble fragments that are clinically useful markers of inflammation. The detection of complement proteins deposited within transplanted tissues has become an indispensible biomarker of AMR, and several assays have recently been developed to measure complement activation by Abs reactive to specific donor HLA expressed within the transplant. Complement inhibitors have entered clinical use and have shown efficacy for the treatment of AMR. New methods of detecting complement activation within transplanted organs will improve our ability to diagnose and monitor AMR, and they will also help guide the use of complement inhibitory drugs.

Cardiac Transplantation in the New Era

The prevalence of heart failure continues to rise due to the aging population and longer survival of people with conditions that lead to heart failure, eg, hypertension, diabetes, and coronary artery disease. Although medical therapy has had an important impact on survival of patients and improving quality of life, heart transplantation remains the definitive therapy for patients that eventually deteriorate. Since the first successful heart transplantation in 1967, significant improvements have been made regarding donor and recipient selection, surgical techniques, and postoperative care. However, the number of potential organ donors has not changed and the growing number of patients in need for transplantation has resulted an increase in waiting list time, and the need for mechanical support. To overcome this issue, the United Network for Organ Sharing implemented an allocation system to prioritize the sickest patients on the list to receive organs. Despite the careful selection of patients, pretransplant immunological screening, and multidrug immunosuppressive regimens, acute and chronic rejections occur and potentially limit graft and patient survival. Treatment for rejection largely depends on the type of rejection, the presence of hemodynamic compromise, and time after transplantation. The limiting factor for long-term graft survival is allograft vasculopathy, an immune-mediated process causing diffuse narrowing of the coronary arteries. Percutaneous coronary intervention and coronary artery bypass surgery are often not an option for this vasculopathy due to the lack of focal lesions, and retransplantation is the only option in appropriate patients.

The 2013 International Society for Heart and Lung Transplantation Working Formulation for the standardization of nomenclature in the pathologic diagnosis of antibody-mediated rejection in heart transplantation

During the last 25 years, antibody-mediated rejection of the cardiac allograft has evolved from a relatively obscure concept to a recognized clinical complication in the management of heart transplant patients. Herein we report the consensus findings from a series of meetings held between 2010-2012 to develop a Working Formulation for the pathologic diagnosis, grading, and reporting of cardiac antibody-mediated rejection. The diagnostic criteria for its morphologic and immunopathologic components are enumerated, illustrated, and described in detail. Numerous challenges and unresolved clinical, immunologic, and pathologic questions remain to which a Working Formulation may facilitate answers.

Human leukocyte antigens and alloimmunization in heart transplantation: an open debate

Considerable advances in heart transplantation outcome have been achieved through the improvement of donor-recipient selection, better organ preservation, lower rates of perioperative mortality and the use of innovative immunosuppressive protocols. Nevertheless, long-term survival is still influenced by late complications. We support the introduction of HLA matching as an additional criterion in the heart allocation. Indeed, allosensitization is an important factor affecting heart transplantation and the presence of anti-HLA antibodies causes an increased risk of antibody-mediated rejection and graft failure. On the other hand, the rate of heart-immunized patients awaiting transplantation is steadily increasing due to the limited availability of organs and an increased use of ventricular assist devices. Significant benefits may result from virtual crossmatch approach that prevents transplantation in the presence of unacceptable donor antigens. A combination of both virtual crossmatch and a tailored desensitization therapy could be a good compromise for a favorable outcome in highly sensitized patients. Here, we discuss the unresolved issue on the clinical immunology of heart transplantation.

Heart transplantation: challenges facing the field

There has been significant progress in the field of heart transplantation over the last 45 years. The 1-yr survival rates following heart transplantation have improved from 30% in the 1970s to almost 90% in the 2000s. However, there has been little change in long-term outcomes. This is mainly due to chronic rejection, malignancy, and the detrimental side effects of chronic immunosuppression. In addition, over the last decade, new challenges have arisen such as increasingly complicated recipients and antibody-mediated rejection. Most, if not all, of these obstacles to long-term survival could be prevented or ameliorated by the induction of transplant tolerance wherein the recipient's immune system is persuaded not to mount a damaging immune response against donor antigens, thus eliminating the need for chronic immunosuppression. However, the heart, as opposed to other allografts like kidneys, appears to be a tolerance-resistant organ. Understanding why organs like kidneys and livers are prone to tolerance induction, whereas others like hearts and lungs are tolerance-resistant, could aid in our attempts to achieve long-term, immunosuppression-free survival in human heart transplant recipients. It could also advance the field of pig-to-human xenotransplantation, which, if successful, would eliminate the organ shortage problem. Of course, there are alternative futures to the field of heart transplantation that may include the application of total mechanical support, stem cells, or bioengineered whole organs. Which modality will be the first to reach the ultimate goal of achieving unlimited, long-term, circulatory support with minimal risk to longevity or lifestyle is unknown, but significant progress in being made in each of these areas.

Alloantibody and complement promote T cell-mediated cardiac allograft vasculopathy through noncanonical nuclear factor-κB signaling in endothelial cells

BACKGROUND

Cardiac allograft vasculopathy is the major cause of late allograft loss after heart transplantation. Cardiac allograft vasculopathy lesions contain alloreactive T cells that secrete interferon-γ, a vasculopathic cytokine, and occur more frequently in patients with donor-specific antibody. Pathological interactions between these immune effectors, representing cellular and humoral immunity, respectively, remain largely unexplored.

METHODS AND RESULTS

We used human panel reactive antibody to form membrane attack complexes on allogeneic endothelial cells in vitro and in vivo. Rather than inducing cytolysis, membrane attack complexes upregulated inflammatory genes, enhancing the capacity of endothelial cells to recruit and activate allogeneic interferon-γ--producing CD4(+) T cells in a manner dependent on the activation of noncanonical nuclear factor-κB signaling. Noncanonical nuclear factor-κB signaling was detected in situ within endothelial cells both in renal biopsies from transplantation patients with chronic antibody-mediated rejection and in panel-reactive antibody--treated human coronary artery xenografts in immunodeficient mice. On retransplantation into immunodeficient hosts engrafted with human T cells, panel-reactive antibody--treated grafts recruited more interferon-γ--producing T cells and enhanced cardiac allograft vasculopathy lesion formation.

CONCLUSIONS

Alloantibody and complement deposition on graft endothelial cells activates noncanonical nuclear factor-κB signaling, initiating a proinflammatory gene program that enhances alloreactive T cell activation and development of cardiac allograft vasculopathy. Noncanonical nuclear factor-κB signaling in endothelial cells, observed in human allograft specimens and implicated in lesion pathogenesis, may represent a target for new pharmacotherapies to halt the progression of cardiac allograft vasculopathy.

Donor-specific antibodies are associated with antibody-mediated rejection, acute cellular rejection, bronchiolitis obliterans syndrome, and cystic fibrosis after lung transplantation

BACKGROUND

Lung transplantation is limited by chronic lung allograft dysfunction. Acute cellular rejection (ACR) is a risk factor for allograft dysfunction; however, the role of antibody-mediated rejection (AMR) is not well characterized.

METHODS

This was a retrospective review from 2007 to 2011 of lung transplant recipients with human leukocyte antigen (HLA) antibody testing using Luminex (Luminex Corp, Austin, TX) single-antigen beads. Statistics included Fisher's exact test for significance.

RESULTS

Donor-specific antibodies (DSA) developed in 13 of 44 patients. Of the 13 with DSA, 12 had cystic fibrosis compared with 18 of 31 in the non-DSA group (p = 0.035). Of those with DSAs, 23.1% occurred within the first year, and 69.2% occurred between 1 and 3 years. Twelve of 13 DSA patients had anti-HLA DQ specificity compared with 2 of 31 non-DSA patients (p = 0.0007). AMR developed in 10 of the 13 DSA patients compared with 1 of 31 non-DSA patients (p = 0.0001). The DSA group experienced 2.6 episodes/patient of cellular rejection vs 1.7 episodes/patient in the non-DSA group (p = 0.059). Bronchiolitis obliterans syndrome developed in 11 of 13 in the DSA group vs 10 of 31 in the non-DSA group (p = 0.0024). In the DSA group, 11.5% HLAs matched compared with 20.4% in the non-DSA group (p = 0.093). AMR developed in 11 of 22 patients in the non-DSA HLA group compared with 0 of 22 in the group without non-DSA HLA antibodies (p = 0.002). Survival at 1 and 3 years was 92% and 36% in the DSA group, respectively, and 97% and 65% in the non-DSA group.

CONCLUSIONS

DSAs and non-DSAs occur frequently after lung transplantation. DSAs are prevalent in the cystic fibrosis population and are associated with AMR, bronchiolitis obliterans syndrome, and possibly, ACR.

Antibody-mediated rejection after intestinal transplantation

PURPOSE OF REVIEW

In spite of impressive improvements in short-term outcomes for intestine transplant recipients, late allograft loss continues to plague the field. Attention has mostly been focused on T-cell-mediated cellular mechanisms of allograft rejection to explain these losses; however, as in other forms of solid-organ transplantation, especially kidney and heart, antibody-mediated mechanisms of acute and chronic allograft injury are increasingly being implicated. In this review, the mechanisms of B-cell- and humoral-mediated allograft injury will be briefly discussed along with the limited evidence that exist for invoking antibody-mediated rejection (AMR) as important in intestine transplantation.

RECENT FINDINGS

The presence of donor-specific antibody has been reported to increase the incidence and severity of intestine allograft rejection and to worsen the overall prognosis for graft and patient. C4d staining in intestine biopsies is unreliable, and currently it is not possible to diagnose AMR with certainty in intestine transplantation. Treatment of presumed AMR in intestine recipients is purely anecdotal at this time.

SUMMARY

Further basic and clinical research needs to be conducted to more confidently diagnose and treat AMR in intestinal transplantation.

A survey of current practice for antibody-mediated rejection in heart transplantation

No evidence based management guidelines exist for antibody mediated rejection (AMR) in heart transplantation. The International Society for Heart and Lung Transplantation (ISHLT) recently introduced standardized pathologic based diagnostic criteria for AMR (pAMR 0-3). We evaluated international practice for the management of AMR focusing on pAMR grade, donor specific antibody (DSA) and allograft function. On-line survey data were analyzed from 184 ISHLT members (physicians-78%, surgeons-20%). The majority were from adult-transplant (84%), medium-large volume centres (transplants/year: 10-25, 61%; 25-50, 19%) across North America (60%) and Europe (26%). Irrespective of pAMR grade and DSA, 83-90% treated a drop in ejection fraction (EF≤45% or >25% decrease). In the presence of stable EF, an increasing number elected treatment for progressively severe pAMR grade (p<0.001) and for accompanying DSA (p<0.05, pAMR 1-3). Intravenous steroid was the most commonly used therapy followed by intravenous immunoglobulin (IVIG) or plasmapheresis, rituximab and thymoglobulin. Plasmapheresis and rituximab were favored for positive versus negative DSA (p<0.05). Using a threshold of ≥70% consensus among respondents, treatment for AMR may be considered for a drop in EF, asymptomatic pAMR 3 or asymptomatic pAMR 2 with DSA. Combination steroid, IVIG and plasmapheresis are suggested as initial therapies.

Immunologic considerations in heart transplantation for congenital heart disease

Children and adults with congenital heart disease (CHD) can require interventions that result in immunologic alterations that are different than those seen in patients with cardiomyopathies. Patients with CHD can be exposed to heart surgeries, blood products, valved and non-valved allograft tissue, and mechanical circulatory support, all of which can alter the immunologic status of these patients. This change in immunologic status is most commonly manifested as the development of anti-human leukocyte antigen (HLA) antibodies. This review will delineate a) the causes of anti-HLA anti-body production (often referred to as allosensitization); b) preventive strategies for anti-HLA antibody production before transplantation; c) treatment strategies for those patients who develop anti-HLA antibodies before transplantation; d) consequences of HLA allosensitization after transplantation; and e) treatment of HLA allosensitization and antibody-mediated rejection after transplantation.

Permanent CNI treatment for prevention of renal allograft rejection in sensitized hosts can be replaced by regulatory T cells

Recent data suggest that donor-specific memory T cells (T(mem)) are an independent risk factor for rejection and poor graft function in patients and a major challenge for immunosuppression minimizing strategies. Many tolerance induction protocols successfully proven in small animal models e.g. costimulatory blockade, T cell depletion failed in patients. Consequently, there is a need for more predictive transplant models to evaluate novel promising strategies, such as adoptive transfer of regulatory T cells (Treg). We established a clinically more relevant, life-supporting rat kidney transplant model using a high responder (DA to LEW) recipients that received donor-specific CD4(+)/ 8(+) GFP(+) T(mem) before transplantation to achieve similar pre-transplant frequencies of donor-specific T(mem) as seen in many patients. T cell depletion alone induced long-term graft survival in naïve recipients but could not prevent acute rejection in T(mem)(+) rats, like in patients. Only if T cell depletion was combined with permanent CNI-treatment, the intragraft inflammation, and acute/chronic allograft rejection could be controlled long-term. Remarkably, combining 10 days CNI treatment and adoptive transfer of Tregs (day 3) but not Treg alone also induced long-term graft survival and an intragraft tolerance profile (e.g. high TOAG-1) in T(mem)(+) rats. Our model allows evaluation of novel therapies under clinically relevant conditions.

Improving survival during heart transplantation: diagnosis of antibody-mediated rejection and techniques for the prevention of graft injury

The diagnosis of antibody-mediated rejection (AMR) has presented a challenge due to the pleiomorphic immunologic responses that represent the condition. A consensus with regard to its pathological diagnosis continues to evolve. Due to an increasing number of sensitized patients undergoing heart transplantation, its incidence appears to be on the rise and the condition is associated with worse outcomes than acute cellular rejection. Treatment of AMR is also more difficult and response to increases in conventional immunosuppression is often limited. Risk factors for AMR include the use of ventricular assist devices, prior exposure to blood products, allografts and multiparity. Detection of alloantibodies with a high specificity and sensitivity allows risk stratification of recipients at potential risk of AMR. Desensitization and AMR treatment strategies are focused on several therapeutic targets, including suppression of T and B cells and elimination or inhibition of circulating antibodies.

Pushing the boundaries: the current status of ABO-incompatible cardiac transplantation

Since the introduction of intentional ABO-incompatible (ABOi) cardiac transplantation in infants in the late 1990's, the number of patients listed for and undergoing ABOi transplants has increased. This practice has been shown to lead to a reduction in waiting list mortality and increased utilisation of donor organs with equivalent outcomes to ABO-compatible transplants. Differences in the infant immune system provide a window of opportunity for ABOi transplantation. However it is increasingly clear that older patients and those with significant amounts of blood group antibody specific isohaemagglutinins may also benefit. Newer research is now focussing on longer term outcomes of ABOi transplants - in particular the development of graft accommodation or tolerance. This review assesses the current status of ABO-incompatible cardiac transplantation both in infants and in sensitized and older patients.

Heart transplantation research in the next decade--a goal to achieving evidence-based outcomes: National Heart, Lung, And Blood Institute Working Group

The National Heart, Lung, and Blood Institute (NHLBI) convened a Working Group (WG) on August 5 to 6, 2010 in Bethesda, Maryland to discuss future directions of research in heart transplantation (HT). The WG was composed of researchers with expertise in the basic science, clinical science, and epidemiological aspects of advanced heart failure and HT. These experts were asked to identify the highest priority research gaps in the field and make recommendations for future research strategies. The WG was also asked to include approaches that capitalize on current scientific opportunities and focus on areas that required unique NHLBI leadership. Finally, the WG was charged with developing recommendations that would have short- and long-term impact on the field of HT. The WG participants reviewed key areas in HT and identified the most urgent knowledge gaps. These gaps were then organized into the following 4 specific research directions: 1) enhanced phenotypic characterization of the pre-transplant population; 2) donor-recipient optimization strategies; 3) individualized immunosuppression therapy; and, 4) investigations of immune and non-immune factors affecting late cardiac allograft outcomes. Finally, because the HT population is relatively small compared with other patient groups, the WG strongly urged concerted efforts to enroll every transplant recipient into a clinical study and to increase collaborative networks to optimize research in this field.

A longitudinal study of the course of asymptomatic antibody-mediated rejection in heart transplantation

BACKGROUND

Growing evidence suggests worse cardiac allograft vasculopathy and mortality in patients with asymptomatic antibody-mediated rejection (AMR). Debate continues about whether therapeutic intervention is warranted to avoid adverse outcomes. In this study we examine the course of individual episodes of untreated asymptomatic AMR on follow-up endomyocardial biopsy (EMB).

METHODS

The U.T.A.H. Cardiac Transplant Program database was queried for transplant recipients between 1985 and 2009 who survived beyond 1 year and had at least 1 episode of lone AMR with a follow-up EMB. All EMBs were screened for AMR by immunofluorescence and graded for severity. Data were analyzed based on time from transplant (early, ≤12 months; late, >12 months).

RESULTS

Nine hundred fifty-eight patients with a total of 15,448 biopsies qualified for the study. Average age at transplant was 46.7 years; 13% of the patients were female. Within the first year post-transplant, asymptomatic AMR was diagnosed in 13.6% of biopsies compared with 5.2% beyond 1 year. AMR resolved in 65% (early) vs 75% (late) on follow-up EMB. More severe AMR was less likely to improve regardless of time from transplant. Furthermore, after an episode of AMR had resolved, the recurrence rate at 3, 6 and 12 months was 44%, 50.1% and 56.2%, respectively.

CONCLUSIONS

The incidence of AMR is higher in the first year post-transplant and the likelihood of resolution is less on follow-up EMB, especially when more severe. A small but significant number of cases became worse or did not change. These new findings may be helpful in planning future studies that test whether therapeutic interventions on asymptomatic AMR favorably impact outcomes.

Biomarkers of heart transplant rejection: the good, the bad, and the ugly!

Acute cellular rejection (ACR), antibody-mediated rejection (AMR), and cardiac allograft vasculopathy (CAV) are important limitations for the long-term survival of heart transplant recipients. Although much progress has been made in reducing ACR with modern immunosuppressive treatments and continuous biopsy surveillance, there is still a long way to go to better understand and treat AMR, to enable early detection of patients at risk of CAV, and to reduce the development and sustained progression of this irreversible disease that permanently compromises graft function. This review considers the advances made in ACR detection and treatment allowing a more prolonged survival and the risk factors leading to endothelial injury, dysfunction, inflammation, and subsequent CAV, as well as new treatment modalities for CAV. The review also evaluates the controversies around the definition, pathogenesis, and treatment of AMR. To date, much progress is still needed to significantly reduce post-transplant complications and increase graft and patient survival.

Antibody-mediated rejection: an evolving entity in heart transplantation

Antibody-mediated rejection (AMR) is gaining increasing recognition as a major complication after heart transplantation, posing a significant risk for allograft failure, cardiac allograft vasculopathy, and poor survival. AMR results from activation of the humoral immune arm and the production of donor-specific antibodies (DSA) that bind to the cardiac allograft causing myocardial injury predominantly through complement activation. The diagnosis of AMR has evolved from a clinical diagnosis involving allograft dysfunction and the presence of DSA to a primarily pathologic diagnosis based on histopathology and immunopathology. Treatment for AMR is multifaceted, targeting inhibition of the humoral immune system at different levels with emerging agents including proteasome and complement inhibitors showing particular promise. While there have been significant advances in our current understanding of the pathogenesis, diagnosis, and treatment of AMR, further research is required to determine optimal diagnostic tools, therapeutic agents, and timing of treatment.