Utility of virtual crossmatch in sensitized patients awaiting heart transplantation

Management of the sensitized heart transplant candidate

PURPOSE OF REVIEW

For sensitized heart transplant candidates who have antibodies to human leukocyte antigens (HLA), finding a suitable donor can be challenging and can lead to adverse waitlist outcomes. In recent years, the number of sensitized patients awaiting heart transplantation has increased likely due to the use of durable and mechanical circulatory support as well as increasing number of candidates with underlying congenital heart disease.

RECENT FINDINGS

Advances in the assessment of HLA antibodies allow for identification of heart transplant candidates who may benefit from desensitization strategies to widen the donor pool and mitigate the risk of adverse posttransplant outcomes.

SUMMARY

Antibody sensitization is a barrier to successful heart transplantation and strategies to identify sensitized patients, stratify their risk, and mitigate this risk through desensitization is crucial to optimize the quality of life and survival of HT recipients.

Detection and management of HLA sensitization in candidates for adult heart transplantation

Heart transplantation (HT) remains the preferred therapy for patients with advanced heart failure. However, for sensitized HT candidates who have antibodies to human leukocyte antigens , finding a suitable donor can be challenging and can lead to adverse waitlist outcomes. In recent years, the number of sensitized patients awaiting HT has increased likely due to the use of durable and mechanical circulatory support as well as increasing number of candidates with underlying congenital heart disease. This State-of-the-Art review discusses the assessment of human leukocyte antigens antibodies, potential desensitization strategies including mechanisms of action and specific protocols, the approach to a potential donor including the use of complement-dependent cytotoxicity, flow cytometry, and virtual crossmatches, and peritransplant induction management.

Donor heart selection: Evidence-based guidelines for providers

The proposed donor heart selection guidelines provide evidence-based and expert-consensus recommendations for the selection of donor hearts following brain death. These recommendations were compiled by an international panel of experts based on an extensive literature review.

Heart transplantation across preformed donor-specific antibody barriers using a perioperative desensitization protocol

Heart transplantation across preformed donor-specific HLA-antibody barriers is associated with impaired short- and long-term survival. Therefore, in recipients with preformed anti-HLA antibodies, waiting for crossmatch-negative donors is standard practice. As an alternative strategy, recipients with preformed anti-HLA donor specific antibodies have been managed at our institutions with a perioperative desensitization regimen. A retrospective analysis was performed comparing heart transplant recipients with preformed donor-specific HLA-antibodies to recipients without donor-specific antibodies. Recipients with a positive virtual crossmatch received a perioperative desensitization protocol including tocilizumab intraoperatively, plasma exchange and rituximab followed by a six-month course of IgGAM. Among the 117 heart-transplanted patients, 19 (16%) patients underwent perioperative desensitization, and the remaining 98 (84%) patients did not. Cold ischemic time, posttransplant extracorporeal life support for primary graft dysfunction, and intensive care unit stay time did not differ between groups. At 1-year follow-up, freedom from pulsed steroid therapy for presumed rejection and biopsy-confirmed acute cellular or humoral rejection did not differ between groups. One-year survival amounted to 94.7% in the treated patients and 81.4% in the control group. Therefore, heart transplantation in sensitized recipients undergoing a perioperative desensitization appears safe with comparable postoperative outcomes as patients with a negative crossmatch.

Contemporary review of heart transplant immunology and immunosuppressive therapy

Background

Survival after heart transplantation (HT) has improved considerably since the first HT was performed in 1967 in Cape Town, South Africa. Understanding immunology behind organ rejection has paved way for advances in the assessment of pre-transplant compatibility, development of newer and more specific immunosuppressive drugs, and management of rejection.

Objectives

Unlike medical therapy for heart failure, transplant protocols vary considerably between different centers. These variations in protocols generally reflect unique population characteristics and the availability of resources. This review article aims to provide a consolidated update on contemporary cardiac transplant medicine. We also aim to highlight local practice and its difference from our international counterparts.

Methods

A literature search was performed on Pubmed and Cochrane Central Register of Controlled Trials to identify trials and review articles that discussed heart transplant immunology and protocols. The International Society for Heart and Lung Transplant (ISHLT) guidelines were also reviewed. We focused on risk factors, prevention strategies, and treatment of cardiac rejection.

Results

A total of 48 articles were selected to provide a comprehensive overview of the contemporary practice of cardiac transplant immunosuppressive therapy. Comparisons were made with local data and practice protocols to highlight key differences.

Conclusion

Heart transplant covers a small subset of cardiac patients and much of the evidence is derived from empirical observations and retrospective analysis. This accounts for the heterogeneity in care and treatment protocols. More studies are needed to select best practices from around the world to further improve outcomes.

Heart transplantation: focus on donor recovery strategies, left ventricular assist devices, and novel therapies

Heart transplantation is advocated in selected patients with advanced heart failure in the absence of contraindications. Principal challenges in heart transplantation centre around an insufficient and underutilized donor organ pool, the need to individualize titration of immunosuppressive therapy, and to minimize late complications such as cardiac allograft vasculopathy, malignancy, and renal dysfunction. Advances have served to increase the organ donor pool by advocating the use of donors with underlying hepatitis C virus infection and by expanding the donor source to use hearts donated after circulatory death. New techniques to preserve the donor heart over prolonged ischaemic times, and enabling longer transport times in a safe manner, have been introduced. Mechanical circulatory support as a bridge to transplantation has allowed patients with advanced heart failure to avoid progressive deterioration in hepato-renal function while awaiting an optimal donor organ match. The management of the heart transplantation recipient remains a challenge despite advances in immunosuppression, which provide early gains in rejection avoidance but are associated with infections and late-outcome challenges. In this article, we review contemporary advances and challenges in this field to focus on donor recovery strategies, left ventricular assist devices, and immunosuppressive monitoring therapies with the potential to enhance outcomes. We also describe opportunities for future discovery to include a renewed focus on long-term survival, which continues to be an area that is under-studied and poorly characterized, non-human sources of organs for transplantation including xenotransplantation as well as chimeric transplantation, and technology competitive to human heart transplantation, such as tissue engineering.

Clinical Outcomes of Perioperative Desensitization in Heart Transplant Recipients

Background.

Sensitization remains a barrier to heart transplantation (HT). Perioperative desensitization strategies have been described; however, a paucity of evidence exists to demonstrate efficacy and safety in HT.

Methods.

This single-center, retrospective study consisted of adults who received an HT. Perioperative desensitization was initiated if virtual crossmatch or flow-cytometry crossmatch was positive. Therapy consisted of plasmapheresis, intravenous immunoglobulin, and rabbit antithymocyte globulin. Historical controls received standard immunosuppression or induction. The primary endpoint was survival at 12 mo. Secondary endpoints included freedom from acute rejection, cardiac allograft vasculopathy (CAV), and infectious complications.

Results.

Of the 104 patients included, 48 received no induction, 46 received induction, and 10 underwent perioperative desensitization. No differences were observed in the primary endpoint at 12 mo (90.0% versus 97.9%, P = 0.25 for desensitization versus no-induction; 90.0% versus 100%, P = 0.72 for desensitization versus induction). Rates of acute rejection were lower with induction and desensitization compared with no-induction. There were no significant differences in CAV between the groups. Infectious complications were also similar among the groups (10.0% versus 16.7%, P = 0.62 for desensitization versus no-induction; 10.0% versus 30.4%, P = 0.34 for desensitization versus induction).

Conclusions.

This study suggests that a perioperative desensitization strategy triggered by positive virtual crossmatch or flow-cytometry crossmatch allows for successful transplantation of sensitized HT recipients and results in acceptable rates of survival, rejection, CAV, and infection at 12 mo.

The meaning of donor-specific antibodies after heart transplant

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) is a major contributor of impaired long-term survival after heart transplantation (HTx). The presence of circulating donor-specific antihuman leukocyte antigen (HLA) antibodies (DSAs) is considered as a mandatory criterion for AMR after HTx. DSA are known prognostic biomarkers of outcome, for example, recipients with de-novo DSA have a three-fold increased risk of mortality.

RECENT FINDINGS

Although the awareness of the impact and prognosis of DSA on the survival has been increased in the HTx community over the last decade, the management of DSA pre and posttransplant varies among centers and is mainly based on the experience of transplant physicians. Thus, firm consensus strategies for each HTx recipient should be established by a center advisory board of experts in the field of HLA genetics, transplantation immunology, and HTx to evaluate the immunological risk preoperatively and also continuously during the posttransplant course. Consequently, the recent advances of invasive and noninvasive diagnostic tools should be applied, according to the risk, laboratory findings, and clinical events of the recipient. Such individual strategy will result in tailored therapeutic options.

SUMMARY

Novel standards for the management of DSA in HTx recipients on the basis of an interdisciplinary approach of experts will improve diagnostics for personalized medicine.

Out with the old, in with the new: Virtual versus physical crossmatching in the modern era

The virtual crossmatch (VXM) is gaining acceptance as an alternative approach to assess donor:recipient compatibility prior to transplantation. In contrast to a physical crossmatch, the virtual crossmatch does not require viable donor cells but rather relies on complete HLA typing of the donor and current antibody assessment of the recipient. Thus, the VXM can be performed in minutes which allows for faster transplant decisions thereby increasing the likelihood that organs can be shipped across significant distances yet safely transplanted. Here, we present a brief review of the past 50 years of histocompatibility testing; from the original complement-dependent cytotoxicity crossmatch in 1969 to the new era of molecular HLA typing, solid-phase antibody testing and virtual crossmatching. These advancements have shaped a paradigm shift in our approach to transplantation. That is, foregoing a prospective physical crossmatch in favor of a VXM. In this review, we undertake an in-depth analysis of the pros- and cons- of physical and virtual crossmatching.Finally, we provide objective data on the selected use of the VXM which demonstrate the value of a VXM in lieu of the traditional physical crossmatch for safe and efficient organ transplantation.

Does the Canadian allocation system for highly sensitized patients work?

PURPOSE OF REVIEW

The number of sensitized heart transplant candidates is rising. Highly sensitized patients are disadvantaged because they encounter longer waiting times to heart transplant. Strategies to reduce their waiting times include waitlist prioritization and desensitization therapies. The purpose of this review is to describe the listing category for highly sensitized patients in the Canadian allocation system, examine the advantages and limitations of this strategy and provide an approach to the management of the highly sensitized patient awaiting heart transplant.

RECENT FINDINGS

Analysis of data from the United Network of Organ Sharing shows that the incidence of death or removal from the waitlist in patients listed for heart transplant increases as the calculated panel reactive antibody (cPRA) increases and is independent of medical urgency. In the Canadian allocation system, patients with cPRA more than 80% have a similar incidence of death on the waitlist as less sensitized patients, suggesting they survive to be transplanted. Furthermore, prioritizing and transplanting highly sensitized patients has been associated with acceptable post-transplant outcomes.

SUMMARY

The Canadian allocation system prioritizes highly sensitized patients to increase equity and access to transplantation while maintaining good post-transplant outcomes. Not all highly sensitized patients can wait for an organ, even if prioritized. A pragmatic individualized approach would consider the medical stability of the patient, the likelihood of transplant with a negative crossmatch and then determine whether waitlist prioritization or desensitization is the more appropriate strategy.

Early immune biomarkers and intermediate-term outcomes after heart transplantation: Results of Clinical Trials in Organ Transplantation-18

Clinical Trials in Organ Transplantation-18 (CTOT-18) is a follow-up analysis of the 200-subject multicenter heart transplant CTOT-05 cohort. CTOT-18 aimed to identify clinical, epidemiologic, and biologic markers associated with adverse clinical events past 1 year posttransplantation. We examined various candidate biomarkers including serum antibodies, angiogenic proteins, blood gene expression profiles, and T cell alloreactivity. The composite endpoint (CE) included death, retransplantation, coronary stent, myocardial infarction, and cardiac allograft vasculopathy. The mean follow-up was 4.5 ± SD 1.1 years. Subjects with serum anti-cardiac myosin (CM) antibody detected at transplantation and at 12 months had a higher risk of meeting the CE compared to those without anti-CM antibody (hazard ratio [HR] = 2.9, P = .046). Plasma VEGF-A and VEGF-C levels pretransplant were associated with CE (odds ratio [OR] = 13.24, P = .029; and OR = 0.13, P = .037, respectively). Early intravascular ultrasound findings or other candidate biomarkers were not associated with the study outcomes. In conclusion, anti-CM antibody and plasma levels of VEGF-A and VEGF-C were associated with an increased risk of adverse events. Although this multicenter report supports further evaluation of the mechanisms through which anti-CM antibody and plasma angiogenesis proteins lead to allograft injury, we could not identify additional markers of adverse events or potential novel therapeutic targets.

Understanding the Correlation Between DSA, Complement Activation, and Antibody-Mediated Rejection in Heart Transplant Recipients

BACKGROUND

Donor-specific HLA antibodies (DSA) are associated with increased rates of rejection and of graft failure in cardiac transplantation. The goal of this study was to determine the association of preformed and posttransplant development of newly detected DSA (ndDSA) with antibody-mediated rejection (AMR) and characterize the clinical relevance of complement-activating DSA in heart allograft recipients.

METHODS

The study included 128 adult and 48 pediatric heart transplant patients transplanted between 2010 and 2013. Routine posttransplant HLA antibody testing was performed by IgG single-antigen bead test. The C3d single-antigen bead assay was used to identify complement-activating antibodies. Rejection was diagnosed using International Society for Heart and Lung Transplantation criteria.

RESULTS

In this study, 22 patients were transplanted with preexisting DSA, and 43 patients developed ndDSA posttransplant. Pretransplant (P < 0.05) and posttransplant (P < 0.001) ndDSA were associated with higher incidence of AMR. Patients with C3d + DSA had significantly higher incidence of AMR compared with patients with no DSA (P < 0.001) or patients with C3d-DSA (P = 0.02). Nine (36%) of 25 patients with AMR developed transplant coronary artery disease compared with 17 (15.9%) of 107 patients without AMR (P < 0.05). Among the 47 patients who received ventricular assistant device (VAD), 7 of 9 VAD+ patients with preformed DSA experienced AMR compared with 7 of 38 VAD+ patients without preformed DSA, indicating presensitization to donor HLA significantly increased the risk of AMR (P < 0.01).

CONCLUSIONS

Preformed and posttransplant ndDSA were associated with AMR. C3d + DSA correlates with complement deposition on the graft and higher risk of AMR which may permit the application of personalized immunotherapy targeting the complement pathway.

Study rationale, design, and pretransplantation alloantibody status: A first report of Clinical Trials in Organ Transplantation in Children-04 (CTOTC-04) in pediatric heart transplantation

Anti-HLA donor-specific antibodies are associated with worse outcomes after organ transplantation. Among sensitized pediatric heart candidates, requirement for negative donor-specific cytotoxicity crossmatch increases wait times and mortality. However, transplantation with positive crossmatch may increase posttransplantation morbidity and mortality. We address this clinical challenge in a prospective, multicenter, observational cohort study of children listed for heart transplantation (Clinical Trials in Organ Transplantation in Children-04 [CTOTC-04]). Outcomes were compared among sensitized recipients who underwent transplantation with positive crossmatch, nonsensitized recipients, and sensitized recipients without positive crossmatch. Positive crossmatch recipients received antibody removal and augmented immunosuppression, while other recipients received standard immunosuppression with corticosteroid avoidance. This first CTOTC-04 report summarizes study rationale and design and relates pretransplantation sensitization status using solid-phase technology. Risk factors for sensitization were explored. Of 317 screened patients, 290 were enrolled and 240 underwent transplantation. Core laboratory evaluation demonstrated that more than half of patients were anti-HLA sensitized. Greater than 80% of sensitized patients had class I (with or without class II) HLA antibodies, and one-third of sensitized patients had at least 1 HLA antibody with median fluorescence intensity of ≥8000. Logistic regression models demonstrated male sex, weight, congenital heart disease history, prior allograft, and ventricular assist device are independent risk factors for sensitization.

Recent advances in heart transplantation

Despite advances in medical and electrical therapies for heart failure, morbidity and mortality remain high and patients often progress to end-stage heart failure. Over the last five decades, heart transplantation is considered a standard therapy for select patients with end-stage heart failure. However, while heart transplantation has become a treatment of choice for end-stage heart failure, challenges still exist for improvement in the short- and long-term outcomes. While there is an increase in the number of patients with end-stage heart failure, the number of donor organs remains a major limiting factor. Heart transplantation candidates in the current era are also more complex: older, antigen-sensitized, and on mechanical circulatory support at the time of listing and transplant. Such potential heart transplant recipients have an increased chance of problems, including antibody-mediated rejection and primary graft dysfunction. Recent advances could address the current challenges and include: 1) attempts to expand the pool of donor hearts; 2) changes in heart transplantation allocation policy allowing for more equitable organ distribution; and 3) advances in the management of antibody sensitization. Developments in these areas could result in improved survival and quality of life for heart transplantation recipients.

The impact of flow PRA on outcome in pediatric heart recipients in modern era: An analysis of the Pediatric Heart Transplant Study database

Data from patients in the Pediatric Heart Transplant Study (PHTS) registry transplanted between 2010 and 2014 were analyzed to determine the association between HLA antibody (PRA) determined by SPA using Luminex or flow cytometry with a positive retrospective cross-match and the post-transplant outcomes of acute rejection and graft survival. A total of 1459 of 1596 (91%) recipients had a PRA reported pretransplant; 26% had a PRA > 20%. Patients with a PRA > 20% were more likely to have CHD, prior cardiac surgery, ECMO support at listing, and waited longer for transplantation than patients with a PRA <20%. Patients with higher PRA% determined by SPA were predictive of a positive retrospective cross-match determined by flow cytometric method (P < .001). A PRA > 50% determined by SPA was independently associated with worse overall graft survival after first month of transplant in both unadjusted and adjusted for all other risk factors. In this large multicenter series of pediatric heart transplant recipients, an elevated PRA determined by SPA remains a significant risk factor in the modern era.

Honoring 50 Years of Clinical Heart Transplantation in Circulation

Heart transplantation has become a standard therapy option for advanced heart failure. The translation of heart transplantation from innovative experiments to long-term clinical success has married prescient insights with discipline and organization in the domains of surgical techniques, organ preservation, immunosuppression, organ donation and transplantation logistics, infection control, and long-term graft surveillance. This review explores the key milestones of the past 50 years of heart transplantation and discusses current challenges and promising innovations on the clinical horizon.

Desensitization strategies in adult heart transplantation-Will persistence pay off?

Strategies are needed to enable successful heart transplantation in highly sensitized patients. Immunologic challenges from sensitization to human leukocyte antigen (HLA) reduce access to compatible donors, extend waiting times to transplant, and increase the risks of antibody-mediated rejection and cardiac allograft vasculopathy after transplant. The prime goal of desensitization is to increase access to transplantation through expansion of the donor organ pool. Existing therapies are directed at key components of the humoral immune response with newer biologically based regimens able to target plasma cells as the source of antibody production, as well as complement activation that has a central role in antibody-mediated injury. Despite the emergence of early promising results for these agents, a significant knowledge gap remains with the current data for desensitization, extrapolated mostly from non-heart solid-organ transplants and small observational studies. Notably, no approach has demonstrated significant and sustainable reductions in HLA antibody pre-transplant, and the ideal desensitization strategy remains elusive. In addition, clinical tools to evaluate the humoral response and efficacy of therapy are limited, focusing almost exclusively on HLA antibody detection. Importantly, desensitization is associated with significant costs and potential risks, and overall long-term outcomes and cost-effectiveness have not been sufficiently evaluated. Investigation is ongoing into the development of a clinically effective desensitization strategy in heart transplantation.

CPRA for allocation of kidneys in the US: More candidates ≥98% CPRA, lower positive crossmatch rates and improved transplant rates for sensitized patients

In 2009 calculated panel reactive antibody (CPRA) replaced PRA as the metric for HLA sensitization in the US kidney allocation system. During the next four years, registrants with at least one unacceptable antigen increased (34-40%) and registrants with ≥98% PRA/CPRA increased from 7% to 9% of the waitlist. These changes were accompanied by a reduction in kidney offers refused for positive crossmatch: 14,137 (1.7%) in 2009 and 3,310 in 2013 (0.4%). Registrants with ≥98% PRA/CPRA had highest rates of refusal but also showed substantial improvement (20% in 2009 vs 8% in 2013). For registrants with ≥98% PRA/CPRA, 45% of accepted offers in 2009 were not transplanted into the intended recipient compared to 11% in 2013. Transplant rates remained low for these patients (∼50/1000 active patient-years), but rates improved for patients with 80-97% PRA/CPRA (223/1000 active patient-years in 2009 vs 354/1000 in 2013). In 2013, 40% regraft candidates had CPRA ≥98% compared to 4% of primary graft candidates. More females than males were ≥98% CPRA (14% vs 7%) and more females had CPRA above 0 (50% vs 28%). In the CPRA era, listing of unacceptable antigens increased, positive crossmatches were diminished and transplant rates for sensitized patients improved.

Changes in the methodology of pre-heart transplant human leukocyte antibody assessment: an analysis of the United Network for Organ Sharing database

BACKGROUND

We sought to investigate temporal trends in the methodology of human leukocyte antibody assessment in heart transplantation.

METHODS

The United Network for Organ Sharing database was queried from June 2004 to March 2013 to obtain pre-heart transplantation human leukocyte antibody results. The % panel reactive antibody for class I and II antibodies was recorded along with the methodology of assessment. Allosensitization was defined as class I and/or II panel reactive antibody of ≥ 10%. The primary outcome measure was graft survival.

RESULTS

During the study period, 12,858 patients with available data underwent heart transplantation. The prevalence of allosensitization increased, with 16.8% in 2005-2006 sensitized at the time of transplantation compared to 23.1% in 2010-2011 (p < 0.001); this occurred in conjunction with an increase in the utilization of flow cytometry (77.2% in 2005-2006; 97.0% in 2010-2011, p < 0.001). Using multivariable analysis, a positive pre-heart transplantation panel reactive antibody by flow cytometry independently predicted graft loss.

CONCLUSIONS

There has been a recent increase in flow cytometric assessment of human leukocyte antibodies prior to heart transplantation, which may be associated with an increase in the prevalence of pre-transplant patients being characterized as allosensitized. Flow cytometry may identify patients with the highest likelihood of graft loss.

Effect of Pretransplant Rituximab Use on Posttransplant Clinical Outcomes in Patients with High Panel Reactive Antibody Scores

BACKGROUND/AIMS

Whether desensitization treatment is needed before kidney transplant (KT) in patients with high panel reactive antibody (PRA) scores but negative cross-matching (XM) tests remains controversial. This study aimed to investigate the effect of pretransplant rituximab (RTX) treatment on clinical outcomes in these patients.

METHODS

The study included 52 patients with PRA >50% but negative XM tests before transplantation. A single dose of RTX was administered before KT in 32 patients (RTX group). The 20 patients without RTX treatment were considered as controls (CON group). We compared the development of acute antibody-mediated rejection (AMR), rejection-free survival, allograft function, allograft and patients' survival rates and infection rates between the 2 groups.

RESULTS

The rate of acute AMR development was significantly lower in the RTX group than in the CON group (p = 0.009), and rejection-free survival in the RTX group was higher than in the CON group (p = 0.042). The 3-year graft and patient survival rates were higher in the RTX group than in the CON group (p = 0.007 and p = 0.037, respectively). There were no significant differences in infection rates between the groups.

CONCLUSIONS

Pretransplant use of RTX improved the post-transplant clinical outcomes in patients with high PRAs but negative XM tests.

Report from a forum on US heart allocation policy

Since the latest revision in US heart allocation policy (2006), the landscape and volume of transplant waitlists have changed considerably. Advances in mechanical circulatory support (MCS) prolong survival, but Status 1A mortality remains high. Several patient subgroups may be disadvantaged by current listing criteria and geographical disparity remains in waitlist time. This forum on US heart allocation policy was organized to discuss these issues and highlight concepts for consideration in the policy development process. A 25-question survey on heart allocation policy was conducted. Among attendees/respondents were 84 participants with clinical/published experience in heart transplant representing 51 US transplant centers, and OPTN/UNOS and SRTR representatives. The survey results and forum discussions demonstrated very strong interest in change to a further-tiered system, accounting for disadvantaged subgroups and lowering use of exceptions. However, a heart allocation score is not yet viable due to the long-term viability of variables (used in the score) in an ever-developing field. There is strong interest in more refined prioritization of patients with MCS complications, highly sensitized patients and those with severe arrhythmias or restrictive physiology. There is also strong interest in distribution by geographic boundaries modified according to population. Differences of opinion exist between small and large centers.

Immunosuppression following heart transplantation: prospects and challenges

Immunosuppression after heart transplantation has significantly reduced the incidence of cellular rejection and improved patient outcomes with the routine use of calcineurin inhibitors. Antimetabolites and proliferation signal inhibitors add to the improvement in patient outcomes, particularly with respect to the reduced burden of cardiac allograft vasculopathy. Patients with antibody sensitization are potentially at higher risk of postoperative complications. Sensitized patients are undergoing heart transplantation with increased frequency, in part due to the emergence of ventricular assist device use as a bridge to heart transplantation. Despite improvements in immunosuppressive therapies, many challenges face physicians and patients, which will further refine and improve care of the post-heart transplant patient.

Association of human leukocyte antigen donor-recipient matching and pediatric heart transplant graft survival

BACKGROUND

The effect of donor-recipient human leukocyte antigen (HLA) matching on outcomes remains relatively unexplored in pediatric patients. The objective of this study was to investigate the effects of donor-recipient HLA matching on graft survival in pediatric heart transplantation.

METHODS AND RESULTS

The UNOS (United Network for Organ Sharing) database was queried for heart transplants occurring between October 31, 1987, and December 31, 2012, in a recipient aged ≤17 years with ≥1 postoperative follow-up visit. Retransplants were excluded. Transplants were divided into 3 donor-recipient matching groups: no HLA matches (HLA-no), 1 or 2 HLA matches (HLA-low), and 3 to 6 HLA matches (HLA-high). Primary outcome was graft loss. Four thousand four hundred seventy-one heart transplants met the study inclusion criteria. High degree of donor-recipient HLA matching occurred infrequently: HLA-high (n=269; 6%) versus HLA-low (n=2683; 60%) versus HLA-no (n=1495; 34%). There were no differences between HLA matching groups in the frequency of coronary vasculopathy (P=0.19) or rejection in the first post-transplant year (P=0.76). Improved graft survival was associated with a greater degree of HLA donor-recipient matching: HLA-high median survival, 17.1 (95% confidence interval, 14.0-20.2) years; HLA-low median survival, 14.2 (13.1-15.4) years; and HLA-no median survival, 12.1 (10.9-13.3 years) years; P<0.01, log-rank test. In Cox-regression analysis, HLA matching was independently associated with decreased graft loss: HLA-low versus HLA-no hazard ratio, 0.86 (95% confidence interval, 0.74-0.99), P=0.04; HLA-high versus HLA-no, 0.62 (95% confidence interval, 0.43-0.90), P<0.01.

CONCLUSIONS

Decreased graft loss in pediatric heart transplantation was associated with a higher degree of donor-recipient HLA matching, although a difference in the frequency of early rejection or development of coronary artery vasculopathy was not seen.

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.

Allosensitization in heart transplantation: an overview

Transplant candidates might manifest circulating antibodies against human leukocyte antigens and nonhuman leukocyte antigens, a condition termed allosensitization. The presence of these antibodies decreases a given candidate's possible donor pool, thereby prolonging the time to transplantation. They are also associated with poorer posttransplant outcomes including increased morbidity and mortality. With the increasing use of ventricular assist devices as a bridge to transplantation, the prevalence of allosensitized transplant candidates has increased. This has implications for transplant programs in terms of donor-recipient matching and managing transplant-related complications, which are more common in this high risk cohort. Controversy exists as to the best approach in managing sensitized patients, before and after transplantation. Transplant centres have used various strategies to reduce antibody loads with mixed results being reported; moreover, it remains unclear as to whether attempts at desensitization translate into better posttransplant outcomes. As an alternative management approach, some centres participate in large organ sharing strategies and allocate organs based on the probability of finding a successful donor-recipient match. In this article, the immunological basis of allosensitization, its causes, implications, and therapeutic strategies to manage sensitized patients are reviewed. The literature in relation to desensitization therapies in heart transplant candidates is also reviewed.

HLA and MICA allosensitization patterns among patients supported by ventricular assist devices

BACKGROUND

Ventricular assist devices (VADs) are increasingly being used as a bridge to transplantation and have been implicated as a risk factor for allosensitization to human leukocyte antigens (HLA). We investigate the association between VAD and allosensitization to human leukocyte antigens (HLA) and major-histocompatibility-complex (MHC) class I-related Chain A (MICA) antigens.

METHODS

We considered all patients who received a VAD at our institution between 2000 and 2009; 89 of them had pre-VAD and post-VAD (≤6 months after implant) HLA antibody screening. A control group of non-VAD heart transplant candidates was constructed with at least 2 pre-transplant panel-reactive antibody (PRA) tests within 8 months. Two controls were randomly selected/VAD patient matched for year (n = 178). Patients and controls with available sera from these time-points were tested by Luminex/flow PRA single-antigen beads and by MICA antibody Luminex single-antigen beads. Medical records were reviewed for comparison of pre-transplant immunologic risk factors and post-transplant outcomes between the 2 groups.

RESULTS

Compared with controls, VAD patients had greater Class I differences between peak and initial PRA (18% vs. 0%, p < 0.0001) and higher peak PRA (24% vs. 6%, p < 0.0001). The differences between the 2 groups in Class II were less pronounced than in Class I. Of patients who had single-antigen testing, VAD implantation was significantly associated with development of new HLA antibody specificities (Class I and/or Class II) post-VAD with an increase in calculated PRA (cPRA) post-VAD compared with controls (16% vs. 0%, p < 0.0001). This risk was still present after adjusting for age, gender, pre-VAD PRA, transfusion and duration of follow-up in a multivariate analysis (p < 0.0001 and 0.02, respectively). There were no differences in development of MICA antibodies between the 2 groups (14% in both). There was no significant difference in the incidence of pre-transplant positive T-cell crossmatch, pre-transplant donor-specific HLA antibodies, rejection episodes or graft survival between the 2 groups.

CONCLUSION

Our results suggest that VAD is associated with significant HLA allosensitization independent of common risk factors.

Occurrence of Fatal and Nonfatal Adverse Outcomes after Heart Transplantation in Patients with Pretransplant Noncytotoxic HLA Antibodies

HLA antibodies (HLA ab) in transplant candidates have been associated with poor outcome. However, clinical relevance of noncytotoxic antibodies after heart transplant (HT) is controversial. By using a Luminex-based HLA screening, we retested pretransplant sera from HT recipients testing negative for cytotoxic HLA ab and for prospective crossmatch. Out of the 173 consecutive patients assayed (52 ± 13y; 16% females; 47% ischemic etiology), 32 (18%) showed pretransplant HLA ab, and 12 (7%) tested positive against both class I and class II HLA. Recipients with any HLA ab had poorer survival than those without (65 ± 9 versus 82 ± 3%; P = 0.02), accounting for a doubled independent mortality risk (P = 0.04). In addition, HLA-ab detection was associated with increased prevalence of early graft failure (35 versus 15%; P = 0.05) and late cellular rejection (29 versus 11%; P = 0.03). Of the subgroup of 37 patients suspected for antibody mediated rejection (AMR), the 9 with pretransplant HLA ab were more likely to display pathological AMR grade 2 (P = 0.04). By an inexpensive, luminex-based, HLA-screening assay, we were able to detect non-cytotoxic HLA ab predicting fatal and nonfatal adverse outcomes after heart transplant. Allocation strategies and desensitization protocols need to be developed and prospectively tested in these patients.

The use of the calculated panel-reactive antibody and virtual crossmatch in heart transplantation

PURPOSE OF REVIEW

Alloantibodies to human leukocyte antigens (HLAs) in patients awaiting heart transplantation are associated with prolonged wait time to transplant, increased risk of posttransplant rejection and cardiac allograft vasculopathy, and decreased survival. Solid-phase assays to determine antibody presence have allowed for the development of a calculated panel-reactive antibody to denote unacceptable antigens. The virtual crossmatch allows for the comparison of recipient HLA antibodies to prospective donor HLA antigens to safely match a patient to an appropriate donor without a prospective crossmatch.

RECENT FINDINGS

Expansion of the donor pool and decreased waiting time for heart transplant may be impacted by further assessment of the functional status of alloantibodies and novel means for desensitization. Sensitized patients who receive left ventricular assist device (LVAD) as a bridge to cardiac transplant appear to have similar postoperative 1-year outcomes compared with nonsensitized patients.

SUMMARY

Antibody sensitization poses an additional hurdle to patients awaiting heart transplantation. Functional antibody assessment, placement of a LVAD as bridge to cardiac transplant, and novel means of desensitization may impact a sensitized patient's ability to safely undergo heart transplantation.

Postoperative care of the transplanted patient

The successful delivery of optimal peri-operative care to pediatric heart transplant recipients is a vital determinant of their overall outcomes. The practitioner caring for these patients must be familiar with and treat multiple simultaneous issues in a patient who may have been critically ill preoperatively. In addition to the complexities involved in treating any child following cardiac surgery, caretakers of newly transplanted patients encounter multiple transplant-specific issues. This chapter details peri-operative management strategies, frequently encountered early morbidities, initiation of immunosuppression including induction, and short-term outcomes.

Thoracic organ transplantation: laboratory methods

Although great progress has been achieved in thoracic organ transplantation through the development of effective immunosuppression, there is still significant risk of rejection during the early post-transplant period, creating a need for routine monitoring for both acute antibody and cellular mediated rejection. The currently available multiplexed, microbead assays utilizing solubilized HLA antigens afford the capability of sensitive detection and identification of HLA and non-HLA specific antibodies. These assays are being used to assess the relative strength of donor specific antibodies; to permit performance of virtual crossmatches which can reduce the waiting time to transplantation; to monitor antibody levels during desensitization; and for heart transplants to monitor antibodies post-transplant. For cell mediated immune responses, the recent development of gene expression profiling has allowed noninvasive monitoring of heart transplant recipients yielding predictive values for acute cellular rejection. T cell immune monitoring in heart and lung transplant recipients has allowed individual tailoring of immunosuppression, particularly to minimize risk of infection. While the current antibody and cellular laboratory techniques have enhanced the ability to manage thoracic organ transplant recipients, future developments from improved understanding of microchimerism and graft tolerance may allow more refined allograft monitoring techniques.

HLA antibody detection and characterization by solid phase immunoassays: methods and pitfalls

Solid phase immunoassays for the detection and characterization of HLA-specific antibodies provide greatly increased sensitivity, specificity, and time and reagent efficiency, compared to the traditionally used cell-based methods. Testing is performed using commercially available test kits. The assays are of two general types: enzyme-linked immunosorbent assays and multianalyte bead. The types vary in both sensitivity and equipment requirements.While these assays afford great improvement over the cell-based assays, they can be confounded by interference from substances within the serum that result in high background reactivity. The high sensitivity of the assays also makes them more susceptible to environmental factors and operator variability. The user must be aware of the capabilities of the various formats, the factors that can affect test results, and lot to lot variability of any single product. Knowledge of the characteristics of each product and thorough and accurate analysis of the results are essential to the utility of these assays.