ABO-INCOMPATIBLE HEART TRANSPLANTATION IN INFANTS: THE FREEMAN HOSPITAL EXPERIENCE

Postoperative Outcomes in Infants Undergoing ABO-incompatible Heart Transplantation in the United States

BACKGROUND

ABO-incompatible heart transplant is a method to increase the infant donor pool. However data on long-term survival and rejection after ABO-incompatible heart transplant in recent era are limited.

METHODS

The United Network for Organ Sharing database was queried for infant heart transplants performed from January 2008 to March 2020. Patient demographics and known risk factors for posttransplant mortality were collected. Statistical analysis using Bayesian additive regression trees was performed to evaluate the association of ABO incompatibility and overall survival, graft survival, acute rejection episodes, and length of stay.

RESULTS

Of 1368 included infants (age < 1 year), 280 (20.47%) were ABO incompatible. ABO incompatibility was not associated with increased all-cause mortality, acute rejection episodes, or length of stay, whereas extracorporeal membrane oxygenation and intubation status of the recipient at the time of transplantation were associated with increased all-cause mortality and graft failure. Idiopathic cardiomyopathy was associated with a decreased likelihood of posttransplant all-cause mortality. One-, 5-, and 10-year survival rates among compatible vs incompatible transplants were estimated to be 90% vs 88%, 82% vs 79%, and 77% vs 73%, respectively.

CONCLUSIONS

ABO-incompatible infant heart transplant does not affect posttransplant survival, incidence of rejection, or postoperative length of stay. Therefore it remains a viable and important strategy to increase the infant donor pool.

Pediatric heart transplantation

Pediatric heart transplantation (pHTx) represents a small (14%) but very important and particular part in the field of cardiac transplantation. This treatment has lifelong impact on children. To achieve the best short and especially long-term survival with adequate quality of life, which is of crucial importance for this young patient population, one has to realize and understand the differences with adult HTx. Indication for transplantation, waitlist management including ABO incompatible (ABOi) transplantation and immunosuppression differ. Although young transplant recipients are ultimately likely to be considered for re-transplantation. One has to distinguish between myopathy and complex congenital heart disease (CHD). The differences in anatomy and physiology make the surgical procedure much more complex and create unique challenges. These recipients need a well-organized and educated team with pediatric cardiologists and intensivists, including a high skilled surgeon, which is dedicated to pHTx. Therefore, these types of transplants are best concentrated in specialized centers to achieve promising outcome.

Failure of neonatal B-cell tolerance induction by ABO-incompatible kidney grafts in piglets

BACKGROUND

ABO-incompatible (ABOi) infant heart transplantation results in B-cell tolerance to graft A/B antigens, confirming human susceptibility to acquired immunologic or "neonatal" tolerance as described originally in murine models. Starting with this clinical observation, we sought to model neonatal ABOi organ transplantation to allow mechanistic studies of tolerance.

METHODS

Plasma anti-A/B antibodies were measured over time in piglets to establish developmental antibody kinetics. Blood group O piglets received kidney allografts from group A (AO-incompatible) or group O (AO-compatible) donors under cyclosporine immunosuppression. Anti-A antibodies were measured serially after transplantation; A/H antigen expression and allograft rejection were assessed in graft biopsies.

RESULTS

Anti-A antibodies developed in naïve piglets in a kinetic pattern analogous to human infants; anti-B remained low. After transplantation, anti-A antibodies developed similarly in AO-incompatible and AO-compatible groups and were not suppressed by cyclosporine. A/H antigen expression was persistent in all graft biopsies; however, A/H antigens were not detected in vascular endothelium. Cellular and antibody-mediated rejection was absent or minimal in early and late biopsies in both groups, with one exception.

CONCLUSIONS

Naturally delayed isohemagglutinin production in piglets is analogous to the developmental kinetics in human infants. However, in contrast to deficient anti-A antibody production as seen long-term after "A-into-O" infant heart transplant recipients, normal anti-A antibody production after "A-into-O" piglet kidney transplantation indicates that tolerance did not develop despite graft A antigen persistence. These findings suggest that the impact on the host immune system of exposure to nonself ABH antigens during early life in human heart versus porcine kidney grafts may depend on expression in vascular endothelium.

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.

ABO-incompatible heart transplantation: analysis of the Pediatric Heart Transplant Study (PHTS) database

BACKGROUND

ABO incompatible (ABOi) heart transplantation is an accepted approach to increasing organ availability for young patients. Previous studies have suggested that early survival for ABOi transplants is similar to ABO compatible (ABOc) transplants. We analyzed the Pediatric Heart Transplant Study (PHTS) database from 1/96 to 12/08 to further assess this strategy.

METHODS

We analyzed the numbers of ABOi and ABOc done at the PHTS centers. We then compared the clinical characteristics, and short-term freedom from death, rejection and infection in the ABOi patients with the patients that had an ABOc heart transplant during the same period. All patients were less than or equal to 15 months of age at listing (the age of the oldest ABOi patient). We adjusted for co-variates shown to increase risk for mortality (age less than 1 month, extracorporeal membrane oxygenation (ECMO), ventilator, previous sternotomy, and congenital heart disease).

RESULTS

There were 931 total transplants done at 34 PHTS centers during the 12 year time period in patients ≤15 months of age. Of these, 502 transplants were performed at 20 PHTS centers that did at least one ABOi heart transplant. Eighty-five of the 502 (17%) were ABOi. At time of transplant, ABOi recipients compared with ABOc were more likely to be on a ventilator (49.4% vs 36.5%, p=0.025), and more often supported with ECMO (23.5% vs 13.4%, p=0.018). There was similar survival at 12 months (82% vs 84%, p=0.7). In risk adjusted analysis ABOi status was not associated with 1 year mortality (HR 0.85, 95% CI 0.45-1.6, p=0.61). The ABOi patients had greater freedom from rejection when compared with ABOc patients for all 34 centers (75% vs 62%, p=0.016), but the difference was not significant when limited only to the 20 centers doing ABOi transplants (75% vs 69%, p=0.4). The ABOi cohort had lower infection rates (23.5% vs 37.9%, p = 0.013). This difference remained after adjusting for center and other covariates.

CONCLUSIONS

In center and risk adjusted analysis, young children who received an ABOi transplant had equivalent one-year survival and freedom from rejection compared with those who received an ABOc transplant. In spite of the favorable outcome for ABOi recipients, many centers appear to reserve ABOi transplantation for sicker patients. These data mandate reexamination of the current United Network for Organ Sharing (UNOS) policy that gives priority to ABOc over ABOi transplantation in the United States.

ABO-incompatible hearts for infant transplantation

PURPOSE OF REVIEW

The practice of offering ABO-incompatible (ABOi) heart transplantation during infancy was initiated based on the rationale that infants are at the highest risk of dying while waiting for a transplant, yet are at low risk of hyperacute antibody-mediated rejection due to immunologic immaturity. Since the first report of intentional ABOi heart transplantation a decade ago, its success has been corroborated in numerous reports and the practice has been widely adopted. This review summarizes clinical results in reports of ABOi transplantation and the evolution of ABOi listing strategies, as well as evidence of immune tolerance after ABOi transplantation.

RECENT FINDINGS

Recent reports have documented comparable midterm and long-term clinical outcomes in ABOi and ABO-compatible (ABOc) heart transplant recipients in terms of survival and posttransplant complications. Despite successful outcomes, however, there are obstacles to widespread implementation of ABOi transplantation in the USA and in some European centers. The notable deficiency in development of antibody production to donor A/B antigens following ABOi transplantation described in early reports has been corroborated, with some exceptions. Potential advantages of ABOi transplantation are emerging as well as innovative strategies that may allow ABOi heart transplantation beyond the age of infancy.

SUMMARY

ABOi heart transplantation is one example in which immunologic immaturity has been exploited to the advantage of pediatric transplant recipients. In-depth exploration of transplant-related immunobiology in the young may reveal further opportunities.

Impact of ABO-incompatible listing on wait-list outcomes among infants listed for heart transplantation in the United States: a propensity analysis

BACKGROUND

The purported advantage of ABO-incompatible (ABO-I) listing is to reduce wait times and wait-list mortality among infants awaiting heart transplantation. We sought to describe recent trends in ABO-I listing for US infants and to determine the impact of ABO-I listing on wait times and wait-list mortality.

METHODS AND RESULTS

In this multicenter retrospective cohort study using Organ Procurement and Transplant Network data, infants<12 months of age listed for heart transplantation between 1999 and 2008 (n=1331) were analyzed. Infants listed for an ABO-I transplant were compared with a propensity score-matched cohort listed for an ABO-compatible transplant through the use of a Cox shared-frailty model. The primary end point was time to heart transplantation. The percentage of eligible infants listed for an ABO-I heart increased from 0% before 2002 to 53% in 2007 (P<0.001 for trend). Compared with infants listed exclusively for an ABO-compatible heart, infants with a primary ABO-I listing strategy (n=235) were more likely to be listed 1A, to have congenital heart disease and renal failure, and to require extracorporeal membrane oxygenation. For the propensity score-matched groups (n=197 matched pairs), there was no difference in wait-list mortality; however, infants with blood type O assigned an ABO-I listing strategy were more likely to undergo heart transplantation by 30 days (31% versus 16%; P=0.007) with a less pronounced effect for infants with other blood types.

CONCLUSIONS

The proportion of US infants listed for an ABO-I heart transplantation has risen dramatically in recent years but still appears to be preferentially used for sicker infant candidates. The ABO-I listing strategy is associated with a higher likelihood of transplantation within 30 days for infants with blood group O and may benefit a broader range of transplantation candidates.

Equivalent outcomes for pediatric heart transplantation recipients: ABO-blood group incompatible versus ABO-compatible

ABO-blood group incompatible infant heart transplantation has had excellent short-term outcomes. Uncertainties about long-term outcomes have been a barrier to the adoption of this strategy worldwide. We report a nonrandomized comparison of clinical outcomes over 10 years of the largest cohort of ABO-incompatible recipients. ABO-incompatible (n = 35) and ABO-compatible (n = 45) infant heart transplantation recipients (< or =14 months old, 1996-2006) showed no important differences in pretransplantation characteristics. There was no difference in incidence of and time to moderate acute cellular rejection. Despite either the presence (seven patients) or development (eight patients) of donor-specific antibodies against blood group antigens, in only two ABO-incompatible patients were these antibodies implicated in antibody-mediated rejection (which occurred early posttransplantation, was easily managed and did not recur in follow-up). Occurrence of graft vasculopathy (11%), malignancy (11%) and freedom from severe renal dysfunction were identical in both groups. Survival was identical (74% at 7 years posttransplantation). ABO-blood group incompatible heart transplantation has excellent outcomes that are indistinguishable from those of the ABO-compatible population and there is no clinical justification for withholding this lifesaving strategy from all infants listed for heart transplantation. Further studies into observed differing responses in the development of donor-specific isohemagglutinins and the implications for graft accommodation are warranted.

Multicenter experience of ABO-incompatible pediatric cardiac transplantation

Although ABO blood group incompatible cardiac transplantation in neonates and infants reduces waiting list mortality without compromising outcome, the technique has not been adopted by all centers, and to date Toronto remains the only center to have published results from a large case series. We present a review of ABO-incompatible heart transplantation in the United Kingdom (UK) where current recipient selection criteria differ somewhat from those used in the United States (US) and Canada. Between February 2000 and November 2006, 21 ABO-incompatible cardiac transplants were performed in children aged 2-40 months (median 10.0). Immunosuppression followed standard regimens. Pretransplant donor-specific isohemagglutinins of >1:4, (the UNOS cutoff), were present in five patients and reduced by plasma exchange. After transplantation, 19/21 recipients demonstrated persisting deficiency of donor-specific isohemagglutinins. Significant donor-specific isohemagglutinins levels were detected repeatedly in 2/21 recipients who have shown no clinical or biopsy evidence of rejection. All recipients survive without retransplantation and there have been no episodes of humoral rejection. We conclude it is possible for other centers to replicate the excellent results achieved in Toronto and that ABO-incompatible transplantation may be performed successfully in some patients beyond infancy with established isohemagglutinin production providing preoperative antibody removing strategies are used.

Influence of Blood Group on Mortality and Waiting Time Before Heart Transplantation in the United Kingdom: Implications for Equity of Access

BACKGROUND

We examined waiting time for adult heart transplantation in the UK and sought to determine whether recipients with particular ABO blood groups were disadvantaged.

METHODS

Data were obtained from the National Transplant Database. Registration outcome data were analyzed for 622 new, non-urgent, adult, heart-only registrations from April 1, 1999 to March 31, 2003. Unadjusted waiting times of the 618 first registrations were summarized using Kaplan-Meier estimates.

RESULTS

Death rates were relatively low, with no significant difference in the proportions of patients among the different blood groups who died while waiting. A smaller proportion of blood group O patients were transplanted at 1 year after registration, with a significant difference in waiting time to transplant between blood groups (p < 0.0001). Blood group A and AB patients were generally transplanted sooner than O and B patients, with median waiting times of 81 days (95% CI: 67 to 114) and 76 days (95% CI: 52 to 178) vs 214 days (95% CI: 162 to 308) and 174 days (95% CI: 78 to 249), respectively.

CONCLUSIONS

Although no particular blood group was disadvantaged in terms of mortality on the heart transplant list, blood group O and B patients waited significantly longer for transplantation. The difference was at least partly due to a large proportion of blood group O hearts being used for non-O patients. To address this imbalance, the UK Transplant Cardiothoracic Advisory Group (CTAG) changed the allocation protocol, so that "out-of-zone" offers of blood group O donors for non-urgent patients are now restricted to O and B recipients.

ABO-histo blood group incompatibility in hematopoietic stem cell and solid organ transplantation

In contrast to solid organ transplantation (SOT), ABO-histo blood group incompatibility is of minor importance for hematopoietic stem cell transplantation (HSCT). Patients receiving ABO-incompatible HSCT are at an increased risk for immune-mediated hematological complications including immediate and delayed hemolysis, late red blood cell engraftment and pure red cell aplasia, but seem not to have a worse overall survival or increased transplant-related mortality. This review gives an overview of the immunological mechanisms leading to complications associated with ABO-incompatible HSCT and describes approaches to prevent them. The current organ shortage in SOT stimulates the exploration of new strategies to expand the donor pool including ABO-incompatible SOT and xenotransplantation. Here, we discuss the hypothesis that ABO-incompatible transplantation may be viewed as a human in vivo model for the humoral immune mechanisms of antigen-mismatched transplantation. ABO-incompatible HSCT and SOT provide excellent possibilities to analyze graft accommodation and transplantation tolerance. Understanding the underlying mechanisms of graft survival in ABO-incompatible transplantation may facilitate new strategies to overcome the immunological barriers in SOT and xenotransplantation.

Exchange Transfusion During Extra Corporeal Membrane Oxygenation Used as a Bridge to ABO-Mismatch Cardiac Transplantation

We report a case in which exchange transfusion was performed directly from the extracorporeal membrane oxygenation circuit in a child being bridged to cardiac transplantation. This allowed preparation for ABO-mismatched heart transplant in a child dependent on extracorporeal support, awaiting a suitable ABO-matched donor. Exchange transfusion or plasmapheresis in this setting would normally be performed on cardiopulmonary bypass to allow exsanguinations under hypothermia immediately pre-transplant. This allows little time for depletion of isohemagglutinins, occasionally leading to prolonged bypass times. We believe our method to be a safe alternative, allowing ample time for immunological preparation for ABO-incompatible transplant.

B-cell Tolerance following ABO-Incompatible Infant Heart Transplantation

Infant heart transplantation is highly successful therapy for severe congenital heart malformations and cardiomyopathies. However the donor organ shortage is particularly crucial for this age group. We have demonstrated that some aspects of human immaturity can be exploited for infant organ transplantation, allowing safe transplantation of organs that would be "incompatible" in mature individuals, and as a window of continued susceptibility to induction of immunologic tolerance.

Anesthesia Considerations for Pediatric Thoracic Solid Organ Transplant

This article discusses the indications, perioperative management, postoperative complications, and patient outcome of pediatric heart transplantation and pediatric lung transplantation. Special emphasis is placed on the anesthetic considerations relevant for children who are undergoing or have received a solid thoracic organ transplant.

Successful ABO-incompatible heart transplantation in two infants

In the pediatric age group shortage of donor hearts leads to mortality rates of 30-50% on the waiting list. Because of the immaturity of the immune system of infants, ABO-incompatible heart transplantation may be an option to increase donor availability. We transplanted two infants with blood type O at the age of 7 and 5 months, respectively, with complex congenital heart disease. Intraoperative plasma exchange was performed during cardiopulmonary bypass followed by standard immunosuppression. Both recipients received a blood type A donor organ. Plasma was exchanged up to six times until anti-A antibodies were eliminated. No hyperacute rejection occurred, ventricular function is excellent and there have been no acute rejection episodes up to 4 months after transplantation. Anti-A antibody titers remained low and eventually disappeared. ABO-incompatible cardiac transplantation shows good short-term results in young infants and appears to be a safe procedure to reduce mortality on the waiting list.