Lost in Regulation: Why Do U.S. Policies for ABO-incompatible Transplantation for Infants Lag Behind?

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.