Biagio A. Pietra

Pediatric heart transplantation after declaration of cardiocirculatory death.

In three infants awaiting orthotopic cardiac transplantation, transplantation was successfully performed with the use of organs from donors who had died from cardiocirculatory causes. The three recipients had blood group O and were in the highest-risk waiting-list category. The mean age of donors was 3.7 days, and the mean time to death after withdrawal from life support was 18.3 minutes. The 6-month survival rate was 100% for the 3 transplant recipients and 84% for 17 control infants who received transplants procured through standard organ donation. The mean number of rejection episodes among the three infants during the first 6 months after surgery was 0.3 per patient, as compared with 0.4 per patient among the controls. Echocardiographic measures of ventricular size and function at 6 months were similar among the three infants and the controls (left ventricular shortening fraction, 43.6% and 44.9%, respectively; P=0.73). No late deaths (within 3.5 years) have occurred in the three infants, and they have had functional and immunologic outcomes similar to those of controls. Mortality while awaiting a transplant is an order of magnitude higher in infants than in adults, and donors who died from cardiocirculatory causes offer an opportunity to decrease this waiting-list mortality.

CD4 T cell–mediated cardiac allograft rejection requires donor but not host MHC class II

Numerous studies indicate that CD4 T cells are required for acute cardiac allograft rejection. However, the precise role for CD4 T cells in this response has remained ambiguous owing to the multipotential properties of this T-cell subpopulation. In the current study, we demonstrate the capacity of CD4 T cells to serve as direct effector cells of cardiac allograft rejection. We show that CD4 T cells are both necessary and sufficient for acute graft rejection, as indicated by adoptive transfer experiments in immune-deficient SCID and rag1(-/-) recipients. We have analyzed the contribution of direct (donor MHC class II restricted) and indirect (host MHC class II restricted) antigen recognition in CD4-mediated rejection. Acute CD4 T cell-mediated rejection required MHC class II expression by the allograft, indicating the importance of direct graft recognition. In contrast, reciprocal experiments indicate that CD4 T cells can acutely reject allogeneic cardiac allografts established in rag1(-/-) hosts that were also MHC class II deficient. This latter result indicates that indirect presentation of donor antigens by host MHC class II is not required for acute CD4-mediated rejection. Taken together, these results indicate that CD4 T cells can serve as effector cells for primary acute cardiac allograft rejection, predominantly via direct donor antigen recognition and independent of indirect reactivity.

Early Predictors of Survival to and After Heart Transplantation in Children With Dilated Cardiomyopathy

Background— The importance of clinical presentation and pretransplantation course on outcome in children with dilated cardiomyopathy listed for heart transplantation is not well defined. Methods and Results— The impact of age, duration of illness, sex, race, ventricular geometry, and diagnosis of myocarditis on outcome in 261 children with dilated cardiomyopathy enrolled in the Pediatric Cardiomyopathy Registry and Pediatric Heart Transplant Study was studied. End points included listing as United Network for Organ Sharing status 1, death while waiting, and death after transplantation. The median age at the time of diagnosis was 3.4 years, and the mean time from diagnosis to listing was 0.62±1.3 years. Risk factors associated with death while waiting were ventilator use and older age at listing in patients not mechanically ventilated (P=0.0006 and P=0.03, respectively). Shorter duration of illness (P=0.04) was associated with listing as United Network for Organ Sharing status 1. Death after transplantation was associated with myocarditis at presentation (P=0.009), nonwhite race (P<0.0001), and a lower left ventricular end-diastolic dimension z score at presentation (P=0.04). In the myocarditis group, 17% (4 of 23) died of acute rejection after transplantation. Conclusions— Mechanical ventilator use and older age at listing predicted death while waiting, whereas nonwhite race, smaller left ventricular dimension, and myocarditis were associated with death after transplantation. Although 97% of children with clinically or biopsy-diagnosed myocarditis at presentation survived to transplantation, they had significantly higher posttransplantation mortality compared with children without myocarditis, raising the possibility that preexisting viral infection or inflammation adversely affects graft survival.

A Two-Step Model of Acute CD4 T-Cell Mediated Cardiac Allograft Rejection

CD4 T cells are both necessary and sufficient to mediate acute cardiac allograft rejection in mice. This process requires “direct” engagement of donor MHC class II molecules. That is, acute rejection by CD4+ T cells requires target MHC class II expression by the donor and not by the host. However, it is unclear whether CD4+ T cell rejection requires MHC class II expression on donor hemopoietic cells, nonhemopoietic cells, or both. To address this issue, bone marrow transplantation in mice was used to generate chimeric heart donors in which MHC class II was expressed either on somatic or on hemopoietic cells. We report that direct recognition of hemopoietic and nonhemopoietic cells are individually rate limiting for CD4+ T cell-mediated rejection in vivo. Importantly, active immunization with MHC class II+ APCs triggered acute rejection of hearts expressing MHC class II only on the somatic compartment. Thus, donor somatic cells, including endothelial cells, are not sufficient to initiate acute rejection; but they are necessary as targets of direct alloreactive CD4 T cells. Taken together, results support a two-stage model in which donor passenger leukocytes are required to activate the CD4 response while direct interaction with the somatic compartment is necessary for the efferent phase of acute graft rejection.

The Hemodynamic Response to Dexmedetomidine Loading Dose in Children With and Without Pulmonary Hypertension

BACKGROUND:Dexmedetomidine, an &agr;-2 receptor agonist, is widely used in children with cardiac disease. Significant hemodynamic responses, including systemic and pulmonary vasoconstriction, have been reported after dexmedetomidine administration. Our primary goal of this prospective, observational study was to quantify the effects of dexmedetomidine initial loading doses on mean pulmonary artery pressure (PAP) in children with and without pulmonary hypertension. METHODS:Subjects were children undergoing cardiac catheterization for either routine surveillance after cardiac transplantation (n = 21) or pulmonary hypertension studies (n = 21). After anesthetic induction with sevoflurane and tracheal intubation, sevoflurane was discontinued and anesthesia was maintained with midazolam 0.1 mg/kg IV (or 0.5 mg/kg orally preoperatively) and remifentanil IV infusion 0.5 to 0.8 &mgr;g/kg/min. Ventilation was mechanically controlled to maintain PCO2 35 to 40 mm Hg. When end-tidal sevoflurane was 0% and fraction of inspired oxygen (FIO2) was 0.21, baseline heart rate, mean arterial blood pressure, PAP, right atrial pressure, pulmonary artery occlusion pressure, right ventricular end-diastolic pressure, cardiac output, and arterial blood gases were measured, and indexed systemic vascular resistance, indexed pulmonary vascular resistance, and cardiac index were calculated. Each subject then received a 10-minute infusion of dexmedetomidine of 1 &mgr;g/kg, 0.75 &mgr;g/kg, or 0.5 &mgr;g/kg. Measurements and calculations were repeated at the conclusion of the infusion. RESULTS:Most hemodynamic responses were similar in children with and without pulmonary hypertension. Heart rate decreased significantly, and mean arterial blood pressure and indexed systemic vascular resistance increased significantly. Cardiac index did not change. A small, statistically significant increase in PAP was observed in transplant patients but not in subjects with pulmonary hypertension. Changes in indexed pulmonary vascular resistance were not significant. CONCLUSION:Dexmedetomidine initial loading doses were associated with significant systemic vasoconstriction and hypertension, but a similar response was not observed in the pulmonary vasculature, even in children with pulmonary hypertension. Dexmedetomidine does not appear to be contraindicated in children with pulmonary hypertension.

Infant heart transplantation: improved intermediate results

OBJECTIVES Our objectives were to (1) review our experience with heart transplants in infants (age < 6 months), (2) delineate risk factors for 30-day mortality, and (3) compare outcomes between our early and recent experience. METHODS Records of all infants listed for transplantation in our center before September 1996 were analyzed. Early and recent comparisons were made between chronologic halves of the accrual period. Univariate analysis was used to analyze potential risk factors for 30-day mortality (categorical variables, Fishers exact test; continuous variables, nonparametric Wilcoxon rank-sum test). Multivariable analysis included univariate variables with p values < or = 0.10. Actuarial survivals were estimated (Kaplan-Meier) and compared by the log-rank test. RESULTS Fifty-one of the 60 infants listed for transplantation were operated on (waiting list mortality 15%). Thirty-day mortality was 18% overall, 30% in the first 3 years and 10% in the last 3 years (p = 0.07). Sepsis was the commonest cause of early death (4/9). Univariate analysis suggested four potential risk factors for early death: preoperative mechanical ventilation (p = 0.01), prior sternotomy (p = 0.002), preoperative inotropic drugs (p = 0.08), and warm ischemia time (p = 0.08). Multivariable analysis indicated that prior sternotomy (p = 0.01) was an independent risk factor for 30-day mortality. Actuarial survivals were 80%, 78%, and 70% at 1, 2, and 3 years, and these figures improved between early and recent groups (p = 0.05). Late deaths were most commonly due to acute rejection (3/5). CONCLUSIONS Results of heart transplantation in infancy improve with experience. Prior sternotomy increases initial risk. Intermediate-term survival for infants with end-stage heart disease is excellent.

Donor IFN-γ Receptors Are Critical for Acute CD4+ T Cell-Mediated Cardiac Allograft Rejection

Recent studies using mouse models demonstrate that CD4+ T cells are sufficient to mediate acute cardiac allograft rejection in the absence of CD8+ T cells and B cells. However, the mechanistic basis of CD4-mediated rejection is unclear. One potential mechanism of CD4-mediated rejection is via elaboration of proinflammatory cytokines such as IFN-γ. To determine whether IFN-γ is a critical cytokine in CD4-mediated acute cardiac allograft rejection, we studied whether the expression of IFN-γ receptors on the donor heart was required for CD4-mediated rejection. To investigate this possibility, purified CD4+ T cells were transferred into immune-deficient mice bearing heterotopic cardiac allografts from IFN-γ receptor-deficient (GRKO) donors. While CD4+ T cells triggered acute rejection of wild-type heart allografts, they failed to trigger rejection of GRKO heart allografts. The impairment in CD4-mediated rejection of GRKO hearts appeared to primarily involve the efferent phase of the immune response. This conclusion was based on the findings that GRKO stimulator cells provoked normal CD4 proliferation in vitro and that intentional in vivo challenge of CD4 cells with wild-type donor APC or the adoptive transfer of in vitro primed CD4 T cells failed to provoke acute rejection of GRKO allografts. In contrast, unseparated lymph node cells acutely rejected both GRKO and wild-type hearts with similar time courses, illustrating the existence of both IFN-γ-dependent and IFN-γ-independent mechanisms of acute allograft rejection.

Acute cardiac allograft rejection by directly cytotoxic CD4 T cells: parallel requirements for Fas and perforin.

Background. CD4 T cells can suffice as effector cells to mediate primary acute cardiac allograft rejection. Although CD4 T cells can readily kill appropriate target cells in vitro, the corresponding role of such cytolytic activity for mediating allograft rejection in vivo is unknown. Therefore, we determined whether the cytolytic effector molecules perforin (PFP) and/or FasL (CD95L) were necessary for CD4 T cell-mediated rejection in vivo. Methods. Wild-type C3H(H-2k) or Fas (CD95)-deficient C3Hlpr (H-2k) hearts were transplanted into immune-deficient C57B6rag−/− (H-2b) mice. Then, recipients were reconstituted with naïve purified CD4 T cells from wild-type, PFP-deficient, or FasL (gld)-deficient T-cell donors. Results. In vitro, alloreactive CD4 T cells were competent to lyse donor major histocompatibility complex class II+ target cells, largely by a Fas-dependent mechanism. In vivo, the individual disruption of donor Fas expression (lpr) or CD4 T-cell-derived PFP had no significant impact on acute rejection. However, FasL-deficient (gld) CD4 T cells demonstrated delayed allograft rejection. Importantly, the simultaneous removal of both donor Fas expression and CD4 T-cell PFP completely abrogated acute rejection, despite the persistence of CD4 T cells within the graft. Conclusions. Results demonstrate that the direct rejection of cardiac allografts by CD4 effector T cells requires the alternative contribution of graft Fas expression and T cell PFP expression. To our knowledge, this is the first demonstration that cytolytic activity by CD4 T cells can play an obligate role for primary acute allograft rejection in vivo.

Mechanical limitation of pulmonary blood flow facilitates heart transplantation in older infants with hypoplastic left heart syndrome

OBJECTIVES Progression of pulmonary vascular disease limits heart transplantation for hypoplastic left heart syndrome (HLHS) to early infancy. Our objective was to assess the impact of bilateral pulmonary artery banding (PAB) on the operative courses of HLHS infants transplanted at ages older than 4 months. METHODS Courses of all HLHS patients in our center who remained listed to age >or=120 days before heart transplantation were assessed. Patients undergoing transplantation after standard management (control group) were compared to patients having a prior pulmonary blood flow limiting procedure (PAB group). RESULTS Of 16 identified patients, one crossed over to stage I Norwood on day 185 and died post-operatively. Fifteen patients were transplanted at age >or=120 days (control group n=9, PAB group n=6). Four PAB patients had open PA band placement. Two PAB patients underwent experimental percutaneous bilateral internal pulmonary artery flow limiting device insertion. The PAB group mean age at banding was 141+/-54 days, and mean interval from PAB to transplant was 35+/-31 days (range 1.5-68 days). No differences in age at transplant, weight at transplant, warm graft ischemia time or total graft ischemia time were detected between groups. Mean times of mechanical ventilation (control 143+/-69h vs. PAB 44+/-13h), inhaled nitric oxide (control 126+/-70h vs. PAB 37+/-9h), inotropic support (control 171+/-64h vs. PAB 87+/-17h), intensive care unit (ICU) stay (control 8.3+/-2.7 days vs. PAB 4.5+/-1.4 days), and hospital stay (control 10.4+/-3.9 days vs. PAB 7.0+/-1.1 days) were all lower in the PAB group (P<0.05 all comparisons). Two control patients died, three required extracorporeal membrane oxygenation (ECMO), and six did not tolerate primary chest closure. No PAB patient died or required ECMO. All PAB patients tolerated primary chest closure. All PAB patients had widely patent branch pulmonary arteries with no re-interventions to date. All hospital survivors remain alive (mean follow-up, control 50.2 months, PAB 11.5 months). CONCLUSIONS Pre-transplant mechanical limitation of pulmonary blood flow simplified management and reduced morbidity for HLHS patients undergoing heart transplantation at ages >or=4 months. This strategy extends the permissible transplant waiting time in older infants with HLHS.

Coronary Artery Vasculopathy in Pediatric Cardiac Transplant Patients

The single largest cause of late graft loss in pediatric cardiac transplantation is transplant coronary artery vasculopathy (CAV). The mechanism of CAV remains unknown; it appears to have both immune and non-immune causes. The final common pathway of these mechanisms is endothelial activation, a prothrombotic environment, and endothelial damage with subsequent diffuse intimal proliferation. The disease process has largely been thought to be progressive and unresponsive to treatment. Re-transplantation has been advocated as the only definitive treatment. The appropriate management is largely unknown; intervention or surgical management has had limited utility, while medical management appears to have the most promise. Improvement in outcome can be achieved by optimizing non-immune factors and aggressive management of the immune mechanisms. Long-term survival of transplant patients after diagnosis with CAV is now being reported.