Prospective randomized trial of azathioprine in cryopreserved valved allografts in children

Immunogenicity of Homologous Heart Valves: Mechanisms and Future Considerations

Pediatric valvar heart disease continues to be a topic of interest due to the common and severe clinical manifestations. Problems with heart valve replacement, including lack of adaptive valve growth and accelerated structural valve degeneration, mandate morbid reoperations to serially replace valve implants. Homologous or homograft heart valves are a compelling option for valve replacement in the pediatric population but are susceptible to structural valve degeneration. The immunogenicity of homologous heart valves is not fully understood, and mechanisms explaining how implanted heart valves are attacked are unclear. It has been demonstrated that preservation methods determine homograft cell viability and there may be a direct correlation between increased cellular viability and a higher immune response. This consists of an early increase in human leukocyte antigen (HLA)-class I and II antibodies over days to months posthomograft implantation, followed by the sustained increase in HLA-class II antibodies for years after implantation. Cytotoxic T lymphocytes and T-helper lymphocytes specific to both HLA classes can infiltrate tissue almost immediately after implantation. Furthermore, increased HLA-class II mismatches result in an increased cell-mediated response and an accelerated rate of structural valve degeneration especially in younger patients. Further long-term clinical studies should be completed investigating the immunological mechanisms of heart valve rejection and their relation to structural valve degeneration as well as testing of immunosuppressant therapies to determine the needed immunosuppression for homologous heart valve implantation.

Antibody-mediated rejection of arterialised venous allografts is inhibited by immunosuppression in rats

Objectives and Design

We determined in a rat model (1) the presence and dynamics of alloantibodies recognizing MHC complexes on quiescent Brown-Norway (BN) splenic cells in the sera of Lewis (LEW) recipients of Brown-Norway iliolumbar vein grafts under tacrolimus immunosuppression; and (2) the presence of immunoglobulins in the wall of acute rejected vein allografts.

Materials and Methods

Flow cytometry was used for the analysis of day 0, 14 and 30 sera obtained from Lewis recipients of isogeneic iliolumbar vein grafts (group A) or Brown-Norway grafts (group B, C) for the presence of donor specific anti-MHC class I and II antibodies. Tacrolimus 0.2 mg/kg daily was administered from day 1 to day 30 (group C). Histology was performed on day 30.

Results

Sera obtained preoperatively and on day 30 were compared in all groups. The statistically significant decrease of anti MHC class I and II antibody binding was observed only in allogenic non-immunosuppressed group B (splenocytes: MHC class I - day 0 (93%±7% ) vs day 30 (66%±7%), p = 0.02, MHC class II - day 0 (105%±3% ) vs day 30 (83%±5%), p = 0.003; B-cells: MHC class I - day 0 (83%±5%) vs day 30 (55%±6%), p = 0.003, MHC class II - day 0 (101%±1%) vs day 30 (79%±6%), p = 0.006; T-cells: MHC class I - day 0 (71%±7%) vs day 30 (49%±5%), p = 0.04). No free clusters of immunoglobulin G deposition were detected in any experimental group.

Conclusion

Arterialized venous allografts induce strong donor-specific anti-MHC class I and anti-MHC class II antibody production with subsequent immune-mediated destruction of these allografts with no evidence of immunoglobulin G deposition. Low-dose tacrolimus suppress the donor-specific antibody production.

Immune response to allograft implantation in children with congenital heart defects

Cryopreserved valved allografts are frequently used in the repair of congenital heart defects in children. Although the longevity of these grafts is generally good in most patients, there continue to be ongoing problems with allograft dysfunction and subsequent failure, particularly in infants and young children. The aim of this review is to discuss the immunogenicity of cryopreserved allograft tissue and measures that may minimize the deleterious effect of the immune system on allograft function and durability.

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.

Exploring the role of cyclosporine in reducing homograft degeneration post-Ross

The Ross procedure is safe and effective for children with aortic valve disease. Pulmonary homograft degeneration, proposed to be immune-mediated, is a major cause of reoperation. Cyclosporine increased homograft valve survival in animals, but has not been studied in humans. To investigate the efficacy of low-dose cyclosporine in preventing homograft degeneration and complications, a retrospective historical-controlled study was performed on data of all children who underwent Ross procedure and received cyclosporine. The primary endpoint was homograft function at the last follow-up; secondary endpoints were readmission, reoperation, death, and safety. Seventeen patients were matched with 16 controls. At the end of the follow-up period (cyclosporine, 6.7 years; controls, 8 years), homograft stenosis and/or regurgitation were present in half of all patients. Three (18%) patients in the cyclosporine group and 5 (29%) in the control group were readmitted. Surgical intervention due to homograft failure was needed in 1 (6%) cyclosporine patient and 3 (19%) of the controls. Although cyclosporine failed to show a significant difference in signs of homograft degeneration, it might decrease the need for reoperation following the Ross procedure. Larger prospective well-designed studies are required to confirm these findings.

Is the Ross operation still an acceptable option in children and adolescents?

BACKGROUND

The Ross operation is increasingly accepted as an alternative to conventional valve prostheses for children, adolescents, and young adults. We review patients younger than 20 years of age.

METHODS

Of 404 Ross operations done before November 2004, 60 were young patients with a median age of 12 years (range, 1 to 20 years). The pulmonary autograft technique universally was as a free root. A cryopreserved pulmonary homograft reconstructed the right ventricular outflow tract.

RESULTS

Early postoperative complications were reentry for bleeding in 2 patients and one pacemaker insertion. No thromboembolic or hemorrhagic events occurred during the follow-up of 42 +/- 27 months. Two late deaths occurred, one from myocardial infarction after 3 months and another sudden death after 5 years, probably from critical pulmonary homograft stenosis. Echocardiographic follow-up revealed a median peak gradient of 6.3 +/- 3 mm Hg across the autograft. The median pulmonary homograft peak gradient of 19.1 +/- 13.7 mm Hg was increased to more than 30 mm Hg in 6 patients. Another 6 patients had moderate but clinically insignificant pulmonary homograft regurgitation. Altogether, 6 patients required reoperation for replacement of stenotic homografts. No autograft related reoperation occurred.

CONCLUSIONS

Young patients with the Ross operation had good mid-term autograft function and no perioperative mortality. Factors that justify the choice of the Ross operation for young patients are the normal physiologic hemodynamics and growth of the autograft as well as freedom from anticoagulation. A 10% reoperation rate, elevated pulmonary homograft gradients, and the surgical complexity remain limiting factors.

Aortoiliac reconstruction in infants and toddlers: replacement with decellularized branched pulmonary artery allograft

BACKGROUND

Aortic reconstruction in infants and small children has been reported with Dacron or polytef prosthetic material, hypogastric artery autograft, and saphenous vein autograft. In children, synthetic grafts are limited by a concern for late infection and a lack of potential growth. Available autogenous vessels have a limited length and diameter. Conventional allografts have not been durable. When the entire infrarenal aorta and aortoiliac bifurcation must be replaced, none of the historic options are optimal.

METHODS

We report 2 cases of infrarenal aorta and aortoiliac bifurcation reconstruction using a new generation of cryopreserved allograft now decellularized for decreased immunogenicity. The branched pulmonary artery allograft is particularly attractive for reconstruction of the aortic bifurcation.

RESULTS

The postoperative course in both cases was uncomplicated. Follow-up with serial abdominal duplex ultrasound has shown no evidence of graft stenosis or calcification at 29 and 32 months, respectively.

CONCLUSIONS

The use of commercially available, decellularized, and antigen-reduced allograft offers a nonsynthetic option for replacement of the pediatric abdominal aorta. We chose this novel approach in hopes of reducing the lifetime risk for graft infection and maintaining the potential for graft ingrowth by the child.

The sensitized pediatric heart transplant candidate: causes, consequences, and treatment options

Sensitization to HLA antigens and the subsequent development of HLA antibodies in children under consideration for heart transplantation is a significant impediment to survival after listing. This is related both to the frequent need for prospective donor-specific crossmatching (thus limiting donor availability and increasing pretransplant morbidity and mortality), and to the increased risk of adverse outcomes after transplantation. This article will review the scope of this problem in children under consideration for heart transplantation, the different methods available for diagnosing HLA sensitization, the known causes of HLA sensitization, the consequences of these preformed antibodies on outcomes before and after heart transplantation, and the different methods of preventing and treating this sensitization that are currently available. Improved methods of diagnosing, preventing, and treating this problem can only lead to better outcomes for children who require heart transplantation.

Panel-Reactive Antibodies Late After Allograft Implantation in Children

BACKGROUND

Circulating human leukocyte antigen (HLA) panel-reactive antibodies (PRA > 10%) have been independently associated with increased risk of rejection and mortality in patients who undergo cardiac transplantation. Cryopreserved allografts used to repair heart defects induce broadly reactive HLA antibodies in children that persist for an undetermined duration of time. The purpose of this study was to prospectively determine the level of HLA sensitization several years after implantation of cryopreserved allografts in children.

METHODS

We conducted late follow-up of 13 children previously screened for PRA before and after implantation of valved and nonvalved allografts who are alive and free from allograft replacement. Panel-reactive antibodies against HLA class I and II antigens were determined using flow cytometry and classified as high reactive (>50% PRA), low reactive (11% to 50%), or absent (0% to 10%). Follow-up PRA was compared with PRA obtained 3 months after initial allograft implantation.

RESULTS

Elevated HLA class I PRA persisted at late follow-up in 12 of 13 children, although it decreased significantly from high to low or from low to absent in 12 of 13 patients (p < 0.001). Elevated HLA class II PRA persisted at late follow-up in 6 of 13 children (46%) and had decreased significantly from prior levels (p = 0.011).

CONCLUSIONS

Circulating HLA antibodies induced by cryopreserved allograft tissue persist up to 8 years after implantation although they decrease with time. Therefore, children who have received cryopreserved allografts before cardiac transplantation may be at greater risk for transplant rejection.

Early failure of a tricuspid valve replacement with a mitral valve homograft in a heart transplant recipient

A 24-year-old woman experienced severe tricuspid valve regurgitation 6 years after heart transplantation. Tricuspid valve replacement was performed using a cryopreserved mitral valve homograft. Severe tricuspid valve regurgitation recurred within 4 months, associated with an increase in the panel reactive antibody titers from zero to 72%. Tricuspid valve replacement was repeated with a porcine bioprosthesis with excellent recovery and function for >2 years. The mitral valve homograft displayed inflammatory features consistent with humoral immune-mediated destruction.

Conduit selection for right ventricular outflow tract reconstruction: contemporary options and outcomes

Reconstruction of the right ventricular outflow tract, with the establishment of an unobstructed pathway between the right ventricle and the pulmonary arteries, is a task that the congenital heart surgeon frequently faces. In situations where this outflow tract is congenitally absent, or the pulmonary valve has been used to replace a dysfunctional left ventricular outflow tract, a conduit is usually required to establish pulmonary blood flow. Cryopreserved homografts are currently favored for this, though these nonviable valved allografts have certain limitations. The following review will further define the problem of right ventricular outflow tract reconstruction, with an emphasis on conduit selection and possible alternatives to conduit repair.

Mycophenolic mofetil reduces the HLA antibody response of children to valved allograft implantation

BACKGROUND

Valved allografts induce a brisk, broadly reactive human leukocyte antigen (HLA) antibody response in children after implantation. Mycophenolic mofetil (MMF) is a powerful immunosuppressant that inhibits the proliferation of both T cells and B cells and has been reported to possibly reduce HLA panel reactive antibody (PRA) in sensitized transplant recipients.

METHODS

The purpose of this study was to determine whether MMF can blunt the HLA antibody response to valved allografts in children. Eight patients completed (of 28 approached) a pilot study to determine the effects of 3 months of twice daily MMF (600 mg/m(2)/dose) on the HLA antibody response measured before surgery, at 1 month, and at 3 months after implantation. Patients were 7.5 +/- 4 yrs old (mean +/- standard deviation [SD]), with 5 patients undergoing repair of tetralogy of Fallot, 2 Ross procedures, and 1 aortic valve replacement.

RESULTS

In contrast to historical controls with a virtual 100% HLA class I PRA response to valved allograft implantation, MMF markedly decreased the HLA class I antibody response at 1 and 3 months postimplantation. In 6 cases where the HLA type of the donor was defined, PRA specificity correlated with incompatible antigens on the allograft. One patient withdrew after 2 weeks due to a sinus infection that was successfully treated with oral antibiotics, and 3 patients had a transient adverse effect of postoperative vomiting.

CONCLUSIONS

This study demonstrates the ability to pharmacologically abrogate the HLA class I antibody response to valved allograft implantation in children using MMF.

Blood group incompatibility and accelerated homograft fibrocalcifications

OBJECTIVE

Cryopreserved valved homograft has become the conduit of choice for right ventricular outflow tract reconstruction in pediatric cardiac surgery. Aortic homografts have been frequently used in pulmonary position, but accelerated aortic homograft fibrocalcification may occur. Blood group incompatibility between receiver and homograft donor may play a central role in this context.

METHODS

Between 1993 and 2000, 59 children (mean age 6.4 +/- 4.4 years) received cryopreserved valved homografts for right ventricular outflow tract reconstruction and were followed from 2 to 10 years clinically, with echocardiography and chest radiography for detection of development of homograft calcifications. Seventeen patients were 3 years or younger. Fifty aortic (85%) and 9 pulmonary homografts were all used in pulmonary position. Thirty-three patients (56%) had the same blood group (ABO) as the homograft donor (iso group), and 26 were blood group-incompatible (non-iso group).

RESULTS

No deaths occurred during follow-up. Six patients (10.2%) required homograft replacement because of severe fibrocalcifications, and another 3 showed moderate homograft calcifications (5.1%) at last examination. Freedom from moderate to severe homograft calcification at 8 years (Kaplan-Meier) was 95.2% for the iso group and 72.9% for the non-iso group (P <.0001). Homograft calcifications occurred within 2 years of implantation in 6/9 patients (67%) in the non-iso group.

CONCLUSIONS

Blood group incompatibility between receiver and homograft donor seems to play an important role in the development of accelerated fibrocalcifications in cryopreserved homografts, particularly in the very young (3 years old or younger). Blood group compatibility should therefore be respected to avoid accelerated homograft fibrocalcifications.

Transplantation of the cryopreserved tracheal allograft in growing rabbits

BACKGROUND

Surgical treatment of the congenital tracheal stenosis is challenging, and the long-term fate of transplanted tracheal allograft remains unclear. The authors evaluated the morphologic changes of the cryopreserved tracheal allograft after transplantation in a growing rabbit model.

METHODS

Each allograft (n = 7) was harvested from 90- to 120-day-old Japanese rabbits, immersed in the preservation solution, and stored in a programmable freezer until reaching -80 degrees c and then kept in liquid nitrogen for 1 week. Orthotopic tracheal transplantation of 7 tracheal rings in an end-to-end fashion was performed in age-matched young rabbits without immunosuppression. The grafts were assessed at 1 week, 1 month, and 3 months after transplantation. As controls, fresh autografts also were evaluated after the same procedure.

RESULTS

Two animals died of pneumonia. The body weight gain was similar in both groups. All grafts were patent, but no allografts showed normal growth in length or diameter. Microscopic findings of the allograft showed calcification of the tracheal cartilage without infiltration of inflammatory cells and marked lymphocyte proliferation in the subepithelium.

CONCLUSIONS

Cryopreserved tracheal allografts without immunosuppressant showed favorable patency of the trachea at 3 months; however, no growth of the allograft occurred in this animal model. The problem of calcification of the allograft remains to be solved.

Immunogenicity of decellularized cryopreserved allografts in pediatric cardiac surgery: comparison with standard cryopreserved allografts

BACKGROUND

Recognition of the immunogenicity of standard cryopreserved allografts has led to the development of new decellularized allografts (CryoValve SG; CryoLife, Inc, Kennesaw, Ga). This preliminary study examined the HLA antibody response to these decellularized allografts and compared it with the response to standard allograft material.

METHODS

We prospectively measured the frequency of panel-reactive HLA class I (HLA-A, HLA-B, and HLA-C) and class II (HLA-DR/DQ) alloantibodies in 14 children (age 8.5 +/- 7.9 years) receiving decellularized, cryopreserved allografts, including 6 undergoing allograft patch insertion and 8 with a valved pulmonary allograft. We compared them with 20 historical control subjects (age 1.7 +/- 2.4 years) undergoing implantation of standard cryopreserved allografts, 8 with valves and 12 with allograft patch. All patients had panel-reactive antibody levels measured before and at 1, 3, and 12 months after the operation. HLA class I and class II panel-reactive antibody levels were determined with a sensitive flow cytometry technique.

RESULTS

We found panel-reactive antibody levels in decellularized allografts to be elevated slightly from preoperative levels for both class I and class II antibodies at 1, 3, and 12 months (P >.05). The panel-reactive antibody level for both class I and class II antibodies were significantly lower for decellularized allografts as compared to standard allografts. Functionally, the allografts were similar with decellularized valved grafts showing a peak echo-determined systolic gradient of 13 +/- 15 mm Hg at 8 +/- 2.6 months postoperatively as compared to a gradient of 24 +/- 18 mm Hg measured 12 +/- 6 months postoperatively in standard allografts (P =.11).

CONCLUSIONS

Decellularized grafts elicited significantly lower levels of class I and class II HLA antibody formation at 1, 3, and 12 months after implantation than did standard cryopreserved allografts. Early hemodynamic function of decellularized grafts was similar to that of standard cryopreserved allograft valves. Further experience is necessary to determine whether the reduced immunogenicity of decellularized allografts will truly allow tissue ingrowth and improved long-term durability in patients.

Homograft replacement of the mitral valve in young recipients: mid-term results

OBJECTIVE

Mitral homograft (MH) can represent an interesting alternative for valve replacement in the young. However, concerns have been expressed about the durability of valve allografts in children. We report our experience with MH replacement in young patients.

METHODS

From 1993 to 1997, 13 young patients aged 3-25 years (mean 15+/-6 years) underwent total mitral valve (MV) replacement with a cryopreserved homograft (CH). All but one had previously undergone one or more cardiac operations. The indications were rheumatic disease (6), acute and subacute endocarditis (2), congenital heart disease (4), and systemic lupus endocarditis (1).

RESULTS

No in hospital deaths are reported. Discharge echocardiogram showed a well-functioning MH in all but one patient. One patient was lost to follow-up. Follow-up ranged from 0.7 to 6.6 years (4.1+/-2.2). On follow-up two patients were doing well. Two patients died without reoperation and both had MV stenosis. Seven patients (54%) required reoperation: mean delay 4.17 years (0.7-7). In all cases, thickening, shrinking and calcification of the allograft were present. None of these seven had contributive histopathologic changes. One patient presenting recurrent MV insufficiency will require a reoperation.

CONCLUSION

MV homograft is a safe and reproducible technique, but does not provide durable results and should not be used in young patients.

Immunology and failure of valved allografts in children

Valved allografts are frequently used in the repair of congenital heart defects in children. Although the longevity of these grafts is generally very good, there continue to be ongoing problems with allograft stenosis, allograft valve insufficiency, and subsequent allograft failure, particularly in younger children. This review presents data on the immunologic and nonimmunologic risk factors implicated in valved allograft failure, in addition to ongoing investigation into the improvement of allograft function.

Homograft conduit failure in infants is not due to somatic outgrowth

OBJECTIVE

It has been assumed that the need for homograft replacement is due to somatic outgrowth, but this has not been adequately studied. Our objective was to identify reasons for homograft conduit failure.

METHODS

The records and imaging studies of 40 patients undergoing homograft conduit replacement of the right ventricular outflow tract from 1996 to 2000 were retrospectively reviewed.

RESULTS

The majority of patients had a diagnosis of tetralogy of Fallot (n = 20) and truncus arteriosus (n = 13). The median age at the initial operation was 8 months (0.25-108 months). The initial homograft sizes ranged from 9 to 22 mm, and 28 conduits were of pulmonary origin. When comparing size of the initial homograft with patients' expected pulmonary valve diameter (z = 0), oversizing was noted to be +3 (range, 0.83-5.4). Median interval to conduit failure was 5.3 years (0.83-11.3 years). At homograft replacement, only 12 patients had an existing conduit that was 1 SD below the homograft conduit size needed (z < or = -1). Most conduits had important regurgitation, but this was rarely a primary reason for reintervention (n = 1). Reoperation was usually required for stenosis, with a median gradient of 53 mm Hg (20-140 mm Hg). Stenosis was further categorized angiographically as follows: homograft valvular stenosis (shrinkage; 21/40 [53%]), distal anastomotic stenosis (4/40 [10%]), conduit kinking (3/40 [8%]), sternal compression (3/40 [8%]), posterior shelf impingement (2/40 [5%]), and somatic outgrowth (3/40 [8%]). Replacement in 2 patients was for proximal hood aneurysm. Several patients (7/40 [18%]) had stenosis at multiple levels. The average decrease in conduit diameter was 47% (28%-73%).

CONCLUSIONS

Somatic outgrowth is seldom a primary reason for homograft conduit replacement of the right ventricular outflow tract. The most common cause for failure is conduit obstruction with thickening and shrinkage at the annular area. Conduit stenosis was responsible for failure in 53% of patients, technical issues were responsible for 30%, and only 8% failed as a result of somatic outgrowth. Placement of a smaller homograft (z = 0) at the initial operation may decrease the incidence of conduit kinking, sternal compression, and posterior shelf impingement.