Bioengineered human and allogeneic pulmonary valve conduits chronically implanted orthotopically in baboons: hemodynamic performance and immunologic consequences

OBJECTIVE

This study assesses in a baboon model the hemodynamics and human leukocyte antigen immunogenicity of chronically implanted bioengineered (decellularized with collagen conditioning treatments) human and baboon heart valve scaffolds.

METHODS

Fourteen baboons underwent pulmonary valve replacement, 8 with decellularized and conditioned (bioengineered) pulmonary valves derived from allogeneic (N = 3) or xenogeneic (human) (N = 5) hearts; for comparison, 6 baboons received clinically relevant reference cryopreserved or porcine valved conduits. Panel-reactive serum antibodies (human leukocyte antigen class I and II), complement fixing antibodies (C1q binding), and C-reactive protein titers were measured serially until elective sacrifice at 10 or 26 weeks. Serial transesophageal echocardiograms measured valve function and geometry. Differences were analyzed with Kruskal-Wallis and Wilcoxon rank-sum tests.

RESULTS

All animals survived and thrived, exhibiting excellent immediate implanted valve function by transesophageal echocardiograms. Over time, reference valves developed a smaller effective orifice area index (median, 0.84 cm(2)/m(2); range, 1.22 cm(2)/m(2)), whereas all bioengineered valves remained normal (effective orifice area index median, 2.45 cm(2)/m(2); range, 1.35 cm(2)/m(2); P = .005). None of the bioengineered valves developed elevated peak transvalvular gradients: 5.5 (6.0) mm Hg versus 12.5 (23.0) mm Hg (P = .003). Cryopreserved valves provoked the most intense antibody responses. Two of 5 human bioengineered and 2 of 3 baboon bioengineered valves did not provoke any class I antibodies. Bioengineered human (but not baboon) scaffolds provoked class II antibodies. C1q(+) antibodies developed in 4 recipients.

CONCLUSIONS

Valve dysfunction correlated with markers for more intense inflammatory provocation. The tested bioengineering methods reduced antigenicity of both human and baboon valves. Bioengineered replacement valves from both species were hemodynamically equivalent to native valves.

Elimination of alpha-gal xenoreactive epitope: alpha-galactosidase treatment of porcine heart valves

BACKGROUND AND AIM OF THE STUDY

Porcine heart valves are among the most widely used tissue valves in clinical heart valve implantation. However, immunologic responses have been implicated as potential causes of the limited durability of xenograft heart valves. The study aim was to determine the effectiveness of alpha-galactosidase treatment used to degrade the major xenoreactive antigens found in xenograft heart valves.

METHODS

Fresh porcine heart valves and pericardium treated with alpha-galactosidase were studied to evaluate the xenoreactive galactose (alpha1,3) galactose (alpha-gal) antigen. Removal of the alpha-gal epitope from the porcine heart valve was monitored via 3,3'-diaminobenzidine staining intensity, while the removal of alpha-gal from N-glycans on porcine heart valves treated with recombinant alpha-galactosidase was determined either qualitatively or quantitatively by mass fingerprinting using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The porcine pericardium was used for monitoring the change in mechanical properties after alpha-galactosidase treatment. In addition, the biomechanical modification property of collagen fiber rearrangement on tissue was assessed using transmission electron microscopy (TEM).

RESULTS

Following a 24-h incubation at pH 7.2, 4 degrees C, employing 0.1 U/ml of Bacteroides thetaiotaomicron-derived recombinant alpha-galactosidase, the enzyme effectively removed the alpha-gal epitopes expressed on porcine heart valves. The identification type of alpha-gal N-glycan on fresh aortic valve, aortic wall, pulmonary valve, and pulmonary wall was 7.1%, 10.3%, 6% and 8%, respectively. In the presence of alpha-galactosidase treatment, alpha-gal-containing N-glycans were converted into alpha-gal-negative N-glycans. Likewise, alpha-gal-containing N-glycans were not detected when MALDI-TOF MS quantitative analysis was used. Furthermore, no significant difference was observed in the mechanical properties and findings from TEM in alpha-galactosidase-treated porcine pericardial tissue when compared to fresh porcine pericardium.

CONCLUSION

Alpha-galactosidase can effectively remove the alpha-gal epitope from porcine heart valves and pericardium. This may possibly alleviate harmful xenoreactive immunologic responses by alpha-gal, without adversely affecting the biomechanical properties of the alpha-galactosidase-processed tissue.

Genipin blues: an alternative non-toxic crosslinker for heart valves?

BACKGROUND AND AIM OF THE STUDY

One approach in tissue-engineering involves the implantation of decellularized, xenogenic scaffolds, with the expectation of repopulation in vivo. However, a major limitation of this method is the propensity to induce a strong immune host response. The study aim was to mitigate this immunogenicity by employing a crosslinking treatment with genipin.

METHODS

Porcine matrices were prepared using a detergent-enzymatic treatment and fixed in 0.01% or 0.001% aqueous genipin. The mechanical properties of the matrices were monitored by tensile strength testing. The survival of chicken fibroblasts was used to determine cell-friendliness of the matrices. Non-fixed, decellularized biological scaffolds (n = 3) were implanted in a sheep model and compared to an equal number of genipin-fixed scaffolds (n = 6). Matrices implanted in the pulmonary position were explanted after six weeks and examined using light and transmission electron microscopy. The antibody reaction against porcine tissue in sheep serum was also determined.

RESULTS

Statistically significant differences were found between non-fixed leaflets, 0.001% genipin-and 0.6% glutaraldehyde (GA)-fixed leaflets for work to maximum load (non-fixed 0.00646 J; genipin-fixed 0.00509 J; GA-fixed 0.00543 J) and stiffness (non-fixed 9281 N/m; genipin-fixed 16214 N/m; GA-fixed 14401 N/m). Genipin-treated matrices were not cytotoxic. For all concentrations of genipin a high proportion of viable cells was present (79-100%). Low-dose GA (10 microg/ml) showed a distinct cytotoxicity (24.8% viability). At explant, an intense chronic inflammatory response was observed in non-fixed matrices, in contrast to genipin-fixed scaffolds. The sheep serum showed a marked decrease in IgG response in both 0.001% and 0.01% genipin-fixed matrices (IgG 30 and 20, respectively) when compared to non-fixed matrices (IgG 40).

CONCLUSION

Genipin crosslinking of the matrices attenuated, but did not eliminate, the inflammatory host reaction. Whether genipin treatment might extend the durability of xenogenic scaffolds remains to be investigated.

Time-related Histopathologic Analyses of Immunologically Untreated Porcine Valved Conduits Implanted in a Porcine-to-Goat Model

This study was performed to evaluate the clinical feasibility of use of immunologically nontreated xenogenic valves, using a porcine-to-goat pulmonary valved conduit implantation model. Porcine pulmonary valve conduits were prepared with no specific immunological treatment and implanted in the right ventricular outflow tract of goats under cardiopulmonary bypass. The goats were assigned at predetermined intervals (1 day, 1 week, and 3, 6, and 12 months) as two animals for each interval. Echocardiographic examinations of the valves were performed before sacrifice. Upon retrieving the xenograft specimens, they were inspected visually and microscopically. Ten of the 12 animals survived the predetermined observation periods. Variable degrees of pulmonary regurgitation were the main findings on echocardiographic evaluations. On gross examination of the explanted specimens, all leaflets, except in one animal that prematurely died, were fairly well preserved. They were slightly shortened but free of thrombosis or vegetation. Aneurysmal dilatations of the anterior wall of the implanted pulmonary artery were observed in one of 12-month-survival animals and in another one of 3-month-survival animals. Microscopically, the three components of implanted xenografts (the pulmonary artery, valve, and infundibulum) were shown to be gradually replaced with host cells in time, while maintaining structural integrity. The nuclei of the donor tissue disappeared through pyknosis and karyolysis. In conclusion, immnunologically untreated xenogenic pulmonary valved conduits can be an alternative potential as valve substitutes with distinctive advantages of providing self-healing potential, despite a few problems observed in the current study such as occurrences of pulmonary regurgitation and sporadic cases of aortic aneurysm.

Factors influencing the survival of cryopreserved homografts. The second homograft performs as well as the first☆

OBJECTIVE

To determine the life span of cryopreserved homografts implanted in the right ventricular outflow tract and the factors influencing it.

METHODS

From 1989 through 2003, we reconstructed the pulmonary valve with 301 homografts in 272 patients (median age 13 years; range 4 days-69 years). Indications were tetralogy of Fallot (136), truncus (23), Rastelli repair (11), double outlet ventricle (13), endocarditis (5), and the Ross operation (84). Median follow-up was 5.7 years (range 0-14). We analyzed possible predictors of graft replacement by simple and multiple Cox regression.

RESULTS

Actuarial survival was 96+/-1.2% at 1, 95+/-1.4% at 5, and 94+/-1.5% at 10 years follow-up. Three homografts were explanted because of endocarditis (excluded from the analysis). Freedom from explantation was 99.6+/-0.4% at 1, 94.5+/-1.7% at 5, and 81.8+/-4.1% at 10 years. Variables, significantly related to explantation in the univariate analysis, were younger age, small graft size, implantation in a non-anatomical position, the aortic donor homograft, a shorter aortic cross-clamp time and the implantation of a second homograft. In the multiple model, non-anatomical position (P=0.001), smaller graft size (P<0.0001) or younger age (on square root scale, P<0.0001) and clamp time (P=0.01) remain as independent risk factors. Immunological variables, like blood group incompatibility, implantation of a second homograft and short warm ischemic time were not significant.

CONCLUSIONS

The life span of a cryopreserved homograft is determined by graft size (correlates with age) and the non-anatomic position (correlates with indication). In a specific patient, the second homograft performs as well as the first.

[The determination of trace amounts of protein in solutions containing surface-active substances]

A rapid and sensitive method for protein determination (0.5-16 micrograms) in samples of any volume containing various surfactants in concentration up to 1% is suggested. The method includes the protein acid denaturation, the solution of acid insoluble precipitate of detergent in ethanol (25-30%), the protein determination on nitrocellulose filter, dyeing by aminoblack 10 B, elution of dyed complex and colorimetric determination at 630 nm.

Immunological and echocardiographic evaluation of decellularized versus cryopreserved allografts during the Ross operation

OBJECTIVE

Compare the immunological and echocardiographic data of decellularized versus cryopreserved allografts used for RVOT reconstruction during Ross operation.

METHODS

From 16/01/03 thru 07/10/03, 20 Ross operations were performed using decellularized (n=11) or cryopreserved (n=9) allografts. Echocardiography was done at discharge, 1, 3, 6 and 12 months and annually thereafter. Samples for determination of antibodies against HLA class I and II were obtained preoperatively and at days 5, 10, 30, 90 and 180 postoperatively. These samples were tested by the ELISA method in LAT-M dishes (unspecific) for identification of circulating antibodies and the results expressed as mean sample values (Is=DO/cutoff). If positive, LAT-E (specific) was performed and PRA levels determined.

RESULTS

There was no mortality. Cryopreserved allografts showed marked Is values elevations for class I and II antibodies which started at the first month and remained elevated up to 6 months. In contrast, of the patients receiving decellularized allografts, seven remained negative, two patients had only marginal elevation of class I antibodies and two patients showed abnormal elevations of PRA levels. This response happened earlier than in the cryopreserved group, starting on the 5th postoperative day and has returned to baseline levels in one case. Echocardiography showed mild, but significant, elevation of gradients in cryopreserved valves but none in the decellularized.

CONCLUSIONS

Decellularized allografts had normal function up to 18 months and showed important reduction of the immunogenic response when compared to cryopreserved valves.

Factors influencing late allograft valve failure

Allograft valves are a valuable valve replacement substitute in the surgical management of heart valve disease. It remains the valve substitute of choice in the reconstruction of the right ventricular outflow tract in children with congenital heart disease and in the Ross procedure. However, its durability remains suboptimal, particularly in children. This article reviews the mechanisms and factors implicated in late allograft dysfunction, with a focus on the evidence for an immunological cause for allograft valve failure. Unravelling the mechanisms of allograft valve failure may allow modification of the allograft to improve its long-term durability.

Mid-term findings on echocardiography and computed tomography after RVOT-reconstruction: comparison of decellularized (SynerGraft) and conventional allografts

OBJECTIVE

The immune response against human-leucocyte-antigens on donor-cells may be an important factor contributing to the degeneration of allograft-valves. We have previously reported that the use of the decellularized allograft SynerGraft (CryoLife) reduces the immunologic response of the allograft-recipient. In this study we compare the echocardiographic and computed tomography angiographic (CTA) findings of SynerGrafts with conventional cryopreserved allografts.

METHODS

22 patients who received a pulmonary SynerGraft (SG-group) (21 during a Ross-procedure) underwent CTA and resting echocardiography (median: 10 months postoperatively). 47 randomly chosen patients who underwent a Ross-procedure served as controls (C-group) (median: 32 months postoperatively).

RESULTS

Neither the pressure gradients (mean: SG=9+/-4 vs C=10+/-4mmHg; P=0.64) across the allograft, nor the effective orifice area (EOAI) (SG=0.93+/-0.80 vs C=0.93+/-0.42cm(2)/m(2); P=0.96) differed between the groups. The EOAI showed a significant correlation with the smallest allograft-conduit-area measured on CTA (r=0.81; P<0.001) which was most frequently (n=34) found in the proximal postvalvular tubular part of the conduit. Calcifications (n=11) or a fibroproliferative reaction (n=15) were rarely observed. Overall, there were no radiologic differences between the groups. On CTA, the smallest diameter of the allograft-conduits was significantly smaller than the diameter given on the cryopreservation protocol (SG=16+/-3 and C=17+/-3mm vs 25mm in both groups; P<0.001 each) whereas the diameter of the distal part of the allograft was not (SG=24+/-2, P=0.066, and C=25+/-3mm, P=0.82).

CONCLUSIONS

Despite a significant shorter follow-up in the SynerGraft-group, no functional or radiologic differences were observed as compared to control-patients. The smallest diameter is located almost exclusively at the proximal level of allograft-conduits.

Congenital semilunar valvulogenesis defect in mice deficient in phospholipase C epsilon

Phospholipase Cepsilon is a novel class of phosphoinositide-specific phospholipase C, identified as a downstream effector of Ras and Rap small GTPases. We report here the first genetic analysis of its physiological function with mice whose phospholipase Cepsilon is catalytically inactivated by gene targeting. The hearts of mice homozygous for the targeted allele develop congenital malformations of both the aortic and pulmonary valves, which cause a moderate to severe degree of regurgitation with mild stenosis and result in ventricular dilation. The malformation involves marked thickening of the valve leaflets, which seems to be caused by a defect in valve remodeling at the late stages of semilunar valvulogenesis. This phenotype has a remarkable resemblance to that of mice carrying an attenuated epidermal growth factor receptor or deficient in heparin-binding epidermal growth factor-like growth factor. Smad1/5/8, which is implicated in proliferation of the valve cells downstream of bone morphogenetic protein, shows aberrant activation at the margin of the developing semilunar valve tissues in embryos deficient in phospholipase Cepsilon. These results suggest a crucial role of phospholipase Cepsilon downstream of the epidermal growth factor receptor in controlling semilunar valvulogenesis through inhibition of bone morphogenetic protein signaling.

Immunological and histological evaluation of decellularized allograft in a pig model: comparison with cryopreserved allograft

BACKGROUND AND AIM OF THE STUDY

The remodeling process of the decellularized allograft after implantation remains unclear. Herein, the hemodynamics, recellularization and immunological response of the decellularized allograft were evaluated at four weeks after implantation in a mini-pig model, and compared with a cryopreserved allograft.

METHODS

Six porcine pulmonary allografts were harvested from mini-pigs, and cryopreserved for four weeks. In two pigs, the grafts were decellularized with Triton X solution, after which static reseeding of the valve surface was performed for 48 h with autologous endothelial cells harvested from a leg artery. Decellularization, but not reseeding, was carried out in two mini-pigs, and cryopreservation alone in two mini-pigs. Whilst under right heart bypass, the right ventricular outflow tract was replaced in six minipigs. The grafts were explanted after four weeks; analysis included direct pressure measurement, echocardiography, macroscopy, light microscopy with hematoxylin and eosin staining, and immunohistochemical studies to identify macrophages, T lymphocytes, and endothelial cells.

RESULTS

Hemodynamically and macroscopically, there were no major differences between the three groups. In the cryopreservation-only group, immunohistochemistry showed an influx of macrophages, and T lymphocytes at the cusps. Endothelial cell coverage was found in the decellularized and decellularized + cell-seeded groups, but no macrophages and T lymphocytes were found at the cusps.

CONCLUSION

Decellularization of the cryopreserved allograft may reduce the inflammatory response and improve its long-term durability.

Role of inflammation and ischemia after implantation of xenogeneic pulmonary valve conduits: histological evaluation after 6 to 12 months in sheep

OBJECTIVE

Commercially available biological heart valve prostheses undergo degenerative changes, which finally lead to complete destruction. Here we evaluate the role of inflammation and ischemia after implantation of xenogeneic heart valve conduits (XPC) generated by novel concepts of tissue engineering.

METHODS

Acellularized (a-)XPC and autologus re-seeded (s-)XPC were implanted into sheep. Samples were taken as follows: after acellularization (n = 2), after re-seeding (n = 2), 6 months (seeded/non-seeded: n = 3/5), 9 months (n = 2/5), and 12 months (n = 3/2) post implantation. Five native porcine conduits served as control. Using histological methods, samples were evaluated for pathological changes and existence/density of microvessels.

RESULTS

Prior to implantation a-XPC were completely free of cells. Six months after implantation, leaflets and pulmonary arteries of s-XPC and a-XPC showed good endothelial surface coverage. Microvessel density within the myocardial cuffs and pulmonary vessel walls were comparable to control in all grafts. However, 6, 9 and 12 months after implantation pathological severe microvessel ingrowth, calcification and cellular infiltrations were observed on a-XPC and s-XPC valves, whereas myocardial cuffs and XPC-artery walls showed only mild degenerative alterations.

CONCLUSIONS

Inflammatory reactions play a pivotal role in the degeneration of a-XPC and s-XPC. Thus, since ischemia seems to have little or no influence on this process, inflammation inductive factors should be the center of interest.

The pathology of fresh and cryopreserved homograft heart valves: an analysis of forty explanted homograft valves

OBJECTIVE

Tissue degeneration reduces the durability of aortic and pulmonary homograft heart valves. Homograft valves can evoke cellular and humoral immune responses that might be detrimental to the valve tissue. Analyzing explanted homograft valves helps in understanding the different factors that eventually lead to tissue degeneration.

METHODS

A total of 40 homografts was acquired from patients whose grafts had been explanted because of stenosis (n = 22), insufficiency (n = 8), paravalvular leakage (n = 4), other technical problems (n = 4), noncardiac death (n = 1), and stenosis with endocarditis (n = 1). The period of implantation varied from 14 days to 16 years (median, 4 years). Cryopreserved valves (n = 31) were, in the majority, derived from beating-heart donors, whereas the fresh valves were sterilized with antibiotics and stored at 4 degrees C for an average of 32 days. Four unimplanted cryopreserved valves, 1 native aortic valve, and 1 native pulmonary valve were used as references. Analysis included macroscopy, light microscopy with routine hematoxylin and eosin staining (cellularity and tissue structure), and immunohistochemical studies to allow identification of macrophages (CD68) and T lymphocytes (CD3), endothelial cells, leukocyte adhesion molecules (CD54, CD106, and CD62E), and immunoglobulin (IgG) and complement factor (C3) depositions. In situ hybridization for the Y chromosome was performed in 10 cases, with host-donor sex mismatch, to distinguish between host and donor cells. The outcomes of histology and immunohistochemistry were related to clinical factors, such as implantation time and reason for explantation.

RESULTS

In the first year after implantation, a strong reduction in cellularity of the valve tissue was observed, with almost acellular tissues after 1 year. Trilaminar tissue architecture disappeared with the same speed, whereas endothelial cells were almost absent in all explants. Macrophages and T lymphocytes were encountered in 85% and 78% of the leaflets, respectively. Expression of leukocyte adhesion molecules was low in almost all grafts, and IgG and C3 depositions were not increased. Valve tissue cellularity consisted mainly of ingrown host cells when the implantation time exceeded 1 year.

CONCLUSIONS

During the first year of implantation, homograft valves rapidly lose their cellular components and normal tissue architecture. A low-grade inflammatory response was observed, but no convincing evidence of immune-mediated injury was found.

The presence of immune stimulatory cells in fresh and cryopreserved donor aortic and pulmonary valve allografts

BACKGROUND AND AIM OF THE STUDY

Heart valve allografts (HVA) used for valve replacement or ventricular outflow tract reconstruction may suffer from structural deterioration due to donor-specific immune responses. The presence of immune stimulatory cells, including dendritic cells and activated endothelial cells, has not been studied thoroughly in aortic or pulmonary HVA. The presence and distribution of these cells in both aortic and pulmonary HVA, before and after cryopreservation, was analyzed immunohistochemically.

METHODS

Aortic (n = 16) and pulmonary (n = 13) HVA, discarded for implantation due to morphological or technical reasons, were obtained from 12 heart-beating and nine non-heart-beating tissue donors. Aortic and pulmonary HVA were dissected longitudinally into two symmetric sections by splicing of the non-coronary aortic and non-facing pulmonary cusps. Each symmetric half contained one-and-a-half valve cusps attached to the vascular wall. Fresh halves were directly fixed in formaldehyde, and analyzed immunohistochemically. The corresponding halves of the valves were decontaminated, cryopreserved, stored for at least three weeks and thereafter thawed according to the Heart Valve Bank protocol before analysis.

RESULTS

Activated endothelial cells, expressing PECAM-1 (CD31), VCAM-1 and HLA class II molecules covered at least 50% of fresh valvular surfaces. A comprehensive vascular network was found in the myocardial rim and adventitial layer, which was covered entirely by activated endothelial cells. HLA class II-positive macrophages (CD68) and T lymphocytes (CD3) were found scattered in the stroma and subendothelial layer of the valve leaflets. Mononuclear cell clusters were found predominantly in relation to native degenerative foci, and more often in aortic valves. No difference in cellular distribution was observed between the two donor types. Dendritic cells positive for both S100 and CD45 were not found in immuno-double-stained sections. Cryopreservation resulted in minor structural alterations in the vascular wall, and an increase of cells with pycnotic nuclei and reduction of adhesion molecule expression on endothelial cells. All fresh and cryopreserved aortic and pulmonary HVA contained abundant HLA class II-positive endothelial cells and sparse distribution of mononuclear cells in the luminal and adventitial layers.

CONCLUSION

Cryopreservation minimally affected the extracellular matrix of HVA and diminished the cellularity of the valve graft, while the HLA class II expression of cells was not abrogated. Aortic valve allografts harbor more mononuclear cells than their pulmonary counterparts. The absence of dendritic cells (professional antigen-presenting cells) is compensated by the preservation of other cells expressing HLA class II molecules predominantly in the endothelium; this may be responsible for the initiation of a specific immune response against HVA.

Humoral immune response to allograft valve tissue pretreated with an antigen reduction process

The humoral immune response to allograft heart valves as measured by PRA was absent in 52 of 57 (91%) patients at 1 month and was absent in 43 of 49 (88%) at 3 months in allograft valves treated with the SynerGraft process for antigen reduction. Short-term valve function is satisfactory. This may be associated with improved durability and long-term function.

Recellularization of heart valve grafts by a process of adaptive remodeling

The objective of this study was to investigate if function and durability of connective tissue grafts stems from in vivo revascularization and recellularization. Viability is important for durable valve performance, demonstrated by pulmonary autografts. A pattern of in vivo recellularization occurs in xenogeneic or allogeneic heart valves decellularized prior to implantation, dictated by the tissue matrix and functional biomechanics. Porcine or sheep heart valves were decellularized with the SynerGraft antigen reduction process (a common treatment process to remove all histologically demonstrable leaflet cells), and implanted as pulmonary (n = 11) or aortic valve (n = 9) replacements in sheep. Sheep allograft pulmonary valves (n = 4) were implanted as pulmonary valve replacements. Recellularization was evaluated histologically after 3, 4, 5, 6, and 11 months, with cell phenotypes identified using specific antibodies. SynerGraft heart valves were progressively recellularized beginning with an initial cellular infiltrate, and subsequent repopulation with mature interstitial cells. This process occurs in the conduit and then in the leaflet, and is associated with revascularization of the graft. Functional, fully developed fibrocytes, actively synthesizing type I procollagen (antibody probe) were present within 3 months. As the process matured cell density and distribution became similar to native valve leaflets with localization of smooth muscle actin positive cells at the ventricularis/spongiosa interface. After 11 months, leaflet explants had no detectable inflammatory cells, were as much as 80% repopulated, and had a distribution of smooth muscle actin positive cells similar to that of the natural leaflet. SynerGraft- treated heart valve implants are repopulated by a process typical of adaptive remodeling following implantation. This antigen reduction treatment is the first successful tissue engineering effort obtaining an implant with mature recipient cells capable of matrix protein synthesis. Normal early valve function and durability is maintained.

Does histocompatibility affect homograft valve function after the Ross procedure?

BACKGROUND

Homograft valves have been shown to be immunogenic, but it is unknown whether this affects valve function. Therefore, we prospectively studied the degree of histoincompatibility (defined as the number of human leukocyte antigen [HLA] mismatches between valve donor and recipient) and the response of the recipient (measured by antibodies against HLA) in relation to echocardiographic parameters of homograft valve function after the Ross procedure.

METHODS AND RESULTS

Twenty-six patients (mean age 41+/-14 years; 20 males, 6 females) and the cryopreserved pulmonary homograft valves that were implanted during a Ross procedure were typed for HLA-A, HLA-B, and HLA-DR. After a mean follow-up of 15+/-6 months, 14 (54%) of the patients were anti-HLA class I antibody positive. In all but 1 patient, these antibodies were shown to be donor specific. During follow-up, there was a significant increase of the maximal (+6.2+/-7.1 mm Hg) and mean (+3.2+/-4.3 mm Hg) transhomograft pressure gradients but not of homograft regurgitation. Neither the number of HLA mismatches nor antibody status was found to have significant impact on homograft valve function. In a multivariate analysis, smaller homograft size (P=0.001) and younger recipient age (P=0.044) were shown to be significantly associated with increased transhomograft pressure gradients.

CONCLUSIONS

Implantation of a cryopreserved pulmonary homograft during the Ross procedure can induce a specific humoral response. We observed a significant increase of the transhomograft pressure gradients within 15+/-6 months after surgery. For this period, we were unable to demonstrate a relationship between this increase and the degree of histoincompatibility.

Anti-HLA class I antibodies and pulmonary homograft function after the Ross procedure

BACKGROUND

The Ross procedure provides excellent long-term results in the majority of patients. However, degeneration of the pulmonary homograft in some patients remains an unresolved problem that may be related to immunologic factors. Therefore, we studied the prevalence of antihuman leukocyte antigen (HLA) class I antibodies and echocardiographic results of homograft function at rest.

METHODS

Forty-seven patients (37 men, 10 women; 47 +/- 15 years) were seen for echocardiography 1.1 to 63.9 months (median, 27 months) postoperatively. The presence of anti-HLA antibodies was tested against a panel of lymphocytes of 50 donors.

RESULTS

Twenty-seven (57%) of the patients produced anti-HLA class I antibodies. No difference in the maximal or mean transhomograft pressure gradient, or in the frequency of homograft regurgitation according to the presence or absence of anti-HLA antibodies was found. However, the right ventricle was slightly but significantly larger in antibody-positive patients (26.3 +/- 4.2 versus 30.7 +/- 3.5 mm; p = 0.001).

CONCLUSIONS

In the first years after the Ross procedure, we could not detect significant evidence of an association between anti-HLA class I antibodies and echocardiographic results of homograft function at rest in adults.

Immunogenic human leukocyte antigen class II antigens on human cardiac valves induce specific alloantibodies

BACKGROUND

The kinetics of panel reactive antibodies (PRA) and incidence of antibodies directed against human leukocyte antigen (HLA) class II were studied in patients who received a cryopreserved cardiac valve allograft.

METHODS

A complement-dependent microlymphocytotoxicity test was used to determine the percentage of panel reactive antibodies. Anti-HLA class II antibodies were measured by two-color fluorescence assays.

RESULTS

The panel reactive antibodies became positive in 25 (78%) of 32 recipients between 1 and 16 months after implantation. Antibodies against HLA class II antigens were detected in 11 (37%) of 30 patients. In 9 (82%) of 11 cases these antibodies were donor specific. The induction of antibodies against donor HLA class II antigens suggests that intact HLA class II antigens are expressed by viable cells within the graft. Dithiothreitol analysis showed that the antibodies were of the immunoglobulin G type. Apparently, the HLA class II antigens are expressed in an immunogenic way, as activation of specific T-helper cells is essential for the switch from immunoglobulin M to immunoglobulin G antibodies.

CONCLUSIONS

Allogeneic valve transplantation is associated with the production of donor-specific anti-HLA class I and II antibodies that could contribute to graft failure. This possibly detrimental effect might be prevented by cross matching in sensitized patients.

Evidence for rejection of homograft cardiac valves in infants

OBJECTIVE

Concern about the durability of small homograft cardiac valves has been expressed by surgeons, and evidence has been found that homograft valves evoke a recipient immune response. We reviewed our experience with homograft valves for evidence of rejection.

METHODS

A search of our files revealed 11 homograft cardiac valves removed at reoperation and one at autopsy. Six valves were from adults, five were from infants, and one was from a 13-year-old child. Immunohistochemical studies with antibodies against smooth muscle actin, CD20, CD43, CD34, and CD68 were performed on the homografts containing inflammatory infiltrates. These valves happened to be the valves from the five infants. These five valves were also stained with Gram and Gomori's methenamine silver stains.

RESULTS

The failed homografts from the adults and 13-year-old child showed leaflet calcification, fibrosis, and degeneration, but no inflammation. The valves from the infants all failed in less than 8 months. The valve leaflets were thickened, and the valve leaflets and aortic sleeves contained a hyperplastic intimal layer with numerous spindle cells positive for smooth muscle actin embedded in a glycosaminoglycan matrix. The homografts contained multiple foci of inflammation consisting of T lymphocytes (in all five infant valves) and B lymphocytes (in three of the five infant valves). Special stains for organisms were negative.

CONCLUSIONS

Rapid failure plus lymphocytic infiltration in valve leaflets and aortic sleeves is consistent with rejection. The hyperplastic intima is similar to coronary arteries in transplant-associated vascular disease. Our observations are consistent with other reports of rapid failure of homograft valves in this age group.

Donor-specific cellular immune response against human cardiac valve allografts

We studied the presence of donor-specific T lymphocytes in explanted human cardiac valve allografts in vivo. From five of seven explants we propagated lymphocyte cultures in an interleukin-2 conditioned medium. Phenotyping revealed the presence of T-cell receptors in more than 95% of the lymphocytes obtained in each culture. Donor-specific cytotoxicity was demonstrated in three patients with known HLA status of the donor. Cytotoxicity was directed against only HLA class I in one patient, and against class I and/or class II in the others. These results indicate that donor-specific cellular reactivity can be induced by transplantation of human cardiac valve allografts.

Stimulation of immune-competent cells in vitro by human cardiac valve-derived endothelial cells

Both fresh and cryopreserved human cardiac valve allografts are transplanted without matching donor and recipient for blood group or human leukocyte antigens (HLA) and without the usual immunosuppressive therapy that follows organ transplantation. Calcification occurs in almost all transplanted valves, and in children acute valve failure is frequently seen. We hypothesized that failure of the human valve allografts could have an immunologic basis. This hypothesis was tested in a functional way by performing lymphocyte stimulation assays using fresh and cryopreserved valve pieces and endothelial cells derived from valve leaflets as stimulator. Human peripheral blood lymphocytes, both matched and mismatched for HLA antigens, were used as responder cells. The results were expressed as the stimulation index. Fresh valve pieces induced a significantly higher stimulation index (median, 9; range, 4 to 117) compared with the cryopreserved pieces (median, 2; range, 0 to 9; p = 0.002 by Wilcoxon test). The stimulation index was significantly reduced when lymphocytes matched for HLA-DR with the valve pieces were used (median, 1; range, 0 to 5) as compared with the HLA-DR-mismatched combination (median, 4; range, 2 to 117; p = 0.006, Wilcoxon test). Valve leaflet-derived endothelial cells were able to induce a median stimulation index of 8 (range, 3 to 15) when incubated with lymphocytes mismatched for HLA-A, -B, and -DR. In conclusion, stimulation of immune-competent cells in vitro is induced by both fresh and cryopreserved human valve pieces and by endothelial cells derived from fresh valve leaflets. The immune response can be reduced by using cryopreserved valves or by matching valve donor and responder lymphocytes for HLA-DR.(ABSTRACT TRUNCATED AT 250 WORDS)