Histologic and immunohistochemical responses after aortic valve allografts in the rat

Cadaver donation: structural integrity of pulmonary homografts harvested 48 h post mortem in the juvenile ovine model

Cryopreserved pulmonary homograft (CPH) implantation remains the gold standard for reconstruction of the right ventricular outflow tract (RVOT). Harvesting homografts < 24-h post mortem is the international norm, thereby largely excluding cadaveric donors. This study examines the structural integrity and stability of ovine pulmonary homografts harvested after a 48-h post mortem period, cryopreserved and then implanted for up to 180 days. Fifteen ovine pulmonary homografts were harvested 48-h post mortem and cryopreserved. Five CPH served as a control group (group 1; n = 5). CPH were implanted in the RVOT of juvenile sheep and explanted after 14 days (group 2; n = 5) and 180 days (group 3; n = 5). Leaflet integrity was evaluated by strength analysis, using tensile strength (TS), Young's modulus (YM) and thermal denaturation temperature (T_d), and morphology, including haematoxylin and eosin (H&E), Picrosirius red staining, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and von Kossa stains. Echocardiography confirmed normal function in all implants. In explants, no reduction in TS, YM or T_d could be demonstrated and H&E showed mostly acellular leaflet tissue with no difference on Picrosirius red. TEM demonstrated consistent collagen disruption after cryopreservation in all three groups, with no morphological deterioration during the study period. von Kossa stains showed mild calcification in group 3. No deterioration of structural integrity could be demonstrated using strength or morphological evaluations between the controls and implant groups over the study period. Extending the post mortem harvesting time of homografts beyond 24 h did not appear to negatively affect the long-term performance of such transplanted valves in this study.

* A Rat Model for the In Vivo Assessment of Biological and Tissue-Engineered Valvular and Vascular Grafts

The demand for an improvement of the biocompatibility and durability of vascular and valvular implants requires translational animal models to study the in vivo fate of cardiovascular grafts. In the present article, a review on the development and application of a microsurgical rat model of infrarenal implantation of aortic grafts and aortic valved conduits is provided. By refinement of surgical techniques and inclusion of hemodynamic considerations, a functional model has been created, which provides a modular platform for the in vivo assessment of biological and tissue-engineered grafts. Through optional addition of procalcific diets, disease-inducing agents, and genetic modifications, complex multimorbidity scenarios mimicking the clinical reality in cardiovascular patients can be simulated. Applying this model, crucial aspects of the biocompatibility, biofunctionality and degeneration of vascular and valvular implants in dependency on graft preparation, and modification as well as systemic antidegenerative treatment of the recipient have been and will be addressed.

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.

Comparison of aortic valve allograft decellularization techniques in the rat

Rodent models have been essential to understanding the immune-mediated failure of aortic valve allografts (AVAs). Decellularization has been proposed to reduce the immunogenicity of AVAs. The objective of this study was to determine the most effective method to decellularize AVAs for use in a rat model. Three different decellularization techniques were compared in Lewis aortic valves. Detergent decellularization involved a series of hypotonic and hypertonic Tris buffers at 4 degrees C for 48 h/buffer containing 0.5% Triton X-100 followed by a 72 h washout in phosphate-buffered saline. Osmotic decellularization was performed in similar manner to the detergent-based technique except without the addition of Triton X-100. Enzymatic decellularization consisted of trypsin/EDTA at 37 degrees C for 48 h. Assessment was performed with light microscopy (H&E, Movat's pentachrome), immunohistochemistry for residual cellular elements, and hydroxyproline assays. Detergent-based methodology effected near-complete decellularization of both the leaflets and aortic wall in addition to preservation of the extracellular matrix (ECM). Osmotic lysis was associated with preservation of ECM and moderate decellularization. Enzymatic decellularization resulted in complete decellularization but extensive degeneration and fragmentation of the ECM. When implanted into the infrarenal aorta of allogeneic rats for 1 week, valves decellularized with detergent-based and osmotic methodology failed to stimulate an allogeneic immune response as evidenced by an absence of T cell infiltrates. Osmotic lysis protocols with low dose detergent appear to be most effective at both removing antigenic cellular elements and preserving ECM.

Decellularization reduces the immune response to aortic valve allografts in the rat

OBJECTIVES

Cryopreserved valve allografts used in congenital cardiac surgery are associated with a significant cellular and humoral immune response. This might be reduced by removal of antigenic cellular elements (decellularization). The aim of this study was to determine the immunologic effect of decellularization in a rat allograft valve model.

METHODS

Brown Norway and Lewis rat aortic valves were decellularized with a series of hypotonic and hypertonic buffers, protease inhibitors, gentle detergents (Triton X-100), and phosphate-buffered saline. Valves were implanted into Lewis rats in syngeneic and allogeneic combinations. Cellular (CD3 and CD8) infiltrates were assessed with morphometric analysis, and the humoral response was assessed with flow cytometry.

RESULTS

Morphometric analysis identified a significant reduction in CD3 + cell infiltrates (cells per square millimeter of leaflet tissue) in decellularized allografts compared with that seen in nondecellularized allografts at 1 (79 +/- 29 vs 3310 +/- 223, P < .001), 2 (26 +/- 11 vs 109 +/- 20, P = .004), and 4 weeks (283 +/- 122 vs 984 +/- 145, P < .001). Anti-CD8 staining confirmed the majority of infiltrates were cytotoxic T cells. Flow cytometric mean channel fluorescence intensity identified a negative shift (abrogated antibody formation) for decellularized allografts compared with nondecellularized allografts at 2 (19 +/- 1 vs 27 +/- 3, P = .033), 4 (35 +/- 2 vs 133 +/- 29, P = .001), and 16 weeks (28 +/- 2 vs 166 +/- 54, P = .017).

CONCLUSIONS

Decellularization significantly reduces the cellular and humoral immune response to allograft tissue. This could prolong the durability of valve allografts and might prevent immunologic sensitization of allograft recipients.

Transcatheter placement of a low-profile biodegradable pulmonary valve made of small intestinal submucosa: A long-term study in a swine model

OBJECTIVE

We sought to investigate a placement of a percutaneous low-profile prosthetic valve constructed of small intestinal submucosa in the pulmonary position in a swine model.

METHODS

Twelve female farm pigs were stented at the native pulmonary valve to induce pulmonary insufficiency. Once right ventricular dilation occurred, the small intestinal submucosa valve was implanted. The pigs were followed up with transthoracic echocardiographic Doppler scanning. One animal died of heart failure before valve replacement. Animals were euthanized at 1 day, 1 month, 3 months, 6 months, and 12 months after valve implantation.

RESULTS

The small intestinal submucosa pulmonary valve showed effective reversal of pulmonary regurgitation. There were no misplacements during deployment. There were no embolizations. One-year echocardiographic follow-up showed minimal regurgitation and no stenosis for a valve/vessel ratio of 0.78 or greater. Histologic examination demonstrated intensive remodeling of the small intestinal submucosal valve. Within 1 month, the surface was covered by endothelium, and fibroblasts invaded the interior. Over the following months, the small intestinal submucosal valve remodeled without apparent graft rejection.

CONCLUSION

The small intestinal submucosa valve has the potential for graft longevity without the need for anticoagulation or immunosuppression. Histologic remodeling of the valve tissue provides a replacement capable of resembling a native valve that can be placed percutaneously with low-profile delivery systems.

Ovine panel reactive antibody assay of HLA responsivity to allograft bioengineered vascular scaffolds

BACKGROUND

Increasing evidence implicates immune response as a contributing factor in the failure of allograft valve transplants. Increases in panel reactive antibodies have been identified in human subjects. To correlate these responses with novel preimplantation processing methods to reduce cellularity, both a relevant panel reactive antibody assay and a chronic implantation animal model are necessary. We modified a human flow cytometric panel reactive antibody assay for ovine model use to detect antibody responses to residual antigen-loading decellularized scaffolds engineered from pulmonary artery tissue.

METHODS

A clinical panel reactive antibody assay was modified with anti-sheep antibodies. Dimethyl sulfoxide cryopreserved (n = 4) and decellularized scaffolds (n = 8) fashioned as patches from pulmonary arteries were implanted for study. Fresh (nonprocessed) tissue implants were used as positive controls (n = 2), and sham-treated animals were used as negative controls (n = 2). Baseline, 10-week, and 20-week blood samples were assayed for panel reactive antibody levels. Immunohistochemistry with anti-major histocompatibility complex antibodies were performed on preimplantation scaffolds.

RESULTS

Chronic implants of fresh tissue stimulated strong panel reactive antibody responses. Classically cryopreserved tissues provoked modest panel reactive antibody responses to major histocompatibility complex I antigen and no response to major histocompatibility complex II antigen. Decellularized tissue scaffolds provoked minimal to no panel reactive antibody responses to either major histocompatibility complex I or II antigen. Immunohistochemistry correlated with the panel reactive antibody results by identifying significant amounts of major histocompatibility complex I and II in fresh tissue, reduced antigen staining in cryopreserved control tissues, and minimal amounts in decellularized tissues.

CONCLUSIONS

These studies with an ovine modified panel reactive antibody assay confirmed minimal immune allosensitization to transplanted decellularized tissue patches. Qualifying criteria for putative tissue-engineered scaffolds should include minimal recipient panel reactive antibody response.

Six-year monitoring of the donor-specific immune response to cryopreserved aortic allograft valves: Implications with valve dysfunction

BACKGROUND

The immune rejection has been anticipated as one of the major causes of allograft aortic valve (AAV) degeneration. The purpose of this study was to prospectively serially measure the magnitude and evolution of the recipient anti-HLA class I antibody response up to 6 years from AAV implant and to correlate serologic data with valve performance by means of a concurrent echocardiographic survey.

METHODS

Cryopreserved AAVs were obtained from multiorgan HLA-typed donors. Nineteen patients younger than 50 years (mean age, 43.3 +/- 8 years) were prospectively studied. After successful surgery, all AAV recipient underwent at 3 and 6 months and each year postoperatively (mean follow-up, 71.9 months) concomitant serum sample collection and two-dimensional transthoracic echocardiography. The presence of anti-HLA antibodies was tested against a panel of lymphocytes obtained from 30 blood donors.

RESULTS

Progressive structural valve deterioration was seen in 6 patients (31.5%) of whom 4 (21%) were reoperated. All pretransplant recipients sera were panel-reactive antibody negative. Seventeen patients (89.4%) demonstrated significant panel-reactive antibody levels, which peaked at 6 months postoperatively, declined from 6 to 24 months, and slowly decreased afterward. In 14 of 19 cases (73.6%) donor-specific HLA antibodies were identified. A strong immunization (6-year persistence of panel-reactive antibody > 70% and peak panel-reactive antibody > 80%) was detected in 31.5% and 36.8% of recipients, respectively. Strong immunization was found to be significantly associated with progressive structural deterioration.

CONCLUSIONS

The immune reaction after cryopreserved AAV implantation is a peculiar long-lasting response occurring in the majority of recipients younger than 50 years of age. An association between a sustained and pronounced immunization and an aggressive AAV degeneration was observed.

Decellularization of rat aortic valve allografts reduces leaflet destruction and extracellular matrix remodeling

OBJECTIVES

Decellularization of aortic valve allografts in advance of transplantation is a promising approach to overcome immune-induced early graft failure. In this study the effects of in vitro cell extraction on extracellular matrix molecules and in vivo remodeling of decellularized aortic valves were investigated in a heterotopic aortic valve rat implantation model.

METHODS

Rat aortic valve conduits were decellularized by a 2-step detergent-enzymatic extraction method involving sodium dodecyl sulfate in combination with RNase and DNase. Cellular and acellular allogeneic (2x, n = 4) and syngeneic valve grafts (2x, n = 3) were grafted infrarenally into the descending aorta for 21 days. Immunohistochemical techniques were used to study extracellular matrix constitution (elastin, collagen, fibronectin, and chondroitin sulfate) and cellular infiltration.

RESULTS

The decellularization procedure resulted in a complete loss of all cellular structures from the entire valve conduit with minimal damage to the extracellular matrix. All transplanted cellular allografts became deformed, swollen, and acellular with major changes in extracellular matrix structure. The transplanted decellularized allografts, however, retained normal preserved valve leaflets comparable to transplanted cellular and acellular syngeneic grafts. With the exception of cellular syngeneic grafts, all other grafts showed retrovalvular thrombi.

CONCLUSIONS

Damage to the valves caused by decellularization technique is much less than the damage caused by the recipient's immune response. In vitro removal of viable cells in (cryopreserved) homografts may decrease graft failure. Seeding with autologous or major histocompatibility complex-matched donor endothelial cells will be necessary to diminish damage induced by an absent blood-tissue barrier.

Early pregnancy factor treatment suppresses the inflammatory response and adhesion molecule expression in the spinal cord of SJL/J mice with experimental autoimmune encephalomyelitis and the delayed-type hypersensitivity reaction to trinitrochlorobenzene in normal BALB/c mice

Early pregnancy factor (EPF) is a secreted protein, present in serum during early pregnancy and essential for maintaining viability of the embryo. It is a homologue of chaperonin 10 (Cpn10) but, unlike Cpn10, it has an extracellular role. EPF has immunosuppressive and growth regulatory properties. Previously we have reported the preparation of recombinant EPF (rEPF) and shown that treatment with rEPF will suppress clinical signs of MBP-EAE in Lewis rats and PLP-EAE in SJL/J mice. In the present study, these findings have been extended to investigate possible mechanisms involved in the action of EPF. Following treatment of mice with rEPF from the day of inoculation, there were fewer infiltrating CD3+ and CD4+ cells in the parenchyma of the spinal cord during the onset of disease and after the initial episode, compared with mice treated with vehicle. Expression of the integrins LFA-1, VLA-4 and Mac-1 and of members of the immunoglobulin superfamily of adhesion molecules ICAM-1 and VCAM-1 was suppressed in the central nervous system (CNS) following rEPF treatment. The expression of PECAM-1 was not affected. To determine if rEPF suppressed T cell activation in the periphery, the delayed-type hypersensitivity (DTH) reaction of normal BALB/c mice to trinitrochlorobenzene (TNCB) following treatment with rEPF was studied. The results showed that treatment with rEPF suppressed the DTH reaction, demonstrating the ability of EPF to downregulate the cell-mediated immune response. These results indicate that suppression of immunological mechanisms by rEPF plays a major role in the reduction of clinical signs of disease in experimental autoimmune encephalomyelitis (EAE).

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.

Aortic valve cusp vessel density: relationship with tissue thickness

OBJECTIVES

The presence of a microvasculature within aortic cusps implies that tissue oxygen requirements exceed the amount deliverable by diffusion from the tissue surfaces alone. For the design of a successful tissue-engineered valve replacement, the effect of diffusion distance (tissue thickness) on oxygen delivery must be considered. We therefore examined in normal aortic valve cusps the relationship between the presence of microvessels and the tissue thickness.

METHODS

Thirty porcine aortic valve cusps were excised and examined after cusp microvessels were pressure filled with a carbon particle solution. Cusp images were captured for stereographic vessel density analysis, and cusp thickness was determined with a radiographic technique. Histologic cross-sections were evaluated to determine vessel depth from the cusp surface.

RESULTS

Cusp basal regions measured 0.69 to 0.86 mm in thickness, significantly thicker (P =.001) than the rest of the cusp, which measured 0.36 to 0.48 mm. In general a vascular bed was present when cusp thickness exceeded 0.5 mm, with a median value of 5.16 vessels/mm(3).

CONCLUSIONS

From published values of arterial wall oxygen consumption and diffusivity, we predicted that the probable maximum oxygen diffusion distance for valve tissue would be about 0.2 mm. This was consistent with our physical findings, which implies that central tissue anoxia is avoided by the capillary bed. An avascular tissue-engineered valve metabolically similar to an aortic valve should therefore not exceed a thickness of approximately 0.40 mm.

Primary aortic valve replacement with cryopreserved aortic allograft: an echocardiographic follow-up study of 570 patients

BACKGROUND

Despite the many advantages of an aortic allograft valve (AAV) over a prosthetic aortic valve, its durability is suboptimal. The aims of the present study were to document characteristic features of AAV dysfunction and to investigate factors influencing the development of such dysfunction.

METHODS AND RESULTS

A group of 570 patients (mean age, 48+/-16 years) with a cryopreserved AAV underwent a follow-up echocardiographic study (mean time after surgery, 6.8 years; range, 1.0 to 22.9 years). Significant AAV regurgitation was present in 14.7% of patients, and AAV stenosis was present in 3.2%. The root replacement subgroup had the smallest number of patients with significant AAV regurgitation (5.0%) compared with the subcoronary (23.0%) or the inclusion cylinder technique subgroup (14.7%). After 10 to 15 years after AAV replacement, grade > or =2 AAV dysfunction was present in 40% of patients in the subcoronary subgroup, but no significant dysfunction was observed in patients in the root replacement subgroup (P<0.001). Smaller host aortic annulus size in both subcoronary (coefficient, -0.145; P=0.013) and root replacement subgroups (coefficient, -0.249; P=0.011) was associated with more frequent AAV dysfunction (grade > or =2). In addition, significant AAV dysfunction was more frequent when patients were younger (coefficient, -0.020; P=0.015) in the subcoronary subgroup and the donor was older (coefficient, 0.054; P=0.019) in the root replacement subgroup.

CONCLUSIONS

The present study indicates that the root replacement technique is associated with less frequent AAV degeneration. Our findings should help in establishing more strict selection criteria for surgical replacement procedure type and patient/donor factors for AAV replacement and, therefore, could lead to improve AAV longevity.

Increased numbers of circulating donor-specific T helper lymphocytes after human heart valve transplantation

Implantation of cryopreserved human donor heart valves for either congenital or acquired cardiac disease has been performed since the last three decades. Although the clinical outcome is good, long-term valve degeneration resulting in dysfunction has been observed. A specific immunological response of the recipient against the allograft has been proposed as one of the factors involved in this process. Helper T lymphocytes play an important intermediate role in cellular and humoral immune response. Increasing numbers of circulating donor-specific helper T lymphocytes precursors (HTLp) correlate with graft rejection after organ transplantation. To investigate whether cryopreserved human donor heart valves are able to induce a donor-specific T helper response, we monitored the HTLp frequencies (HTLpf) in peripheral blood samples of 13 patients after valve allograft transplantation by use of a limiting dilution assay followed by an interleukin-2 bioassay. Prior to transplantation, HTLpf specific for donor and third-party antigens showed individual baseline levels. After allografting, the antidonor frequencies significantly increased in 11 of the 13 patients (P = 0.02). This was not found for stimulation with third-party spleen cells (P = 0.68), which indicates a donor-specific response. Maximal donor-specific HTLpf were already found at 1--2 months after operation. Valve allograft transplantation induces an increase in the numbers of donor-specific HTLp in peripheral blood of the patients. Analogous to organ transplantation, these HTLp may play a crucial role in events that lead to valve damage. Therefore, monitoring of HTLp in peripheral blood samples might be informative for donor valve degeneration (rejection) and subsequently valve allograft failure.

Circulating donor-specific cytotoxic T lymphocytes with high avidity for donor human leukocyte antigens in pediatric and adult cardiac allograft valved conduit recipients

OBJECTIVE

Specific immunological responses may be involved in the process of cryopreserved allograft valved conduit (AVC) degeneration, which is more frequently seen in young recipients. Rejection of heart and corneal allografts is preceded by an increase in the fraction of cytotoxic T lymphocytes (CTL) with high avidity for donor human leukocyte antigens (HLA) circulating in both peripheral blood and the affected graft. These donor-specific high-avidity CTLs are regarded as the destructive cells capable of causing graft damage. To monitor the precursors of these cells (CTLp) in young and adult AVC recipients, in vitro quantitative tests were performed on sequentially taken blood samples to quantitate CTLp frequencies and their avidity for donor antigens.

METHOD

Six children and nine adults who received a cryopreserved AVC in the period between 1994 and 1997 were included in the study. From these patients, two to six blood samples were obtained up to 3 years after valve implantation. The number of circulating CTLp present within the peripheral blood mononuclear cell (PBMC) population was determined by limiting dilution analysis (LDA). The fraction of CTLp with high avidity for donor HLA class I was determined by addition of CD8 monoclonal antibodies (mAb) during the cytotoxic phase of the assay. Third-party stimulator cells were used to verify the donor-specificity of the response.

RESULTS

The number of donor-specific CTLp increased significantly in the period 6-12 months after AVC implantation, while third-party-specific CTLp frequencies were not affected. Additionally, we found a significant increase of the high-avidity fraction of CTLp directed against donor antigens as early as during the first 6 months after AVC implantation. The fraction of high-avidity CTLp remained significantly higher post- compared with pre-implantation, even after 12 months. We observed no significant difference in the kinetics of CTLp frequencies between pediatric and adult AVC recipients.

CONCLUSION

Implantation of cryopreserved human AVC induces an increase in the total number of circulating CTLp directed against donor HLA class I in both adults and children. The shift towards more destructive high-avidity CTLp in the peripheral blood indicates their potential damaging effect towards the heart valve allograft.

T lymphocytes mediate leaflet destruction and allograft aortic valve failure in rats

BACKGROUND

Allograft heart valves are commonly used in cardiac surgery but ultimately fail. This situation is most acute in children. This study addresses the role of T cell-mediated immune damage in allograft valve failure.

METHODS

Syngeneic (Lewis to Lewis) or allogeneic (Brown Norway to Lewis) aortic valve grafts were implanted infrarenally into Lewis rat recipients (n = 24). Allogeneic valve grafts were also implanted into T cell-deficient rats (nude; n = 12). At 7, 14, and 28 days the valves were explanted and examined for structural integrity and cellular infiltration.

RESULTS

Syngeneic grafts maintained normal leaflet structure with little leaflet immune infiltration. Allografts showed leaflet infiltration (7 days), significant leaflet thickening, progressively decreased cellularity (14 days), and leaflet destruction (28 days). Infiltrates contained CD43+, CD3+, and CD8+ cells. Allografts in T cell-deficient rats showed none of the above changes and maintained normal structural integrity.

CONCLUSIONS

Allograft heart valves in the rat model undergo T cell-mediated immune rejection, resulting in structural failure.

A size-matching heterotopic aortic valve implantation model in the rat

BACKGROUND

Structural failure of cardiac valve allografts may be related to technical factors such as size mismatch, resulting in early intimal proliferation and fibrosis or immunological reactions against the transplanted valves, featuring lymphocytic infiltration.

OBJECTIVE

To develop a heterotopic aortic valve implantation model in the rat to study the immunological factors leading to graft failure in the setting of a technical adaptation for size mismatch.

METHODS

Syngeneic (WAG-WAG or DA-DA) and allogeneic (WAG-BN or WAG-DA) rat strain combinations were used to study the effect of the allogeneic response on valve properties. An end-to-side anastomosis was made between the U-shaped aortic root graft and the recipient's abdominal aorta to resolve the problems of size matching.

RESULTS

No animals suffered from ischemic or neurological complications during the study period. One hundred percent survival and patency of the aortic grafts were achieved at the end of a 21-day observation period. In the syngeneic group 9 of 10 valves were still competent when assessed during retrograde injection. In contrast, 2 of 10 allogeneic valve grafts were competent on postoperative Day 21. Microscopic evaluation revealed no fibrosis or intimal thickening in the syngeneic valve grafts while the allogeneic valve grafts demonstrated rejection-like morphology.

CONCLUSION

The absence of fibrosis and intimal thickening in the syngeneic transplanted valve grafts indicates that this implantation model is not influenced by nonimmunological-based structural changes. Therefore, this new model enables us to study the association between donor-directed immune responses and allograft degeneration in a technically unbiased manner.