Ann Thorac Surg 2005;79:1755-1758
© 2005 The Society of Thoracic Surgeons
Case report
Histopathologic Findings in a Novel Decellularized Pulmonary Homograft: An Autopsy Study
Friedhelm Sayk, MDa,*,
Inge Bos, MDa,
Ute Schubert, BSb,
Thilo Wedel, MDc,
Hans-H. Sievers, MDb
a Institute of Pathology, University of Luebeck, Luebeck, Germany
b Clinic for Cardiovascular Surgery, University of Luebeck, Luebeck, Germany
c Institute of Anatomy, University of Luebeck, Luebeck, Germany
Accepted for publication November 7, 2003.
* Address reprint requests to Dr Sayk, Institute of Pathology, University of Luebeck, Ratzeburger Allee 160, 23538 Luebeck, Germany
friedhelm.sayk{at}innere1.uni-luebeck.de
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Abstract
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Innovative valved conduits for reconstruction of the right ventricular outflow tract are desirable to overcome the risk of valve deterioration of conventional homografts. This study describes the histopathology of a novel decellularized pulmonary homograft (SynerGraft) implanted in the right ventricular outflow tract of a 60-year-old man 5 weeks before death. The histomorphology of this decellularized homograft showed integrity of its extracellular matrix and a gradual cellular infiltrate consisting predominantly of macrophages, resembling an early nonspecific inflammatory phase of recellularization without any signs of a specific immunologic interference 5 weeks after implantation. Whether this morphologic appearance precedes the desired repopulation with autologous fibroblasts remains to be established.
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Introduction
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Reconstruction of the right ventricular outflow tract (RVOT) using a valved substitute remains a major challenge in cardiovascular surgery. Conventional cryopreserved homografts are at risk for chronic immunologic rejection and structural valve deterioration [1–3]. A novel decellularized homograft (SynerGraft; CryoLife, Kennesaw, GA) offers the perspective of autologous recellularization and thus enhanced durability without immunologic interference [4]. SynerGrafts are allogenic pulmonary valve transplants treated by a proprietary process to substantially reduce valve cellularity (Fig 1), thereby diminishing antigenic tissue components and, hypothetically, leaving an ideal scaffold for infiltration and colonization with host derived cells. Whereas results in animal experiments are encouraging [5, 6], clinical experience is still rare. This is the first histopathologic assessment of a SynerGraft in humans.
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Fig 1. SynerGraft tissue before implantation showing a cell-free but structurally intact matrix (hematoxylin & eosin [HE] stain).
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We describe a 60-year-old man with congenital pulmonary stenosis, who underwent pulmonary comissurotomy in 1969 and implantation of a conventional cryopreserved pulmonary homograft in 1996. Owing to recurrent pulmonary valve insufficiency, reconstruction of the RVOT using a SynerGraft was performed recently. After implantation, the hemodynamic function of the SynerGraft as evaluated by transesophageal echocardiography was excellent without signs of stenosis or insufficiency. However, the patient died of bronchopneumonia 5 weeks postoperatively. At autopsy, the SynerGraft did not reveal any sign of valvular disease (no stenosis, incompetence, dehiscence, vegetations, tears, or abrasions); the leaflets were pliable, coaptive, and without thickening (Fig 2).
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Fig 2. Macroscopic aspect of the right ventricular outflow tract showing the dorsal part of the pulmonary trunk with the distal (arrow) and proximal (asterisk) suture line of the SynerGraft (formalin-fixation).
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To characterize the cellular composition and extracellular matrix of the SynerGraft, standard histochemical stainings were applied on paraffin sections, including hematoxylin-eosin (HE), elastic van Gieson (EVG), Masson's trichrome stain, and naphthol-AS-D-chloracetatesterase (ASD). Immunohistochemical labeling was performed using antibodies against T and B lymphocytes (OPD4 and CD20), hematopoetic progenitor and endothelial cells (CD34), dendritic cells (S100), macrophages (CD68), vimentin (Vim), and 
-smooth muscle actin (-SMA).
The extracellular matrix of the SynerGraft leaflet, sinus, and pulmonary artery wall showed marked edema without any degenerative changes of the extracellular matrix (eg, elastic fragmentation or fibrosis) according to de Sa and coworkers [7]. The cellularity of the SynerGraft sinus and pulmonary artery wall was sparse, with the free edge of the leaflet being completely acellular. Immunohistochemical characterization disclosed that virtually all cells detected in the SynerGraft were either macrophages (CD68) or polymorphonuclear cells (PMN [ASD]; Fig 3). Both macrophages and PMN formed a thin cellular layer covering the luminal surface of the leaflet, sinus, and pulmonary wall with focal gradual infiltration into the stroma. Moreover, immigration of PMN and spindle-shaped macrophages into the SynerGraft was observed at the distal and proximal suture line with focal transformation into foam cells resorbing remnants of nonviable donor myocardium. No -SMA-immunoreactive cells, dendritic cells (S100), progenitor cells/stem cells, or neovascularization of the SynerGraft (CD34) were found either in the leaflet or in the sinus or the pulmonary artery wall of the SynerGraft. In particular, neither T lymphocytes (OPD4) nor B lymphocytes (CD20) were detected within the SynerGraft. The valve sinuses and leaflets were free of thrombotic aggregates. In the native pulmonary artery wall, cellularity and extracellular matrix composition were normal with abundant smooth muscle cells and only moderate intimal fibrosis.
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Fig 3. SynerGraft: pulmonary sinus showing gradual recellularization from the lumen, consisting of (A) polymorphonuclear cells (ASD) and (B) macrophages (CD68).
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Comment
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With the exception of the pulmonary autograft, no other heart valve graft is currently viable after long-term implantation [1, 5]. Cryopreserved allograft valves are suspected of eliciting an immunologic response with infiltration of lymphocytes [3] and serologic elevation of antibodies [8, 9]. This process may result in destruction of the cellular components and fibrotic changes of the extracellular matrix of the graft, as previously demonstrated by Goffin and associates and other authors [1–3, 10, 11].
A novel decellularized homograft (SynerGraft) offers the perspective of autologous recellularization and thus enhanced durability without immunologic interference. In animal experiments, the recellularization process of decellularized SynerGrafts was reported by Elkins and colleagues [5] to evolve in two distinct phases, starting with a nonspecific inflammatory reaction characterized by an infiltration of macrophages during the first few months, and followed by colonization with fibroblasts. In the present case, the patient died from nonvalve-related disease 5 weeks after implantation of the SynerGraft. In accordance with the time course described by Elkins and colleagues [5] macrophages and PMN, but no progenitor, fibroblastic, or -SMA-immunoreactive cells were found in the leaflet attachment, the sinus, and the pulmonary artery wall of the SynerGraft. Infiltration with autologous macrophages and PMN in the absence of infective endocarditis was also observed in cryopreserved allografts 30 days after implantation in a sheep model [11]. However, that finding is in contrast to findings in human cryopreserved allografts at a similar interval which were characterized by extreme loss of interstitial cells and, notably, absence of any inflammatory infiltrate [10].
In accordance with animal studies, we could not detect a histomorphologic correlate of a specific immunologic cellular response against the SynerGraft. That the SynerGraft had only been in place for 5 weeks, however, does not allow conclusions about its long-term resistance to immunologic attack. In contrast to the results of other investigators [5], we could demonstrate that repopulating cells did not only migrate into the SynerGraft from the adjacent host tissue; instead, the valve was also infiltrated by inflammatory cells forming a cellular gradient from the luminal surface toward the valvular matrix (Fig 3), thereby indicating their blood-borne and, thus, recipient-derived origin.
Further investigation is necessary to determine long-term SynerGraft cellularity, its resistance to immunologic interference, and its hemodynamic longevity.
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References
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Abstract
Introduction
