Inhibition of adenosine deaminase and nucleoside transport

Interstitial cellular and matrix restoration of cardiac valves after cryopreservation

OBJECTIVES

We previously characterized the porcine aortic leaflet interstitial cell phenotype as having both synthetic and contractile characteristics; that is, it is a myofibroblast. In this study we hypothesized (1) that the cryopreservation of aortic valves causes a significant reduction in cell density, (2) that it simultaneously causes alterations in representative components of extracellular matrix, and (3) that both of these processes are reversible.

METHODS

Seventy-two leaflets from 24 porcine aortic valves were studied. Whole valves were subjected to variable lengths of preharvest ischemia (group 1), ischemia followed by processing analogous to clinical methods (group 2), and ischemia followed processing with an organ culture type of resuscitation (group 3). Vital dye exclusion by cells enzymatically dispersed from leaflets was used to quantify viability. Electron and light microscopy, immunohistochemical assay, and a silicone rubber substratum contractility assay were used both in dispersed cell preparations and in leaflet cross sections to examine structural, ultrastructural, and functional changes across the 3 groups through a range of preharvest ischemic times.

RESULTS

Results indicated that harvest ischemic periods between 2 and 24 hours after donor death were not responsible for cell number reductions. During this interval overt dissolution of chondroitin sulfate simultaneous with a relative sparing of fibronectin was evidenced by immunohistochemical staining. Although not reduced in number, ischemic interstitial cells did show significant ultrastructural evidence of injury and suppressed monoclonal binding to vimentin and alpha-smooth muscle actin. After cryopreservation, viable cell numbers were always markedly reduced at all ischemic intervals and damage to both soluble extracellular matrix components and cell ultrastructure was increased. At all time and processing points, however, some retention of matrix secretory and cellular contractile capabilities was observed among the surviving cells. After the extended periods of preharvest ischemia (2-24 hours) followed by processing, a restitution of functioning cells was accomplished by means of whole-leaflet incubation in 15% fetal bovine serum.

CONCLUSIONS

After application of the described methods, new cells within restored intact leaflets as well as in single-cell preparations demonstrated normal ultrastructure and contractile and synthetic functions (normal phenotypic expression). If functioning leaflet interstitial cells can contribute to homograft durability, bioengineering methods for pretransplantation cell repopulation could be refined with these techniques and applied to clinical valve transplantation.

Metabolic detriment in donor heart valves induced by ischemia and cryopreservation

BACKGROUND

The injury resulting from postmortem ischemia is a critical deterrent to the availability of donor valves. Using the reduction of XTT-tetrazolium salt as a marker of metabolic sequelae, we assessed the injurious effect of ischemia and the metabolic sequelae in 156 porcine semilunar leaflets.

METHODS

The leaflets were randomly allocated to noncryoprocessed (n = 72) or cryoprocessed (n = 72) groups. At each preservation temperature of 4 degrees C, 24 degrees C, or 37 degrees C, 24 leaflets each were exposed to one of four storage periods of 9, 17, 30, or 60 hours. Twelve fresh aortic leaflets served as baseline reference samples.

RESULTS

There was a progressive loss in the metabolic functioning of valve leaflet cells in both noncryopreserved and cryopreserved tissue as the storage times increased. Cryopreserved tissue showed a greater loss of function than noncryopreserved tissue did. The metabolic injury was mainly a consequence of cryoprocessing. The greatest loss in metabolic functioning occurred in the valves stored for 60 hours. The least favorable combination of variables was cryopreservation and a precryopreservation storage time of 60 hours.

CONCLUSIONS

We conclude that 30- to 60-hour delays do not have a significant metabolic effect on cardiac leaflets. Thus it may be possible to safely extend the permissible ischemic periods after organ harvest.

Effect of cryopreservation and histocompatibility on type I procollagen gene expression in aortic valve grafts

BACKGROUND

Allograft valves are excellent substitutes for diseased or absent valves but undergo primary tissue degeneration. Fibroblast viability may determine resistance to valve deterioration. This study evaluated gene expression for procollagen by valve grafts and studied the effects of cryopreservation and histocompatibility on this property.

METHODS AND RESULTS

Fresh and cryopreserved rat aortic valves were implanted heterotopically into syngeneic or allogeneic recipients. Nonviable, cryothermally injured valves were used as negative controls. The grafts and native aortic roots were excised 3 days after implantation. Northern hybridization with a human procollagen alpha 1 (I) complementary DNA probe was used to assess the expression of type I procollagen mRNA. The content of procollagen mRNA relative to 18S ribosomal RNA was evaluated by means of scanning densitometry. In situ hybridization was used to locate the areas of procollagen mRNA expression in the grafts. Both fresh and cryopreserved grafts exhibited greater expression than the native valve. This increase in expression was observed in both syngeneic and allogeneic grafts, but not in the negative control group. In situ hybridization showed a strong signal for procollagen in the aortic wall and a weak signal in the leaflet and myocardium in the viable grafts and in native tissues.

CONCLUSIONS

Regardless of preservation or allogenicity, fibroblast viability in aortic valve grafts persists after implantation. Increased gene expression for procollagen suggests a capacity for repair and regeneration.

Structure-function correlations in cryopreserved allograft cardiac valves

This communication briefly summarizes a morphologic investigation of explanted cryopreserved heart valves and discusses the findings in the context of ongoing debates regarding modes of failure, cellular viability, durability of the extracellular matrix, and the contribution of immune responses. We studied 20 cryopreserved human heart valve allografts functioning up to 9 years as either orthotopic aortic valves/root replacements or right ventricle-to-pulmonary artery conduits explanted for valve failure, infection, or growth-related conduit or valve stenosis. Implanted valves had progressively severe loss of normal layered structure and were devoid of stainable deep connective tissue cells. Inflammation was minimal. Other late findings included minimal inflammation, mild cuspal hematomas, mural thrombus, and calcification, most prominent in the aortic wall. Transmission electron microscopy of late explants revealed nonviable cells and their debris, and a collagenous skeleton that was largely intact. We conclude that cryopreserved allograft heart valves have minimal, if any, viable cells, but largely retain the original collagen network; preservation of the autolysis-resistant collagenous skeleton likely provides the structural basis of function. Our results also suggest that immune responsiveness has little, if any, impact on late allograft function or degradation.

In situ hybridization: a new technique to determine the origin of fibroblasts in cryopreserved aortic homograft valve explants

Tissue degeneration reduces the durability of cryopreserved homografts. Earlier studies indicated that the presence of fibroblasts in homograft leaflets may contribute to increased valve longevity. These fibroblasts may be of recipient origin or represent surviving donor cells. We developed a method, based on in situ hybridization, to determine the origin of fibroblasts in homograft explants. In young pigs we performed aortic valve replacement with a cryopreserved porcine aortic homograft. A male homograft was implanted in a female pig, whereas two male recipients received a female homograft. After 3 to 4 months the homografts were explanted. Frozen sections were made and alternately examined with hematoxylin-eosin staining and in situ hybridization. With a biotinylated porcine Y chromosome-specific deoxyribonucleic acid probe, male fibroblasts could be clearly distinguished from female fibroblasts. In all leaflets we observed both donor and recipient fibroblasts. The distribution of these populations was marked in schematic drawings. Recipient fibroblasts mostly spread onto the leaflet surface but also penetrated the leaflet tissue. Remaining donor fibroblasts did not show morphologic signs of decreased viability on hematoxylin-eosin staining. In situ hybridization may become a useful technique in homograft research. In this porcine model, the fibroblasts in the aortic homograft explants were of both donor and recipient origin.