Endothelial cell replication in an in vivo model of aortic allografts

Characterization of decellularized implants for ECM integrity and immune response elicitation

Biological scaffold is a popular choice for the preparation of tissue-engineered organs and has the potential to address donor shortages in clinics. However, biological scaffolds prepared by physical or chemical agents cause damage to the extracellular matrix by potentially inducing immune responses after implantation. The current study explores the fate of the decellularized scaffolds using a cocktail of chemicals following implantation without using immunosuppressants. Using the syngeneic (Lewis male- Lewis female) and allogeneic (Brown Norway male- Lewis female) models and different tissue routes (subcutaneous vs omentum) for implantation, we applied in-depth quantitative proteomics, genomics along with histology and quantitative image analysis tools to comprehensively describe and compare the proteins following decellularization and post-implantation. Our data helped to identify any alteration post decullarization as well implantation. We could also monitor route-specific modulation of the Extracellular matrix (ECM) and regulation of the immune responses (macrophage and T cells) following implantation. The current approach opens up the possibility to monitor the fate of biological scaffolds in terms of the ECM and immune response against the implants. In addition, the identification of different routes helped us to identify differential immune responses against the implants. This study opens up the potential to identify the changes associated with chemical decellularization both pre and post-implantation, which could further help to promote research in this direction.

Comparison of healing in fresh and preserved arterial allografts in the dog

The use of aortic allografts for the management of vascular prosthetic infections has recently been reintroduced. Impressive results have been obtained; however, the possibility of late degeneration remains a major concern. The healing behavior of aortic allografts, either fresh or preserved, in antibiotic-supplemented nutrient medium at 4 degrees C for 1 week and used as thoracic aorta substitutes in dogs was investigated after 6 months of implantation. Four dogs received a fresh aortic allograft from four different donors, and four dogs received a preserved allograft from two different donors. Autografts in two dogs were performed as controls. The in vivo investigation was conducted to describe (1) the histological characteristics of the arterial wall, (2) the macroscopic and thrombogenic aspect of the luminal surface, (3) the integrity of the endothelial lining by scanning electron microscopy, and (4) its biochemical function by prostacyclin (PGI2) and thromboxane A2 (TXA2) secretion. Immune-mediated reactions directed toward the grafts were measured by sequential screening of donor-specific serum antibody development. All donor-recipient pairs of dogs were major histocompatibility complex (MHC)-incompatible according to a mixed lymphocyte reaction (MLR) assay. From the results of this study we concluded that although preserved arterial allografts exhibited similar surface characteristics as those of fresh allografts in terms of re-endothelialization and long-term graft function, an elicited immune response, a degenerative process in the media, and a hyperplasic reaction in the intima could not be prevented using this method of preservation.

Histologic and immunohistochemical responses after aortic valve allografts in the rat

BACKGROUND

Human aortic valve allografts elicit a cellular and humoral immune response. It is not clear whether this is important in promoting valve damage. We investigated the changes in morphology, cell populations, and major histocompatibility complex antigen distribution in the rat aortic valve allograft.

METHODS

Fresh heart valves from Lewis rats were transplanted into the abdominal aorta of DA rats. Valves from allografted, isografted, and presensitized recipient rats were examined serially with standard morphologic and immunohistochemical techniques.

RESULTS

In comparison with isografts, the allografts were infiltrated and thickened by increased numbers of CD4+ and CD8+ lymphocytes, macrophages, and fibroblasts. Thickening of the valve wall and leaflet and the density of the cellular infiltrate was particularly evident after presensitization. Endothelial cells were frequently absent in presensitized allografts whereas isografts had intact endothelium. Cellular major histocompatibility complex class I and II antigens in the allograft were substantially increased. A long-term allograft showed dense fibrosis and disruption of the media with scattered persisting donor cells.

CONCLUSIONS

The changes in these aortic valve allograft experiments are consistent with an allograft immune response and confirm that the response can damage aortic valve allograft tissue.

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.

Mechanisms underlying degeneration of cryopreserved vascular homografts

OBJECTIVE

To analyze the mechanism(s) underlying homograft degeneration, we designed an experimental model in which the behavior of cryopreserved autografts and homografts, as well as fresh autografts, implanted in the same animal was compared.

METHODS

A cryopreserved homograft was implanted in the aorta of 14 sheep. The excised aortic autologous segment was then subjected to cryopreservation, and 1 to 8 weeks later it was implanted 1 to 2 cm below the cryopreserved homograft. The intermediate segment of the native aorta, the fresh autograft, was dissected at this point. Animals were put to death at different times and the implanted segments were harvested together with a portion of native aorta. Histologic and immunohistochemical analyses, as well as cell viability assessments, were then performed on the explanted segments. Similar studies were also conducted on fragments of cryopreserved autografts and homografts before implantation.

RESULTS

With the exception of a partial loss of the endothelium, cryopreserved specimens retained cell viability and morphologic integrity before implantation. Explanted cryopreserved homografts showed profound changes affecting all strata, as well as a decline in cell viability. Lymphocyte infiltrates were found up to 12 months after implantation. Endothelium was always absent in cryopreserved homografts. However, a reendothelialization of the cryopreserved autografts was observed. After an initial period of neuronal degeneration, reenervation of the cryopreserved autograft segment occurred 6 to 12 months after the operation. Findings regarding the fresh autografts were similar to those of the cryopreserved autografts.

CONCLUSION

Our results suggest that the immunologic reaction rather than the cryopreservation process is responsible for the degenerative process occurring in cryopreserved homografts.

Procollagen production in fresh and cryopreserved aortic valve grafts

Long-term durability of aortic valve allografts may be enhanced by cellular capacities for regeneration and repair. To evaluate aortic valve graft production of an important structural protein, rat aortic roots were implanted heterotopically into the abdominal aorta of recipient rats. Grafts were either syngeneic or strongly allogeneic, were implanted either fresh or after cryopreservation, and were left in place 2 to 21 days after implantation. A total of 80 aortic valve grafts and the corresponding native aortic valves were examined. The grafts were retrieved and immunocytochemically stained for the presence of procollagen, a precursor to collagen. Regardless of histocompatibility or preservation, grafts exhibited consistent procollagen presence that equaled or exceeded that seen in the corresponding native valves. Positive procollagen staining was predominantly in the aortic wall. The most prominent staining was near the hinge point of the valve leaflets, with no staining in the free portion of the leaflets. Staining with alpha-actin demonstrated vascular smooth muscle in sites remote from the areas positive for procollagen, which suggests that vascular smooth muscle was not responsible for the procollagen production. These findings indicate that cryopreservation is compatible with persistent fibroblast viability and in vivo protein synthesis by both syngeneic and allogeneic aortic valve grafts.

A comparison of failure modes of glutaraldehyde-treated versus antibiotic-preserved mitral valve allografts implanted in sheep

Morphologic studies and calcium analyses were made on mitral valve allografts from 12 juvenile sheep surviving 12 to 24 weeks after mitral valve replacement. Before implantation, the allografts were treated with 0.625% glutaraldehyde (group I, n = 4) or with cold antibiotic solution (group II, n = 8). Three group I animals died 12 to 19 weeks after implantation because of dysfunction of calcified valves; the surviving animal also had extensive allograft calcification. One group II animal died of mitral regurgitation; the valves of the other seven (including five with regurgitation shown by Doppler and ventriculographic studies) were explanted at 19 to 24 weeks. Chordal rupture related to calcific deposits was found in all group I valves. Leaflet perforations (n = 4) and ruptured chordae (n = 4), each caused by connective tissue deterioration, were found in group II valves. Inflammatory reaction was absent or minimal in group I valves but moderate or severe in group II valves. Fibrous sheaths were thicker in group II than in group I valves. Calcium levels were much higher in group I than in group II valves. Calcification in group I valves was diffuse and involved collagen, elastic fibers, and connective tissue cells and matrix; in group II valves, it was localized in connective tissue cells. Thus glutaraldehyde-treated allografts failed because of extensive calcification, whereas antibiotic-preserved allografts underwent deterioration of connective tissue and infiltration by inflammatory cells.