Heterologous antigenicity induced in human aortic homografts during preservation

Guided tissue regeneration in heart valve replacement: from preclinical research to first-in-human trials

Heart valve tissue-guided regeneration aims to offer a functional and viable alternative to current prosthetic replacements. Not requiring previous cell seeding and conditioning in bioreactors, such exceptional tissue engineering approach is a very fascinating translational regenerative strategy. After in vivo implantation, decellularized heart valve scaffolds drive their same repopulation by recipient's cells for a prospective autologous-like tissue reconstruction, remodeling, and adaptation to the somatic growth of the patient. With such a viability, tissue-guided regenerated conduits can be delivered as off-the-shelf biodevices and possess all the potentialities for a long-lasting resolution of the dramatic inconvenience of heart valve diseases, both in children and in the elderly. A review on preclinical and clinical investigations of this therapeutic concept is provided with evaluation of the issues still to be well deliberated for an effective and safe in-human application.

Effects of storage temperature and fetal calf serum on the endothelium of porcine aortic valves

Endothelial integrity and function may be an important determinant for long-term success of allograft heart valves. To determine the optimal storage temperatures for preservation of long-term endothelial function in porcine aortic valves, different storage temperatures and times were investigated. Fresh valves were either (1) stored at 4 degrees C, with or without 10% fetal calf serum supplement, for 1, 2, 4, 7, 14, 21, or 28 days; (2) cryopreserved for 2, 4, or 8 weeks at -80 degrees C or -170 degrees C; (3) cryopreserved in long-term storage (as long as 1 year), with or without fetal calf serum, at -170 degrees C. Viability of endothelial cells was assessed through measurement of the production of prostacyclin in basal and bradykinin-stimulated conditions, during in vitro incubation of the valve cusps at 37 degrees C. Endothelial morphologic variations in valves stored at 4 degrees C were evaluated by scanning electron microscopy. With storage at 4 degrees C, after 4 days the valves already produced significantly less (p < 0.05) prostacyclin than fresh preparations in both basal (0.21 +/- 0.04 versus 3.56 +/- 0.03 ng.ml-1.cm-2) and stimulated conditions (4.17 +/- 0.36 vs 24.23 +/- 1.83). Morphologic changes could not yet be distinguished with scanning electron microscopy at that time. When the storage period was extended, the levels of prostacyclin further diminished; after 14 days, prostacyclin release could no longer be detected. In cryopreserved valves, prostacyclin production was similar for as long as 2 weeks of storage either at -80 degrees C or at -170 degrees C in basal (2.69 +/- 0.63 vs 2.93 +/- 0.51) and stimulated (16.43 +/- 3.19 vs 16.50 +/- 2.57, = 6) conditions. After 8 weeks, no prostacyclin release could be detected in valves stored at -80 degrees C. After 6 months storage at -170 degrees C, the prostacyclin production was significantly (p < 0.05) reduced compared with fresh valves; it then remained constant for as long as 1 year. The valves stored with fetal calf serum produced significantly (p < 0.05) less prostacyclin than did those without fetal calf serum. For longer cryopreserved banking, we recommend storing heart valves at -170 degrees C instead of at -80 degrees C to maintain viability of endothelial cells. Fetal calf serum would harm endothelial viability during long-term cryopreservation.

Effect of immunological differences on rat aortic valve allograft calcification

Calcification may be a cause of allograft valve degeneration. To determine whether immunological differences between donor and recipient affect the degree of calcification that occurs, adult Lewis rats received aortic valve allografts transplanted heterotopically into the abdominal aorta. All valves were transplanted immediately after harvest. The valves were not exposed to antibiotics or albumin before insertion. Valve donors were of the Lewis (syngeneic), F344 (weakly allogeneic, RT1 compatible, non-RT1 incompatible), LBN F1 (moderately allogeneic, one haplotype identical, one haplotype incompatible at the RT1 and non-RT1 loci), and Brown Norway (strongly allogeneic, RT1 and non-RT1 incompatible) strains. Valves were harvested 3-12 weeks following transplantation. Scanning electron microscopy and energy dispersion x-ray microanalysis were performed on one leaflet of each valve to evaluate calcium content. Calcium content expressed in counts (mean +/- standard error) according to donor strain were: Lewis, 1642 +/- 233; F344, 4853 +/- 1412; LBN F1, 4714 +/- 823; and Brown Norway, 4358 +/- 835. Significant differences (p less than 0.05) existed between valves from Lewis donors and those from each other strain. No differences among the other strains were statistically significant. It is concluded that syngeneic valve allografts calcify less than allogeneic grafts. However, the degree of allogenicity did not influence the magnitude of calcification.

Immunogenicity, antigenicity, and endothelial viability of aortic valves preserved at 4 degrees C in a nutrient medium

In patients undergoing aortic valve replacement, allograft valves stored at 4 degrees C in a nutrient medium have been associated with excellent immediate and long-term results. The effects of this method of prolonged storage on the antigenic, immunological and cellular characteristics of these grafts are incompletely understood. This study was designed to study these phenomena in rat aortic valves subjected to antibiotic sterilization and stored for up to 3 weeks in RPMI containing 10% fetal calf serum. Selected valves from Brown Norway rats were implanted heterotopically into the abdominal aorta of Lewis rats. Other valves were studied prior to transplantation. Antigenicity was determined by immunocytochemical staining using monoclonal mouse antibodies directed at Class I and Class II rat antigens. Immunogenicity was determined by duration of second-set skin graft survival following heterotopic aortic valve implant. Endothelial cell viability was determined by flow cytometric analysis of endothelial cells harvested from aortic valve allografts by collagenase digestion. Only fresh valves and valves stored for 1 day were positive for Class I antigens; no valves were positive for Class II antigens. Duration of skin graft survival was prolonged with greater duration of storage, but grafts remained immunogenic after 21 days of storage. Endothelial cell viability declined from 95% in the fresh valves to 64% after 21 days of storage. With prolonged duration of allograft valve storage at 4 degrees C, there is an attenuation of antigenicity, immunogenicity, and endothelial cell viability. Loss of endothelial cells may contribute to the changes in immunological responses to the valve allografts. The expression of antigens on the endothelial surface is not a reliable predictor of immunological response.

Endothelial cell viability in the rat aortic wall

Aortic allografts may offer advantages over prosthetic materials for aortic valve replacement or reconstruction with a valved aortic conduit. Cellular viability may partly determine long-term allograft performance. To evaluate endothelial cell viability in a rat model, valved aortic conduits were subjected to collagenase digestion. The resulting endothelial cell suspension was labeled with Griffonia simplicifolia agglutinin-fluorescein isothiocyanate (GSA-FITC), a marker specific for vascular endothelial cells of the rat. The cells were then incubated with propidium iodide, which is excluded by viable cells. Flow cytometry evaluated endothelial cell viability by determination of percentage of GSA-FITC-positive cells that were negative for propidium iodide. Aortas were studied immediately after harvest or after storage at 4 degrees C in a nutrient medium for 3 to 21 days. Percentage of viable endothelial cells showed a progressive decline with increasing duration of storage. These results demonstrate flow cytometric measurement of endothelial cell viability, a factor of possible importance in assessing allograft storage methods, and show that endothelial viability declines with prolonged storage at 4 degrees C in a nutrient medium.

Viable and nonviable aortic homografts in the subcoronary position: a comparative study

One hundred fifty-five freeze-dried, 63 frozen, and 337 Hanks'-antibiotic solution preserved or nutrient-antibiotic solution preserved homografts used for isolated aortic valve replacement have been followed for 1 to 20 years (mean, 5.3 years), a total of 2,931 patient-years of follow-up information. Overall survival, valve-related death, primary tissue failure, failure due to surgical technical error, infective endocarditis, and overall event-free survival have been assessed and compared. Overall survival 20 years after operation was 51.6% +/- 8.1% with a low incidence of sudden death. The method and length of preservation did not have any effect on the long-term performance or the mode of failure of the homografts. The rate of primary tissue failure was apparently higher with valves preserved in a solution containing calf serum, but the difference was not significant. It is concluded that long-term patient survival and quality of life after aortic valve replacement with a homograft are excellent. The current study, however, could not verify the existence of a significant difference between the three assessed methods of homograft preservation. Furthermore, it could not prove the importance of cellular viability or the existence of clinically significant immunological factors other than the calf serum content of the nutrient medium.