Mona Bracher

Off-the-shelf human decellularized tissue-engineered heart valves in a non-human primate model

Heart valve tissue engineering based on decellularized xenogenic or allogenic starter matrices has shown promising first clinical results. However, the availability of healthy homologous donor valves is limited and xenogenic materials are associated with infectious and immunologic risks. To address such limitations, biodegradable synthetic materials have been successfully used for the creation of living autologous tissue-engineered heart valves (TEHVs) in vitro. Since these classical tissue engineering technologies necessitate substantial infrastructure and logistics, we recently introduced decellularized TEHVs (dTEHVs), based on biodegradable synthetic materials and vascular-derived cells, and successfully created a potential off-the-shelf starter matrix for guided tissue regeneration. Here, we investigate the host repopulation capacity of such dTEHVs in a non-human primate model with up to 8 weeks follow-up. After minimally invasive delivery into the orthotopic pulmonary position, dTEHVs revealed mobile and thin leaflets after 8 weeks of follow-up. Furthermore, mild-moderate valvular insufficiency and relative leaflet shortening were detected. However, in comparison to the decellularized human native heart valve control - representing currently used homografts - dTEHVs showed remarkable rapid cellular repopulation. Given this substantial in situ remodeling capacity, these results suggest that human cell-derived bioengineered decellularized materials represent a promising and clinically relevant starter matrix for heart valve tissue engineering. These biomaterials may ultimately overcome the limitations of currently used valve replacements by providing homologous, non-immunogenic, off-the-shelf replacement constructs.

Injectable living marrow stromal cell-based autologous tissue engineered heart valves: first experiences with a one-step intervention in primates

AIMS A living heart valve with regeneration capacity based on autologous cells and minimally invasive implantation technology would represent a substantial improvement upon contemporary heart valve prostheses. This study investigates the feasibility of injectable, marrow stromal cell-based, autologous, living tissue engineered heart valves (TEHV) generated and implanted in a one-step intervention in non-human primates. METHODS AND RESULTS Trileaflet heart valves were fabricated from non-woven biodegradable synthetic composite scaffolds and integrated into self-expanding nitinol stents. During the same intervention autologous bone marrow-derived mononuclear cells were harvested, seeded onto the scaffold matrix, and implanted transapically as pulmonary valve replacements into non-human primates (n = 6). The transapical implantations were successful in all animals and the overall procedure time from cell harvest to TEHV implantation was 118 ± 17 min. In vivo functionality assessed by echocardiography revealed preserved valvular structures and adequate functionality up to 4 weeks post implantation. Substantial cellular remodelling and in-growth into the scaffold materials resulted in layered, endothelialized tissues as visualized by histology and immunohistochemistry. Biomechanical analysis showed non-linear stress-strain curves of the leaflets, indicating replacement of the initial biodegradable matrix by living tissue. CONCLUSION Here, we provide a novel concept demonstrating that heart valve tissue engineering based on a minimally invasive technique for both cell harvest and valve delivery as a one-step intervention is feasible in non-human primates. This innovative approach may overcome the limitations of contemporary surgical and interventional bioprosthetic heart valve prostheses.

Energy charge as an indication of liver viability : a comparison of changes in livers that remained intact with those subjected to autografting

As efforts to assess the viability of liver grafts continue, the recent description of noninvasive measurement by fluorimetry or magnetic resonance spectrometry of adenine nucleotides has brought energy charge into focus again as an index of viability. Many previous studies have been conducted in human donor livers that have clinical relevance but which cannot be standardized, or in rats in which the hepatic artery is not anastomosed. In the present study, pig livers were definitively rendered ischemic for 1 or 2 hr. In one group the livers were then revascularized (intact) while in the other, the livers were removed during the final 20-30 min of the ischemic period and were subjected to autograft. There was a marked difference in survival between the intact and the autograft groups. One hour of ischemia in the intact group was associated with survival comparable to that of autograft controls (8-100 days); 2 hr of ischemia caused shortened survival, ranging from 2 to 18 days. In the recipients of autografts, survival after 1 hr of ischemia ranged from 3 to 16 days; after 2 hr of ischemia no autograft recipient survived overnight. The energy charge returned to the preoperative level after 2 hr of ischemia in both intact and autograft groups. The concentrations remained depressed after 2 hr of ischemia in autografted animals, thus being associated with survival. However, the patterns of total adenine nucleotide and adenosine triphosphate were not always similar to those of energy charge. The concentrations of aspartate aminotransferase were similarly elevated in all ischemic groups irrespective of duration or subsequent survival. There was, however, a close association between euglobulin lysis times (ELT) and survival. In the autograft recipients of livers subjected to 2 hr of ischemia that did not survive overnight the ELT remained significantly shortened. It is concluded that adenine nucleotide metabolism is important as an index of viability, but that concentrations of total and individual adenine nucleotides and the energy change all need to be computed. There does, however, appear to be an absolute relationship between survival and euglobulin lysis time that would be clinically useful in patients undergoing liver transplantation or hepatic vascular exclusion.

Cell specific ingrowth hydrogels

Extracellular mimetic hydrogels formed from peptide crosslinkers and polyethylene glycol monomers permit cell-controlled invasion. The use of matrix metalloproteinase specific peptides might further allow for selective control of different cell-type invasion. In this study, the invasion of fibroblasts and vascular smooth muscle cells (VSMC) into hydrogels polymerised with either a peptide generally permissive for matrix metalloproteinase (MMP) degradation or peptides preferentially cleaved by MMP-14 or MMP-9 enzymes were compared. The two cell-types invaded the MMP permissive hydrogel equally. However, invasion of VSMC into MMP-14 selective peptide crosslinked hydrogels was diametrically opposite in nature to that of fibroblasts whereby VSMC showed a two-fold increase into these hydrogels relative to that observed in permissive hydrogels whilst fibroblasts had a relative two-fold decrease (p < 0.01). These findings are suggestive that invasion and growth of different cell-types in engineered synthetic extracellular matrix mimics may be controlled selectively by the choice of protease specific peptide crosslinker and this could have general utility in tissue regenerative and engineering approaches.

Mitigation of bioprosthetic heart valve degeneration through biocompatibility: in vitro versus spontaneous endothelialization.

BACKGROUND Glutaraldehyde-related cytotoxicity and transanastomotic ingrowth inhibition prevent the spontaneous endothelialization of bioprosthetic heart valves. In order to evaluate the ability of improved biocompatibility to reduce tissue degeneration, conventionally fixed aortic root prostheses were both glutaraldehyde-detoxified and in vitro endothelialized. METHODS Entire aortic roots were fixed in 0.2% glutaraldehyde (GA) (control group) and either detoxified in acetic acid-buffered urazole (0.1 M) or detoxified and in vitro lined with cultured, autologousjugular vein endothelial cells. The valved roots were inserted in the distal aortic arch of 15 juvenile Merino sheep for a period of 12 weeks. Upon explant, leaflets, sinuses and aortic wall of the prostheses were analysed by SEM to assess the surface endothelium, histologically regarding tissue inflammation, and by atomic absorption spectrophotometry to determine the content of tissue calcium. RESULTS There was no endothelium on control grafts, except for a short anastomotic pannus. The detoxified group showed an incomplete patchy endothelium on the aortic wall but hardly any on the leaflets, whereas, the in vitro lined group had aortic wall, sinuses and most of the leaflets confluently endothelialized. Tissue inflammation was prominent in the control group and least expressed in the endothelialized group (p < 0.05). Detoxification significantly reduced leaflet calcification. In the aortic wall, both detoxification and endothelial lining were required to significantly mitigate calcification. CONCLUSION In the 12 week circulatory sheep model, the calcium mitigating effect of detoxification was more pronounced than that of in vitro endothelialization. Nevertheless, there was a distinct overall benefit if detoxification was combined with endothelialization.

Effect of total hepatectomy on coagulation and glucose homeostasis in the pig.

It has been suggested recently that patients with fulminant liver failure should be prepared for transplantation by early hepatectomy, yet the acute effects of removal of the liver upon the coagulation profile and certain hormones are not known. This study was conducted on totally hepatectomized pigs that survived up to 27 hr. Measurements were made of serum insulin, plasma glucagon (IRG and GLI), glucose, catecholamines, and the coagulation profile. The increase in serum insulin was directly related to levels of plasma glucosethere was a 100-fold increase in animals with plasma glucose levels>400 mg/100ml and none when blood glucose was <100 mg/100 ml. Plasma glucagon showed a sharp transient increase within 1 hr of hepatectomy and a slow rise thereafter with levels apparently unrelated to serum insulin or plasma glucose. There was a transient increase in plasma adrenaline but a sharp continous increase in plasma norepinephrine. No changes of note occurred in the coagulation profile-even levels of fibrinogen only declined by 20% in 27 hr. The study has shown that early total hepatectomy is safe as far as changes in coagulation are concerned but changes in serum insulin and especially plasma norepinephrine may be of more significance.

Six-hour porcine liver storage without flushing or perfusion

Livers from normal porcine donors were preserved by surface cooling only, without flushing or perfusion, for periods up to 24 hr. All recipients of livers stored for 6 hr survived until sacrifice at 7 days. In a separate, similar group, survival up to 21 days was noted. Only 2 of 6 recipients survived after 9-hr liver storage, but one of these lived for > 120 days. No animals survived longer than 2 days after transplantation of livers stored for 12 or 24 hr. The changes in plasma levels of aspartate aminotransferase of recipients of 6-hr surface-cooled livers were not significantly different from AST levels of recipients of livers stored in University of Wisconsin or Euro-Collins solution as observed in previous studies in this laboratory. At sacrifice after 7 days, there was no histologic evidence of damage after surface cooling. In the light of recent reports of evidence of endothelial and reticuloendothelial damage caused by flushing solutions, it is suggested that surface cooling of the liver may provide adequate preservation for 6 hr in appropriate circumstances. Further studies will be needed to confirm that this method of preservation is applicable to livers removed from brain-dead donors and that it does not cause endothelial damage.

Ang‐2 and PDGF‐BB cooperatively stimulate human peripheral blood monocyte fibrinolysis

Angiopoietin‐2 (Ang‐2) is a growth factor, which was identified originally as playing a critical role in vessel remodeling during angiogenesis. More recent evidence has indicated additional involvement in vascular homeostatic responses such as coagulation and inflammation, which are central to wound healing. We therefore determined whether a relationship existed between Ang‐2 and monocytes, one of the initial cell types to be recruited to a wound, in the context of fibrin clot invasion. Ang‐2 significantly increased monocyte invasion of fibrin in the presence of serum. In the absence of serum, it required a combination of Ang‐2 and platelet‐derived growth factor BB (PDGF‐BB) to increase invasion by threefold. Furthermore, it was shown that the heightened invasion was dependent on serine proteases and matrix metalloproteinases (MMPs) and that the combination of Ang‐2 and PDGF‐BB increased urokinase plasminogen‐activator receptor expression, as well as MMP‐9 and membrane type 1 MMP expression. These data give further credence to the concept of Ang‐2 as a key regulator of several essential phases of wound healing.

Disturbances of Reticulo-Endothelial Function following Experimental Liver Transplantation

Reticulo-endothelial function was studied in liver-grafted pigs using intravenous carbon clearance at 24 h and 7 days postgraft. Flushing of auto- or allografts with Ringers lactate and with or without 4-hour storage caused no change in carbon clearance. Flushing of auto- or allografts with Eurocollins solution and with or without 4-hour storage resulted in delayed carbon clearance at 7 days. The changes appeared to be related to the composition of preservation fluid rather than to its volume or temperature, the duration of storage or the development of rejection.

The Anticalcific Effect of Glutaraldehyde Detoxification on Bioprosthetic Aortic Wall Tissue in the Sheep Model

Background: Increasing concentrations of glutaraldehyde (GA) lead to a decreased rather than increased calcification of bioprosthetic aortic wall tissue. This study determined to what extent the benefit of better cross‐linking is masked by the intrinsic propensity of GA towards calcification. Materials and Methods: Porcine aortic roots were immediately fixed at the abattoir at three different concentrations of GA (0.2%, 1.0%, and 3.0% for 1 week at 4°C). Subsequently, roots underwent a GA extraction process using high volumes of Urazole solution (acetic acid buffer, pH 4.5, 37°C, 1 week) followed by NaBH4 reduction (2 days, 37°C). Roots were implanted in the distal aortic arch of young sheep for 6 weeks and 6 months. Calcium analysis was quantitatively done by atomic absorption spectrophotometry and qualitatively assessed by light microscopy on Von Kossa stains. Results: There was a distinct anticalcification effect of GA detoxification after 6 weeks (56.8% to 97.9%; 95% confidence interval [CI]), which stabilized on a more moderate level after 6 months of implantation (19.1% to 31.6%; 95% CI). The most pronounced effect of GA extraction was seen in 0.2% fixed tissue, where aortic wall calcification was mitigated by 97% and 32% after 6 weeks and 6 months, respectively. Mitigation of aortic wall calcification was 71% (6 weeks) and 21% (6 months) in the 3.0% GA group. The combined effect of higher cross‐link density and detoxification achieved an 82% (6 weeks) and 48% (6 months) reduction of calcium levels in the 3.0% GA group. In long‐term implants (6 months), detoxification alone on top of standard 0.2% GA fixation was as effective (from 174.1 ± 11.9 μg/mg without detoxification to 119.3 ± 19.3 μg/mg with detoxification) as 3.0% fixation (114.8 ± 10.0 μg/mg without detoxification to 91.3 ± 11.5 μg/mg with detoxification). Conclusion: We were able to determine in the circulatory sheep model to what degree the intrinsic procalcific effect of GA counteracts the protective effect of higher cross‐link density. Our study also established that the effect of detoxification is particularly pronounced in commercial low‐grade fixation.