Current pediatric heart valve replacement options are suboptimal because they are incapable of somatic growth. Thus, children typically have multiple surgeries to replace outgrown valves. In this study, we present the in vivo function and growth potential of our tissue-engineered pediatric tubular valve. The valves were fabricated by sewing two decellularized engineered tissue tubes together in a prescribed pattern using degradable sutures and subsequently implanted into the main pulmonary artery of growing lambs. Valve function was monitored using periodic ultrasounds after implantation throughout the duration of the study. The valves functioned well up to eight weeks, four weeks beyond the suture strength half-life, after which their insufficiency index worsened. Histology from the explanted valves revealed extensive host cell invasion occurred within the engineered root and commenced from the leaflet surfaces. These cells expressed multiple phenotypes, including endothelial, and deposited elastin and collagen IV. Although the tubes fused together along the degradable suture line, as designed, the leaflets shortened compared to their original height. This shortening is hypothesized to result from inadequate fusion at the commissures prior to suture degradation. With appropriate commissure reinforcement, this novel heart valve may provide the somatic growth potential desired for a pediatric valve replacement.

Diseased aortic valves often require replacement, with over 30% of the current aortic valve surgeries performed in patients who will outlive a bioprosthetic valve. While many promising tissue-engineered valves have been created in the lab using the cell-seeded polymeric scaffold paradigm, none have been successfully tested long-term in the aortic position of pre-clinical model. The high pressure gradients and dynamic flow across the aortic valve leaflets require engineering a tissue that has the strength and compliance to withstand high mechanical demand without compromising normal hemodynamics. A long-term preclinical evaluation of an off-the-shelf tissue-engineered aortic valve in sheep model is presented here. The valves were made from a tube of decellularized cell-produced matrix mounted on a frame. The engineered tissue matrix is primarily composed of collagen, with strength and organization comparable to native valve leaflets. In vitro testing showed excellent hemodynamic performance with low regurgitation, low systolic pressure, and large orifice area. The implanted valves showed large-scale leaflet motion and maintained effective orifice area throughout the duration of the 6-month implant, with no calcification. After 24 weeks implantation (over 17 million cycles), the valves showed no change in tensile mechanical properties or collagen content. In addition, histology and DNA quantitation showed repopulation of the engineered matrix with interstitial-like cells and endothelialization. New extracellular matrix deposition, including elastin, further demonstrates positive tissue remodeling in addition to recellularization and valve function. Long-term implantation in the sheep model resulted in functionality, matrix remodeling, and recellularization, unprecedented results for a tissue-engineered aortic valve.

Treatment of congenital heart defects in children requiring right ventricular outflow tract reconstruction typically involves multiple open-heart surgeries because all existing graft materials have no growth potential. Here we present an ‘off-the-shelf' vascular graft grown from donor fibroblasts in a fibrin gel to address this critical unmet need. In a proof-of-concept study, the decellularized grafts are implanted as a pulmonary artery replacement in three young lambs and evaluated to adulthood. Longitudinal ultrasounds document dimensional growth of the grafts. The lambs show normal growth, increasing body weight by 366% and graft diameter and volume by 56% and 216%, respectively. Explanted grafts display physiological strength and stiffness, complete lumen endothelialization and extensive population by mature smooth muscle cells. The grafts also show substantial elastin deposition and a 465% increase in collagen content, without signs of calcification, aneurysm or stenosis. Collectively, our data support somatic growth of this completely biological graft.

Abstract-We have previously demonstrated the feasibility of fabricating a fibrin-based tissue-engineered heart valve (TEHV) using neonatal human dermal fibroblasts (nhDF), including leaflets with structural and mechanical anisotropy similar to native leaflets. The aim here was to evaluate the performance of this TEHV in a pilot study using the sheep model. Bi-leaflet TEHV were conditioned in a cyclic stretching bioreactor, then implanted within a polymeric sleeve interpositionally into the pulmonary artery of four sheep, with the pulmonary valve either left intact or rendered incompetent. Heparin and immunosuppression were administered for the duration. Echocardiography was performed at implantation and at 4 and 8 weeks. Explants were examined histologically, biochemically, and mechanically. In all sheep, echocardiography at implantation showed coapting leaflets, with minimal valve regurgitation and no turbulence. Orifice area and pressure gradients at systole approached the native pulmonary valve values. Echocardiography at 4 weeks revealed both leaflets functional with moderate regurgitation and turbulence in three sheep; in one sheep, only one leaflet was evident. Explanted leaflets had thickness and tensile properties comparable to the implanted leaflets. There was extensive endothelialization of the root lumenal surface. In the two sheep continued to 8 weeks, only one shortened leaflet remained in both cases. Immunocytochemistry indicated this was due to sustained tissue contraction caused by the nhDF and not by the invading host cells, which included a subpopulation consistent with bone marrow-derived cells. Short-term success was thus achieved in terms of excellent valve function at implantation and some valve function for at least 4 weeks; however, an apparent progressive tissue contraction needs to be resolved for long-term success.

There is a need for replacement heart valves that can grow with children. We fabricated tubes of fibroblast-derived collagenous matrix that have been shown to regenerate and grow as a pulmonary artery replacement in lambs and implemented a design for a valved conduit consisting of three tubes sewn together. Seven lambs were implanted with tri-tube valved conduits in sequential cohorts and compared to bioprosthetic conduits. Valves implanted into the pulmonary artery of two lambs of the first cohort of four animals functioned with mild regurgitation and systolic pressure drops <10 mmHg up to 52 weeks after implantation, during which the valve diameter increased from 19 mm to a physiologically normal ~25 mm. In a second cohort, the valve design was modified to include an additional tube, creating a sleeve around the tri-tube valve to counteract faster root growth relative to the leaflets. Two valves exhibited trivial-to-mild regurgitation at 52 weeks with similar diameter increases to ~25 mm and systolic pressure drops of <5 mmHg, whereas the third valve showed similar findings until moderate regurgitation was observed at 52 weeks, correlating to hyperincrease in the valve diameter. In all explanted valves, the leaflets contained interstitial cells and an endothelium progressing from the base of the leaflets and remained thin and pliable with sparse, punctate microcalcifications. The tri-tube valves demonstrated reduced calcification and improved hemodynamic function compared to clinically used pediatric bioprosthetic valves tested in the same model. This tri-tube valved conduit has potential for long-term valve growth in children.

Hemocompatibility of tissue-engineered vascular grafts remains a major hurdle to clinical utility for small-diameter grafts. Here we assessed the feasibility of using autologous blood outgrowth endothelial cells to create an endothelium via lumenal seeding on completely biological, decellularized engineered allografts prior to implantation in the sheep femoral artery. The 4-mm-diameter, 2- to 3-cm-long grafts were fabricated from fibrin gel remodeled into an aligned tissue tube in vitro by ovine dermal fibroblasts prior to decellularization. Decellularized grafts pre-seeded with blood outgrowth endothelial cells (n=3) retained unprecedented (>95 %) monolayer coverage 1 h post-implantation and had greater endothelial coverage, smaller wall thickness, and more basement membrane after 9-week implantation, including a final week without anti-coagulation therapy, compared with contralateral non-seeded controls. These results support the use of autologous blood outgrowth endothelial cells as a viable source of endothelial cells for creating an endothelium with biological function on decellularized engineered allografts made from fibroblast-remodeled fibrin.

There was a significant mortality benefit to the use of dabigatran as thromboembolic prophylaxis when compared with warfarin in the setting of mechanical heart valve replacement in the swine model. There was also a decreased incidence of bleeding complications in the dabigatran group compared with the warfarin group. Valve thrombus was observed in all study groups. Any conclusions regarding the rate of thrombus formation are outside the scope of this study and merit further investigation.

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