Cardiac tissue engineering: remodeling and recellularization of cardiovascular grafts

Heart valve diseases and blood vessel lesions are important causes of mortality and morbidity worldwide. As one of the possible solutions, tissue engineering aims to create viable substitutes to face the well-known problems after implantation of prostheses and to present a solution for children. However, currently used tissue engineering paradigms, generally comprising of an in vitro maturation step, are associated with several problems, costs and risks. Therefore, our group oriented their efforts to the creation or recellularization of cardiovascular prostheses by using the patient s body as its own bioreactor and more specifically by making use of the immature foreign body reaction (FBR) or parts thereof.Cellular dynamic studies showed a peak of primitive cells attracted after 3 days of implantation, which could differentiate into (myo)fibroblasts. Macrophages remained present during the different stages and so we further unraveled the role of macrophages during the early FBR in a first part of this thesis. From the gene expression, it was clear that monocytes and macrophages attracted by fibrinogen were first activated to perform a pure pro-inflammatory role and then switched to a more wound healing phenotype with added anti-inflammatory effects.Furthermore, the development of the early FBR could be altered by the application of cyclic stretching forces to the scaffold where the developing FBR took place. Forces induced a more than twofold increase in tissue strength and produced changes in extracellular matrix organization and some contractile protein expression.In previous experiments, we made use of a peritoneal implantation step to preseed the matrix with FBR cells. This supplemental implantation could be avoided by using homing factors detected within this system to attract relevant primitive FBR cells from the circulation. Some identified candidate homing molecules, amongst which stem cell factor (SCF) and stromal cell derived factor 1 alpha (SDF-1α) were used in coating experiments performed in a rat model of carotid artery implantation. This showed significantly higher attraction of primitive cells through fibronectin (FN) and SDF-1α coating compared with the uncoated controls. In a third part of this thesis, we found that both SDF-1α/CXCR-4 and FN/α4-integrin-VCAM1 axes were necessary during the early FBR, and could determine that these factors were important in the phase where the peak of stem cell attraction was seen. Consequently, the possibility to coat photo-oxidized bovine pericardium (POP) with FN and/or SDF-1α was shown. In a next step, POP valves were coated with FN, SDF-1α or the combination thereof, and subsequently implanted in sheep. Co-impregnation of the valves showed that the different types and relative proportion of valve interstitial cells approached those values observed in native valves. From this study, we could conclude that the combination of FN and SDF-1α had the most potential to be used inour recellularization paradigm.Wewere confident that our approach was not exclusively applicable to heart valves and could also be used in other cardiovascular applications, such as vessel replacement. So, the FN-SDF-1α coating was evaluated on small-caliber vascular grafts. Short-term implants again showed the effects of the coating on the attraction of primitive cells (CD34+ and CD117+). This was associated with an improved endothelial coverage in parallel with a reduction in intimal hyperplasia and thrombosis on long-term.Eventually, we have evaluated the coating on decellularized homografts and compared them with homografts and decellularized homografts. Decellularization reduced calcification potential of homografts, while coating of decellularized homografts induced a reduction in regurgitation, the attraction and differentiation of appropriate valve interstitial cells and even collagen organization. In conclusion, the development of a useful technology to recellularize cardiovascular prostheses by coating them with the combination of FN and SDF-1α was shown. Its valuable improvements on acellular xenografts, small-caliber synthetic vascular grafts and decellularized homografts were proven.status: publishe