Prof. Dr. med. Mathias Wilhelmi and PD Dr. rer. nat Ulrike Böer
Director of the Clinic/Institute: Prof. Dr. med. Axel Haverich; Clinic for Cardiac, Thoracic, Transplant and Vascular Surgery
Representative at the NIFE: Ulrike Böer
Dr. med. Thomas Aper, Sarah Zippusch (M.Sc.), Dr. med. Julia Meier, Dr. med. Claudia Schrimpf
Florian Helms, Alena Richter
Vascular protheses, cardiac patches, decellularization, xeno immunogenicity, fibrin-based implants, atherogenic pathological mechanisms, (pre) vascularization, autologous vascular cell types
Decellularization, fibrin compaction, cell culture, Reseeding strategies of implants, immunohistochemistry, capillary formation assays, bioreactor technology
Short summary of scientific expertise
Tissue engineered (TE) vascular prostheses are promising tools for the replacement of damaged or lacking arteries and veins since they combine a low risk for thrombosis and graft infection and the potential to be integrated and remodeled after implantation. Here, two approaches were followed to generate biological scaffolds for TE vascular grafts. First, decellularization of xenogeneic arteries (from the horse) results in cell-free matrices with sufficient biomechanical stability and an extremely low immunogenicity. Second, vascular grafts can be manufactured using the blood clotting protein fibrin by a special molding technique that compresses the gel-like fibrin to a solid structure. Fibrin grafts can be reseeded with vascular cells like endothelial and smooth muscle cells isolated from autologous tissue sources to mimic the natural structure of the arterial wall. In this context, the pre-vascularization of the outer vessel wall is of particular importance. Other projects are the generation of fibrin-based cardiac patches for the surgical therapy of congenital heart defects, the characterization of metallic stent materials with respect to cellular reseeding, the determination of graft-specific serum antibodies and the establishment of an ex-vivo perfusion system to study patho-mechanisms of atherosclerosis development.
Research focusses: Development of bioartificial vascular prostheses
Cardiovascular diseases are the major cause of death in the western industrial countries due to atherosclerotic changes in the vasculature. Surgical therapy often requires the replacement of arteries by vascular prostheses such as for the generation of bypasses at the heart or in the periphery. Vascular prostheses should bear a low risk for thrombosis and infections and also should not evoke any adverse tissue or immune reaction. The currently used materials for vascular prostheses are exclusively alloplastic polymers which display a sufficient biocompatibility but are prone to thrombi formation and bear a high risk for infections which also includes biofilm formation. A new approach is the generation of bioartificial vascular grafts by means of tissue engineering. Tissue engineered grafts consists of biological scaffold materials and can be reseeded with vascular cells leading to remodeling and full integration of the graft after implantation. Moreover, due to a potentially rapid neovascularization the implant will be supplied with oxygen and nutrients and also immune cells will be able to infiltrate the graft to eliminate infectious bacteria.
Group Böer/ Wilhelmi:
We follow different approaches for the generation and characterization of scaffolds for vascular tissue engineering:
Decellularization of equine carotid arteries
Carotide arteries from the horse are ideal scaffolds for large diameter vascular grafts such as hemodialysis shunts due to their size and the low number of outlets. To avoid rejection after implantation, xenogeneic (= from a different species) cells in the native carotid arteries have to be removed by treatment with detergents and enzyme leaving behind a cell-free scaffold (Fig. 1). We developed an intensified decellularization protocol that extensively reduced the immunogenicity of the equine carotid arteries but maintained their biomechanical properties. Intensified decellularization of equine carotids resulted in scaffolds with excellent biocompatibility and a high potential to be reseeded and remodeled as shown in vitro and in several animal models (mouse, rat, and sheep).
Determination of the immunogenicity of xenogeneic bioartificial grafts
To determine graft-specific antibody formation in patients after implantation of xenogeneic bioprosthetic heart valves, we developed an enzyme-linked immuno sorbent assay (ELISA). Tissue-specific antibody formation was most pronounced in patients implanted with glutaraldehyde-fixed porcine heart valves up to 5 years and was directed against cellular and extracellular proteins of the heart valve. After implantation of decellularized porcine valves this immune response was significantly reduced and even absent in patients implanted with human decellularized valves. This underlines the recommendation to use the latter in particular for valve replacement in young patients.
Manufacturing of fibrin-based vascular grafts
Fibrin is a natural polymer involved in the blood clotting cascade which also can serve as material for cardiovascular grafts. However, the low stability of fibrin has limited its use for the generation of vascular prostheses so far. Here, we developed a method to compact fibrin by high speed rotation resulting in vascular grafts (Fig. 2) comprising a sufficient stability and an enormous potential to be reseeded and remodeled in vivo as already shown in an animal model.
Fibrin-based cardiac patches
Also cardiac patches as required for constructive surgical therapy of congenital heart defects requires a material which is stable, which can be remodeled and bears no immunogenic risk. Thus, fibrin was used to generate patches which was compacted by pressure and thus gains patches with sufficient stability (Fig. 3). Fibrin patches may be a promising alternative to xenogeneic cardiac patch materials. Currently, for the first time fibrin patches are tested in a sheep model.
Seeding of fibrin grafts with vascular cells
To mimic the natural structure of arteries, fibrin grafts can be seeded in vitro with vascular cells. Endothelial cells, smooth muscle cells and mesenchymal stem cells were isolated from autologous tissue sources and used to form the three layers of an artery comprising the endothelial lining, a contractile medial layer and network of capillaries in the outer layer. Structures as such can be visualized by fluorescence microscopy (Fig. 4).
Ex vivo perfusion as disease model for atherosclerosis
The pathogenesis of atherosclerosis involves changes in the so-called vasa vasorum which are necessary for oxygen and nutrient supply of the vessel wall. Damage or obstruction of vasa vasorum results in local hypoxic zonation and subsequently in inflammation, lipid accumulation and intimal thickening. To study the early mechanisms of atherosclerosis, porcine aortas were exposed to low oxygen conditions in an in vitro perfusion system and the effects of arterial wall hypoxia on cellular and molecular structures in the vessel wall will be investigated (Fig. 5).
Evaluation of nano-structured metals for the generation of vascular stents
Nitinol-based materials for the generation of stents which were electropolished and structured by the deposition of nano particles were tested for their repopulation properties by vascular cells (Cooperation project with Prof. Dr. Stefan Barcikowski, Technical Universitity of Duisburg-Essen).
AG Schrimpf/ Wilhelmi
Our group works on vascular remodeling during atherogenesis and vascular tissue engineering. We are focusing on microcirculation with special interest in the dissection of molecular mechanisms between pericytes and endothelial cells.