In an in-situ approach towards tissue engineered cardiovascular replacement grafts, cell-free scaffolds are implanted that engage in endogenous tissue formation. Bioactive molecules can be incorporated into such grafts to facilitate cellular recruitment. Stromal cell derived factor 1α (SDF1α) is a powerful chemoattractant of lymphocytes, monocytes and progenitor cells and plays an important role in cellular signaling and tissue repair. Short SDF1α-peptides derived from its receptor-activating domain are capable of activating the SDF1α-specific receptor CXCR4. Here, we show that SDF1α-derived peptides can be chemically modified with a supramolecular four-fold hydrogen bonding ureido-pyrimidinone (UPy) moiety, that allows for the convenient incorporation of the UPy-SDF1α-derived peptides into a UPy-modified polymer scaffold. We hypothesized that a UPy-modified material bioactivated with these UPy-SDF1α-derived peptides can retain and stimulate circulating cells in an anti-inflammatory, pro-tissue formation signaling environment. First, the early recruitment of human peripheral blood mononuclear cells to the scaffolds was analyzed in vitro in a custom-made mesofluidic device applying physiological pulsatile fluid flow. Preferential adhesion of lymphocytes with reduced expression of inflammatory factors TNFα, MCP1 and lymphocyte activation marker CD25 was found in the bioactivated scaffolds, indicating a reduction in inflammatory signaling. As a proof of concept, in-vivo implantation of the bioactivated scaffolds as rat abdominal aorta interposition grafts showed increased cellularity by CD68+ cells after 7 days. These results indicate that a completely synthetic, cell-free biomaterial can attract and stimulate specific leukocyte populations through supramolecular incorporation of short bioactive SDF1α derived peptides.

Inflammation is a natural phase of the wound healing response, which can be harnessed for the in situ tissue engineering of small-diameter blood vessels using instructive, bioresorbable synthetic grafts. This process is dependent on colonization of the graft by host circulating cells and subsequent matrix formation. Typically, vascular regeneration in small animals is governed by transanastomotic cell ingrowth. However, this process is very rare in humans and hence less relevant for clinical translation. Therefore, a novel rat model was developed, in which cell ingrowth from the adjacent tissue is inhibited using Gore-Tex sheathing. Using this model, our aim here was to prove that functional blood vessels can be formed in situ through the host inflammatory response, specifically by blood-borne cells. The model was validated by implanting sex-mismatched aortic segments on either anastomoses of an electrospun poly(ɛ-caprolactone) (PCL) graft, filled with fibrin gel, into the rat abdominal aorta. Fluorescent in situ hybridization analysis revealed that after 1 and 3 months in vivo, over 90% of infiltrating cells originated from the bloodstream, confirming the effective shielding of transanastomotic cell ingrowth. Using the validated model, PCL/fibrin grafts were implanted, either or not loaded with monocyte chemotactic protein-1 (MCP-1), and cell infiltration and tissue development were investigated at various key time points in the healing cascade. A phased healing response was observed, initiated by a rapid influx of inflammatory cells, mediated by the local release of MCP-1. After 3 months in vivo, the grafts consisted of a medial layer with smooth muscle cells in an oriented collagen matrix, an intimal layer with elastin fibers, and confluent endothelium. This study proves the regenerative potential of cells in the circulatory system in the setting of in situ vascular tissue engineering.

Synthetic replacement grafts for heart valves and small-diameter blood vessels such as coronary arteries have the potential to circumvent many of the limitations of currently available autologous grafting materials. Cell-free material incorporating biologically active compounds may guide the formation of fully autologous new tissue in situ derived from host cells after implantation. Inspiration for such bioactive compounds and their dynamics can be found in in vivo repair processes. Molecules such as stromal cell-derived factor 1α (SDF1α) that can attract progenitor cells from the bloodstream and modulate immune responses may be able to improve neotissue development in cell-free vascular and valvular grafts. Advances in the development of fully synthetic molecules and scaffold materials allow the spatial and temporal control of biologically active factors, enabling tissue engineers to mimic complex cellular signalling. This review focuses on combining knowledge of the molecular dynamics of factors involved in in vivo damage repair with the possibilities offered by newly developed synthetic materials. This approach has lead to encouraging results in the field of in situ vascular tissue engineering, and can ultimately lead to the development of off-the-shelf available vascular and valvular replacement grafts.