Gene expression study of monocytes/macrophages during early foreign body reaction and identification of potential precursors of myofibroblasts.

Foreign body reaction (FBR), initiated by adherence of macrophages to biomaterials, is associated with several complications. Searching for mechanisms potentially useful to overcome these complications, we have established the signaling role of monocytes/macrophages in the development of FBR and the presence of CD34(+) cells that potentially differentiate into myofibroblasts. Therefore, CD68(+) cells were in vitro activated with fibrinogen and also purified from the FBR after 3 days of implantation in rats. Gene expression profiles showed a switch from monocytes and macrophages attracted by fibrinogen to activated macrophages and eventually wound-healing macrophages. The immature FBR also contained a subpopulation of CD34(+) cells, which could be differentiated into myofibroblasts. This study showed that macrophages are the clear driving force of FBR, dependent on milieu, and myofibroblast deposition and differentiation.

The remodeling of cardiovascular bioprostheses under influence of stem cell homing signal pathways.

Optimizing current heart valve replacement strategies by creating living prostheses is a necessity to alleviate complications with current bioprosthetic devices such as calcification and degeneration. Regenerative medicine, mostly in vitro tissue engineering, is the forerunner of this optimization search, yet here we show the functionality of an in vivo alternative making use of 2 homing axes for stem cells. In rats we studied the signaling pathways of stem cells on implanted bioprosthetic tissue (photooxidized bovine pericardium (POP)), by gene and protein expression analysis. We found that SDF-1alpha/CXCR4 and FN/VLA4 homing axes play a role. When we implanted vascular grafts impregnated with SDF-1alpha and/or FN as carotid artery interpositions, primitive cells were attracted from the circulation. Next, bioprosthetic heart valves, constructed from POP impregnated with SDF-1alpha and/or FN, were implanted in pulmonary position. As shown by CD90, CD34 and CD117 immunofluorescent staining they became completely recellularized after 5 months, had a normal function and biomechanical properties and specifically the combination of SDF-1alpha and FN had an optimal valve-cell phenotype.

The recruitment of primitive Lin(-) Sca-1(+), CD34(+), c-kit(+) and CD271(+) cells during the early intraperitoneal foreign body reaction.

Implanted materials, such as medical devices, provoke the body to initiate an inflammatory reaction, known as the foreign body reaction (FBR), which causes several complications for example in hip prostheses, silicone implants, peritoneal dialysis catheters and left ventricular assist devices. FBR is initiated by macrophage adherence and results in granulation tissue formation. The early immunobiology and development of this tissue is not completely understood, but there are indications from related myofibroblast-forming diseases such as vascular repair and fibrosis that primitive stem cells also play a role in the formation of FBR-tissue. To investigate this, acellular photo-oxidized bovine pericardium patches were implanted intraperitoneally in rats and retrieved at time-points ranging from 6h to 7 days. A significant fraction of Sca-1(+) (6h-2 days), c-kit(+), CD34(+) and CD271(+) (2-3 days) stem/progenitor cells were detected. Colony-forming and differentiation capacity of the primitive stem cells into adipo-, osteo-, and myofibroblasts were shown. The presence of these primitive cells and their myofibroblastic differentiation potential were also confirmed at RNA level. The identification of specific primitive cells during FBR may have important implications for the inflammatory responses to inert materials and their use in tissue prostheses.

In vivo cellularization of a cross-linked matrix by intraperitoneal implantation: a new tool in heart valve tissue engineering.

AIM: To use in vivo instead of in vitro cell seeding in heart valve tissue engineering. METHODS AND RESULTS: Intraperitoneally preseeded, photo-oxidized bovine pericardial pulmonary valve constructs (group 1) were compared with non-preseeded constructs (group 2) implanted in sheep. All valves functioned normally and were macroscopically intact at explantation [1 week (n = 6) and 1 month (n = 6) in each group], except for one thrombosed leaflet in a group-2 valve at 1 month. Almost 10-fold higher neomatrix deposition and doubling of the leaflet thickness were found in group 1 vs. 2 (P < 0.05). A concomitant significant decrease in leaflet length (15%) was found at 1 month in group 1. The total cross-sectional surface and total amount of collagen of the original matrix remained unchanged in both groups at all times. Immunohistochemistry showed a low immune response, stem/progenitor cell infiltration, appropriated differentiation, and spontaneous endothelialization of the valves. Significantly, increased re-cellularization was found after IP preseeding compared with spontaneous seeding: cell coverage of the leaflet was 71-100 vs. 8-26% (P < 0.05), respectively. CONCLUSION: Complete re-cellularization can be obtained by IP preseeding of an acellularized cross-linked matrix. Well-functioning valve constructs show cellularization and differentiation into myofibroblast phenotype and concomitant neomatrix deposition.