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Trendsetters
The endothelium of healthy blood vessels serves anticoagulant, antiproliferative, and vasodilator functions. Prosthetic vascular grafts, widely used for reconstruction of large vessels and to provide access to blood vessels for hemodialysis, lack endothelial cells and often fail because unopposed proliferation of myofibroblasts leads to stenosis of the graft and consequent thrombosis. Similarly, small-diameter prosthetic vascular grafts, badly needed for distal limb bypass procedures, cannot currently be used clinically because they thrombose rapidly in the absence of an adequate endothelium. Because adding functional endothelium to artificial grafts potentially could overcome these problems, endothelialization of prosthetic vascular grafts has been a goal in clinical medicine for well over two decades.
Up to now, however, this goal has not been achieved. Recently, it has been proposed that stably modified endothelial cells carrying various transgenes (to produce, for example, anticoagulant or vasodilator substances) might be implanted on prosthetic vascular grafts. Thus far, however, this approach has not worked because autologous cells grown in culture tend to lose their differentiated phenotype and therefore lack the adhesive strength to resist the force of flowing blood when the graft is implanted in vivo. Hence, loss of the cells from the implanted graft is one of the most vexing problems in this field.
One approach for overcoming incomplete seeding and poor cell retention on prosthetic grafts was reported by Pasic et al. (3). They harvested endothelial cells from the omentum of dogs and then seeded those cells directly onto the luminal surface of small-diameter Dacron grafts, using centrifugal force to promote even seeding. They achieved a remarkably high cell density prior to graft implantation and found, furthermore, that more than 95% of their grafts remained patent 1 yr after being implanted into dogs, whereas only 13% of grafts not seeded with endothelial cells remained patent for 1 yr. The striking success of this experiment may well be due to the high seeding density of relatively well-differentiated endothelial cells on the grafts.
An alternate method of accomplishing the same goal may be to apply shear stress to cultured epithelial cells in vitro. Application of shear stress in vitro for 9 days promoted formation of a highly adherent endothelium on the luminal surface of polypropylene hollow fibers, and these adherent cells were more differentiated than cells not exposed to the stress (2). Most notably, the density of Weibel-Palade bodies, which are specific storage sites for both von Willebrand factor and P-selectin and which are only found in differentiated endothelial cells, was nearly 40-fold higher in aortic endothelial cells exposed to chronic arterial shear stress than in cells not exposed to the stress. In addition, shear stress stimulated the organization of an extensive actin cytoskeleton, anchored to focal adhesions in the cells.
More recently, endothelial cell adhesiveness to artificial vascular graft material was increased before implanting the grafts into the circulation by first exposing the cells to shear stress for several days in vitro (3). The strength of endothelial cell adhesion was proportional to the shear stress load during the preconditioning period, and the grafts treated with the highest level of shear stress (25 dyn/cm2) retained a fully confluent luminal monolayer in vivo both 1 day and 1 wk after implantation into the aorta of rats. By contrast, preconditioning with the much lesser venous shear stress (1 dyn/cm2) resulted in significant loss of cells. Three months after grafting, neointimal hyperplasia was much less in grafts that contained endothelial monolayers than in grafts without endothelium.
Taken together, these studies suggest that endothelialization of prosthetic vascular grafts can be improved if differentiated cells with high adhesive strength are used. Harvesting of differentiated cells from omentum (or from fat tissue) seems suitable, but the number of cells that can be obtained with this approach is still quite limited. Cell number can be scaled up by culture in vitro, and fully endothelialized prosthetic grafts may become a reality if such cells, notorious for their loss of adhesive strength and dedifferentiation in culture, were preconditioned with shear stress in order to regain the differentiated functions of healthy endothelium before implantation in vivo. Endothelialization with cells in which stable genetic alterations have been made should then become possible.
References
| Occasionally, the Editor of the Trendsetters section invites contributions from the authors of published scientific articles that have been identified as being of special interest. All précis to Trendsetters are by invitation only. |
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