Optimization of recombinant t-PA secretion from seeded vascular grafts

Seeding of vascular grafts with genetically engineered endothelial cells (EC) secreting anticoagulant or fibrinolytic agents offers a potential means of improving patency rates. Prior to the initiation of in vivo studies, we examined cell retention and tissue plasminogen activator (t-PA) secretion from small-diameter synthetic graft segments seeded with sheep venous EC genetically engineered to secrete human t-PA. Following retroviral-mediated gene transfer, EC were seeded at varying densities onto 4-mm-diameter synthetic graft segments of different composition, achieving confluent coverage of all materials. t-PA production from seeded grafts was evaluated under both static conditions and after flow exposure, for up to 3 days after seeding. t-PA secretion varied directly with increasing seeding density for all graft types, reaching a maximum of 20 ng/cm2/24 hr. t-PA secretion correlated highly with the number of seeded cells as determined by measurement of DNA from lysates of seeded grafts (r2 = 0.90, P < 0.0001). For all graft types tested, approximately 50% of seeded cells were retained after exposure to flow in vitro. Retained EC remained viable as determined by t-PA secretion. The rate of t-PA secretion from collagen-impregnated Dacron grafts was higher than that obtained with other materials both under static conditions and after flow exposure. This higher rate was most likely due to the higher surface area presented by the Dacron grafts. These data demonstrate that small-diameter prosthetic graft materials can be coated with a layer of EC that (i) remains metabolically active and capable of secreting a fibrinolytic agent, and (ii) remains adherent to the graft surface after exposure to flow. These experiments provide a foundation for in vivo studies in which grafts are seeded with EC genetically engineered to increase local fibrinolysis.