Expression of tissue factor mRNA in cardiac xenografts - Clues to the pathogenesis of acute vascular rejection

Background. Acute vascular rejection destroys vascularized xenografts over a period of hours to days and is now considered the major hurdle to the cli nical application of xenotransplantation. The hallmark of acute vascular re jection is diffuse intravascular coagulation; however, the pathogenesis of coagulation is a matter of controversy. One line of evidence points to acti vated endothelial cells and another to activated inflammatory cells as a so urce of tissue factor and thus as a primary cause of this lesion. The disti nction between the two mechanisms inducing coagulation in the xenograft pro vides an opportunity for specific intervention. Methods. To explore these mechanisms, we studied the expression of tissue f actor mRNA by in situ reverse transcriptase-polymerase chain reaction in re lation to the histopathologic manifestations of acute vascular rejection in guinea pig hearts transplanted into rats treated by cobra venom factor to avoid the hyperacute rejection. Results. Three hours after transplantation and before the deposition of fib rin, tissue factor mRNA was expressed in the endothelial cells lining small and medium blood vessels and in smooth muscle cells of guinea pig cardiac xenografts. Sixteen hours after transplantation, while rat tissue factor mR NA was expressed only in occasional infiltrating cells, cardiac xenografts showed prominent deposits of fibrin in small vessels. Maximum expression of tissue factor on rat infiltrating cells was observed 48 hr after transplan tation. Conclusions. These results suggest that in acute vascular rejection, coagul ation is initiated on the donor vascular system, while the procoagulant cha racteristics of infiltrating cells may reflect a response to tissue injury rather than a cause.