Recent studies have suggested that the platelet fibrinogen (Fg) receptor, platelet membrane glycoprotein IIbIIIa (GPIIbIIIa, or integrin alpha(IIb)beta(3)) is also an adenosine triphosphate (ATP) binding site, and that the binding of ATP can directly inhibit the Fg-binding function of GPIIbIIIa. However, any direct effect of ATP on GPIIbIIIa function in intact fresh platelets is difficult to distinguish from indirect inhibitory effects via competition with ADP or elevation of platelet cyclic AMP levels. We therefore studied effects of ATP on Fg binding to activated GPIIbIIIa on the following model particles: externally activated and fixed platelets, as well as latex particles and liposomes containing functionally competent activated GPIIbIIIa receptors for Fg. These particles have 'normal', activated GPIIbIIIa in terms of: (1) binding affinity, (2) specificity to Fg, and (3) conformational change(s) after Fg binding. These particles neither require nor respond to further activation in order to bind Fg. With these model particles, we showed that ATP does not have any direct effect on the binding of Fg to platelet GPIIbIIIa and platelet aggregation. These simplified model particles are useful tools in the mechanistic study of platelet GPIIbIIIa function and the interaction between platelet GPIIbIIIa and its ligands.
The pathophysiological state of rejection in liver xenotransplantation is poorly understood. Data from clinical pig liver perfusion suggest that pig livers might be rejected less vigorously than pig hearts or kidneys. Pig livers used in clinical xenoperfusions were exposed to blood from patients with liver failure. We have shown in an animal model that transplant recipients with liver failure are less capable of initiating hyperacute rejection of a xenografted liver than a healthy transplant recipient. The goal of this report is to examine the pathological characteristics of pig livers used in 2 clinical pig liver perfusions and combine this information with in vitro studies of pig-to-human liver xenotransplantation to determine whether the findings in the perfused pig livers could be explained in part by the diminished capacity of the patient with liver failure to respond to xenogeneic tissue. Pathological analysis of the perfused pig livers showed immunoglobulin M deposition in the sinusoids with little evidence of complement activation. Our in vitro studies showed that serum from patients with liver failure caused less injury to pig liver endothelium than serum from healthy subjects. Serum from patients with liver failure had similar levels of xenoreactive antibodies as serum from healthy humans. Incubation of serum from patients with liver failure with pig hepatic endothelial cells generated less iC3b, Bb fragment, and C5b-9 than serum from healthy subjects. We conclude that the altered injury in the perfused pig livers can be attributed to the relative complement deficiency that accompanies liver failure.
