Plasma fibrinogen inhibits platelets adhesion in flowing blood to immobilized fibrinogen.

The influence of variations in plasma fibrinogen concentration on platelet adhesion to immobilized fibrinogen was investigated in a parallel-plate perfusion chamber. At a shear rate of 1600 s-1 platelet adhesion decreased when increasing concentrations of purified fibrinogen were added to the plasma (IC50 = 1.5 +/- 0.2 g/L fibrinogen, n = 24). Washed platelets reconstituted in a human albumin solution with red blood cells were more sensitive for soluble fibrinogen (IC50 = 0.4 +/- 0.1 g/L, n = 5, P < .05). When platelet activation during circulation of the blood was minimized by using a single-passage perfusion system, an IC50 of 2.0 +/- 0.2 g/L was found (n = 9). To exclude the possibility that the inhibition of fibrinogen was caused by irreversible changes in the fibrinogen molecule during the purification procedure, normal plasma was mixed in different ratios with plasma from a patient with congenital afibrinogenemia. Under these conditions, the plasma fibrinogen IC50 was 1.5 +/- 1.1 g/L. Absence of endogenous fibrinogen in the platelets of the patient resulted in an IC50 of 1.2 +/- 0.5 g/L for plasma fibrinogen. These results demonstrate that increased plasma fibrinogen concentrations inhibit platelet adhesion to fibrinogen under flow.

On the role of von Willebrand factor in promoting platelet adhesion to fibrin in flowing blood.

Platelet adhesion to fibrin at high shear rates depends on both the glycoprotein (GP) IIb:IIIa complex and a secondary interaction between GPIb and von Willebrand factor (vWF). This alternative link between platelets and vWF in promoting platelet adhesion to fibrin has been examined in flowing whole blood with a rectangular perfusion chamber. Optimal adhesion required both platelets and vWF, as shown by the following observations. No binding of vWF could be detected when plasma was perfused over a fibrin surface or when coated fibrinogen was incubated with control plasma in an enzyme-linked immunosorbent assay. However, when platelets were present during perfusion, interactions between vWF and fibrin could be visualized with immunoelectron microscopy. Exposure of fibrin surfaces to normal plasma before perfusion with severe von Willebrand's disease blood did not compensate for the presence of plasma vWF necessary for adhesion. vWF mutants in which the GPIIb:IIIa binding site was mutated or the GPIb binding site was deleted showed that vWF only interacts with GPIb on platelets in supporting adhesion to fibrin and not with GPIIb:IIIa. Complementary results were obtained with specific monoclonal antibodies against vWF. Thus, vWF must first bind to platelets before it can interact with fibrin and promote platelet adhesion. Furthermore, only GPIb, but not GPIIb:IIIa is directly involved in this interaction of vWF with platelets.

Evidence that fibrin alpha-chain RGDX sequences are not required for platelet adhesion in flowing whole blood.

The role of the RGDX putative receptor-recognition sites, which are present on the alpha chains of fibrin, in promoting platelet adhesion has been examined in flowing whole blood using the rectangular perfusion chamber at wall shear rates of 340 and 1,600/s. Platelets adhered to a comparable extent to surfaces coated with native fibrin and surfaces coated with fragment X-fibrin, a product of limited fibrinolysis that lacks the RGDS sites normally present at positions 572 to 575 of the alpha chains. The strengths of these adhesive interactions were comparable based on the concentrations of the antiadhesive peptide D-RGDW required to block platelet deposition to native and fragment X-fibrin at both low and high wall shear rate. Blocking either or both RGDX sequences with peptide-specific monoclonal antibodies did not inhibit platelet deposition in perfusion experiments performed with normal blood at 340/s, indicating that neither RGD motif is required for adhesion. However, adhesion was partly inhibited by anti-RGDX antibodies when perfusions were performed with blood from an afibrinogenemic patient, suggesting the RGDX sequences may play a limited role in platelet deposition. Exposure of fibrin surfaces to plasminogen/tissue-type plasminogen activator did cause a time-dependent loss of adhesiveness, but this effect was only weakly correlated with proteolysis of the fibrin alpha chains. These observations provide evidence that neither RGDX sequence is required for platelets to adhere avidly to fibrin in flowing blood. These results further suggest that incomplete fibrinolysis yields a highly thrombogenic surface.

Platelet adhesion to fibrin(ogen).

We have found that glycoprotein IIb:IIIa (GPIIb:IIIa) expressed on nonstimulated platelets is the primary receptor for platelet adhesion to immobilized fibrinogen or fibrin. At low shear rates of the blood the interaction between GPIIb:IIIa and fibrin(ogen) is strong enough to resist the shear forces exerted on the platelet as was shown with experiments with antibodies against platelet membrane glycoproteins and perfusion studies with blood from patients lacking platelet membrane receptors. Impaired platelet adhesion to fibrin(ogen) was found with blood from a patient with Glanzmann's thrombasthenia (lacking GPIIb:IIIa), blood from patients with the Bernard-Soulier syndrome (lacking GPIb) and blood from patients with severe von Willebrand's disease. This indicates that at higher shear rates additional interactions via GPIb on the platelet and von Willebrand factor originating from plasma or platelets are necessary to increase the affinity of the platelet for fibrin(ogen).

Inhibition of platelet adhesion to fibrin(ogen) in flowing whole blood by Arg-Gly-Asp and fibrinogen gamma-chain carboxy terminal peptides.

We have employed synthetic peptides with sequences corresponding to the integrin receptor-recognition regions of fibrinogen as inhibitors of platelet aggregation and adhesion to fibrinogen- and fibrin-coated surfaces in flowing whole blood, using a rectangular perfusion chamber at wall shear rates of 300 s-1 and 1,300 s-1. D-RGDW caused substantial inhibition of platelet aggregation and adhesion to fibrinogen and fibrin at both shear rates, although it was least effective at blocking platelet adhesion to fibrin at 300 s-1. RGDS was a weaker inhibitor, and produced a biphasic dose-response curve; SDRG was inactive. HHLGGAKQAGDV partially inhibited platelet aggregation and adhesion to fibrin(ogen) at both shear rates. These results support the identification of an RGD-specific receptor, most likely the platelet integrin glycoprotein IIb:IIIa, as the primary receptor responsible for platelet:fibrin(ogen) adhesive interactions under flow conditions, and indicate that platelet adhesion to surface bound fibrin(ogen) is stabilized by multivalent receptor-ligand contacts.