The Dual Role Of Platelets In Thrombus Formation

The formation of a hemostatic thrombus during blood vessel injury is a highly regulated event which ensures that a blood clot is sufficiently stable but not overly robust to cause thrombus propagation and vessel occlusion. We have previously identified two anti-adhesive processes operating on the surface of fibrin clots that prevent excessive accumulation of blood cells such as platelets and leukocytes. In particular, binding of fibrinogen and plasminogen to fibrin creates a mechanically unstable surface through non-proteolytic and proteolytic mechanisms, respectively, which limits integrin-mediated cell adhesion. The proteolytic process depends on activation of plasminogen, which accumulates in a thin superficial layer of fibrin clots, by the plasminogen activation system assembled on transiently adherent blood cells. This is followed by plasmin generation and decomposition of the fibrin surface resulting in cell detachment under flow. We have further demonstrated that plasminogen activators tPA or uPA pre-bound to platelets can activate fibrin-bound plasminogen through the interfacial mechanism, i.e. when the components of the plasminogen activation system are assembled on different surfaces. Furthermore, plasminogen activators pre-bound to one population of platelets are able to activate plasminogen pre-bound to a separate pool of platelets. These data indicate that platelets activate both platelet- or fibrin-bound plasminogen and, potentially, could use their fibrinolytic potential during adhesion to fibrin clots. However, platelets are known also to contain high concentrations of PAI-1 which is secreted during clot formation. Therefore, it is uncertain whether the proteolytic anti-adhesive mechanism is functional when platelets encounter platelet-rich fibrin clots. To begin to address this question, we have compared fibrinolytic activity of tPA- or uPA-treated platelets placed on the surface of plasminogen-coated fibrin clots with that of platelets incorporated inside the gels prepared by mixing of fibrinogen with thrombin in the presence of uPA. The fibrinolytic activity was determined by detecting the release of ¹²⁵Iodine-labeled fibrin degradation products or measuring plasmin amidolytic activity. While fibrinolysis by platelets inside the gel was completely inhibited, platelets adherent to the surface of fibrin clots retained their ability to activate plasminogen and degrade fibrin. Also, when both platelets and monocytic U937 cells, a cell line expressing endogenous uPA, were included in fibrin clots, platelets completely suppressed fibrinolytic activity of U937 cells. At the same time, proteolytic activity of U937 cells added on the top of platelet-rich gels was not affected. Confocal microscopy and Western blot analyses showed that by contrast with platelets incorporated in the gel which released substantial amounts of PAI-1, no PAI-1 was secreted from platelets adherent to the surface of the gel. These data may explain the well-known but hitherto unexplained capacity of platelets to either promote or inhibit fibrinolysis by introducing the idea that two different functions of platelets are relevant to different regions of the thrombus. Thus, platelets inside the clot, via the release of PAI-1, block fibrinolysis aiding in thrombus integrity. However, platelets transiently contacting fibrin are profibrinolytic enabling the anti-adhesive mechanism which prevents their excessive accumulation and, consequently, thrombus propagation.