Plasminogen on the surfaces of fibrin clots prevents adhesion of leukocytes and platelets

Abstract

Summary Background and Objectives Although leukocytes and platelets adhere to fibrin with alacrity in vitro, these cells do not readily accumulate on the surfaces of fibrin clots in vivo. The difference in the capacity of blood cell integrins to adhere to fibrin in vivo and in vitro is striking and implies the existence of a physiologic antiadhesive mechanism. The surfaces of fibrin clots in the circulation are continually exposed to plasma proteins, several of which can bind fibrin and influence cell adhesion. Recently, we have demonstrated that adsorption of soluble fibrinogen on the surface of a fibrin clot results in its deposition as a soft multilayer matrix, which prevents attachment of blood cells. In the present study, we demonstrate that another plasma protein, plasminogen, which is known to accumulate in the superficial layer of fibrin, exerts an antiadhesive effect. Results After being coated with plasminogen, the surfaces of fibrin clots became essentially non-adhesive for U937 monocytic cells, blood monocytes, and platelets. The data revealed that activation of fibrin-bound plasminogen by the plasminogen-activating system assembled on adherent cells resulted in the generation of plasmin, which decomposed the superficial fibrin layer, resulting in cell detachment under flow. The surfaces generated after the initial cell adhesion remained non-adhesive for subsequent attachment of leukocytes and platelets. Conclusion We propose that the limited degradation of fibrin by plasmin generated by adherent cells loosens the fibers on the clot surface, producing a mechanically unstable substrate that is unable to support firm integrin-mediated cell adhesion.

Membrane microvesicles: a circulating source for fibrinolysis, new antithrombotic messengers

“…Plasminogen thus bound adopts an open conformation that is readily transformed into plasmin. Recently published studies [4][5][6] are in agreement with these hitherto unknown fibrinolytic pathways and potentially novel biomarkers in clinical practice.…”

The Assembly of Nonadhesive Fibrinogen Matrices Depends on the αC Regions of the Fibrinogen Molecule

“…Although self-association of fibrinogen is readily induced, suggesting its potential significance, no physiologic role has been ascribed to this process. We have recently proposed that the fibrinogen matrix deposited on the surface of fibrin clots may play an important role in normal hemostasis by preventing excessive adhesion of circulating blood cells, thus limiting thrombus propagation (6,43). The finding of the present study that the fibrinogen molecules without the ␣C regions are incapable of developing the multilayer with ensuing sustained cell adhesion may have significant implications with respect to the role of these derivatives in several pathological conditions.…”

Leukocyte integrin Mac-1 (CD11b/CD18, αMβ2, CR3) acts as a functional receptor for platelet factor 4

“…The THP-1 cells were cultured in RPMI containing 10% FBS, antibiotics, and 0.05 mM 2-mercaptoethanol. The THP-1 were differentiated by adding 10 ng/ml of PMA into the medium for 48 h. Human neutrophils were isolated under sterile conditions from peripheral blood obtained from healthy volunteers as described (51). Human monocytes were isolated from peripheral blood obtained from healthy volunteers using the EasySep Human monocyte isolation kit (StemCell Technologies, Cambridge, MA) according to the manufacturer's protocol.…”