High accumulation of plasminogen and tissue plasminogen activator at the flow surface of mural fibrin in the human arterial system

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

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.

Vascular tissue engineering and vascularized 3D tissue regeneration

Vascularized tissue regeneration has a great deal of potential in clinical medicine. Appropriate 3D tissue regeneration that yields tissue with the desired function and shape requires both growth signals and vascularization. In this paper, we discuss vascularized tissue regeneration using various vessel systems: artificial vessel, autologous vascular graft, autologous vascular bundle transfer and tissue engineered vessel. Vascularized 3D tissue regeneration will require a great deal of additional research before it can be applied to clinical situations. Several promising studies of vascularized tissue regeneration have been reported. However, additional studies into the maturation of neovascularization, the development of effective biomaterial, and the possibility of using stem cells will be needed before these techniques can be used in the clinical situation.

Topology of the fibrinolytic system within the mural thrombus of human abdominal aortic aneurysms

Development and progression of acquired abdominal aortic aneurysms (AAAs) involve proteolytic activity. In the present study, we investigate the distribution of fibrinolytic system components within mural thrombi of human AAAs. 20 mural thrombi and the remaining AAA walls were dissected. The luminal, intermediate and abluminal thrombus layers, and media and adventitia were separately incubated in cell culture medium. Conditioned media were then analysed for plasminogen activators (PAs), plasminogen activator inhibitor-1 (PAI-1), free-plasmin, plasmin alpha(2)-antiplasmin complexes (PAPs) and D-dimers release. In parallel, PA and PAI-1 mRNA expression analysis was performed by RT-PCR. The study was completed by immunohistochemical localization of these components in AAA, ex vivo functional imaging using (99m)Tc-aprotinin as a ligand and measurement of PAP and D-dimer plasma levels. All fibrinolytic system components were present in each aneurysmal layer. However, the mural thrombus was the main source of active serine-protease release. Interestingly, the luminal layer of the thrombus released greater amounts of PAPs and D-dimers. This paralleled the preferential immunolocalization of plasminogen and PAs, and the (99m)Tc-aprotinin scintigraphic signal observed in the luminal pole of the thrombus. In contrast, mRNA expression analysis showed an exclusive synthesis of tPA and PAI-1 within the wall, whereas uPA mRNA was also expressed within the thrombus. Taken together, these results suggest that the increased plasma concentrations of PAPs and D-dimers found in AAA patients are related to mural thrombus proteolytic activity, thus explaining their known link with AAA progression. Components of the fibrinolytic system could also represent a target for functional imaging of thrombus activities in AAA.

Hybrid Small-Caliber Vascular Prosthesis for Coronary Artery Bypass Grafting: A Preliminary Study of Plasmin-Treated Fibrin-Coated Vascular Prosthesis

The potential use of plasmin-treated fibrin-coated vascular prosthesis (PF-V) for coronary artery bypass grafting (CABG) in animal models was investigated. PF-V grafts, 3 mm in internal diameter, were studied on 5 sheep in off-pump CABG model and on 18 rabbits in abdominal aortic bypass grafting (AABG) model. Patency, blood flow, angiography, Indium-111 platelet scintigraphy, and histology of the graft were evaluated. In the sheep CABG model, the PF-V grafts were patent for a range of 12 to 22 days without postoperative antiplatelet therapy. Graft flows ranged 58 to 90 ml/min until the day before graft occlusion by thrombus. In rabbit AABG model, the fibrin coating of the PF-V grafts was completely absorbed and replaced with neofibrin net between 7 and 14 days after implantation. Platelet depositions on the graft between 7 and 14 days after implantation were significantly higher than those at other periods (p < 0.05). The small-caliber PF-V graft in sheep CABG model had a good blood flow with high antithrombogenicity in acute phase, but occluded over 2 to 3 weeks without antiplatelet agents after implantation. The current problem of the PF-V graft was a thrombus formation at the time of the degradation of fibrin coating. Further improvements are needed.

Expression of fibrinolysis activators and their inhibitor in neointima of polyester vascular grafts

The aim of the study was to evaluate dynamic changes in the expression of fibrinolytic system components in neointima forming in polyester vascular grafts. The study was carried out on 18 mongrel dogs divided into three groups, that underwent replacement of abdominal aorta with a polyester double velour prosthesis. Grafts were removed at 1, 4 and 12 months. The specimens were fixed according to AMeX method. Immunohistochemical labeling for von Willebrand factor (vWf), tissue plasminogen activator (t-PA), urokinase (u-PA), its receptor (u-PAR), plasminogen activator inhibitor type 1 (PAI-1) and D-dimer (DD) was performed. Increasing intensity of vWf expression on the graft luminal surface was found in successive periods of the study. A light positive t-PA and u-PA staining was shown in neointima at 1 month and its intensity was significantly increased at 4 and 12 months. Expression of u-PAR appeared at 4 months. A light positive PAI-1 and DD staining was demonstrated in neointima in all periods of the study. The results demonstrated increasing expression of fibrinolysis activators in neointima of polyester vascular grafts. Intensive expression of plasminogen activators, when compared to their inhibitor may reduce thrombotic properties of graft neointima particularly in the late period after prosthesis implantation.

Basic principles in thrombolysis: regulatory role of plasminogen

During thrombolytic therapy, patients are treated with a plasminogen activator in order to stimulate the fibrinolytic system by converting the precursor plasminogen into the active enzyme plasmin. The fibrinolytic process can be divided into two phases. In the first phase, plasminogen binds to intact fibrin and initial fibrinolysis takes place. As a result, carboxyterminal lysine residues are generated, which represent new binding sites for plasminogen. In the second phase, plasminogen binds to these sites and fibrinolysis is accelerated because the local plasminogen concentration is strongly enhanced and because this plasminogen has a higher reactivity. For instance, both single-chain urokinase-type plasminogen activator (scu-PA) and staphylokinase have a high preference for this type of plasminogen, which explains their fibrin-selective action. A recently discovered thrombin-activatable fibrinolysis inhibitor (TAFI) eliminates carboxyterminal lysine residues from partially degraded fibrin and, thus, inhibits the second phase of fibrinolysis. These mechanisms show that plasminogen plays an important regulatory role in fibrinolysis and thrombolysis.