Pauline F. Marx

Thrombin-Activatable Fibrinolysis Inhibitor (TAFI, Plasma Procarboxypeptidase B, Procarboxypeptidase R, Procarboxypeptidase U)

Recently, a new inhibitor of fibrinolysis was described, which downregulated fibrinolysis after it was activated by thrombin, and was therefore named TAFI (thrombin-activatable fibrinolysis inhibitor; EC 3.4.17.20). TAFI turned out to be identical to the previously described proteins, procarboxypeptidase U, procarboxypeptidase R, and plasma procarboxypeptidase B. Activated TAFI (TAFIa) downregulates fibrinolysis by the removal of carboxy-terminal lysines from fibrin. These carboxy-terminal lysines are exposed upon limited proteolysis of fibrin by plasmin and act as ligands for the lysine-binding sites of plasminogen and tissue-type plasminogen activator (t-PA). Elimination of these lysines by TAFIa abrogates the fibrin cofactor function of t-PA-mediated plasminogen activation, resulting in a decreased rate of plasmin generation and thus downregulation of fibrinolysis. In this review, the characteristics of TAFI are summarized, with an emphasis on the pathways leading to activation of TAFI and the role of TAFIa in the inhibition of fibrinolysis. However, it cannot be ruled out that TAFI has other, as yet undefined, functions in biology.

Development of a Genotype 325–Specific proCPU/TAFI ELISA

Objective—A Thr/Ile polymorphism at position 325 in the coding region of proCPU has been reported. Immunological assays, fully characterized (including genotype dependency), are required for the quantitation of proCPU levels. Methods and Results—We have generated a panel of monoclonal antibodies against human, plasma-derived proCPU. Two combinations exhibiting distinct reactivities were selected for measurement of proCPU in plasma. T12D11/T28G6-HRP yielded values of 10.1±3.1 &mgr;g/mL (mean±SD, n=86; normal donors), and T32F6/T9G12-HRP yielded values of 5.4±3.0 &mgr;g/mL. Grouping according to the 325 genotype demonstrated that T12D11/T28G6-HRP was independent to this polymorphism whereas T32F6/T9G12-HRP revealed a complete lack of reactivity with the Ile/Ile genotype (ie, 0.0±0.0, 4.2±1.7, and 7.3±2.9 &mgr;g/mL for the Ile/Ile, Ile/Thr, and Thr/Thr isoforms, respectively). Commercially available antigen assays appeared to be partially dependent on the 325 genotype (eg, 44±8.9% and 100±30% for the Ile/Ile and Thr/Thr isoforms, respectively). Conclusions—Our data demonstrate that great care should be taken when evaluating proCPU antigen values as a putative causative agent or as a diagnostic risk marker for cardiovascular events.

Thrombin-activatable Fibrinolysis Inhibitor Binds to Streptococcus pyogenes by Interacting with Collagen-like Proteins A and B

Regulation of proteolysis is a critical element of the host immune system and plays an important role in the induction of pro- and anti-inflammatory reactions in response to infection. Some bacterial species take advantage of these processes and recruit host proteinases to their surface in order to counteract the host attack. Here we show that Thrombin-activatable Fibrinolysis Inhibitor (TAFI), a zinc-dependent procarboxypeptidase, binds to the surface of group A streptococci of an M41 serotype. The interaction is mediated by the streptococcal collagen-like surface proteins A and B (SclA and SclB), and the streptococcal-associated TAFI is then processed at the bacterial surface via plasmin and thrombin-thrombomodulin. These findings suggest an important role for TAFI in the modulation of host responses by streptococci.

Activation of TAFI on the Surface of Streptococcus pyogenes Evokes Inflammatory Reactions by Modulating the Kallikrein/Kinin System

Bacteria-controlled regulation of host responses to infection is an important virulence mechanism that has been demonstrated to contribute to disease progression. Here we report that the human pathogen Streptococcus pyogenes employs the procarboxypeptidase TAFI (thrombin-activatable fibrinolysis inhibitor) to modulate the kallikrein/kinin system. To this end, bacteria initiate a chain of events starting with the recruitment and activation of TAFI. This is followed by the assembly and induction of the contact system at the streptococcal surface, eventually triggering the release of bradykinin (BK). BK is then carboxyterminally truncated by activated TAFI, which converts the peptide from a kinin B2 receptor ligand to a kinin B1 receptor (B1R) agonist. Finally, we show that streptococcal supernatants indirectly amplify the B1R response as they act on peripheral blood mononuclear cells to secrete inflammatory cytokines that in turn stimulate upregulation of the B1R on human fibroblasts. Taken together our findings implicate a critical and novel role for streptococci-bound TAFI, as it processes BK to a B1R agonist at the bacterial surface and thereby may redirect inflammation from a transient to a chronic state.

Recent developments in thrombin-activatable fibrinolysis inhibitor research.

Thrombin-activatable fibrinolysis inhibitor (TAFI) provides an important molecular link between the coagulation and fibrinolytic systems. In this review, recent major advances in TAFI research, including the elucidation of crystal structures, the development of small inhibitors and the role of TAFI in systems other than hemostasis, are described and discussed.

TAFI: regulating the cross talk between coagulation and fibrinolysis TAFI: Regulierung der Wechselwirkung zwischen Gerinnung und Fibrinolyse

Abstract Thrombin activates thrombin-activatable fibrinolysis inhibitor (TAFI) that removes C-terminal lysines or arginines. The activity of active TAFI (TAFIa) is lost rapidly. TAFIa is not inactivated by proteolysis but converted to an inactive state by a conformational transition. Numerous polymorphisms were identified in the TAFI gene. The TAFI-325Ile variant seems to influence TAFI levels. TAFIa retards plasmin formation and makes plasmin more susceptible to inhibition by antiplasmin; it prevents the conversion of the fibrin fragment DD(E) to fragment DD that impairs fibrin polymerization. The complement-derived factors C3a and C5a as well as bradykinin are further substrates for TAFIa. Elevated TAFI levels were associated with an increased risk of venous thrombosis. TAFI deficiency has been shown to be associated with an enhanced leucocyte migration. The absence of TAFI results in delayed wound healing with disturbed keratinocyte migration. Pro-inflammatory properties of osteopontin are downregulated by TAFIa. Hence, TAFI plays a role – besides in regulation of fibrinolysis – in wound healing, angiogenesis, and inflammation.