The lectin complement pathway serine proteases (MASPs) represent a possible crossroad between the coagulation and complement systems in thromboinflammation.

UNLABELLED: ESSENTIALS: The lectin pathway's MASP-1/2 activates coagulation factors but the trigger of the activation is unknown. MASP-1/2 activation was assessed by quantifying complexes between MASPs and antithrombin/C1-inhibitor. Activated platelets and fibrin were demonstrated to activate MASP-1 and MASP-2 both in vitro and in vivo. These findings may represent a crossroad between the complement and the coagulation systems. BACKGROUND: The activated forms of the complement lectin pathway (LP) proteases MASP-1 and MASP-2 are able to cleave the coagulation factors prothrombin, fibrinogen, factor XIII and thrombin-activatable fibrinolysis inhibitor in vitro. In vivo studies also show that MASP-1 is involved in thrombogenesis. OBJECTIVES: To clarify the not yet identified mechanisms involved in triggering activation of the LP during thrombotic reactions. METHODS: Novel sandwich-ELISAs for detection of complexes between MASP-1 or MASP-2 and the serpins C1 inhibitor (C1-INH) or antithrombin (AT), were used to specifically detect and quantify the activated forms of MASP-1 and MASP-2. RESULTS: Activated platelets were shown by flow cytometry to bind Ficolin-1, -2 and -3 but not MBL, which was associated with activation of MASP-1 and MASP-2. We also demonstrated that fibrin and the plasmin-generated fibrin fragment DD in plasma, bind and activate MASP-1 and MASP-2. As demonstrated by the ELISA and SDS-PAGE/Western blotting, the fibrin-associated activation was reflected in a specific inactivation by AT during clotting without the assistance of heparin. In all other cases the MASPs were, as previously reported, inactivated by C1-INH. In systemic lupus erythematosus patients with thrombotic disease and in polytrauma patients, the levels of activated MASP-1 and MASP-2 in complex with both AT and C1-INH were associated with markers of thrombotic disease and contact/coagulation system activation. CONCLUSIONS: MASP-1 and MASP-2 are activated during blood clotting. This activation is triggered by activated platelets and by the generation of fibrin during thrombotic reactions in vitro and in vivo, and may represent a novel activation/amplification mechanism in thromboinflammation.

Complement activation triggered by chondroitin sulfate released by thrombin receptor-activated platelets.

BACKGROUND: Chondroitin sulfate (CS) is a glycosaminoglycan released by activated platelets. OBJECTIVE: Here we test the hypothesis that CS released by activated platelets can trigger complement activation in the fluid phase. METHODS AND RESULTS: Thrombin receptor-activating peptide (TRAP)-6 was used to activate platelets in platelet-rich plasma and blood, anticoagulated with the thrombin inhibitor lepirudin. TRAP activation induced fluid-phase complement activation, as reflected by the generation of C3a and sC5b-9, which could be attenuated by the C3 inhibitor compstatin. Chondroitinase ABC treatment of supernatants from activated platelets totally inhibited the activation, indicating that platelet-derived CS had initiated the complement activation. Furthermore, addition of purified CS to plasma strongly triggered complement activation. C1q was identified as the recognition molecule, as it bound directly to CS, and CS-triggered complement activation could be restored in C1q-depleted serum by adding purified C1q. TRAP activation of whole blood increased the expression of CD11b on leukocytes and generation of leukocyte-platelet complexes. It was demonstrated that these leukocyte functions were dependent on C3 activation and signaling via C5a, as this expression could be inhibited by compstatin and by a C5aR antagonist. CONCLUSIONS: We conclude that platelets trigger complement activation in the fluid phase by releasing CS, which leads to inflammatory signals mediated by C5a.