Thrombin generates previously unidentified C5 products that support the terminal complement activation pathway

Abstract

The coagulation and complement pathways simultaneously promote homeostasis in response to injury but cause tissue damage when unregulated. Mechanisms by which they cooperate are poorly understood. To delineate their interactions, we studied the effects of thrombin and C5 convertase on C5 in purified and plasmabased systems, measuring release of the anaphylatoxin C5a, and generation of C5b, the initial component of the lytic membrane attack complex. Thrombin cleaved C5 poorly at R751, yielding minimal C5a and C5b. However, thrombin efficiently cleaved C5 at a newly identified, highly conserved R947 site, generating previously undescribed intermediates C5 T and C5b T . Tissue factor-induced clotting of plasma led to proteolysis of C5 at a thrombin-sensitive site corresponding to R947 and not R751. Combined treatment of C5 with thrombin and C5 convertase yielded C5a and C5b T , the latter forming a C5b T -9 membrane attack complex with significantly more lytic activity than with C5b-9. Our findings provide a new paradigm for complement activation, in which thrombin and C5 convertase are invariant partners, enhancing the terminal pathway via the generation of newly uncovered C5 intermediates. Delineating the molecular links between coagulation and complement will provide new therapeutic targets for diseases associated with excess fibrin deposition and complement activation. (Blood. 2012;120(8):1717-1725)

Targeting complement component 5a promotes vascular integrity and limits airway remodeling

“…Thrombin has been reported to be able to substitute for C5 convertase and activate C5 directly (10,24,25). However, recent studies indicate that direct activation of C5 by thrombin can occur but is inefficient; instead of cleaving C5 at the C5 convertase cleavage site, thrombin cleaves C5 at a unique site, generating a unique C5 cleavage product (C5b T ), which is further cleaved by C5 convertase, resulting in a C5b T -9 membrane attack complex with enhanced lytic activity (26). Neutrophil elastase (27), plasmin, macrophage serine proteases (28), Factors Xa, IXa, and XIa (22) have all been reported to have C5 convertase activity.…”

Combined Inhibition of Complement and CD14 Efficiently Attenuated the Inflammatory Response Induced byStaphylococcus aureusin a Human Whole Blood Model

“…However, the applicability of the wholeblood model is limited by the neutralized thrombin. Indeed, it has been postulated that thrombin may activate complement directly at the level of C5 (52). Thus, our findings have to be tested in animal models by using species-reactive complement inhibitors and CD14 blockers to better address the complexity of a biological system.…”

Antigen Modulation Confers Protection to Red Blood Cells from Antibody through Fcγ Receptor Ligation

“…Thus, these data do not rule out the involvement of C3 but do demonstrate that C3 is not required for clearance in this system. Although C3 engagement is thought to be the primary complement effector pathway whereby Abs initiate complement, several C3-independent pathways of complement activation have been described (32), suggesting that downstream targets of com- plement activation may be involved in cellular clearance. To test this, we next transferred Ag + cells into immunized FVB recipients, which are deficient in the C5 component of complement required for initiation of the membrane attack complex (33).…”