Complement C5 but not C3 is expendable for tissue factor activation by cofactor-independent antiphospholipid antibodies

Abstract

The complement and coagulation cascades interact at multiple levels in thrombosis and inflammatory diseases. In venous thrombosis, complement factor 3 (C3) is crucial for platelet and tissue factor (TF) procoagulant activation dependent on protein disulfide isomerase (PDI). Furthermore, C5 selectively contributes to the exposure of leukocyte procoagulant phosphatidylserine (PS), which is a prerequisite for rapid activation of monocyte TF and fibrin formation in thrombosis. Here, we show that monoclonal cofactor-independent antiphospholipid antibodies (aPLs) rapidly activate TF on myelomonocytic cells. TF activation is blocked by PDI inhibitor and an anti-TF antibody interfering with PDI binding to TF, and requires C3 but unexpectedly not C5. Other prothrombotic, complement-fixing antibodies, for example, antithymocyte globulin, typically induce TF activation dependent on C5b-7-mediated PS exposure on the outer membrane of monocytes. We show that aPLs directly induce procoagulant PS exposure independent of C5. Accordingly, mice deficient in C3, but not mice deficient in C5, are protected from in vivo thrombus formation induced by cofactor-independent aPLs. Only immunoglobulin G (IgG) fractions with cofactor-independent anticardiolipin reactivity from patients with antiphospholipid syndrome (APS) induce complement-independent monocyte PS exposure and PDI-dependent TF activation. Neither a human monoclonal aPL directed against β2-glycoprotein I (β2GPI) nor patient IgG with selective reactivity to β2GPI rapidly activated monocyte TF. These results indicate that inhibitors of PDI and TF, but not necessarily clinically available drugs targeting C5, have therapeutic benefit in preventing thrombosis associated with APS caused by pathogenic aPLs primarily reactive with lipid, independent of β2GPI.

Tissue factor at the crossroad of coagulation and cell signaling

“…These aPLs, which are not reactive with the β2‐glycoprotein I cofactor, induce thrombosis independent of C5 and low‐density lipoprotein receptor‐related protein 8 (Lrp8) interacting with β2‐glycoprotein I. In contrast, C3 deficiency prevents amplified venous thrombus formation in an inferior vena cava thrombosis model in mice, consistent with the in vitro properties of aPLs in TF activation . Complement activation is also crucial in flow‐restricted venous thrombosis models in the mouse.…”

“…A distinction between these two types of EV is the affinity of TF for its ligand FVIIa. The PDI chaperone function enhances TF‐FVIIa affinity and is blocked by an antibody to TF, 10H10 , which reduces EV TF‐FVIIa affinity and markedly diminishes EV prothrombotic activity . Similarly, monocyte TF is activated by antiphospholipid antibodies (aPLs) that expose PS independent of PDI, but TF activation nevertheless requires the PDI oxidoreductase function and TF interactions that are blocked by anti‐TF 10H10 .…”

Protein disulfide isomerase in cardiovascular disease

“…Complement factors are crucial for venous thrombosis; C3 contributes to platelet and tissue factor procoagulant activation, and C5 is crucial for exposure of leukocyte procoagulant phosphatidylserine 89 . Muller-Calleja et al demonstrated that tissue factor activation is reduced by 16F16 (a PDI inhibitor) or 10H10 (an anti-tissue factor antibody that blocks PDI-tissue factor binding) and requires C3 but not C5 90 . Although this report is consistent with the hypothesis that PDI decrypts tissue factor by oxidizing the Cys186-Cys209 disulfide bond or through its chaperone activity, several studies undermined the hypothesis [91][92][93] .…”

Cofactor-Independent Antiphospholipid Antibodies: Implications for Pathogenesis, Diagnosis, and Treatment of Antiphospholipid Syndrome

“…[43][44][45][46] From the perspective of pathophysiology HCQ, rapamycin and complement inhibitors are of particular interest, because these substances have been shown to inhibit signalling induced by aCL and anti-β2GPI, respectively. 33,45,47 Due to the high costs, specific complement inhibition by Eculizumab has been used only in few selected patients with variable results. 46 The lack of controlled studies also raises a concern of publication bias.…”

“…Accordingly, C3deficient mice but not C5-deficient mice were protected from the prothrombotic effects of such aCL. 47 Therefore, an inhibitor of C5 like Eculizumab would be expected to be ineffective against TF activation by aCL, but might still be effective in preventing activation of the lytic complement complex.…”