Antibodies to FXa and thrombin in patients with SLE differentially regulate C3 and C5 cleavage

Abstract

ObjectivesThe significance of antibodies directed against activated factor X (FXa) and thrombin (Thr) in patients with SLE and/or antiphospholipid syndrome (APS) is unknown. FXa and Thr are coregulated by antithrombin (AT) and activate complement. Therefore, we studied the ability of anti activated factor X (aFXa) and/or anti-(a)Thr IgG from patients with SLE±APS to modulate complement activation.MethodsPatients with SLE±APS were selected on the basis of known aThr and/or aFXa IgG positivity, and the effects of affinity-purified aFXa/aThr IgG on FXa and Thr-mediated C3 and C5 activation were measured ±AT. Structural analyses of FXa and Thr and AT–FXa and AT–Thr complexes were analysed in conjunction with the in vitro ability of AT to regulate aFXa–FXa and aThr–Thr-mediated C3/C5 activation.ResultsUsing affinity-purified IgG from n=14 patients, we found that aThr IgG increased Thr-mediated activation of C3 and C5, while aFXa IgG did not increase C3 or C5 activation. Structural analysis identified potential epitopes and predicted a higher likelihood of steric hindrance of AT on FXa by aFXa IgG compared with the AT–Thr–aThr IgG complex that was confirmed by in vitro studies. Longitudinal analysis of 58 patients with SLE (±APS) did not find a significant association between positivity for aFXa or aTHr IgG and C3 levels or disease activity, although there was a trend for patients positive for aFXa IgG alone or both aFXa and aThr IgG to have lower levels of C3 compared with aThr IgG alone during clinical visits.ConclusionsWe propose a novel method of complement regulation in patients with SLE±APS whereby aFXa and aThr IgG may have differential effects on complement activation.

A Preclinical Investigation on the Role of IgG Antibodies against Coagulant Components in Multiple Sclerosis

“…When the reactive center loop (RCL) of the antithrombin III anticoagulant molecule binds to the heparin-like glycosaminoglycan-rich domains of the clotting antigen, the protease cleaves the scissile P1-P1 bond of the RCL to form the P1-protease complex that results in the inactivation of the protease. Antibodies against coagulant components, on the other hand, can act as antagonists for antithrombin III binding capability to thrombin, preventing the formation of an antithrombin III-protease complex [ 35 ], inhibiting protease inactivation, and contributing to the thrombosis-inflammation interplay [ 36 ]. For instance, antibodies against FXa have been detected in patients diagnosed with SLE and APS, interfering with the antithrombin-mediated FXa inhibition and causing further thrombin generation [ 15 ].…”

An update on inflammation in antiphospholipid syndrome

“…Interestingly, aThr potentiated thrombin-mediated activation of C3 and C5, whereas aFXa IgG did not increase C3 or C5 activation [49]. Having said that, the longitudinal evaluation of 58 patients with SLE ± APS did not reveal a significant association between positivity for aFXa or aThr and C3 levels or disease activity [49]; one wonders if a study of primary APS patients might show something different given the many factors that likely contribute to complement activation in SLE. A 2021 multicenter retrospective study evaluated preconception complement levels (C3 and C4) in 197 pregnant patients with APS or aPL and found that low preconception C3 and C4 were associated with a significantly higher prevalence of pregnancy losses [50 ▪ ].…”

“…It was also shown that in patients with thrombotic APS, evidence of these circulating immune complexes was associated with thrombocytopenia, heart valve thickening/dysfunction, and lower levels of C3 and C4 [48 ▪▪ ]. A recent 2022 study evaluated the significance of antibodies directed against factor Xa and thrombin [49]. Antifactor Xa antibodies (aFXa) and antithrombin antibodies (aThr) were affinity purified from patients with SLE ± APS [49].…”

“…A recent 2022 study evaluated the significance of antibodies directed against factor Xa and thrombin [49]. Antifactor Xa antibodies (aFXa) and antithrombin antibodies (aThr) were affinity purified from patients with SLE ± APS [49]. Interestingly, aThr potentiated thrombin-mediated activation of C3 and C5, whereas aFXa IgG did not increase C3 or C5 activation [49].…”

“…Antifactor Xa antibodies (aFXa) and antithrombin antibodies (aThr) were affinity purified from patients with SLE ± APS [49]. Interestingly, aThr potentiated thrombin-mediated activation of C3 and C5, whereas aFXa IgG did not increase C3 or C5 activation [49]. Having said that, the longitudinal evaluation of 58 patients with SLE ± APS did not reveal a significant association between positivity for aFXa or aThr and C3 levels or disease activity [49]; one wonders if a study of primary APS patients might show something different given the many factors that likely contribute to complement activation in SLE.…”