Complement C5a receptors and neutrophils mediate fetal injury in the antiphospholipid syndrome

Abstract

Antiphospholipid syndrome (APS) is defined by recurrent pregnancy loss and thrombosis in the presence of antiphospholipid (aPL) Ab's. Currently, therapy for pregnant women with APS is focused on preventing thrombosis, but anticoagulation is only partially successful in averting miscarriage. We hypothesized that complement activation is a central mechanism of pregnancy loss in APS and tested this in a model in which pregnant mice receive human IgG containing aPL Ab's. Here we identify complement component C5 (and particularly its cleavage product C5a) and neutrophils as key mediators of fetal injury, and we show that Ab's or peptides that block C5a-C5a receptor interactions prevent pregnancy complications. The fact that F(ab)′2 fragments of aPL Ab's do not mediate fetal injury and that C4-deficient mice are protected from fetal injury suggests that activation of the complement cascade is initiated via the classical pathway. Studies in factor B-deficient mice, however, indicate that alternative pathway activation is required and amplifies complement activation. In contrast, activating FcγRs do not play an important role in mediating aPL Ab-induced fetal injury. Our findings identify the key innate immune effectors engaged by pathogenic autoantibodies that mediate poor pregnancy outcomes in APS and provide novel and important targets for prevention of pregnancy loss in APS.

Requirement of activation of complement C3 and C5 for antiphospholipid antibody–mediated thrombophilia

“…In previous studies from our group, the findings suggested that C3 activation is required for (4). Subsequently, Girardi et al showed that complement activation, specifically C5a-C5aR interaction, is required for aPL-induced pregnancy loss and suggested that C5a promotes neutrophil infiltration of decidual tissue (5). Evidence that neutrophils activated by C5a release procoagulant substances and that monocytes activated by C5a release tissue factor suggests that infiltrating leukocytes stimulated by complement split products can initiate placental infarction and ultimately cause fetal death (8).…”

“…Passive transfer of IgG from aPL-positive sera (IgG-APS) has been found to induce fetal loss, thrombosis, and endothelial cell activation in mice, suggesting a direct pathogenic role of aPL (1)(2)(3). Complement activation is a necessary intermediary event in the pathogenesis of fetal loss associated with aPL in this model (4,5).…”

“…Anti-C5 mAb have been shown to prevent C5 activation in vivo and in vitro and to protect mice from aPL-induced pregnancy loss (5,13). To further investigate the role of C5 in thrombophilia induced by aPL, male CD1 mice (weight 25-30 grams; Charles River Laboratories) were injected intraperitoneally with 500 g of IgG-APS or IgG-NHS (n ϭ 10 mice per group) at time 0 and at 48 hours later.…”

The Central Role of the Alternative Complement Pathway in Human Disease

“…Although the pathogenesis of this syndrome had been attributed to the ability of these Abs to directly modify clotting-related activities or cell activation (34), using a murine model of aPL syndrome in which pregnant mice are injected with human aPL Ab-containing IgG Abs, it was found that inhibition of the complement cascade in vivo using the C3 convertase inhibitor Crry-Ig, or using mice deficient in complement C3, resulted in the absence of fetal loss and growth retardation (35). In a subsequent study, the authors demonstrated that several factors are necessary for the development of disease in this model, including C4, factor B, C5a, and neutrophils (21). Interestingly, the depletion of neutrophils prevented C3 deposition in the deciduas, perhaps because infiltrating neutrophils (initially recruited by classical pathway activation) fuel complement activation by releasing alternative pathway components.…”

“…It may be that the initial insult induces deficiencies of regulatory proteins in necrotic cells or regions of injury, setting the stage for amplification of injury by the alternative pathway. Alternatively, infiltrating cells such as neutrophils bring in C3 and properdin that increase activation specifically at that site by providing additional substrate for the alternative pathway (21). It may also be that the amplification loop is much more robust in vivo than previously appreciated.…”