Antibody-induced Pregnancy Loss Research Articles

Tissue factor in antiphospholipid antibody-induced pregnancy loss: a pro-inflammatory molecule.

Fetal loss in patients with antiphospholipid antibodies (aPL) has been ascribed to thrombosis of placental vessels. However, we have shown that inflammation, specifically complement activation with generation of the anaphylotoxin C5a, is an essential mediator of fetal injury. We have analysed the role of tissue factor (TF) in a mouse model of aPL-induced pregnancy loss. TF is the major cellular activator of the coagulation cascade but also has cell signaling activity. Mice that received aPL-IgG showed strong TF staining throughout the decidua and on embryonic debris. This TF staining was not associated with either fibrin staining or thrombi in deciduas. The absence of fibrin deposition and thrombi suggests that TF-dependent activation of coagulation does not mediate aPL-induced pregnancy loss.We found that either blockade of TF with a monoclonal antibody in wild type mice or a genetic reduction of TF prevented aPL-induced inflammation and pregnancy loss indicated a pathogenic role for TF in aPL-induced pregnancy complications. In response to aPL-generated C5a, neutrophils express TF potentiating inflammation in the deciduas and leading to miscarriages. Importantly, we showed that TF in myeloid cells, but not fetal-derived cells (trophoblasts), was associated with fetal injury, suggesting that the site for pathologic TF expression is neutrophils. We found that TF expression in neutrophils contributes to respiratory burst and subsequent trophoblast injury and pregnancy loss induced by aPL. The identification of TF, acting as an important pro-inflammatory mediator in aPL-induced fetal injury, provides a new target for therapy to prevent pregnancy loss in the aPL syndrome.

Tissue factor: A link between inflammation and thrombosis in antiphospholipid antibody-induced pregnancy loss

Tissue factor: A link between inflammation and thrombosis in antiphospholipid antibody-induced pregnancy loss

TNF-α Is a Critical Effector and a Target for Therapy in Antiphospholipid Antibody-Induced Pregnancy Loss

The antiphospholipid syndrome (APS) is characterized by recurrent fetal loss, intrauterine growth restriction, and vascular thrombosis in the presence of antiphospholipid (aPL) Abs. Our studies in a murine model of APS induced by passive transfer of human aPL Abs have shown that activation of complement and recruitment of neutrophils into decidua are required for fetal loss, and emphasize the importance of inflammation in aPL Ab-induced pregnancy loss. In this study, we examine the role of TNF-alpha in pregnancy complications associated with aPL Abs in a murine model of APS. We show that aPL Abs are specifically targeted to decidual tissue and cause a rapid increase in decidual and systemic TNF-alpha levels. We identify the release of TNF-alpha as a critical intermediate that acts downstream of C5 activation, based on the fetal protective effects of TNF-alpha deficiency and TNF blockade and on the absence of increased TNF-alpha levels in C5-deficient mice treated with aPL Abs. Our results suggest that TNF-alpha links pathogenic aPL Abs to fetal damage and identify TNF blockade as a potential therapy for the pregnancy complications of APS.

A novel inhibitor of the alternative complement pathway prevents antiphospholipid antibody-induced pregnancy loss in mice

Studies in gene-targeted mice have demonstrated that factor B of the alternative complement pathway plays an important role in several disease models, but an exogenous inhibitor of factor B has not previously been available. We have developed an inhibitory monoclonal antibody directed against a critical epitope on mouse factor B and have tested it in a model of antiphospholipid (aPL) antibody (Ab)-induced fetal loss. Gene-targeted factor B-deficient mice (fB −/−) were injected with a fusion protein comprised of the second and third short consensus repeat (SCR) domains of mouse factor B linked to a mouse IgG 1 Fc domain. Hybridomas were made from splenocytes of the immunized mouse. One mAb, designated 1379, produced an IgG 1 antibody that inhibited alternative pathway activation in vitro and in vivo by preventing formation of the C3bBb complex. Strikingly, this mAb inhibited alternative pathway activation in serum from mice, rats, humans, monkeys, pigs and horses. Fab fragments made from this mAb also inhibited alternative pathway activation. Epitope mapping demonstrated that this antibody binds to factor B within the third SCR domain. When mAb 1379 was administered to mice that also received human IgG containing antiphospholipid antibodies, it provided significant protection from antiphospholipid antibody-induced complement activation and fetal loss. Thus, this mAb to factor B has broad species reactivity and effectively inhibits alternative pathway activation. The mAb protects mice in an in vivo model of antiphospholipid antibody syndrome, demonstrating the therapeutic potential for the inhibition of factor B in this disease.

Inhibition of complement activation: a novel mechanism for the protective effects of heparin in antiphospholipid antibody-induced pregnancy loss

The antiphospholipid syndrome (APS), defined by thrombosis and recurrent pregnancy loss in the presence of antiphospholipid (aPL) antibodies, is generally treated with anticoagulation. Because complement activation is essential and causative in aPL antibody-induced fetal injury, we hypothesized that heparin protects mice from fetal loss in APS by preventing complement activation on trophoblasts and that anticoagulation, per se, is not sufficient to prevent miscarriage. We used a murine model of APS in which pregnant mice are injected with human IgG-containing aPL antibodies (aPL-IgG) or IgG from healthy individuals (NH-IgG). Passive transfer of aPL-IgG caused a 43.3 ± 3.5% frequency of fetal resorption (P < 0.001 versus NH-IgG) and treatment with either unfractionated heparin (UFH) (10 U or 20 U subcutaneously twice daily) or low molecular weight heparin (LMWH) (enoxaparin) reduced the frequency of fetal resorption to that of mice treated with NH-IgG: UFH10 + aPL, 9.6 ± 3.2; UFH20 + aPL 10.4 ± 2.1; LMWH + aPL, 11.8 ± 2.8; NH-IgG, 10.5 ± 2.5% (P < 0.01 versus aPL). Even in the absence of detectable anticoagulation, as in mice treated with 10 U UFH, heparins prevented aPL-induced pregnancy loss and inhibited systemic complement activation evidenced by generation of C3adesArg (aPL, 1212 ± 101* ng/ml; UFH10U + aPL, 178 ± 25; LMWH + aPL, 180 ± 15; NH-IgG, 215 ± 381, * P < 0.05 versus NH-IgG). Heparin limited C3 deposition in deciduas of mice treated with aPL. Neither fondaparinux nor hirudin inhibited the generation of complement split products or prevented pregnancy loss, despite anticoagulation levels comparable with heparin, indicating that anticoagulation is insufficient therapy for APS-associated miscarriage. We assessed the effects of anticoagulants on complement activation in vitro using BeWo cells, trophoblast-like cells with externalized phosphatidylserine recognized by aPL. APL-opsonized BeWo cells cultured with mouse serum activated complement, evidenced by increased surface-bound C3 fragments (detected as C3 staining by FACS) and generation of soluble C3adesArg in supernatants. In the presence of UFH or LMWH, however, complement activation was completely inhibited (C3adesArg [ng/ml]: aPL, 680 ± 30; UFH + aPL, 120 ± 20*; LMWH + aPL, 105 ± 18*; % C3-positive BeWo cells by FACS: aPL, 31 ± 11; aPL + UFH, 9 ± 5*; LMWH + aPL, 5 ± 3; * P < 0.01 versus aPL). In contrast, neither fondaparinux nor hirudin inhibited complement activation in vitro. In conclusion, heparins inhibit complement activation in vitro and in vivo. Heparins may prevent obstetrical complications in APS by blocking activation of complement induced by aPL targeted to deciduas – not by preventing placental thrombosis. These findings explain how subanticoagulant doses of heparin can limit antibody-mediated tissue injury.

Activation of complement mediates antiphospholipid antibody-induced pregnancy loss.

Although it is clear that the specific antigenic reactivity of antiphospholipid (aPL) antibodies is critical to their effect, the pathogenic mechanisms that result in injury in vivo are incompletely understood. We hyphothesized that aPL antibodies targeted to the placenta activate complement locally, generating split products that mediate placental injury and lead to foetal loss and growth retardation. To test this hypothesis, we used a murine model of APS in which pregnant mice are injected with human IgG containing aPL antibodies. Mice treated with inhibitors of complement activation and mice deficient in complement components were protected from aPL antibody-induced foetal damage. Although the cause of tissue injury in this disease is probably multifactoral, we have shown that complement activation is an absolute requirement for foetal loss and growth restriction and, therefore, thatthis pathway acts upstream of other important effector mechanisms. Identification of complement activation as a mechanism that is necessary for aPL-induced tissue damage and definition ofthe complement components necessary to trigger such injury is likely to lead to a better understanding of the pathogenesis of vascular and tissue injury in SLE and to new and improved treatments.

The role of complement in the antiphospholipid syndrome.

The anti-phospholipid syndrome (APS) is characterized by recurrent fetal loss, vascular thrombosis and thrombocytopenia occurring in the presence of antiphospholipid (aPL) antibodies. We hypothesized that aPL antibodies activate complement in the placenta, generating split products that mediate placental injury and lead to fetal loss and growth restriction, and that complement activation by aPL antibodies in other vascular areas causes inflammation and thrombophilia. Here we review studies in a murine model of APS in which human aPL antibodies are passively transferred into pregnant mice. Blockade of complement activation using a C3 convertase inhibitor or genetic deletion of C3 protected mice from pregnancy complications induced by aPL antibodies. These findings demonstrate that complement activation is a central mechanism in aPL antibody-induced pregnancy loss and fetal growth restriction. Although the cause of tissue injury in APS is likely to prove multifactorial, we have shown that complement activation is an absolute requirement for the most serious phenotypic outcomes, pregnancy complications and thrombosis, and, therefore, that this pathway acts upstream of other important effector mechanisms.

Intercellular adhesion molecule 1 expression is required for antiphospholipid antibody-induced pregnancy loss.

Intercellular adhesion molecule 1 expression is required for antiphospholipid antibody-induced pregnancy loss.