Blockade of autoantibody-initiated tissue damage by using recombinant fab antibody fragments against pathogenic autoantigen

Activation of the complement cascade via the classical pathway is required for the development of tissue injury in many autoantibody-mediated diseases. It therefore makes sense to block the pathological action of autoantibodies by preventing complement activation through inhibition of autoantibody binding to the corresponding pathogenic autoantigen using targeted Fab antibody fragments. To achieve this, we use bullous pemphigoid (BP) as an example of a typical autoimmune disease. Recombinant Fabs against the non-collagenous 16th-A domain of type XVII collagen, the main pathogenic epitope for autoantibodies in BP, were generated from antibody repertoires of BP patients by phage display. Two Fabs, Fab-B4 and Fab-19, showed marked ability to inhibit the binding of BP autoantibodies and subsequent complement activation in vitro. In the in vivo experiments using type XVII collagen humanized BP model mice, these Fabs protected mice against BP autoantibody-induced blistering disease. Thus, the blocking of pathogenic epitopes using engineered Fabs appears to demonstrate efficacy and may lead to disease-specific treatments for antibody-mediated autoimmune diseases.

Low-molecular-weight heparin inhibition in classical complement activation pathway during pregnancy

INTRODUCTION

Low-molecular-weight heparin is used clinically for the prevention of pregnancy complications associated with prothrombotic disorders, particularly anti-phospholipid syndrome. Nevertheless, recent studies have suggested that heparin may exert direct effects on the placental trophoblast, independently of its anticoagulant activity. In addition, heparin prevents complement activation in vivo and protects mice from pregnancy complications.

MATERIALS AND METHODS

The inhibition of the classical complement activation pathway by heparin was analyzed by means of in vitro assays and in pregnant women receiving prophylaxis with therapeutic doses (40 mg/day) of subcutaneous low molecular weight heparin by haemolysis of antibody-sensitized sheep erythrocytes (CH(50) assay).

RESULTS

The specific interaction between low-molecular-weight heparin and the C1q subunit of the C1 complex of the complement cascade allowed the isolation of a small subpopulation of heparin ( 8.03+/-1.20 microg %), with an anti-activated factor X activity more than four times greater than the starting material. This subpopulation could be responsible for the in vitro inhibition of the classical complement activation pathway evaluated by the total haemolysis of antibody-sensitized sheep erythrocytes. About 60 microg/ml of low molecular weight heparin was needed to achieve 50% of haemolysis. The detection of the classical complement pathway inhibition in pregnant women treated with heparin required a first activation with aggregated human IgG.

CONCLUSIONS

We concluded that the interaction between low-molecular-weight heparin and C1q could be relevant not only in the complement-dependent, but also in the complement-independent inflammation mechanisms responsible for the prevention of pregnancy loss.

The complement system in systemic autoimmune disease

Complement is part of the innate immune system. Its major function is recognition and elimination of pathogens via direct killing and/or stimulation of phagocytosis. Activation of the complement system is, however, also involved in the pathogenesis of the systemic autoimmune diseases. Activation via the classical pathway has long been recognized in immune complex-mediated diseases such as cryoglobulinemic vasculitis and systemic lupus erythematosus (SLE). In SLE, the role of complement is somewhat paradoxical. It is involved in autoantibody-initiated tissue damage on the one hand, but, on the other hand, it appears to have protective features as hereditary deficiencies of classical pathway components are associated with an increased risk for SLE. There is increasing evidence that the alternative pathway of complement, even more than the classical pathway, is involved in many systemic autoimmune diseases. This is true for IgA-dominant Henoch Schönlein Purpura, in which additional activation of the lectin pathway contributes to more severe disease. In anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis the complement system was considered not to be involved since immunoglobulin deposition is generally absent in the lesions. However, recent studies, both in human and animal models, demonstrated complement activation via the alternative pathway as a major pathogenic mechanism. Insight into the role of the various pathways of complement in the systemic autoimmune diseases including the vasculitides opens up new ways of treatment by blocking effector pathways of complement. This has been demonstrated for monoclonal antibodies to C5 or C5a in experimental anti-phospholipid antibody syndrome and ANCA-associated vasculitis.

Identification of a new class of small molecule C5a receptor antagonists

C5a is a terminal product of the complement cascade that activates and attracts inflammatory cells including granulocytes, mast cells and macrophages via a specific GPCR, the C5a receptor (C5aR). Inhibition of C5a/C5aR interaction has been shown to be efficacious in several animal models of autoimmune diseases, including RA, SLE and asthma. This account reports the discovery of a new class of C5aR antagonists through high-throughput screening. The lead compounds in this series are selective and block C5a binding, C5a-promoted calcium flux in human neutrophils with nanomolar potency.

The detrimental role of angiotensin receptor agonistic autoantibodies in intrauterine growth restriction seen in preeclampsia

Growth-restricted fetuses are at risk for a variety of lifelong medical conditions. Preeclampsia, a life-threatening hypertensive disorder of pregnancy, is associated with fetuses who suffer from intrauterine growth restriction (IUGR). Recently, emerging evidence indicates that preeclamptic women harbor AT₁ receptor agonistic autoantibodies (AT₁-AAs) that contribute to the disease features. However, the exact role of AT₁-AAs in IUGR and the underlying mechanisms have not been identified. We report that these autoantibodies are present in the cord blood of women with preeclampsia and retain the ability to activate AT₁ receptors. Using an autoantibody-induced animal model of preeclampsia, we show that AT₁-AAs cross the mouse placenta, enter fetal circulation, and lead to small fetuses with organ growth retardation. AT₁-AAs also induce apoptosis in the placentas of pregnant mice, human villous explants, and human trophoblast cells. Finally, autoantibody-induced IUGR and placental apoptosis are diminished by either losartan or an autoantibody-neutralizing peptide. Thus, these studies identify AT₁-AA as a novel causative factor of preeclampsia-associated IUGR and offer two possible underlying mechanisms: a direct detrimental effect on fetal development by crossing the placenta and entering fetal circulation, and indirectly through AT₁-AA–induced placental damage. Our findings highlight AT₁-AAs as important therapeutic targets.

Evidence for complement activation in the amniotic fluid of women with spontaneous preterm labor and intra-amniotic infection

OBJECTIVE

The complement system plays an important role in host defense against infection. Concentrations of complement split products or anaphylatoxins (C3a, C4a, and C5a) in biological fluids are considered to reflect complement activation. The purpose of this study was to determine if term and preterm parturition are associated with evidence of complement activation in the amniotic fluid.

STUDY DESIGN

Amniotic fluid (AF) samples were collected from 270 women in the following groups: (1) normal pregnant women in midtrimester (n = 70), (2) term not in labor (n = 23), (3) term in labor (n = 48), and (4) preterm labor (PTL) (n = 129). PTL was categorized into: (a) PTL without microbial invasion of the amniotic cavity (MIAC) who delivered at term (n = 42), (b) PTL who delivered preterm without MIAC (n = 57), and (c) PTL with MIAC (n = 30). C5a, C4a, and C3a concentrations in amniotic fluid were determined by ELISA. Nonparametric tests were used for statistical analysis.

RESULTS

(1) The median AF C5a concentration was higher in women at term than that of those in the midtrimester (p = 0.02); (2) Spontaneous labor at term was not associated with changes in AF concentrations of anaphylatoxins C3a, C4a, and C5a (all p > 0.05); (3) Among patients with PTL who delivered preterm, those with MIAC had higher AF C4a and C5a concentrations than those without infection (p < 0.01); and (4) AF C3a, C4a, and C5a concentrations were higher in patients with PTL with MIAC than in those with PTL without MIAC who delivered at term.

CONCLUSION

Patients with spontaneous preterm labor and intact membranes with microbial invasion of the amniotic cavity had higher median amniotic fluid concentration of complement split products C3a, C4a, and C5a than patients without intra-amniotic infection. These findings suggest that preterm labor in the context of infection is associated with activation of the complement system.

Targeted inhibition of the complement alternative pathway with complement receptor 2 and factor H attenuates collagen antibody-induced arthritis in mice

The alternative pathway (AP) of complement is required for the induction of collagen Ab-induced arthritis (CAIA) in mice. The objective of this study was to examine the effect of a recombinant AP inhibitor containing complement receptor 2 and factor H (CR2-fH) on CAIA in mice. CR2 binds to tissue-fixed activation fragments of C3, and the linked fH is a potent local inhibitor of the AP. CAIA was induced in C57BL/6 mice by i.p. injections of 4 mAb to type II collagen (CII) on day 0 and LPS on day 3. PBS or CR2-fH (250 or 500 microg) were injected i.p. 15 min after the mAb to CII on day 0 and 15 min after LPS on day 3; the mice were sacrificed on day 10. The disease activity score (DAS) was decreased significantly (p < 0.001) in both groups receiving CR2-fH compared with the PBS. Histology scores for inflammation, pannus, bone damage, and cartilage damage decreased in parallel with the DAS. C3 deposition in the synovium and cartilage was significantly reduced (p < 0.0001) in the mice treated with CR2-fH. In vitro studies with immune complexes containing type II collagen and mAb to CII showed that CR2-fH specifically inhibited the AP with minimal effect on the classical pathway (CP) and no effect on the lectin pathway (LP). The relative potency of CR2-fH in vitro was superior to mAbs to factor B and C5. Thus, CR2-fH specifically targets and inhibits the AP of complement in vitro and is effective in CAIA in vivo.

Antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an autoimmune systemic disease that is diagnosed when there is vascular thrombosis and/or pregnancy morbidity occurring with persistently positive antiphospholipid antibodies (aPL) (lupus anticoagulant test, anticardiolipin antibodies, and/or anti-beta(2)-glycoprotein I antibodies). Although International APS Classification Criteria have been formulated to provide a uniform approach to APS research, aPL may cause a spectrum of clinical manifestations, some of which are not included in these criteria. The main aPL-related cardiac manifestations include valve abnormalities (vegetations and/or thickening), myocardial infarction (MI), intracardiac thrombi, and myocardial microthrombosis. In this article, we will review the definition, etiopathogenesis, clinical manifestations, diagnosis, and treatment of aPL-related clinical events with emphasis on cardiac manifestations.

Both complement and IgG fc receptors are required for development of attenuated antiglomerular basement membrane nephritis in mice

To elucidate the mechanisms of glomerulonephritis, including Goodpasture's syndrome, mouse models are used that use heterologous Abs against the glomerular basement membrane (GBM) with or without preimmunization with foreign IgG from the same species. These studies have revealed the requirement of either FcgammaR or complement, depending on the experimental model used. In this study, we provide evidence that both FcgammaR and complement are obligatory for a full-blown inflammation in a novel attenuated passive model of anti-GBM disease. We demonstrate that administration of subnephritogenic doses of rabbit anti-GBM Abs followed by a fixed dose of mouse mAbs to rabbit IgG, allowing timing and dosing for the induction of glomerulonephritis, resulted in reproducible complement activation via the classical pathway of complement and albuminuria in wild-type mice. Because albuminuria was absent in FcR-gamma-chain(-/-) mice and reduced in C3(-/-) mice, a role for both FcgammaR and complement is postulated. Because C1q(-/-) and C4(-/-) mice lacking a functional classical and lectin pathway did develop albuminuria, we suggest involvement of the alternative pathway of complement. Anti-GBM glomerulonephritis occurs acutely following the administration of mouse anti-rabbit IgG, and proceeds in a chronic fashion dependent on both FcgammaR and complement. This novel attenuated model allows elucidating the relative contribution of different mediator systems of the immune system to the development of renal injury, and also provides a platform for the assessment of different treatment protocols and evaluation of drugs that ultimately may be beneficial for the treatment of anti-GBM mediated glomerulonephritides.

Mechanisms of antiphospholipid antibody-associated pregnancy complications

Women with antiphospholipid syndrome (APS) and antiphospholipid antibodies (aPL) are at high risk for recurrent spontaneous miscarriage and late pregnancy complications, such as preeclampsia and preterm labor. Recent clinical and experimental observations suggest that the pathophysiology of pregnancy failure in patients with APS may involve inflammation at the maternal-fetal interface and disruption of normal trophoblast function and survival, rather than a pro-thrombotic event. While treatment with heparin and aspirin from early pregnancy has been shown to significantly increase the live birth rate in recurrent miscarriage patients with APS, the incidence of severe late pregnancy complications still remains high. This review will discuss what is currently known about the mechanisms by which aPL may compromise pregnancy outcome.

Antiphospholipid antibodies induce a pro-inflammatory response in first trimester trophoblast via the TLR4/MyD88 pathway

PROBLEM

Women with antiphospholipid antibodies (aPL) are at risk for recurrent miscarriage, pre-eclampsia, and pre-term labor. aPL target the placenta directly by binding to beta(2)-glycoprotein I (beta(2)GPI) expressed on the surface of trophoblast cells. The objective of this study was to determine the effects of aPL on trophoblast function and the mechanisms involved.

METHOD OF STUDY

First trimester trophoblast cells were treated with anti-beta(2)GPI monoclonal antibodies and patient-derived aPL, after which cell survival and function was evaluated.

RESULTS

We report that anti-beta(2)GPI antibodies trigger an inflammatory response in trophoblast, characterized by increased secretion of interleukin (IL)-8, MCP-1, GRO-alpha, and IL-1beta, and that this occurs in a TLR-4/MyD88-dependent manner. At high concentrations, these antibodies also induce caspase-mediated cell death. This was attenuated upon disabling of the MyD88 pathway, suggesting that anti-beta(2)GPI-induced inflammatory mediators compromise trophoblast survival by acting in an autocrine/paracrine manner. Enhanced IL-8, GRO-alpha, and IL-1beta secretion also occurred when trophoblast cells were incubated with antibodies from patients with antiphospholipid syndrome. Heparin, which acts as a pro-survival factor in human trophoblast, attenuated the anti-beta(2)GPI antibody-mediated cell death, and also the pro-inflammatory response, but only at high concentrations.

CONCLUSION

These findings demonstrate that aPL triggers a placental inflammatory response via the TLR-4/MyD88 pathway, which in turn compromises trophoblast survival. Thus, the TLR-4/MyD88 pathway may provide a new therapeutic target to improve pregnancy outcome in antiphospholipid syndrome patients.

Neonatal and pregnancy outcome in primary antiphospholipid syndrome: a 10-year experience in one medical center

BACKGROUND

Antiphospholipid syndrome (APS) in pregnancy is characterized by the presence of maternal autoantibodies in association with recurrent fetal loss and severe obstetric complications such as prematurity, intrauterine growth retardation, or placental insufficiency. This study aimed to assess the perinatal outcomes in neonates born to mothers with APS.

METHODS

The medical records of pregnant women with APS and their offspring were retrospectively collected between January 1997 and July 2007. Maternal and perinatal histories including demographic data, medications, obstetric histories, and neonatal clinical manifestations and laboratory data were analyzed.

RESULTS

Eleven women with a diagnosis of primary APS were included. Eight of these patients had experienced frequent spontaneous abortions (72.7%), and four had unexplained fetal deaths (36.3%). None of them had vascular thrombosis. Specific autoimmune antibodies were detected, including anticardiolipin antibody (n=6), anti-beta2 glycoprotein I (n=3), and antiphospholipid antibody (n=7). Among the pregnancies, five had preterm births (45.4%), two had intrauterine growth retardation (18.1%), and one had intrauterine fetal demise (9.1%). Thrombocytopenia was noted in three babies, all of whose mothers had lower platelet counts. One patient with neonatal thrombocytopenia developed intracranial hemorrhage, seen on brain images.

CONCLUSIONS

This limited study suggests that neonates born to mothers with primary APS are at risk of prematurity, being small for gestational age, and having thrombocytopenia. Further large, prospective studies are required to better define the perinatal outcomes.

Coagulation and innate immune responses: can we view them separately?

The horseshoe crab is often referred to as a "living fossil," representative of the oldest classes of arthropods, almost identical to species in existence more than 500 million years ago. Comparative analyses of the defense mechanisms used by the horseshoe crab that allowed it to survive mostly unchanged throughout the millennia reveal a common ancestry of the coagulation and innate immune systems that are totally integrated-indeed, almost inseparable. In human biology, we traditionally view the hemostatic pathways and those regulating innate immune responses to infections and tissue damage as entirely separate entities. But are they? The last couple of decades have revealed a remarkable degree of interplay between these systems, and the linking cellular and molecular mechanisms are rapidly being delineated. In this review, we present some of the major points of intersection between coagulation and innate immunity. We attempt to highlight the potential impact of these findings by identifying recently established paradigms that will hopefully result in the emergence of new strategies to treat a range of inflammatory and hemostatic disorders.

Intrauterine growth restriction, human placental development and trophoblast cell death

Intrauterine growth restriction (IUGR) is a failure to achieve the growth potential of a fetus that is promised by the genetic constitution and environmental influences endogenous to the pregnancy. Optimal placental development and the ability of the placenta to compensate for stimulus-induced injury are central in promotion of normal fetal growth. In this review, we will overview placental development with a focus on how villous structure relates to function. We will also describe the differentiation and turnover of villous trophoblast while highlighting selected features of microscopic placental injury. Histopathological studies of the placenta in IUGR indicate that abnormalities of the maternal spiral arterioles, dysregulated villous vasculogenesis, and abundant fibrin deposition are characteristic of the injuries associated with this condition. We identify selected insults, including oxidative stress and complement activation, and key pathways that regulate apoptosis in villous trophoblast, including increased p53 activity, altered translation of AKT and mTOR proteins, and the stress response of the endoplasmic reticulum. We surmise that trophoblast dysregulation at a subcellular level and loss of functional mass of villous trophoblast via cell death pathways are key contributors to the suboptimal placental performance that yields IUGR. We predict that a better understanding of placental dysfunction in IUGR will lead to targeted therapeutic options for this important clinical condition.

Molecular characterization of three new mutations causing C5 deficiency in two non-related families

Deficiencies in complement components are rare diseases whose diagnosis is often underestimated. In addition, in only a few cases molecular studies have been carried out for the characterization of the underlying genetic defects. To date, studies involving C5-deficient patients are scarce. The aim of the present report is to characterize the biochemical and molecular complement deficiency in two non-related families with one or more members showing no detectable hemolytic complement activity (CH50<50 U/ml) and reporting a history of several episodes of meningitis. Protein deficiency was assessed by means of hemolytic assays, bi-dimensional double immunodiffusion, ELISA and Western blot of patients' sera. Molecular studies were carried out by PCR and RT-PCR of DNA and RNA, respectively, both extracted from fresh blood samples of each family member. In Family A, only the propositus had complete C5 deficiency. Molecular studies showed that he was heterozygous for two changes in the C5 gene. One of the mutations was also carried by the father (c.1883_1884AG<CTCT) and the second (c.2536T>C, Y846H) was a de novo mutation. In Family B, the two C5-deficient members share the homozygous nonsense mutation c.892C>T (Q298X) in exon 9. The characterization of these new mutations is interesting in order to elucidate structure-function relationships in the C5 gene and it also helps to understand the molecular basis of this uncommon deficiency. Moreover, this report highlights the importance of complement screening in cases of repeated meningococcal infections in order to establish its involvement and to consider adequate clinical recommendations such as prophylactic antibiotics or meningococcal vaccines.

Twenty-year follow-up: an unusual case of nephropathy of antiphospholipid syndrome

Nephropathy of antiphospholipid antibody syndrome (NAPS) is an increasingly well-recognized aspect of antiphospholipid syndrome. The most characteristic histopathology is that of thrombotic microangiopathy, and thrombosis occurring in the renal vasculature is thought to be the initiating event. Other less common pathologies have been reported, and the mechanisms of these are unclear. Therapy has been largely empiric. We report a case of NAPS in a patient with atypical pathology, who has declined therapy with immunosuppressive agents and anticoagulants and who has maintained normal renal function in 20 years of follow-up.

Neonatal effects of maternal antiphospholipid syndrome

Antiphospholipid antibodies (aPL) can impair the physiologic development of a fetus during pregnancy not only by causing thrombosis of the placental vessels, but also by directly binding throphoblast cells and modifying their functions. Consequently, the presence of aPL in pregnant women is linked to an increased rate of pregnancy complications. These include recurrent early miscarriages, late fetal losses, and hypertensive disorders of gestation. In this clinical setting, preeclampsia is usually early and severe and can be complicated by the syndrome of hemolysis, elevated liver enzymes, and low platelet count (HELLP syndrome). The close association between aPL and obstetric pathology supports the inclusion of these manifestations in the clinical classification criteria of antiphospholipid syndrome. About 30% of children born to mothers with aPL passively acquire these autoantibodies; fortunately, the occurrence of thrombosis seems extremely rare in these babies. The prospective ongoing studies of children born to antiphospholipid syndrome patients reassure us about their general good health; however, some data suggest that learning difficulties might occur, possibly related to in utero exposure to aPL.

Toll-like receptors: another player in the pathogenesis of the anti-phospholipid syndrome

The anti-phospholipid syndrome (APS) is a systemic autoimmune disease characterized by an adaptive immune response against self phospholipid (PL)--binding proteins. Although APS is considered as an autoantibody-mediated disease, there is now evidence that anti-phospholipid antibodies (aPL) are necessary but not sufficient to trigger some of the clinical manifestations of the syndrome. For example, mediators of the innate immunity are recognized to be additional second hits able to induce the thrombotic events in the presence of aPL. Finally, environmental agents - in particular infectious ones - were reported to act as triggers for the production of autoantibodies cross-reacting with PL-binding proteins as well as inflammatory stimuli that potentiate the aPL thrombogenic effect. Altogether, these findings suggest a role for the innate immunity in APS pathogenesis. Toll-like receptors (TLR) are receptors that induce prompt inflammatory responses and mediate functional activation in immune effector cells. There is evidence that aPL, and in particular anti-beta(2) glycoprotein I (beta(2)GPI) antibodies, may activate endothelial cells and monocytes through TLR-4-dependent signalling. Whether or not TLR may behave as surface receptors for beta(2)GPI is still matter of research. Drugs or molecules able to interfere with TLR involvement may represent new therapeutic approaches for APS.

How we diagnose the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an acquired thrombophilia, characterized by the occurrence of venous and arterial events. This article examines the laboratory and key clinical aspects of APS. Particular focus is given to anti–beta 2-glycoprotein I (β2GPI) antibodies in view of their recent inclusion in the APS classification criteria. The clinical utility of using the β2GPI enzyme-linked immunosorbent assay, in conjunction with the established lupus anticoagulant assays and cardiolipin enzyme-linked immunosorbent assay, for diagnosing and risk stratifying patients suspected of having APS is discussed. The relative importance of the various assays in diagnosing obstetric APS (early and late gestation miscarriages) is explored. The implications of recent epidemiologic findings for possibly understanding the underlying pathophysiologic mechanisms of obstetric APS are highlighted. Insights into which patients with obstetric APS may be at most risk of thrombotic complications are presented.

Sixth meeting of the European Forum on antiphospholipid antibodies. How to improve the understanding of the antiphospholipid syndrome?

The main objective of these meetings is to promote international collaboration in various clinical and research projects. This paper is the summary of the 2007 Ljubljana meeting, and offers an overview of the proposed projects. The technical and methodological details of the projects will be published on the forum’s web site (http://www.med.ub.es/MIMMUN/FORUM/STUDIES.HTM).

Neutrophil activation by the tissue factor/Factor VIIa/PAR2 axis mediates fetal death in a mouse model of antiphospholipid syndrome

Women with antiphospholipid syndrome (APS), a condition characterized by the presence of antiphospholipid antibodies (aPL), often suffer pregnancy-related complications, including miscarriage. We have previously shown that C5a induction of tissue factor (TF) expression in neutrophils contributes to respiratory burst, trophoblast injury, and pregnancy loss in mice treated with aPL. Here we analyzed how TF contributes to neutrophil activation and trophoblast injury in this model. Neutrophils from aPL-treated mice expressed protease-activated receptor 2 (PAR2), and stimulation of this receptor led to neutrophil activation, trophoblast injury, and fetal death. An antibody specific for human TF that has little impact on coagulation, but potently inhibits TF/Factor VIIa (FVIIa) signaling through PAR2, inhibited aPL-induced neutrophil activation in mice that expressed human TF. Genetic deletion of the TF cytoplasmic domain, which allows interaction between TF and PAR2, reduced aPL-induced neutrophil activation in aPL-treated mice. Par2-/- mice treated with aPL exhibited reduced neutrophil activation and normal pregnancies, which indicates that PAR2 plays an important role in the pathogenesis of aPL-induced fetal injury. We also demonstrated that simvastatin and pravastatin decreased TF and PAR2 expression on neutrophils and prevented pregnancy loss. Our results suggest that TF/FVIIa/PAR2 signaling mediates neutrophil activation and fetal death in APS and that statins may be a good treatment for women with aPL-induced pregnancy complications.

Tracing the molecular pathogenesis of antiphospholipid syndrome

Fetal loss induced by antiphospholipid antibodies (aPLs) in mice is a complement-driven inflammatory condition. Engagement of the complement receptor C5aR on neutrophils induces expression of the principal initiator of the blood clotting mechanism, tissue factor (TF), and blocking this downstream event of complement activation prevents antibody-induced fetal loss. In this issue of the JCI, the study by Redecha et al. clarifies that in mice, the contribution of TF to this pathogenic mechanism is independent of its role in coagulation and thrombosis, but involves inflammatory signaling through the receptor PAR2 (see the related article beginning on page 3453). The study not only sheds light on a critical effector mechanism of aPL-induced fetal loss, but also suggests that treatment with statins, which decrease TF and PAR2 expression, may hold promise as a therapeutic approach to antiphospholipid syndrome-associated pregnancy complications.

Complement inhibition keeps mothers calm and avoids fetal rejection

The paternal antigens presented by the fetus could be considered foreign by the mother's immune system and elicit an immune response. Here we show that the complement system functions as an effector in fetal rejection in two different mouse models of pregnancy loss. In a mouse model of fetal loss and growth restriction (IUGR) induced by antiphospholipid antibodies (aPL), we found that complement activation is a crucial and early mediator of pregnancy loss. We demonstrated that C5a-C5aR interaction and neutrophils are key mediators of fetal injury. We identified tissue factor (TF) as a critical intermediate that, acting downstream of C5 activation, enhances neutrophil activity and trophoblast injury. In an antibody-independent mouse model of spontaneous miscarriage and IUGR (CBAxDBA) we also identified C5a as an essential mediator. Complement activation caused dysregulation of the angiogenic factors (deficiency of free vascular endothelial growth factor (VEGF) and elevated levels of soluble VEGF receptor 1) required for normal placental development. Inhibition of complement activation prevented angiogenesis failure and rescued pregnancies. Our studies in antibody-dependent and antibody-independent models of pregnancy complications identified complement activation as the crucial mediator of damage and will allow development of new interventions to prevent pregnancy loss and IUGR.

Updating on the pathogenic mechanisms 5 of the antiphospholipid antibodies-associated pregnancy loss

Anti-phospholipid antibodies (aPL) are risk factor for recurrent pregnancy loss and obstetrical complications. The mechanisms of aPL-mediated pregnancy failure are still a matter of research. Although aPL are associated with thrombosis, thrombotic events cannot explain all the miscarriages. There is evidence for a direct in vitro aPL effect on the trophoblast as shown by their binding; reduction of proliferation, human chorionic gonadotrophin release, in vitro invasiveness, adhesion molecule expression; and increased apoptosis. Such a direct reactivity is supported by the expression of beta2 glycoprotein (beta 2GP) I on trophoblast cell membranes. aPL/anti-beta 2GPI antibodies also bind to human decidual/endometrial cells in vitro and induce a pro-inflammatory phenotype. APL-mediated inflammatory processes at the placental level are apparently responsible for fetal loss at least in animal models. Both complement activation and pro-inflammatory cytokine/chemokine secretion have been shown to play a role. More recently, complement-induced tissue factor expression on infiltrating neutrophils was described as an additional pathogenic mechanisms mediated by aPL. As a whole, these findings do suggest that aPL may induce a defective placentation by acting at different levels without involving necessarily thrombotic events.

Influence of different IgG anticardiolipin antibody cut-off values on antiphospholipid syndrome classification

BACKGROUND

While medium to high titers of anticardiolipin (aCL) antibodies, defined as >40 GPL units or >99th percentile, is a laboratory criteria for the 'definite' diagnosis of antiphospholipid syndrome (APS), agreement between the two cut-offs has not been validated.

OBJECTIVE

To validate the current aCL laboratory criterion by verifying the effect of the two cut-offs on APS classification.

PATIENTS/METHODS

Ninety aCL positive APS patients were selected on the basis of their GPL values above the 99th percentile (17.4 GPL), which was calculated by testing 100 age- and sex-matched healthy subjects.

RESULTS

A significant difference in the IgG positivity (P < 0.0001) was found between the APS laboratory profiles as 20 out of the 24 (83.3%) patients with single positivity (aCL alone), six out of the 23 (26.1%) with double positivity (aCL plus lupus anticoagulant or anti-beta(2)glycoprotein I), and none out of the 43 with triple positivity (aCL plus lupus anticoagulant and anti-beta(2)glycoprotein I) had titers between the 99th percentile and 40 GPL units. Moreover, the rate of aCL values between the 99th percentile and 40 GPL units was significantly higher (P < 0.0001) in patients with pregnancy morbidity (73.7%) as compared to those with vascular thrombosis (16.9%) and those with both conditions (16.7%).

CONCLUSION

The 99th percentile cut-off level seems more sensitive than the >40 GPL value for APS classification, as it includes subjects with aCL positivity alone as well as patients with pregnancy morbidity.

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.

The C5b-9 membrane attack complex of complement activation localizes to villous trophoblast injury in vivo and modulates human trophoblast function in vitro

The complement system plays an important role in normal human pregnancy. Uncontrolled activation of this system has been associated with many disease states. We tested the hypothesis that the C5b-9 membrane attack complex (MAC) localizes to sites of villous injury and modulates trophoblast function. Placental sections from pregnancies with no complications, intrauterine growth restriction, or preeclampsia were immunostained and the surface density for MAC and fibrin was determined by morphometric analysis. Primary cytotrophoblasts from term placentas were cultured in a FiO(2) of <1%, 8% and 20% with 10% human serum containing active MAC or heat-inactivated control serum. Immunofluorescent MAC binding to trophoblast was quantified, and the neoepitopes formed in cytokeratin 18 filaments and poly-ADP-ribose polymerase during apoptosis were used to measure cell death. Trophoblast differentiation was assessed by HCG secretion, formation of syncytia, and expression of syncytin. MAC localized to fibrin deposits in normal placentas, and especially in placentas from IUGR and preeclampsia. MAC binding to cytotrophoblasts was inversely proportional to FiO(2) and enhanced apoptosis. MAC increased markers of differentiation in cultures at 72h (medium HCG, syncytia and syncytin expression). Our findings demonstrate that MAC associates with fibrin deposits at sites of villous injury in vivo. Hypoxia also enhances MAC deposition in cultured trophoblasts and MAC alters trophoblast function in a phenotype specific manner.

Mechanisms behind flare of renal lupus during murine pregnancy

The outcome of pregnancy in systemic lupus erythematosus is still controversial. The authors recently reported the disappearance of the manifestation of the skin disease but a diminished survival rate in lupus-prone animals undergoing several pregnancies. It was postulated that lupus-prone animals must have subclinical renal symptoms at an early age and that immune and hormonal changes during pregnancy exacerbate immune reactions in the kidneys, leading to a shortened life span. Here, the authors analysed changes at day 14 of pregnancy in lupus-prone LPR (MRL/lpr) mice and MRL controls regarding cytokines, regulatory T (Treg) cells and deposition of immunocomplexes. Worsened kidney function was observed during pregnancy, even in the absence of lupus signs. This was accompanied by renal inflammation and higher interferon-gamma and interleukin-10 levels. C3 and immunoglobulin G deposition was enhanced in kidney and placenta from lupus-prone pregnant animals. Pregnancy enhanced the levels of Treg cells in control animals but not in lupus-prone animals. As pregnancy-induced Treg cells were shown to be specific for paternal antigens it is not to be expected that these Treg cells can help to destroy autoreactive cells. The authors conclude that early subclinical kidney disease in lupus-prone animals exacerbates during pregnancy. Albeit obtained with an experimental animal model, their data are potentially of importance for lupus patients of reproductive age.

Properdin: New roles in pattern recognition and target clearance

Properdin was first described over 50 years ago by Louis Pillemer and his collaborators as a vital component of an antibody-independent complement activation pathway. In the 1970s properdin was shown to be a stabilizing component of the alternative pathway convertases, the central enzymes of the complement cascade. Recently we have reported that properdin can also bind to target cells and microbes, provide a platform for convertase assembly and function, and promote target phagocytosis. Evidence is emerging that suggests that properdin interacts with a network of target ligands, phagocyte receptors, and serum regulators. Here we review the new findings and their possible implications.

Maternal Par4 and platelets contribute to defective placenta formation in mouse embryos lacking thrombomodulin

Absence of the blood coagulation inhibitor thrombomodulin (Thbd) from trophoblast cells of the mouse placenta causes a fatal arrest of placental morphogenesis. The pathogenesis of placental failure requires tissue factor, yet is not associated with increased thrombosis and persists in the absence of fibrinogen. Here, we examine the role of alternative targets of coagulation that might contribute to the placental failure and death of Thbd(-/-) embryos. We demonstrate that genetic deficiency of the protease-activated receptors, Par1 or Par2, in the embryo and trophoblast cells does not prevent the death of Thbd(-/-) embryos. Similarly, genetic ablation of the complement pathway or of maternal immune cell function does not decrease fetal loss. In contrast, Par4 deficiency of the mother, or the absence of maternal platelets, restores normal development in one-third of Thbd-null embryos. This finding generates new evidence implicating increased procoagulant activity and thrombin generation in the demise of thrombomodulin-null embryos, and suggests that platelets play a more prominent role in placental malfunction associated with the absence of thrombomodulin than fibrin formation. Our findings demonstrate that fetal prothrombotic mutations can cause localized activation of maternal platelets at the feto-maternal interface in a mother with normal hemostatic function.

Pathogenic role of antiphospholipid antibodies

The antiphospholipid antibody syndrome (APS) is characterized by recurrent arterial and venous thrombosis and/or pregnancy in association with antiphospholipid (aPL) antibodies. The pathogenic mechanisms in APS that lead to in vivo injury are incompletely understood. Recent evidence suggests that APL antibodies alter regulation of haemostasis and induce activation of complement. We will discuss the current knowledge on how aPL antibodies trigger increased inflammation and enhanced thrombotic tendency, and thereby lead to tissue damage.

The alternative complement pathway revisited

Alternative pathway amplification plays a major role for the final effect of initial specific activation of the classical and lectin complement pathways, but the quantitative role of the amplification is insufficiently investigated. In experimental models of human diseases in which a direct activation of alternative pathway has been assumed, this interpretation needs revision placing a greater role on alternative amplification. We recently documented that the alternative amplification contributed to 80–90% of C5 activation when the initial activation was highly specific for the classical pathway. The recent identification of properdin as a recognition factor directly initiating alternative pathway activation, like C1q in the classical and mannose-binding lectin in the lectin pathway, initiates a renewed interest in the reaction mechanisms of complement. Complement and Toll-like receptors, including the CD14 molecule, are two main upstream recognition systems of innate immunity, contributing to the inflammatory reaction in a number of conditions including ischaemia-reperfusion injury and sepsis. These systems act as ‘double-edged swords’, being protective against microbial invasion, but harmful to the host when activated improperly or uncontrolled. Combined inhibition of complement and Toll-like receptors/CD14 should be explored as a treatment regimen to reduce the overwhelming damaging inflammatory response during sepsis. The alternative pathway should be particularly considered in this regard, due to its uncontrolled amplification in sepsis. The alternative pathway should be regarded as a dual system, namely a recognition pathway principally similar to the classical and lectin pathways, and an amplification mechanism, well known, but quantitatively probably more important than generally recognized.

Antiphospholipid antibodies: laboratory and pathogenetic aspects

Antiphospholipid antibodies (aPL) constitute a heterogeneous group of autoantibodies that share the ability to bind phospholipids (PL) alone, protein-PL complexes, or PL-binding proteins. They have been detected in isolation, in association with autoimmune diseases such as systemic lupus erythematosus (SLE), and during the course of different infections. aPL have been associated with an array of clinical manifestations in virtually every organ, although deep vein and arterial thrombosis as well as pregnancy morbidity are predominant. The co-occurrence of these clinical findings with aPL constitutes the so-called antiphospholipid syndrome (APS). aPL can be detected by immunological methods [e.g., anticardiolipin antibodies (aCL)] or by functional methods that exploit the effect of aPL on blood coagulation [lupus anticoagulant (LA)]. Since aPL are heterogeneous, numerous immunological and coagulation assays have been developed. These assays have not been fully standardized, and, therefore, problems such as high interlaboratory variation are relatively frequent. Recently, recommendations have been published regarding LA and aCL testing. Not all aPL are pathogenic. However, when they are not associated with infections, they have a role in the pathogenesis of APS. Clinical and experimental data have shown that aPL exert their pathogenic activity by interfering with the function of coagulation factors, such as thrombin and factors X, XI and XII, and with the function of anticoagulant proteins of the protein C system. In addition, aPL interaction with platelets and endothelial cells induces a pro-adhesive activated phenotype.

Alternative complement pathway activation fragment Bb in early pregnancy as a predictor of preeclampsia

OBJECTIVE

Preeclampsia is a multisystem disease classically defined on the basis of hypertension and proteinuria. As shown in animal studies, complement activation is associated with inflammation in the placenta and adverse pregnancy outcomes. The association between complement activation in humans and adverse pregnancy outcomes is unclear. The purpose of this study was to determine whether elevated levels of the activation fragment Bb in early pregnancy are predictive of preeclampsia.

STUDY DESIGN

This prospective study of 701 women was conducted in Denver, CO. A single plasma sample was obtained from each woman before 20 weeks' gestation. The cohort was followed up throughout pregnancy for the development of preeclampsia. Analysis included multivariate logistic regression to adjust for established risk factors for preeclampsia.

RESULTS

Preeclampsia developed in 4.6% of the cohort. Women with elevated Bb (90th or greater percentile) were substantially more likely to develop preeclampsia than women who had levels less than the 90th percentile (unadjusted relative risk [RR], 3.3, 95% confidence interval [CI] 1.6 to 7, P = .0009). Other significant risk factors for preeclampsia included nulliparity (RR, 2.1, 95% CI, 1-4), a high body mass index (P = .006 for trend), and maternal medical (preexisting maternal hypertension, type 1 diabetes, systemic lupus erythematosus) disease (RR, 4.4, 95% CI, 2-10). Significant risk factors among multiparous women included a history of hypertension in a previous pregnancy (RR, 5, 95% CI, 1.6 to 16) and a change of paternity (RR, 5.1, 95% CI, 1.6 to 15). Adjustment for risk factors did not attenuate the association between an elevated Bb and preeclampsia (adjusted odds ratio [OR], 3.8, 95% CI, 1.6 to 9, P = .002) in the cohort. After removing women with plasma obtained before 10 weeks, the adjusted OR of Bb in the top decile for preeclampsia was 6.1 (95% CI 2.2, 17, P = .0005).

CONCLUSION

The complement activation product Bb in early pregnancy is a biomarker for elevated risk of preeclampsia. This observation suggests that events linked to activation of complement in early pregnancy are associated with the pathogenesis of preeclampsia.