Complement Activation: A Novel Pathogenic Mechanism in the Antiphospholipid Syndrome

Targeting the tissue factor coagulation initiation complex prevents antiphospholipid antibody development

ABSTRACT

Antiphospholipid antibodies (aPL) in primary or secondary antiphospholipid syndrome (APS) are a major cause for acquired thrombophilia, but specific interventions preventing autoimmune aPL development are an unmet clinical need. Although autoimmune aPL cross react with various coagulation regulatory proteins, lipid-reactive aPL, including those derived from patients with COVID-19, recognize the endolysosomal phospholipid lysobisphosphatidic acid presented by the cell surface-expressed endothelial protein C receptor. This specific recognition leads to complement-mediated activation of tissue factor (TF)-dependent proinflammatory signaling and thrombosis. Here, we show that specific inhibition of the TF coagulation initiation complex with nematode anticoagulant protein c2 (NAPc2) prevents the prothrombotic effects of aPL derived from patients with COVID-19 in mice and the aPL-induced proinflammatory and prothrombotic activation of monocytes. The induction of experimental APS is dependent on the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase complex, and NAPc2 suppresses monocyte endosomal reactive oxygen species production requiring the TF cytoplasmic domain and interferon-α secretion from dendritic cells. Latent infection with murine cytomegalovirus causes TF cytoplasmic domain-dependent development of persistent aPL and circulating phospholipid-reactive B1 cells, which is prevented by short-term intervention with NAPc2 during acute viral infection. In addition, treatment of lupus prone MRL-lpr mice with NAPc2, but not with heparin, suppresses dendritic-cell activation in the spleen, aPL production and circulating phospholipid-reactive B1 cells, and attenuates lupus pathology. These data demonstrate a convergent TF-dependent mechanism of aPL development in latent viral infection and autoimmune disease and provide initial evidence that specific targeting of the TF initiation complex has therapeutic benefits beyond currently used clinical anticoagulant strategies.

Anticardiolipin Antibody Plays a More Important Role Than Anti-β2-Glycoprotein I Antibody in Activating Complement in Patients with Lupus Nephritis

Objective

This research aimed to explore the correlation between antiphospholipid antibodies (aPLs) and complement activation in lupus nephritis (LN) patients.

Methods

A retrospective analysis was carried out on patients diagnosed with LN based on renal biopsy from June 2019 to June 2022. The study assessed levels of IgM, IgA, and IgG subtypes of anticardiolipin antibodies (aCLs) and anti-β2-glycoprotein I (anti-β2-GPI) antibodies. Pathological and clinical data were collected concurrently with the renal biopsy.

Results

The analysis included 76 LN patients, with 44.7% testing positive for aPLs. LN patients with positive aPLs exhibited increased hematuria, higher SLEDAI scores, reduced serum C3 and C4 levels, and more C1q deposits in the glomerulus compared to those with negative aPLs (P<0.05). Correlation analysis demonstrated the inverse relationships between IgG-aCL levels and serum C3 and C4 levels (r=-0.29, P=0.005; r=-0.24, P=0.016, respectively), as well as a positive correlation with C4 deposits in the glomerulus (r=0.20, P=0.041).

Conclusion

This investigation suggests that aPLs, particularly IgG-aCLs, may be associated with the severity of LN and could contribute to the activation of classical complement pathways.

Platelets and Thrombotic Antiphospholipid Syndrome

Antiphospholipid antibody syndrome (APS) is an autoimmune disorder characterised by thrombosis and the presence of antiphospholipid antibodies (aPL): lupus anticoagulant and/or IgG/IgM anti-β2-glycoprotein I and anticardiolipin antibodies. APS carries significant morbidity for a relatively young patient population from recurrent thrombosis in any vascular bed (arterial, venous, or microvascular), often despite current standard of care, which is anticoagulation with vitamin K antagonists (VKA). Platelets have established roles in thrombosis at any site, and platelet hyperreactivity is clearly demonstrated in the pathophysiology of APS. Together with excess thrombin generation, platelet activation and aggregation are the common end result of all the pathophysiological pathways leading to thrombosis in APS. However, antiplatelet therapies play little role in APS, reserved as a possible option of low dose aspirin in addition to VKA in arterial or refractory thrombosis. This review outlines the current evidence and mechanisms for excessive platelet activation in APS, how it plays a central role in APS-related thrombosis, what evidence for antiplatelets is available in clinical outcomes studies, and potential future avenues to define how to target platelet hyperreactivity better with minimal impact on haemostasis.

Antiphospholipid Syndrome and Kidney Involvement

BACKGROUND

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by the development of autoantibodies and the impairment of the coagulation system. Knowledge about this syndrome is increasing over time, but kidney involvement, especially APS nephropathy, still represents a challenge for physicians.

SUMMARY

A "two hit" model has been hypothesized to explain APS pathophysiology, and the role played by some factors, such as the complement system, is becoming more and more clear. From a clinical point of view, along with thrombosis in any site and/or obstetric morbidities, that are the hallmarks of APS, a constellation of several other clinical symptoms is related to APS. These symptoms alone are not sufficient to fulfill Sydney criteria for APS and this could potentially lead to omitting some diagnoses. The mainstay of management of APS is antithrombotic therapy, but there are expectations for new drugs that regulate the immune system. APS could affect the kidneys in many ways and among them, APS nephropathy is an intriguing entity that has been overlooked in recent years. Novel studies on APS nephropathy are lacking.

KEY MESSAGES

In this review, we discuss what we currently know about APS and its relationship with the kidney, with an eye toward the future perspectives. Multicenter studies on APS nephropathy are necessary in order to develop targeted therapies.

Structural Proteomic Profiling of Cerebrospinal Fluids to Reveal Novel Conformational Biomarkers for Alzheimer's Disease

Alzheimer's disease (AD) is the most common representation of dementia, with brain pathological hallmarks of protein abnormal aggregation, such as with amyloid beta and tau protein. It is well established that posttranslational modifications on tau protein, particularly phosphorylation, increase the likelihood of its aggregation and subsequent formation of neurofibrillary tangles, another hallmark of AD. As additional misfolded proteins presumably exist distinctly in AD disease states, which would serve as potential source of AD biomarkers, we used limited proteolysis-coupled with mass spectrometry (LiP-MS) to probe protein structural changes. After optimizing the LiP-MS conditions, we further applied this method to human cerebrospinal fluid specimens collected from healthy control, mild cognitive impairment (MCI), and AD subject groups to characterize proteome-wide misfolding tendencies as a result of disease progression. The fully tryptic peptides embedding LiP sites were compared with the half-tryptic peptides generated from internal cleavage of the same region to determine any structural unfolding or misfolding. We discovered hundreds of significantly up- and down-regulated peptides associated with MCI and AD indicating their potential structural changes in AD progression. Moreover, we detected 53 structurally changed regions in 12 proteins with high confidence between the healthy control and disease groups, illustrating the functional relevance of these proteins with AD progression. These newly discovered conformational biomarker candidates establish valuable future directions for exploring the molecular mechanism of designing therapeutic targets for AD.

Association of antiphospholipid antibodies with clinical activity and renal pathological activity in patients with lupus nephritis

OBJECTIVES

This study aimed to investigate the association of antiphospholipid antibodies (aPL) with clinical activity and renal pathological activity in patients with lupus nephritis (LN).

METHODS

Levels of anticardiolipin () antibodies, anti-β2-glycoprotein I (anti-β2-GPI) antibodies and lupus anticoagulant (LAC) were measured, and other clinical and pathological data were also obtained during the same period before renal biopsy.

RESULTS

A total of 83 patients with LN were included in this study, 40 patients (48.2%) in the s positive group and 43 patients in the aPL negative group. LN patients with positive aPL had significantly higher SLEDAI (p = 0.012), more hematuria (p = 0.043), lower serum C3 (p = 0.003) and C4 (p = 0.014), and a higher pathological activity index (p = 0.012), more micro-thrombosis (p = 0.046) and more C3 deposits (p = 0.038) in the glomerulus than patients with negative aPL The level of IgG- was significantly correlated with SLEDAI and serum level of C3 (r = 0.44, p < 0.001; r = -0.39, p = 0.003, respectively). The level of IgM- was significantly correlated with SLEDAI, and serum levels of C3 and C4 (r = 0.27, p = 0.014; r = -0.22, p = 0.041; r = -0.23, p = 0.035, respectively).

CONCLUSIONS

Our work suggests that aPL, especially, are correlated with both clinical activity and renal pathological activity in patients with LN.

Antiphospholipid syndrome: Complement activation, complement gene mutations, and therapeutic implications

Antiphospholipid syndrome (APS) is an acquired thromboinflammatory disorder characterized by the presence of antiphospholipid antibodies as well as an increased frequency of venous or arterial thrombosis and/or obstetrical morbidity. The spectrum of disease varies from asymptomatic to a severe form characterized by widespread thrombosis and multiorgan failure, termed catastrophic APS (CAPS). CAPS affects only about ∼1% of APS patients, often presents as a thrombotic microangiopathy and has a fulminant course with >40% mortality, despite the best available therapy. Animal models have implicated complement in the pathophysiology of thrombosis in APS, with more recent data from human studies confirming the interaction between the coagulation and complement pathways. Activation of the complement cascade via antiphospholipid antibodies can cause cellular injury and promote coagulation via multiple mechanisms. Finally, analogous to classic complement-mediated diseases such as atypical hemolytic uremic syndrome, a subset of patients with APS may be at increased risk for development of CAPS because of the presence of germline variants in genes crucial for complement regulation. Together, these data make complement inhibition an attractive and potentially lifesaving therapy to mitigate morbidity and mortality in severe thrombotic APS and CAPS.

The correlation of anti-β2-glycoprotein I antibody with complement activation in patients with idiopathic membranous nephropathy

PURPOSE

This study evaluated the correlation of anti-β2-glycoprotein I (anti-β2-GPI) antibodies with complement activation in patients with idiopathic membranous nephropathy (IMN).

METHODS

Thirty-two IMN patients with positive anti-β2-GPI antibody were enrolled, and 32 age- and sex-matched IMN patients with negative anti-β2-GPI antibody were randomly enrolled as controls. The frozen serum samples of these 64 patients were collected for detection of anti-phospholipase A2 receptor (PLA2R) antibody and the activity of three complement pathways. Paraffin specimens of the kidney tissues of these 64 patients were collected for immunohistochemical staining of C4d.

RESULTS

IMN patients with positive anti-β2-GPI antibody had a significant decline of the residual complement activity of alternative pathway than those with negative anti-β2-GPI antibody (37.4 ± 21.2% vs 65.7 ± 50.5%, P = 0.021). The positive rate of kidney C4d staining in IMN patients with and without anti-β2-GPI was 65.6 and 81.2%, with no significant difference (P = 0.257). Patients with and without anti-β2-GPI had the same positive rate of anti-PLA2R antibody.

CONCLUSION

Anti-β2-GPI antibody was associated with alternative complement activation in patients with IMN.

Complement deposition, C4d, on platelets is associated with vascular events in systemic lupus erythematosus

OBJECTIVE

Complement components, including C4d, can be found on activated platelets, a process associated with vascular disease in SLE. We investigated whether platelet C4d (PC4d) adds additional value to traditional and known lupus-associated risk factors when identifying SLE patients with vascular disease.

METHODS

This cross-sectional study included 308 well-characterized SLE patients and 308 matched general population controls. PC4d deposition was analysed using flow cytometry. Values >95% of controls were considered as PC4d positive (+). aPL were determined by Luminex, and the LA test was performed by DRVVT. History of vascular disease (composite and as separate outcomes) was defined at inclusion.

RESULTS

SLE patients had increased PC4d deposition as compared with population controls (50 vs 5%, P < 0.0001). PC4d+ positively associated with any vascular events, and separately with venous and cerebrovascular events, and also with all investigated aPL profiles. The association for any vascular event remained statistically significant after adjustment for traditional and SLE-associated risk factors (odds ratio: 2.3, 95% CI: 1.3, 4.3, P = 0.008). Compared with patients negative for both PC4d and LA, patients with double positivity were more likely to have vascular disease (odds ratio: 12.3, 95% CI: 5.4, 29.3; attributable proportion due to interaction 0.8, 95% CI: 0.4, 1.1).

CONCLUSION

PC4d+ is associated with vascular events in SLE, independently of traditional and SLE-associated risk factors. Concurrent presence of PC4d and LA seem to interact to further increase the odds for vascular events. Prospective studies should examine whether the aPL/PC4d combination can improve prediction of vascular events in SLE and/or APS.

16th International Congress on Antiphospholipid Antibodies Task Force Report on Antiphospholipid Syndrome Treatment Trends

Antiphospholipid syndrome (APS), an acquired autoimmune thrombophilia, is characterised by thrombosis and/or pregnancy morbidity in association with persistent antiphospholipid antibodies. The 16th International Congress on Antiphospholipid Antibodies Task Force on APS Treatment Trends reviewed the current status with regard to existing and novel treatment trends for APS, which is the focus of this Task Force report. The report addresses current treatments and developments since the last report, on the use of direct oral anticoagulants in patients with APS, antiplatelet agents, adjunctive therapies (hydroxychloroquine, statins and vitamin D), targeted treatment including rituximab, belimumab, and anti-TNF agents, complement inhibition and drugs based on peptides of beta-2-glycoprotein I. In addition, the report summarises potential new players, including coenzyme Q10, adenosine receptor agonists and adenosine potentiation. In each case, the report provides recommendations for clinicians, based on the current state of the art, and suggests a clinical research agenda. The initiation and development of appropriate clinical studies requires a focus on devising suitable outcome measures, including a disease activity index, an optimal damage index, and a specific quality of life index.

17β-Estradiol Promotes Proinflammatory and Procoagulatory Phenotype of Innate Immune Cells in the Presence of Antiphospholipid Antibodies

Antiphospholipid syndrome (APS) is the most common cause of acquired thrombophilia and recurrent spontaneous miscarriages associated with extended persistence of antiphospholipid antibodies (aPL). How circulating aPL and high-17β-estradiol (E2) environment contribute to the pregnancy complications in APS is poorly defined. Therefore, we aimed to analyse whether E2 could be responsible for the immune cell hyperactivation in aPL- positive (lupus anticoagulant, anti-cardiolipin, anti-β2-glycoprotein) in women. For this, peripheral blood mononuclear cells (PBMCs) from 14 aPL- positive and 13 aPL- negative women were cultured in the presence or absence of E2, LPS or E2+LPS and cell immunophenotype and cytokine release were analysed. In the aPL+ group, E2 presence markedly increased the percentage of NK cells positive for CD69 (p < 0.05), monocytes positive for tissue factor (TF, CD142) (p < 0.05), and B cells expressing PD-L1 (p < 0.05), as well as the elevated production of IL-1β comparing to aPL- women (p < 0.01). Regardless of aPL positivity, E2 augmented the procoagulatory response elicited by LPS in monocytes. Our findings show the ability of E2 to promote proinflammatory and procoagulatory phenotype of innate immune cells in individuals with aPL positivity. Our data highlights the significant impact of female hormones on the activation of immune cells in the presence of aPL.

Antiphospholipid antibodies in patients with proliferative and membranous lupus nephritis

The aim of this study was to investigate the role of antiphospholipid antibodies (aPLs) in patients with proliferative and membranous lupus nephritis (LN). Patients hospitalized with LN from July 2014 to December 2017 were selected. Levels of serum IgG, IgA, and IgM anticardiolipin (aCL) antibodies, anti-β2-glycoprotein I (anti-β2-GPI) antibodies, and lupus anticoagulant (LAC) were measured. Other clinical and pathological data were obtained at the time of hospitalization for diagnosis. Overall, 75 patients with proliferative LN and 31 patients with membranous LN were analyzed. Significant differences were found between the two groups in the detection rates of aCL antibodies (45.3% vs 22.7%, p = 0.029) and IgG-aCL antibodies (40% vs 12.9%, p = 0.007). The aCL antibody-positive proliferative LN patients (n = 34) had lower serum complement C3 and C4 levels (p = 0.010 and p = 0.021, respectively) and higher intensity of complement C1q deposition in kidney tissue than the aCL antibody-negative proliferative LN patients (n = 41) (p = 0.003). Our work suggests that aCL antibodies, especially IgG-aCL antibodies, may play a role in the damage caused by proliferative LN, and this process may involve the classical pathway of complement activation.

Association between antiphospholipid antibodies and factor Bb in lupus nephritis patients with glomerular microthrombosis

BACKGROUND AND PURPOSE

The mechanism of glomerular microthrombosis (GMT) in patients with lupus nephritis (LN) is largely unknown. The aim of this study is to investigate the association between antiphospholipid antibodies (aPLs) and factor Bb in LN patients with GMT.

METHODS

Patients with biopsy-proven LN hospitalized from July 2015 to July 2018 in our hospital were selected for this study. Levels of lupus anticoagulant (LAC), anticardiolipin antibodies (aCLs), anti-β2-glycoprotein I (anti-β2-GPI) antibodies and factor Bb were measured, and other clinical and pathological data were also obtained during the same period before renal biopsy.

RESULTS

A total of 25 LN patients with GMT and 76 LN patients without GMT were included in this study. In LN patients with GMT, the presence of anti-β2GPI and LAC were both significantly higher than in those without GMT (P < .001 and P = .039, respectively). The level of factor Bb was also higher in LN patients with GMT than in those without GMT (P = .021). In the correlation analysis, Bb level was positively correlated with serum creatinine (r = 0.28, P = .014), activity index (r = 0.24, P = .021) GMT (r = 0.65, P < .001) and IgG-anti-β2GPI (r = 0.771, P < .001).

CONCLUSIONS

Our work suggests that aPLs, especially IgG-anti-β2GPI, may play a role in the progress of GMT, and this process might involve alternative complement activation.

The non-haemostatic role of platelets in systemic lupus erythematosus

Dysregulation of lymphocyte function, accumulation of autoantibodies and defective clearance of circulating immune complexes and apoptotic cells are hallmarks of systemic lupus erythematosus (SLE). Moreover, it is now evident that an intricate interplay between the adaptive and innate immune systems contributes to the pathogenesis of SLE, ultimately resulting in chronic inflammation and organ damage. Platelets circulate in the blood and are chiefly recognized for their role in the prevention of bleeding and promotion of haemostasis; however, accumulating evidence points to a role for platelets in both adaptive and innate immunity. Through a broad repertoire of receptors, platelets respond promptly to immune complexes, complement and damage-associated molecular patterns, and represent a major reservoir of immunomodulatory molecules in the circulation. Furthermore, evidence suggests that platelets are activated in patients with SLE, and that they could contribute to the circulatory autoantigenic load through the release of microparticles and mitochondrial antigens. Herein, we highlight how platelets contribute to the immune response and review evidence implicating platelets in the pathogenesis of SLE.

Complement in the Pathophysiology of the Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is characterized by thrombosis and pregnancy morbidity in the presence of antiphospholipid antibodies (aPL). Complement is a system of enzymes and regulatory proteins of the innate immune system that plays a key role in the inflammatory response to pathogenic stimuli. The complement and coagulation pathways are closely linked, and expanding data indicate that complement may be activated in patients with aPL and function as a cofactor in the pathogenesis of aPL-associated clinical events. Complement activation by aPL generates C5a, which induces neutrophil tissue factor-dependent procoagulant activity. Beta-2-glycoprotein I, the primary antigen for pathogenic aPL, has complement regulatory effects in vitro. Moreover, aPL induce fetal loss in wild-type mice but not in mice deficient in specific complement components (C3, C5). Antiphospholipid antibodies also induce thrombosis in wild type mice and this effect is attenuated in C3 or C6 deficient mice, or in the presence of a C5 inhibitor. Increased levels of complement activation products have been demonstrated in sera of patients with aPL, though the association with clinical events remains unclear. Eculizumab, a terminal complement inhibitor, has successfully been used to treat catastrophic APS and prevent APS-related thrombotic microangiopathy in the setting of renal transplant. However, the mechanisms of complement activation in APS, its role in the pathogenesis of aPL related complications in humans, and the potential of complement inhibition as a therapeutic target in APS require further study.

Obstetric antiphospholipid syndrome

The present clinical and laboratory classification criteria for antiphospholipid syndrome (APS) were established in Sydney, Australia, in 2006. In this review, we focus on the obstetric subset of APS (OAPS), defined by persistent positivity for antiphospholipid antibodies together with either early recurrent pregnancy loss, early fetal death, stillbirth or premature birth <34 gestational weeks due to pre-eclampsia, eclampsia and placental insufficiency. It is important to diagnose these cases since most women suffering from OAPS can, when given appropriate treatment, have successful pregnancies. Furthermore, patients with OAPS may, depending on the antibody profile, be at enhanced risk of thrombotic events later in life. We present an update on the present knowledge of possible underlying pathogenesis, risk factors and risk estimations for adverse pregnancy outcomes before and during pregnancy, current treatment concepts, and long-term outcomes for women with OAPS and their children.

Antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disease characterized by the presence of antiphospholipid antibodies, such as lupus anticoagulant, anticardiolipin antibodies and anti-β2-glycoprotein 1 antibodies. APS can present with a variety of clinical phenotypes, including thrombosis in the veins, arteries and microvasculature as well as obstetrical complications. The pathophysiological hallmark is thrombosis, but other factors such as complement activation might be important. Prevention of thrombotic manifestations associated with APS includes lifestyle changes and, in individuals at high risk, low-dose aspirin. Prevention and treatment of thrombotic events are dependent mainly on the use of vitamin K antagonists. Immunosuppression and anticomplement therapy have been used anecdotally but have not been adequately tested. Pregnancy morbidity includes unexplained recurrent early miscarriage, fetal death and late obstetrical manifestation such as pre-eclampsia, premature birth or fetal growth restriction associated with placental insufficiency. Current treatment to prevent obstetrical morbidity is based on low-dose aspirin and/or low-molecular-weight heparin and has improved pregnancy outcomes to achieve successful live birth in >70% of pregnancies. Although hydroxychloroquine and pravastatin might further improve pregnancy outcomes, prospective clinical trials are required to confirm these findings.

Mechanisms of thrombosis in systemic lupus erythematosus and antiphospholipid syndrome

The presence of antiphospholipid antibodies is one of the most common acquired risk factors for thrombosis. Antiphospholipid antibodies is a collective term for a set of autoantibodies with closely related but different specificity. Experiments in which isolated patient antibodies were injected into mice have shown that a specific subset of autoantibodies, those directed against the first domain of plasma protein β₂-glycoprotein I, can explain the increased risk of thrombosis. Experiments performed with these mice have shown that autoantibodies against β₂-glycoprotein I bind to and activate cells such as endothelial cells, monocytes, and platelets. Activation of these cells, all involved in the regulation of hemostasis, results in a shift towards a prothrombotic state. How this process is regulated, whether this is the only mechanism involved, and whether this is the only subpopulation responsible for the increased thrombotic risk is unknown. In this review, we will critically discuss what is known and what is debatable on the pathophysiology of antiphospholipid syndrome.

Antibodies Against Complement Components: Relevance for the Antiphospholipid Syndrome-Biomarkers of the Disease and Biopharmaceuticals

PURPOSE OF REVIEW

Laboratory criterion for the diagnosis of antiphospholipid syndrome (APS) is the presence of antiphospholipid antibodies (aPL Abs). Complement system has a role in mediating aPL Abs-induced thrombosis in animal models. The importance of antibodies against complement components (potential biomarkers of APS) and the importance of antibodies with beneficial anti-complement effects in APS (as biopharmaceuticals) are reviewed.

RECENT FINDINGS

Antibodies against complement components described in APS patients, so far, are anti-C1q and anti-factor H Abs, although anti-factor B Abs and anti-C5a Abs were described in animal models of APS. Clinical studies in APS patients are limited to a small number of case reports. Studies that would confirm potential role of Abs against complement components (as potential biomarkers of APS) are lacking. Lack of randomized clinical trials (that would provide complete data for confirmation of beneficial effects of biopharmaceuticals in complement inhibition) in APS is alarming.

Distinct contributions of complement factors to platelet activation and fibrin formation in venous thrombus development

Expanding evidence indicates multiple interactions between the hemostatic system and innate immunity, and the coagulation and complement cascades. Here we show in a tissue factor (TF)-dependent model of flow restriction-induced venous thrombosis that complement factors make distinct contributions to platelet activation and fibrin deposition. Complement factor 3 (C3) deficiency causes prolonged bleeding, reduced thrombus incidence, thrombus size, fibrin and platelet deposition in the ligated inferior vena cava, and diminished platelet activation in vitro. Initial fibrin deposition at the vessel wall over 6 hours in this model was dependent on protein disulfide isomerase (PDI) and TF expression by myeloid cells, but did not require neutrophil extracellular trap formation involving peptidyl arginine deiminase 4. In contrast to C3^-/- mice, C5-deficient mice had no apparent defect in platelet activation in vitro, and vessel wall platelet deposition and initial hemostasis in vivo. However, fibrin formation, the exposure of negatively charged phosphatidylserine (PS) on adherent leukocytes, and clot burden after 48 hours were significantly reduced in C5^-/- mice compared with wild-type controls. These results delineate that C3 plays specific roles in platelet activation independent of formation of the terminal complement complex and provide in vivo evidence for contributions of complement-dependent membrane perturbations to prothrombotic TF activation on myeloid cells.

Potential influences of complement factor H in autoimmune inflammatory and thrombotic disorders

Complement system homeostasis is important for host self-protection and anti-microbial immune surveillance, and recent research indicates roles in tissue development and remodelling. Complement also appears to have several points of interaction with the blood coagulation system. Deficiency and altered function due to gene mutations and polymorphisms in complement effectors and regulators, including Factor H, have been associated with familial and sporadic autoimmune inflammatory - thrombotic disorders, in which autoantibodies play a part. These include systemic lupus erythematosus, rheumatoid arthritis, atypical haemolytic uremic syndrome, anti-phospholipid syndrome and age-related macular degeneration. Such diseases are generally complex - multigenic and heterogeneous in their symptoms and predisposition/susceptibility. They usually need to be triggered by vascular trauma, drugs or infection and non-complement genetic factors also play a part. Underlying events seem to include decline in peripheral regulatory T cells, dendritic cell, and B cell tolerance, associated with alterations in lymphoid organ microenvironment. Factor H is an abundant protein, synthesised in many cell types, and its reported binding to many different ligands, even if not of high affinity, may influence a large number of molecular interactions, together with the accepted role of Factor H within the complement system. Factor H is involved in mesenchymal stem cell mediated tolerance and also contributes to self-tolerance by augmenting iC3b production and opsonisation of apoptotic cells for their silent dendritic cell engulfment via complement receptor CR3, which mediates anti-inflammatory-tolerogenic effects in the apoptotic cell context. There may be co-operation with other phagocytic receptors, such as complement C1q receptors, and the Tim glycoprotein family, which specifically bind phosphatidylserine expressed on the apoptotic cell surface. Factor H is able to discriminate between self and nonself surfaces for self-protection and anti-microbe defence. Factor H, particularly as an abundant platelet protein, may also modulate blood coagulation, having an anti-thrombotic role. Here, we review a number of interaction pathways in coagulation and in immunity, together with associated diseases, and indicate where Factor H may be expected to exert an influence, based on reports of the diversity of ligands for Factor H.

Marked inflammation in catastrophic longitudinal myelitis associated with systemic lupus erythematosus

Catastrophic longitudinal myelitis is an extremely rare neurologic manifestation of collagen vascular disease, described heretofore in 11 cases of SLE and 1 of Sjogren's Syndrome. This report documents markedly abnormal and worsening CSF findings on sequential CSF examinations over a period of three days (WBC >1500 cells/μL, >80% neutrophils, markedly elevated protein, and extremely low glucose levels) in the absence of infection. These abnormalities cleared rapidly with institution of immunosuppressive therapy so that a third CSF exam done within three days revealed almost complete normalization of CSF values. These findings suggest that in some cases of CLM a strong inflammatory component may be present, while in others, other pathogenic factors may predominate. Lupus (2007) 16, 823—826.

Complement levels and anti-C1q autoantibodies in patients with neuropsychiatric systemic lupus erythematosus

Objective

The objective of this paper is to analyse serum levels of anti-C1q, C1q circulating immune complexes (CIC), complement activation and complement components in systemic lupus erythematosus (SLE) patients during the first central nervous system neuropsychiatric (NP) event and to define the possible association between these results and clinical and laboratory characteristics.

Methods

A total of 280 patients suspected of having NP involvement due to SLE were recruited in the Leiden NPSLE-clinic. All SLE patients were classified according to the ACR 1982 revised criteria for the classification of SLE. The clinical disease activity was measured by the SLE Disease Activity Index 2000 (SLEDAI-2K) and NP diagnoses were classified according to the 1999 ACR case definitions for NPSLE. We measured in serum of all patients anti-C1q and C1q CIC levels, the activation capacity of complement (CH50 and AP50) and different complement components (C1q, C3, C4).

Results

In 92 patients the symptoms were attributed to SLE. NPSLE patients consisted of 63 patients with focal NPSLE and 34 patients with diffuse NPSLE. Anti-C1q antibodies were significantly higher and CH50, AP50 and C3 were significantly lower in NPSLE patients compared with SLE patients without NPSLE. This association was specially marked for diffuse NPSLE while no differences were found for focal NPSLE. After using potential predictors, decreased C4 remained significantly associated with focal NPSLE, but only when antiphospholipid antibodies (aPL) were included in the model. C3 and AP50 were independently associated with diffuse NPSLE. When SLEDAI-2K was included in the model these two associations were lost. When individual NPSLE syndromes were analysed, psychosis and cognitive dysfunction showed significantly lower values of complement activation capacity and all complement components. No significant associations were seen for other individual NPSLE syndromes.

Conclusion

The associations between diffuse NPSLE and anti-C1q, C3/AP50 and focal NPSLE and C4 may be explained by disease activity and the presence of aPL, respectively. The role of complement activation and complement components in lupus psychosis and cognitive dysfunction merits further research.

Human IgG Antinuclear Antibodies Induce Pregnancy Loss in Mice by Increasing Immune Complex Deposition in Placental Tissue: In Vivo Study

PROBLEM

A threefold higher prevalence of antinuclear antibodies (ANA) has been reported in patients with recurrent pregnancy loss (RPL). Nevertheless, the role of ANA in reproductive failure is still unclear. The aim of this study was to investigate the role of ANA during early pregnancy in vivo.

METHOD OF STUDY

We used pregnant mice treated with immunoglobulin G (IgG) obtained from normal healthy subjects (NHS); ANA(+) sera of patients with RPL; and ANA(+) sera from women with uncomplicated pregnancies (HW). Placental immunohistochemical/immunofluorescence staining was performed to detect complement and immune complex deposition. ELISA was performed to evaluate complement levels.

RESULTS

ANA(+) IgG from RPL women significantly increased embryo resorption rate, reduced C3, and increased C3a serum levels compared to NHS IgG or ANA(+) -HW IgG. Increased C3 deposition and increased immune complex staining in placental tissues from mice treated with ANA(+) -RPL IgG fraction compared to NHS- and ANA(+) -HW-IgG-treated mice were found.

CONCLUSION

ANA(+) IgG injection in mice is able to induce fetal resorption and complement activation. The presence on placental tissues of immune complexes and complement fragments suggests the complement activation as a possible mechanism of placental damage.

Immunotherapy in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is a disorder characterized by the association of arterial or venous thrombosis and/or pregnancy morbidity with the presence of antiphospholipid antibodies (anticardiolipin antibodies, lupus anticoagulant antibodies, and/or anti-β2-glycoprotein I antibodies). Thrombosis is the major manifestation in patients with aPLs, but the spectrum of symptoms and signs associated with aPLs has broadened considerably, and other manifestations, such as thrombocytopenia, non-thrombotic neurological syndromes, psychiatric manifestations, livedo reticularis, skin ulcers, hemolytic anemia, pulmonary hypertension, cardiac valve abnormality, and atherosclerosis, have also been related to the presence of those antibodies. Several studies have contributed to uncovering the basis of antiphospholipid antibody pathogenicity, including the targeted cellular components, affected systems, involved receptors, intracellular pathways used, and the effector molecules that are altered in the process. Therapy for thrombosis traditionally has been based on long-term oral anticoagulation; however, bleeding complications and recurrence despite high-intensity anticoagulation can occur. The currently accepted first-line treatment for obstetric APS (OAPS) is low-dose aspirin plus prophylactic unfractionated or low-molecular-weight heparin (LMWH). However, in approximately 20% of OAPS cases, the final endpoint, i.e. a live birth, cannot be achieved. Based on all the data obtained in different research studies, new potential therapeutic approaches have been proposed, including the use of new oral anticoagulants, statins, hydroxychloroquine, coenzyme Q10, B-cell depletion, platelet and TF inhibitors, peptide therapy or complement inhibition among others. Current best practice in use of these treatments is discussed.

Targeted therapy in antiphospholipid syndrome

PURPOSE OF REVIEW

To review novel therapeutic targets that are currently under investigation to develop safer, targeted therapies for antiphsopholipid antibody (aPL)-mediated clinical manifestations.

RECENT FINDINGS

Novel therapeutic options potentially available include anti-CD20 monoclonal antibodies and new-generation anticoagulants (such as direct thrombin and anti-Xa inhibitors). Research focusing on interfering with aPL-mediated cell activation, targeting complement components and the innovative concept of blocking the pathogenic subpopulation of aPL with tailored peptides are currently being explored.

SUMMARY

Antiphospholipid syndrome is an autoimmune disease characterized by thrombosis and pregnancy morbidity occurring in patients persistently positive for aPL. Current therapeutic options remain confined to long-term anticoagulation with vitamin K antagonists. The future holds much promise with the identification of novel potential targets, many of which are currently under investigation. The challenge will be to design prospective randomized controlled clinical trials to provide the evidence necessary to support integration of these therapies into clinical practice.

Platelet activation and anti-phospholipid antibodies collaborate in the activation of the complement system on platelets in systemic lupus erythematosus

Anti-phospholipid (aPL) antibodies are important contributors to development of thrombosis in patients with the autoimmune rheumatic disease systemic lupus erythematosus (SLE). The underlying mechanism of aPL antibody-mediated thrombosis is not fully understood but existing data suggest that platelets and the complement system are key components. Complement activation on platelets is seen in SLE patients, especially in patients with aPL antibodies, and has been related to venous thrombosis and stroke. The aim of this study was to investigate if aPL antibodies could support classical pathway activation on platelets in vitro as well as in SLE patients. Furthermore, we investigated if complement deposition on platelets was associated with vascular events, either arterial or venous, when the data had been adjusted for traditional cardiovascular risk factors. Finally, we analyzed if platelet complement deposition, both C1q and C4d, was specific for SLE. We found that aPL antibodies supported C4d deposition on platelets in vitro as well as in SLE patients (p = 0.001 and p<0.05, respectively). Complement deposition on platelets was increased in SLE patients when compared with healthy individuals (p<0.0001). However, high levels of C4d deposition and a pronounced C1q deposition were also seen in patients with rheumatoid arthritis and systemic sclerosis. In SLE, C4d deposition on platelets was associated with platelet activation, complement consumption, disease activity and venous (OR = 5.3, p = 0.02), but not arterial, thrombosis, observations which were independent of traditional cardiovascular risk factors. In conclusion, several mechanisms operate in SLE to amplify platelet complement deposition, of which aPL antibodies and platelet activation were identified as important contributors in this investigation. Complement deposition on platelets was identified as a marker of venous, but not arterial thrombosis, in SLE patients independently of traditional risk factors and aPL antibodies. Further studies are needed to elucidate the role of complement deposition on platelets in development of venous thrombosis.

Adaptive immune responses during pregnancy

It has long been believed that there is no immune interaction between mother and conceptus during pregnancy. This concept changed after evidence was provided that the maternal immune system is aware of the semiallogeneic conceptus and develops strategies to tolerate it. Since then, finely regulated mechanisms of active tolerance toward the fetus have been described. This Special Issue of the American Journal of Reproductive Immunology deals with these mechanisms. It begins with the description of minor histocompatibility antigens in the placenta; it further goes through adaptive immune responses toward paternal fetal antigens, mostly concentrating on regulatory T cells and molecules modulating the Th1/Th2 balance. The participation of antibody-producing B cells in normal and pathological pregnancies is also discussed. This introductory chapter resumes the concepts presented throughout the Issue and discusses the clinical applications raised from these concepts.

Organ specific complement proteins inhibition can reduce the risk of antiphospholipid antibodies mediated fetal loss

Antiphospholipid syndrome is a pregnancy related, systemic autoimmune disorder in which antibodies directed against cell membrane phospholipids with multiple venous/arterial thromboses. Though the etiology of antiphospholipid syndrome has not been identified exactly, experimental evidences suggest the possible mechanisms involved in the pathogenicity of this syndrome. Antiphospholipid antibodies mediate deposition of complement proteins in placenta and over expression of tissue factor on the surface of neutrophils which are reported to be the prominent cause of prothrombotic phenotype. The activation complement components C3 and C5 by antiphospholipid antibodies would eventually activates blood coagulation pathway that leads to thrombosis. Inhibition of activated complement components C3a and C5a by anticomplement agents protects from antiphospholipid antibody mediated fetal loss. Since the interference of complement pathway may lead to deleterious effects, we hypothesis that local inhibition of complement proteins C3a and C5a at placenta would reduce the risk of antiphospholipid antibody mediated placental thrombosis and pregnancy complications.

Increased C1q, C4 and C3 deposition on platelets in patients with systemic lupus erythematosus--a possible link to venous thrombosis?

OBJECTIVE

Patients with systemic lupus erythematosus (SLE) have an increased risk of developing vascular diseases (VD) such as myocardial infarction, stroke and venous thrombosis, which can only partly be explained by traditional risk factors. The role of platelets in this process has not been extensively studied. Platelet activation supports complement binding to the platelet surface, and increased C4d has been seen on platelets in SLE patients as well as in non-rheumatic patients with stroke. In this study we investigated in vivo platelet deposition of the classical complement pathway components C1q, C4d and C3d in relation to VD in SLE patients. Furthermore, the ability of serum to support in vitro complement deposition on fixed heterologous platelets was analyzed.

METHODS

Blood from 69 SLE patients and age- and sex-matched healthy individuals was collected in sodium-citrate tubes and platelets isolated by centrifugation. Complement deposition on platelets was detected by flow cytometry.

RESULTS

We could demonstrate that SLE patients had increased C1q, C3d and C4d deposition on platelets as compared to healthy controls (p < 0.0001). SLE patients with a history of venous thrombosis had increased complement deposition on platelets as compared to SLE patients without this manifestation (p < 0.05). In vitro studies demonstrated that serum from patients with lupus anticoagulant, venous thrombosis or antiphospholipid antibody syndrome supported increased platelet C4d deposition in vitro as compared to SLE patients without these manifestations (p < 0.05). Our data support the hypothesis that platelet activation and the subsequent complement deposition on platelets are central in the development of venous thrombosis in SLE.

CONCLUSIONS

Altogether we suggest that complement deposition on platelets could reflect important pathogenetic events related to the development of venous thrombosis in SLE and might be used as a marker for venous thrombosis in SLE.

Antiphospholipid antibodies promote leukocyte-endothelial cell adhesion and thrombosis in mice by antagonizing eNOS via β2GPI and apoER2

In antiphospholipid syndrome (APS), antiphospholipid antibodies (aPL) binding to β2 glycoprotein I (β2GPI) induce endothelial cell-leukocyte adhesion and thrombus formation via unknown mechanisms. Here we show that in mice both of these processes are caused by the inhibition of eNOS. In studies of cultured human, bovine, and mouse endothelial cells, the promotion of monocyte adhesion by aPL entailed decreased bioavailable NO, and aPL fully antagonized eNOS activation by diverse agonists. Similarly, NO-dependent, acetylcholine-induced increases in carotid vascular conductance were impaired in aPL-treated mice. The inhibition of eNOS was caused by antibody recognition of domain I of β2GPI and β2GPI dimerization, and it was due to attenuated eNOS S1179 phosphorylation mediated by protein phosphatase 2A (PP2A). Furthermore, LDL receptor family member antagonism with receptor-associated protein (RAP) prevented aPL inhibition of eNOS in cell culture, and ApoER2-/- mice were protected from aPL inhibition of eNOS in vivo. Moreover, both aPL-induced increases in leukocyte-endothelial cell adhesion and thrombus formation were absent in eNOS-/- and in ApoER2-/- mice. Thus, aPL-induced leukocyte-endothelial cell adhesion and thrombosis are caused by eNOS antagonism, which is due to impaired S1179 phosphorylation mediated by β2GPI, apoER2, and PP2A. Our results suggest that novel therapies for APS can now be developed targeting these mechanisms.

What nephrologists need to know about antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disorder characterized by recurrent arterial or venous thrombosis and/or pregnancy losses, in the presence of persistently elevated levels of anticardiolipin antibodies (aCL) and/or evidence of circulating lupus anticoagulant (LA). The kidney is a major target organ in both primary and secondary APS. With the expanding spectrum of renal diseases associated with APS, and the impact of APS in ESRD care, this subject is of increasing relevance to nephrologists. This review describes the various clinical manifestations and histological features of this syndrome, with reference to the kidney.

Antiphospholipid antibody profiles in lupus nephritis with glomerular microthrombosis: a prospective study of 124 cases

Introduction

Glomerular microthrombosis (GMT) is a common vascular change in patients with lupus nephritis (LN). The mechanism underlying GMT is largely unknown. Although several studies have reported the association of antiphospholipid antibodies (aPL) with GMT, the relation between GMT and aPL remains controversial. Previous studies have demonstrated that some aPL could bind to several hemostatic and fibrinolytic proteases that share homologous enzymatic domains. Of the protease-reactive aPL, some can inhibit the anticoagulant activity of activated protein C and the fibrinolytic function of plasmin, and hinder the antithrombin inactivation of thrombin. The purpose of this study was to investigate the prevalence of GMT in LN patients and examine the relation between the aPL profiles (including some protease-reactive aPL) and GMT.

Methods

Renal biopsy specimens were examined for the presence of glomerular microthrombi. Plasma samples from 25 LN patients with GMT (LN-GMT group) and 99 LN patients without GMT (LN-non-GMT group) were tested for lupus anticoagulant and antibodies against cardiolipin, β2 glycoprotein I, plasmin, thrombin, tissue plasminogen activator, and annexin II.

Results

The prevalence of GMT in LN patients was 20.2%. Compared with the LN-non-GMT group, the LN-GMT group had an elevated systemic lupus erythematosus disease activity index; elevated renal tissue injury activity and chronicity indices; elevated serum creatinine, blood urea nitrogen, and proteinuria levels; a lower serum C3 level and much intense glomerular C3, C1q staining; and a higher frequency of hypertension (P < 0.05 for all). Additionally, the detection rate of lupus anticoagulant, immunoglobulin G (IgG) anti-β2 glycoprotein I and anti-thrombin antibodies were higher in the LN-GMT group than in the LN-non-GMT group (P < 0.05 for all). No statistical differences were found in the detection rates of IgG anti-cardiolipin, plasmin, tissue plasminogen activator, or annexin II antibodies (P > 0.05 for all). No detectable difference in IgM autoantibodies to the above antigens was observed between the two groups.

Conclusions

GMT occurs in approximately 20.2% of LN patients. Patients with GMT have severer renal tissue injuries and poorer renal functions than patients without GMT. The lupus anticoagulant and antibodies against β2 glycoprotein I and thrombin may play a role in GMT.

Immunohistochemical detection of intravascular platelet microthrombi in patients with lupus nephritis and anti-phospholipid antibodies

OBJECTIVES

To evaluate whether the use of platelet immunohistochemistry (IHC) markers improves the sensitivity of histological methods to detect microthrombosis in SLE nephritis and aPLs and to analyse the clinicopathological correlations of microthrombosis in this setting.

METHODS

Kidney biopsy specimens from 65 patients with SLE, including 36 with positive aPLs, were studied by IHC using antibodies against platelet glycoproteins CD41 and CD61. Clinical data at the time of kidney biopsy and during a mean follow-up of 7.5 years after biopsy were recorded and analysed with regard to histological or IHC data.

RESULTS

Histological lesions previously defined as APS nephropathy were found in 33% of the SLE kidney biopsies and were not associated with positive aPLs. Microthrombi detected as intravascular CD61(+) platelet deposits were present in 43% of the tissues and were significantly associated with positive aPLs, but not with histological APS nephropathy, nephritis manifestations nor with renal outcome. Histological APS lesions but not CD61(+) microthrombi correlated with an older age at nephritis presentation, previous cardiovascular risk factors and worse renal outcome.

CONCLUSIONS

Immunodetection of intravascular CD61(+) platelet aggregates is more sensitive than histological evaluation to detect acute microthrombosis and provides a better correlation with aPLs in SLE patients. In contrast, histological lesions consistent with APS nephropathy were not associated with aPLs but with cardiovascular risk factors and worse renal outcome.

Obstructive uropathy as the only manifestation of flare in a patient with systemic lupus erythematosus and anti-phospholipid syndrome

Chronic interstitial cystitis and ureteral stenosis has occasionally been reported in systemic lupus erythematosus, mostly associated with gastrointestinal symptoms. We report a case of obstructive uropathy associated to chronic interstitial cystitis as the only manifestation of lupus flare in a patient with SLE and anti-phospholipid syndrome (APS) who had been in remission for many years. The development of chronic interstitial cystitis in patients with SLE and APS has not been previously reported. Histopathological study of her urinary bladder and ureteral meatus showed chronic inflammatory infiltrate in the subepithelium. Lack of significant lower urinary tract symptoms and gastrointestinal involvement were some of the factors that could have prevented an earlier diagnosis. Obstructive uropathy and renal insufficiency initially improved with immunosuppressive treatment and endoureteral protheses, but poor compliance to the therapy led to ominous ending.

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.

The obstetric antiphospholipid syndrome

The association of persistent presence of circulating antiphospholipid antibodies and thromboembolic events, (recurrent) pregnancy loss or both is termed antiphospholipid syndrome. Pregnancies in women with the syndrome should be regarded as at high-risk for complications. Optimal management consisting of close follow-up and pharmacological treatment can result in about 70-80% live births. Apart from the laboratory diagnosis of the syndrome and pathophysiology, this review will focus on treatment during pregnancy.

Role of complement in antiphospholipid antibody-mediated thrombosis

The mechanisms by which an antibody that reacts with phospholipids (universal components of mammalian membranes) causes thrombosis are not immediately obvious. The development of an animal model of antiphospholipid antibody syndrome has moved the field forward in dissecting the pathogenesis of antiphospholipid antibody syndrome and has implicated the complement system in the mechanism of disease. Understanding complement's role in promoting thrombosis will be important in designing safer, more effective approaches to the treatment of patients with antiphospholipid antibodies, and may shed light on which patients are at greatest risk for thrombosis, perhaps permitting primary prophylaxis before irreversible tissue and organ damage occur.

Complement and coagulation: strangers or partners in crime?

The convergence between complement and the clotting system extends far beyond the chemical nature of the complement and coagulation components, both of which form proteolytic cascades. Complement effectors directly enhance coagulation. These effects are supplemented by the interactions of complement with other inflammatory mediators that can increase the thrombogenicity of blood. In addition, complement inhibits anticoagulant factors. The crosstalk between complement and coagulation is also well illustrated by the ability of certain coagulation enzymes to activate complement components. Understanding the interplay between complement and coagulation has fundamental clinical implications in the context of diseases with an inflammatory pathogenesis, in which complement-coagulation interactions contribute to the development of life-threatening complications. Here, we review the interactions of the complement system with hemostasis and their roles in various diseases.