Antiphospholipid syndrome: laboratory detection, mechanisms of action and treatment

Identification of the binding site for fondaparinux on Beta2-glycoprotein I

Antiphospholipid syndrome (APS) is an autoimmune disease with clinical manifestations of thrombosis and pregnancy complications. Beta2-glycoprotein I (β2GPI) is the major antigen for the APS-related antibodies. Heparin, low-molecular weight heparin and the synthetic pentasaccharide fondaparinux are commonly used for prophylaxis and treatment of thrombosis in patients with antiphospholipid syndrome. These antithrombotic drugs bind and activate antithrombin III to inactivate blood clotting proteases. Heparin and heparin derivatives might have a direct beneficial effect in APS via binding to β2GPI and interfering with prothrombotic properties of β2GPI/antibody complexes. We compared fondaparinux to heparin regarding its ability to bind β2GPI and inhibit the binding of β2GPI/antibody complexes to negatively charged phospholipids and endothelial cells. Although heparin and fondaparinux bind β2GPI at therapeutically relevant doses, neither fondaparinux nor heparin was efficient in inhibition of the binding of β2GPI/antibody complexes to negatively charged phospholipids and endothelial cells. Our studies suggest that these drugs do not act on pathological properties of β2GPI/antibody complexes, emphasizing the need for a new treatment specific for β2GPI-related thrombosis in APS. We observed that the binding interface of fondaparinux on β2GPI does not include the lysine residues known to be critical for binding of heparin. The docking model of the β2GPI complex with fondaparinux is in agreement with multiple experimental observations.

Antiphospholipid antibody syndrome

PURPOSE OF REVIEW

Antiphospholipid syndrome is a rare cause of ocular vaso-occlusive disease, but is associated with significant systemic morbidity and mortality. Thus, early diagnosis and treatment is essential.

RECENT FINDINGS

Although the pathophysiology of antiphospholipid syndrome continues to be poorly understood, there has been continued progress with regard to the relationship between antiphospholipid antibody and its target, β-2-glycoprotein I. Due to numerous limitations with standard serologic evaluation, new approaches to the evaluation of patient serum are being considered. New guidelines for the treatment and management of antiphospholipid antibody syndrome have been established by the 13th International Committee on Antiphospholipid Antibodies.

SUMMARY

A better understanding of the pathophysiology behind antiphospholipid antibody syndrome has led to novel approaches in the diagnosis and treatment of this disease.

The NF-κB specific inhibitor DHMEQ prevents thrombus formation in a mouse model of antiphospholipid syndrome

BACKGROUND

β2-glycoprotein I (β2GPI)-dependent antiphospholipid antibodies (aPLs) are considered to play a pivotal pathogenic role in antiphospholipid syndrome (APS) by inducing the expression of tissue factor, inflammatory cytokines, and chemokines, most of which are dependent upon the NF-κB pathway. Therefore, the NF-κB is regarded as a promising target for the development of a novel therapeutic strategy. However, progress has been limited owing to the fact that there are no widely-used in vivo models, or highly specific inhibitors.

OBJECTIVE

This study aimed to test the effects of an NF-κB-specific inhibitor, DHMEQ, in preventing thrombus formation using an original mouse model of APS.

MATERIALS AND METHODS

Specificity of a monoclonal aPL WB-6 was examined by ELISA. WB-6 was injected into normal BALB/c mice with or without DHMEQ treatment. A pulse laser was radiated to a cutaneous vein in the window of a dorsal skinfold chamber attached to the mouse and thrombus formation was observed and recorded under a microscope.

RESULTS

WB-6 bound preferentially to the caldiolipin (CL)-β2GPI complex rather than to CL alone, or β2GPI alone. WB-6, but not isotype-matched control antibody, induced a prothrombotic state in the mice by inducing tissue factor expression upon circulating monocytes, resulting in thrombus formation at the site of laser-induced endothelial injury. This diathesis was almost completely ameliorated by DHMEQ treatment.

CONCLUSIONS

Inhibition of the NF-κB pathway is a promising strategy for the development of a novel treatment for APS.

GAPSS: the Global Anti-Phospholipid Syndrome Score

OBJECTIVE

To develop and validate a risk score [global APS score (GAPSS)] derived from the combination of independent risk for thrombosis and pregnancy loss (PL), taking into account the aPL profile, conventional cardiovascular risk factors and the autoimmune antibody profile.

METHODS

This cross-sectional study included 211 consecutive SLE patients. Data on clinical manifestations, conventional cardiovascular risk factors, aPL profile, ANAs, ENA and anti-dsDNA were collected. Long-term low-dose aspirin, oral anticoagulant and HCQ treatment were also included in the analysis. Patients were randomly divided into two sets by a computer-generated randomized list. We developed GAPSS in the first set of patients (n = 106), assigning the risk factors identified by multivariate analysis weighted points proportional to the β-regression coefficient values. GAPSS was validated in the second set of patients (n = 105). The relationship between GAPPS and thrombosis and/or PL was analysed.

RESULTS

In the first set, higher values of GAPSS were seen in patients who experienced thrombosis and/or PL compared with those without clinical events [GAPSS 9.3 (4.8) (range 1-19) and 5.3 (4) (range 0-16), P < 0.001]. Also taken separately, patients who experienced thrombosis or PL showed higher GAPSS compared with those without clinical events [GAPSS 9.6 (4.8) (range 1-19) vs 4.9 (5) (range 0-14), P = 0.027 for thrombosis; 7.3 (5) vs 3.9 (5.1) (range 0-16), P = 0.024 for PL, respectively]. In the second set, the results were similar, with statistically higher values of GAPSS in patients with a clinical history of thrombosis and/or PL compared with those without events [GAPSS 9.5 (5.6) (range 0-20) and 3.9 (4.1) (range 0-17), P < 0.001). Higher values were also seen when subclassifying the patients according to the clinical manifestation, thrombosis or PL [GAPSS 9.5 (5.6) (range 0-20) vs 4.8 (5.4) (range 0-17), P = 0.036 for thrombosis; 7.9 (3.3) vs 3.8 (5.4) (range 0-16), P = 0.037 for PL, respectively).

CONCLUSION

These data propose a substantial improvement in risk prediction of thrombosis or PL in SLE based on assessment of the GAPSS, a quantitative scoring system.

Anti-cardiolipin from periodontitis patients induces MCP-1 production by human umbilical vein endothelial cells

AIM

Periodontal diseases are associated with a variety of systemic diseases, including cardiovascular disease and stroke, and patients with periodontitis demonstrate elevated levels of anti-cardiolipin antibodies. We sought to determine if anti-cardiolipin antibodies from periodontitis patients induced monocyte chemotactic protein-1 production by human vascular endothelial cells.

MATERIALS AND METHODS

IgG was purified from sera from 53 subjects, including chronic and aggressive periodontitis patients and periodontally healthy controls, with elevated or normal IgG anti-cardiolipin levels. In addition, anti-cardiolipin antibodies were specifically removed from some sera by immunoabsorption.

RESULTS

We found that, irrespective of diagnostic category, IgG from subjects with elevated anti-cardiolipin induced significantly greater monocyte chemotactic protein-1 production by human vascular endothelial cells than IgG from those subjects with normal anti-cardiolipin titres. Removal of anti-cardiolipin from IgG preparations from periodontitis patients significantly reduced their ability to induce monocyte chemotactic protein-1.

CONCLUSIONS

Since elevated titres of anti-cardiolipin are found in a significantly greater proportion of patients with periodontitis than in periodontally healthy individuals, and these antibodies activate endothelial cells to produce monocyte chemotactic protein-1, they may explain some of the associations noted between periodontal infections and systemic conditions.

Thrombotic recurrences and bleeding events in APS vascular patients: a review from the literature and a comparison with the APS Piedmont Cohort

In APS vascular patients, thrombotic recurrences are more frequent than in non-APS thrombotic patients. To better define this clinical setting, a systematic review of the literature after 1999 was performed: 8 cohort studies (including the recent APS Piedmont Cohort) and 6 intervention studies were selected and evaluated. Thrombotic recurrences, bleeding events, therapeutic strategies, antiphospholipid (aPL) profile, inherited and acquired risk factors (when present) were calculated and compared. Emerging risk factors for thrombotic recurrences include withdrawal of oral anticoagulant therapy (OAT), high intensity OAT (INR range 3-4), aPL profile (triple positivity, Miyakis types 1 and 2a profiles) and association with inherited or acquired pro-thrombotic risk factors. Moreover, there are evidences that high risk (mainly for aPL profile) APS vascular patients have a high recurrence rate in spite of correct OAT treatment. Clinical trials in this clinical setting are needed.

Clinical manifestations and anti-phospholipid antibodies in 712 patients with systemic lupus erythematosus: evaluation of two diagnostic assays

OBJECTIVES

To evaluate the agreement and performance of two tests for aPLs with regard to association with manifestations of the APS in patients with SLE.

METHODS

We investigated 712 SLE patients and 280 population controls. Cardiolipin and β(2) glycoprotein-I antibodies were measured with routine ELISA and a new automated method. Three positivity cut-offs (99%, 90% of controls and recommended cut-off by manufacturers) were used. Associations with previous thrombotic events, thrombocytopenia and, in a subgroup of patients, obstetric morbidity (n = 296) were evaluated. Results were compared with the LA test, performed in 380 patients.

RESULTS

Inter-test agreement was moderate (demonstrated by κ-values 0.16-0.71). Performance of the two tests was similar: at the 99th percentile cut-off, sensitivity for any thrombotic event ranged from 3.7% to 24.8%, while specificity was 84.7-97.7%. Regardless of assay, IgG isotypes were associated with venous thrombosis and ischaemic cerebrovascular disease, whereas aPLs of IgM isotype were weakly associated with ischaemic heart disease. Associations were greatly affected by aPL level. LA performed better than the specific aPL tests. LA was associated with any thrombotic event, odds ratio 5.4 (95% CI 3.1, 9.4), while the specific aPL tests ranged from non-significant to an odds ratio of 1.9 (95% CI 1.03, 3.4) using criteria cut-off. LA was also convincingly associated with other APS manifestations.

CONCLUSION

In relation to thrombotic manifestations, there was moderate agreement but no clear advantages when comparing a routine aPL ELISA with an automated method. APL isotype and titre as well as LA positivity are important for risk assessment in SLE patients.

Coagulation and the fibrin network in rheumatic disease: a role beyond haemostasis

Activation of the immune system has been increasingly recognised to be associated with procoagulatory status in patients with inflammatory rheumatic disease. Changes in endothelial cell and platelet activation, blood flow, expression and activity of different coagulation factors, and impaired fibrinolysis serve as pathophysiological basis for enhanced risk of venous thromboembolism in inflammatory rheumatic diseases, such as rheumatoid arthritis (RA), connective tissue diseases and vasculitides. Recent studies identifying mechanisms for a functional role of coagulation factors beyond haemostasis have provided examples of interesting links between the coagulation system and innate immune activation. Furthermore, citrullinated fibrinogen is an important and early autoantigen in patients with RA carrying the HLA-DRβ1 shared epitope allele, which demonstrates an adaptive immune response to a coagulation factor in an inflammatory rheumatic disease. Additional studies have provided strong evidence that a multitude of different components of the haemostatic system (such as thrombin, fibrinogen, coagulation factor XIII and factors of the fibrinolytic system) are relevant mediators of inflammatory processes as well as of inflammatory control. Understanding the interactions between coagulation and the immune system in inflammatory rheumatic diseases will not only improve our knowledge of disease mechanisms, but could also permit the development of innovative therapeutic interventions.

Pathophysiology of thrombotic APS: where do we stand?

The antiphospholipid syndrome (APS) is diagnosed when patients with thrombotic complications or foetal losses have elevated levels of antiphospholipid antibodies in their plasmas. The term APS is confusing, because the pathogenic auto-antibodies are not directed against phospholipids but towards a plasma protein, β(2)-glycoprotein I. For many years the reason why auto-antibodies against β(2)-glycoprotein I were pro-thrombotic was unclear, because man and mice deficient in β(2)-glycoprotein I do not express a clear phenotype. Animal models in which passive transfer of patient antibodies into mice resulted in an increased thrombotic response have provided novel insights in the importance of this protein in the pathology of APS.

Circulating β2 glycoprotein I-IgG anti-β2 glycoprotein I immunocomplexes in patients with definite antiphospholipid syndrome

Beta2-glycoprotein I (β(2)GPI), a relevant antigen in Antiphospholipid Syndrome (APS), binds anionic macromolecules including heparin (Hep). A possible formation of ternary complexes between β(2)GPI, antibodies and Hep in APS is thus possible. The aim of this study was to evaluate Hep-β(2)GPI interaction in patients with APS. The affinity of Heps of different length, including unfractionated Hep (UFH), low-molecular weight Hep (enoxaparin) and pentasaccharide (fondaparinux), to human β(2)GPI was estimated by fluorescence spectroscopy, yielding dissociation constant (K(d)) values of 1.1, 24.0 and 89.4 µM, demonstrating that the longer UFH binds to β(2)GPI far more tightly than the shorter ones. Plasma and protein G-purified IgGs from eight patients with APS (i.e. five with thromboembolic disease and three with catastrophic APS), were fractionated by affinity chromatography using a Hep (UFH)-bound column, eluted with a linear NaCl gradient. For each chromatographic analysis, fractions were collected in the whole NaCl gradient and tested by ELISA for the presence of β(2)GPI and anti-β(2)GPI IgG. The results of Hep-affinity chromatography and ELISAs concurrently indicate that either β(2)GPI and anti-β(2)GPI IgG elute from the Hep column in the same chromatographic peak, at a retention time identical to that of the purified, isolated β(2)GPI, thus suggesting that circulating immunocomplexes containing β(2)GPI are present in patients with APS.

Anti-cardiolipin and anti-β2-glycoprotein I antibodies: normal reference ranges in northwestern Italy

Laboratory tests for anticardiolipin antibodies (aCL) and anti-β2glycoprotein I antibodies (a-β2GPI) face problems common to many autoantibody assays: the lack of a reference standard and the need for each laboratory to assess assay-specific cut-off values. The aims of the study were to evaluate the reference range upper limits (99th percentile) used for aCL and a-β2GPI in the northwest of Italy and to investigate the analytical performances of these assays with the newly obtained reference ranges. We assayed aCL and a-β2GPI in 104 serum samples from patients without a history of thrombosis, pregnancy morbidity, tumours, infections and/or autoimmune diseases (30 males and 74 non-pregnant females). We tested all the commercial assays available in our regions (i.e. Orgentec Diagnostika, Aesku Diagnostics and Inova Diagnostics ELISA; CliA Zenit-RA and EliA Phadia Laboratory Systems). A further 30 serum samples, including 10 from healthy subjects, 10 from antiphospholipid syndrome (APS) patients and 10 from septic patients were assessed to investigate the analytical performance of the obtained cut-off limits. Reference range upper limits obtained with the commercial kits differ among assays and from the values reported by the manufacturer. Moreover, normal reference ranges calculated for IgG and IgM aCL differed from the arbitrary selected laboratory classification values suggested in the guidelines of 40 GPL and MPL.

Recurrent venous thromboembolism while on anticoagulant therapy

There has been immense progress in the management of venous thromboembolism in recent years with increased awareness and adequate thromboprophylaxis proving successful in reducing the morbidity and mortality associated with this condition. One of the commonest complications of an initial venous thrombosis is the development of recurrent thrombosis. Unlike in the case of the first clot, the diagnosis and management of the recurrent episode remain a difficult issue. Even more challenging is the clinical situation where a new thrombus develops while the patient is being treated with anticoagulant medication for a previous clot. The clinical approach and management of these patients are complex, and require understanding of the differences in thrombus development in the different clinical circumstances.

Recent developments in our understanding of the antiphospholipid syndrome

The antiphospholipid syndrome is an autoimmune disease that manifests clinically as recurrent thrombotic complications or foetal losses and serologically with elevated levels of antiphospholipid antibodies in the plasmas of these patients. The term 'antiphospholipid syndrome' is confusing, because the auto-antibodies are not directed against phospholipids but towards a plasma protein, β(2) -glycoprotein I. For many years, the reason why auto-antibodies against β(2) -glycoprotein I were pro-thrombotic was unclear, because β(2) -glycoprotein I seems to be an obsolete protein in our circulation. Human and mice deficient in this protein do not express a clear phenotype. Recent studies on the structure and function of β(2) -glycoprotein I have provided novel insights into the importance of this protein in physiology and its role in the pathology of the antiphospholipid syndrome.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

The complexity of cardiolipin in health and disease

Cardiolipin, the signature phospholipid of mitochondria, is a lipid dimer that is important for a diverse range of mitochondrial activities beyond the process of ATP production. Thus not surprisingly, derangements in cardiolipin metabolism are now appreciated to contribute to an assortment of pathological conditions. A comprehensive inventory of enzymes involved in cardiolipin biosynthesis and remodeling was just recently obtained. Post-biosynthesis, the acyl chain composition of cardiolipin is modified by up to three distinct remodeling enzymes that produce either a homogeneous tissue-specific mature form of cardiolipin or alternatively 'bad' cardiolipin that has been linked to mitochondrial dysfunction. In this review, we initially focus on the newly identified players in cardiolipin metabolism and then shift our attention to how changes in cardiolipin metabolism contribute to human disease.

β(2) -Glycoprotein I: evolution, structure and function

β(2) -Glycoprotein I (β(2) -GPI) is a protein that circulates in blood at high concentrations. The function of β(2) -GPI has long been an enigma. More than 20 years ago, it was discovered that β(2) -GPI is the major antigen for the circulating antibodies in the antiphospholipid syndrome. However, this knowledge has not advanced our understanding of the physiologic role of the protein. In recent years, new insights have suggested an important function of this protein in innate immunity. β(2) -GPI was found to scavenge lipopolysaccharide and was able to clear unwanted anionic cellular remnants such as microparticles from the circulation. The function of β(2) -GPI seems to depend on the structural conformation of the protein, and it has been established that β(2) -GPI can exist in at least two conformations. In this review, we will highlight and summarize the current knowledge on this protein.