Exposure of anionic phospholipids upon platelet activation permits binding of beta 2 glycoprotein I and through it that of IgG antiphospholipid antibodies. Studies in platelets from patients with antiphospholipid syndrome and normal subjects

Βeta-2-glycoprotein I exerts antithrombotic function through its domain V in mice

Little is known about the physiological role of beta-2-glycoprotein I (β2GPI) despite it being the major auto-antigen in the antiphospholipid syndrome. A systematic study of the role of β2GPI in thrombus formation in vivo has not been performed to date. Herein, we report that β2GPI deficient (-/-) mice have enhanced thrombus formation compared to wild type (WT) mice in a laser-induced arteriole and venule model of thrombosis. Furthermore, neutrophil accumulation and elastase activity was enhanced in thrombi of β2GPI -/- compared with WT mice. The antithrombotic function of β2GPI is dependent on its fifth domain (domain V); intravenous administration of the β2GPI domain deletion mutant lacking domain V (human recombinant domain I-IV) had no effect on platelet and fibrin thrombus size in β2GPI -/- or WT mice. On the contrary, intravenous administration of human recombinant domain V significantly inhibited platelet and fibrin thrombus size in both β2GPI -/- mice and WT mice. These findings reveal a major role for β2GPI as a natural anticoagulant and implicate domain V of β2GPI as a potential antithrombotic therapy.

Cardiolipin interacts with beta-2-glycoprotein I and forms an open conformation-Mechanisms analyzed using hydrogen/deuterium exchange

Beta‐2‐glycoprotein I (β₂GPI) is the major antigen for the antiphospholipid antibodies in the antiphospholipid syndrome. The exposed epitope in domain I of β₂GPI can be recognized by the anti‐β₂GPI antibody. Here, we prepared the anionic di‐oleoyl‐phosphatidylserine (DOPS) and cardiolipin (CL) liposomes to interact with the β₂GPI. The conformational changes of β₂GPI upon binding with the liposomes were analyzed using hydrogen/deuterium exchange mass spectrometry. The exchange level of sequences 21–27 significantly increased after β₂GPI had interacted with DOPS. This change indicated a reduced interaction between domain I and domain V, inferring to a protrusion of the sequences 21–27 from the ring conformation. After β₂GPI had interacted with CL for 30 min, the exchange levels in 4 of the 5 domains increased significantly. The deuteration levels of sequences 1–20, 21–27, 196–205, 273–279 and 297–306 increased, suggesting that these regions had become more exposed, and the domain I was no longer in contact with domain V. The increasing deuteration levels in sequences 70–86, 153–162, 191–198, 196–205 and 273–279 indicated β₂GPI undergoing conformational changes to expose these inner regions, suggesting a structural transition. Overall, DOPS and CL induced minor conformational changes of β₂GPI at sequences 21–27 and forms an intermediate conformation after 10 min of interaction. After a complete protein–lipid interaction, high negatively charged CL membrane induced a major conformation transition of β₂GPI.

The interaction of β2-glycoprotein I with lysophosphatidic acid in platelet aggregation and blood clotting

β₂-Glycoprotein I (β₂-GPI) is a plasma protein that binds to oxidized low-density lipoprotein (LDL) and negatively charged substances, and inhibits platelet activation and blood coagulation. In this study, we investigated the interaction of β₂-GPI with a negatively charged lysophosphatidic acid (LPA) in platelet aggregation and blood clotting. Two negatively charged lysophospholipids, LPA and lysophosphatidylserine, specifically inhibited the binding of β₂-GPI to oxidized LDL in a concentration-dependent manner. Intrinsic tryptophan fluorescence studies demonstrated that emission intensity of β₂-GPI decreases in an LPA-concentration-dependent manner without a shift in wavelength maxima. LPA specifically induced the aggregation of β₂-GPI in phosphate-buffered saline, and in incubated plasma and serum, both of which are known to accumulate LPA by the action of lecithin-cholesterol acyltransferase and lysophospholipase D/autotaxin. β₂-GPI aggregated by LPA did not inhibit activated von Willebrand factor-induced aggregation, and did not prolong the activated partial thromboplastin time in blood plasma, in contrast to non-aggregated β₂-GPI. These results suggest that β₂-GPI aggregated by the binding to LPA fails to inhibit platelet aggregation and blood clotting in contrast to non-aggregated β₂-GPI.

Thrombocytopenia as a thrombotic risk factor in patients with antiphospholipid antibodies without disease criteria

INTRODUCTION

The antiphospholipid syndrome (APS) is an acquired immune disorder defined by the presence of thrombosis (arterial and/or venous) and/or pregnancy morbidity along with the presence of positive antiphospholipid antibodies (aPL). There is a clear relationship between aPL and some events not included in the clinical criteria, including haematologic.

OBJECTIVES

a) to study the probability of developing clinical APS in patients with positive aPL and thrombopenia; b) to identify potential risk factors for thrombosis, and c) to study the association between thrombocytopenia and aPL.

METHODS

A retrospective study of 138 patients with positive aPL without fulfilling clinical criteria for APS. Thrombocytopenia was defined as a platelet count≤100,000/μl. Patients with other causes of thrombocytopenia were excluded.

RESULTS

Seventeen of the 138 (12%) patients in the study had thrombocytopenia. The mean platelet count was 60,000/μl. The risk of developing thrombocytopenia was higher in smokers (OR 2.8; P=.044), in those with lupus anticoagulant (OR 13.5; P<.001) and those with higher burden of aPL (OR 50.8; P<.001). After a mean follow-up of 146±60.3 months, 5 patients with thrombocytopenia (29.4%) developed thrombosis.

CONCLUSIONS

In our series, the incidence of thrombocytopenia is 12%. aPL-positive patients who develop thrombocytopenia have a potential risk of developing thrombosis. Tobacco could be a risk factor for thrombocytopenia. Autoantibodies load is a risk factor for the development of thrombocytopenia.

Task Force on Catastrophic Antiphospholipid Syndrome (APS) and Non-criteria APS Manifestations (II): thrombocytopenia and skin manifestations

The objectives of the ‘Task Force on Catastrophic Antiphospholipid Syndrome (APS) and Non-criteria APS Manifestations’ were to assess the clinical utility of the international consensus statement on classification criteria and treatment guidelines for the catastrophic APS, to identify and grade the studies that analyze the relationship between the antiphospholipid antibodies and the non-criteria APS manifestations, and to present the current evidence regarding the accuracy of these non-criteria APS manifestations for the detection of patients with APS. This article summarizes the studies analyzed on thrombocytopenia and skin manifestations, and presents the recommendations elaborated by the Task Force after this analysis.

Antiphospholipid-associated thrombocytopenia or autoimmune hemolytic anemia in patients with or without definite primary antiphospholipid syndrome according to the Sapporo revised classification criteria: a 6-year follow-up study

The updated Sapporo classification criteria for antiphospholipid syndrome (APS) only include thrombosis or pregnancy morbidity as clinical criteria. To test this notion, we studied 55 patients (80% women) with hematologic manifestations. All fulfilled the laboratory criteria for primary APS. Thirty-five patients (64%) had thrombocytopenia, 14 (25%) had autoimmune hemolytic anemia, and 6 (11%) had both. Twenty-five patients (22 women, 88%) also fulfilled one clinical criterion for APS after a median follow-up of 13.2 years (range, 1.45-37 years), whereas the remaining 30 patients (22 women, 73%) have not had any thrombotic event nor pregnancy morbidity after a median follow-up of 5.4 years (range, 0.12-24 years). No patient developed systemic lupus erythematosus during follow-up. The hematologic manifestation was asynchronous with the APS onset in 84% of patients. The response to treatment was similar regardless of the APS status. Patients with definite APS were more frequently positive for the lupus anticoagulant (63%) than lupus anticoagulant-positive patients without APS (30%; odds ratio, 3.5; 95% confidence interval, 1.07-11.4; P < .02). Anticardiolipin or anti–β2-glycoprotein-I antibodies were highly prevalent among the study groups. Our study suggests that, depending upon their antiphospholipid profile, patients with hemocytopenias appear to comprise a peculiar subset of patients with APS; some develop thrombotic and/or obstetric APS whereas others continue with hematologic APS.

Thrombophilic patterns of coagulation factors in lupus

Our aim was to better define the coagulation abnormalities in patients with systemic lupus erythematosus who had thrombosis or high-risk clinical settings for thrombosis. Clinical and laboratory data of 111 patients with lupus referred for coagulation assessment because of thrombosis, pregnancy loss or high-risk clinical settings for thrombosis were reviewed retrospectively. Increased activity of procoagulant factors and decreased activity of anti-coagulant factors were observed well above the expected 5% prevalence. All comparisons were significant at the P < 0.001 level. Anticardiolipin antibodies were present in 70.5% of patients tested (55/78) in this high-risk group, but usually in low titres. Platelet hyperfunction was detected in the majority of patients tested (85.7%, 78/91). Hypercoagulability in lupus is complex and is better defined by assessing multiple haemostatic factors in addition to platelet function. Platelet hyperfunction contributes significantly to thrombophilia in lupus and this is the key finding of our study.

Liposome Opsonization

Adsorption of serum proteins to the liposomal surface plays a critical role in the clearance of liposomes from the blood circulation. In this review, we will discuss the role of the liposomal opsonins proposed so far in liposome clearance. Additional, related topics that will be addressed are the cell-surface receptors that might be involved in liposome elimination from the blood compartment and the effect of poly(ethylene glycol) (PEG) modification on prevention of liposome opsonization.

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.

Prevalence of antiphospholipid antibodies in Chilean patients with rheumatoid arthritis

Antiphospholipid (aPL) antibodies found in patients with autoimmune diseases are also detected in those with inflammatory diseases. The purpose of this study was to examine the prevalence of these antibodies in patients with rheumatoid arthritis (RA), and to evaluate the association of these antibodies with thrombosis and/or other clinical characteristics of this inflammatory disorder. Eighty-four patients with RA and 82 normal controls were studied. Anticardiolipin (aCL), anti-beta(2) glycoprotein I (anti-beta(2)GPI), and antiprothrombin (aPT) antibodies and the lupus anticoagulant (LA) activity were determined. Seven out of 84 (8.3%) patients were positive for aCL, six out of 84 (7.2%) for anti-beta(2)GPI, and six out of 84 (7.2%) for aPT, while in controls the overall prevalence of aPL antibodies was 3.6% (3 out of 82). All patients and controls were LA negative. There was no correlation between the presence of aPL with thrombosis and/or other clinical features of the antiphospholipid syndrome. We found aPL antibodies in 19.1% (16 out of 84) of the patients with rheumatoid arthritis and this prevalence was statistically higher than in normal controls (P<0.003). In this study, the presence of aPL antibodies was not associated with the development of thrombosis and/or thrombocytopenia. Whether the presence of aPL antibodies implies an increased risk for thrombosis and atherosclerosis in these patients should be studied further.

Circulating levels of beta2-glycoprotein I in thrombotic disorders and in inflammation

Beta2-glycoprotein I (beta2GPI) is a plasma protein suspected to have a role in inhibition of thrombosis. This suspicion is reinforced by the observation that beta2GPI is the major target for autoantibodies in the antiphospholipid syndrome. However, little is known about its circulating levels in common thrombotic diseases or inflammation. We measured beta2GPI levels in 344 healthy controls, 58 normal pregnancies, 102 patients with non-haemorrhagic stroke, 121 patients with acute coronary syndrome and 200 patients with elevated C-reactive protein (CRP). In healthy individuals, we found a strong positive correlation between age and beta2GPI concentration (r = 0.274, P < 0.001) and that beta2GPI levels fall significantly after the eighth week of pregnancy (P = 0.002). We also found significantly reduced levels of beta2GPI in patients with stroke and in elderly patients with myocardial syndrome (P = 0.013 and 0.043). However, in neither group did beta2GPI levels change in the following six months, suggesting that the reduced levels were not a transient post-event phenomenon. In patients with inflammation, beta2GPI levels showed a significant negative correlation with CRP (r = -0.284, P < 0.001) and positively correlated with albumin and transferrin (r = 0.372 and 0.453, respectively with P < 0.001 for both). Furthermore, the largest reduction in beta2GPI levels occurred in patients with the highest CRP values (P < 0.001).

Antiphospholipid antibodies in young adults with stroke

BACKGROUND

Antiphospholipid antibodies have been associated with a clinical syndrome consisting thrombosis and recurrent, unexplained fetal loss.

METHODS

The literature pertaining to stroke associated with antiphospholipid antibodies, with emphasis on stroke in young adults, was reviewed.

RESULTS

Antiphospholipid antibodies are an independent risk factor for stroke in young adults in five of six studies. Multiple antiphospholipid specificities or the Lupus Anticoagulant were tested in addition to anticardiolipin antibody in these studies. In the single study that found no increased risk for stroke, only anticardiolipin antibody was tested. Only one of these studies evaluated for risk of recurrent stroke in young adults with antiphospholipid antibodies and found it to be increased. No treatment trials have been conducted in young adults with antiphospholipid antibodies and stroke. In the single treatment trial comparing aspirin and low-INR producing doses of warfarin to prevent recurrent stroke, both were found to be equally effective.

CONCLUSIONS

Antiphospholipid antibodies, particularly Lupus Anticoagulant, is an independent risk factor for first and possibly recurrent ischemic stroke in young adults. The best therapeutic strategy for preventing antiphospholipid antibody-associated recurrent stroke is not clear.

Prevalence of antibodies against oxidised LDL in a cohort of 163 patients with positive anti-phospholipid antibodies and recent thrombosis

We evaluated the prevalence of anti-beta2-glycoprotein I (a-beta2-GPI), anti-phosphatidylserine/prothrombin (a-PP), anti-prothrombin (a-PT), and anti-oxidised low-density lipoprotein (a-oxLDL) antibodies in a cohort of patients with a recent thrombotic event. Among consecutive sera sent to an autoimmune laboratory for routine testing for anti-cardiolipin antibodies (aCL) 473 were found positive for IgG and/or IgM aCL. The referring physicians were requested for clinical information about thrombo-embolic events occurring within 6 months prior to testing. One hundred and sixty-three individuals of which 82 had suffered from cerebro-vascular infarction and 49 from deep venous thromboses or pulmonary emboli were included in the study. Ninety-four sera were positive for IgG aCL and 69 were negative. There was a strong correlation between the presence of IgG aCL and the three other phospholipid-related antibodies: a-beta2-GPI, a-PP, and a-PT. However, IgG a-oxLDL antibodies were almost equally distributed among sera positive and negative for IgG aCL. The presence of antibodies of all subgroups was most pronounced among patients with venous thrombo-embolic disease. In this cohort antibodies to beta2-GPI, PP, PT seem to coexist with aCL. However, a-oxLDL antibodies appear to define a subset of patients, who were older, had more arterial vascular disease, and with no apparent association to the presence of IgG aCL.

IgG and IgM isotypes of anti-cardiolipin and anti-beta2-glycoprotein i antibodies reflect different forms of recent thrombo-embolic events

We correlated the distribution and levels of serum anti-cardiolipin (aCL) and anti-beta(2)-glycoprotein-1 antibodies (anti-beta(2)-GPI) of the IgG and IgM isotypes to the clinical spectrum of recent (<6 months) thrombo-embolic events in a cohort of 162 patients. Clinical information was obtained by questionnaires from the referring physicians. Cerebro-vascular infarction (CVI) had taken place in 82 patients, deep venous thrombosis (DVT) in 34, pulmonary embolism (PE) in 14, myocardial infarction (MI) in four, and other thromboses in 28 patients. SLE was the most commonly associated rheumatic disease and accounted for 20 (12%) patients. In 124 (77%) patients no underlying rheumatic disease was identified. Isolated IgG aCL was found in 31 of 48 patients with DVT/PE (65%), but in only 21 of 82 patients with CVI (26%); p<0.0001. IgG anti-beta(2)-GPI were detected in 23 (48%) DVT/PE patients, but in only 13 (16%) CVI patients; p<0.001. The IgG class anti-beta(2)-GPI positive patients had significantly higher levels of IgG aCL (mean 65 units) compared to IgG anti-beta(2)-GPI negative patients (mean 29 units); p<0.0001. In contrast, isolated IgM aCL was found in nine (19%) patients with DVT/PE, but in 46 (56%) CVI patients; p<0.0001. Only ten patients had IgM anti-beta(2)-GPI. The present study shows that the IgG and IgM aCL isotypes seem to define different clinical subsets of patients with thrombo-embolic events with IgG aCL being most prevalent in the group having DVT/PE, IgM aCL being found primarily among CVI patients.

Increased platelet deposition on extracellular matrix under flow conditions in patients with antiphospholipid syndrome who experience thrombotic events

OBJECTIVE

To assess platelet function under defined flow conditions in patients with antiphospholipid syndrome (APS) and to correlate the results with thrombotic complications and the presence of subsets of antiphospholipid antibodies (aPL), lupus anticoagulant (LAC), and/or anticardiolipin antibodies (aCL).

METHODS

We studied 88 randomized APS patients with or without a history of thrombosis. Seventeen patients with other thrombosis (no APS) and 26 healthy subjects served as controls. Platelet adhesion and aggregation on the extracellular matrix were measured with a cone-and-plate(let) analyzer (CPA) by examining the percentage of total area covered with platelets (surface coverage [SC]) and the mean size of surface-bound objects (average size [AS]) and were compared with platelet responses to different ADP concentrations by conventional aggregometry.

RESULTS

Under defined flow conditions, SC and AS were significantly higher for venous thrombosis and arterial thrombosis in APS patients compared with no thrombosis, other thrombosis, and healthy control groups. The increased platelet adhesion and aggregation in APS patients with thrombotic events was associated with higher levels of von Willebrand factor (vWF) antigen (mean +/- SD 230.6 +/- 51.2%) and ristocetin cofactor activity (181.0 +/- 36.0%). No change in CPA and vWF parameters was found in APS patients with positive results for aPL who did not undergo thrombotic events or in patients with other thrombosis. The CPA parameters were neither associated with the high response of platelets to ADP nor associated with the presence of LAC, aCL, or both. The CPA parameters were similarly increased irrespective of aspirin use. The results suggest that the increased adhesion properties of platelets in APS patients could be mediated by high levels and activity of vWF. This complements the known ability of APS antibodies to enhance platelet response to agonists in conventional aggregometry.

CONCLUSION

The CPA test was found to be valuable in differentiating APS patients with and without thrombotic complications.

?2-glycoprotein I protects J774A.1 macrophages and human coronary artery smooth muscle cells against apoptosis

beta(2)-Glycoprotein I (beta(2)-GPI) is a plasma glycoprotein with multifactorial relevance to clinical consequences. It was previously indicated that beta(2)-GPI can selectively bind to apoptotic cells. This study was designed to determine the role of beta(2)-GPI in apoptosis. Using an immunohistochemical study, we observed that beta(2)-GPI was co-localized with the apoptotic macrophages and smooth muscle cells (SMCs) of human coronary arteries. The contribution of beta(2)-GPI to apoptotic death was then investigated in vascular cells. Two nitric oxide (NO) donors, S-nitrosoglutathione (GSNO) and S-nitroso-N-acetyl penicillamine (SNAP) were used in this study to trigger apoptosis in J774A.1 macrophages and human coronary artery smooth muscle cells (HCASMC). Cell viability was significantly improved in beta(2)-GPI-treated cells. It was also possible to detect a remarkable inhibitory effect by beta(2)-GPI on the NO-induced apoptosis by preventing nuclear shrinkage. Furthermore, the NO-induced apoptosis was associated with increase in caspase-3 activity and in the protein levels of caspase-3, c-Fos, and c-Jun. However, all these apoptosis-related events were inhibited in vascular cells treated with 200 microg/ml beta(2)-GPI. This is the first study to show that beta(2)-GPI may be important in the prevention of apoptosis in vascular cells.

Current debates in antiphospholipid syndrome: the acquired antibody-mediated thrombophilia

Antiphospholipid (APL) syndrome is the most common form of acquired thrombophilia. It can cause significant morbidity and even mortality. The term "APL antibodies" represents a heterogeneous group of antibodies associated with this disorder. Currently no single assay can identify every APL antibody. Clinically relevant APL antibodies are mainly anticardiolipin antibodies (ACA) detected by solid phase enzyme-linked immunosorbent assay (ELISA) and lupus anticoagulants (LA) demonstrated by in vitro coagulation assay. However, there are some other antibodies associated with the APL syndrome (i.e., subgroup APL antibodies). ACAs, LAs, and subgroup APL antibodies represent intersecting, but non-identical, subsets of autoantibodies. Thus, those autoantibodies may coexist or may occur independently. Any organ system and any size of vessel can be affected during the clinical course of the disease. Therefore, the APL syndrome can manifest itself in a wide variety of clinical thrombotic features. Fetal loss and pregnancy morbidity represent a specific challenge. Despite tremendous advances in the understanding of the pathogenesis of APL syndrome during the past decade, the mainstay of management is still anticoagulation. However, there is no general agreement regarding the duration and intensity of anti-coagulant therapy. In this review, we focused on the current dilemmas and their present clarifications in the wide clinicopathologic spectrum of APL syndrome and APL antibody-related distinct pathologic conditions.

Prevalence of antiphospholipid and antiplatelet antibodies in human immunodeficiency virus (HIV)-infected Chilean patients

Antiphospholipid (aPL) and antiplatelet (aPlt) antibodies, found in patients with autoimmune diseases, are also detected in infectious diseases. The purpose of this study was to examine the prevalence of these antibodies in HIV patients and to evaluate an association of these antibodies with thrombocytopenia and/or thrombosis. Sixty-three HIV-seropositive patients and 52 normal controls were studied. Anti-cardiolipin (aCL), anti-beta(2) glycoprotein I (anti-beta(2)GPI), and antiprothrombin (aPT) antibodies were determined and the lupus anticoagulant (LA) test was performed. Antiplatelet antibodies (aPlt) were also determined. Seven out of 63 (12.7%) HIV patients were positive for aCL, four of 63 (6.3%) for anti-beta(2)GPI, and five of 63 (7.9%) for aPT. No patients studied were LA positive. Six out of 63 (9.5%) patients were positive for aPlt. One of them showed weak reactivity for GPIb-IX. The platelet count of patients (202+/-63 x 10(3) platelets/microL) was significantly lower than in the controls (343+/-6 x 10(3) platelets/microL) (P<0.001). There was no correlation between the presence of aPL and/or aPlt and thrombocytopenia. Of the HIV-infected patients, 22.2% presented aPL and 9.4% aPlt antibodies. In this study, the presence of aPL and aPlt antibodies was not associated with the development of thrombosis and/or thrombocytopenia.

Pathogenic role of anti-beta 2-glycoprotein I antibodies in antiphospholipid associated fetal loss: characterisation of beta 2-glycoprotein I binding to trophoblast cells and functional effects of anti-beta 2-glycoprotein I antibodies in vitro

BACKGROUND

Antiphospholipid antibodies reacting with beta2-glycoprotein I (beta 2GPI) have been associated with recurrent fetal loss and pregnancy complications.

OBJECTIVE

To investigate whether specific mutations in the phospholipid binding site of beta 2GPI might affect its binding to trophoblast and in turn the anti-beta 2GPI antibody induced functional effects.

METHODS

beta 2GPI adhesion to trophoblast was evaluated as human monoclonal IgM or polyclonal IgG anti-beta 2GPI antibody binding to trophoblast monolayers cultured (1) in complete medium; (2) in serum-free medium; (3) after serum starvation in the presence of purified human beta 2GPI; or (4) in the presence of beta 2GPI with single or multiple mutations in the amino acid loop Cys(281)-Lys-Asn-Lys-Glu-Lys-Lys-Cys(288). The effect of anti-beta 2GPI binding to trophoblast was evaluated as chorionic gonadotropin (hCG) mRNA expression, and protein release by RT-PCR and radioimmunoassay, respectively.

RESULTS

beta 2GPI adhesion to trophoblast and its consequent recognition by the specific antibodies were inversely proportional to the mutation number in the phospholipid binding site. Anti-beta 2GPI antibodies reduced gonadotropin release, hormone dependent hCG mRNA expression, and protein synthesis in the presence of beta 2GPI, while the addition of the mutants or the absence of beta 2GPI had no effect.

CONCLUSIONS

beta 2GPI binds to trophoblast in vitro through its fifth domain, as reported for endothelial cells, and can be recognised by anti-beta 2GPI antibodies; the antibody binding downregulates trophoblast hCG synthesis and secretion. Such a mechanism might contribute to defective placentation in women with fetal loss associated with the antiphospholipid syndrome.

Circulating oxidized LDL forms complexes with beta2-glycoprotein I: implication as an atherogenic autoantigen

Beta2-glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies (Abs, aPL) present in patients with antiphospholipid syndrome (APS). We recently reported (J. Lipid Res., 42: 697, 2001; J. Lipid Res., 43: 1486, 2002) that beta2-GPI specifically binds to Cu2+-oxidized LDL (oxLDL) and that the beta2-GPI ligands are omega-carboxylated 7-ketocholesteryl esters. In the present study, we demonstrate that oxLDL forms stable and nondissociable complexes with beta2-GPI in serum, and that high serum levels of the complexes are associated with arterial thrombosis in APS. A conjugated ketone function at the 7-position of cholesterol as well as the omega-carboxyl function of the beta2-GPI ligands was necessary for beta2-GPI binding. The ligand-mediated noncovalent interaction of beta2-GPI and oxLDL undergoes a temperature- and time-dependent conversion to much more stable but readily dissociable complexes in vitro at neutral pH. In contrast, stable and nondissociable beta2-GPI-oxLDL complexes were frequently detected in sera from patients with APS and/or systemic lupus erythematodes. Both the presence of beta2-GPI-oxLDL complexes and IgG Abs recognizing these complexes were strongly associated with arterial thrombosis. Further, these same Abs correlated with IgG immune complexes containing beta2-GPI or LDL. Thus, the beta2-GPI-oxLDL complexes acting as an autoantigen are closely associated with autoimmune-mediated atherogenesis.

Antiphospholipid antibodies and stroke in young women

BACKGROUND AND PURPOSE

Antiphospholipid antibodies have been associated with ischemic stroke in some but not all studies.

METHODS

We performed a population-based case-control study examining antiphospholipid antibodies (anticardiolipin antibodies and lupus anticoagulants) using stored frozen sera and plasma in 160 cases and 340 controls enrolled in the Stroke Prevention in Young Women study. We evaluated for the presence of anticardiolipin antibody (IgG, IgM, and IgA isotypes) by an enzyme-linked immunosorbent assay and for the lupus anticoagulant using several phospholipid-dependent coagulation tests (activated partial thromboplastin time, dilute Russell's viper venom time) with mixing studies. If mixing studies were prolonged, confirmatory tests were performed.

RESULTS

A positive anticardiolipin antibody level of any isotype was seen in 43 cases (26.9%) and 62 controls (18.2%) (P=0.03), lupus anticoagulant in 29 cases (20.9%) and 38 controls (12.8%) (P=0.03), and either anticardiolipin antibody or lupus anticoagulant in 61 cases (42.1%) and 86 controls (27.9%) (P=0.003). After adjustment for age, current cigarette smoking, hypertension, diabetes, angina, ethnicity, body mass index, and high-density lipoprotein levels, the relative odds of stroke for women with anticardiolipin antibody immunoreactivity of any isotype or a lupus anticoagulant was 1.87 (95% confidence interval, 1.24 to 2.83; P=0.0027).

CONCLUSIONS

The results from this study support the importance of antiphospholipid antibodies as an independent risk factor for stroke in young women.

Omega-carboxyl variants of 7-ketocholesteryl esters are ligands for beta(2)-glycoprotein I and mediate antibody-dependent uptake of oxidized LDL by macrophages

beta(2)-Glycoprotein I (beta(2)-GPI) is a major antigen for anticardiolipin antibodies (aCL, Abs) present in patients with antiphospholipid syndrome. We recently reported that beta(2)-GPI specifically binds to oxidized LDL (oxLDL) and that the beta(2)-GPI's major ligand, oxLig-1 is 7-ketocholesteryl-9-carboxynonanoate (Kobayashi, K., E. Matsuura, Q. P. Liu, J. Furukawa, K. Kaihara, J. Inagaki, T. Atsumi, N. Sakairi, T. Yasuda, D. R. Voelker, and T. Koike. 2001. A specific ligand for beta(2)-glycoprotein I mediates autoantibody-dependent uptake of oxidized low density lipoprotein by macrophages. J. Lipid Res. 42: 697-709). In the present study, we demonstrate that omega-carboxylated 7-ketocholesteryl esters are critical for beta(2)-GPI binding. A positive ion mass spectrum of a novel ligand, designated oxLig-2, showed fragmented ions at m/z 383 and 441 in the presence of acetone, which share features of oxLig-1 and 7-ketocholesterol. In the negative ion mode, ions at m/z 627, 625, and 243 were observed. oxLig-2 was most likely 7-ketocholesteryl-12-carboxy (keto) dodecanoate. These ligands were recognized by beta(2)-GPI. Liposome binding to macrophages was significantly increased depending on the ligand's concentration, in the presence of beta(2)-GPI and an anti-beta(2)-GPI Ab. Synthesized variant, 7-ketocholesteryl-13-carboxytridecanoate (13-COOH-7KC), also showed a significant interaction with beta(2)-GPI and a similar binding profile with macrophages. Methylation of the carboxyl function diminished all of the specific ligand interactions with beta(2)-GPI. Thus, omega-carboxyl variants of 7-ketocholesteryl esters can mediate anti-beta(2)-GPI Ab-dependent uptake of oxLDL by macrophages, and autoimmune atherogenesis linked to beta(2)-GPI interaction with oxLDL.

What causes the antiphospholipid syndrome?

The antiphospholipid syndrome (APS) is characterized by unpredictable, sporadic, thrombotic events. The cause of the thrombosis is probably multifactorial and may involve disparate effects of the autoantibodies associated with the syndrome, which are known to interfere with various protein regulators of hemostasis. An integrated theory of pathogenesis that accounts for the diversity of autoantibodies and their effects suggests that cellular inflammation or apoptosis within the vasculature may lead to oxidation or turnover in phospholipid membranes. Thus, normally cryptic, functionally important epitopes of phospholipid-binding proteins are subjected to increased exposure to immune surveillance.

Immunopathogenesis of the antiphospholipid antibody syndrome: an update

The primary antiphospholipid antibody syndrome is characterized clinically by the presence of venous and arterial thrombosis, recurrent fetal loss, and thrombocytopenia. The presence of antiphospholipid antibodies is a central serologic finding in primary antiphospholipid antibody syndrome, and plays a critical role in diagnosis. Contrary to initial reports, it is now widely accepted that these autoantibodies are directed predominantly against two antigens: phospholipid-binding plasma protein beta2-glycoprotein I and prothrombin. The mechanism by which antiphospholipid antibodies cause disease is under vigorous investigation. It is hypothesized that antiphospholipid antibodies induce a procoagulant state by binding to antigens on endothelial cells and trophoblast cell surfaces. Indeed, beta2-glycoprotein I appears to function as a cofactor that facilitates this interaction. The resulting endothelial cell activation is associated with cell-surface expression of adhesion molecules that lead to monocyte adhesion - the first steps in thrombosis. Although the precise mechanism that mediates endothelial cell-platelet interaction have not been fully elucidated, platelet binding to the endothelium appears to be the next phase in thrombosis. Thus, the antiphospholipid antibody may be a triggering or activating factor in placental spiral artery thrombosis and subsequent placental infarction. More recently, a role for annexin V has emerged. Studies suggest that thrombosis in the antiphospholipid syndrome may be due to disruption of the annexin shield by antiphospholipid (and cofactor) antibodies, which results in the increased exposure of trophoblasts and endothelial cells to thrombogenic phospholipids.

Antiphospholipid syndrome and recurrent miscarriages

Sixty percent of recurrent spontaneous abortions are unexplained. Antiphospholipid syndrome is a multisystem disease with the predominant features of venous and arterial thrombosis, recurrent pregnancy loss, foetal death and the presence of antiphospholipid antibodies. Many epidemiological studies focus on antiphospholipid autoantibodies syndrome (APS) as a cause of recurrent spontaneous abortion (RSA). It is found that 7-25% of RSA would have APS as the main risk factor. 'Association not being synonymous with cause', the proportion of abortions due to the APS is difficult to estimate for several reasons: definition of recurrent abortion is variable, the assays for antiphospholipid antibodies are not well standardised, inclusion of patients in the study group according to the antibodies titre is author dependent. Recent studies suggest association of antiphospholipid antibodies syndrome not only with recurrent abortions but also with infertility. New mechanisms are described by which antiphospholipid antibodies could cause placental thrombosis and infarction, acting directly on the surface anticoagulant expressed on trophoblastic cells. Only lupus anticoagulant (LA) and anticardiolipin antibodies (aCL) assays are sufficiently standardised to be usable in routine. Testing for other antiphospholipid antibodies (aPLs) should remain investigational. Several treatments have been proposed: low doses of aspirin, low or immunosuppressive doses of corticosteroids, and preventive or effective dose of heparin, intravenous immunoglobulin.

Anti-beta2 glycoprotein I antibodies prevent the De-activation of platelets and sustain their phagocytic clearance

Exposure to phosphatidylserine (PS) tags dying and senescent cells for removal and identifies activated platelets. In this study we followed the fate of PS-exposing platelets in the presence of antibodies purified from Systemic Lupus Erythematosus (SLE) and primary Anti-phospholipid Syndrome (APS) patients' sera by beta2GPI affinity chromatography. Thrombin-activated platelets exposed PS and associated to beta2GPI. Both events were required for recognition by antibodies. Human monocyte-derived macrophages phagocytosed activated platelets only. Each macrophage internalized an average of 3.16+/-0.2 platelets after 60 min at 37 degrees C. Phagocytosis did not increase after longer incubations (4.65+/-0.26 platelets internalized by each macrophage after 300 min). Recognition of platelets by anti-beta2GPI antibodies significantly increased phagocytosis (P< 0.01). Upon withdrawal of thrombin, platelets downregulated PS (PS exposure t(1/2): 242 min) and the ability to be recognized by macrophages. Purified beta2GPI bound to PS-exposing platelets (association t(1/2): 250 min). Phosphatidyl serine exposure and beta2GPI association had virtually identical kinetics. Antibody binding prolonged the exposure of the beta2GPI/PS complex (t(1/2): >1200 min). The ability to phagocytose opsonized platelets was accordingly sustained (5.3+/-0.2 opsonized platelets were internalized by each macrophage after 60 min and 9.4+/-0.3 after 300 min). Anti-beta2GPI antibodies therefore poise activated platelets in a PS-exposing status, preventing the recycling of their function and favoring their phagocytic clearance.

Antiphospholipid antibodies and platelets

The most distinguishing serologic feature of antiphospholipid syndrome (APS) is the moderate to high blood titers of antiphospholipid-binding antibodies (aPL). The pathogenic mechanisms of APS are poorly understood, but may occur as a result of the interaction between anticardiolipin antibodies (aCL), beta-2 glycoprotein-I (beta(2)GP-I) (the aCL cofactor) and blood platelets. However, the relationship between aCL/beta(2)GP-I complexes and platelet aggregation has yet to be clearly elucidated. This article will briefly review aPL, beta(2)GP-I and platelet physiology with respect to recent hypotheses relating aCL/beta(2)GP-I complexes and platelets.

Phosphatidylserine expression on cell surfaces promotes antibody-dependent aggregation and thrombosis in beta2-glycoprotein I-immune mice

Beta-2-glycoprotein I (beta2GP1) has been implicated as the primary antigenic target in antiphospholipid syndrome. To study the role beta2GP1 antibodies play in thrombosis associated with this syndrome, the clearance and binding of phosphatidylserine (PS)-containing target membranes were monitored in beta2GP1-immune mice. Clearance in immune mice (T(1/2)4.8 min) was faster than in normal mice (T(1/2)11.0 min). Analysis of PS vesicles recovered from immune mice by sequencing and Western blotting showed the presence of bound beta2GP1 and autologous antibody, respectively. Bleeding times in immune mice were approximately 30% shorter than in control mice. In vitro clotting times, however, were the same in both populations. To determine if the in vivo results could be attributed to the interaction of autoantibodies with the vascular endothelium, the binding of PS-containing target membranes to normal and apoptotic endothelial cells was studied. While endothelial cells bound PS vesicles, beta2GP1 reduced uptake by approximately 50% in both normal and apoptotic endothelium. In the presence of beta2GP1 antibodies, however, uptake in apoptotic cells, but not normal cells, increased by more than two-fold. These results suggest that thrombosis in antiphospholipid syndrome could, in part, be due to antibody-dependent cross-linking of beta2GP1 bound to PS-expressing cells and the vascular endothelium.

Serum anti-beta2-glycoprotein-I and anticardiolipin antibodies during thrombosis in systemic lupus erythematosus patients

PURPOSE

Antibodies to beta2-glycoprotein-I are more strongly associated with clinical antiphospholipid syndrome than are anticardiolipin antibodies. We previously found a decrease in anticardiolipin antibodies at the time of thrombosis in 6 patients with systemic lupus erythematosus (SLE). We therefore sought to determine the prevalence and levels of antibodies to beta2-glycoprotein-I and to cardiolipin before, during, and after thrombosis in patients with SLE, and to compare them with patients who did not have thrombosis.

METHODS

We studied 24 patients with SLE who had at least one episode of thrombosis and 102 patients with SLE without thrombosis. Serum anticardiolipin antibodies were measured by conventional enzyme-linked immunosorbent assay (ELISA) using newborn calf serum as the blocking agent. Serum anti-beta2-glycoprotein-I antibodies were measured by ELISA on nonirradiated plates, using purified human beta2-glycoprotein-I without phospholipid.

RESULTS

All patients with thrombosis had anti-beta2-glycoprotein-I antibodies, compared with only 17% of controls (P <0.0001). We observed a significant decrease in serum anti-beta2-glycoprotein-I levels at the time of thrombosis, as compared with previous and subsequent samples. The prevalence and levels of IgG and IgM anticardiolipin antibodies were similar in patients with and without thrombosis. A decrease in IgG or IgM anticardiolipin titers occurred during thrombosis in 6 patients. Anticoagulant, corticosteroid, and immunosuppressive treatments did not appear to affect anti-beta2-glycoprotein-I levels at the time of thrombosis.

CONCLUSION

Anti-beta2-glycoprotein-I antibodies are strongly associated with thrombosis in patients with SLE. The decrease of anti-beta2-glycoprotein-I levels at the time of thrombosis may indicate a pathogenic role. This antibody may also be a marker of predisposition for thrombosis in these patients.

Apoptotic cells as immunogen and antigen in the antiphospholipid syndrome

Apoptotic cell antigens have been increasingly recognized as the targets of autoantibodies across a broad spectrum of autoimmune diseases, including systemic lupus erythematosus (SLE) and the antiphospholipid (aPL) syndrome. In this review, we will focus on one set of apoptotic antigens, namely, those targeted in the aPL syndrome. Here we discuss the biology of aPL autoantibodies and recent work from our and other laboratories demonstrating that apoptotic cells express unique antigen(s) that serve(s) as both immunogen and antigen for aPL autoantibodies. Specific features or events occurring at the surface of apoptotic cells, which may influence immunogenicity and/or antigenicity, will also be discussed. Finally, we will speculate on the broader implications of these findings for the development of systemic autoimmunity as seen in SLE.

Beta2-glycoprotein I (beta2-GPI) mRNA is expressed by several cell types involved in anti-phospholipid syndrome-related tissue damage

We report here the expression of beta2-GPI mRNA by cell types involved in the pathophysiology of the anti-phospholipid syndrome (APS), i.e. endothelial cells as a target of autoantibodies in the APS, astrocytes and neurones involved in APS of the central nervous system (CNS). Lymphocytes were also included in the study, as it has been demonstrated that patients with systemic lupus erythematosus-associated CNS diseases have serum anti-lymphocyte antibodies cross-reacting with brain antigens, and intrathecally synthesized anti-neurone antibodies. Reverse transcriptase-polymerase chain reaction followed by restriction enzyme digestion of the product obtained demonstrated the presence of beta2-GPI mRNA in all cell types here tested, cultured both in presence and absence of fetal calf serum. In both culture conditions, the same cell types were immunoreactive to an anti-beta2-GPI MoAb, as determined by indirect immunofluorescence technique. Taken together, these results indicate a direct cell synthesis of beta2-GPI, suggesting an antigenic function of beta2-GPI in the APS, including the CNS disease that occurs in this syndrome.

Lupus anticoagulants and antiphospholipid antibodies

In recent years, clinical syndromes involving lupus anticoagulants and antiphospholipid antibodies have come into increasing clinical prominence. Since the discovery that most antiphospholipid antibodies require the presence of anionic phospholipid-binding proteins such as B2-glycoprotein I and prothrombin, a large number of studies have attempted to delineate the specificity of these antibodies. Several mechanisms have been proposed to explain the hypercoagulable state associated with these antibodies. This review attempts to summarize these data and the challenges that confront efforts to delineate the pathogenesis of the prothrombotic state associated with the presence of these antibodies.

Antiphospholipid antibodies bind to rat cerebellar granule cells: the role of N-methyl-D-aspartate receptors

IgGs from sera containing antiphospholipid antibodies (aPL), detected as antibodies to cardiolipin, or control sera were incubated with rat cerebellar granule cells in primary culture. Using a mitochondrial dehydrogenase activity assay (MTT test), aPL IgGs were shown to decrease MTT metabolism after 24 h incubation with the cells, and to cause non-toxic amounts of glutamate to become neurotoxic when added to the cells for 45 min. Acute and chronic aPL toxicity were prevented by MK-801. Sera containing aPL bound to intact cerebellar neurons, as revealed by an immunofluorescent technique. These results suggest that antiphospholipid antibodies interfere with excitatory pathways in glutamatergic cerebellar granule cells by a mechanism involving overactivation of the NMDA glutamate receptor.

Neurological manifestations of antiphospholipid antibody syndrome

Thrombosis, thrombocytopenia, recurrent fetal loss and a variety of non-thrombotic neurological disorders have all been associated with antiphospholipid antibodies (aPL). Cerebral ischemia associated with aPL is the most common arterial thrombotic manifestation. Depression, cognitive dysfunction, depression and psychosis have all been associated with aPL. The presumed pathophysiologic mechanism underlying these manifestations is thought to be a result of cerebral ischemia in some, but not all cases. Seizures, chorea and transverse myelitis all appear to be associated with aPL. An interaction between aPL and central nervous system cellular elements rather than aPL-associated thrombosis seems to be a more plausible mechanism for these clinical manifestations. Migraine on the other hand, does not appear to be associated with aPL in either lupus or non-lupus populations. Neuroimaging studies show an increased frequency of brain abnormalities in patients with aPL, but none appear to be specific. The best treatment strategy for preventing neurological manifestations of aPL is not fully defined. For thrombotic manifestations, both antiplatelet and anticoagulant therapies have been suggested. In some patients, immunosuppressant therapy has been used. For non-thrombotic manifestations, some combination of immunosuppressant therapy and symptomatic treatment may be warranted.

Anti-endothelial cell antibody binding makes negatively charged phospholipids accessible to antiphospholipid antibodies

OBJECTIVE

Anti-endothelial cell autoantibodies (AECA) are often associated with antibodies to anionic phospholipids (PL), such as phosphatidylserine (PS). Yet, beta2-glycoprotein I (beta2GPI)-dependent anti-PL antibodies (aPL) do not have access to their target antigens on the membrane of endothelial cells (EC). Given that AECA are capable of exposing PS and, thereby, initiating apoptosis, we explored the relationships between AECA, beta2GPI, and aPL on the surface of EC.

METHODS

Human EC were incubated with mouse AECA monoclonal antibodies, and the translocation of PS was established through the binding of annexin V, which binds specifically to PS. A rabbit anti-beta2GPI antibody and biotin-conjugated F(ab')2 aPL derived from 3 patients were also used to detect beta2GPI on the cells.

RESULTS

Twenty percent to 36% of the cells expressed anionic PL following incubation with AECA, as revealed by the binding of annexin V and beta2GPI. The proportion of anionic PL-expressing EC (up to 90%) correlated with the period of incubation of EC with AECA and depended on the dose of AECA. Bound aPL resided exclusively within the AECA-positive EC population.

CONCLUSION

Based on our findings, AECA may be pathogenic. Some of them may even have the potential to induce production of aPL.

IgG from patients with antiphospholipid syndrome binds to platelets without induction of platelet activation

The clinical manifestations of the antiphospholipid syndrome (APLS) include arterial and venous thrombosis, thrombocytopenia and fetal loss, but the pathogenic mechanisms remain unclear. It has been hypothesized that platelet activation by autoantibody may be a pathogenic mechanism. We studied IgG binding, microparticle (mp) formation and P-selectin expression by flow cytometry in normal platelets after incubation in serum from 11 patients with antiphospholipid antibodies and that from 10 normal healthy subjects. Levels of platelet-associated IgG were significantly higher after incubation in patient sera (mean 17.2, range 2.0-75.0%) compared with normal sera (mean 2.0, range 1.2-3.7%, P<0.05). Incubation of normal platelets in serum led to increased microparticle formation (P<0.01) and P-selectin expression (P < 0.05), compared with unstimulated platelets. There was no significant difference, however, between microparticle formation nor P-Selectin expression induced by patient serum (mp 3.0 (1.6-5.0)%; P-selectin 8.0 (4.0-16.6)%) versus normal serum (mp 3.2 (2.1-4.5)%; P-selectin 10.1 (4.0-15.6); median (range)). Pre-activation of platelets with sub-threshold ADP concentrations or thrombin receptor activator peptide resulted in a small increase in microparticle formation, but there was still no significant difference between the effects of patient and control sera. Despite the presence of platelet membrane binding IgG in serum from 5/11 patients with antiphospholipid antibodies, there was no evidence for associated enhanced platelet-activating ability. This study supports antiplatelet reactivity in antiphospholipid syndrome, but not a direct platelet-activating role for platelet-directed autoantibodies.

Theoretical considerations of RES-avoiding liposomes: Molecular mechanics and chemistry of liposome interactions

The development of long-circulating, RES-avoiding liposomes has become a remarkable milestone in the progress of contemporary pharmacology. Drugs incorporated in such liposomes are protected from fast metabolization and clearance, and can be further targeted to a desired tissue site. Ideally, future developments should result in drug carriers which can identify and act upon their targets with even higher efficiency and selectivity, preferably close to or exceeding that of the natural immune cells.Further increasing carrier 'inertness' with regard to the normal biological milieu is the major requirement for future success. The ability of natural blood components to circulate with blood for several days and weeks presents both the motivation and the challenge for further research. Today, even the best available preparations are inferior to natural proteins and cells with regard to their ability to remain in circulation by approximately two orders of magnitude.In view of the above, it seems vitally important to determine the mechanisms responsible for glycolipid- or polymer-modified liposome protection against RES, and whether any potentially useful mechanisms have been underutilized. Furthermore, identification of quantitative dependencies between liposome structure and pharmacokinetics (and mechanisms underlying such dependencies) would benefit future research and reduce the cost of development.This paper discusses the relationships between liposome structure and circulation with respect to the theoretical mechanistic models of mass transfer, liposome interactions with cells and blood proteins, and boundary effects resulting from surface modification. Special attention is paid to the practical application and limitations of the models.

Human β2-Glycoprotein I Binds to Endothelial Cells Through a Cluster of Lysine Residues That Are Critical for Anionic Phospholipid Binding and Offers Epitopes for Anti-β2-Glycoprotein I Antibodies

β2-Glycoprotein I (β2GPI) is a phospholipid-binding protein recognized by serum autoantibodies from the anti-phospholipid syndrome both in cardiolipin- and β2GPI-coated plates. We found that: 1) recombinant wild-type β2GPI bound to HUVEC and was recognized by both human monoclonal IgM and affinity-purified polyclonal IgG anti-β2GPI anti-phospholipid syndrome Abs; and 2) a single amino acid change from Lys286 to Glu significantly reduced endothelial adhesion. Double and triple mutants (from Lys284,287 to Glu284,287, from Lys286,287 to Glu286,287, and from Lys284,286,287 to Glu284,286,287) completely abolished endothelial binding. A synthetic peptide (P1) spanning the sequence Glu274–Cys288 of the β2GPI fifth domain still displayed endothelial adhesion. Another peptide (P8), identical with P1 except that Cys281 and Cys288 were substituted with serine residues, did not bind to HUVEC. Anti-β2GPI Abs, once bound to P1 adhered to HUVEC, induced E-selectin expression and up-regulated IL-6 secretion. Control experiments conducted with irrelevant Abs as well as with the P8 peptide did not show any endothelial Ab binding nor E-selectin and IL-6 modulation. Our results suggest that: 1) β2GPI binds to endothelial cells through its fifth domain; 2) the major phospholipid-binding site that mediates the binding to anionic phospholipids is also involved in endothelial binding; 3) HUVEC provide a suitable surface for β2GPI binding comparable to that displayed by anionic phospholipids dried on microtiter wells; and 4) the formation of the complex between β2GPI and the specific Abs leads to endothelial activation in vitro.

Possible pathogenetic role of activated platelets in the primary antiphospholipid syndrome involving the central nervous system

Neurological disorders occurring in the primary antiphospholipid syndrome (neuro-PAPS) have not yet been completely understood. Platelet activation has been suggested to play a crucial role in the pathogenesis of hemostatic disorders in the antiphospholipid syndrome, but no association with neuro-PAPS has been investigated so far. Therefore, we investigated 16 patients with PAPS by flow cytometry in the presence of circulating activated platelets as defined by the surface expression of activation-dependent glycoprotein CD62. In addition, the relationship among activated platelets and anticardiolipin antibodies (aCL) was evaluated. Compared to normal subjects CD62 was found significantly increased in these patients. Furthermore, a significantly increased percentage of CD62-positive platelets was found in the neuro-PAPS group (nine patients) compared to the non-neuro-PAPS patients (seven subjects). On the contrary, no significant difference was found between the two groups with regard to aCL IgG and platelet number. Furthermore, within the neuro-PAPS group, no difference was evidenced, in the CD62-positive platelet percentage, between the four subjects with thrombocytopenia and the five with the normal blood platelet count. Similarly, neuro-PAPS subjects with previous peripheral arterial and/or venous thrombosis did not show a significantly more elevated level of CD62-positive platelets. Finally, a linear correlation was found between the aCL IgG level and the CD62-positive platelet percentage in all the patients and, more significantly, in the neuro-PAPS group, but not within the non-neuro-PAPS patients. Our data demonstrate that circulating activated platelets are detectable by flow cytometry in the majority of PAPS patients and suggest the existence of a relationship among activated platelets, aCL, and neurological disease that patients affected by PAPS might undergo.

Flow cytometric detection of circulating activated platelets in primary antiphospholipid syndrome. Correlation with thrombocytopenia and anticardiolipin antibodies

Platelet activation has been suggested to play a crucial role in the pathogenesis of haemostatic disorders in antiphospholipid syndrome (APS). In 16 patients with primary APS (PAPS) we investigated by flow cytometry the presence of circulating activated platelets as defined by the surface expression of activation-dependent glycoproteins CD62 and CD63. In addition, the relationships among activated platelets, thrombocytopenia, antiphospholipid antibodies (aPL) and platelet associated IgG (PalgG) were evaluated. Compared to normal subjects CD62, but not CD63 expression, was found significantly increased in patients. All thrombocytopenic subjects showed a percentage of CD62 expressing platelets above the cut off. In thrombocytopenics a significantly increased percentage of CD62 and higher levels of aCL IgG were found compared to PAPS patients with normal platelet count. No correlation was found between activated platelets and both lupus anticoagulant antibodies and PalgG. Our data demonstrate that circulating activated platelets are detectable by flow cytometry in the majority of PAPS patients and suggest the existence of a relationship among activated platelets, thrombocytopenia and aPL levels.

The antiphospholipid/cofactor syndromes: a primary variant with antibodies to beta 2-glycoprotein-I but no antibodies detectable in standard antiphospholipid assays

BACKGROUND

Most systemic lupus erythematosus (SLE) patients with two or more clinical manifestations of the antiphospholipid syndrome (APS) and negative antiphospholipid antibodies (aPL) have antibodies to beta 2-glycoprotein-I (a beta 2 GP-I). Herein we describe a similar set of circumstances, but in patients without evidence of SLE.

PATIENTS AND METHODS

We studied 6 patients with recurrent venous and/or arterial thromboses without aPL as detected by routine assays nor clinical or serological evidence of other autoimmune disease. Immunoglobin (Ig) G and IgM antibodies to bovine and human phospholipid-free beta 2 GP-I were studied by Western blot test and by enzyme-linked immunosorbent assay (ELISA) utilizing radiated and nonirradiated plates. We also tested antibodies to cardiolipin, phosphatidylserine, and phosphatidylethanolamine by ELISA. As controls, 54 normal sera were studied.

RESULTS

All 6 patients had recurrent arterial and/or venous thromboses. Three also had thrombocytopenia, 1 had livedo reticularis, and 2 had valvular heart disease. None of the patients had aPL, but all had serum IgG reactivity against human and bovine beta 2 GP-I (P < 0.001 versus controls for both). Titers of anti-bovine beta 2 GP-I were higher when studied in irradiated plates but were also higher than normal in nonirradiated plates (P < 0.001). These antibodies did not recognize human or bovine beta 2 GP-I bound to cardiolipin in solid phase. We confirmed by Western blot that these autoantibodies recognize human beta 2 GP-I. We found no IgM a beta 2 GP-I.

CONCLUSIONS

We describe a primary condition akin to the antiphospholipid syndrome with negative aPL, but with serum IgG antibodies to human and bovine beta 2 GP-I. These antibodies recognize beta 2 GP-I epitopes that are not accessible when beta 2 GP-I is bound to cardiolipin.

Pathogenesis of the antiphospholipid antibody syndrome

Endothelial and/or platelet activation likely initiates thrombus formation. Whether antiphospholipid antibody (aPL) is an activator, a toxic response, or a protective response is not clear, nor is it certain whether aPL is germ-line encoded or antigen-driven. The pregnancy model is particularly informative. Alternative hypotheses about thrombogenicity which relegate aPL to the role of bystander have not yet been excluded.

Treatment of neurologic complications of antiphospholipid antibody syndrome

Thrombosis associated with antiphospholipid antibodies (aPL) occurs in both venous and the arterial circulation. The most common arterial thrombo-occlusive event is cerebral infarction. We briefly review treatment strategies aimed at patients with cerebrovascular disease and aPL. Besides general treatment issues, we discuss primary prevention and secondary prevention. Most regimens include antithrombotics or immune modulation. Prospective studies (currently underway) are required to better estimate the rate of recurrent thrombo-occlusive events on standardized therapy before one therapy can be recommended over another with reasonable evidence.

Neurological aspects of antiphospholipid antibody syndrome

Antiphospholipid antibodies (aPL) have been associated with a variety of neurological disorders, mostly linked to focal neuroparenchymal ischemia or infarction. Cerebral ischemia associated with the antiphospholipid syndrome (APS) occurs at a younger age than typical atherothrombotic cerebrovascular disease, is often recurrent, and high positive GPL values are usually linked to the presence of a lupus anticoagulant. When other features of the syndrome are not present and cerebral ischemia occurs only associated with anticardiolipin immunoreactivity, there appears to be no discerning features of these patients unless GPL > 40 for which recurrent thrombo-occlusive events appear to occur more frequently. Other neurological manifestations associated with aPL include cerebral venous sinus thrombosis, ocular ischemia, dementia, including ischemic encephalopathy, and chorea. The role of aPL in migrainous events is controversial and may not play a role in recent, large case-controlled studies. Most seizures in patients harboring aPL are associated with focal brain infarction.