Anticardiolipin antibodies in patients with chronic hepatitis C virus infection: characterization in relation to antiphospholipid syndrome

The antiphospholipid syndrome (APS) is usually defined by the association of clinical manifestations that comprise venous and/or arterial thrombosis, recurrent fetal losses, and thrombocytopenia, along with the presence of anticardiolipin (aCL) antibodies and/or lupus anticoagulant. Various infectious diseases can induce aCL; however, these antibodies are not usually associated with thrombotic events, as happens with autoimmune diseases, in which these antibodies need the presence of beta(2)-glycoprotein I. Levels of immunoglobulin G (IgG) and IgM aCL antibodies were determined by enzyme-linked immunosorbent assay for 243 patients with chronic hepatitis C virus (HCV) infection and 100 healthy controls. Clinical events of APS, the level of beta(2)-glycoprotein dependence of aCL, the presence of cryoglobulins and other autoantibodies, and cross-reactivity between purified aCL and HCV were evaluated. Positive results for aCL antibodies were found more frequently (3. 3%) for the patients with HCV infection than for healthy controls (0%). All positive aCL antibodies were beta(2)-glycoprotein I independent. No significant association was found between aCL antibodies and clinical manifestations of APS, neither was one found between the presence of other autoantibodies or cryoglobulins and that of aCL. Finally, no cross-reactivity between aCL antibodies and HCV antigens was observed. As previously reported, aCL antibodies seem to be an epiphenomenon, and they do not have clinical or laboratory significance in HCV patients.     

Abstract 1-10

1. Usefulness of a sensitive and insensitive aPTT reagent as a confirmatory test in the diagnosis of lupus anticoagulant (LA) in oral anticoagulated patients2. Human monoclonal antibodies against the complex of phosphatidylserine and prothrombin from patients with antiphospholipid syndrome3. Antibodies to factor XII: a possible predictive marker for recurrent foetal loss4. TINU syndrome and antiphospholipid antibodies5. Annexin V (AxV) increases the immunogenicity of phosphatidylserine (PS) exposing cells6. The orientation of b2-glycoprotein I (b2-GPI) on the plate is important for the binding of antiphospholipid antibodies7. Prevalence of antiphospholipid, antiendothelial and anti-oxidized LDL auto-antibodies in patients with Cushing‘s syndrome8. High incidence of antiphospholipid antibodies (APA) in patients with fibromyalgia syndrome (FMS)9. Anti-high density lipoprotein and anticardiolipin antibodies inversely correlate with paraoxonase activity and total antioxidant capacity in patients with SLE and APS10. Neonatal antiphospholipid syndrome – does it really exist?     

Antiprothrombin antibodies are associated with pregnancy loss in patients with the antiphospholipid syndrome

OBJECTIVE: To document the clinical association between the history of pregnancy loss in patients with the diagnosis of primary or secondary antiphospholipid syndrome (APS) and the presence of different antiprothrombin antibody subtypes [immunoglobulin G (IgG), IgM and IgA] in a cohort of patients with APS. METHODS: Records of 170 female patients with primary APS, or APS secondary to systemic lupus erythematosus (SLE) or secondary to other autoimmune diseases were studied. RESULTS: In female APS patients with IgG antiprothrombin antibodies (n = 105) significant associations to pregnancy loss (p < 0.0001), early pregnancy loss (p < 0.0001) and a negative association to thrombocytopenia (p < 0.01) could be identified. In the group of patients with IgG antiprothrombin antibodies and at least one pregnancy (n = 84) a significant association with pregnancy loss (p < 0.005) and especially with early pregnancy loss (p < 0.0001) was demonstrated. No association with other immunoglobulin subtypes of antiprothrombin antibodies could be documented. In the subgroup of patients with primary APS and at least one pregnancy in the history, pregnancy loss (p < 0.005) and early pregnancy loss (p < 0.0001) were found to be highly associated with the presence of IgG antiprothrombin antibodies. IgG antiprothrombin antibodies represent the highest independent risk factor for pregnancy loss with an odds ratio of 4.5. There was no statistically significant association with venous or arterial thrombosis in all IgG antiprothrombin antibody positive patients. CONCLUSION: The results of this study document the association of IgG antiprothrombin antibodies with pregnancy loss and in particular early pregnancy loss in a large and well-characterized cohort of patients. We would recommend routine testing for antiprothrombin antibodies in young female patients with APS.     

Effect of antiphospholipid antibodies on the formation and lysis of fibrin network in patients with recurrent miscarriage

The present work was intended to study the process of fibrin formation and lysis and plasmin generation in a group of patients with recurrent miscarriage (RM), due to the presence of antiphospholipid antibodies (N = 10); as well as in women with RM without the antiphospholipid syndrome (APS) (N = 6), compared with those of a group of healthy women (N= 8). In the group of patients with APS, nine were positive for antibodies against cardiolipin (aCL), five for anti-beta2-glycoprotein I (anti-beta2GPI), four for both antibodies, and one for antibodies against prothrombin (aPT) and lupus anticoagulant (LA). Fibrin formation and lysis was followed by turbidity and plasmin generation using chromogenic substrate S2251. The polymerization curves from RM patients without APS and the LA patient showed an increased slope and maximum turbidity compared to those of the control group. The speed of lysis was higher in the LA patient (21 +/- 0) 10(-4) deltaOD/seg and the RM patients without APS (19.6 +/- 5.7) 10(-4) deltaDO/seg, compared to that of the control group (14.5 +/- 2.8) 10(-4) deltaDO/seg. Plasmin generation increased only in RM patients without APS (85 +/- 24%) against the control group (52 +/- 3%), p = 0.005. The changes observed in the fibrin polymerization and lysis process of women with RM without APS and LA seem to be related to their higher fibrinogen levels, while the increased plasmin generation was related to the patients' morbidity.     

Detection of 'antiphospholipid' antibodies: a single chromogenic assay of thrombin generation sensitively detects lupus anticoagulants, anticardiolipin antibodies, plus antibodies binding beta(2)-glycoprotein I and prothrombin

The diagnosis of the antiphospholipid syndrome (APS) requires both a typical clinical event plus a persistently positive test in an assay for either anticardiolipin (aCL) antibodies or a lupus anticoagulant (LA). Enzyme linked immunosorbent assays (ELISA) specific for autoantibodies against beta(2)-glycoprotein I (beta(2)GPI) or prothrombin are also used, but none of the tests are adequately sensitive or specific. A chromogenic assay was developed that measures the effect of test antibody or plasma samples on in vitro thrombin formation. It is able to detect both LA and beta(2)GPI-dependent aCL antibodies and may have greater specificity for APS than currently available tests. Using this method various monoclonal antibodies (MoAbs) were examined, from mice immunized with beta(2)GPI, mice with a spontaneous animal model of APS, and from three humans with APS. Plasma and affinity purified antibodies from patients with APS and control groups were also examined. Thrombin inhibition was more sensitive to perturbation by MoAbs than a combination of tests for LA (P < 0.05) and at lower antibody concentrations (12.5 microg/ml versus 100 microg/ml). There was a significant correlation between inhibition of thrombin generation and the level of MoAb reactivity to beta(2)GPI (r = 0.90; P < 0.001) but not to CL (r = 0.06; P = 0.76). Plasma and affinity purified antibodies from patients with APS also inhibited thrombin generation, and significantly more so than patients with aPL from causes other than APS. APS patient samples showed thrombin inhibition in the presence of anti-beta(2)GPI or antiprothrombin antibodies. All MoAbs binding beta(2)GPI showed inhibition of thrombin generation, while MoAbs binding domain I of beta(2)GPI had more LA effect.     

Assay principles of antiphospholipid antibodies and heterogeneity of the antibodies

The antiphospholipid syndrome(APS) is characterized by predominant clinical features of venous and arterial thrombosis and recurrent pregnancy loss accompanied by antiphospholipid antibodies(aPL) such as anticardiolipin antibodies(aCL) and lupus anticoagulant(LA). In 1990, three individual research groups, including us, first reported that a 50 kD plasma cofactor is required for the binding of aCL to cardiolipin(CL) and now, beta 2-glycoprotein I(beta 2-GPI), which binds to anionic phospholipids(PLs), is widely believed to be the major antigen for aCL. It was also reported that epitopes for such aCL are cryptic and that they appear only when beta 2-GPI interacts with lipid membranes containing anionic PLs, such as CL and phosphatidylserine, or with a polyoxygenated polystyrene surface. In contrast, prothrombin was recently identified as the "true" antigen for LA. In this review paper, we would like to describe on specificity of aPL and also on a possible mechanism on autoantibody-dependent development of atherosclerosis.     

Autoantibody explosion in antiphospholipid syndrome

Antiphospholipid syndrome (APS) is characterized by thrombosis and pregnancy loss in the presence of antiphospholipid antibodies (aPL), mainly anticardiolipin, anti-beta2-glycoprotein I and lupus anticoagulant. However, similar to systemic lupus erythematosus, APS is also characterized by multiple other autoantibodies including 'non-classical' aPL, as well as other antibodies. Herein we describe the autoantigen properties, prevalence and clinical importance of 30 different antibodies in APS. Among the other antibodies characterizing APS are autoantibodies directed to platelets, glycoproteins, various coagulation factors, lamins, mitochondrial antigens and cell surface markers. Few of these autoantibodies are correlated with the presence of other antibodies, and some may have an additive role in the pro-thrombotic tendency of the syndrome. This autoantibody explosion might be important in early identification of the syndrome and its manifestations.     

Anticardiolipin Antibodies in Patients with Chronic Hepatitis C Virus Infection: Characterization in Relation to Antiphospholipid Syndrome

The antiphospholipid syndrome (APS) is usually defined by the association of clinical manifestations that comprise venous and/or arterial thrombosis, recurrent fetal losses, and thrombocytopenia, along with the presence of anticardiolipin (aCL) antibodies and/or lupus anticoagulant. Various infectious diseases can induce aCL; however, these antibodies are not usually associated with thrombotic events, as happens with autoimmune diseases, in which these antibodies need the presence of β₂-glycoprotein I. Levels of immunoglobulin G (IgG) and IgM aCL antibodies were determined by enzyme-linked immunosorbent assay for 243 patients with chronic hepatitis C virus (HCV) infection and 100 healthy controls. Clinical events of APS, the level of β₂-glycoprotein dependence of aCL, the presence of cryoglobulins and other autoantibodies, and cross-reactivity between purified aCL and HCV were evaluated. Positive results for aCL antibodies were found more frequently (3.3%) for the patients with HCV infection than for healthy controls (0%). All positive aCL antibodies were β₂-glycoprotein I independent. No significant association was found between aCL antibodies and clinical manifestations of APS, neither was one found between the presence of other autoantibodies or cryoglobulins and that of aCL. Finally, no cross-reactivity between aCL antibodies and HCV antigens was observed. As previously reported, aCL antibodies seem to be an epiphenomenon, and they do not have clinical or laboratory significance in HCV patients.     

Analysis of T cell responses to the beta 2-glycoprotein I-derived peptide library in patients with anti-beta 2-glycoprotein I antibody-associated autoimmunity

Antiphospholipid syndrome (APS) is an autoimmune disease that accompanies anti-phospholipid antibodies measured as either anti-cardiolipin antibodies (aCL) or lupus anticoagulant. beta(2)-glycoprotein I (beta(2)GPI) is the most common and apparently the best-characterized antigenic target for aCL. To investigate T-cell responses to beta(2)GPI, we stimulated PBMC of 18 APS or systemic lupus erythematosus (SLE) patients carrying anti-beta(2)GPI and 10 healthy controls, using a peptide library covering the beta(2)GPI sequence. We established seven CD4(+) T cell lines reactive with beta(2)GPI peptide. Three of four epitopes for patient-derived T cell lines were p244-264, whereas one T cell line from a control subject also recognized p244-264. Furthermore, there was no tendency for particular HLA class II molecules to present beta(2)GPI peptides. However, cytokine producing patterns were significantly different between T cell lines from patients and those from healthy individuals (p =.028); patients' T cells tend to exhibit higher IL-4 and lower IFN-gamma responses. These T cell lines did not react to beta(2)GPI purified from human plasma. These results indicate that beta(2)GPI-reactive CD4(+) T cells of APS/SLE patients mainly recognize cryptic p244-264 in the context of various HLA class II molecules, and exhibit Th0-Th2-type responses. Our findings may provide a clue to the pathogenesis of APS.     

Anticardiolipin and anti-beta-2-glycoprotein I antibodies

The anticardiolipin (aCL) antibody test was first established in 1983, using cardiolipin (negatively charged phospholipid) as an antigen in a solid-phase immunoassAy. It was first applied to the study of systemic lupus erythematosus patients, and was found associated with thromboses and recurrent pregnancy losses. The wide use of this test was determinant in the definition of the "aCL or antiphospholipid syndrome" (APS).Later, it was demonstrated that aCL antibodies do not recognize anionic phospholipids but are directed against plasma proteins bound to anionic phospholipids, mainly beta-2-glycoprotein I, which is now considered as the autoantigen in APS. Anti-beta-2-glycoprotein I (anti-beta2GPI) is not yet accepted as a serological criterion for APS, but most investigators would consider a patient with anti-beta2GPI antibodies and clinical features of APS to have the syndrome. aCL and anti-beta2GPI are a heterogeneous group of antibodies with different clinical significances and can be present in different autoimmune diseases as well as in infectious diseases.     

Anti-Beta 2-glycoprotein I antibody isotype and IgG subclass in antiphospholipid syndrome patients

Beta 2-glycoprotein I (beta2-GPI) is an antigenic target recognised by antiphospholipid antibodies found in association with the antiphospholipid syndrome (APS). In this study, the prevalence of Immunoglobulin M (IgM) and IgA anti-beta2-GPI antibodies was examined in APS patients and compared with IgG antibodies. In addition the value of measuring antibody isotypes and IgG subclass was investigated in the laboratory diagnosis of APS. A solid phase enzyme linked immunosorbent assay was established to measure IgG, IgM and IgA and IgG subclass antibodies to beta2-GPI in patients with APS and a variety of other thrombotic and non-thrombotic disorders. Raised levels of IgM anti-beta2-GPI antibodies were observed in 65% of patients with APS, 21% with systemic lupus erythematosus (SLE), 23% with rheumatoid factor, 4% with stroke, 5% carotid artery stenosis (CAS), 17% with a biological false positive serology for syphilis, 43% with infectious mononucleosis (IM) and 27% with human immunodeficiency virus (HIV). The median value for IgM antibodies to beta2-GPI for all these groups ranged from 2 to 7 arbitrary units (AU). Elevated levels of IgA antibodies to beta2-GPI were found in patients with APS (47%), SLE (13%), rheumatoid factor (26%), CAS (48%), stroke (25%), VDRL false positive serology for syphilis (33%), IM (47%) and HIV (7%). The median value of IgA antibodies to beta2-GPI in all of these groups ranged from 2 to 4 AU. Conversely the median value for IgG anti-beta2-GPI in APS patients was 112 AU compared to 1-4 AU in the other conditions examined. The presence of IgM and IgA antibodies to beta2-GPI was much less specific and sensitive for APS than IgG, with raised levels of these isotypes seen in a variety of thrombotic and non-thrombotic disorders. Elevated levels of IgG1, IgG2, IgG3 and IgG4 antibodies to beta2-GPI were detected in APS patients. While all four IgG anti-beta2-GPI antibody subclasses were represented in APS patients there appeared to be a significant overall skewing towards to the IgG2 subclass.     

Clinical significance of anticardiolipin and anti-beta2-glycoprotein I antibodies

BACKGROUND: Anticardiolipin (aCl) and anti-beta2-glycoprotein I (anti-beta2-gpI) antibodies are autoantibodies associated with the antiphospholipid syndrome (APS), which is characterized by both arterial and venous thrombosis and miscarriages. The scope of this study was to explore the clinical characteristics of patients with aCl and anti-beta2-gpI antibodies. METHODS: ACl were tested in 3,600 consecutive sera in our laboratory between January 1999 and June 2001. The clinical diagnosis and prevalence of thrombosis and pregnancy morbidity were retrospectively reviewed in aCl-positive patients. Furthermore, the frequency of anti-beta2-gpI antibodies, lupus anticoagulant (LA), prolonged activated partial thromboplastin time (aPTT), and thrombocytopenia were investigated in aCl-positive patients. RESULTS: 147 aCl-positive patients, 110 women and 37 men with a mean age of 41 years (range 7.8-82.5), were identified. 42 (28.6%) aCl-positive patients fulfilled the criteria for APS which was secondary to a connective tissue disorder in 8 patients. The frequency of anti-beta2-gpI antibodies and LA, prolonged aPTT, and thrombocytopenia in aCl-positive patients was 23.8, 27.2, 25.7 and 9.2%, respectively. The presence of both aCl and anti-beta2-gpI antibodies was strongly associated with clinical symptoms of APS (p = 0.007) compared to p = 0.008 for LA. CONCLUSION: Our data suggest that assessment of anti-beta2-gpI antibodies in addition to aCl is a valuable diagnostic tool in the workup of patients with APS.     

New autoantigens in the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is an autoimmune disease characterized by arterial and venous thrombosis, recurrent miscarriages or fetal loss, and circulating antiphospholipid antibodies (aPL). Enzyme-linked immunosorbent assays for anticardiolipin and anti-β2-glycoprotein I antibodies and clotting assays for the lupus anticoagulant are the tests recommended for detecting aPL. However, the aPL are a heterogeneous group of antibodies directed against anionic phospholipids but also toward phospholipid-binding plasma proteins or phospholipid-protein complexes. β2-glycoprotein I (β2GPI) is the playmaker antigen of APS, however during apoptosis, lysophospholipids can become exposed on the cell surface, and mainly through their interaction with β2GPI, they can become targets of aPL. Some CL metabolites are likely to escape from the remodeling cycle. This would account for the progressive loss of mitochondrial CL during apoptosis, as well as for the presence of CL and lyso-CL at the cell surface, where they can interact with β2GPI and become targets of aPL. Other recognized targets of aPL are represented by phosphatidylserine, lyso(bis)phosphatidic acid, Phosphatidylethanolamine, vimentin, and annexin A5. These molecules may allow improving the knowledge on the pathogenesis, and the early identification of APS. Although several studies have shown the presence of antibodies directed against other antigens in APS, their clinical relevance is still a matter of debate, and it needs to be confirmed with experimental data and longitudinal studies.     

Phosphatidylserine-dependent anti-prothrombin antibody as a new marker for the diagnosis of antiphospholipid syndrome

Antiphospholipid syndrome (APS) is an autoimmune disorder in which vascular thrombosis and recurrent pregnancy loss occur in patients with antiphospholipid antibodies(aPL). Measurements of the beta2-glycoprotein I-dependent anticardiolipin antibody(aCL) and lupus anticoagulant(LAC) are the only laboratory tests available for the diagnosis of APS. Recently, phosphatidylserine-dependent antiprothrombin antibody(aPS/PT) has been detected. aPS/PT was measured by ELISA using the phosphatidylserine-prothrombin complex as an antigen immobilized on ELISA plates in the presence of CaCl2. In our study of 219 patients with APS and autoimmune diseases, the prevalence of aPS/PT-IgG in those with APS was 42.2%, which was significantly higher than that(4.6%) in patients with autoimmune diseases. Furthermore, aPS/PT was closely associated with APS manifestations with an odds ratio (OR) of 2.92 (95% confidence interval (95% CI): 1.33 to approximately 6.40), whereas the OR for aCL was 2.06 (95% CI: 0.91 to approximately 4.66). In addition, aPS/PT-IgG was strongly correlated with the presence of LAC as detected with a diluted Russell viper venom time test (dRVVT) (OR: 38.2, 95% CI: 13.4 to approximately 109.1). The monoclonal antibody (23-1D) of aPS/PT also prolonged the clotting time in LAC tests (aPTT, dRVVT, and kaolin clotting time) in a concentration-dependent manner. In conclusion, aPS/PT is more closely associated with manifestations of APS and LAC, and positive results from an aPS/PT test can mark thrombotic events in APS patients. The determination of aPS/PT in clinical practice, in conjunction with that of other aPL, may improve the likelihood of recognizing APS.     

Research around beta 2-glycoprotein I: a major target for antiphospholipid antibodies

Beta2-glycoprotein I (beta2GPI), a phospholipid-binding protein, is one of the major target antigens for antiphospholipid antibodies (aPL) found in patients with antiphospholipid syndrome (APS). Thrombophilic disorders in APS patients are strongly associated with aPL, and their pathogenic properties depend on the presence of beta2GPI. Procoagulant cell stimulation by aPL, via beta2GPI, is one of the most plausible mechanisms of thrombosis in APS, and p38 mitogen activated protein kinase (MAPK) pathway plays a crucial role in such activation. beta2GPI is proteolytically cleaved in domain V by activated factor X or plasmin, leading to the generation of the nicked form of beta2GPI. Recently, increasing attention is focused on the role of nicked-beta2GPI as a regulator of extrinsic fibrinolysis pathway.     

Antiphospholipid antibodies in pediatric patients with prolonged activated partial thromboplastin time during infection

OBJECTIVE: To investigate the close association between different antiphospholipid antibodies (aPL) caused by infection and their appearance together with a prolonged activated partial thromboplastin time (aPTT). METHODS: Sera from 122 children were evaluated in this study. Thirty-seven children with mild to medium prolonged aPTT (>37.2s) and elevated C-reactive protein (CRP) levels during various forms of infections (group 2), 18 children without infections (group 3) but with mild to medium prolonged aPTT and 13 children with infections (group 4) and with elevated CRP-level as well as a control group (group 1) of 54 patients without any infection and normal aPTT and negative CRP levels were investigated with commercially available ELISA tests (AESKU.Diagnostics, Wendelsheim, Germany) for the presence of antibodies directed against cardiolipin (CL), phosphatidylserine (PS) and beta2-glycoprotein I (beta 2GPI). The cutoff for positive results was defined with the healthy, aged matched control group (group 1) using the mean OD values plus 2 standard deviations. The lupus anticoagulant (dilute Russell's Viper Venom time, dRVVT) and coagulation Factor XII were determined with routine tests (Dade Behring). RESULTS: Detection of at least one antibody to phospholipids was possible in 89.2% of group 2. It could be shown that IgM anti-beta 2GPI antibodies were found in 27 (59.5%) of group 2, but only in 1 (5.6%) of group 3 (p=0.024) and only in 4 (7.4%) of the controls (p=0.014). The presence of IgG-anti-beta 2GPI antibodies showed no significant difference in the different groups. Furthermore, children of groups 2, 3 and 4 had statistically significant higher levels of antibodies against PS IgG and PS IgM than controls. Also, antibodies to CL of the IgG-type were more frequently detected in children of group 2 than in controls (p=0.038). Detection of CL-IgM antibodies did not reach a significant level in the comparison of the different groups. CONCLUSION: During commonly acquired infections elevation of aPL of nearly all types seems to be a common process. Mild prolongation of aPTT might reflect this presence of aPL in the course of the infectious disease. Our data suggest that there exists no differences in specificity in comparison to the "pathogenic" aPL but the presence over time might be the trigger for the autoimmune activity to begin.     

Abstract 11-20

11. Six different APs and b2-GPI in pregnancy with pathology12. Domain 1-specific anti-b2-GPI antibodies from APS patients contribute to lupus anticoagulant activity13. Nodular regenerative hyperplasia (NRH) of the liver – a manifestation of organ-specific antiphospholipid syndrome?14. Practical and beneficial value of comprehensive testing for anti- phospholipid antibodies (aPL)15. Anti-b2-glycoprotein I antibodies as the sole antibodies: about 11 patients16. Frequency and specificities of antiphospholipid antibodies (aPL) in volunteer blood donors17. Interest of dRVVT in antiphospholipid syndrome18. Carboxyl-variants of cholesteryl-linoleate as a ligand for b2- glycoprotein I and a possible mechanism on autoantibody-mediated atherosclerosis19. Antiphospholipid syndrome (APS) presenting as progressive supranuclear palsy (PSP)20. Autoantibodies to cardiolipin and b2-glycoprotein-I in coronary artery disease     

Isotype distribution and clinical relevance of anti-beta2-glycoprotein I (beta2-GPI) antibodies: importance of IgA isotype.

The aim of this study was to evaluate the prevalence of IgG, IgA and IgM anti-beta2-GPI antibodies in anti-phospholipid syndrome (APS), and to establish the clinical significance of IgA type antibodies compared with the other isotypes. Anti-beta2-GPI antibodies were measured in the sera of 70 patients by solid-phase enzyme immunoassay in gamma-irradiated polystyrene plates coated with human purified beta2-GPI. Thirty-three out of the 70 patients were classified as having APS: three of them had primary, and 30 had secondary APS related to systemic lupus erythematosus (SLE). The remaining 37 patients had SLE without APS. Anti-beta2-GPI antibodies of IgG, IgA and IgM isotypes were present in 84.8%, 59.3% and 51.5% of patients with APS. Both the frequency and the level of each isotype were significantly higher in patients with APS. This association was very strong for IgA (P = 0.0004 for the antibody frequency and P < 0.0001 for the antibody level), as well as for IgG type antibodies (P < 0.0001 and P < 0.0001), whereas it was weaker for IgM (P = 0.01 and P = 0.04). A strong relationship was demonstrated between increased IgA anti-beta2-GPI antibody levels and a history of venous thrombosis, thrombocytopenia, heart valve disease, livedo reticularis and epilepsy. IgG anti-beta2-GPI antibodies were associated with the presence of lupus anticoagulant (LA) in addition to the main features of APS. However, antibodies of IgM isotype were related only to thrombocytopenia and heart valve disease. We recommend the evaluation of anti-beta2-GPI antibodies of IgA isotype in addition to IgG in patients with clinical suspicion of APS.