Autoantibody-mediated atherosclerosis

Beta2-glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies (aPL) present in patients with antiphospholipid syndrome (APS). Oxidized low-density lipoprotein (oxLDL) is subsequently targeted by beta2-GPI and anti-beta2-GPI autoantibodies. Ligands specific for beta2-GPI derived from oxLDL have been characterized as oxidized forms of cholesteryl linoleate, such as 7-ketocholesterol-9-carboxynonanoate, i.e. 9-oxo-9-(7-ketocholest-5-en-3beta-yloxy) nonanoic acid, (namely oxLig-1). The in vitro phenomenon that it is significantly increased in binding of oxLig-1 containing liposomes to macrophages via an interaction with beta2-GPI and an anti-beta2-GPI autoantibody (via the Fcgamma receptor) may propose a novel mechanism on 'autoantibody-mediated atherosclerosis'. Furthermore, autoantibodies against a complex of beta2-GPI and oxLig-1 are detected in sera of APS patients and appearance of the antibodies is associated with episodes of thrombosis, especially, arterial thrombosis. Thus, autoimmune atherogenesis linked to beta2-GPI interaction with oxLDL and autoantibodies may be present in APS.     

Atherogenic autoantigen: oxidized LDL complexes with beta2-glycoprotein I

Beta2-Glycoprotein I (beta2-GPI) is a major antigen for antiphospholipid antibodies present in patients with antiphospholipid syndrome (APS). In 1997, we demonstrated that beta2-GPI specifically binds to Cu2+-oxidized low-density lipoprotein (oxLDL) and that the beta2-GPI-oxLDL complex is subsequently targeted by anti-beta2-GPI antibodies in vitro. Then ligands for beta2-GPI were purified from oxLDL and characterized as omega-carboxylated 7-ketocholesteryl esters, such as 7-ketocholesteryl-9-carboxynonanoate (oxLig-1) and 7-ketocholesteryl-12-carboxy (keto) dodecanoate (oxLig-2). These ligands mediate to form oxLDL-beta2-GPI complexes, and the complexes are taken up avidly by macrophages via anti-beta2-GPI autoantibody-mediated phagocytosis. We recently demonstrated that appearance of autoantibodies against a complex of beta2-GPI and oxLig-1 are highly associated with a history of arterial thrombosis. Serum oxLDL-beta2-GPI complex and their IgG immune complexes are also risk factors arterial thrombosis in APS patients. There is increasing circumstantial evidence of autoimmune mechanism involving beta2-GPI and oxLDL in the atherogenesis in APS.     

Anti-β 2 -Glycoprotein I Autoantibodies and Atherosclerosis

β-Glycoprotein I (β 2 -GPI) is a major antigen for antiphospholipid antibodies (aPL) present in patients with antiphospholipid syndrome (APS). We previously reported that β 2 -GPI specifically binds to oxidized low-density lipoprotein (oxLDL). Further, a ligand specific for β 2 -GPI, oxLig-1, purified from the extracted lipids of oxLDL was identified as 7-ketocholesterol-9-carboxynonanoate (i.e., 9-oxo-9-(7-ketocholest-5-en-3β-yloxy) nonanoic acid) OxLig-1 was recognized by β 2 -GPI and subsequently by anti-&beta 2 -GPI autoantibodies. Binding of liposomes containing oxLig-1 to macrophages were significantly enhanced in the presence of both β 2 -GPI and an anti-β 2 -GPI autoantibody derived from (NZW×BXSB) F1 mouse, an animal APS model, or from APS patients. Anti-β 2 -GPI autoantibodies derived from APS patients with episodes of arterial thrombosis were detected in ELISA, using a solid phase &beta 2 -GPI complex with oxLig-1. It was also reported that LDL-receptor-deficient mice that were fed a chow diet and immunized with β 2 -GPI had an accelerated atherosclerosis and that β 2 -GPI was abundantly expressed within subendothelial regions and intimal-medial borders of human atherosclerotic plaques. All of these observations strongly suggest that autoimmune atherogenesis linked to β 2 -GPI interaction with oxLDL and autoantibodies may be present in APS.     

IgG autoantibodies against beta2-glycoprotein I complexed with a lipid ligand derived from oxidized low-density lipoprotein are associated with arterial thrombosis in antiphospholipid syndrome

We recently reported [J. Lipid Res. 42 (2001), 697; 43 (2002), 1486; 44 (2003), 716] that [beta2-glycoprotein I (beta2GPI) forms complexes with oxidized LDL (oxLDL) and autoantibodies against these complexes are present in patients with SLE and antiphospholipid syndrome (APS). The relationship of beta2GPI/oxLDL complexes and IgG autoantibodies against beta2GPI complexed with oxLig-1 (an oxLDL-derived ligand) with clinical manifestations of APS was studied in 150 APS and SLE patients. The beta2GPI/oxLDL levels of APS patients were similar to those of SLE patients without APS, but they were significantly higher than healthy individuals. There was no difference in the complex levels among the patients with arterial, venous thrombosis, or pregnancy morbidity. IgG anti-beta2GPI/oxLig-1 levels of APS were significantly higher than those of SLE without APS and healthy individuals. Further, antibody levels of APS patients with arterial thrombosis were significantly higher than those patients with venous thrombosis and pregnancy morbidity. Thus, oxidation of LDL leads the complex formation with beta2GPI in SLE and APS patients. In contrast, anti-beta2GPI/oxLig-1 autoantibodies were generated only in APS and were strongly associated with arterial thrombosis. These results suggest that autoantibodies against beta2GPI/oxLDL complexes are etiologically important in the development of atherosclerosis in APS.     

Oxidized low-density lipoprotein/beta2-glycoprotein I complexes and autoantibodies to oxLig-1/beta2-glycoprotein I in patients with systemic lupus erythematosus and antiphospholipid syndrome

Oxidized low-density lipoprotein (oxLDL) interacts with beta2-glycoprotein I (beta2-GPI) via oxLDL-derived specific ligands (oxLig-1) forming complexes. The prevalence and significance of oxLDL/alpha2-GPI complexes and antibodies to oxLig-1/alpha2-GPI were evaluated in patients with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). The oxLDL/beta2-GPI complex was 69% positive (above mean + 3 SD of control subjects) in 97 consecutive patients with SLE, 62% in 40 patients with SLE with secondary APS, and 60% in 50 control patients with SLE without APS. IgG anti-oxLig-1/beta2-GPI antibody was positive in 31 (32%) of 97 consecutive patients with SLE, in 26 (65%) of 40 patients with SLE with secondary APS, and in 6 (19%) of 32 control patients with SLE. Anti-oxLig-1/beta2-GPI antibodies were 93.7% specific with a positive predictive value of 90.0% for APS, better than anticardiolipin antibodies (80.0% specific, 71.4% predictive value). These results confirm that oxLDL/beta2-GPI complexes are common in SLE and suggest a possible immunogenic role in APS. In contrast, IgG anti-oxLig-1/beta2-GPI antibodies not only are associated with but also are clinically useful risk factors for APS.     

Patients with atherosclerotic syndrome, negative in anti-cardiolipin assays, make IgA autoantibodies that preferentially target domain 4 of beta2-GPI

Autoantibodies targeting beta2-glycoprotein l (beta2-GPI), a component of the atherosclerotic plaque, are commonly found in patients with acute ischemic syndromes. Serum samples from APS (antiphospholipid syndrome) patients and from cardiovascular patients exhibiting acute atherosclerotic syndromes were analyzed for IgG and IgA antibodies in both anti-beta2-GPI and anticardiolipin (aCL) ELISA assays. All of the APS samples used here were positive in both assays. Serum samples from 382 atherosclerosis patients were also analyzed for IgG and IgA antibodies in the same assays. In sharp contrast to the APS samples, we found that only 1% of the samples from atherosclerosis patients were positive for IgA aCL, and 1.6% positive for IgG aCL, whereas 35.6% were positive for IgA anti-beta2-GPI and only 1.6% for IgG anti-beta2-GPI. The antigenic specificity of 29 serum samples from atherosclerosis patients was evaluated. Six different recombinant domain-deleted mutants (DM) of human beta2-GPI and full-length human beta2-GPI (wild-type) were used in competitive inhibition assays to inhibit the autoantibodies from binding in the anti-beta2-GPI ELISA assays. Domain-deleted mutants D--345 and D--45 inhibited the binding in the IgA anti-beta2-GPI assay, suggesting that these autoantibodies recognize domain 4 of the beta2-GPI molecule. These results clearly show that IgA anti-beta2-GPI autoantibodies from atherosclerotic patients are distinct from IgA autoantibodies found in APS samples.     

Proteolytic cleavage of beta(2)-glycoprotein I: reduction of antigenicity and the structural relationship

Binding of beta(2)-glycoprotein I (beta(2)-GPI)-dependent anticardiolipin antibodies (aCL) derived from antiphospholipid syndrome (APS) is significantly reduced in aCL ELISA due to loss of the phospholipid (PL) binding property of beta(2)-GPI by plasmin treatment. In the present study, the treatment generated a nicked form of beta(2)-GPI and resulted in loss of antigenicity for the autoantibodies detected in ELISA, using an beta(2)-GPI directly adsorbed polyoxygenated carboxylated plate, the assay system of which was not related to PL binding. The nicked form bound to neither Cu(2+)-oxidized low-density lipoprotein (oxLDL) nor to beta(2)-GPI-specific lipid ligands isolated from oxLDL, the result being a complete loss of subsequent binding of anti-beta(2)-GPI autoantibodies. The conformational change in the nicked domain V was predicted from its intact structure determined by an X-ray analysis (implemented in Protein Data Bank: 1C1Z), molecular modeling and epitope mapping of a monoclonal anti-beta(2)-GPI antibody, i.e. Cof-18, which recognizes the related structure. The analysis revealed that novel hydrophobic and electrostatic interactions appeared in domain V after the cleavage, thereby affecting the PL binding of beta(2)-GPI. Such a conformational change may have important implications for exposure of cryptic epitopes located in the domains such as domain IV.     

Recombinant Domain V of β2-Glycoprotein I Inhibits the Formation of Atherogenic oxLDL/β2-Glycoprotein I Complexes

β2-glycoprotein I (β2-GPI) is a plasma protein that interacts with oxidized low-density lipoproteins (oxLDL) via β2-GPI domain V to form oxLDL/β2-GPI complexes, potential autoantigens promoting atherogenesis in patients with antiphospholipid syndrome (APS). Such a interaction would expose β2-GPI domain I or/and IV, structures recognized by anti-β2-GPI autoantibodies. IgG immune complexes with oxLDL/β2-GPI complexes can interact with macrophages via Fcγ receptor, causing oxLDL/β2-GPI endocytosis and foam cell formation, contributing to atherosclerosis. Here, we use recombinant domain V to study the interaction between oxLDL and β2-GPI and hypothesized that domain V would interfere with this interaction thereby reducing oxLDL macrophage uptake and foam cell formation. The β2-GPI domain V sequence was expressed by using the Pichia pastoris expression system to obtain recombinant domain V of β2-GPI (P.rβ2-GPI DV). ELISA tests demonstrated that P.rβ2-GPI DV interacted with oxLDL via 7-ketocholesteryl-9-carboxynonanoate (oxLig-1), a negatively charged lipid moiety of oxLDL. The ω-carboxyl residue of oxLig-1 is required for the interaction. Serologic tests showed a significant increase in oxLDL and oxLDL/β2-GPI levels in patients with APS (p?P.rβ2-GPI DV was able to bind oxLDL in high affinity and competitively inhibited native β2-GPI (nβ2-GPI) binding to free oxLDL as well as to oxLDL from the oxLDL/β2-GPI complexes. These observations suggest that P.rβ2-GPI DV may be used to inhibit the formation of the oxLDL/β2-GPI complexes, a potential approach for reducing foam cell development and mitigating atherogenesis in patients with APS. The present work provides a new effective strategy to prevent the progression of atherothrombotic vascular complications in APS patients.     

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.     

Epitope studies with anti-beta 2-glycoprotein I antibodies from autoantibody and immunized sources

This paper examines the methodology of anti-beta(2)-glycoprotein I (beta(2)-GPI) epitope determination and provides further epitope studies using human sera containing anti-beta(2)-GPI autoantibodies. Studies in this field may be misleading as the antigen coating density using mutant forms of beta(2)-GPI may be below the threshold required for monogamous divalent binding by low affinity anti-beta(2)-GPI autoantibodies, while being easily detected by high affinity anti-beta(2)-GPI from immunized animals. The antigen density threshold effect is found in anti-beta(2)-GPI autoantibodies from humans and from monoclonal anti-beta(2)-GPI derived from mice with models of autoimmune disease. Anti-beta(2)-GPI from an autoimmune mouse and from 18/21 human sera did not bind above background levels to a domain-I-deleted mutant. In addition, single point mutations in domain I result in dramatic changes in the binding of many human sera containing anti-beta(2)-GPI. These findings support a conclusion that domain I of beta(2)-GPI contains significant epitopes for the anti-beta(2)-GPI antibodies found in the antiphospholipid syndrome.     

beta2-glycoprotein I, the playmaker of the antiphospholipid syndrome

From its discovery in the early 60s till the beginning of the 90s, there was not much interest in plasma protein beta2-glycoprotein I (beta2-GPI). The finding that beta2-GPI acts as an essential cofactor for the detection of antiphospholipid antibodies (aPL) tremendously increased the interest in beta2-GPI [Lancet 335 (1990) 1544; Lancet 336 (1990) 177; Proc. Natl. Acad. Sci. U. S. A. 87 (1990) 4120]. It is now generally accepted that autoantibodies directed towards beta2-GPI are not only a serological marker but that they are involved in the pathology of the antiphospholipid syndrome (APS). In this review, we will first discuss the biochemistry of the protein beta2-GPI and the influence that the antibodies have on the function of beta2-GPI. Next, we will discuss the problems that are faced when assays to detect the presence of the autoantibodies are performed, emphasizing the urgent need for standardization of the anti-beta2-GPI-ELISA. Finally, we will discuss our latest insights into beta2-GPI and its role in the pathology of APS. Thereby, we will focus on the role of dimerized beta2-GPI on platelet and endothelial cell function.     

Avidity of anti-beta-2-glycoprotein I antibodies

The terms affinity and avidity are often used indiscriminately, despite clearly differing. Since affinity refers to monovalent binding of antibodies to a monovalent epitope, the majority of data on the binding of anti-beta2-glycoprotein I antibodies (anti-beta2-GPI) characterized their avidity rather than affinity. Anti-beta2-GPI were generally believed to be of low avidity, but heterogeneous avidity of patients' IgG anti-beta2-GPI has been demonstrated. High avidity anti-beta2-GPI monoclonals were reported to possess higher pathogenicity than low avidity anti-beta2-GPI. Polyclonal high avidity anti-beta2-GPI were found to be more common in patients with antiphospholipid syndrome (APS) and associated with thrombosis. Some conformational changes of beta2-GPI are required for the binding of polyclonal anti-beta2-GPI to the antigen: neither high density of the antigen nor high avidity of the anti-beta2-GPI alone is sufficient for the recognition. Avidity of anti-beta2-GPI should be considered in any attempt of inter-laboratory standardisation and/or evaluation of anti-beta2-GPI enzyme-linked immunosorbent assay (ELISA).     

Antibodies against beta2-glycoprotein I complexed with an oxidised lipoprotein relate to intima thickening of carotid arteries in primary antiphospholipid syndrome

To explore whether antibodies against beta2-glycoprotein I (beta2GPI) complexed to 7-ketocholesteryl-9-carboxynonanoate (oxLig-1) and to oxidised low-density lipoproteins (oxLDL) relate to paraoxonase activity (PONa) and/or intima media thickness (IMT) of carotid arteries in primary antiphospholipid syndrome (PAPS). As many as 29 thrombotic patients with PAPS, 10 subjects with idiopathic antiphospholipid antibodies (aPL) without thrombosis, 17 thrombotic patients with inherited thrombophilia and 23 healthy controls were investigated. The following were measured in all participants: beta2GPI-oxLDL complexes, IgG anti-beta2GPI-oxLig-1, IgG anti-beta2GPI-oxLDL antibodies (ELISA), PONa, (para-nitrophenol method), IMT of common carotid (CC) artery, carotid bifurcation (B), internal carotid (IC) by high resolution sonography. Beta2GPI-oxLDL complex was highest in the control group (p < 0.01), whereas, IgG anti-beta2GPI-oxLig1 and IgG anti-beta2GPI-oxLDL were highest in PAPS (p < 0.0001). In healthy controls, beta2GPI-oxLDL complexes positively correlated to IMT of the IC (p = 0.007) and negatively to PONa after correction for age (p < 0.03). PONa inversely correlated with age (p = 0.008). In PAPS, IgG anti-beta2GPI-oxLig-1 independently predicted PONa (p = 0.02) and IMT of B (p = 0.003), CC, (p = 0.03) and of IC (p = 0.04). In PAPS, PONa inversely correlated to the IMT of B, CC and IC (p = 0.01, 0.02 and 0.003, respectively). IgG anti-beta2GPI-oxLig-1 may be involved in PAPS related atherogenesis via decreased PON activity.     

Pathogenic roles of anti-beta2-GPI antibody in patients with antiphospholipid syndrome]

Antiphospholipid syndrome (APS) is characterized by arterial and/or venous thrombosis and pregnancy morbidity in the presence of antiphospholipid antibodies (aPL). Beta2-glycoprotein I (beta2-GPI) and prothrombin are representative autoantigens, the former more extensively investigated. Anti-beta2-GPI antibodies are not only markers of APS, but also are considered to be pathogenic. Possible roles of anti-beta2-GPI antibodies are, 1) enhancement the binding of beta2-GPI to anionic phospholipid and inhibition of protein C activation/activated protein C, 2) to form anti-beta2-GPI antibody-beta2-GPI-oxidized LDL complex and to promote uptake by sub-endothelial macrophage, resulting in atherosclerosis, 3) to dimerize beta2-GPI on the surface of platelets and to activate platelets via apoE receptor 2 and subsequent signal transduction, 4) stimulation of monocytes via p38 MAP kinase pathway and induction of tissue factor production. In pregnancy morbidity, activation of complement cascade plays an important role. These findings may provide a novel target in the management of APS.     

Anti-beta(2)-glycoprotein I autoantibody expression as a potential biomarker for strokes in patients with anti-phospholipid syndrome

Anti-phospholipid syndrome (APS) is an autoimmune disease. Cerebral ischemia associated with APS occurs at a younger age than typical atherothrombotic cerebrovascular disease, is often recurrent, and is associated with high positive IgG anti-phospholipid (GPL) unit levels. This study sought to determine the frequency rates of anti-cardiolipin (aCL) dependent on the presence of beta(2)-GPI, anti-beta(2)-glycoprotein I (abeta(2)-GPI), and anti-phosphatidyl serine (aPS) IgG autoantibodies among stroke patients, and thus demonstrate the importance of testing for abeta(2)-GPI autoantibodies. For these study, stroke patients and control subjects recruited from Mosul, Erbil, and Dohuk provinces in Northeren Iraq between March 2004 and March 2005 were evaluated. All cases were under 50 years-of-age and had no recognizable risk factors. Using ELISA to evaluate the presence of IgG isotype of aCL, abeta(2)-GPI, and aPS autoantibodies in their blood, the results indicated that the frequency of abeta(2)-GPI was 14/50 (28%), aCL was 11/50 (22%), and aPS was 9/50 (18%) among stroke patients. In contrast, aCL was detected in 2/30 (6.7%) of control subjects; each of the other anti-phospholipid antibodies (APLA) was never observed. Of all the abeta(2)-GPI(+) cases, the incidence of stroke patients having the combined profile of abeta(2)-GPI + aCL was 11/14 (78.6%) and of abeta(2)-GPI + aPS was 9/14 (64.3%). Only 2/14 (14.3%) of these abeta(2)-GPI(+) patients also expressed aCL in the absence of aPS. The frequency of patients expressing all three markers was only 9/14 (64.3 %). In none of the APS/stroke patients were aCL or aPS expressed in the absence of the abeta(2)-GPI. Conversely, abeta(2)-GPI as a sole marker was seen in 3/14 (21.4%) of these patients (i.e., in absence of either other marker). It can be concluded from these studies that the among the three major forms of APLA examined, the presence of abeta(2)-GPI IgG autoantibodies appeared to correlate best with stroke in patients who were concurrently suffering APS.     

Intracellular trafficking of beta2-glycoprotein I complexes with lipid vesicles in macrophages: implications on the development of antiphospholipid syndrome

Beta(2)-glycoprotein I (beta(2)GPI) is known as a major autoantigen for antiphospholipid antibodies. Our recent data show that binding of beta(2)GPI to oxidized low-density lipoprotein (oxLDL) or to liposomes containing anionic phospholipid(s) may facilitate the presentation of beta(2)GPI's epitope by macrophages/dendritic cells to autoreactive T cells. In the present study, we investigated intracellular trafficking of beta(2)GPI and its complexes with oxLDL or liposomes containing phosphatidylserine (PS-liposomes) in mouse macrophage-like J774 cells. A relatively small amount of non-complexed beta(2)GPI was taken up and stagnated in the late endosome after incubating for 16h. In contrast, beta(2)GPI complexes with oxLDL or PS-liposomes were transported into the lysosome. In the presence of the IgG anti-beta(2)GPI autoantibody, WB-CAL-1, beta(2)GPI/oxLDL complexes were rapidly incorporated into intracellular space and were finally localized in the lysosome. Interestingly, in vitro pulses by beta(2)GPI/oxLDL complexes together with WB-CAL-1 led to the expression of membranous CD36 as well as Fcgamma type I receptors (FcgammaRI). These observations suggest that IgG immune complexes of beta(2)GPI/oxLDL provide not only FcgammaRI- but also scavenger receptor-mediated uptake of beta(2)GPI/oxLDL complexes by macrophages. Thus, beta(2)GPI/oxLDL complexes as a major atherogenic autoantigen and IgG anti-beta(2)GPI autoantibodies may facilitate antigen presentation and foam cell formation in antiphospholipid syndrome.     

Which are the best biological markers of the antiphospholipid syndrome?

The diagnosis of antiphospholipid syndrome (APS) requires the presence of both clinical and biological features. Due to the heterogeneity of anti-phospholipid antibodies (aPL) the laboratory approach for their detection includes clotting-based tests for lupus anticoagulant (LA) as well as solid-phase assays for anticardiolipin antibodies (aCL). In addition, as it has been shown that autoimmune aPL recognize epitopes on phospholipid (PL)-binding plasma proteins, assays detecting antibodies to beta 2-glycoprotein I (beta 2-GPI) or prothrombin have been developed. The association between venous or arterial thrombosis and recurrent fetal loss with the presence of conventional aPL (LA and/or aCL) has been confirmed by many studies. The LA and IgG aCL at moderate/high titre seem to exhibit the strongest association with clinical manifestations of the APS. Several reports indicate that LA is less sensitive but more specific than aCL for the APS. Assays against PLs other than CL as well as the use of mixtures of PLs have been proposed to improve the detection of APS-related aPL. Concerning antibodies to PL-binding proteins (detected in the absence of PLs), there is evidence that anti-beta 2-GPI are closely associated with thrombosis and other clinical features of the APS. Moreover, these antibodies may be more specific in the recognition of the APS and in some cases may be present in the absence of aPL detected by standard tests. Many issues are still under debate and are discussed in this review, such as the problems of standardization of anti-beta 2-GPI assays, detection of the IgA isotype of aCL and anti-beta 2-GPI, the coagulation profiles of LA in the recognition of the thrombotic risk and the association of particular markers with subsets of patients with APS.     

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.     

Antiphospholipid antibodies: effects on trophoblast and endothelial cells

PROBLEM: Antiphospholipid syndrome (APS) may affect placental functions through several possible mechanisms. Interaction of antiphospholipid antibodies (aPL) with cells involved in the coagulation cascade is thought to produce a procoagulant state. Thrombotic placental pathology is however not specific for the APS. METHOD OF STUDY: An analysis of published data. RESULTS: It is now generally accepted that the clinically relevant aPL bind to proteins with affinity for phospholipids (PL), such as beta2-glycoprotein I (beta2-GPI). Following the attachment of beta2-GPI to trophoblast anionic PL, both molecules undergo conformational changes resulting in the exposure of cryptic epitopes within the structure of beta2-GPI. This may allow the subsequent binding of antibodies hence affecting trophoblast functions directly. Moreover anti-beta2-GPI antibodies induce the activation of endothelial cells (ECs), resulting in a proinflammatory state which favours the prothrombotic diathesis of the syndrome. CONCLUSION: Numerous ameliorations in the APS knowledge have been introduced in the last few years. To have clarified the mechanism of antibody mediated damage on trophoblast and ECs represents an important step to explain the cellular events leading to pregnancy complications.     

Antiphospholipid Syndrome Infectious Origin

Antiphospholipid syndrome (APS) is characterized by the presence of pathogenic autoantibodies against beta 2-glycoprotein-I (beta 2GPI). The factors causing production of anti-beta 2GPI remain unidentified, but an association with infectious agents has been reported. Studies on experimental APS models proved that molecular mimicry between beta 2GPI-related synthetic peptides and structures within bacteria, viruses, tetanus toxoid, and CMV are a cause for experimental APS. Any explanation of how microbial infections might set off APS must take into account the observation that all individuals appear to harbor potentially autoreactive lymphocytes, as well as natural antiphospholipid antibodies, but that these cells or antibodies remain innocuous unless somehow activated. Herein, we discuss the association of antiphospholipid antibodies in the infectious state, molecular mimicry as a proposed cause for development of APS, and the contribution of the database to this topic.