Correspondence *Department of Plasma Proteins/Blood coagulation, Sanquin Research/Diagnostic Services, Plesmanlaan 125, 1066CX Amsterdam, The Netherlands

Email b.delaat@sanquin.nl

Introduction

The antiphospholipid syndrome (APS) is an autoimmune disease that is characterized clinically by vascular thrombosis and pregnancy morbidity, and serologically by the presence of antiphospholipid antibodies in the plasma of patients.¹ APS is highly related to other autoimmune diseases, especially systemic lupus erythematosus (SLE). Approximately 20–35% of patients with SLE fulfill the criteria for APS. Although the clinical criteria that characterize the disease occur frequently, the incidence of APS is low. The importance of the presence of antiphospholipid antibodies for the confirmation of the syndrome highlights the need for highly specific diagnostic assays to detect these antibodies. At present, several assays, such as phospholipid-dependent coagulation assays, anti-₂-glycoprotein I (₂ GPI)-enzyme-linked immunosorbent assay (ELISA)-based assays and anticardiolipin ELISA-based assays, show a variable, but mediocre, correlation with thrombosis when studied in prospective cohort studies. The consequence of the low specificity of the assays is the suboptimal treatment of patients.^{2, 3} This Review outlines newly developed assays for the detection of antiphospholipid antibodies, the correlation of these assays with antiphospholipid-related clinical symptoms, and pathogenetic mechanisms that could explain the occurrence of these symptoms.

History of APS

Over the years, APS has developed from an incomprehensible correlation between a prothrombotic phenotype and an in vitro assay for the detection of a bleeding tendency (lupus anticoagulant [LA]) to a well-recognized syndrome (Figure 1). A commonly used assay to detect the presence of antiphospholipid antibodies has its origin in 1906, when Wasserman described a method to detect syphilis.⁴ Pangborn found that the antigen that was recognized by reagin (antibodies produced by syphilis patients) was a phospholipid that could be extracted from beef heart, which is now known to be cardiolipin.⁵ During that time it became evident that some patients who tested positive for reagin did not suffer from syphilis but occasionally displayed a prothrombotic phenotype.^{6, 7} This observation was the basis for the development of the anticardiolipin antibody ELISA that is still used to detect patients with APS.

Figure 1 Chronology of significant events in research into antiphospholipid syndrome.

 

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Almost 50 years after the introduction of the reagin assay, the existence of what was subsequently called LA was observed by Conley and Hartman.⁸ They described two patients with SLE whose plasma contained a unique coagulation inhibitor; this inhibitor prolonged whole-blood clotting time and prothrombin time, even when the plasma was diluted with normal pool plasma. As this inhibitor was predominantly found in patients with SLE, the in vitro anticoagulant phenomenon was called lupus anticoagulant. Despite the fact that APS is mainly known for its correlation between antiphospholipid antibodies and thrombosis, pregnancy morbidity was the first clinical manifestation that was reported to be associated with LA.⁹ The correlation between vascular thrombosis and LA was first described in 1963 by Bowie et al. and was followed by many others.¹⁰ However, it took until the early 1990s to discover that antiphospholipid antibodies did not recognize phospholipids directly but indirectly via phospholipid-binding plasma proteins.^{11, 12, 13, 14} Although many plasma proteins have been found to be involved in APS, antibodies against ₂ GPI are thought to be the antibodies with most clinical significance.¹⁵

Diagnosing patients with APS

The clinical criteria for diagnosing a patient with APS are clear and can be established objectively. By contrast, the laboratory criteria initially seem to be clear, but in practice the assays that are defined to measure the presence of antiphospholipid antibodies in plasma are controversial, and their outcome depends on the laboratory in which the assay is performed. Despite the many attempts to increase the specificity of the laboratory criteria and the establishment of consensus criteria for the serology (Box 1), a high number of patients are still inaccurately diagnosed when the revised Sapporo classification criteria for APS are followed.^{16, 17} According to these criteria for APS, fulfillment of at least one clinical and one laboratory criterion are required.

The anticardiolipin ELISA is generally considered to have high sensitivity but low specificity. The aspecificity of this assay can be explained by the fact that infections and drugs sometimes coincide with the presence of anticardiolipin antibodies.^{18, 19, 20} These infection-related anticardiolipin antibodies bind to cardiolipin directly, independently of plasma proteins, and their presence is not correlated with a prothrombotic phenotype. The high sensitivity of the anticardiolipin antibody ELISA for infection-related antibodies generates significant numbers of false-positive results, which makes the assay unsuitable for the detection of patients at risk of thrombosis. This unsuitability was recently confirmed by a meta-analysis by Galli et al.,³ who showed that antiphospholipid antibodies detected with an anticardiolipin ELISA did not correlate significantly with thrombosis in general. These authors also reported that the LA assay (outlined above) correlates best with thrombosis.³ Surprisingly, the latest addition to the diagnostic criteria, the anti-₂ GPI ELISA, failed to show a significant correlation with thrombosis.² This finding was unexpected, because ₂ GPI is thought to be the most clinically important antigen in APS.

The major reason for the poor correlation of anti-₂ GPI antibodies with clinical symptoms is probably the lack of standardization of the anti-₂ GPI antibody ELISA. Despite numerous attempts to standardize the anticardiolipin antibody ELISA and the anti-₂ GPI antibody ELISA, large differences are still found when assays from different manufacturers are used. Different types of plate, different sources of ₂ GPI and the lack of an international calibrator are thought to be the main causes of the discrepancies. However, the lack of correlation with thrombosis could also be explained by the observation that not all antibodies against ₂ GPI are pathologic; only antibodies that target a specific domain of the protein correlate with thrombosis.²¹ The anti-₂ GPI antibody ELISA detects all antibodies that are reactive against ₂ GPI, which makes it less specific, and probably not very suitable as a general diagnostic test.

Although anti-₂ GPI antibodies seem to be important in the correlation with thrombosis, this is not the case for antiphospholipid-related pregnancy morbidity. A meta-analysis by Opatrny et al. reports that LA best correlated with recurrent fetal loss, as it does with thrombosis, but that there was no correlation between pregnancy morbidity and anti-₂ GPI antibodies.²² Anticardiolipin antibodies did show a significant correlation with recurrent fetal loss, but not with thrombosis. This finding might indicate a different pathogenetic mechanism for thrombosis and pregnancy morbidity.

Antiphospholipid antibodies and thrombosis

Lupus anticoagulant and thrombosis

As a result of the poor specificity of all assays used to diagnose patients with APS, there are conflicting opinions on which antibodies should be measured to detect patients at risk of thrombosis. As mentioned above, the LA assay is generally accepted as the assay that correlates best with thrombosis,^{2, 3} possibly because this assay measures functional activity (prolongation of phospholipid-dependent coagulation) instead of simply physically recognizing a protein coated to an ELISA plate. Nevertheless, LA can be caused by antibodies with different reactivity (to ₂ GPI, prothrombin, annexin A5, etc.). Because it is generally accepted that ₂ GPI is the main antigen in APS, an assay was developed to discriminate between LA caused by anti-₂ GPI antibodies and LA caused by antibodies with other specificity.²³ The addition of cardiolipin to an assay based on the activated partial thromboplastin time (a measure of coagulation) resulted in a correction (a decrease) of the clotting time when LA was caused by anti-₂ GPI antibodies but not by antibodies with other specificity. By using this discriminating power, we have found, in a cohort of 198 patients, almost all of whom were diagnosed with SLE, that the odds ratio for vascular thrombosis from a positive assay result increased from 10.2 for the classic LA assay and 6.8 for the classic anti-₂ GPI antibody ELISA to 42.3 for LA caused by anti-₂ GPI antibody activity. Pengo et al. also showed that the combination of positive LA with a positive anti-₂ GPI antibody ELISA dramatically increased the correlation with thrombosis compared with the two separate assays.²⁴ Several other methods have been described that discriminate between a ₂ GPI-dependent LA and a ₂ GPI-independent LA.^{25, 26} We can now say that patients at high risk of thrombosis have circulating antibodies in their plasma that recognize ₂ GPI and cause LA (Figure 2). These antibodies can be detected using a combination of a ₂ GPI antibody ELISA and an LA assay. Whether this combination identifies all patients at high risk is not yet known.

Figure 2 Correlation between antibody specificity and thrombosis.

The graph shows the odds ratios of antibodies used in regular assays to detect thrombosis (LA, anti-₂ GPI ELISA, antiprothrombin antibody, anticardiolipin antibody ELISA) compared with more recently developed assays (anti-domain I ELISA, ₂ GPI-dependent LA).^{21, 23} The higher the odds ratio, the more likely the indication of thrombosis. Abbreviations: Abs, antibodies; aPTT, activated partial thromboplastin time; ₂ GPI, ₂-glycoprotein I; dRVVT, dilute Russell viper venom time; ELISA, enzyme-linked immunosorbent assay; LA, lupus anticoagulant.

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Anti-domain I antibodies and thrombosis

Since the discovery of ₂ GPI as major antigen for the antiphospholipid antibodies, many groups have investigated the epitope specificity of anti-₂ GPI antibodies. Anti-₂ GPI antibodies form a rather heterogeneous group of antibodies; some patients with these antibodies suffer from thrombosis whereas others do not. This heterogeneity is also highlighted by the varied presentation of clinical symptoms seen in patients with APS.²⁷ As outlined in Table 1, all domains of ₂ GPI have been described to be targeted by antiphospholipid antibodies, but most studies (especially regarding clinical significance) point to antibodies to domain I as being important in APS.^{19, 28, 29, 30, 31, 32, 33, 34, 35} The ₂ GPI plasma protein consists of 326 amino acids that are arranged into five short consensus repeats. Domain V is different to the other four domains as it consists of 84 amino acids, instead of approximately 60. These extra amino acids form a large hydrophilic loop through which the domain binds to anionic phospholipids. The surface of domain I also contains a hydrophilic patch, which is described to be highly immunogenic, as most antibodies bind to it.²¹ Furthermore, in a population of 198 patients suffering from a variety of autoimmune diseases, antibodies that recognized domain I of ₂ GPI were detected most commonly in those patients with a history of thrombosis.²¹ Antibodies that recognized other parts of ₂ GPI (mostly domain V) were predominantly found in patients with no history of thrombosis. By discriminating between these antibodies, we could increase the odds ratio for thrombosis from 6.7 for a positive classic anti-₂ GPI antibody ELISA to 18.9 for the anti-domain I ELISA. In the same study, only antibodies with affinity towards domain I showed LA. To investigate whether the anti-domain I ELISA would be a legitimate addition to the diagnostic criteria for APS, we have recently initiated a European study in which several centers send in samples to test for domain specificity.

Table 1 Heterogeneity of anti-₂ glycoprotein I antibodies.

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Pathogenetic actions of LA-causing anti-₂ GPI antibodies

As a result of the differences in opinion regarding assay choice for detecting patients at risk of thrombosis, different groups have used different populations of antiphospholipid antibodies to study the pathogenesis of APS. This variety of approaches has led to numerous theories on how antiphospholipid antibodies could induce clinical symptoms related to the syndrome.³⁶ As mentioned, we have found that LA caused by antiphospholipid antibodies with an affinity for domain I of ₂ GPI correlates highly with thrombosis.^{21, 23} To gain more insight into the pathogenesis of antiphospholipid antibodies, we have focused on this specific subpopulation of antibodies.

Several mechanisms have been proposed to explain the pathophysiology of antiphospholipid antibodies. An increased resistance against annexin A5 is one of them. Rand et al. first reported that antiphospholipid antibodies conferred increased resistance to the anticoagulant capacity of annexin A5.³⁷ It is thought that annexin A5 circulates at low concentrations in the blood until platelets or other cells become activated, at which point annexin A5 binds to the exposed negatively charged phospholipids on the surface of these cells and thereby prevents coagulation factors from binding and becoming activated. Rand et al. hypothesized that antiphospholipid antibodies could cause thrombosis by competing with the binding of annexin A5 to the cells and thereby reducing its anticoagulant properties. On the basis of this hypothesis, Rand et al. have developed an assay to detect what they have called 'increased resistance against annexin A5' in plasma and tested several populations of patients with antiphospholipid antibodies.³⁸ An increased resistance to annexin A5 was common among patients who were positive for anti-₂ GPI antibodies, but there was no clear correlation with the incidence of thrombosis in these patients.

On testing our patient population for the presence of increased annexin A5 resistance, anti-₂ GPI domain I antibodies conferred increased resistance to the anticoagulant properties of annexin A5.³⁹ Plasma that contained antibodies with other specificities did not confer increased resistance to annexin A5. An important role for annexin A5 in hemostasis is somewhat controversial owing to its low concentration in plasma and the absence of a detectable phenotype in annexin A5^-/- mice; however, this is the first time that a strong correlation has been found between an assay based on a proposed pathogenetic mechanism and a subpopulation of antibodies that are detected in patients, almost all of whom have a history of thrombosis. Rand and colleagues have also shown that resistance to annexin A5 might be one of the mechanisms that is responsible for recurrent pregnancy morbidity.²² Although this result was not shown by the previously mentioned meta-analysis of Opatrny et al., perhaps one subpopulation of antibodies is responsible for both recurrent fetal loss and thrombosis.²²

An increased resistance to protein C of antiphospholipid antibodies, independent of factor V Leiden (a factor known to cause hypercoagulation), has also been postulated to explain APS.^{40, 41} Several groups have reported that antiphospholipid antibodies can inhibit the anticoagulant properties of protein C either by competing for phospholipid-binding spaces on the surface of platelets or by disruption of the activated protein C (APC) complex. Increased APC resistance has been described for anti-₂ GPI antibodies as well as for antiphospholipid antibodies with reactivity to other cofactors. Resistance to APC correlates highly with venous thrombosis and, to a lesser extent, with arterial thrombosis, irrespective of the presence of factor V Leiden. Our studies showed that a ₂ GPI-dependent LA, and, thereby, the presence of antibodies against domain I, showed an increased correlation with the occurrence of venous thrombosis compared to classic LA, but that the correlation with arterial thrombosis was comparable to that of classic LA. These observations prompted the study of the correlation between increased resistance to APC and ₂ GPI-dependent LA. Indeed, our data show that only patients that display a ₂ GPI-dependent LA showed an increased resistance to the anticoagulant properties of protein C.⁴² In addition, the level of increased APC resistance correlated with the potency of the ₂ GPI-dependent LA.

As ₂ GPI-dependent LA (and hence anti-domain I antibodies) causes increased resistance to annexin A5 and APC, it could be hypothesized that the clinical symptoms of APS are caused by different pathogenetic mechanisms rather than by one mechanism (Figure 3). Research over the past few years has been focused on cellular involvement in APS. Several receptors have been proposed to be involved in the activation of platelets, endothelial cells and monocytes. LDL receptors might have a crucial role in antiphospholipid-related thrombosis, as it was shown that antibody-bound ₂ GPI could bind and activate platelets through LDL receptor-related protein.⁴³ Another receptor on platelets, glycoprotein Ib, might also be involved in this process, as was shown by Pennings et al.⁴⁴ Annexin A2 and Toll-like receptors 2 and 4 have been described to be involved in the activation of endothelial cells by anti-₂ GPI antibodies; annexin A2 was also proposed to have a role in the antiphospholipid-mediated activation of monocytes.⁴⁵

Figure 3 Possible pathogenetic mechanisms of anti-domain I antibodies.

Antiphospholipid antibodies directed against domain I of ₂ GPI seem to test positive in assays that relate to different pathologic mechanisms, which implies that thrombosis (related to anti-domain I antibodies) in antiphospholipid syndrome is caused by different pathologic mechanisms. Abbreviations: APC, activated protein C; ₂ GPI, ₂-glycoprotein I; vWF, von Willebrand factor.

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This list of examples is far from complete, and the future will tell us which mechanism (or mechanisms) is important in antiphospholipid antibody-induced thrombosis and pregnancy morbidity in APS. The diversity of possible receptors and different mechanisms urges the need for investigation of anti-₂ GPI domain I antibodies (as these are almost uniformly correlated with thrombosis) with respect to different mechanisms of disease as stated above. Although anti-₂ GPI domain I antibodies are highly correlated with venous thrombosis, it is possible that other pathogenetic populations of antiphospholipid antibodies will also be detected.

Conclusion and future directions

APS is still seen as a rather obscure disease despite extensive research; this view is mainly the result of the unreliability of the current assays for detecting the presence of antiphospholipid antibodies. The consequences are a poor correlation between serological markers and clinical manifestations, and a lack of clarity about the pathogenetic mechanism causing the syndrome. As mentioned above, we found that the presence of LA-inducing anti-₂ GPI antibodies (with an affinity for domain I) correlates almost uniformly with the occurrence of thrombosis.^{21, 23} Our next aim is to investigate whether the presence of anti-domain I antibodies correlates with recurrent fetal loss.

Research focused specifically on anti-domain I antibodies will hopefully provide information about the mechanism that causes APS. We have found that these antibodies cause increased (factor V Leiden-independent) APC resistance, increased resistance against the anticoagulant properties of annexin A5 and increased levels of activated von Willebrand factor (Figure 3).^{39, 42, 46} It is anticipated that this population is also involved in disease-causing mechanisms other than thrombosis in APS.

It seems strange that new roles for a protein with no previous clear function come to light in the presence of an autoantibody. Perhaps native ₂ GPI already possesses these functions, but requires activation (such as a conformational change) before it can perform them. Native ₂ GPI could potentially already exhibit these actions at sites of cellular damage.⁴⁷ Under apoptotic conditions, the antigen would be concentrated on the anionic phospholipids of the exposed cellular membranes and inhibit the unwanted consequences of the exposure of negatively-charged surfaces to components of the circulating bloodstream. In the presence of anti-₂ GPI antibodies, the active conformation of the ₂ GPI protein is stabilized and its function is no longer limited to sites of damage. The consequence would be an imbalance in the hemostatic system, resulting in thrombosis at any vascular site in the body.

In the future, more insight is needed into whether there are more antibody subpopulations that can cause the (specific) clinical manifestations that are observed in APS. Future investigations into the pathology should be performed with antibody populations with precisely defined epitopes.

Acknowledgments

Désirée Lie, University of California, Irvine, CA, is the author of and is solely responsible for the content of the learning objectives, questions and answers of the Medscape-accredited continuing medical education activity associated with this article.

References

1. Levine JS et al. (2002) The antiphospholipid syndrome. N Engl J Med 346: 752–763 | Article | PubMed | ISI | ChemPort |
2. Galli M et al. (2003) Anti-2-glycoprotein I and anti-prothrombin antibodies and the risk of thrombosis in the antiphospholipid syndrome. Blood 102: 2717–2723 | Article | PubMed | ISI | ChemPort |
3. Galli M et al. (2003) Lupus anticoagulants are stronger risk factors for thrombosis than anticardiolipin antibodies in the antiphospholipid syndrome: a systematic review of the literature. Blood 101: 1827–1832 | Article | PubMed | ISI | ChemPort |
4. Wasserman A et al. (1906) Eine serodiagnostiche reaction bei syphilis [German]. Dtsch Med Wochenschr 32: 745
5. Pangborn MC (1941) A new serologically active phospholipid from beef heart. Proc Soc Exp Biol Med 48: 484–486 | ChemPort |
6. Moore JE and Mohr GF (1952) Biologically false positive tests for syphilis: type, incidence, and cause. J Am Med Association 150: 467–473 | ChemPort |
7. Moore JE and Lutz WB (1955) Natural history of systemic lupus erythematosus: approach to its study through chromic biological false positive reactors. J Chronic Dis 1: 297–316 | Article | PubMed | ChemPort |
8. Conley CL and Hartman RCA (1952) A hemorrhagic disorder caused by circulating anticoagulant in patients with disseminated lupus erythematosus. J Clin Invest 31: 621–622 | ISI |
9. Beaumont JL (1954) Syndrome hemorrhagic acquis du a un anticoagulant [French]. Sang 25: 1–5 | PubMed | ChemPort |
10. Bowie EJW et al. (1963) Thrombosis in systemic lupus erythematosus despite circulating anticoagulants. J Lab Clin Med 62: 416–430 | PubMed | ISI | ChemPort |
11. Galli M et al. (1990) Anticardiolipin antibodies (ACA) directed not to cardiolipin but to a plasma protein cofactor. Lancet 335: 1544–1547 | Article | PubMed | ISI | ChemPort |
12. Matsuura H et al. (1990) Anticardiolipin cofactor(s) and differential diagnosis of autoimmune disease. Lancet 336: 117–118 | Article | PubMed | ChemPort |
13. McNeil HD et al. (1990) Antiphospholipid antibodies are directed against a complex antigen that includes a lipid-binding inhibitor of coagulation: 2-glycoprotein I (apolipoprotein H). Proc Natl Acad Sci USA 87: 4120–4124 | Article | PubMed | ChemPort |
14. Bevers EM et al. (1991) Lupus anticoagulant IgG's (LA) are not directed to phospholipids only, but to a complex of lipid-bound human prothrombin. Thromb Haemost 66: 629–632 | PubMed | ChemPort |
15. De Laat B et al. (2004) 2-glycoprotein I, playmaker in the antiphospholipid syndrome. Clin Immunol 112: 161–168 | Article | PubMed | ChemPort |
16. Wilson WA et al. (1999) International consensus statement on preliminary classification criteria for definite antiphospholipid syndrome: report of an international workshop. Arthritis Rheum 42: 1309–1311 | Article | PubMed | ISI | ChemPort |
17. Miyakis S et al. (2006) International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS). J Thromb Haemost 4: 295–306 | Article | PubMed | ISI | ChemPort |
18. Consigny PH et al. (2002) High prevalence of co-factor independent anticardiolipin antibodies in malaria exposed individuals. Clin Exp Immunol 127: 158–164 | Article | PubMed | ChemPort |
19. Abuaf N et al. (1997) Autoantibodies to phospholipids and to the coagulation proteins in AIDS. Thromb Haemost 77: 856–861 | PubMed | ChemPort |
20. Uhtman IW et al. (2002) Viral infections and antiphospholipid antibodies. Semin Arthritis Rheum 31: 256–263 | Article | PubMed | ISI |
21. De Laat B et al. (2005) IgG antibodies that recognize epitope Gly40-Arg43 in domain I of 2-glycoprotein I cause LAC and their presence correlates strongly with thrombosis. Blood 105: 1540–1545 | Article | PubMed | ChemPort |
22. Opatrny L et al. (2006) Association between antiphospholipid antibodies and recurrent fetal loss in women without autoimmune disease: a metaanalysis. J Rheumatol 33: 2214–2221 | PubMed | ChemPort |
23. De Laat HB et al. (2004) 2-glycoprotein I-dependent lupus anticoagulant highly correlates with thrombosis in the antiphospholipid syndrome. Blood 104: 3598–3602 | Article | PubMed | ChemPort |
24. Pengo V et al. (2005) Antibody profiles for the diagnosis of antiphospholipid syndrome. Thromb Haemost 93: 1147–1152 | PubMed | ChemPort |
25. Devreese KM (2007) A functional coagulation test to identify anti-2-glycoprotein I dependent lupus anticoagulants. Thromb Res 119: 753–759 | Article | PubMed | ChemPort |
26. Pengo V (2004) A two-step coagulation test to identify anti-glycoprotein I lupus anticoagulants. J Thromb Haemost 2: 702–707 | Article | PubMed | ChemPort |
27. Cervera R et al. (2002) Antiphospholipid syndrome: clinical and immunologic manifestations and patterns of disease expression in a cohort of 1,000 patients. Arthritis Rheum 46: 1019–1027 | Article | PubMed | ISI |
28. Yang CD et al. (1998) Detection of anti-recombinant 2-glycoprotein I fifth-domain antibodies in sera from patients with systemic lupus erythematosus. Rheumatol Int 18: 5–10 | Article | PubMed | ChemPort |
29. Igarashi M et al. (1996) Human 2-glycoprotein I as an anticardiolipin cofactor determined using mutants expressed by a baculovirus system. Blood 87: 3262–3270 | PubMed | ChemPort |
30. George J et al. (1998) Target recognition of 2-glycoprotein I (2GPI)-dependent anticardiolipin antibodies: evidence for involvement of the fourth domain of 2GPI in antibody binding. J Immunol 160: 3917–3923 | PubMed | ISI | ChemPort |
31. McNeely PA et al. (2001) 2-glycoprotein I-dependent anticardiolipin antibodies preferentially bind the amino terminal domain of 2-glycoprotein I. Thromb Haemost 86: 590–595 | PubMed |
32. Reddel SW et al. (2000) Epitope studies with anti-2-glycoprotein I antibodies from autoantibody and immunized sources. J Autoimmun 15: 91–96 | Article | PubMed | ChemPort |
33. Iverson GM et al. (1998) Anti-2-glycoprotein I (2GPI) autoantibodies recognize an epitope on the first domain of 2GPI. Proc Natl Acad Sci USA 95: 15542–15546 | Article | PubMed | ChemPort |
34. Iverson GM et al. (2002) Use of single point mutations in domain I of 2GPI to determine fine antigenic specificity of antiphospholipid autoantibodies. J Immunol 169: 7097–7103 | PubMed | ChemPort |
35. Blank M et al. (1999) Prevention of experimental antiphospholipid syndrome and endothelial cell activation by synthetic peptides. Proc Natl Acad Sci USA 96: 5164–5168 | Article | PubMed | ChemPort |
36. Giannakopoulos B et al. (2007) Current concepts on the pathogenesis of the antiphospholipid syndrome. Blood 109: 422–430 | Article | PubMed | ChemPort |
37. Rand JH et al. (1997) Pregnancy loss in the antiphospholipid-antibody syndrome—a possible thrombogenic mechanism. N Engl J Med 337: 154–160 | Article | PubMed | ISI | ChemPort |
38. Rand JH et al. (2004) Detection of antibody-mediated reduction of annexin A5 anticoagulant activity in plasmas of patients with the antiphospholipid syndrome. Blood 104: 2783–2790 | Article | PubMed | ChemPort |
39. De Laat B et al. (2007) Correlation between antiphospholipid antibodies that recognize domain I of 2-glycoprotein I and a reduction in the anticoagulant activity of annexin A5. Blood 109: 1490–1494 | Article | PubMed | ChemPort |
40. Gardiner C et al. (2006) Detection of acquired resistance to activated protein C associated with antiphospholipid antibodies using a novel clotting assay. Blood Coagul Fibrinolysis 17: 477–483 | Article | PubMed | ChemPort |
41. Nojima J et al. (2005) Acquired activated protein C resistance associated with IgG antibodies against 2-glycoprotein I and prothrombin as a strong risk factor for venous thromboembolism. Clin Chem 51: 545–552 | Article | PubMed | ChemPort |
42. De Laat B. et al. (2007) Correlation between the potency of a 2-glycoprotein I dependent lupus anticoagulant and the level of resistance to activated protein C [abstract]. Abstract P-S-539, XXI^st congress of the ISTH, Geneva
43. van Lummel M et al. (2005) The binding site in 2-glycoprotein I for ApoER2' on platelets is located in domain V. J Biol Chem 280: 36729–36736 | Article | PubMed | ChemPort |
44. Pennings MT et al. (2007) Platelets express three different splice variants of ApoER2 that are all involved in signalling. J Thromb Haemost 5: 1538–1544 | Article | PubMed | ChemPort |
45. Urbanus RT et al. (2007) Current insight into diagnostics and pathophysiology of the antiphospholipid syndrome. Blood Rev [doi:doi: 10.1016/j.blre.2007.09.001] | Article |
46. Hulstein JJ et al. (2007) 2-Glycoprotein I inhibits von Willebrand factor dependent platelet adhesion and aggregation. Blood 110: 1483–1491 | Article | PubMed | ChemPort |
47. Pittoni V and Valesini G (2002) The clearance of apoptotic cells: implications for autoimmunity. Autoimmun Rev 1: 154–161 | Article | PubMed | ChemPort |

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Subject areas under which this article appears: Immunology