Journal of Thrombosis and Haemostasis, 5: 1825–1827
COMMENTARY
Antiphospholipid syndrome: new light comes from in vivo studies R. FORASTIERO Division of Hematology, Thrombosis and Hemostasis, Institute of Cardiology and Cardiovascular Surgery, Favaloro Foundation, Favaloro University, Buenos Aires, Argentina
To cite this article: Forastiero R. Antiphospholipid syndrome: new light comes from in vivo studies. J Thromb Haemost 2007; 5: 1825–7. See also Vega-Ostertag ME, Ferrara DE, Romay-Penabad Z, Liu X, Taylor WR, Colden-Stanfield M, Pierangeli SS. Role of p38 mitogen-activated protein kinase in antiphospholipid antibody-mediated thrombosis and endothelial cell activation. This issue, pp 1828–34.
Antiphospholipid syndrome (APS) is a clinical autoimmune disorder characterized by thrombosis, venous or arterial, and recurrent pregnancy morbidity in association with the persistence of antiphospholipid antibodies (aPL). The detection of lupus anticoagulant, and/or high levels of anticardiolipin and anti-b2 glycoprotein I (anti-b2GPI) antibodies, is a mandatory laboratory feature for the diagnosis to be made [1]. It is well known that aPL comprise a heterogeneous family of immunoglobulins. Antibodies directed against anionic phospholipids per se are mainly found in infectious disorders and do not tend to be associated with the clinical features of APS. On the other hand, APS-related aPL target phospholipid-binding proteins, such as b2GPI and prothrombin [2,3]. Accumulating evidence has demonstrated that the presence of immunoglobulin G (IgG) anti-b2GPI and antiprothrombin antibodies predicts a higher risk of first or recurrent thromboembolic events. This information was found not only by retrospective studies [4,5] but also by recent prospective studies [6,7]. The common denominator for all the last published data is that the risk of thrombosis progressively increases with the number of positive aPL tests. It is now well recognized that autoimmune aPL are pathogenic in vivo. Given that b2GPI and prothrombin are the most relevant antigenic targets in APS, it is thought that the main pathogenic mechanisms involve antibodies reacting with these two proteins. Several hypotheses have been proposed to explain the mechanisms by which autoimmune aPL promote thrombosis [8,9]. The strongest evidence, however, was provided by in vivo experiments carried out in animal models of thrombosis and pregnancy loss. In this issue of the Journal, Vega-Ostertag et al. [10] report data on the pathogenic effects of aPL. This study significantly Correspondence: R. Forastiero, Hematologı´ a, Universidad Favaloro, Solı´ s 453, (C1078AAI) Buenos Aires, Argentina. Tel.: +54 11 4378 1145; fax: +54 11 4378 1311; e-mail: rforastiero@favaloro.edu.ar 2007 International Society on Thrombosis and Haemostasis
advances our knowledge of the mechanisms that explain how these autoimmune aPL play a direct role in thrombus formation. Vega-Ostertag et al. were able to demonstrate that purified aPL injected in mice induces expression of tissue factor (TF) and vascular cell adhesion molecule-1 (VCAM-1) in vivo by means of novel techniques. The results indicate that IgG aPL induce TF activity in carotid artery tissue and in peritoneal macrophages as well as VCAM-1 expression in aortic endothelial cells. The innovative technique used for VCAM-1 expression in mouse aortas ex vivo is a promising method to quantify the effects of aPL on the endothelium. Their findings confirm previous experimental data showing aPL-induced upregulation of adhesion molecules in vitro. The crucial role of cellular TF expression in the pathophysiology of APS is usually accepted as one of the key events in this disorder. We and other authors showed raised plasma levels of soluble TF in APS patients [11,12]. Although the origin of soluble TF is not clear, the circulating levels of TF may reflect to some degree the TF expression within the vasculature and/or circulating cell-derived microparticles. In the last few years, the intracellular mechanisms involved in aPL-mediated cell activation have been explored. Activation of nuclear factor-kappa B (NF-jB) has been shown to have an essential role in cellular activation. In vitro experiments have demonstrated that IgGs purified from APS patients are able to induce the nuclear translocation of NF-jB, leading to the transcription of a large number of genes such as those of adhesion molecules, cytokines and also TF [9]. Activation of p38 mitogen-activated protein kinase (MAPK) has also been involved in upregulation of adhesion molecules, cytokines and TF. In a recent study, it was shown that aPL induce TF expression in monocytes from APS patients by activating, simultaneously and independently, the phosphorylation of MEK-1/ERK proteins, and the p38 MAPK-dependent nuclear translocation and activation of NF-jB/Rel proteins [13]. By using specific inhibitors, the involvement of both p38 MAPK phosphorylation and NF-jB activation in in vitro cellular activation was confirmed. In particular, the specific p38 MAPK inhibitor SB 203580
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abrogated platelet and monocyte activation mediated by aPL [9,12,13]. In a recent report, it was also shown for the first time that monocytes from APS patients have an increased overexpression of vascular endothelial growth factor (VEGF) and its receptor Flt-1, which may stimulate TF expression [14]. Furthermore, in vitro results indicated that the p38 MAPK signaling pathway plays an important role because SB 203580 significantly inhibited the aPL-dependent expression of VEGF, Flt-1 and TF. Vega-Ostertag et al. [10] add to these insights with studies showing for the first time that the in vivo upregulation of VCAM-1 and TF in mice treated with IgG aPL was significantly decreased by pretreatment with the specific p38 MAPK inhibitor SB 203580. In this elegant study, the authors show that this inhibitor attenuated IgG APSinduced thrombosis in vivo and endothelial cell activation in vivo and in vitro. Another important finding in the study of Vega-Ostertag et al. [10] is that pretreatment with SB 203580 completely abrogated platelet aggregation activated by low doses of thrombin in aPL-treated mice. This effect closely correlated with a reduction in thrombus size induced in the same mice. In previous studies we demonstrated that APS patients with IgG anti-b2GPI antibodies have increased excretion of plateletderived thromboxane urinary metabolites, suggesting that platelet activation could contribute to thrombosis in APS [15]. In vivo, the infusion of anti-b2GPI antibodies into hamsters produced platelet-rich thrombi in carotid arteries primed with a photochemical injury [16]. Binding of anti-b2GPI antibodies to b2GPI bound to platelet receptors, such as apolipoprotein E receptor 2¢ and GPIba, induces activation of the p38 MAPK/phospholipase A2 pathway, leading to thromboxane production [17]. Hence, there are numerous data supporting platelet activation as an important pathway in thrombosis pathogenesis in APS. Interestingly, the authors also show that using the same IgG aPL cellular alterations were observed in venous as well as in arterial endothelium. These findings represent the first suggestion of a common mechanism being involved in both venous and arterial thromboses in patients with APS. However, it is likely that no single mechanism explains thrombosis because it is multifactorial and venous and arterial thrombotic episodes have clear pathogenic differences. Cellular activation is one of the major pathogenic mechanisms underlying the thrombotic tendency in APS, but other important hypotheses have also been proposed. Complement activation, disruption of antithrombotic pathways such as the protein C pathway, the TF pathway inhibitor, protein Z/protein Z-dependent protease inhibitor, annexin A5 and impairment of fibrinolysis are some of the hypotheses [8]. One of the strengths of the study of VegaOstertag et al. [10] is the simultaneous evaluation of cell activation in monocytes, endothelial cells and platelets in various in vivo and in vitro experiments using the same IgG APS. The usefulness of selective inhibition of p38 MAPK in reversing pathogenic effects of autoimmune aPL is proved in this APS murine model as well as in other autoimmune and inflammatory diseases. Nevertheless, the effectiveness of new
therapeutic approaches such as MAPK inhibitors must be clearly demonstrated in clinical studies before their use as potential antithrombotic therapies in addition to oral anticoagulation. Whether or not MAPK inhibitors could be used therapeutically for preventing APS-related clinical events remains to be elucidated. This study, showing the intracellular signaling induced by aPL and how to hamper it in vivo, is relevant because it adds new light on the pathophysiology of APS. Disclosure of Conflict of Interests The author states that he has no conflict of interest. References 1 Miyaki S, Lockshin MD, Atsumi T, Branch DW, Brey RL, Cervera R, Derksen RH, de Groot PG, Koike T, Meroni PL, Reber G, Shoenfeld Y, Tincani A, Vlachoyiannopoulos PG, Krilis SA. International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome (APS). J Thromb Haemost 2006; 4: 295– 306. 2 Forastiero R, Martinuzzo M. Antigen specificity and clinical relevance of antiphospholipid syndrome-related autoantibodies. Curr Rheumatol Rev 2005; 1: 177–87. 3 Levine JS, Branch DW, Rauch J. The antiphospholipid syndrome. N Engl J Med 2002; 346: 752–63. 4 Galli M, Borrelli G, Jacobsen EM, Marfisi RM, Finazzi G, Marchioli R, Wisloff FG, Marziali S, Morboeuf O, Barbui T. Clinical significance of different antiphospholipid antibodies in the WAPS (Warfarin in the Anti- Phospholipid Syndrome) study. Blood 2007; doi:10.1182/ blood-2007-01-066043. 5 de Laat B, Derksen RH, Urbanus RT, de Groot PG. IgG antibodies that recognize epitope Gly40-Arg43 in domain I of beta 2-glycoprotein I cause LAC, and their presence correlates strongly with thrombosis. Blood 2005; 105: 1540–5. 6 Forastiero R, Martinuzzo M, Pombo G, Puente D, Rossi A, Celebrı´ n L, Bonaccorso S, Aversa L. A prospective study of antibodies to b2 glycoprotein I and prothrombin, and risk of thrombosis. J Thromb Haemost 2005; 3: 1231–8. 7 Bizzaro N, Ghirardello A, Zampieri S, Iaccarino L, Tozzoli R, Ruffatti A, Villalta D, Tonutti E, Doria A. Anti-prothrombin antibodies predict thrombosis in patients with systemic lupus erythematosus: a 15year longitudinal study. J Thromb Haemost 2007; 5: 1158–64. 8 Giannakopoulos B, Passam F, Rahgozar S, Krilis SA. Current concepts on the pathogenesis of the antiphospholipid syndrome. Blood 2007; 109: 422–30. 9 Vega-Ostertag M, Pierangeli SS. Mechanisms of aPL-mediated thrombosis: effects of aPL on endothelium and platelets. Curr Rheumatol Rep 2007; 9: 190–7. 10 Vega-Ostertag ME, Ferrara DE, Romay-Penabad Z, Liu X, Taylor WR, Colden-Stanfield M, Pierangeli SS. Role of p38 mitogen-activated protein kinase in antiphospholipid antibody-mediated thrombosis and endothelial cell activation. J Thromb Haemost 2007; 5: 1828– 34. 11 Forastiero RR, Martinuzzo ME, de Larran˜aga G. Circulating levels of tissue factor and proinflammatory cytokines in patients with primary antiphospholipid syndrome or leprosy-related antiphospholipid antibodies. Lupus 2005; 14: 129–36. 12 Koike T, Bohgaki M, Amengual O, Atsumi T. Antiphospholipid antibodies: lessons from the bench. J Autoimmun 2007; 28: 129–33. 13 Lopez-Pedrera C, Buendia P, Cuadrado MJ, Siendones E, Aguirre MA, Barbarroja N, Montiel-Duarte C, Torres A, Khamashta M, Velasco F.
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Antiphospholipid syndrome 1827 Antiphospholipid antibodies from patients with the antiphospholipid syndrome induce monocyte tissue factor expression through the simultaneous activation of NF-kappaB/Rel proteins via the p38 mitogen-activated protein kinase pathway, and the MEK-1/ERK pathway. Arthritis Rheum 2006; 54: 301–11. 14 Cuadrado MJ, Buendia P, Velasco F, Aguirre MA, Barbarroja N, Torres A, Khamashta M, Lopez-Pedrera C. Vascular endothelial growth factor expression in monocytes from patients with primary antiphospholipid syndrome. J Thromb Haemost 2006; 4: 2461–9. 15 Forastiero R, Martinuzzo M, Carreras LO, Maclouf J. Anti-b2 glycoprotein I antibodies and platelet activation in patients with
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antiphospholipid antibodies: association with increased excretion of platelet-derived thromboxane urinary metabolites. Thromb Haemost 1998; 79: 42–5. 16 Jankowski M, Vreys I, Wittevrongel C, Boon D, Vermylen J, Hoyaerts MF, Arnout J. Thrombogenicity of beta-2-glycoprotein I-dependent antiphospholipid antibodies in a photochemically induced thrombosis model in the hamster. Blood 2003; 101: 157–62. 17 Pennings MT, Derksen RH, van Lummel M, Adelmeijer J, Van Hoorelbeke K, Urbanus RT, Lisman T, de Groot PG. Platelet adhesion to dimeric beta-glycoprotein I under conditions of flow is mediated by at least two receptors: glycoprotein Ibalpha and apolipoprotein E receptor 2Õ. J Thromb Haemost 2007; 5: 369–77.