Recent advances in the antiphospholipid antibody syndrome

PURPOSE OF REVIEW

The antiphospholipid syndrome (APS) is a systemic autoimmune disorder characterized by recurrent thrombosis and/or obstetrical morbidity in the presence of persistently positive antiphospholipid antibodies. Recent insights into the pathogenesis of APS have begun to elucidate pathophysiology and led to the identification of potential therapeutic interventions. The objective of this review is to examine the advances in this field and highlight the areas of further investigation.

RECENT FINDINGS

Several mechanisms of thrombosis and pregnancy loss in APS have been proposed. These include activation of endothelial cells, monocytes, and platelets, and/or inhibition of natural anticoagulant and fibrinolytic systems by antiphospholipid antibodies. However, in many cases the underlying molecular mechanisms and their relevance to the human disorder remain uncertain. New therapeutic agents such as statins, hydroxychloroquine, rituximab, complement inhibitors, and interventions aimed at disruption of intracellular signaling pathways have shown promise in preclinical and clinical studies.

SUMMARY

Indefinite anticoagulation remains the mainstay of treatment for thrombotic APS. Despite advances in diagnostic techniques, it remains difficult to predict thrombotic risk in asymptomatic patients with antiphospholipid antibodies. Further mechanistic and clinical studies are needed to predict thrombotic risk and develop improved therapies for this devastating illness.

Altered fibrin clot structure/function in patients with antiphospholipid syndrome: association with thrombotic manifestation

We tested the hypothesis that plasma fibrin clot structure/function is unfavourably altered in patients with antiphospholipid syndrome (APS). Ex vivo plasma clot permeability, turbidity and susceptibility to lysis were determined in 126 consecutive patients with APS enrolled five months or more since thrombotic event vs 105 controls. Patients with both primary and secondary APS were characterised by 11% lower clot permeability (p<0.001), 4.8% shorter lag phase (p<0.001), 10% longer clot lysis time (p<0.001), and 4.7% higher maximum level of D-dimer released from clots (p=0.02) as compared to the controls. Scanning electron microscopy images confirmed denser fibrin networks composed of thinner fibres in APS. Clots from patients with "triple-antibody positivity" were formed after shorter lag phase (p=0.019) and were lysed at a slower rate (p=0.004) than in the remainder. Clots from APS patients who experienced stroke and/or myocardial infarction were 8% less permeable (p=0.01) and susceptible to lysis (10.4% longer clot lysis time [p=0.006] and 4.5% slower release of D-dimer from clots [p=0.01]) compared with those following venous thromboembolism alone. Multivariate analysis adjusted for potential confounders showed that in APS patients, lupus anticoagulant and "triple-positivity" were the independent predictors of clot permeability, while "triple-positivity" predicted lysis time. We conclude that APS is associated with prothrombotic plasma fibrin clot phenotype, with more pronounced abnormalities in arterial thrombosis. Molecular background for this novel prothrombotic mechanism in APS remains to be established.

Antibodies against thrombin in dengue patients contain both anti-thrombotic and pro-fibrinolytic activities

Dengue virus (DENV) infection may result in severe life-threatening Dengue haemorrhagic fever (DHF). The mechanisms causing haemorrhage in those with DHF are unclear. In this study, we demonstrated that antibodies against human thrombin were increased in the sera of Dengue patients but not in that of patients infected with other viruses. To further characterise the properties of these antibodies, affinity-purified anti-thrombin antibodies (ATAs) were collected from Dengue patient sera by thrombin and protein A/L affinity columns. Most of the ATAs belonged to the IgG class and recognized DENV nonstructural protein 1 (NS1). In addition, we found that dengue patient ATAs also cross-reacted with human plasminogen (Plg). Functional studies in vitro indicated that Dengue patient ATAs could inhibit thrombin activity and enhance Plg activation. Taken together, these results suggest that DENV NS1-induced thrombin and Plg cross-reactive antibodies may contribute to the development of haemorrhage in patients with DHF by interfering with coagulation and fibrinolysis.

Impaired fibrinolysis in the antiphospholipid syndrome

The antiphospholipid syndrome (APS) is characterized by venous and/or arterial thrombosis, or recurrent fetal loss, in the presence of antiphospholipid antibodies (APL). The pathogenesis of APS is multifaceted and involves numerous mechanisms including activation of endothelial cells, monocytes, and/or platelets; inhibition of natural anticoagulant pathways such as protein C, tissue factor inhibitor, and annexin A5; activation of the complement system; and impairment of the fibrinolytic system. Fibrinolysis--the process by which fibrin thrombi are remodeled and degraded--involves the conversion of plasminogen to plasmin by tissue plasminogen activator (tPA) or urokinase-type plasminogen activator, and is tightly regulated. Although the role of altered fibrinolysis in patients with APS is relatively understudied, several reports suggest that deficient fibrinolytic activity may contribute to the pathogenesis of disease in these patients. This article discusses the function of the fibrinolytic system and reviews studies that have reported alterations in fibrinolytic pathways that may contribute to thrombosis in patients with APL. Some of these mechanisms include elevations in plasminogen activator inhibitor-1 levels, inhibitory antibodies against tPA or other components of the fibrinolytic system, antibodies against annexin A2, and finally, antibodies to beta(2)-glycoprotein-I (beta(2)GPI) that block the ability of beta(2)GPI to stimulate tPA-mediated plasminogen activation.

Antiphospholipid antibody profiles in lupus nephritis with glomerular microthrombosis: a prospective study of 124 cases

Introduction

Glomerular microthrombosis (GMT) is a common vascular change in patients with lupus nephritis (LN). The mechanism underlying GMT is largely unknown. Although several studies have reported the association of antiphospholipid antibodies (aPL) with GMT, the relation between GMT and aPL remains controversial. Previous studies have demonstrated that some aPL could bind to several hemostatic and fibrinolytic proteases that share homologous enzymatic domains. Of the protease-reactive aPL, some can inhibit the anticoagulant activity of activated protein C and the fibrinolytic function of plasmin, and hinder the antithrombin inactivation of thrombin. The purpose of this study was to investigate the prevalence of GMT in LN patients and examine the relation between the aPL profiles (including some protease-reactive aPL) and GMT.

Methods

Renal biopsy specimens were examined for the presence of glomerular microthrombi. Plasma samples from 25 LN patients with GMT (LN-GMT group) and 99 LN patients without GMT (LN-non-GMT group) were tested for lupus anticoagulant and antibodies against cardiolipin, β2 glycoprotein I, plasmin, thrombin, tissue plasminogen activator, and annexin II.

Results

The prevalence of GMT in LN patients was 20.2%. Compared with the LN-non-GMT group, the LN-GMT group had an elevated systemic lupus erythematosus disease activity index; elevated renal tissue injury activity and chronicity indices; elevated serum creatinine, blood urea nitrogen, and proteinuria levels; a lower serum C3 level and much intense glomerular C3, C1q staining; and a higher frequency of hypertension (P < 0.05 for all). Additionally, the detection rate of lupus anticoagulant, immunoglobulin G (IgG) anti-β2 glycoprotein I and anti-thrombin antibodies were higher in the LN-GMT group than in the LN-non-GMT group (P < 0.05 for all). No statistical differences were found in the detection rates of IgG anti-cardiolipin, plasmin, tissue plasminogen activator, or annexin II antibodies (P > 0.05 for all). No detectable difference in IgM autoantibodies to the above antigens was observed between the two groups.

Conclusions

GMT occurs in approximately 20.2% of LN patients. Patients with GMT have severer renal tissue injuries and poorer renal functions than patients without GMT. The lupus anticoagulant and antibodies against β2 glycoprotein I and thrombin may play a role in GMT.