Antiphospholipid antibody-mediated effects in an arterial model of thrombosis are dependent on Toll-like receptor 4

Abstract

Patients with antiphospholipid syndrome (APS) produce antiphospholipid antibodies (aPL) and develop vascular thrombosis that may occur in large or small vessels in the arterial or venous beds. On the other hand, many individuals produce aPL and yet never develop thrombotic events. Toll-like receptor 4 (TLR4) appears to be necessary for aPL-mediated prothrombotic effects in venous and microvascular models of thrombosis, but its role in arterial thrombosis has not been studied. Here, we propose that aPL alone are insufficient to cause thrombotic events in an arterial model of APS, and that a concomitant trigger of innate immunity (e.g. TLR4 activation) is required. We show specifically that anti-β2-glycoprotein I (anti-β2GPI) antibodies, a subset of aPL, accelerated thrombus formation in C57BL/6 wild-type, but not TLR4-deficient, mice in a ferric chloride-induced carotid artery injury model. These aPL bound to arterial and venous endothelial cells, particularly in the presence of β2GPI, and to human TLR4 by enzyme-linked immunoassay. Arterial endothelium from aPL-treated mice had enhanced leukocyte adhesion, compared to control IgG-treated mice. In addition, aPL treatment of mice enhanced expression of tissue factor (TF) in leukocytes induced by the TLR4 ligand lipopolysaccharide (LPS). aPL also enhanced LPS-induced TF expression in human leukocytes in vitro. Our findings support a mechanism in which aPL enhance TF expression by leukocytes, as well as augment adhesion of leukocytes to the arterial endothelium. The activation of TLR4 in aPL-positive individuals may be required to trigger thrombotic events.

Activated signature of antiphospholipid syndrome neutrophils reveals potential therapeutic target

“…Interestingly, a number of the upregulated genes have already been defined as playing a role in APS, such as TLR4 (13), the complement component 5a receptor (47), protease-activated receptor 2 (48), and Fms-related tyrosine kinase 1 (FLT1, a receptor for vascular endothelial growth factor) (49). Furthermore, TLR and IFN-mediated signaling stood out in the meta-group analysis -both are receiving active attention in APS (50)(51)(52)(53).…”

“…Finally, we felt that our recently described animal model was particularly amenable to assessing the role of neutrophil adhesion in antiphospholipid antibody-mediated thrombosis (21). Notably, the majority of models used to study APS in mice have relied on explicit vessel wall damage such as femoral vein pinch injury (13,54), laser injury to the cremaster microcirculation (55,56), and ferric chloride application (57). The flow restriction model used by our While the PSGL-1 -/-mice used here are global KOs, we added specificity to the model with adoptive transfer experiments.…”

Cross Talk Pathways Between Coagulation and Inflammation

“…Research on TLR2 and TLR4 in models of thrombosis, however, has yielded many interesting findings relevant to coagulation and thrombosis. TLR4 has been shown to mediate TF expression in antiphospholipid antibody (anti-β2GPI)-mediated thrombosis, 77 in diabetes mellitus, in hypercholesterolemia in mice and monkeys, in lipopolysaccharide-induced endotoxemia and microvascular thrombosis, 78 and in arterial thrombosis in mice.…”

Antiphospholipid syndrome: an update for clinicians and scientists

“…On this front, particular attention has been given to the cell-surface TLRs, TLR2 and TLR4. In mouse models, TLR4 deletion protects against venous and arterial thrombosis in some [31–33], but not all [34]*, studies (it is worth pointing out that the latter study utilized cofactor-independent aPL). Studies of obstetric APS have also yielded mixed results with an older study demonstrating no role for TLR4 in an in vivo model of pregnancy loss [35].…”