NF-κB is activated from endosomal compartments in antiphospholipid antibodies-treated human monocytes.

BACKGROUND: The antiphospholipid antibody syndrome (APS) is an autoimmune disease associated with arterial or venous thrombosis and/or recurrent fetal loss and is caused by pathogenic antiphospholipid antibodies (aPLA). We recently demonstrated that Toll-like receptor 2 (TLR2) and CD14 contribute to monocyte activation of aPLA.

OBJECTIVE: To study the mechanisms of cell activation by aPLA, leading to pro-coagulant and pro-inflammatory responses.

METHODS AND RESULTS: For this study, we used purified antibodies from the plasmas of 10 different patients with APS and healthy donors. We demonstrate that aPLA, but not control IgG, co-localizes with TLR2 and TLR1 or TLR6 on human monocytes. Blocking antibodies to TLR2, TLR1 or TLR6, but not to TLR4, decreased TNF and tissue factor (TF) responses to aPLA. Pharmacological and siRNA approaches revealed the importance of the clathrin/dynamin-dependent endocytic pathway in cell activation by aPLA. In addition, soluble aPLA induced NF-κB activation, while bead-immobilized aPLA beads, which cannot be internalized, were unable to activate NF-κB. Internalization of aPLA in monocytes and NF-κB activation were dependent on the presence of CD14.

CONCLUSION: We show that TLR2 and its co-receptors, TLR1 and TLR6, contribute to the pathogenicity of aPLA, that aPLA are internalized via clathrin- and CD14-dependent endocytosis and that endocytosis is required for NF-κB activation. Our results contribute to a better understanding of the APS and provide a possible therapeutic approach.