Mechanism of Action of Hydroxychloroquine in the Antiphospholipid Syndrome

Objective.The antiphospholipid syndrome (APS) is characterized by thromboembolic events and/or recurrent abortions in the presence of pathogenic antiphospholipid antibodies (aPL). Hydroxychloroquine (HCQ) is recommended for thromboprophylaxis in patients with systemic lupus erythematosus and aPL. We aimed at identifying mechanisms responsible for the beneficial effects of HCQ in the APS.

Methods.Monocytic cells were stimulated with human aPL in vitro and expression of tissue factor (TF) and tumor necrosis factor (TNFα) was quantified by qRT-PCR. For intracellular ROS detection, cells were stained with OxyBurst and analyzed by flow cytometry. Live cell imaging was performed by confocal laser scanning microscopy to show translocation of NADPH oxidase 2 (NOX2) and TLR8 on stimulation. Finally, the effect of HCQ was tested in an in vivo model of venous thrombosis.

Results.HCQ prevents the induction of TF and TNFα by aPL in MonoMac1 cells, by blocking the activation of endosomal NOX2. This latter effect is not mediated by direct interference with the previously described uptake of aPL into the endosome but by inhibiting the recruitment of the catalytic subunit of NOX2 (gp91phox) into the endosome. Furthermore, HCQ protects mice from aPL induced and NOX2 mediated thrombus formation in vivo.

Conclusion.We propose that interference with the assembly of endosomal NOX2 is a major effect of HCQ responsible for its antithrombotic effect in the APS. Since most if not all APS patients harbor aPL that activate endosomal NOX2 this probably explains the well established efficacy of HCQ in APS patients.