C6-Deficient Mice Are Protected From the Pathogenic Effects of Antiphospholipid Antibodies.

The pathogenic mechanisms mediated by antiphospholipid (aPL) antibodies are only partially understood. Tissue factor (TF) upregulation has been shown to play an important role and recent studies have demonstrated that mice deficient in complement C3, C5 or C5a receptor (R) are resistant to the thrombogenic effects of aPL antibodies. The complement membrane attack (C5b-9 MAC) complex has been shown to bind specific receptors, to induce TF expression and to exert procoagulant effect in various cell types (i.e. endothelial cells). However whether C5b-9 MAC plays a role in the pathogenesis of thrombosis in Antiphospholipid Syndrome (APS) is unknown. Here we addressed that question by studying the effects of human aPL antibodies on thrombosis and TF upregulation in C6 deficient (−/−) mice.

C6 deficient (−/−) mice and the corresponding wild-type (WT) mice C3H/HeJ (C6+/+) were injected twice with 500γg/ml IgG isolated from a patient with APS (IgG-APS) or with the corresponding control IgG-NHS. Seventy-two h after the first injection the size of induced thrombi in the femoral vein, IgG anticardiolipin (ACL) activity in the sera were determined. TF activity was determined in homogenates of carotid arteries and in peritoneal macrophages using a chromogenic assay.

Thrombus sizes were significantly larger in WT mice C6 (+/+) treated with IgG-APS when compared with C6 (+/+) mice treated with IgG-NHS (p= <0.001) The sizes of thrombi were significantly smaller in the C6 (−/−) mice injected with IgG-APS compared to their WT C6 (+/+) counterparts (p= <0.001) showing an important abrogation of thrombus formation in mice lacking C6. Thrombus sizes in C6 (−/−) mice injected with IgG-NHS were not different when compared to C6 (+/+) mice treated with IgG-NHS (p= 0.786), Similarly, upregulation of TF activity in carotid arteries homogenates and in peritoneal macrophages in C6 (+/+) mice treated with IgG-APS were significantly diminished in C6 (−/−) mice (p=0.036 and p=0.041, respectively). All mice injected with IgG-APS had medium-high titers of aCL in their serum (see Table)

These data indicate that the C6 component of the complement system mediates aPL-thrombogenic effects and not only underscore the importance of complement activation by aPL antibodies, but also propose its inhibition as a possibly novel therapeutic target for thrombosis in APS .

No relevant conflicts of interest to declare.