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Background —Antiphospholipid (aPL) antibodies are associated with thrombosis in patients diagnosed with antiphospholipid syndrome (APS) and enhance thrombus formation in vivo in mice, but the mechanism of thrombosis by aPL is not completely understood. Although aPL antibodies have been shown to inhibit protein C activation and activate endothelial cells (ECs) in vitro, no study has examined whether these antibodies activate ECs in vivo. Therefore, human affinity-purified aPL (ap aPL) antibodies from APS patients were tested in a mouse model of microcirculation using the cremaster muscle that allows direct microscopic examination of thrombus formation and adhesion of white blood cells (WBCs) to ECs as an indication of EC activation in vivo. Adhesion molecule expression on human umbilical vein endothelial cells (HUVECs) after aPL exposure was performed to confirm EC activation in vitro. Methods and Results —All 6 ap aPL antibodies significantly increased the expression of VCAM-1 (2.3- to 4.4-fold), with one of the antibodies also increasing the expression of E-selectin (1.6-fold) on HUVECs in vitro. In the in vivo experiments, each ap aPL antibody except for 1 preparation increased WBC sticking (mean number of WBCs ranged from 22.7 to 50.6) compared with control (14.4), which correlated with enhanced thrombus formation (mean thrombus size ranged from 1098 to 6476 versus 594 μm 2 for control). Conclusions —Activation of ECs by aPL antibodies in vivo may create a prothrombotic state on ECs, which may be the first pathophysiological event of thrombosis in APS.

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SummaryAntiphospholipid syndrome is a disorder of recurrent thrombosis and pregnancy losses associated with production of anticardiolipin antibodies and lupus anticoagulant positivity. Recently, we have adapted a mouse model of induced venous thrombosis to study the role of autoantibodies in thrombus formation. To determine whether immunoglobulins from patients with the antiphospholipid syndrome play a role in thrombosis, we injected groups of CDI mice either with immunoglobulins purified from seven patients with the antiphospholipid syndrome (nine preparations studied: four IgG, three IgM and two IgA) or with immunoglobulins of the same isotype from healthy controls. Seventy- two h after injection, a non-occlusive thrombus was induced in the femoral veins of experimental mice by a pinch injury; the thrombus areas as well as times of formation and disappearance of the thrombi were measured. Eight of the nine antiphospholipid syndrome immunoglobulin preparations caused a significant increase in mean thrombus area and a significant delay in mean thrombus disappearance time as compared with normal controls. To determine whether anticardiolipin antibodies might be involved, separate groups of mice were injected with affinity-purified IgG (n = 2) or IgM (n = 2) anticardiolipin antibodies or with normal immunoglobulins of the same isotype, and the effects on thrombus formation compared. Mean thrombus area and mean disappearance times were again significantly increased in all four groups injected with affinity-purified antibodies. This is the first study to show that anticardiolipin antibodies of IgG, IgM and IgA isotypes may play a role in thrombosis in vivo.

Abstract-Recent studies have shown that antiphospholipid (aPL) enhances expression of intercellular cell adhesion molecule-1 (ICAM-1), vascular cell adhesion molecule-1 (VCAM-1), and E-selectin on endothelial cells (ECs) and that these effects are correlated with increased adhesion of leukocytes to endothelium in cremaster muscle in vivo and with thrombosis in a mouse model. Activation of ECs by aPL may create a hypercoagulable state that precedes and contributes to thrombosis in patients with aPL syndrome (APS). This study proposed to examine whether this in vivo activation of ECs and enhanced thrombosis by aPL are mediated by ICAM-1, P-selectin, or VCAM-1. The dynamics of thrombus formation and the number of adhering leukocytes were studied in ICAM-1-deficientmice treated with affinity-purified aPL antibodies (ap IgG-APS) or with control IgG and compared with wild-type mice treated in a similar fashion. In another set of experiments, the adhesion of leukocytes to cremaster muscle and the dynamics of thrombus formation were studied in CD1 mice treated with aPL or control IgG before and 30 minutes after intravenous infusion with 100 g monoclonal antibody anti-VCAM-1. The results indicate that the enhanced adhesion of leukocytes to endothelium in wild-type mice was significantly reduced in ICAM-1 Ϫ/Ϫ and completely abrogated in ICAM-1

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To determine the predictive value of the IgG anticardiolipin antibody (ACA) test for thrombosis, recurrent fetal loss, and thrombocytopenia, the clinical features of 121 patients with varying antibody levels were studied. When patients were grouped into high-positive, low-positive, and normal groups according to their ACA levels, there were strong statistical correlations with arterial thrombosis, venous thrombosis, fetal loss, thrombocytopenia, and a positive Coombs' test. At levels of 7 SD and above, the test was highly specific (greater than 80%) and predictive (greater than 70%) for thrombosis, thrombocytopenia, and recurrent fetal loss. This study suggests that the IgG ACA test may be a useful predictor for thrombosis, recurrent fetal loss, and thrombocytopenia in patients with autoimmune disorders.

Objective. To characterize the binding and functional properties of antiphospholipid antibodies (aPL) induced by immunization with a viral peptide and to determine whether aPL are pathogenic in vivo.Methods. Ten murine monoclonal aPL were generated from spleen cells of PL/J mice immunized with TIFI, a phospholipid-binding peptide spanning Thr 101 -Thr 120 of ULB0-HCMVA from human cytomegalovirus (CMV), which shares structural similarity with the phospholipid-binding site of ␤ 2 -glycoprotein I (␤ 2 GPI).Results. The antibodies generated had aPL activity that was inhibited by cardiolipin liposomes, and this inhibition was enhanced in the presence of ␤ 2 GPI. Some of the antibodies exhibited binding to cultured endothelial cells in vitro, and some had lupus anticoagulant activity. Injection with 2 of the monoclonal aPL in mice resulted in a significant increase in the number of leukocytes adhering to endothelial cells and enhanced thrombus formation in vivo.Conclusion. These results indicate that aPL induced by immunization with a phospholipid-binding CMV peptide are pathogenic in vivo. The results also suggest a mechanism (molecular mimicry) by which pathogenic aPL may be generated in patients with antiphospholipid syndrome.