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Identification of an Fc gamma receptor-independent mechanism by which intravenous immunoglobulin ameliorates antiphospholipid antibody-induced thrombogenic phenotype.

OBJECTIVE
Patients with the antiphospholipid antibody syndrome (APS) often experience recurrent arterial and venous thrombosis and pregnancy losses. Intravenous immunoglobulin (IVIG) therapy has prevented pregnancy loss in some women with APS and has reversed fetal resorption rates in murine models of pregnancy loss. Although the basis for these effects is unknown, effector mechanisms of pathogenic antibodies often involve receptors for IgG (Fc gamma receptors [Fc gammaR]). We examined the potential mechanisms of action of WIG in an in vivo murine model of antiphospholipid antibody (aPL)-induced thrombosis and endothelial cell activation.


METHODS
Mice infused with IgG containing human anticardiolipin antibodies (aCL) were treated with IVIG (36 microg i.v.), saline, or ovalbumin. Surgically induced thrombus formation and in vivo leukocyte adhesion to endothelial cells were measured. Circulating levels of aCL were measured by enzyme-linked immunosorbent assay. To determine whether Fc gammaR are required for the effects of IVIG, we treated mice deficient in stimulatory Fc gammaR. To examine the effects of IVIG on endogenously generated antibody, we treated mice immunized with beta2-glycoprotein I (beta2GPI).


RESULTS
IVIG treatment inhibited aPL-induced endothelial cell activation and enhancement of thrombosis in mice passively infused with human aPL-containing IgG, and this was associated with a decrease in aPL levels. Similarly, IVIG lowered aPL levels and inhibited thrombogenesis in mice immunized with beta2GPI. The thrombophilic effects of aPL were evident in Fc gammaR-deficient mice.


CONCLUSION
Treatment with IVIG inhibits the thrombogenic effects of aPL in vivo and reduces the levels of aCL in the circulation. Blockade of stimulatory Fc gammaR on inflammatory cells is not necessary for this effect. The mechanism of action of IVIG is more likely saturation of the IgG transport receptor, leading to accelerated catabolism of pathogenic aPL. These results have implications in the management of thrombosis in APS and may have applications for pregnant patients with a history of APS.