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Antiphospholipid antibodies (aPL) are associated with an increased risk of thrombosis; however, the mechanism remains unknown. Recent studies have focused on the impediment of protein C anticoagulant activity by anti-β2-glycoprotein I (β2GPI) antibodies (aβ2GPI Ab). We purified IgG fractions containing a high concentration of aβ2GPI Ab from patients with antiphospholipid syndrome (APS) and then investigated the effect of purified aβ2GPI Ab on the activity of activated protein C (APC). Using a three-step chromatography method (DEAE-sepharose column, phosphatidylserine polyacrylamide gel column dependent on the presence of β2GPI, and protein G column chromatography), we successfully isolated anti-β2GPI IgG from nine patients with APS. Seven of nine samples inhibited APC activity in a concentration-dependent manner only in the presence of β2GPI, as observed by a chromogenic assay that was able to determine thrombin activity even in the presence of APC. The extent of APC inhibition by these fractions appeared to be related to aβ2GPI Ab titers of the purified IgG. However, the inhibitory effect of IgG from patients was not detected in the absence of β2GPI. IgG purified from three normal subjects did not affect APC activity. Herein, we show a useful method for the isolation of IgG containing a high concentration of aβ2GPI Ab. Moreover, the present findings indicate that inhibition by aβ2GPI Ab on APC anticoagulant activity could explain one of the mechanisms for the thrombotic state in APS. [ABSTRACT FROM AUTHOR]