β2GPI complexed with HLA class II molecules is recognized by autoantibodies in APS patients. Autoantibody binding to β2GPI bound to HLA class II in aPL antibody-positive and -negative APS patients and healthy controls. Diluted sera were mixed with cells transfected with β2GPI and HLA-DR7, and IgG Ab binding to the cells was assessed (open histogram). Cells transfected with GFP alone were stained as a control (shaded histogram). Levels of aCL antibody-IgG, anti-β2GPI antibody-IgG, and lupus anticoagulant (LA) of each sample are indicated. Levels above the normal ranges (aCL antibody-IgG, 18.5 IgG phospholipid; anti-β2GPI antibody-IgG, 2.2 U; LA, 1.3) are indicated in bold underlined numbers. N.D., not detected. Data are representative of at least 3 independent experiments.