aPL antibody binds to β2GPI complexed with different HLA-DR alleles. N-terminal His-tagged β2GPI and GFP were cotransfected into 293T cells together with different HLA-DR alleles. The transfectants were stained with anti-His mAb (β2GPI, solid bars) or aPL antibody (EY2C9 mAb, open bars), followed by APC-labeled anti-mouse IgG or anti-human IgM Ab, respectively. MFIs of APC on GFP-positive cells are shown. Data are representative of 3 independent experiments.