Binding of β2-GPI-mAb complex to liposomes does not necessarily require PE or oxidation. Nonoxidized (▪, •, ▴) or oxidized (□, ○, ▵) liposomes (10 μg/mL) were used to study the binding of β2-GPI–anti-β2-GPI complexes in the presence of 2 mM calcium. Complexes were formed by mixing the mAb no. 1522 with 2.25 times its concentration of β2-GPI to ensure saturation of the mAb before addition to the liposomes. Concentrations indicated on the x-axis refer to those of the mAb. (A) PS/PC vesicles; (B) PE/PS/PC vesicles; (C) vesicles contain 40% PE plus 2% PS (○, •), 5% PS (▵, ▴), or 20% PS (□, ▪), with the remainder of vesicles comprised of PC. Binding isotherms of panel C were performed with a different set of liposomes than panels A and B, resulting in slightly different maximum binding values for the 20%PS/PE/PC vesicles (□, ▪) than observed in panel B. Similar results were obtained with the other mAbs. Is/lo is the ratio of signal intensity after reagent addition (Is) to the baseline value (lo).
