β₂-GPI in its elongated form interacts with C3b and enhances degradation of C3b. (A) β₂-GPI (black columns) shows minor binding to C3b, but pH-modified β₂-GPI (gray columns) gained binding activities to C3b. Binding of the β₂-GPI antibody to C3b is shown (background). (B) β₂-GPI (black columns) shows minor binding to C3b, but recombinant β₂-GPI/I-IV (patterned columns) bound to immobilized C3b. The binding was dose-dependent. β₂-GPI/IV-V (light gray columns) did not bind to C3b. Data are mean ± SD of 3 independent experiments. *P < .05. ***P < .001. (C) NHS (20%) was preincubated with β₂-GPI, β₂-GPI/I-IV, or β₂-GPI/IV-V (each 0.1-0.4μM), surface activated via the alternative complement pathway, and assayed for presence of C5b-9. β₂-GPI (black columns) and β₂-GPI/I-IV (patterned columns) reduced C5b-9 deposition. Fragment β₂-GPI/IV-V (light gray columns) and BSA (gray columns) showed no effect on pathway activation. Data are mean plus or minus SD of 4 independent experiments. *P < .05. **P < .01. ***P < .001. (D) β₂-GPI/I-IV showed low cofactor enhancing activities (lanes 2-5). (E) β₂-GPI/IV-V did not modulate C3b cofactor activities (lanes 2-5).