Effects of TACI siRNA and heparitinase on APRIL and BAFF binding to B cells. (A) Down-regulation of TACI by siRNA. Control or TACI siRNA was transfected into human peripheral blood B cells, as described in “Materials and methods,” and was subjected to immunoblot analysis probed by polyclonal anti-TACI antibody. β-Actin represents the loading control. (B) Cell surface expression of TACI and HSPG. B cells transfected with TACI siRNA or control siRNA were stained with anti-TACI mAb (11H3; left) or anti–heparan sulfate side chain of HSPG mAb (10E4; right) in the presence or absence of heparitinase (10 U/mL) for 10 minutes at 37°C. (C) Reduced binding ability of APRIL and BAFF by the defect of TACI and HSPG. Cells were treated as in panel B and were stained with FLAG-tagged APRIL (left) or BAFF (right). Stained cells were analyzed by flow cytometry. Ctr indicates control siRNA; hep, heparitinase treatment. Data are representative of 3 independent experiments with similar results.