Complement does not effect the ability of anti-CD20 mAb to deplete B cells in vivo. (A) BALB/c human CD20 transgenic mice⁷  (WT or C1q^−/−) received 250 μg of anti–human CD20 mAb (Rit m2a⁷) or mAb lacking C1q binding activity (K322A mutation⁷ ) intravenously on day 0. The number of circulating B cells was then assessed by flow cytometry for CD19 and B220. The results are expressed as percentage of B cells observed at time 0. (B) Experiments show adoptive transfer of hCD20 Tg (target: high carboxyfluorescein succinimidyl ester [Sigma-Aldrich]) and WT (nontarget: low carboxyfluorescein succinimidyl ester) splenic B cells into recipient mice carrying various effector defects (C1q^−/−, C3^−/− [Jackson Laboratories], Fcγ chain^−/−, and clodronate [Sigma-Aldrich], treated). Twenty-four hours later, mice received control or Rit m2a mAb (10 μg, intravenously), and 16 hours later splenic B cells were analyzed by flow cytometry to determine the target:nontarget ratio. Bars represent mean ± SD, n = 3; each condition is representative of at least 2 independent experiments. The data clearly demonstrate that Rit m2a only deletes target cells (low target:nontarget ratio), and that activatory FcR (absent in Fcγ chain^−/− mice) and macrophages (deleted in clodronate-treated mice), but not complement components C1q or C3, are important for target-cell deletion.