Figure 1. Bim is required for BCR ligation–induced killing of immature and mature B lymphocytes. (A) Immature B cells sorted from spleen as B220⁺IgM⁺IgD^- were cultured overnight in the presence of F(ab')₂ anti-IgM antibodies and then assessed for viability. Cell survival after 15 h BCR ligation was calculated as a percentage of B cell survival in unstimulated cultures. B cells show a dose-dependent killing reaching 60% at 50 µg/mL for controls, 30% for bim^-/-, and <10% for bcl-2 transgene-expressing cells. (B) Same as for A, except mature B cells were negatively sorted from LNs as Thy1^-Mac-1^-Gr-1^-Ter119^- cells. Control B cells show 50% death at 50 µg/mL F(ab')₂ anti-IgM antibodies, whereas bim^-/- and bcl-2 transgenic B cells show 15 and 5%, respectively. (C) Same as for B, except B cells were stimulated with plate-bound F(ab')₂ anti-IgM antibodies, inducing more severe cell death as evidenced by the 80% cell death of control B cells, 40% death of bim^-/- B cells, and 20% death of bcl-2 transgene-expressing B cells at 50 µg/mL anti-IgM antibody concentration.