CD22/cal has B-cell-specific in vitro cytotoxicity. (A) Cy34.1 mouse IgG1 mAb is conjugated to N-acetyl-γ-calicheamicin dimethyl acid. CD22/cal has an average loading of 17 to 30 μg calicheamicin/mg antibody protein. (B) Biochemical conjugation of Cy34.1 mAb to calicheamicin (CD22/cal) had no effect on the binding properties of Cy34.1. On staining, both Cy34.1 and CD22/cal bound similarly to CD22 on A20 cells. (C) ³H-thymidine incorporation by primary mouse CD19⁺ B cells stimulated with LPS and incubated for 48 hours with increasing concentrations of Cy34.1 or CD22/cal. Unconjugated Cy34.1 had no effect, 3 μg/mL CD22/cal completely inhibited B-cell proliferation. (D) ³H-thymidine incorporation by primary mouse B cells stimulated with LPS and incubated for 48 hours with increasing concentrations of CD22/cal or control GG5/cal antibody. GG5/cal had no cytotoxicity against B cells. (E) ³H-thymidine incorporation by primary mouse CD3⁺ T cells after TCR costimulation and incubation for 48 hours with increasing concentrations of CD22/cal or GG5/cal. CD22/cal had no cytotoxicity against T cells. ³H-thymidine incorporation results are representative of 2 independent experiments and are shown as the mean and SD of triplicate culture.