Titration of CD3x19 bs ab and apoptosis induction in vivo. In analogy to the experiment shown in Table 1, induction of T-cell apoptosis was measured on the single-cell level by ISNT after CD3x19 bs ab targeting in vivo. Groups of three SCID mice were injected intraperitoneally with irradiated Nalm-6 (10⁷) and activated T cells (5 × 10⁷ ). Control groups were mock-injected with PBS (not shown, values were in the range of the anti-CD3 + anti-CD19 group), anti-CD28 (white circles; mab titrated from 200 down to 25 μg per mouse), or monospecific control antibodies (white squares, anti-CD19 [HD37] plus anti-CD3 [OKT3], titrated from 100 μg down to 12.5 μg each). The other groups were treated with CD3x19 bs ab (black squares; titrated from 200 μg down to 25 μg), or CD3x19 plus anti-CD28 (black circles; 200 μg CD3x19 titrated down to 25 μg plus anti-CD28 at a fixed amount of 50 μg/animal). After 16 hours, the cells were washed out of the peritoneal cavity, B cells were depleted as described above, and cells were stained for DNA strand-breaks using ISNT. Percentages of strand-break–positive cells as a marker for apoptosis were measured on the single level using a FACSort flow cytometer.