Visualizing T cell–mediated B-cell killing using bispecific antibodies. (A) CMFDA-labeled BJAB cells (green) were incubated with the bispecific antibody and anti-CD8–APC-labeled T cells (blue) on the Drop-Array plate. T cell–mediated killing of the BJAB cells is seen as positive PI staining (red) in a green cell adjacent to a blue cell. Over time, the number of PI-positive cells increases. The inset regions indicate the areas magnified in panel B. Scale bar = 70 μm. (B) Three different interaction events are indicated. Panel i shows an event where a T cell is adjacent to a BJAC cell and causes the death of that cell as indicated by positive PI staining. Panel ii shows an event in which the T cells cause cell killing of the BJAB cell but the PI positive cell does not burst until much later than the 8-hour time point. Panel iii shows an event in which a T cell is adjacent to 2 BJAB cells and fails to kill either of them. Scale bar = 14 μm. (C) Quantitation of the number of BJAB cells (green) remaining at 28 hours with increasing concentrations of the bispecific antibody. (D) Quantitation of the number of PI-positive cells (red) after 28 hours with increasing concentrations of the bispecific antibody.