An inhibitory anti-S1P mAb blocks apoptotic cell extrusion. (A and B) Extrusion in an MDCK monolayer treated with short-wave UV to induce apoptosis in the presence of a mouse IgG isotype control (A) or 10 µg/ml anti-S1P mAb (B). (C) Quantification of nonextruded apoptotic cells from three independent experiments; n = 100 active caspase-3–positive cells where error bars are SDs; **, P < 0.01. (D and E) Alexa Fluor 488–labeled cell fragments (green) prepared from MDCK cells were added to an intact MDCK monolayer in the presence of a mouse IgG isotype control (D) or 10 µg/ml anti-S1P mAb (E). (F) The percentage of cell fragments causing actin assembly from three independent experiments; n = 100 cell fragments per experiment and error bars = SDs. ***, P < 0.001. Bars, 10 µm.