Tiam1 is dispensable for T-cell arrest but essential for subsequent T-cell migration. (A) WT and Tiam1^−/− T cells were subjected to shear flow for 15 minutes after an accumulation phase on activated bEnd.3 monolayer. The average number of adherent cells per field (×20 objective) at the end time point was determined from 4 independent experiments (2 fields per experiment). Error bars indicate SD; p, P value. No significant difference was observed between WT and Tiam1^−/− T cells. (B) WT and Tiam1^−/− T-cell blasts were subjected to shear flow on ICAM-1 substrate. At the indicated time points, average number of adherent cells per field (×10 objective) was calculated from 3 independent experiments (2 fields per experiment). Error bars indicate SD; p, P value between number of adherent WT and Tiam1^−/− T cells for the same time point. No significant difference was observed. (C) Blasts from WT and Tiam1^−/− mice were either nonstimulated or stimulated by adhesion to ICAM-1 for 5 minutes. Subsequently, Rac activity was determined. (Top panel) GTP-bound Rac. (Middle panel) Total Rac. (Bottom panel) Tiam1. (D) Locomotion of adherent WT and Tiam1^−/− blasts on an ICAM-1 substrate under shear flow was recorded for 15 minutes, and the percentage of migrating cells was determined at the indicated time points. At least 3 independent experiments were performed and 2 fields for each time point per experiment were recorded. Values indicate mean plus or minus SD; p, P value between percentage of migrating WT and Tiam1^−/− T cells, for each time point. (E) Representative confocal images of adherent WT and Tiam1^−/− T-cell blasts on an ICAM-1 substrate under shear flow. Images are recorded every 20 seconds. Scale bar represents 5 μm. Arrows indicate the direction of polarization. Note the switches in direction of polarization of Tiam1^−/− T cells. (F) Polarization of WT or Tiam1^−/− T-cell blasts was monitored after 10 minutes on ICAM-1–coated coverslips under shear flow. At least 100 cells were monitored in each group. Two independent experiments were performed and 2 fields per condition were analyzed. Values indicate mean ± SD; p, P value between percentage of polarized WT and Tiam1^−/− T cells.