Tiam1 is required for chemokine- and S1P-induced Rac activation and chemotaxis. (A) Lymphocytes from WT and Tiam1^−/− mice were either nonstimulated or stimulated with 250 ng/mL SDF1α for the indicated time period. Subsequently Rac activity was determined. (Top panel) Tiam1. (Bottom panels) GTP-bound Rac and total Rac. Sizes are indicated in kilodaltons. Representative results of 3 independent experiments are shown. (B) Chemotaxis of WT and Tiam1^−/− T lymphocytes toward SDF1α (100 ng/mL) was measured in transwells. After 1 hour, cells present in the lower chamber were counted. Results are derived from 3 independent experiments and presented as the percentage of the input cells. Error bars indicate SD; p indicates P value. (C) Lymphocytes from WT and Tiam1^−/− mice were either nonstimulated or stimulated with 250 ng/mL SLC for the indicated time period. Subsequently Rac activity was determined. (Top panel) Tiam1. (Bottom panels) GTP-bound Rac and total Rac. Sizes are indicated in kilodaltons. Representative results of 2 independent experiments are shown. (D) Chemotaxis of WT and Tiam1^−/− T lymphocytes toward SLC (100 ng/mL) was measured in transwells. After 1 hour, cells present in the lower chamber were counted. Results are derived from 2 independent experiments and presented as the percentage of the input cells. Error bars indicate SD; p indicates P value. (E) Lymphocytes from WT and Tiam1^−/− mice were either nonstimulated or stimulated with 150 nM S1P for the indicated time period. Subsequently Rac activity was determined. (Top panel) Tiam1. (Bottom panels) GTP-bound Rac and total Rac. Sizes are indicated in kilodaltons. Representative results of 3 independent experiments are shown. (F) Chemotaxis of WT and Tiam1^−/− T lymphocytes toward S1P (15 nM) was measured in transwells. After 1 hour, cells present in the lower chamber were counted. Results are derived from 3 independent experiments and presented as the percentage of the input cells. Error bars indicate SD; p, P value.