ICAM-2 regulates activation of the small GTPase Rac during endothelial tube formation. (A) Rac activation during Matrigel tube formation. MCECs plated onto Matrigel were analyzed for Rac activation at spreading (40 minutes) and tube formation stage (4 hours) by GST-PBD pull-down assays. During spreading (40 minutes), ICAM-2^–/– and WT cells had comparable levels of Rac activation; however, during tube formation (4 hours), ICAM-2^–/– cells were unable to sustain Rac activation. *P < .05, unpaired t test. n = 4. (B) ICAM-2 signaling induces Rac activation. HUVECs grown to 80% confluence were starved for 16 hours and incubated with goat anti–ICAM-2 Ab at 15 μg/mL (pAb) for 30 minutes, followed by cross-linking with anti–goat IgG Ab. Rac activation was measured at 10, 20, 30, and 90 minutes following cross-linking. Negative control samples were incubated with either the cross-linking Ab or primary Ab alone. ICAM-2 cross-linking induced Rac activation, which peaked at 20 minutes and returned to baseline after 30 minutes. *P < .05, unpaired t test. n = 5. (A-B) Error bars indicate mean ± SEM.