Unligated integrin α5β1 regulates endothelial cell survival. HUVECs were plated on poly-L-lysine–coated, fibronectin-coated (a and b), or vitronectin-coated (c and d) culture plates in culture medium (med) or culture medium containing anti-α5β1, anti-αvβ3, anti-α2β1 Ab’s. After 1 hour, cell attachment was determined (b and d). After 24 hours, the percentage of FITC-annexin V–positive (a and c) cells was determined. (e) HUVECs plated on vitronectin-coated plates in the presence of function-blocking anti-α5β1 or control Ab’s were collected at regular intervals from 0 to 8 hours and PARP cleavage assessed by Western blotting. Relative PARP cleavage levels were determined by densitometry.