CD31 stimulation of endothelial cells increases the adhesive function of vβ3 integrin to RGD peptide.⁵¹Cr-labeled HUVECs were preincubated with anti-CD31 MoAb or control mouse IgG (10 μg/mL each) for 10 minutes at 37°C and were then washed. ⁵¹Cr-labeled HUVECs (5 × 10⁵) were then added to the RGD peptide-coated well of tissue culture plates and were incubated for 10 minutes at 37°C in the presence or absence of anti-vβ3 integrin MoAb, anti-vβ5 integrin MoAb, or anti-5β1 integrin MoAb (10 μg/mL each). After unbound cells were washed off, bound cells were lysed by the addition of HBSS containing 1% Triton X-100 and the radioactivity of the cells was counted in a gamma counter. Data are the means ± SD for 6 experiments. *Significantly different from the mean value of the control response (*P < .001). **Significantly different from the mean value of the control response of anti-CD31 MoAb-pretreated HUVECs (**P < .001).