β2 integrin mAbs suppress the CD25 expression synergistically with sICAM-5. T cells were incubated in the presence of 10 μg/mL sICAM-5 D1–2-Fc together with 5 μg/mL of mAbs against integrin αL, β2, and β1 chains, respectively, in anti-CD3–coated plates for 48 hours. The αL mAbs TS2/4 and TS1/22 did not significantly inhibit the CD25 expression on T cells by themselves but showed additional suppressive effects together with sICAM-5. The activating αL mAb MEM83 neither decreased the CD25 expression alone nor counteracted the suppressive effect of sICAM-5. By contrast, the β2 mAb R7E4 inhibited the CD25 expression per se and also showed additional suppressive effects together with sICAM-5 (A). The β1 integrin activating mAb TS2/16 did not inhibit the CD25 expression by itself and significantly counteracted the suppressive effect of sICAM-5. On the other hand, the β1 integrin inhibitory mAb 2253 significantly inhibited the CD25 expression per se but showed no additional suppressive effect when applied together with sICAM-5 (B). Moreover, 5 mM EDTA significantly down-regulated the CD25 expression and acted synergistically with sICAM-5 (B). Data are shown as means plus or minus SD of triplicates (*P < .05, **P < .01).