CD11b is required for the anti-HIV activity of M25 but not of uPA. (A) Expression of both uPAR and CD11b in unstimulated and PMA-stimulated U1 cells before (top panels) and after (bottom panels) transfection with their respective siRNA and control mock siRNA. The uPAR siRNA showed only a partial inhibitory effect on its target, whereas the CD11b siRNA abolished the expression of the integrin chain. (B) CD11b siRNA abolished the M25 inhibitory effects on HIV expression but did not affect uPA-dependent inhibition, whereas decreased uPAR expression abrogated the inhibitory effects of both agents. (C) Lack of CD11b expression did not interfere with uPA/uPAR–dependent adhesion of PMA-stimulated U1 cells to the plastic substrate, an adhesion that was otherwise inhibited by uPAR siRNA (*P < .001). No effect of M25 on the adhesion of PMA-stimulated U1 cells was observed. The results shown were derived from one experiment representative of 4 independently performed experiments. cpm indicates counts per minute.