Induction of CD11b expression in U1 cells and uPA-mediated inhibition of HIV-1 expression. (A) U1 cells were gated based on their light scatter and analyzed for both uPAR (R4 mAb) and CD11b (αM44 mAb) expression 20 hours after stimulation with PMA, TNF-α, or IL-6; the integrin β₂ chain CD18 is constitutively expressed on U1 cells (Table 1). All stimuli up-regulated the levels of uPAR on the cell surface, but only PMA and, to a lesser extent, TNF-α, promoted the expression of CD11b on U1 cells. (B) uPA inhibits HIV expression in U1 cells stimulated with PMA and TNF-α, but not with IL-6, IFN-γ, or GM-CSF plus LPS. The results were obtained from a single experiment representative or 5 or more independently performed experiments. Error bars indicate the SD of duplicate samples; and cpm, counts per minute.