IL-16 binding to HMC-1 cells. HMC-1 cells were incubated with anti-CD9 (A), anti-CD63 (C), or anti-CD81 (E) mAbs. Alternately, the cells were exposed to CD9-specific antisense (B) or sense (D) oligonucleotides. The ability of IL-16 to bind to the surfaces of the 5 populations of cells was then evaluated. The blackline in each panel represents the binding of labeled IL-16 to the untreated control cells. Shown in gray are the IL-16–binding data obtained with the mAb- or oligonucleotide-treated cells. Shown in dark gray in panels A, C, and E are the control data obtained when cells are incubated with an antitetraspanin mAb in the absence of the labeled cytokine. The bar graph in panel F summarizes the obtained data from 3 different antibody-blocking experiments. Data represent the mean ± SD of 3 experiments.