Analysis of P-gp–mediated drug efflux activity in freshly isolated PBMCs from HTLV and non-HTLV–infected subjects. Rhodamine 123 (R123) dye efflux assays were used to measure the extent of drug resistance. This figure shows the results of a single representative sample; data from other subjects examined is summarized in Table 1. (Left panel) FACscan analysis of cells from a typical HTLV-negative control subject. No decrease in fluorescence is evident after a 90-minute incubation, indicating no P-gp activity. (Center panel) FACscan analysis of cells from a typical HTLV-I–infected subject. After a 90-minute incubation, two populations of cells are present, a high efflux, low fluorescence population and a low efflux, high fluorescence population. (Right panel) Inhibition of R123 efflux by P-gp inhibitors. In the presence of 5 μmol/L reserpine or 5 μmol/L verapamil (shown here), the dye efflux is blocked, indicating the specificity of this assay for P-gp.