Activation of IL-13 synthesis in CD4⁺ T cells infected with HTLV-1 in vivo. Blood was drawn and PBMCs were prepared from 3 HAM/TSP patients, 3 asymptomatic carriers (AC), and 1 HTLV-1-negative donor (NC). Cells were cultured for 24 hours in medium containing brefeldin A and concanamycin A but lacking mitogens. Cells were then stained with fluorescent-tagged antibodies directed against cell surface CD4⁺ or intracellular IL-13 and HTLV-1 Tax proteins and analyzed by flow cytometry. Fluorescence intensities of Tax versus IL-13 are plotted for cells gated as CD4⁺. Neither Tax nor IL-13 signals were observed using matched isotype control antibodies. Patient designations are shown in the upper-right corner of each quadrant.