Surface expression of 4-1BB protein on HTLV-1–transformed human T cells. Expression of 4-1BB was determined in ATL-derived (ATL-3, JuanaW), HAM/TSP-derived (Eva, Xpos), and HTLV-1 in vitro–transformed cell lines (MT-2, C91-PL). Uninfected CCRF-CEM and PBL cells from a healthy donor served as negative controls. For positive control, CCRF-CEM and PBLs were treated with PMA/ionomycin (stim). Extracellular 4-1BB was stained with specific antibodies (black curves) and analyzed by flow cytometry; gray curves represent an unspecific isotype-matched control antibody. One representative experiment of at least 3 is shown.