Presence of the 4-1BB ligand (4-1BBL) in HTLV-1/Tax-positive cells. (A) Transcripts of 4-1BBL and β-actin (ACTB) were detected by RT-PCR. Vertical lines have been inserted to indicate a repositioned gel lane. (B) Transcripts of 4-1BBL were quantified by real-time RT-PCR. Relative copy number was determined by normalizing the 4-1BBL transcripts to those of β-actin. The means of 3 independent experiments plus or minus standard error are shown. (C) Surface expression of 4-1BBL was analyzed in HTLV-1–positive cell lines and in uninfected controls (HuT-78 and CEM). Extracellular 4-1BBL 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.