Expression of the mRNA of IL-15R α on ATL cell lines. RT-PCR for IL-15R α mRNA (A) and Southern blot analysis of the products (B), and RT-PCR for β-actin mRNA as a control of PCR products (C) were performed. The results indicated that all four ATL cell lines (lane 2, SO4; lane 3, ST1; lane 4, KK1; lane 5, OMT) had the mRNA of IL-15R α (778 bp). Three samples of fresh ATL cells (lanes 7, 8, and 10) had smaller amounts of the mRNA equivalent to those of the resting normal PBMC (lanes 11 through 14). One sample of fresh ATL cells (lane 9) and the HL-60 cells (lane 6) did not have the IL-15R α mRNA. The activated normal PBMC (lane 1) had an amount of the mRNA larger than that of the resting PBMC.