Expression of the mRNA of IL-15 on the ATL cell lines. Using the same RNA used for IL-15R α, the RT-PCR for IL-15 mRNA was done. The results showed that the four ATL cell lines (lane 2, SO4; lane 3, ST1; lane 4, KK1; lane 5, OMT), the fresh ATL cells (lanes 7 through 10), the activated PBMC (lane 1), and the resting PBMC (lanes 11 through 14) had almost identical amounts of IL-15 mRNA. The activation of PBMC did not cause an increase of the mRNA. HL-60 (lane 6) did not have any IL-15 mRNA.