Induction of OX40, a Receptor of gp34, on T Cells by Trans‐acting Transcriptional Activator, Tax, of Human T‐Cell Leukemia Virus Type I

gp34, which we had identified as a target molecule of the trans‐activation by Tax of human T‐cell leukemia virus type I (HTLV‐I), has been found to bind OX40, a member of the tumor necrosis factor receptor family, resulting in growth stimulation of activated T cells. We here demonstrate that not only gp34 (OX40L), but also OX40 can be transcriptionally activated by Tax. Three Tax‐producing human T‐cell lines carrying the HTLV‐I genome expressed OX40 on their surfaces. Furthermore, Tax‐induced transcriptional activation of OX40 was shown in Tax‐inducible JPX‐9 cells. These results demonstrate that both OX40 and its ligand (gp34) are constitutively expressed on the surfaces of Tax‐expressing T lymphocytes, suggesting that the OX40L/OX40 system contributes to growth stimulation of the virus‐infected T cells.