THE HUMAN OX40 GP34 SYSTEM DIRECTLY MEDIATES ADHESION OF ACTIVATED T-CELLS TO VASCULAR ENDOTHELIAL CELLS/

Fresh leukemic cells from patients with adult T cell leukemia (ATL) an d some ATL-derived T cell lines show adhesion to human umbilical vein endothelial cells (HUVECs) mainly through E-selectin, but a proportion of this binding remains unaffected by the addition of combinations of antibodies against known adhesion molecules. By immunizing mice with one of such cell lines, we established monoclonal antibodies (mAbs), t ermed 131 and 315, that recognize a single cell surface antigen (Ag) a nd inhibit the remaining pathway of the adhesion. These mAbs did not r eact with normal resting peripheral blood mononuclear cells (PBMC) or most of the cell lines tested except for two other human T cell leukem ia virus type I (HTLV-I)-infected T cell lines. After stimulation with phytohemagglutinin (PHA), PBMC expressed Ag 131/315 transiently, indi cating that these mAbs define a T cell activation Ag. Western blotting and immunoprecipitation revealed that Ag 131/315 has an apparent mole cular mass of 50 kD. Expression cloning was done by transient expressi on in COS-7 cells and immunological selection to isolate a cDNA clone encoding Ag 131/315. Sequence analysis of the cDNA indicated that it i s identical to human OX40, a member of the tumor necrosis factor/nerve growth factor receptor family. We then found that gp34, the ligand of OX40, was expressed on HUVECs and other types of vascular vascular en dothelial cells. Furthermore, it was shown that the adhesion of CD4(+) cells of PHA-stimulated PBMC to unstimulated HUVECs was considerably inhibited by either 131 or 315. Finally, OX40 transfectants of Kit 225 , a human interleukin 2-dependent T cell line, were bound specifically to gp34 transfectants of MMCE, a mouse epithelial cell line, and this binding was blocked by either 315 or 5A8, an anti-gp34 mAb. These res ults indicate that the OX40/gp34 system directly mediates adhesion of activated T cells or OX40(+)-transformed T cells to vascular endotheli al cells.