CD70 is a surface Ag found on activated but not resting T and B lympho
cytes. The biologic activity of this Ab-defined cell surface molecule
on lymphocytes has not been established. Therefore, in an effort to un
derstand the function of the CD70 protein, a mAb defining the CD70 Ag
was used to isolate by expression cloning the cDNA responsible for the
CD70 molecule. The predicted protein product is a type II transmembra
ne protein. Bioassays demonstrated that the CD70 cDNA clone expressed
in African green monkey kidney cells would induce the proliferation of
PHA-costimulated T cells. Comparison with known sequences indicates i
dentity with the CD27 ligand. Therefore the molecule defining the CD70
Ag is identical to the recently defined ligand for CD27.