Identification of distinct domains in CD40 involved in B7-1 induction or growth inhibition

Binding of CD40 ligand to CD40 on a murine B lymphoma M12 induces B7-1 and inhibits cell growth. We have used M12 lymphomas transfected with wild-type and mutant human CD40 molecules to identify regions of the CD40 cytoplasmic tail involved in these signal transduction events. We find that threonine residues at positions 227 and 234 in the cytoplasmic domain play important role in B7-1 induction, but have lesser importance in CD40-mediated growth inhibition. In contrast, a deletion mutant with only six amino acids in the cytoplasmic tail retains some growth-inhibitory function, but has no detectable B7-1 induction capacity. We also find that cAMP synergizes with CD40 signaling to induce high level B7-1 induction, and that the synergistic signal through CD40 requires either T227 or T234, but not both. Thus, we provide evidence for at least two distinct signaling domains in the CD40 molecule.