DIFFERENT CD40-MEDIATED SIGNALING EVENTS REQUIRE DISTINCT CD40 STRUCTURAL FEATURES

Signals delivered to B cells through CD40 are critical to the developm ent of humoral immune responses, In this study we characterize regions of the 62-amino acid cytoplasmic domain of human CD40 (hCD40) that ar e essential for signal transduction and examine the functional consequ ences of mutations in these regions, A panel of mutant hCD40 molecules was stably expressed in mouse B cell lines and tested for its ability to stimulate Ab secretion, homotypic adhesion, and increased surface expression of B7, Fas, CD23, LFA-1, and intracellular adhesion molecul e-1. Our results indicate that CD40 contains at least two major signal ing determinants in the cytoplasmic domain: one disrupted by a truncat ion of 22 amino acids, and a second disrupted by the removal of 10 add itional amino acids. The second determinant includes threonine 234, a residue previously shown to be important in CD40 signal transduction, Our functional analysis of alanine and serine substitutions at positio n 234 indicates that phosphorylation of this residue may be important for full CD40 signaling activity.