Characterization of CD40 signaling determinants regulating nuclear factor-kappa B activation in B lymphocytes

Abstract

CD40 signaling to B cells is important for generating an effective humoral immune response. CD40 ligation leads to B cell activation events such as proliferation, Ig secretion, isotype switching, and up-regulation of cell surface molecules, as well as the generation of memory B cells. Many of these events are dependent upon the ability of CD40 to activate the transcription factor NF-kappa B (NF-kappa B). To define the CD40 signaling components upstream of NF-kappa B activation and the functional consequences downstream of NF-kappa B activation, we examined mouse B cell transfectants expressing wild-type or mutant human CD40. Analysis of CD40 cytoplasmic domain truncation and point mutants defined a 10-amino acid CD40 cytoplasmic signaling determinant required for NF-kappa B activation. A threonine residue at position 234, previously shown to be important for CD40 association with TNF receptor-associated factor 2 (TRAF2), TRAF3, and TRAF5, was not required for NF-kappa B activation. This suggests that in B cells, CD40-induced NF-kappa B activation can occur independently of TRAF2 and TRAF5 association. NF-kappa B activation was independent of the transmembrane domain of CD40, suggesting that it is independent of p23, a molecule that associates with CD40 in a region other than the cytoplasmic domain. Proteasome-dependent inhibitory kappa B alpha (I kappa B alpha) and I kappa B beta degradation occurred downstream of CD40 ligation and preceded CD40-mediated NF-kappa B nuclear translocation. CD40- or pervanadate-mediated I kappa B tyrosine phosphorylation was not detected. NF-kappa B activation correlated with the ability of CD40 to induce Ab secretion and the up-regulation of ICAM-1 and LFA-1. However, NF-kappa B activation was insufficient for CD40-mediated up-regulation of B7-1, Fas, and CD23.