CD40 translocation to lipid rafts: signaling requirements and downstream biological events.

CD40, a member of the TNF receptor family, is expressed on a variety of immune and non-immune cells. Its interaction with its ligand, CD154, plays a pivotal role in humoral and cell-mediated immunity. A low level of CD40 is constitutively associated within membrane lipid rafts and, upon engagement, this level is significantly enhanced. In this study, our objective is to evaluate the process of CD40/lipid raft association in terms of the signals required for its initiation and the resulting biological outcomes. Here, we show the CD40/lipid raft association to be independent of PI-3-kinase, Src family kinases and p38 MAPK pathways. Moreover, CD40 lacking its intracellular domain, which is usually required for CD40-mediated signaling, still localizes to lipid rafts upon engagement, confirming that the CD40/lipid raft association is independent of signaling events. As to the biological outcomes of the CD40/lipid raft association, we show that disrupting lipid raft integrity selectively abolishes CD40-mediated Akt phosphorylation. In addition, replacing the transmembrane domain of CD40 with that of CD45 (a protein excluded from lipid rafts) dramatically reduced CD40-mediated Akt phosphorylation and B7.1 upregulation, while not influencing p38, ERK and JNK activation. Together, these findings clarify the requirements for CD40/lipid raft association and the signals triggered upon CD40 engagement by CD154.