MHC class II signal transduction in human dendritic cells induced by a natural ligand, the LAG-3 protein (CD223)

On encountering a danger signal, dendritic cells (DCs) undergo a complex maturation process and become specialized in antigen presentation. We previously reported that engagement of major histocompatibility complex (MHC) class 11 molecules located on immature DCs in membrane rafts by lymphocyte activation gene-3 (LAG-3; CD223) leads to DC maturation. In contrast, exposure of DCs to class U-specific monoclonal antibodies (mAbs) did not lead to maturation. Here, we have investigated the signal transduction pathways involved in the LAG-3-induced maturation of human monocyte-derived DCs. We first show that areas of raft aggregation (both cholesterol rich and CDw78 microdomains) could be visualized using a soluble LAG-3 protein and confocal microscopy. Engagement of class 11 molecules by both its natural ligand LAG-3 and class 11 mAb induces rapid protein phosphorylation of phospholipase Cgamma2 (PLCgamma2) and p72syk as well as activation of phosphatidyl inositol 3-kinase/Akt, p42/44 extracellular signal-regulated protein kinase, and p38 mitogen-activated protein kinase pathways. Studies using inhibitors demonstrate that these 3 pathways are all important in inducing the maturation process of LAG-3-stimulated DCs. When class 11 molecules were ligated with LAG-3 versus specific antibody, differences in the phosphorylation pattern of c-Akt were observed. Thus, MHC class 11 signaling in DCs involves several pathways that have to be finely regulated to lead to cell activation and maturation. (C) 2003 by The American Society of Hematology.