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

On encountering a danger signal, den-dritic cells (DCs) undergo a complex matu-ration process and become specialized in antigen presentation. We previously re-ported that engagement of major histo-compatibility complex (MHC) class II mol-ecules located on immature DCs in membrane rafts by lymphocyte activation gene-3 (LAG-3; CD223) leads to DC matu-ration. In contrast, exposure of DCs to class II–specific monoclonal antibodies (mAbs) did not lead to maturation. Here, we have investigated the signal transduc-tion 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 visu-alized using a soluble LAG-3 protein and confocal microscopy. Engagement of class II molecules by both its natural ligand LAG-3 and class II mAb induces rapid protein phosphorylation of phos-pholipase C2 (PLC2) and p72syk as well as activation of phosphatidyl inositol 3-kinase/Akt, p42/44 extracellular signal-regulated protein kinase, and p38 mito-gen-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 II mol-ecules were ligated with LAG-3 versus specific antibody, differences in the phos-phorylation pattern of c-Akt were ob-served. Thus, MHC class II signaling in DCs involves several pathways that have to be finely regulated to lead to cell activa-tion and maturation. (Blood. 2003;102