Ly49Q ligand expressed by activated B cells induces plasmacytoid DC maturation

Ly49Q, a type II C-type lectin expressed on mouse plasmacytoid DC (pDC), contains a single carbohydrate recognition domain in its extracellular region and an ITIM in its cytoplasmic domain. We have identified the MHC class I molecule H-2Kb as a Ly49Q ligand, confirming prior reports. Although H-2Kb is expressed on essentially all hematopoietic cells, we found that only CpG-stimulated B cells were able to activate Ly49Q. This discovery correlated with our finding that although H-2Kb forms clusters on CpG-activated B cells, it is diffusely expressed on resting B cells. Furthermore, CpG-stimulated, but not resting, B cells up-regulated co-stimulatory molecules on pDC. This finding was confirmed by the fact that binding by anti-Ly49Q mAb to Ly49Q led to pDC maturation in vitro. Our results suggest that clustered H-2Kb on activated B cells act as ligands for Ly49Q and induce pDC maturation in vitro.