IL-12 and IL-18 down-regulate B cell migration in an Ly49D-dependent manner.

In order to complete their maturation and participate in the humoral immune response, immature B cells that leave the bone marrow are targeted to specific areas in the spleen, where they differentiate into mature cells. Previously, we showed that immature B cells actively down-regulate their integrin-mediated migration to LN or to sites of inflammation, enabling their targeting to the spleen. This inhibition is mediated by IFN-gamma, which is transcribed and secreted at low levels by these immature B cells; its expression is subsequently down-regulated following B cell maturation. The activating and inhibitory MHC class I receptors, Ly49D and Ly49G2, regulate IFN-gamma secretion in B cells, preventing their migration to antigen-enriched sites and their premature encounter with an antigen, while enabling their entry into the LN when mature. In the present study, we elucidate the pathways by which the Ly49 receptors regulate IFN-gamma levels. We show that Ly49D stimulation triggers a signaling cascade that increases transcription of both IL-12B and IL-18; these, in turn, can interact with their specific receptors, which are expressed at elevated levels on immature B cells. Ligation of the IL-12B and IL-18 receptors induces the secretion of IFN-gamma, thereby regulating their cytoskeleton rearrangement and migration.