Model of G-CSF–induced HSPC mobilization. Under basal conditions, monocytic cells in the bone marrow (here shown as macrophages) provide signals that help maintain CXCL12 expression from multiple mesenchymal stromal cell populations, including osteoblasts (Ob) and CAR cells. In response to infection, systemic levels of G-CSF are often increased. G-CSF signaling in monocytic cells leads to a loss of macrophages in the bone marrow. This, in turn, results in reduced CXCL12 expression from mesenchymal stromal cells and HSPC mobilization.