CXCR4 identifies a TpMo population in the BM and mediates monocyte cellular responses in peripheral tissues. (top) Monocytes arise from cMoPs and undergo a developmental pathway that ends with Ly6C^lo monocytes as the terminally differentiated form of monocytes. In this study, we propose that cMoP-differentiated Ly6C^hi monocytes undergo a developmental transition, which involves a transitional premonocyte (TpMo) stage, before giving rise to mature Ly6C^hi monocytes. TpMos are identified based on their CXCR4^hi expression compared with mature Ly6C^hi monocytes that are CXCR4^lo. Furthermore, TpMos can be distinguished from cMoPs based on their negative expression for c-kit. Functionally, TpMos are immobilized in the BM to serve as an immediate precursor for the active pool of mature CXCR4^lo Ly6C^hi monocytes. (Bottom) Upon entering the circulation, CXCR4 plays a significant role in monocyte margination in the pulmonary vasculature. Exposure to subclinical LPS doses triggers increased pulmonary monocyte margination, leading to a transient state of monocytopenia that can be reversed with CXCR4 inhibition using AMD3100. Disruption in CXCR4 signaling also attenuated lung injury and prevented sepsis mortality.