Figure 1.

Monocyte differentiation into DCs and tissue macrophages. Macrophage-DC progenitors (MDPs) give rise to Ly6C⁺ bone marrow monocytes, which exit the bone marrow, in part guided by CCR2-dependent signals. Ly6C⁺ monocytes convert into CX3CR1⁺ monocytes, although the location of this event, in the circulation or bone marrow, remains incompletely understood. Black arrows indicate differentiation steps into tissue DCs and macrophages that occur under homeostatic conditions. Red arrows indicate differentiation steps that occur under inflammatory conditions (UV-induced skin injury, intratracheal LPS administration, or depletion of autologous CD11c⁺ cells). Dashed arrows represent steps that remain uncertain. In the case of splenic cDCs, splenic pre-DCs are the most significant upstream precursor in numeric terms (bold arrow), although MDPs and CX3CR1⁺ monocytes may contribute as well.