Lung Interstitial Macrophages: Past, Present, and Future

Figure 1

Mouse lung interstitial macrophage phenotype and origin at the steady state and upon exposure to LPS, CpG-DNA, or HDM. For clarity, the ratio between the numbers of depicted AM, IM, and monocytes does not reflect the reality. By definition, IM are located in the lung interstitium, while AM reside in the airway lumen. IM can produce IL-10 at baseline, a phenomenon that is potentiated by an exposure to LPS, CpG-DNA [26], or HDM [50]. Phenotypically, IM are non-autofluorescent SiglecF⁻CD11b⁺CX3CR1⁺Ly6C⁻ cells, while AM are autofluorescent SiglecF⁺CD11c⁺CD11b⁻CX3CR1⁻Ly6C⁻ cells. Steady-state IM, as well as LPS- or HDM-induced IM, are thought to be maintained or expanded by the recruitment of CCR2-dependent Ly6C⁺ classical blood monocytes, at least in part. Local proliferation may also account for the maintenance of steady-state IM. Following exposure to CpG-DNA, CCR2-independent lung-resident and splenic Ly6C⁺ monocytes contribute to a large extent to the expansion of the IM pool endowed with enhanced immunoregulatory properties.