Bone marrow Ly6Chigh monocytes are selectively recruited to injured kidney and differentiate into functionally distinct populations.

Roles for monocyte/macrophages (Mphi) in directing the development of tissue fibrosis are increasingly recognized. Macrophages form a heterogeneous group of inflammatory leukocytes, and the mechanisms by which they acquire heterogeneity and its functional significance are unclear. We used the unilateral ureteral obstruction model of progressive kidney fibrosis to explore macrophage heterogeneity and function further. Unilateral ureteral obstruction kidney Mphis form three distinct subpopulations defined by the marker Ly6C, all of which are derived from a single Ly6C(high) bone marrow monocyte population selectively recruited to the kidney. Conditional ablation of these Mphis in vivo in CD11b-DTR mice is potently antifibrotic. The mRNA transcription profile of these populations is consistent with differential functional roles for each subpopulation, with Ly6C(low) macrophages transcribing genes consistent with selective profibrotic or M2-type function. Furthermore, bone marrow chimerism studies indicate that although resident kidney macrophages proliferate markedly to comprise up to 40% of the inflammatory macrophage population, they do not contribute to fibrosis. Our data identify Ly6C as a marker of functionally discrete tissue macrophage subsets and support a model of selective recruitment of Ly6C(high) bone marrow monocytes to the kidney that differentiate into three populations of kidney macrophages, including a profibrotic Ly6C(low) population.