Resident and pro-inflammatory macrophages in the colon represent alternative context-dependent fates of the same Ly6Chi monocyte precursors

TLDR

Macrophages are crucial for intestinal homeostasis and inflammatory bowel disease, yet it is unclear whether distinct macrophage populations mediate these roles or whether resident macrophages alter during inflammation. We demonstrate that resident colon macrophages are CX3CR1^hi, phagocytic, IL‑10–producing, TLR‑resistant cells derived from Ly6C^hi monocytes, while a CX3CR1^int subset expands during colitis and gives rise to TLR‑responsive pro‑inflammatory macrophages, indicating that resident and inflammatory macrophages are alternative fates of the same precursor and suggesting therapeutic targets for IBD.

Abstract

Macrophages (mφ) are essential for intestinal homeostasis and the pathology of inflammatory bowel disease (IBD), but it is unclear whether discrete mφ populations carry out these distinct functions or if resident mφ change during inflammation. We show here that most resident mφ in resting mouse colon express very high levels of CX3CR1, are avidly phagocytic and MHCII(hi), but are resistant to Toll-like receptor (TLR) stimulation, produce interleukin 10 constitutively, and express CD163 and CD206. A smaller population of CX3CR1(int) cells is present in resting colon and it expands during experimental colitis. Ly6C(hi)CCR2(+) monocytes can give rise to all mφ subsets in both healthy and inflamed colon and we show that the CX3CR1(int) pool represents a continuum in which newly arrived, recently divided monocytes develop into resident CX3CR1(hi) mφ. This process is arrested during experimental colitis, resulting in the accumulation of TLR-responsive pro-inflammatory mφ. Phenotypic analysis of human intestinal mφ indicates that analogous processes occur in the normal and Crohn's disease ileum. These studies show for the first time that resident and inflammatory mφ in the intestine represent alternative differentiation outcomes of the same precursor and targeting these events could offer routes for therapeutic intervention in IBD.