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

Macrophages (m phi) are essential for intestinal homeostasis and the pathology of inflammatory bowel disease (IBD), but it is unclear whether discrete m phi populations carry out these distinct functions or if resident m phi change during inflammation. We show here that most resident m phi 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 m phi subsets in both healthy and inflamed colon and we show that the CX3CR1int pool represents a continuum in which newly arrived, recently divided monocytes develop into resident CX3CR1 hi m phi. This process is arrested during experimental colitis, resulting in the accumulation of TLR-responsive pro-inflammatory m phi. Phenotypic analysis of human intestinal m phi indicates that analogous processes occur in the normal and Crohn's disease ileum. These studies show for the first time that resident and inflammatory m phi in the intestine represent alternative differentiation outcomes of the same precursor and targeting these events could offer routes for therapeutic intervention in IBD.