The monocyte-macrophage axis in the intestine

•Mϕ are involved in gut homeostasis and the pathogenesis of intestinal inflammation.•Resident and proinflammatory intestinal Mϕ both derive from Ly6Chi blood monocytes.•Local environmental factors guide monocyte differentiation in the gut mucosa.•Monocyte differentiation is disrupted by inflammation resulting in the accumulation of proinflammatory cells. Macrophages are one of the most abundant leucocytes in the intestinal mucosa where they are essential for maintaining homeostasis. However, they are also implicated in the pathogenesis of disorders such as inflammatory bowel disease (IBD), offering potential targets for novel therapies. Here we discuss the function of intestinal monocytes and macrophages during homeostasis and describe how these populations and their functions change during infection and inflammation. Furthermore, we review the current evidence that the intestinal macrophage pool requires continual renewal from circulating blood monocytes, unlike most other tissue macrophages which appear to derive from primitive precursors that subsequently self-renew.