Many tissue-resident macrophages are derived from embryonic precursors. Mowat and colleagues show that embryonic precursor cells seed gut tissues but at weaning transition to a bone marrow–derived macrophage population that requires continual replenishment. The paradigm that macrophages that reside in steady-state tissues are derived from embryonic precursors has never been investigated in the intestine, which contains the largest pool of macrophages. Using fate-mapping models and monocytopenic mice, together with bone marrow chimera and parabiotic models, we found that embryonic precursor cells seeded the intestinal mucosa and demonstrated extensive in situ proliferation during the neonatal period. However, these cells did not persist in the intestine of adult mice. Instead, they were replaced around the time of weaning by the chemokine receptor CCR2–dependent influx of Ly6Chi monocytes that differentiated locally into mature, anti-inflammatory macrophages. This process was driven largely by the microbiota and had to be continued throughout adult life to maintain a normal intestinal macrophage pool.

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^hiCCR2⁺ 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.

Summary: The intestine contains the largest pool of macrophages in the body which are essential for maintaining mucosal homeostasis in the face of the microbiota and the constant need for epithelial renewal but are also important components of protective immunity and are involved in the pathology of inflammatory bowel disease (IBD). However, defining the biological roles of intestinal macrophages has been impeded by problems in defining the phenotype and origins of different populations of myeloid cells in the mucosa. Here, we discuss how multiple parameters can be used in combination to discriminate between functionally distinct myeloid cells and discuss the roles of macrophages during homeostasis and how these may change when inflammation ensues. We also discuss the evidence that intestinal macrophages do not fit the current paradigm that tissue‐resident macrophages are derived from embryonic precursors that self‐renew in situ, but require constant replenishment by blood monocytes. We describe our recent work demonstrating that classical monocytes constantly enter the intestinal mucosa and how the environment dictates their subsequent fate. We believe that understanding the factors that drive intestinal macrophage development in the steady state and how these may change in response to pathogens or inflammation could provide important insights into the treatment of IBD.

Dendritic cells (DCs) and monocyte‐derived macrophages (MΦs) are key components of intestinal immunity. However, the lack of surface markers differentiating MΦs from DCs has hampered understanding of their respective functions. Here, we demonstrate that, using CD64 expression, MΦs can be distinguished from DCs in the intestine of both mice and humans. On that basis, we revisit the phenotype of intestinal DCs in the absence of contaminating MΦs and we delineate a developmental pathway in the healthy intestine that leads from newly extravasated Ly‐6C^hi monocytes to intestinal MΦs. We determine how inflammation impacts this pathway and show that T cell‐mediated colitis is associated with massive recruitment of monocytes to the intestine and the mesenteric lymph node (MLN). There, these monocytes differentiate into inflammatory MΦs endowed with phagocytic activity and the ability to produce inducible nitric oxide synthase. In the MLNs, inflammatory MΦs are located in the T‐cell zone and trigger the induction of proinflammatory T cells. Finally, T cell‐mediated colitis develops irrespective of intestinal DC migration, an unexpected finding supporting an important role for MLN‐resident inflammatory MΦs in the etiology of T cell‐mediated colitis.

Peritoneal macrophages are one of the most studied macrophage populations in the body, yet the composition, developmental origin and mechanisms governing the maintenance of this compartment are controversial. Here we show resident F4/80^hiGATA6⁺ macrophages are long-lived, undergo non-stochastic self-renewal and retain cells of embryonic origin for at least 4 months in mice. However, Ly6C⁺ monocytes constitutively enter the peritoneal cavity in a CCR2-dependent manner, where they mature into short-lived F4/80^loMHCII⁺ cells that act, in part, as precursors of F4/80^hiGATA6⁺ macrophages. Notably, monocyte-derived F4/80^hi macrophages eventually displace the embryonic population with age in a process that is highly gender dependent and not due to proliferative exhaustion of the incumbent embryonic population, despite the greater proliferative activity of newly recruited cells. Furthermore, although monocyte-derived cells acquire key characteristics of the embryonic population, expression of Tim4 was impaired, leading to cumulative changes in the population with age.