Intestinal macrophages: well educated exceptions from the rule

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Intestinal macrophages: well educated exceptions from the rule Ehud Zigmond1,2 and Steffen Jung1 1 2

Department of Immunology, The Weizmann Institute of Science, Rehovot, Israel Gastroenterology and Hepatology Institute, Tel Aviv-Sourasky Medical Center, Tel Aviv, Israel

Macrophages are the most abundant mononuclear phagocytes in the healthy intestinal lamina propria and have emerged as crucial sentinels for the maintenance of tissue homeostasis. Matching the dynamic mucosal landscape, CX3C chemokine receptor (CX3CR)1-expressing macrophages are relatively short lived, and as opposed to most other tissue macrophages, are continuously replaced from blood monocytes that acquire in the healthy tissue context a robust noninflammatory gene expression signature. By contrast, during gut inflammation, monocytes differentiate in the gut into proinflammatory effector cells, as well as migratory antigen-presenting cells. Manipulation of monocyte fates in the intestine might hold promise for the disease management of inflammatory bowel disorders. The challenging dynamic gut landscape The human intestinal tract is densely populated with up to 1012 microorganisms per gram of luminal content, representing a biomass 10 times as large as the body’s own cells [1,2]. This diverse and adaptable bacterial community complements its host for digestive efficiency [3] and also restrains pathogen colonization. Although these commensals can reach gut-draining lymph nodes (LNs) [4], peaceful coexistence of the microbiota with the host and prevention of sepsis is ensured by ‘firewalls’ that prevent systemic bacterial spread [5]. Before these walls, however, and in particular in the connective tissue underlying the epithelia (referred to as the lamina propria), the host actively engages the flora in a dynamic way that allows for rapid adaptation to the constantly changing microflora and food challenge. The immune cell composition thus adjusts to acute requirements and local needs. Constant tissue renewal is one example of the dynamic gut landscape. The single cell layer of the epithelium is replaced every 5 days from crypt-resident stem cells [6]. The composition of the immune cells residing in the lamina propria is also dynamic and locally adjusted to the flora challenge [7]. There is intimate crosstalk between the host and its microflora. Host recognition of commensal microflora by Toll-like receptors (TLRs) is required for intestinal homeostasis and protection from injury [8]. Moreover, specific commensals shape the gut-associated immune system, for Corresponding author: Jung, S. ([email protected]). Keywords: intestine; macrophages; monocytes; inflammatory bowel disorder. 1471-4906/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.it.2013.02.001

example, segmented filamentous bacteria (SFBs) and Clostridium spp. regulate the prevalence of distinct helper and regulatory T cell populations [9,10]. Conversely, the host actively controls the steady state microbiota composition by secretion of antimicrobial peptides [11] and immunoglobulins [12], highlighting the state of a highly active dynamic symbiosis. Here, we focus on the role of intestinal macrophages in the maintenance of gut homeostasis and its breakdown. Specifically, we discuss recent progress in understanding the origin of these cells and their derivation from Ly6C+ monocytes. Furthermore, we review emerging data on alternative monocyte fates in the inflamed gut. Intestinal macrophages: an exception from the rule In accordance with the historical subdivision of the mammalian mononuclear phagocyte compartment, intestinal mononuclear phagocytes are also subdivided into dendritic cells (DCs) and macrophages. DCs in the intestine that are marked by expression of the integrins CD11c (aX), CD11b (aM) and CD103 (aEb7) have been discussed by Schulz et al. [13]. Here, we focus on the population of monocyte-derived macrophages that represent the most abundant mononuclear phagocytes in the steady state lamina propria of the intestine and which constitute the largest reservoir of macrophages in the body [14]. These cells are characterized by surface expression of the integrins CD11c and CD11b, as well as the F4/80 antigen (EGF-like module-containing mucin-like hormone receptor-like 1; EMR1) [14]. Intestinal macrophages also express CD64, the Fc-g receptor 1 (FcgRI) [15], and unique high levels of the chemokine receptor CX3CR1 [16]. The latter renders these cells readily detectable by flow cytometry and histology in mice harboring a GFP reporter gene insertion in their CX3CR1 gene [17]. In fact, although F4/80+ cells have been long known to reside in the gut tissue [18], it was the documentation of their shear abundance and strategic positioning in the intestine of CX3CR1gfp mice alongside their unique morphological features that attracted major attention to the study of these cells [16]. Of note, CX3CR1hi macrophages in mice were originally considered DCs [16,19,20]. However, with the emergence of functional data (see below) and the establishment of the distinct origin of intestinal CD103+ DCs and CX3CR1hi cells [19,20], the latter cells are now widely regarded as macrophages [21,22]. Recent data have revealed a striking dichotomy in the mammalian macrophage compartment [23–25]. Major tissue-resident macrophages, including brain microglia, liver Trends in Immunology xx (2012) 1–7

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Kupffer cells, and lung macrophages derive from cells generated during development in the yolk sac, an extraembryonic tissue from primitive macrophages. Some of these macrophages might receive further input from the fetal liver [26], however, after birth these populations are maintained through longevity and limited self-renewal independent of further monocyte contributions. With the emergence of hematopoietic stem cells (HSCs) in definitive hematopoesis [27], a second macrophage system is established. HSCs give continuous rise to ephemeral Ly6C+ monocytes that circulate in the blood and are recruited to tissues in physiological and injury-associated inflammation to complement transiently the resident tissue macrophage compartment. Ly6C+ monocytes and their macrophage descendants are highly plastic and can promote inflammation or assist the healing process depending on the injury and tissue context, or even route and timing of their recruitment [28,29]. Surprisingly, intestinal CX3CR1hi macrophages – although seemingly a stable homeostatic tissue-resident cell population – display with 3 weeks [30] a rather limited half-life for macrophages. Moreover, these cells are also unique among tissue macrophages with respect to their origin. Thus, we and others have shown using a conditional cell ablation model [31] in combination with precursor cell transfers, that CX3CR1hi macrophages derive from Ly6C+ blood monocytes [19,20,32] (Figure 1). The notion that intestinal steady state CX3CR1hi macrophages originate from Ly6C+ blood monocytes is also supported by the finding that in mixed bone marrow chimeras generated with wild type and chemokine CC receptor (CCR)2-deficient marrow, CX3CR1hi macrophages are exclusively of wild type genotype, whereas CD103+ DCs are equally represented of the two genotypes [15]. The fact that monocyte recruitment to the gut requires

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Figure 1. The unique origin of intestinal macrophages. Most tissue-resident macrophages are established before birth from so-called primitive macrophages or fetal liver cells, and after birth maintain themselves through longevity and limited self-renewal [23–25]. By contrast, gut-resident CX3C chemokine receptor (CX3CR)1hi macrophages that seed the steady state lamina propria derive from Ly6C+ blood monocytes, like macrophages associated with inflammatory phenotype. Thus, similar to short-lived intestinal CD103+ dendritic cells (DCs), CX3CR1hi macrophages are continuously renewed throughout adulthood from hematopoietic stem cells (HSCs) via macrophage/DC precursors (MDPs).

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expression of the chemokine receptor CCR2, the sensor of the inflammation-associated chemokine (C–C motif) ligand (CCL)1, suggests that it is driven by an ongoing low-grade tonic inflammation in this unique tissue [33]. In line with this assumption, germ-free mice have been reported to harbor fewer CX3CR1hi macrophages [34], although this issue remains disputed [22]. The colonic lamina propria is prior to birth seeded with CX3CR1hi macrophages; however, it remains to be investigated whether these cells are – as opposed to monocyte-derived postnatal CX3CR1hi macrophages – related to the HSC-independent primitive macrophage lineage [23–25]. Taken together, among tissue-resident macrophages, lamina propria CX3CR1hi macrophages represent with their monocytic origin an intriguing exception from the rule. Their ongoing derivation from monocytes likely allows a unique plasticity of this cellular compartment. Macrophages thereby match the dynamic nature of the other components of the system, such as microflora and epithelia; a feature likely related to the unique requirements in this challenging environment. Functional contributions of resident CX3CR1hi macrophages in the steady state When considering the role of intestinal CX3CR1hi macrophages in the healthy gut, it is probably fair to assume that the steady state of the lower gastrointestinal tract represents a unique metastable equilibrium between the dynamic gut flora and the constantly remodeled host tissue. The intestinal steady state might thus be better considered as a type of primed homeostasis. Rescigno and colleagues were the first to report intestinal lamina propria resident cells that projected dendritic extensions into the gut lumen to sense and potentially sample its bacterial content [35]. Subsequent studies ascribed these transepithelial dendrites (TEDs) specifically to CX3CR1hi macrophages and reported that TED formation in the terminal ileum depended on macrophage expression of CX3CR1 [16,36] or epithelial expression of the unique membrane-tethered CX3CR1 ligand CX3CL1/fractalkine [37]. The functional significance of TEDs for the uptake or even mere sensing of luminal content by CX3CR1hi macrophages, however, remains unclear. The intact epithelium itself allows considerable penetrance of luminal material for ample sampling within the lamina propria [38]. In addition, emerging data indicate that CD103+ CD11b+ DCs have ready access to the epithelial layer and thus do not rely on CX3CR1hi macrophages for antigen delivery [39]. Cxc3r1 / and Cx3cl1 / mice are relatively protected in the dextran sodium sulfate (DSS) colitis model [37,40], whereas, Cx3cl1 transgene mediated TED restoration in Cx3cl1 / mice restores colitis, suggesting a direct link between the ability of CX3CR1hi macrophages to breach the epithelium and the development of DSS-induced colitis [37]. A specific role of CX3CR1hi macrophages in the uptake of bacteria from the gut lumen is supported by the fact that these cells provide a myeloid differentiation primary response gene (Myd) 88-independent alternative entry route for noninvasive Salmonella typhimurium mutants [41]. Consistent with their involvement in the sampling of luminal antigens, ileal CX3CR1hi macrophages are intimately

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Review associated with the intestinal epithelium [42]. Yet, absence of TEDs in CX3CR1-deficient animals seems to impair neither pathogen entry nor subsequent T cell priming in the mesenteric lymph nodes [42]; further suggesting the existence of alternative pathways of antigen entry into the intestinal lamina propria. Interestingly though, CX3CR1deficient animals fail to suppress efficiently local and systemic antigen-specific immune responses upon food antigen exposure, that is, establish oral tolerance [43]. This defect has, however, been linked to an impaired ability of CX3CR1deficient mice to maintain a lamina propria T regulatory (Treg) cell compartment and produce interleukin (IL)-10, rather than to an absence of TEDs [43]. Collectively, intestinal macrophages are closely associated with epithelium and can furthermore gain direct information on luminal content by virtue of TEDs. They are hence strategically located to instruct other immune cells in the lamina propria or recruit leukocytes in case of damage. Intestinal macrophage crosstalk with T cells Since the early pioneering studies by Ralph Steinman [44], DCs and macrophages have been discriminated by their distinct ability to prime naı¨ve T cells. Accordingly, despite high MHC class II expression and propensity to take up antigen, intestinal CX3CR1hi macrophages are inferior to DCs in priming naı¨ve T cells [42]. This might be related to the fact that macrophages rapidly shred their cargo and hence are deprived of antigen for T cell stimulation [45]. Like T cell responses in other tissues, the initiation of mucosal T cell immunity is believed to be restricted to organized lymphoid tissues, such as the Peyer’s patches (PPs) and mesenteric LNs, but not to occur in the lamina propria proper. T cell priming towards luminal antigens would hence require transport of antigen to gut-associated lymphoid tissue (GALT). Notably, CX3CR1hi macrophages residing in the healthy gut lack expression of the chemokine receptor CCR7 that is required for migration to the mesenteric LNs [20,42,46]. Rather, using multiphoton microscopy it was revealed that only CD103+ DCs migrate in afferent lymphatic vessels draining the small intestine under the conditions tested [42]. Accordingly, CD103+ DCs are critical to induce effector and Treg cell differentiation in the mesenteric LNs and imprint on them the expression of the small intestine homing receptors CCR9 and a4b7 [30,47–49]. By contrast, CX3CR1hi macrophages that reside in the lamina propria seem to promote the maintenance of functional Treg cells that returned from the LNs to the tissue. This scenario is supported by the abovementioned deficiency of CX3CR1-mutant mice to maintain their tissue Treg cells [43]. Moreover, IL-10 secreting CD11c+CD11b+F4/80+ cells, likely representing CX3CR1hi macrophages, are crucial for the in vivo maintenance of forkhead box (Fox)P3 expression and suppressive activity of Treg cells during colitis [50]. In in vitro cultures [and the presence of exogenous transforming growth factor (TGF)b], CX3CR1hi macrophages have been found to promote the differentiation of naı¨ve CD4+ T cells into FoxP3+ Treg cells [51]. Whereas these data support a link between Treg cells and CX3CR1hi macrophages, the mononuclear phagocyte responsible for the induction and maintenance of T helper (Th)17 cells, the second prevalent effector T cell

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type in the steady state lamina propria remains less well defined. The generation of Th17 cells depends on the presence of luminal commensal microbiota [52,53], and is in fact boosted by a single commensal microbe, segmented filamentous bacterium (SFB) [9]. Th17 cells are rare in the GALT and have hence been proposed to be generated in the lamina propria [54]. Commensal-derived ATP has been shown to activate colonic CD70hi CX3CR1+ cells to promote Th17 cell differentiation [54]. By contrast, it has been proposed that Th17 cell generation might be related to CD11b+CD103+ DCs [7]. This notion is supported by the finding that mice lacking CD11b+CD103+ DCs due to a Notch2 deficiency display decreased numbers of Th17 cells [55]. In this respect, it is interesting that the regional distribution of CD11b+CD103+ DCs in the digestive tract correlates with that of Th17 cells, whose abundance declines from the duodenum towards the ileum, and which are hardly found in healthy colon [7]. Conversely, both CX3CR1hi macrophages and FoxP3+ Treg cells are most abundant in the colon, whereas FoxP3+ Treg cells are scarce in the duodenum [7]. Collectively, intestinal macrophages seem specialized in the maintenance of Treg cells in the gut. However, much remains to be learned about their interactions with other lymphoid cells, in particular the emerging enigmatic innate lymphoid cells (ILCs) and mucosa-associated invariant T cells [56,57]. Functions beyond immunity Macrophages are highly phagocytic cells that can clear bacteria, cellular debris, and nonmicrobial foreign material, which is essential for the maintenance of tissue homeostasis [58]. Intestinal lamina propria macrophages are also involved in wound-healing processes. Macrophages associated with the colonic crypts of Lieberkuhn promote MyD88-dependent survival and proliferation of epithelial progenitor cells during colonic wound healing – an activity relying on the secretion of mitogenic factors, as well as metalloprotease inhibitors and extracellular matrix stabilizers [59]. Moreover, in an acute murine colonic epithelialregeneration model, triggering receptor expressed on myeloid cells (TREM)-2-expressing macrophages assisted wound repair by inhibiting tumor necrosis factor (TNF) and interferon (IFN)g secretion and increasing IL-4 and IL-13 levels in the lamina propria [60]. Monocyte education in the healthy gut CX3CR1hi macrophages are continuously generated from highly plastic Ly6C+ blood monocytes in a process that includes the acquisition of MHCII, loss of Ly6C, and upregulation of F4/80, CD64, CD11c, and CX3CR1 [15,33,61]. A striking feature of the mucosal system is that the intestinal tissue context instructs these cells to acquire a characteristic noninflammatory gene expression signature [22,33]. This includes induction of IL-10 and triggering receptor expressed on myeloid cells (TREM)-2, interleukin-1 receptor-associated kinase (IRAK)-M, and TNF alpha-induced protein (TNFAIP)3 (A20) expression, as well as TNFa, which exerts both pro-and anti-inflammatory activities [62]. This steady-state expression profile is robust in that it seems to withstand acute challenges, such as disturbances induced by epithelial perforation by DSS exposure 3

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Review [63] that are otherwise associated with prominent inflammatory cell infiltrates [33]. Ly6C+ blood monocytes grafted into macrophage-depleted animals also acquire this distinct noninflammatory profile [33]. It will be interesting to compare expression signatures of CX3CR1hi macrophages isolated from colon to cells isolated from duodenum and ileum. If profiles differ, this would suggest that also the instructive cues monocytes receive in these regions likely differ. This would emphasize the functional organization of the gastrointestinal tract into anatomic regions, which host distinct populations of commensals [64] and display distinct compositions of immune cells in their lamina propria [7]. A particular challenge in the future will then be to define the molecular cues that imprint the specific expression profiles onto the CX3CR1hi macrophages in the small and large intestine. Given its flora exposure, the epithelium is likely to play a major role in this education process. Accordingly, intestinal epithelial cells (IECs) have been proposed to influence intestinal mononuclear phagocytes, including DCs and CX3CR1hi macrophages via their secretion of immunoregulatory molecules such as thymic stromal lymphopoietin (TSLP), TGFb, and prostaglandin E2 [65]. A critical role of TGFb in establishing gut homeostasis is supported by the fact that mice lacking the TGFb-activating integrin a(v)b8 in the intestinal CD11c+ compartment (including both DCs and macrophages) develop severe gut inflammation [66,67]. Moreover, recently it was proposed that the characteristic IL-10 expression by intestinal macrophages is induced by epithelial cell-derived semaphorin 7A that engages on aVb1 integrin on their surface [68]. Emerging evidence suggests that monocyte education is important for maintenance of metastable gut homeostasis. Disturbances, such as genetic impairment of sensory components of the CX3CR1hi macrophages or antibiotica-induced dysbiosis [69] can lead to potentially harmful deviations from the steady state, such as expression of CCR7 and migration to the LNs, or induction of proinflammatory cytokines in response to exposure to bacterial products. Macrophages in the human intestine With the advent of mouse mutants and ever-refined new imaging strategies, studies on the murine intestine have recently gained considerable pace. However, in parallel to these efforts, considerable insights have accumulated from the study of human tissue. Reciprocal translation of the data from mice and humans is the major challenge ahead and should be assisted by recent alignment of human and mouse mononuclear phagocytes [70]. The healthy human colon was reported to harbor a population of HLA+CD68+ 25F9+ macrophages localized in close proximity to the epithelium [71]. As in mice, these cells have been proposed to derive from blood monocytes recruited by TGFb, IL-8, and chemokines produced by IECs and mast cells [72]. Under steady state conditions, human lamina propria macrophages display an anergic phenotype, as demonstrated by their inability to produce proinflammatory cytokines. Again, macrophages harbor a unique expression profile characterized by an absence of innate receptors, such as CD14, Fc receptors, and integrins, co-stimulatory molecules, as well as reduced levels of 4

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MyD88, TIR domain containing adaptor protein (TIRAP) and TNF receptor associated factor (TRAF)6. This anergic signature has been proposed to result from conditioning of the cells by TGFb [73,74]. Interestingly, the concept of a hyporeactive macrophage phenotype in the healthy human colon has recently been associated with downregulation of the ubiquitin–proteasome system in colonic macrophages [75]. Despite their anergy with respect to the sensing of bacterial products, these macrophages display avid phagocytic and bactericidal activity [73]. Assessment of lamina propria macrophages isolated from inflammatory bowel disease (IBD) patients has revealed major differences compared with the healthy phenotype. Although CD68+ macrophages expressing the myelomonocytic L1 antigen (calprotectin) are rare under homeostasis, in IBD they accumulate near small vessels and correlate with the degree of inflammation [76]. Moreover a proportion of subepithelial CD40hiCD68+ macrophages, which is more frequent in the colon than in the small bowel, has been reported to increase in biopsies taken from IBD patients [77]. Furthermore, macrophages from IBD colons show increased expression of the co-stimulatory molecules CD80/ CD86, which are rarely seen on colonic macrophages from normal mucosa [78]. A greater proportion of macrophages isolated from inflamed colonic mucosa undergo respiratory burst and upregulate NADPH oxidase expression, compared to macrophages isolated from healthy tissue [79,80]. Macrophages isolated from IBD patients also have altered cytokine profiles [81]. More IL-1b is produced spontaneously by mononuclear cells from inflamed mucosa, compared with normal colonic mucosa, and stimulation with lipopolysaccharide (LPS) further enhances production [79]. Moreover, colonic IBD macrophages also produce active caspase 1 (IL-1b converting enzyme; ICE) [82]. The difference in the surface markers on resident macrophages in the healthy gut compared with macrophages isolated from IBD patients suggests that the latter are newly recruited cells. Direct support for this hypothesis comes from a seminal experiment showing that in adoptive transfer, autologous radiolabeled monocytes harvested from IBD patients migrate to the inflamed mucosa [83]. Recent flow cytometry analysis has shown that the population of migrating cells are TREM-1+, CX3CR1+, dendritic cell-specific intercellular adhesion molecule-3-grabbing non-integrin (DC-SIGN)+ and TLR-2+ and these cells can secrete proinflammatory cytokines such as IL-23 and IL-6 [84,85]. New kids on the block: monocyte-derived cells in the inflamed intestine Studies on human mononuclear phagocytes have traditionally focused on the disease state, that is, surgical specimens obtained by resection or endoscopy-assisted biopsies. By contrast, studies in mice have provided a comprehensive picture of the intestinal mononuclear phagocyte compartment in the healthy state, but only recently moved to the inflammatory state, which is prominently associated with neutrophilic and monocytic tissue infiltrates. At steady state, Ly6C+ monocytes give rise to resident noninflammatory CX3CR1hi macrophages, but when recruited to inflamed tissue they acquire distinct fates (Figure 2). Below, we discuss recent findings concerning these cells, thus widening

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LN TRENDS in Immunology

Figure 2. Alternative monocyte fates in healthy and inflamed intestine. Ly6C+ monocytes entering the healthy intestinal lamina propria rapidly acquire a characteristic noninflammatory gene expression profile and contribute to the maintenance of homeostasis [33]. Establishment of this gene signature can be impaired in dysbiosis. Ly6C+ monocytes entering inflamed colon acquire two alternative fates [33]. They become pro-inflammatory effector monocytes promoting inflammation and migratory antigenpresenting cells (APCs) that potentially affect T cell responses in the draining lymph nodes (LNs).

the focus of this review from bona fide resident steady state CX3CR1hi macrophages to monocyte-derived cells in healthy and inflamed gut. Taking advantage of the CX3CR1gfp mice, it has been shown that the colon of DSS-challenged animals harbors CX3CR1int cells that transiently dominate mononuclear phagocytes in the lamina propria [86]. In these mice, CX3CR1/GFPhi and CX3CR1/GFPint populations are also found to coexist in the T cell transfer colitis model [87], and these cells display non- and proinflammatory gene expression signatures, respectively [88]. Focusing on the small intestine, it has been established that CX3CR1int cells are similar to the resident CX3CR1hi macrophages in that they are Ly6C+ monocyte-derived and express proinflammatory cytokines [22]. Recently this finding has been corroborated in the inflamed colon, and in vitro, bacterial TLR or nucleotide-binding oligomerization domain containing protein (NOD)-like receptor (NLR) ligands trigger expression of proinflammatory cytokines in these cells, but not in resident CX3CR1hi macrophages [33]. Moreover, using a conditional cell ablation strategy, depletion of CCR2expressing CX3CR1int cells alongside their immediate Ly6C+ monocyte precursors has been found to ameliorate DSS-induced colitis [33]. This establishes CX3CR1int cells as proinflammatory effector monocytes that actively drive inflammation and suggests that monocyte manipulation might have therapeutic value for the management of IBD flares. Notably, proinflammatory monocyte-derived cells have also been detected in the mesenteric LNs of colitic mice, although these cells probably enter the LNs from the blood [89], and the relation to lamina propria resident cells remains to be explored. Under inflammatory conditions, Ly6C+ monocytes can in addition give rise to a CCR7-expressing CX3CR1int

Ly6Clo cell population [22,33,61] capable of migrating in lymph and priming T cells towards the Th1 or Th17 lineage [22,33,90]. Notably, however these CX3CR1int Ly6Clo cells are heterogeneous and contain a further-to-be-defined population of cells exhibiting a DC-like phenotype, including higher expression of CD11c, lower expression of CD64 [22,61], as well as expression of the newly described DC marker Zbtb46 [33,91,92]. Activated monocytes stimulated in vitro have been shown to express Zbtb46 [91,92]. However, the exact in vivo origin of the Zbtb46-expressing CX3CR1int Ly6Clo cells seen in colitis and their relation to previously reported inflammatory DCs or classical DCcommitted precursors remains to be determined. Both Zbtb46+ and Zbtb46– CX3CR1int Ly6Clo cells are affected by administration of depleting CCR2 antibody, suggesting that all CX3CR1int Ly6Clo cells originate from CCR2+ precursors [33]. Collectively, these studies suggest that monocytes and the manipulation of their differentiation potential might be attractive targets for the management of IBD flares. Concluding remarks Mammalian mononuclear phagocytes play central roles in the maintenance of tissue integrity during development and after injury, as well as in the initiation and resolution of innate and adaptive immunity. The intestine is unique in that it houses the complex and rapidly changing commensal microflora. This inherent dynamic environment is confronted with an equally unique macrophage compartment that is continuously renewed from blood monocytes and hence is highly versatile. Emerging data show that newly arriving monocytes must undergo a rapid local education process to support homeostasis, otherwise they adopt alternative fates that can promote inflammation. 5

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Review Understanding the cues that guide the generation of quiescent intestinal macrophages might bear potential for IBD therapy. Arguably, however, much remains to be learned about specific cellular interactions in the unique gut tissue context. However, the recent pace of the current data accumulation seems to justify cautious optimism that, in the not too distant future, we might be able to add to our medical IBD treatment options strategies that not only target molecules and pathways, but also specific cells. Given their central role in the maintenance of gut inflammation, intestinal macrophages and other monocyte-derived cells have emerged as prime targets for such efforts. Acknowledgments This work was supported by the Helmsley Foundation and the United States–Israel Binational Science Foundation (BSF). S.J. is a Helmsley Scholar at the Crohn’s & Colitis Foundation.

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