Dendritic cells (DCs) and macrophages are critical to innate and adaptive immunity to the intestinal bacterial microbiota. Here, we identify a myeloid-derived mucosal DC in mice, which populates the entire lamina propria of the small intestine. Lamina propria DCs were found to depend on the chemokine receptor CX3CR1 to form transepithelial dendrites, which enable the cells to directly sample luminal antigens. CX3CR1 was also found to control the clearance of entero-invasive pathogens by DCs. Thus, CX3CR1-dependent processes, which control host interactions of specialized DCs with commensal and pathogenic bacteria, may regulate immunological tolerance and inflammation.

Adult neovascularization relies on the recruitment of circulating cells, but their angiogenic roles and recruitment mechanisms are unclear. We show that the endothelial growth factor VEGF is sufficient for organ homing of circulating mononuclear myeloid cells and is required for their perivascular positioning and retention. Recruited bone marrow-derived circulating cells (RBCCs) summoned by VEGF serve a function distinct from endothelial progenitor cells. Retention of RBCCs in close proximity to angiogenic vessels is mediated by SDF1, a chemokine induced by VEGF in activated perivascular myofibroblasts. RBCCs enhance in situ proliferation of endothelial cells via secreting proangiogenic activities distinct from locally induced activities. Precluding RBCCs strongly attenuated the proangiogenic response to VEGF and addition of purified RBCCs enhanced angiogenesis in excision wounds. Together, the data suggest a model for VEGF-programmed adult neovascularization highlighting the essential paracrine role of recruited myeloid cells and a role for SDF1 in their perivascular retention.

The mononuclear phagocyte (MP) system is a body-wide macrophage (MΦ) and dendritic cell (DC) network, which contributes to tissue homeostasis, inflammation, and immune defense. The in vivo origins of MPs remain poorly understood. Here, we use an adoptive precursor cell transfer strategy into MP-depleted mice to establish the in vivo differentiation sequence from a recently identified MΦ/DC-restricted bone marrow (BM) precursor (MDP) via BM and blood intermediates to peripheral MΦs and DCs. We show that MDPs are in vivo precursors of BM and blood monocytes. Interestingly, grafted Gr1high “inflammatory” blood monocytes shuttle back to the BM in the absence of inflammation, convert into Gr1low monocytes, and contribute further to MP generation. The grafted monocytes give rise to DCs in the intestinal lamina propria and lung, but not to conventional CD11chigh DCs in the spleen, which develop during homeostasis from MDPs without a monocytic intermediate.

CX 3 CR1 is a chemokine receptor with a single ligand, the membrane-tethered chemokine CX 3 CL1 (fractalkine). All blood monocytes express CX 3 CR1, but its levels differ between the main 2 subsets, with human CD16 ؉ and murine Gr1 low monocytes being CX 3 CR1 hi . Here, we report that absence of either CX 3 CR1 or CX 3 CL1 results in a significant reduction of Gr1 low blood monocyte levels under both steady-state and inflammatory conditions. Introduction of a Bcl2 transgene restored the wild-type phenotype, suggesting that the CX 3 C axis provides an essential survival signal. Supporting this notion, we show that CX 3 CL1 specifically rescues cultured human monocytes from induced cell death. Human CX 3 CR1 gene polymorphisms are risk factors for atherosclerosis and mice deficient for the CX 3 C receptor or ligand are relatively protected from atherosclerosis development. However, the mechanistic role of CX 3 CR1 in atherogenesis remains unclear. Here IntroductionChemokines are a family of chemotactic cytokines that activate specific G-protein-coupled 7-transmembrane receptors 1 and have been categorized into C, CC, CXC, and CX 3 C families. The only known CX 3 C chemokine, CX 3 CL1, also known as fractalkine, 1-3 is expressed by activated vascular endothelial cells, 3 neurons, 4 epithelial cells, 5,6 smooth muscle cells, 7 dendritic cells (DCs), 8 and macrophages. 9 The single known CX 3 CL1 receptor, CX 3 CR1, 10 is expressed by T-cell and natural killer (NK) cell subsets, 10,11 brain microglia, 4,12,13 DC subsets 13-15 as well as blood monocytes. 10,13 Classical small-molecular-weight chemokines are secreted proteins considered to form gradients by binding to extracellular matrix proteoglycans. In contrast, CX 3 CL1 is synthesized as a transmembrane protein with its chemokine domain presented on an extended mucin-like stalk. 2,3 In this form, CX 3 CL1 promotes tight, integrin-independent adhesion of CX 3 CR1-expressing leukocytes. 7,16 In addition, constitutive and inducible cleavage by metalloproteases can result in release of a soluble CX 3 CL1 entity from the cell membrane. [17][18][19] CX 3 CL1 thus potentially acts as an adhesion molecule and a chemoattractant; albeit the differential importance of these activities for the physiologic role of CX 3 CL1 remains unknown. In cell lines and cultured microglia, CX 3 CR1 engagement triggers the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway resulting in cell survival and proliferation. [20][21][22][23] However, the significance of this activity for the in vivo role of the CX 3 C chemokine system remains to be determined.Atherosclerosis is characterized by the accumulation of lipids and fibrous elements in the large arteries and involves diverse factors, including components of the immune system. 24 Human CX 3 CR1 gene polymorphisms were shown to be genetic risk factors for coronary artery diseases and atherosclerosis. 25,26 Moreover, mice deficient for either CX 3 CR1 or CX 3 CL1 display a relative resistance to atherosclerosis development in the r...

Peripheral blood monocytes are a population of circulating mononuclear phagocytes that harbor potential to differentiate into macrophages and dendritic cells. As in humans, monocytes in the mouse comprise two phenotypically distinct subsets that are Gr1 high CX 3 CR1 int and Gr1 low CX 3 CR1 high , respectively. The question remains whether these populations contribute differentially to the generation of peripheral mononuclear phagocytes. In this study, we track the fate of adoptively transferred, fractionated monocyte subsets in the lung of recipient mice. We show that under inflammatory and noninflammatory conditions, both monocyte subsets give rise to pulmonary dendritic cells. In contrast, under the conditions studied, only Gr1 low CX 3 CR1 high monocytes, but not Gr1 high CX 3 CR1 int cells, had the potential to differentiate into lung macrophages. However, Gr1 high CX 3 CR1 int monocytes could acquire this potential upon conversion into Gr1 low CX 3 CR1 high cells. Our results therefore indicate an intrinsic dichotomy in the differentiation potential of the two main blood monocyte subsets.

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Alveolar macrophages are a unique type of mononuclear phagocytes that populate the external surface of the lung cavity. Early studies have suggested that alveolar macrophages originate from tissue-resident, local precursors, whereas others reported their derivation from blood-borne cells. However, the role of circulating monocytes as precursors of alveolar macrophages was never directly tested. In this study, we show through the combined use of conditional cell ablation and adoptive cell transfer that alveolar macrophages originate in vivo from blood monocytes. Interestingly, this process requires an obligate intermediate stage, the differentiation of blood monocytes into parenchymal lung macrophages, which subsequently migrate into the alveolar space. We also provide direct evidence for the ability of both lung and alveolar macrophages to proliferate.

The lamina propria that underlies and stabilizes the gut lining epithelium is densely populated with strategically located mononuclear phagocytes. Collectively, these lamina propria macrophages and dendritic cells (DC) are believed to be crucial for tissue homeostasis as well as the innate and adaptive host defense. Lamina propria DC were recently shown to gain direct access to the intestinal lumen by virtue of epithelium-penetrating dendrites. However, the role of these structures in pathogen uptake remains under debate. In this study, we report that entry of a noninvasive model pathogen (Aspergillus fumigatus conidia) into the murine small intestinal lamina propria persists in the absence of either transepithelial dendrites or lamina propria DC and macrophages. Our results suggest the existence of multiple pathogen entry pathways and point at the importance of villus M cells in the uptake of gut lumen Ags. Interestingly, transepithelial dendrites seem altogether absent from the small intestine of BALB/c mice suggesting that the function of lamina propria DC extensions resides in their potential selectivity for luminal Ags, rather than in general uptake or gut homeostasis.