Interleukin-10 (IL-10) is a pleiotropic anti-inflammatory cytokine produced and sensed by most hematopoietic cells. Genome-wide association studies and experimental animal models point at a central role of the IL-10 axis in inflammatory bowel diseases. Here we investigated the importance of intestinal macrophage production of IL-10 and their IL-10 exposure, as well as the existence of an IL-10-based autocrine regulatory loop in the gut. Specifically, we generated mice harboring IL-10 or IL-10 receptor (IL-10Rα) mutations in intestinal lamina propria-resident chemokine receptor CX3CR1-expressing macrophages. We found macrophage-derived IL-10 dispensable for gut homeostasis and maintenance of colonic T regulatory cells. In contrast, loss of IL-10 receptor expression impaired the critical conditioning of these monocyte-derived macrophages and resulted in spontaneous development of severe colitis. Collectively, our results highlight IL-10 as a critical homeostatic macrophage-conditioning agent in the colon and define intestinal CX3CR1(hi) macrophages as a decisive factor that determines gut health or inflammation.

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...

Dendritic cells are critically involved in the promotion and regulation of T cell responses. Here, we report a mouse strain that lacks conventional CD11c(hi) dendritic cells (cDCs) because of constitutive cell-type specific expression of a suicide gene. As expected, cDC-less mice failed to mount effective T cell responses resulting in impaired viral clearance. In contrast, neither thymic negative selection nor T regulatory cell generation or T cell homeostasis were markedly affected. Unexpectedly, cDC-less mice developed a progressive myeloproliferative disorder characterized by prominent extramedullary hematopoiesis and increased serum amounts of the cytokine Flt3 ligand. Our data identify a critical role of cDCs in the control of steady-state hematopoiesis, revealing a feedback loop that links peripheral cDCs to myelogenesis through soluble growth factors, such as Flt3 ligand.

Beyond its established function in hematopoiesis, the bone marrow hosts mature lymphocytes and acts as a secondary lymphoid organ in the initiation of T cell and B cell responses. Here we report the characterization of bone marrow-resident dendritic cells (bmDCs). Multiphoton imaging showed that bmDCs were organized into perivascular clusters that enveloped blood vessels and were seeded with mature B lymphocytes and T lymphocytes. Conditional ablation of bmDCs in these bone marrow immune niches led to the specific loss of mature B cells, a phenotype that could be reversed by overexpression of the antiapoptotic factor Bcl-2 in B cells. The presence of bmDCs promoted the survival of recirculating B cells in the bone marrow through the production of macrophage migration-inhibitory factor. Thus, bmDCs are critical for the maintenance of recirculating B cells in the bone marrow.

Plasmacytoid dendritic cells (PDCs) play a pivotal role as cytokine-secreting accessory cells in the antimicrobial immune defense. In contrast, the capacity of PDCs to act as antigen-presenting cells in naive T cell priming remains unclear. By studying T cell responses in mice that lack conventional DCs (cDCs), and by the use of a PDC-specific antigen-targeting strategy, we show that PDCs can initiate productive naive CD4+ T cell responses in lymph nodes, but not in the spleen. PDC-triggered CD4+ T cell responses differed from cDC-driven responses in that they were not associated with concomitant CD8+ T cell priming. Our results establish PDCs as a bona fide DC subset that initiates unique CD4+ Th cell–dominated primary immune responses.