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.

Lymphoid organs are characterized by a complex network of phenotypically distinct dendritic cells (DC) with potentially unique roles in pathogen recognition and immunostimulation. Classical DC (cDC) include two major subsets distinguished in the mouse by the expression of CD8α. Here we describe a subset of CD8α + DC in lymphoid organs of naïve mice characterized by expression of the CX 3 CR1 chemokine receptor. CX 3 CR1 + CD8α + DC lack hallmarks of classical CD8α + DC, including IL-12 secretion, the capacity to crosspresent antigen, and their developmental dependence on the transcriptional factor BatF3. Gene-expression profiling showed that CX 3 CR1 + CD8α + DC resemble CD8α − cDC. The microarray analysis further revealed a unique plasmacytoid DC (PDC) gene signature of CX 3 CR1 + CD8α + DC. A PDC relationship of the cells is supported further by the fact that they harbor characteristic D-J Ig gene rearrangements and that development of CX 3 CR1 + CD8α + DC requires E2-2, the critical transcriptional regulator of PDC. Thus, CX 3 CR1 + CD8α + DC represent a unique DC subset, related to but distinct from PDC. Collectively, the expression-profiling data of this study refine the resolution of previous DC definitions, sharpen the border of classical CD8α + and CD8α − DC, and should assist the identification of human counterparts of murine DC subsets.

Classical DC (cDC) are required for efficient protective T-cell immunity. Moreover, recent data indicate that cDC also play a critical role in mediating homeostatic proliferation and maintenance of peripheral Treg. Here, we corroborate these findings by defining CD80/ CD86 costimulation as an essential molecular component required for the cDC-Treg interactions. In contrast to earlier reports, the reduced Treg compartment of mice lacking cDC or selective CD80/86 expression on cDC, as such, did not render the respective animals prone to systemic lymphocyte hyperactivation or autoimmunity. Rather, we provide evidence that elevated immunoglobulin titers, as well as changes in T-cell subset prevalence and activation status are strictly associated with the nonmalignant myeloproliferative disorder triggered by the absence of cDC. IntroductionProductive T-cell activation requires, in addition to the TCR stimulus, a second signal provided by costimulatory molecules, the best characterized of which are CD80 (B7-1) and CD86 (B7-2). CD80 and CD86, which are expressed mainly on B cells, DC and medullary thymic epithelial cells (mTEC) [1], are the only known ligands of CD28 and CTLA-4 receptors on T cells. Functions of CD28 and CTLA-4 are distinct with CD28 promoting T-cell activation and CTLA-4-negative regulating T-cell responses.Peripheral self-tolerance and immune homeostasis are maintained, at least in part, by a delicate balance of T effector and Treg. CD25 1 CD41 Treg, which arise spontaneously as the so-called In vitro studies have revealed that BM culture-derived DC selected for high expression of CD86 are particularly effective in driving Treg proliferation [10] and that conversely DC isolated from CD80/86 double knockout mice poorly promote Treg division [4]. Moreover, emerging evidence supports a direct correlation between DC numbers and the proliferation rate of peripheral Treg. Thus, Fms-like tyrosine kinase 3 ligand (Flt3L) treatment, which results in the in vivo expansion of classical DC (cDC) [11] leads to a concomitant increase in peripheral Treg [12,13]. Furthermore, it was recently demonstrated that the conditional ablation of cDC from otherwise intact animals results in reduced numbers and impaired homeostatic proliferation of peripheral Treg [13].Here, we readdressed the role of cDC in the maintenance of peripheral Treg focusing on the role of CD80/86 costimulation. Using constitutive and conditional cDC ablation strategies, we established that peripheral Treg maintenance critically depends on the presence of cDC expressing CD80/86. Surprisingly however and defying earlier notions [13,14], the reduction of Treg in animals lacking cDC as such was not inherently associated with lymphocyte activation. Rather than resulting from a tolerance failure, the autoinflammatory signatures reported for cDCdeficient mice are thus a consequence of the nonmalignant myeloproliferative disorder these animals develop.

Recent years have seen a major advance in our understanding of the organization of the dendritic cell (DC) compartment. Particularly rewarding in this respect have been studies investigating DC origins, based on the identification of transcription factor and growth factor requirements, as well as direct demonstrations of precursor/progeny relationships by adoptive cell transfers. However, to fully understand the organization of the DC compartment, functional definitions of DC subsets must be provided and potential task divisions revealed that distinguish DC from other immune cells, including the closely related mononuclear phagocytes, such as macrophages. In fact, functional definitions might eventually replace the current distinction between DC and macrophages, which is in part based on arbitrary historic considerations, i.e. mononuclear phagocytes identified before the advent of DC in the mid 1970s generally termed macrophages. In this article, we review recent insight in the functions of classical DC in the mouse, focusing on our own work involving conditional and constitutive cell ablation.