Florent Ginhoux

Fate Mapping Analysis Reveals That Adult Microglia Derive from Primitive Macrophages

Primitive Origins for Microglia Microglia are the resident macrophages of the central nervous system and are associated with neurodegeneration and brain inflammatory diseases. Although the developmental origins of other tissue macrophage populations are well established, the origins of microglia remain controversial. Ginhoux et al. (p. 841, published online 21 October) used in vivo lineage tracing studies to show that microglia arise early in mouse development and derive from primitive macrophages in the yolk sac. This is in contrast to other cells of the mononuclear phagocyte system, which arise later in development from a distinct progenitor population. The developmental origins of adult microglia are revealed. Microglia are the resident macrophages of the central nervous system and are associated with the pathogenesis of many neurodegenerative and brain inflammatory diseases; however, the origin of adult microglia remains controversial. We show that postnatal hematopoietic progenitors do not significantly contribute to microglia homeostasis in the adult brain. In contrast to many macrophage populations, we show that microglia develop in mice that lack colony stimulating factor-1 (CSF-1) but are absent in CSF-1 receptor–deficient mice. In vivo lineage tracing studies established that adult microglia derive from primitive myeloid progenitors that arise before embryonic day 8. These results identify microglia as an ontogenically distinct population in the mononuclear phagocyte system and have implications for the use of embryonically derived microglial progenitors for the treatment of various brain disorders.

Origin of the Lamina Propria Dendritic Cell Network

CX(3)CR1(+) and CD103(+) dendritic cells (DCs) in intestinal lamina propria play a key role in mucosal immunity. However, the origin and the developmental pathways that regulate their differentiation in the lamina propria remain unclear. We showed that monocytes gave rise exclusively to CD103(-)CX(3)CR1(+) lamina propria DCs under the control of macrophage-colony-stimulating factor receptor (M-CSFR) and Fms-like thyrosine kinase 3 (Flt3) ligands. In contrast, common DC progenitors (CDP) and pre-DCs, which give rise to lymphoid organ DCs but not to monocytes, differentiated exclusively into CD103(+)CX(3)CR1(-) lamina propria DCs under the control of Flt3 and granulocyte-macrophage-colony-stimulating factor receptor (GM-CSFR) ligands. CD103(+)CX(3)CR1(-) DCs but not CD103(-)CX(3)CR1(+) DCs in the lamina propria constitutively expressed CCR7 and were the first DCs to transport pathogenic Salmonella from the intestinal tract to the mesenteric lymph nodes. Altogether, these results underline the diverse origin of the lamina propria DC network and identify mucosal DCs that arise from pre-DCs as key sentinels of the gut immune system.

Origin, homeostasis and function of Langerhans cells and other langerin-expressing dendritic cells

Langerhans cells (LCs) are a specialized subset of dendritic cells (DCs) that populate the epidermal layer of the skin. Langerin is a lectin that serves as a valuable marker for LCs in mice and humans. In recent years, new mouse models have led to the identification of other langerin+ DC subsets that are not present in the epidermis, including a subset of DCs that is found in most non-lymphoid tissues. In this Review we describe new developments in the understanding of the biology of LCs and other langerin+ DCs and discuss the challenges that remain in identifying the role of different DC subsets in tissue immunity.

Langerhans cells arise from monocytes in vivo.

Langerhans cells (LCs) are the only dendritic cells of the epidermis and constitute the first immunological barrier against pathogens and environmental insults. The factors regulating LC homeostasis remain elusive and the direct circulating LC precursor has not yet been identified in vivo. Here we report an absence of LCs in mice deficient in the receptor for colony-stimulating factor 1 (CSF-1) in steady-state conditions. Using bone marrow chimeric mice, we have established that CSF-1 receptor–deficient hematopoietic precursors failed to reconstitute the LC pool in inflamed skin. Furthermore, monocytes with high expression of the monocyte marker Gr-1 (also called Ly-6c/G) were specifically recruited to the inflamed skin, proliferated locally and differentiated into LCs. These results identify Gr-1hi monocytes as the direct precursors for LCs in vivo and establish the importance of the CSF-1 receptor in this process.

The receptor tyrosine kinase Flt3 is required for dendritic cell development in peripheral lymphoid tissues

Dendritic cell (DC) development begins in the bone marrow but is not completed until after immature progenitors reach their sites of residence in lymphoid organs. The hematopoietic growth factors regulating these processes are poorly understood. Here we examined the effects of signaling by the receptor tyrosine kinase Flt3 on macrophage DC progenitors in the bone marrow and on peripheral DCs. We found that the macrophage DC progenitor compartment was responsive to superphysiological amounts of Flt3 ligand but was not dependent on Flt3 for its homeostatic maintenance in vivo. In contrast, Flt3 was essential to the regulation of homeostatic DC development in the spleen, where it was needed to maintain normal numbers of DCs by controlling their division in the periphery.

Hepatic Recruitment of the Inflammatory Gr1+ Monocyte Subset Upon Liver Injury Promotes Hepatic Fibrosis

In addition to liver‐resident Kupffer cells, infiltrating immune cells have recently been linked to the development of liver fibrosis. Blood monocytes are circulating precursors of tissue macrophages and can be divided into two functionally distinct subpopulations in mice: Gr1hi (Ly6Chi) and Gr1lo (Ly6Clo) monocytes. The role of these monocyte subsets in hepatic fibrosis and the mechanisms of their differential recruitment into the injured liver are unknown. We therefore characterized subpopulations of infiltrating monocytes in acute and chronic carbon tetrachloride (CCl4)‐induced liver injury in mice using flow cytometry and immunohistochemistry. Inflammatory Gr1hi but not Gr1lo monocytes are massively recruited into the liver upon toxic injury constituting an up to 10‐fold increase in CD11b+F4/80+ intrahepatic macrophages. Comparing wild‐type with C‐C chemokine receptor (CCR2)‐deficient and CCR2/CCR6–deficient mice revealed that CCR2 critically controls intrahepatic Gr1hi monocyte accumulation by mediating their egress from bone marrow. During chronic liver damage, intrahepatic CD11b+F4/80+Gr1+ monocyte‐derived cells differentiate preferentially into inducible nitric oxide synthase–producing macrophages exerting proinflammatory and profibrogenic actions, such as promoting hepatic stellate cell (HSC) activation, T helper 1–T cell differentiation and transforming growth factor β (TGF‐β) release. Impaired monocyte subset recruitment in Ccr2−/− and Ccr2−/−Ccr6−/− mice results in reduced HSC activation and diminished liver fibrosis. Moreover, adoptively transferred Gr1hi monocytes traffic into the injured liver and promote fibrosis progression in wild‐type and Ccr2−/−Ccr6−/− mice, which are otherwise protected from hepatic fibrosis. Intrahepatic CD11b+F4/80+Gr1+ monocyte‐derived macrophages purified from CCl4‐treated animals, but not naïve bone marrow monocytes or control lymphocytes, directly activate HSCs in a TGF‐β–dependent manner in vitro. Conclusion: Inflammatory Gr1+ monocytes, recruited into the injured liver via CCR2‐dependent bone marrow egress, promote the progression of liver fibrosis. Thus, they may represent an interesting novel target for antifibrotic strategies. (HEPATOLOGY 2009;50:261–274.)

IRF8 Mutations and Human Dendritic-Cell Immunodeficiency

BACKGROUNDnThe genetic analysis of human primary immunodeficiencies has defined the contribution of specific cell populations and molecular pathways in the host defense against infection. Disseminated infection caused by bacille Calmette-Guérin (BCG) vaccines is an early manifestation of primary immunodeficiencies, such as severe combined immunodeficiency. In many affected persons, the cause of disseminated BCG disease is unexplained.nnnMETHODSnWe evaluated an infant presenting with features of severe immunodeficiency, including early-onset disseminated BCG disease, who required hematopoietic stem-cell transplantation. We also studied two otherwise healthy subjects with a history of disseminated but curable BCG disease in childhood. We characterized the monocyte and dendritic-cell compartments in these three subjects and sequenced candidate genes in which mutations could plausibly confer susceptibility to BCG disease.nnnRESULTSnWe detected two distinct disease-causing mutations affecting interferon regulatory factor 8 (IRF8). Both K108E and T80A mutations impair IRF8 transcriptional activity by disrupting the interaction between IRF8 and DNA. The K108E variant was associated with an autosomal recessive severe immunodeficiency with a complete lack of circulating monocytes and dendritic cells. The T80A variant was associated with an autosomal dominant, milder immunodeficiency and a selective depletion of CD11c+CD1c+ circulating dendritic cells.nnnCONCLUSIONSnThese findings define a class of human primary immunodeficiencies that affect the differentiation of mononuclear phagocytes. They also show that human IRF8 is critical for the development of monocytes and dendritic cells and for antimycobacterial immunity. (Funded by the Medical Research Council and others.).

Blood Monocyte Subsets Differentially Give Rise to CD103+ and CD103− Pulmonary Dendritic Cell Populations

There are two major myeloid pulmonary dendritic cell (DC) populations: CD103+ DCs and CD11bhigh DCs. In this study, we investigated in detail the origins of both myeloid DC pools using multiple experimental approaches. We show that, in resting lung, Ly-6ChighCCR2high monocytes repopulated CD103+ DCs using a CCR2-dependent mechanism, and these DCs preferentially retained residual CCR2 in the lung, whereas, conversely, Ly-6ClowCCR2low monocytes repopulated CD11bhigh DCs. CX3CR1 was required to generate normal numbers of pulmonary CD11bhigh DCs, possibly because Ly-6Clow monocytes in the circulation, which normally express high levels of CX3CR1, failed to express bcl-2 and may have diminished survival in the circulation in the absence of CX3CR1. Overall, these data demonstrate that the two circulating subsets of monocytes give rise to distinct tissue DC populations.

The sphingosine 1-phosphate receptor 1 causes tissue retention by inhibiting the entry of peripheral tissue T lymphocytes into afferent lymphatics

Although much is known about the migration of T cells from blood to lymph nodes, less is known about the mechanisms regulating the migration of T cells from tissues into lymph nodes through afferent lymphatics. Here we investigated T cell egress from nonlymphoid tissues into afferent lymph in vivo and developed an experimental model to recapitulate this process in vitro. Agonism of sphingosine 1-phosphate receptor 1 inhibited the entry of tissue T cells into afferent lymphatics in homeostatic and inflammatory conditions and caused the arrest, mediated at least partially by interactions of the integrin LFA-1 with its ligand ICAM-1 and of the integrin VLA-4 with its ligand VCAM-1, of polarized T cells at the basal surface of lymphatic but not blood vessel endothelium. Thus, the increased sphingosine 1-phosphate present in inflamed peripheral tissues may induce T cell retention and suppress T cell egress.

Origin and functional heterogeneity of non-lymphoid tissue dendritic cells in mice

Summary:u2002 Dendritic cells (DCs) have been extensively studied in mice lymphoid organs, but less is known about the origin and the mechanisms that regulate DC development and function in non‐lymphoid tissues. Here, we discuss recent evidence establishing the contribution of the DC‐restricted lineage to the non‐lymphoid tissue DC pool and discuss the mechanisms that control the homeostasis of non‐lymphoid tissue DCs. We also review recent results underlining the functional specialization of tissue DCs and discuss the potential implications of these findings in tissue immunity and in the development of novel vaccine strategies.