Differential expression of toll-like receptor mRNA in selected tissues of goat (Capra hircus)

Pattern recognition receptors (PRRs) expressed by various immune cells and tissues have been shown to play a pivotal role in the recognition of pathogens by the host. The present study was carried out to identify toll-like receptors (TLRs) 1-10 mRNA in goat peripheral blood mononuclear cells (PBMCs) and selected tissues including jejunum, lung, lymph node, skin, spleen and uterus using reverse transcriptase polymerase chain reaction (RT-PCR). Our results confirm earlier reports regarding the evolutionarily conserved nature of these receptors as successful amplification of the goat TLR mRNAs could be obtained with bovine TLR mRNA-specific primers. The partial sequences of the purified TLR PCR amplicons had 93.8-99.7% nucleotide identity with sheep TLR cDNA sequences available in the GenBank. Semi-quantification of the expression levels of the TLR mRNAs was done using densitometric analysis of band intensities. All the TLR mRNAs (1-10) were expressed in high amounts in the lymph node while spleen showed lower expression of TLR 6 and 10 mRNAs. PBMC and lung expressed all TLR mRNAs in high amounts except TLR 10 mRNA. In uterus and jejunum, lower expression of TLR 3, 4 and 10 mRNAs was seen. Skin had the lowest repertoire of TLR mRNA expression with lower or no expression of TLR 2, 3, 4, 8, 9 and 10 mRNAs. Another interesting observation was that tissues such as uterus, lung and skin that exhibited lower levels of TLR 2 had higher levels of TLR 6 mRNAs.

Resident macrophages influence stem cell activity in the mammary gland

Introduction

Macrophages in the mammary gland are essential for morphogenesis of the ductal epithelial tree and have been implicated in promoting breast tumor metastasis. Although it is well established that macrophages influence normal mammopoiesis, the mammary cell types that these accessory cells influence have not been determined. Here we have explored a role for macrophages in regulating mammary stem cell (MaSC) activity, by assessing the ability of MaSCs to reconstitute a mammary gland in a macrophage-depleted fat pad.

Methods

Two different in vivo models were used to deplete macrophages from the mouse mammary fat pad, allowing us to examine the effect of macrophage deficiency on the mammary repopulating activity of MaSCs. Both the Csf1^op/op mice and clodronate liposome-mediated ablation models entailed transplantation studies using the MaSC-enriched population.

Results

We show that mammary repopulating ability is severely compromised when the wild-type MaSC-enriched subpopulation is transplanted into Csf1^op/op fat pads. In reciprocal experiments, the MaSC-enriched subpopulation from Csf1^op/op glands had reduced regenerative capacity in a wild-type environment. Utilizing an alternative strategy for selective depletion of macrophages from the mammary gland, we demonstrate that co-implantation of the MaSC-enriched subpopulation with clodronate-liposomes leads to a marked decrease in repopulating frequency and outgrowth potential.

Conclusions

Our data reveal a key role for mammary gland macrophages in supporting stem/progenitor cell function and suggest that MaSCs require macrophage-derived factors to be fully functional. Macrophages may therefore constitute part of the mammary stem cell niche.

Functional interaction of HTLV-1 tax protein with the POZ domain of the transcriptional repressor BCL6

The Tax protein encoded by human T-cell leukaemia virus type 1 (HTLV-1) has a pivotal role in T-cell transformation by deregulating cellular signalling pathways. Using the yeast two-hybrid system to screen a human leukocyte cDNA library, we identified BCL6 (B-cell lymphoma 6) as a cellular protein, which interacts with Tax 1. The BCL6 gene encodes a sequence-specific transcriptional repressor that contains a conserved N-terminal poxvirus and zinc finger (POZ) repressor domain and a C-terminal Kruppel-like zinc finger DNA binding domain. Using both in vivo and in vitro methods, we demonstrate that the POZ domain of BCL6 is sufficient for its interaction with Tax 1. Using functional assays, we demonstrate that Tax 1 enhanced the repressive activity of BCL6 and increased the levels of apoptosis induced by BCL6 in osteosarcoma cells indicating that both proteins cooperate in vivo to cause a physiological affect. Furthermore, BCL6 recruited Tax 1 into punctate nuclear structures, which suggests that Tax 1 colocalizes with BCL6 in repressor complexes in vivo. BCL6 expression significantly downregulated both basal and Tax-induced nuclear factor-kappaB and long terminal repeat activation. This suggests that the expression of BCL6 in HTLV infected cells may contribute to the silencing of viral gene expression and to the long clinical latency associated with HTLV infection.

Mechanisms underlying neutrophil-mediated monocyte recruitment

Extravasation of polymorphonuclear leukocytes (PMNs) to the site of inflammation precedes a second wave of emigrating monocytes. That these events are causally connected has been established a long time ago. However, we are now just beginning to understand the molecular mechanisms underlying this cellular switch, which has become even more complex considering the emergence of monocyte subsets, which are affected differently by signals generated from PMNs. PMN granule proteins induce adhesion as well as emigration of inflammatory monocytes to the site of inflammation involving beta(2)-integrins and formyl-peptide receptors. Furthermore, modification of the chemokine network by PMNs and their granule proteins creates a milieu favoring extravasation of inflammatory monocytes. Finally, emigrated PMNs rapidly undergo apoptosis, leading to the discharge of lysophosphatidylcholine, which attracts monocytes via G2A receptors. The net effect of these mechanisms is the accumulation of inflammatory monocytes, thus promoting proinflammatory events, such as release of inflammation-sustaining cytokines and reactive oxygen species. As targeting PMNs without causing serious side effects seems futile, it may be more promising to aim at interfering with subsequent PMN-driven proinflammatory events.

MafB restricts M-CSF-dependent myeloid commitment divisions of hematopoietic stem cells

While hematopoietic stem cell (HSC) self-renewal is well studied, it remains unknown whether distinct control mechanisms enable HSC divisions that generate progeny cells with specific lineage bias. Here, we report that the monocytic transcription factor MafB specifically restricts the ability of M-CSF to instruct myeloid commitment divisions in HSCs. MafB deficiency specifically enhanced sensitivity to M-CSF and caused activation of the myeloid master-regulator PU.1 in HSCs in vivo. Single-cell analysis revealed that reduced MafB levels enabled M-CSF to instruct divisions producing asymmetric daughter pairs with one PU.1(+) cell. As a consequence, MafB(-/-) HSCs showed a PU.1 and M-CSF receptor-dependent competitive repopulation advantage specifically in the myelomonocytic, but not T lymphoid or erythroid, compartment. Lineage-biased repopulation advantage was progressive, maintained long term, and serially transplantable. Together, this indicates that an integrated transcription factor/cytokine circuit can control the rate of specific HSC commitment divisions without compromising other lineages or self-renewal.

The concerted action of GM-CSF and Flt3-ligand on in vivo dendritic cell homeostasis

Dendritic cell (DC) development is efficiently supported by Flt3-ligand or GM-CSF in vitro, and lymphoid-organ DC maintenance in vivo is critically dependent on Flt3-ligand. However, the relevance of GM-CSF for lymphoid-tissue DC maintenance and the importance of both cytokines for nonlymphoid organ DC homeostasis are not defined. Here, we show that, although Gm-csfr and Flt3 are both expressed in DC progenitors, Gm-csfr is expressed predominantly in monocytes, classical DCs (cDCs), and skin DCs, whereas Flt3 is expressed in both cDCs and plasmacytoid DCs (pDCs). In accordance with the respective cytokine receptor expression, DC progenitor and pDC numbers are primarily affected by Flt3-ligand deficiency, whereas both splenic and lymph node cDCs and dermal DCs are reduced in the absence of either GM-CSF or Flt3-ligand. Combined lack of GM-CSF and Flt3-ligand in newly generated double-deficient mice leads to further significant reductions of DC progenitors and dermal DCs. In line with the decrease of respective DC subsets, T-cell and antigen-specific IgG responses decline progressively, from wild-type to GM-CSF– to Flt3-ligand– to double-deficient mice, upon subcutaneous antigen delivery. These data thus show the concerted action of GM-CSF and Flt3-ligand on DC homeostasis in vivo.

Bone marrow angiotensin AT1 receptor regulates differentiation of monocyte lineage progenitors from hematopoietic stem cells

BACKGROUND

The angiotensin II (Ang II) type 1 (AT(1)) receptor is expressed in bone marrow (BM) cells, whereas it remains poorly defined how Ang II regulates differentiation/proliferation of monocyte-lineage cells to exert proatherogenic actions.

METHODS AND RESULTS

We generated BM chimeric apoE(-/-) mice repopulated with AT(1)-deficient (Agtr1(-/-)) or wild-type (Agtr1(+/+)) BM cells. The atherosclerotic development was significantly reduced in apoE(-/-)/BM-Agtr1(-/-) mice compared with apoE(-/-)/BM-Agtr1(+/+) mice, accompanied by decreased numbers of BM granulocyte/macrophage progenitors (GMP:c-Kit(+)Sca-1(-)Lin(-)CD34(+)CD16/32(+)) and peripheral blood monocytes. Macrophage-colony-stimulating factor (M-CSF)-induced differentiation from hematopoietic stem cells (HSCs:c-Kit(+)Sca-1(+)Lin(-)) to promonocytes (CD11b(high)Ly-6G(low)) was markedly reduced in HSCs from Agtr1(-/-) mice. The expression of M-CSF receptor c-Fms was decreased in HSCs/promonocytes from Agtr1(-/-) mice, accompanied by a marked inhibition in M-CSF-induced phosphorylation of PKC-delta and JAK2. c-Fms expression in HSCs/promonocytes was mainly regulated by TNF-alpha derived from BM CD45(-)CD34(-) stromal cells, and Ang II specifically regulated the TNF-alpha synthesis and release from BM stromal cells.

CONCLUSIONS

Ang II regulates the expression of c-Fms in HSCs and monocyte-lineage cells through BM stromal cell-derived TNF-alpha to promote M-CSF-induced differentiation/proliferation of monocyte-lineage cells and contributes to the proatherogenic action.

Hematopoietic cytokines can instruct lineage choice

The constant regeneration of the blood system during hematopoiesis requires tightly controlled lineage decisions of hematopoietic progenitor cells (HPCs). Because of technical limitations, differentiation of individual HPCs could not previously be analyzed continuously. It was therefore disputed whether cell-extrinsic cytokines can instruct HPC lineage choice or only allow survival of cells that are already lineage-restricted. Here, we used bioimaging approaches that allow the continuous long-term observation of individual differentiating mouse HPCs. We demonstrate that the physiological cytokines, macrophage colony-stimulating factor and granulocyte colony-stimulating factor, can instruct hematopoietic lineage choice.

Phospho-ablated Id2 is growth suppressive and pro-apoptotic in proliferating myoblasts

Inhibitor of differentiation protein-2 (Id2) is a dominant negative helix-loop-helix (HLH) protein, and a positive regulator of proliferation, in various cells. The N-terminal region of Id2 contains a consensus cdk2 phosphorylation sequence SPVR, which may be involved with the induction of apoptosis, at least in myeloid 32d.3 cells. However, the role of Id2 phosphorylation at serine 5 in skeletal muscle cells is unknown. The objective of this study was to determine if the phosphorylation of Id2 at serine 5 alters its cellular localization and its role in apoptosis in C2C12 myoblasts. Overexpression of wild type Id2 decreased MyoD protein expression, which corresponded to the increased binding of Id2 to basic HLH proteins E47 and E12. Bromodeoxyuridine incorporation was significantly decreased by the overexpression of phospho-ablated Id2 (S5A); conversely, overexpression of wild type Id2 increased cellular proliferation. The subcellular localization of Id2 and phospho-mimicking Id2 (S5D) were predominantly nuclear compared to S5A. The decreased nuclear localization of S5A corresponded to a decrease in cellular proliferation, and an increase in apoptosis. These data suggest that unphosphorylated Id2 is primarily localized in the cytosol, where it is growth suppressive and potentially pro-apoptotic. These results imply that reducing unphosphorylated Id2 may improve the pool of myoblasts available for differentiation by increasing proliferation and inhibiting apoptosis.

Primed innate immunity leads to autoinflammatory disease in PSTPIP2-deficient cmo mice

The mouse Lupo (I282N) mutation in proline-serine-threonine phosphatase-interacting protein 2 (PSTPIP2) leads to reduced expression of PSTPIP2 that is associated with a macrophage-mediated autoinflammatory disease. Another mutation in PSTPIP2, L98P, termed chronic multifocal osteomyelits (cmo), leads to a disease in mice that resembles chronic recurrent multifocal osteomyelits in humans. The cellular basis of cmo disease was investigated. cmo disease develops independently of lymphocytes and is cured by bone marrow transplantation. Macrophages, mast cells, and osteoclasts from cmo mice fail to express detectable PSTPIP2 protein. Asymptomatic Pstpip2(cmo/cmo) mice have increased circulating levels of macrophage inflammatory protein 1-alpha and interleukin-6, and their macrophages exhibit increased production of these inflammatory mediators, which is normalized by retroviral expression of wild-type PSTPIP2. Spleens of asymptomatic cmo mice contain increased numbers of macrophage precursors, and cmo mice mobilize more macrophage precursors in response to a sterile inflammatory stimulus. Signal transducer and activator of transcription 1 is elevated in cmo splenic macrophages, which also exhibit increased colony-stimulating factor-1-stimulated proliferation and increased extracellular signal-regulated kinase 1/2 phosphorylation. PSTPIP2 overexpression in macrophages leads to the opposite phenotype. Thus, PSTPIP2 deficiency causes both an expansion of macrophage progenitors and increased responsiveness of mature macrophages to activating stimuli, which together prime the organism for exaggerated and sustained responses leading to autoinflammatory disease.

CSF-1 signals directly to renal tubular epithelial cells to mediate repair in mice

Tubular damage following ischemic renal injury is often reversible, and tubular epithelial cell (TEC) proliferation is a hallmark of tubular repair. Macrophages have been implicated in tissue repair, and CSF-1, the principal macrophage growth factor, is expressed by TECs. We therefore tested the hypothesis that CSF-1 is central to tubular repair using an acute renal injury and repair model, ischemia/reperfusion (I/R). Mice injected with CSF-1 following I/R exhibited hastened healing, as evidenced by decreased tubular pathology, reduced fibrosis, and improved renal function. Notably, CSF-1 treatment increased TEC proliferation and reduced TEC apoptosis. Moreover, administration of a CSF-1 receptor-specific (CSF-1R-specific) antibody after I/R increased tubular pathology and fibrosis, suppressed TEC proliferation, and heightened TEC apoptosis. To determine the contribution of macrophages to CSF-1-dependent renal repair, we assessed the effect of CSF-1 on I/R in mice in which CD11b+ cells were genetically ablated and determined that macrophages only partially accounted for CSF-1-dependent tubular repair. We found that TECs expressed the CSF-1R and that this receptor was upregulated and coexpressed with CSF-1 in TECs following renal injury in mice and humans. Furthermore, signaling via the CSF-1R stimulated proliferation and reduced apoptosis in human and mouse TECs. Taken together, these data suggest that CSF-1 mediates renal repair by both a macrophage-dependent mechanism and direct autocrine/paracrine action on TECs.

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: Gr1(hi) (Ly6C(hi)) and Gr1(lo) (Ly6C(lo)) 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 (CCl(4))-induced liver injury in mice using flow cytometry and immunohistochemistry. Inflammatory Gr1(hi) but not Gr1(lo) 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 Gr1(hi) 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 beta (TGF-beta) release. Impaired monocyte subset recruitment in Ccr2(-/-) and Ccr2(-/-)Ccr6(-/-) mice results in reduced HSC activation and diminished liver fibrosis. Moreover, adoptively transferred Gr1(hi) 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 CCl(4)-treated animals, but not naïve bone marrow monocytes or control lymphocytes, directly activate HSCs in a TGF-beta-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.

Colony stimulating factor-1 dependence of paneth cell development in the mouse small intestine

BACKGROUND & AIMS

Paneth cells (PCs) secrete defensins and antimicrobial enzymes that contribute to innate immunity against pathogen infections within the mucosa of the small intestine. We examined the role of colony stimulating factor-1 (CSF-1) in PC development.

METHODS

CSF-1-deficient and CSF-1 receptor (CSF-1R)-deficient mice and administration of neutralizing anti-CSF-1R antibody were used to study the requirement of CSF-1 for the development of epithelial cells of the small intestine. CSF-1 transgenic reporter mice and mice that express only the membrane-spanning, cell-surface CSF-1 isoform were used to investigate regulation by systemic versus local CSF-1.

RESULTS

Mice deficient in CSF-1 or CSF-1R had greatly reduced numbers of mature PCs. PCs express the CSF-1R, and administration of anti-CSF-1R antibody to neonatal mice significantly reduced the number of PCs. Analysis of transgenic CSF-1 reporter mice showed that CSF-1-expressing cells are in close proximity to PCs. CSF-1/CSF-1R-deficient mice also had reduced numbers of the proliferating epithelial cell progenitors and lamina propria macrophages. Expression of the membrane-spanning, cell-surface CSF-1 isoform in CSF-1-deficient mice completely rescued the deficiencies of PCs, proliferating progenitors, and lamina propria macrophages.

CONCLUSIONS

These results indicate local regulation by CSF-1 of PC development, either directly, in a juxtacrine/paracrine manner, or indirectly, by lamina propria macrophages. Therefore, CSF-1R hyperstimulation could be involved in hyperproliferative disorders of the small intestine, such as Crohn's disease and ulcerative colitis.

Increased connective tissue extracellular matrix in the op/op model of osteopetrosis

Mice that are homozygous for the recessive osteopetrosis spontaneous mutation (op/op) develop severe osteopetrosis due to a defect in the production of macrophage colony-stimulating factor (M-CSF) and a deficiency in monocyte-derived osteoclasts. Our study describes a novel soft tissue finding in an osteopetrosis (B6C3Fe a/a-Csf1(op)/J) mouse model. Tissues were obtained from B6C3Fe a/a-Csf1(op)/J mice and age-matched wild-type mice, processed for hematoxylin and eosin sections, and comprehensive light microscopic tissue evaluation was performed. Mutant mice had characteristic traits of op/op deficiency including missing incisors and domed skulls. Histologically, the bone marrow cavity was effaced by interweaving thick bony trabeculae consistent with osteopetrosis. An increase in a finely granular, basophilic interstitial extracellular matrix (ECM) was observed in the subcutaneous connective tissue of the op/op mice when compared with controls. Histochemically, the ECM was negative with periodic acid Schiff and stained dark blue with alcian blue at a pH of 2.5, indicating that it is composed primarily of nonsulfated glycosaminoglycans (GAGs). This work suggests an increased ECM that is composed mainly of GAGs located in the subcutaneous tissue in op/op mice. This increase in ECM may be related to altered matrix production or turnover because of changes in M-CSF production.

Ectodomain shedding of FLT3 ligand is mediated by TNF-alpha converting enzyme

FLT3 ligand (FLT3L) has diverse roles in the hematopoietic system, which include stimulating proliferation of hematopoietic precursors and development of NK cells and dendritic cells. FLT3L is initially synthesized as a membrane-bound protein, which must be cleaved to become a soluble growth factor. However, little is known about the enzyme involved in the proteolytic release of FLT3L. In the current study, we show that shedding of FLT3L is metalloprotease-dependent, and that this proteolytic activity was abolished in fibroblasts lacking TNF-alpha converting enzyme (TACE) and could be rescued by reintroducing wild-type TACE in these cells. Moreover, we found that cells derived from the thymus of conditional TACE-deficient mice produce less FLT3L, and that serum FLT3L levels in these TACE mutant mice are significantly lower, both after LPS treatment and in the absence of such a challenge, further corroborating the relevance of TACE as FLT3L sheddase in vivo. Considering the involvements of FLT3 and FLT3L in hematopoietic malignancies and stem cell mobilization, the identification of the enzyme involved in FLT3L shedding may have important clinical implications.

Colony stimulating factor 1 is an extrinsic stimulator of mouse spermatogonial stem cell self-renewal

Self-renewal and differentiation of spermatogonial stem cells (SSCs) provide the foundation for testis homeostasis, yet mechanisms that control their functions in mammals are poorly defined. We used microarray transcript profiling to identify specific genes whose expressions are augmented in the SSC-enriched Thy1(+) germ cell fraction of mouse pup testes. Comparisons of gene expression in the Thy1(+) germ cell fraction with the Thy1-depleted testis cell population identified 202 genes that are expressed 10-fold or higher in Thy1(+) cells. This database provided a mining tool to investigate specific characteristics of SSCs and identify novel mechanisms that potentially influence their functions. These analyses revealed that colony stimulating factor 1 receptor (Csf1r) gene expression is enriched in Thy1(+) germ cells. Addition of recombinant colony stimulating factor 1 (Csf1), the specific ligand for Csf1r, to culture media significantly enhanced the self-renewal of SSCs in heterogeneous Thy1(+) spermatogonial cultures over a 63-day period without affecting total germ cell expansion. In vivo, expression of Csf1 in both pre-pubertal and adult testes was localized to clusters of Leydig cells and select peritubular myoid cells. Collectively, these results identify Csf1 as an extrinsic stimulator of SSC self-renewal and implicate Leydig and myoid cells as contributors of the testicular stem cell niche in mammals.

CCR2+Ly-6Chi monocytes are crucial for the effector phase of autoimmunity in the central nervous system

The chemokine receptor CCR2 plays a vital role for the induction of autoimmunity in the central nervous system. However, it remains unclear how the pathogenic response is mediated by CCR2-bearing cells. By combining bone marrow chimerism with gene targeting we detected a mild disease-modulating role of CCR2 during experimental autoimmune encephalomyelitis, a model for central nervous system autoimmunity, on radio-resistant cells that was independent from targeted CCR2 expression on endothelia. Interestingly, absence of CCR2 on lymphocytes did not influence autoimmune demyelination. In contrast, engagement of CCR2 on accessory cells was required for experimental autoimmune encephalomyelitis induction. CCR2+Ly-6Chi monocytes were rapidly recruited to the inflamed central nervous system and were crucial for the effector phase of disease. Selective depletion of this specific monocyte subpopulation through engagement of CCR2 strongly reduced central nervous system autoimmunity. Collectively, these data indicate a disease-promoting role of CCR2+Ly-6Chi monocytes during autoimmune inflammation of the central nervous system.

Direct binding of pRb/E2F-2 to GATA-1 regulates maturation and terminal cell division during erythropoiesis

Cell differentiation is often coupled with cell cycle arrest. Here, we show that direct binding of the erythroid transcription factor GATA-1 to the retinoblastoma protein and the pRb/E2F transcription factor complex is critical for red blood cell formation.

How cell proliferation subsides as cells terminally differentiate remains largely enigmatic, although this phenomenon is central to the existence of multicellular organisms. Here, we show that GATA-1, the master transcription factor of erythropoiesis, forms a tricomplex with the retinoblastoma protein (pRb) and E2F-2. This interaction requires a LXCXE motif that is evolutionary conserved among GATA-1 orthologs yet absent from the other GATA family members. GATA-1/pRb/E2F-2 complex formation stalls cell proliferation and steers erythroid precursors towards terminal differentiation. This process can be disrupted in vitro by FOG-1, which displaces pRb/E2F-2 from GATA-1. A GATA-1 mutant unable to bind pRb fails to inhibit cell proliferation and results in mouse embryonic lethality by anemia. These findings clarify the previously suspected cell-autonomous role of pRb during erythropoiesis and may provide a unifying molecular mechanism for several mouse phenotypes and human diseases associated with GATA-1 mutations.

Red blood cell production, or erythropoiesis, proceeds by a tight coupling of proliferation and differentiation. The earliest erythroid progenitor identifiable possesses remnant stem cell characteristics as it both self-renews and differentiates. Each progenitor gives rise to more than 10,000 cells, including secondary progenitors. Yet, during the next stage of differentiation, much of this renewal capability is lost, and terminal erythroid differentiation progresses in a stepwise manner through several stages separated by a single mitosis. The transcription factor GATA-1 is essential for erythroid differentiation because it induces the expression of all the known erythroid-specific genes. Here, we show that GATA-1 directly interacts with proteins that are central to the process of cell division: the retinoblastoma protein pRb and the transcription factor E2F. Specifically, E2F becomes inactivate after engaging in a GATA-1/pRb/E2F tricomplex. Another erythroid transcription factor, termed FOG-1, is able to displace pRb/E2F from this complex in vitro upon binding to GATA-1. We hypothesize that the liberated pRb/E2F can then be the target of subsequent regulation to ultimately release free E2F, which triggers cell division. The physiological role of this new pathway is evidenced by transgenic mouse experiments with GATA-1 mutants unable to bind pRb/E2F, which result in embryonic lethality by anemia.

Local communication on and within bone controls bone remodeling

Bone remodeling is required for healthy calcium homeostasis and for repair of damage occurring with stress and age. Osteoclasts resorb bone and osteoblasts form bone. These processes normally occur in a tightly regulated sequence of events, where the amount of formed bone equals the amount of resorbed bone, thereby restoring the removed bone completely. Osteocytes are the third cell type playing an essential role in bone turnover. They appear to regulate activation of bone remodeling, and they exert both positive and negative regulation on both osteoclasts and osteoblasts. In this review, we consider the intricate communication between these bone cells in relation to bone remodeling, reviewing novel data from patients with mutations rendering different cell populations inactive, which have shown that these interactions are more complex than originally thought. We highlight the high probability that a detailed understanding of these processes will aid in the development of novel treatments for bone metabolic disorders, i.e. we discuss the possibility that bone resorption can be attenuated pharmacologically without a secondary reduction in bone formation.

Rac1 signaling modulates BCL-6-mediated repression of gene transcription

Rac1 is a member of the Rho family of small GTPases that not only regulates signaling pathways involved in cell adhesion and migration but also regulates gene transcription. Here we show that the transcriptional repressor BCL-6 is regulated by Rac1 signaling. Transfection of active Rac1 mutants into colorectal DLD-1 cells led to increased expression of a BCL-6-controlled luciferase reporter construct. Conversely, inhibition of endogenous Rac1 activation by the Rac1 inhibitor NSC23766 decreased reporter activity. Moreover, BCL-6 lost its typical localization to nuclear dots upon activation of Rac1 and became predominantly soluble in a non-chromatin-bound cell fraction. Rac1 signaling also regulated the expression of endogenous BCL-6-regulated genes, including the p50 precursor NF-kappaB1/p105 and the cell adhesion molecule CD44. Interestingly, these effects were not stimulated by the alternative splice variant Rac1b. The mechanism of BCL-6 inhibition does not involve formation of a stable Rac1/BCL-6 complex and is independent of Rac-induced reactive oxygen species production or Jun NH(2)-terminal kinase activation. We show that PAK1 mediates inhibition downstream of Rac and can directly phosphorylate BCL-6. Together, these data provide substantial evidence that Rac1 signaling inhibits the transcriptional repressor BCL-6 in colorectal cells and reveal a novel pathway that links Rac1 signaling to the regulation of gene transcription.

Blood monocytes: development, heterogeneity, and relationship with dendritic cells

Monocytes are circulating blood leukocytes that play important roles in the inflammatory response, which is essential for the innate response to pathogens. But inflammation and monocytes are also involved in the pathogenesis of inflammatory diseases, including atherosclerosis. In adult mice, monocytes originate in the bone marrow in a Csf-1R (MCSF-R, CD115)-dependent manner from a hematopoietic precursor common for monocytes and several subsets of macrophages and dendritic cells (DCs). Monocyte heterogeneity has long been recognized, but in recent years investigators have identified three functional subsets of human monocytes and two subsets of mouse monocytes that exert specific roles in homeostasis and inflammation in vivo, reminiscent of those of the previously described classically and alternatively activated macrophages. Functional characterization of monocytes is in progress in humans and rodents and will provide a better understanding of the pathophysiology of inflammation.

Both cell-surface and secreted CSF-1 expressed by tumor cells metastatic to bone can contribute to osteoclast activation

Tumors metastatic to the bone produce factors that cause massive bone resorption mediated by osteoclasts in the bone microenvironment. Colony stimulating factor (CSF-1) is strictly required for the formation and survival of active osteoclasts, and is frequently produced by tumor cells. Here we hypothesize that the CSF-1 made by tumor cells contributes to bone destruction in osteolytic bone metastases. We show that high level CSF-1 protected osteoclasts from suppressive effects of transforming growth factor beta (TGF-beta). r3T cells, a mouse mammary tumor cell line that forms osteolytic bone metastases, express abundant CSF-1 in vitro as both a secreted and a membrane-spanning cell-surface glycoprotein, and we show that both the secreted and the cell-surface form of CSF-1 made by r3T cells can support osteoclast formation in co-culture experiments in the presence of RankL. Mice with r3T bone metastases have elevated levels of both circulating and bone-associated CSF-1, and the majority of CSF-1 found in bone metastases is associated with the tumor cells. These results support the idea that tumor-cell produced CSF-1 contributes to osteoclast development and survival in bone metastasis.

Vascular endothelial growth factor promotes macrophage apoptosis through stimulation of tumor necrosis factor superfamily member 14 (TNFSF14/LIGHT)

Resolution of inflammation is critical for normal wound healing. Inflammation is prolonged and fails to resolve properly in chronic wounds. We used in vivo and in vitro approaches to show that vascular endothelial growth factor (VEGF) induces macrophage apoptosis and to delineate mechanisms involved in this process. VEGF inhibition during wound healing leads to an increased number of macrophages remaining in wounds, suggesting the involvement of VEGF in removal of these cells from the wound. If this effect has physiological relevance, it likely occurs via apoptosis. We show that VEGF increases apoptosis of macrophages in vitro using Annexin V-FITC staining and caspase activation. Microarray analysis, reverse transcription-polymerase chain reaction, and immunoblotting showed that VEGF increases the expression of tumor necrosis factor superfamily member 14 (TNFSF14/LIGHT) in macrophages. We also show that in macrophages LIGHT promotes apoptosis through the lymphotoxin beta receptor. Moreover, inhibition of LIGHT prevents VEGF-induced death, suggesting that LIGHT mediates VEGF-induced macrophage apoptosis. Taken together, our results identify a novel role for VEGF and for LIGHT in macrophage apoptosis during wound healing, an event critical in the resolution of inflammation. This finding may lead to the development of new strategies to improve resolution of inflammation in problematic wounds.

Differential migration of epidermal and dermal dendritic cells during skin infection

Dendritic cells (DCs) are extremely heterogeneous, most evident in the skin where a variety of different subsets have been identified in recent years. DCs of healthy skin include a number of distinct populations in the dermal layer as well as the well-characterized Langerhans cells (LCs) of the epidermis. These steady-state populations are augmented during bouts of local inflammation by additional monocyte-derived DCs. In an effort to better understand the distinction between the different subsets, we examined their behavior following skin infection with HSV. LC emigration rapidly followed appearance of virus in the skin and resulted in depopulation of regions in areas surrounding infected nerve endings. A separate DC population was found to accumulate within the dermis under patches of active epidermal infection with at least some derived from blood monocyte precursors. Ag-positive DCs could occasionally be found in these dermal accumulations, although they represented a minority of DCs in these areas. In addition, infected DCs appeared compromised in their trafficking capabilities and were largely absent from the migrating population. On resolution of skin disease, LCs repopulated the reformed epidermis and these were of mixed origin, with around half entering from the circulation and the remainder derived from local progenitors. Overall, our results show a range of migrational complexities between distinct skin DC populations as a consequence of localized infection.

A data integration framework for prediction of transcription factor targets

We present a computational framework for predicting targets of transcription factor regulation. The framework is based on the integration of a number of sources of evidence, derived from DNA-sequence and gene-expression data, using a weighted sum approach. Sources of evidence are prioritized based on a training set, and their relative contributions are then optimized. The performance of the proposed framework is demonstrated in the context of BCL6 target prediction. We show that this framework is able to uncover BCL6 targets reliably when biological prior information is utilized effectively, particularly in the case of sequence analysis. The framework results in a considerable gain in performance over scores in which sequence information was not incorporated. This analysis shows that with assessment of the quality and biological relevance of the data, reliable predictions can be obtained with this computational framework.

NF-kappaB pathways in the immune system: control of the germinal center reaction

The NF-kappaB signaling pathway plays a critical role in regulating innate and adaptive immunity. This is clearly evident as mouse models deficient for numerous NF-kappaB subunits and upstream activators exhibit defects in the immune system ranging from impaired development of lymphocytes to defective adaptive immune responses. In this review, we focus on the role that NF-kappaB plays in the germinal center (GC) reaction. Specifically, we discuss the major NF-kappaB subunits and the IkappaB homolog, Bcl-3. Recent findings reveal that Bcl-6, an unrelated transcriptional repressor, is functionally similar to Bcl-3 as both factors may suppress p53 activity to allow for efficient GC formation to occur. We discuss potential mechanisms of action for Bcl-3 and Bcl-6 in this highly complex, but important process of B-cell affinity maturation.

Dendritic cells in the pathogenesis of sarcoidosis

Sarcoidosis is a noncaseating granulomatous disease, likely of autoimmune etiology, that causes inflammation and tissue damage in multiple organs, most commonly the lung, but also skin, and lymph nodes. Reduced dendritic cell (DC) function in sarcoidosis peripheral blood compared with peripheral blood from control subjects suggests that blunted end organ cellular immunity may contribute to sarcoidosis pathogenesis. Successful treatment of sarcoidosis with tumor necrosis factor (TNF) inhibitors, which modulate DC maturation and migration, has also been reported. Together, these observations suggest that DCs may be important mediators of sarcoidosis immunology. This review focuses on the phenotype and function of DCs in the lung, skin, blood, and lymph node of patients with sarcoidosis. We conclude that DCs in end organs are phenotypically and functionally immature (anergic), while DCs in the lymph node are mature and polarize pathogenic Th1 T cells. The success of TNF inhibitors is thus likely secondary to inhibition of DC-mediated Th1 polarization in the lymph node.

Trophic macrophages in development and disease

Specialized phagocytes are found in the most primitive multicellular organisms. Their roles in homeostasis and in distinguishing self from non-self have evolved with the complexity of organisms and their immune systems. Equally important, but often overlooked, are the roles of macrophages in tissue development. As discussed in this Review, these include functions in branching morphogenesis, neuronal patterning, angiogenesis, bone morphogenesis and the generation of adipose tissue. In each case, macrophage depletion impairs the formation of the tissue and compromises its function. I argue that in several diseases, the unrestrained acquisition of these developmental macrophage functions exacerbates pathology. For example, macrophages enhance tumour progression and metastasis by affecting tumour-cell migration and invasion, as well as angiogenesis.

Ex vivo large-scale generation of human red blood cells from cord blood CD34+ cells by co-culturing with macrophages

We generated red blood cells (RBC) from cord blood (CB) CD34+ cells using a four-phase culture system. We first cultured CB CD34+ cells on telomerase gene-transduced human stromal cells in serum-free medium containing stem cell factor (SCF), Flt-3/Flk-2 ligand, and thrombopoietin to expand CD34+ cells (980-fold) and the total cells (10,400-fold) (first phase). Expanded cells from the first phase were liquid-cultured with SCF, interleukin-3 (IL-3), and erythropoietin (EPO) to expand (113-fold) and differentiate them into erythroblasts (second phase). To obtain macrophages for the next phase, we expanded CD34+ cells from a different donor using the same coculture system. Expanded cells from the first phase were liquid-cultured with granulocyte-macrophage colony stimulating factor, macrophage-colony stimulating factor (M-CSF), IL-3, and SCF to generate monocytes/macrophages (75-fold), which were incubated with type AB serum and M-CSF to fully differentiate them into macrophages. Erythroblasts were then co-cultured with macrophages in the presence of EPO to expand (threefold) and fully differentiate them (61% orthochromatic erythroblasts plus 39% RBC) (third phase). RBC were purified from erythroblasts and debris through a deleukocyting filter to generate 6.0 x 10(12) RBC from 1.0 unit of CB (3.0 transfusable units). Qualitatively, these RBC showed a hemoglobin content, oxygenation of hemoglobin, and in vivo clearance similar to those of adult peripheral RBC. Finally, an almost complete enucleation of orthochromatic erythroblasts (99.4%) was achieved by the cultivation method recently described by Miharada et al. in the absence of macrophages and cytokines (fourth phase). RBC were purified from remnant erythroblasts and debris by passage through a deleukocyting filter to generate 1.76 x 10(13) RBC from 1.0 unit of CB (8.8 transfusable units), the highest yield ever reported. Thus, this method may be useful for generating an alternative RBC supply for transfusions, investigating infectious agents that target erythroid cells, and as a general in vitro hematopoietic model system.

CD1a expression defines an interleukin-12 producing population of human dendritic cells

Human and murine dendritic cell (DC) subsets are often defined by phenotypic features that predict their functional characteristics. In humans and mice, DC have been shown to have the ability to polarize naive CD4 T cells to a T helper type 1 (Th1) or Th2 phenotype. However, human myeloid DC generated from monocytes (monocyte-derived DC) have often been regarded as a homogeneous population, both phenotypically and functionally. Monocytes give rise to subpopulations of DC in vitro that can be separated on the basis of their expression of CD1a, a well-described DC subset marker. Importantly, we show that the CD1a(+) DC subset produces significant quantities of interleukin-12p70 (IL-12p70) upon stimulation and, similar to the murine CD8 alpha(+) DC subset, can polarize naive CD4(+) T cells to a Th1 phenotype. In contrast, CD1a(-) DC, similar to murine CD8 alpha(-) DC, do not produce significant amounts of IL-12p70 upon stimulation or polarize T cells to a Th1 phenotype. Like monocyte-derived DC, CD1a(+) and CD1a(-) DC subsets obtained from CD34(+) haematopoietic progenitors under distinct culture conditions were found to have these same features, suggesting that CD1a expression is a marker for myeloid DC that are a major source of IL-12 and Th1 CD4(+) T cell polarization in man.

Development of macrophages of cyprinid fish

The innate immune responses of early vertebrates, such as bony fishes, play a central role in host defence against infectious diseases and one of the most important effector cells of innate immunity are macrophages. In order for macrophages to be effective in host defence they must be present at all times in the tissues of their host and importantly, the host must be capable of rapidly increasing macrophage numbers during times of need. Hematopoiesis is a process of formation and development of mature blood cells, including macrophages. Hematopoiesis is controlled by soluble factors known as cytokines, that influence changes in transcription factors within the target cells, resulting in cell fate changes and the final development of specific effector cells. The processes involved in macrophage development have been largely derived from mammalian model organisms. However, recent advancements have been made in the understanding of macrophage development in bony fish, a group of organisms that rely heavily on their innate immune defences. Our understanding of the growth factors involved in teleost macrophage development, as well as the receptors and regulatory mechanisms in place to control them has increased substantially. Furthermore, model organisms such as the zebrafish have emerged as important instruments in furthering our understanding of the transcriptional control of cell development in fish as well as in mammals. This review highlights the recent advancements in our understanding of teleost macrophage development. We focused on the growth factors identified to be important in the regulation of macrophage development from a progenitor cell into a functional macrophage and discuss the important transcription factors that have been identified to function in teleost hematopoiesis. We also describe the findings of in vivo studies that have reinforced observations made in vitro and have greatly improved the relevance and importance of using teleost fish as model organisms for studying developmental processes.

Targeted overexpression of the two colony-stimulating factor-1 isoforms in osteoblasts differentially affects bone loss in ovariectomized mice

Colony-stimulating factor-1 (CSF1) is one of two cytokines required for normal osteoclastogenesis. There are two major isoforms of CSF1, the cell-surface or membrane-bound isoform (mCSF1) and soluble CSF1 (sCSF1). Whether these isoforms serve nonredundant functions in bone is unclear. To explore this question, we generated transgenic mice expressing human sCSF1, human mCSF1, or both (s/mCSF1) in osteoblasts using the 2.3-kb rat alphaI-collagen promoter. Bone density determined by peripheral quantitative computed tomography was significantly reduced in mCSF1, sCSF1, and s/mCSF1 transgenic mice compared with wild-type animals. When analyzed by sex, sCSF1, and s/mCSF1, female animals but not mCSF1 female mice were found to have greater bone loss than their male littermates (-20 vs. -9.2%; P<0.05 for sCSF1 and -21.6 vs. -11.2% for s/mCSF1; P<0.01). By breeding CSF1 isoform-selective transgenic mice to an op/op background, mice were generated in which a single CSF1 isoform was the only source of the cytokine (sCSF1op/op and mCSF1op/op). Unlike osteoblast-targeted overexpression of mCSF1, selective transgenic expression of sCSF1 did not completely correct the op/op phenotype in 5-mo-old animals. Interestingly, compared with sham-ovariectomized mice of the same genotype, ovariectomy in sCSF1op/op mice led to a greater loss of spinal bone mineral density (22.1%) than was seen in either mCSF1op/op mice (12.9%) or in wild-type animals (10.9%). Our findings support the conclusion that sCSF1 and mCSF1 serve nonredundant functions in bone and that sCSF1 may play a role in mediating estrogen-deficiency bone loss.

The role of inflammation in insulitis and beta-cell loss in type 1 diabetes

Type 1 diabetes mellitus (T1DM) is a chronic autoimmune disease with a strong inflammatory component. The latest studies indicate that innate immunity and inflammatory mediators have a much broader role in T1DM than initially assumed. Inflammation might contribute to early induction and amplification of the immune assault against pancreatic beta cells and, at later stages, to the stabilization and maintenance of insulitis. Inflammatory mediators probably contribute to the suppression of beta-cell function and subsequent apoptosis; they may also inhibit or stimulate beta-cell regeneration and might cause peripheral insulin resistance. The different effects of inflammation take place in different phases of the course of T1DM, and should be considered in the context of a 'dialog' between invading immune cells and the target beta cells. This dialog is mediated both by cytokines and chemokines that are released by beta cells and immune cells, and by putative, immunogenic signals that are delivered by dying beta cells. In this Review, we divided the role of inflammation in T1DM into three arbitrary stages: induction, amplification and maintenance or resolution of insulitis. These stages, and their progression or resolution, might depend on a patient's genetic background, which contributes to disease heterogeneity.

Early chromatin unfolding by RUNX1: a molecular explanation for differential requirements during specification versus maintenance of the hematopoietic gene expression program

At the cellular level, development progresses through successive regulatory states, each characterized by their specific gene expression profile. However, the molecular mechanisms regulating first the priming and then maintenance of gene expression within one developmental pathway are essentially unknown. The hematopoietic system represents a powerful experimental model to address these questions and here we have focused on a regulatory circuit playing a central role in myelopoiesis: the transcription factor PU.1, its target gene colony-stimulating-factor 1 receptor (Csf1r), and key upstream regulators such as RUNX1. We find that during ontogeny, chromatin unfolding precedes the establishment of active histone marks and the formation of stable transcription factor complexes at the Pu.1 locus and we show that chromatin remodeling is mediated by the transient binding of RUNX1 to Pu.1 cis-elements. By contrast, chromatin reorganization of Csf1r requires prior expression of PU.1 together with RUNX1 binding. Once the full hematopoietic program is established, stable transcription factor complexes and active chromatin can be maintained without RUNX1. Our experiments therefore demonstrate how individual transcription factors function in a differentiation stage-specific manner to differentially affect the initiation versus maintenance of a developmental program.

High expression of PGE2 enzymatic pathways in cervical (pre)neoplastic lesions and functional consequences for antigen-presenting cells

Although human papillomavirus (HPV) DNA is detected in the majority of squamous intraepithelial lesions (SIL) and carcinoma (SCC) of the uterine cervix, the persistence or progression of cervical lesions suggest that viral antigens are not adequately presented to the immune system. This hypothesis is reinforced by the observation that most SIL show quantitative and functional alterations of Langerhans cells (LC). The aim of this study was to determine whether prostaglandins (PG) may affect LC density in the cervical (pre)neoplastic epithelium. We first demonstrated that the epithelial expression of PGE(2) enzymatic pathways, including cyclooxygenase-2 (COX-2) and microsomal prostaglandin E synthase 1 (mPGES-1), is higher in SIL and SCC compared to the normal exocervical epithelium and inversely correlated to the density of CD1a-positive LC. By using cell migration assays, we next showed that the motility of immature dendritic cells (DC) and DC partially differentiated in vitro in the presence of PGE(2) are differentially affected by PGE(2). Immature DC had a lower ability to migrate in the presence of PGE(2) compared to DC generated in vitro in the presence of PGE(2). Finally, we showed that PGE(2) induced a cytokine production profile and phenotypical features of tolerogenic DC, suggesting that the altered expression of PGE(2) enzymatic pathways may promote the cervical carcinogenesis by favouring (pre)cancer immunotolerance.

Bacterial signalling overrides cytokine signalling and modifies dendritic cell differentiation

Heterogeneity of dendritic cells (DC) is evident in the gut-associated lymphoid tissue and determined, in part, by incompletely understood local environmental factors. Bacterial signalling is likely to be a dominant influence on precursor cells when recruited to the mucosa. We assessed the influence of commensal bacteria on DC differentiation and function. Murine bone marrow progenitors were exposed to Lactobacillus salivarius, Bifidobacterium breve or Bifidobacterium infantis. Differences in cell surface phenotype and function were assessed. Myeloid differentiation factor 88(-/-) (MyD88) cells were used to determine the influence of Toll-like receptor signalling. While bacterial strains varied in impact, there was a consistent dose-dependent inhibition of DC differentiation with a shift toward a Gr-1(+) CD11b(+) monocyte-like phenotype. A single bacterium on a per cell basis (1 : 1) was sufficient to alter cell phenotype. The effect was only evident in early precursors. Enhanced interleukin-10 production correlated with increased Forkhead box P3 expression and reduced T-cell proliferation. The bacterial effect on DC differentiation was found to be MyD88-dependent. Signalling by enteric commensals through pattern recognition receptors on precursor cells alters DC differentiation and results in cells that are phenotypically monocyte-like and functionally suppressive. This may account for some of the features of mucosal immune tolerance to the microbiota.

MHC class I/peptide transfer between dendritic cells overcomes poor cross-presentation by monocyte-derived APCs that engulf dying cells

In vivo data suggest that monocytes participate critically in cross-presentation, but other data suggest that lymph node resident dendritic cells (DCs) mainly cross-present. Here, we utilized a three-dimensional model of a blood vessel wall that endogenously supports DC development from human monocytes, and we incorporated dying autologous cells in the subendothelial matrix of the model. Flu-infected dying cells promoted monocytes to become mature DCs and cross-present cell-associated Ags for the activation of CTLs. Similar responses were induced by loading the dying cells with the TLR7/8 ligand ssRNA, whereas dying cells loaded with TLR3 ligand were less efficient. Monocyte-derived DCs that developed in this model cross-presented Ag to T cells efficiently regardless of whether they engulfed detectable amounts of labeled dying cells. Unexpectedly, the monocyte-derived cells that directly engulfed dying cells in vitro were not the major APCs stimulating CD8(+) lymphocytes. Instead, bystander DCs acquired more robust capacity to cross-prime through receipt of MHC class I/peptide from the phagocytic, monocyte-derived cells. In mice, lymph node-homing monocyte-derived DCs processed Ags from engulfed cells and then transferred MHC class I/peptide complexes to confer cross-priming capacity to MHC class I-deficient lymph node resident CD8alpha(+) DCs. Thus, natural or synthetic TLR7/8 agonists contained within dying cells promote the conversion of monocytes to DCs with capacity for cross-presentation and for "cross-dressing" other DCs. These data reveal a way in which migratory monocyte-derived DCs and other DCs, like lymph node resident DCs, both mediate cross-presentation.

CX3CR1+ CD115+ CD135+ common macrophage/DC precursors and the role of CX3CR1 in their response to inflammation

CX₃CR1 expression is associated with the commitment of CSF-1R⁺ myeloid precursors to the macrophage/dendritic cell (DC) lineage. However, the relationship of the CSF-1R⁺ CX₃CR1⁺ macrophage/DC precursor (MDP) with other DC precursors and the role of CX₃CR1 in macrophage and DC development remain unclear. We show that MDPs give rise to conventional DCs (cDCs), plasmacytoid DCs (PDCs), and monocytes, including Gr1⁺ inflammatory monocytes that differentiate into TipDCs during infection. CX₃CR1 deficiency selectively impairs the recruitment of blood Gr1⁺ monocytes in the spleen after transfer and during acute Listeria monocytogenes infection but does not affect the development of monocytes, cDCs, and PDCs.

1,25-dihydroxyvitamin D3 is an autonomous regulator of the transcriptional changes leading to a tolerogenic dendritic cell phenotype

Activation of vitamin D receptor (VDR) by 1,25-dihydroxyvitamin D(3) (1,25-vitD) reprograms dendritic cells (DC) to become tolerogenic. Previous studies suggested that 1,25-vitD could inhibit the changes brought about by differentiation and maturation of DCs. Underpinning the described phenotypic and functional alterations, there must be 1,25-vitD-coordinated transcriptional events. However, this transcriptional program has not been systematically investigated, particularly not in a developmental context. Hence, it has not been explored how 1,25-vitD-regulated genes, particularly the ones bringing about the tolerogenic phenotype, are connected to differentiation. We conducted global gene expression analysis followed by comprehensive quantitative PCR validation to clarify the interrelationship between 1,25-vitD and differentiation-driven gene expression patterns in developing human monocyte-derived and blood myeloid DCs. In this study we show that 1,25-vitD regulates a large set of genes that are not affected by differentiation. Interestingly, several genes, impacted both by the ligand and by differentiation, appear to be regulated by 1,25-vitD independently of the developmental context. We have also characterized the kinetics of generation of 1,25-vitD by using three early and robustly regulated genes, the chemokine CCL22, the inhibitory receptors CD300LF and CYP24A1. We found that monocyte-derived DCs are able to turn on 1,25-vitD sensitive genes in early phases of differentiation if the precursor is present. Our data collectively suggest that exogenous or endogenously generated 1,25-vitD regulates a large set of its targets autonomously and not via inhibition of differentiation and maturation, leading to the previously characterized tolerogenic state.

The balance between immunity and tolerance: the role of Langerhans cells

Langerhans cells are immature skin-homing dendritic cells that furnish the epidermis with an immune surveillance system, and translate information between the internal and external milieu. Dendritic cells, in particular Langerhans cells, are gaining prominence as one of the potential principal players orchestrating the decision between immunity and tolerance. Langerhans cells capture aberrant self-antigen and pathogen-derived antigen for display to the efferent immune response. Recent evidence suggests redundancy in the antigen-presenting function of Langerhans cells, with dermal dendritic subsets capable of fulfilling an analogous role. There is mounting evidence that Langerhans cells can cross-prime T cells to recognize antigens. Langerhans cells are proposed to stimulate T regulatory cells, and are implicated in the pathogenesis of cutaneous T cell lymphoma.The phenotype of Langerhans cells, which may be tolerogenic or immunogenic, appears to depend on their state of maturity, inciting immunogen and cytokine environment, offering the potential for manipulation in immunotherapy.

Mobilization and margination of bone marrow Gr-1high monocytes during subclinical endotoxemia predisposes the lungs toward acute injury

The specialized role of mouse Gr-1(high) monocytes in local inflammatory reactions has been well documented, but the trafficking and responsiveness of this subset during systemic inflammation and their contribution to sepsis-related organ injury has not been investigated. Using flow cytometry, we studied monocyte subset margination to the pulmonary microcirculation during subclinical endotoxemia in mice and investigated whether marginated monocytes contribute to lung injury in response to further septic stimuli. Subclinical low-dose i.v. LPS induced a rapid (within 2 h), large-scale mobilization of bone marrow Gr-1high monocytes and their prolonged margination to the lungs. With secondary LPS challenge, membrane TNF expression on these premarginated monocytes substantially increased, indicating their functional priming in vivo. Zymosan challenge produced small increases in pulmonary vascular permeability, which were markedly enhanced by the preadministration of low-dose LPS. The LPS-zymosan-induced permeability increases were effectively abrogated by pretreatment (30 min before zymosan challenge) with the platelet-activating factor antagonist WEB 2086 in combination with the phosphatidylcholine-phospholipase C inhibitor D609, suggesting the involvement of platelet-activating factor/ceramide-mediated pathways in this model. Depletion of monocytes (at 18 h after clodronate-liposome treatment) significantly attenuated the LPS-zymosan-induced permeability increase. However, restoration of normal LPS-induced Gr-1high monocyte margination to the lungs (at 48 h after clodronate-liposome treatment) resulted in the loss of this protective effect. These results demonstrate that mobilization and margination of Gr-1high monocytes during subclinical endotoxemia primes the lungs toward further septic stimuli and suggest a central role for this monocyte subset in the development of sepsis-related acute lung injury.

The proliferative human monocyte subpopulation contains osteoclast precursors

Introduction

Immediate precursors of bone-resorbing osteoclasts are cells of the monocyte/macrophage lineage. Particularly during clinical conditions showing bone loss, it would appear that osteoclast precursors are mobilized from bone marrow into the circulation prior to entering tissues undergoing such loss. The observed heterogeneity of peripheral blood monocytes has led to the notion that different monocyte subpopulations may have special or restricted functions, including as osteoclast precursors.

Methods

Human peripheral blood monocytes were sorted based upon their degree of proliferation and cultured in macrophage colony-stimulating factor (M-CSF or CSF-1) and receptor activator of nuclear factor-kappa-B ligand (RANKL).

Results

The monocyte subpopulation that is capable of proliferation gave rise to significantly more multinucleated, bone-resorbing osteoclasts than the bulk of the monocytes.

Conclusions

Human peripheral blood osteoclast precursors reside in the proliferative monocyte subpopulation.

Dendritic cell antigen presentation drives simultaneous cytokine production by effector and regulatory T cells in inflamed skin

Effector (Teff) and regulatory (Treg) T cells produce cytokines that balance immunity and immunopathology at sites of infection. It is not known how this balance is achieved. Here, we show that Treg and Teff cells specific for the same foreign peptide:major histocompatibility complex II (pMHCII) ligand accumulated preferentially in a subcutaneous site injected with the relevant antigen plus an adjuvant. Some of the Treg cells in this site were producing IL-10 12 days after injection, whereas a similar fraction of the Teff cells were producing IFN-gamma. Acute ablation of Treg cells increased the fraction of IFN-gamma-producing Teff cells, indicating that Teff function was limited by the Treg cells. Production of cytokines by both populations was driven by pMHCII presentation by local CD11b(hi) dermal dendritic cells. Therefore, balanced production of microbicidal and suppressive cytokines in inflamed skin is achieved by simultaneous dendritic cell antigen presentation to Teff and Treg cells.