Differential involvement of PU.1 and Id2 downstream of TGF-beta1 during Langerhans-cell commitment

Yinxieling decoction ameliorates psoriasis by regulating the differentiation and functions of Langerhans cells via the TGF-β1/PU.1/IL-23 signal axis

Langerhans cells (LCs) play a critical role in skin immune responses and the development of psoriasis. Yinxieling (YXL) is a representative Chinese herbal medicine for the treatment of psoriasis in South China. It was found to improve psoriasis without obvious side effects in the clinic. Here we attempted to clarify whether and how YXL regulates the differentiation and functions of LCs in Imiquimod (IMQ)-induced psoriasis in vivo and induced LCs in vitro. The Psoriasis Area Severity Index (PASI) score was used to evaluate the efficacy of YXL for IMQ-induced psoriasis-like mice. Flow cytometry was utilized to analyze the effects of YXL, to regulate the differentiation, migration, maturation, and antigen presentation of LCs. The results show that YXL significantly alleviated skin inflammation, as reduced in PASI score and classic psoriasis characteristics in pathological sections. Although there was no effect on the proportion of total DCs in the skin-draining lymph nodes, the expression of epidermal LCs and its transcription factor PU.1 were both markedly inhibited. LCs were also prevented from migrating from epidermal to skin-draining lymph nodes and mature. In addition, the number of LCs carrying antigens in the epidermis increased, which suggested that YXL could effectively prevent LCs from presenting antigens. In vitro, YXL had a significant impact on inhibiting the differentiation of LCs. Further data showed that YXL decreased the relative expression of transforming growth factor-β (TGFβ) messenger RNA (mRNA) and interleukin-23 (IL-23) mRNAs. Thus, YXL alleviates psoriasis by regulating differentiation, migration, maturation, and antigen presentation via the TGFβ/PU.1/IL-23 signal axis.

Circulating extracellular vesicles in human cardiorenal syndrome promote renal injury in a kidney-on-chip system

BACKGROUNDCardiorenal syndrome (CRS) - renal injury during heart failure (HF) - is linked to high morbidity. Whether circulating extracellular vesicles (EVs) and their RNA cargo directly impact its pathogenesis remains unclear.METHODSWe investigated the role of circulating EVs from patients with CRS on renal epithelial/endothelial cells using a microfluidic kidney-on-chip (KOC) model. The small RNA cargo of circulating EVs was regressed against serum creatinine to prioritize subsets of functionally relevant EV-miRNAs and their mRNA targets investigated using in silico pathway analysis, human genetics, and interrogation of expression in the KOC model and in renal tissue. The functional effects of EV-RNAs on kidney epithelial cells were experimentally validated.RESULTSRenal epithelial and endothelial cells in the KOC model exhibited uptake of EVs from patients with HF. HF-CRS EVs led to higher expression of renal injury markers (IL18, LCN2, HAVCR1) relative to non-CRS EVs. A total of 15 EV-miRNAs were associated with creatinine, targeting 1,143 gene targets specifying pathways relevant to renal injury, including TGF-β and AMPK signaling. We observed directionally consistent changes in the expression of TGF-β pathway members (BMP6, FST, TIMP3) in the KOC model exposed to CRS EVs, which were validated in epithelial cells treated with corresponding inhibitors and mimics of miRNAs. A similar trend was observed in renal tissue with kidney injury. Mendelian randomization suggested a role for FST in renal function.CONCLUSIONPlasma EVs in patients with CRS elicit adverse transcriptional and phenotypic responses in a KOC model by regulating biologically relevant pathways, suggesting a role for EVs in CRS.TRIAL REGISTRATIONClinicalTrials.gov NCT03345446.FUNDINGAmerican Heart Association (AHA) (SFRN16SFRN31280008); National Heart, Lung, and Blood Institute (1R35HL150807-01); National Center for Advancing Translational Sciences (UH3 TR002878); and AHA (23CDA1045944).

Distinct human Langerhans cell subsets orchestrate reciprocal functions and require different developmental regulation

Langerhans cells (LCs) play a pivotal role in skin homeostasis, and the heterogeneity of LCs has long been considered. In this study, we have identified two steady-state (LC1 and LC2) and two activated LC subsets in the epidermis of human skin and in LCs derived from CD34⁺ hemopoietic stem cells (HSC-LCs) by utilizing single-cell RNA sequencing and mass cytometry. Analysis of HSC-LCs at multiple time-points during differentiation revealed that EGR1 and Notch signaling were among the top pathways regulating the bifurcation of LC1 and LC2. LC1 were characterized as classical LCs, mainly related to innate immunity and antigen processing. LC2 were similar to monocytes or myeloid dendritic cells, involving in immune responses and leukocyte activation. LC1 remained stable under inflammatory microenvironment, whereas LC2 were prone to being activated and demonstrated elevated expression of immuno-suppressive molecules. We revealed distinct human LC subsets that require different developmental regulation and orchestrate reciprocal functions.

Lack of a site-specific phosphorylation of Presenilin 1 disrupts microglial gene networks and progenitors during development

Microglial cells play a key role in brain homeostasis from development to adulthood. Here we show the involvement of a site-specific phosphorylation of Presenilin 1 (PS1) in microglial development. Profiles of microglia-specific transcripts in different temporal stages of development, combined with multiple systematic transcriptomic analysis and quantitative determination of microglia progenitors, indicate that the phosphorylation of PS1 at serine 367 is involved in the temporal dynamics of microglial development, specifically in the developing brain rudiment during embryonic microgliogenesis. We constructed a developing brain-specific microglial network to identify transcription factors linked to PS1 during development. Our data showed that PS1 functional connections appear through interaction hubs at Pu.1, Irf8 and Rela-p65 transcription factors. Finally, we showed that the total number of microglia progenitors was markedly reduced in the developing brain rudiment of embryos lacking PS1 phosphorylation compared to WT. Our work identifies a novel role for PS1 in microglial development.

Transcriptional control of dendritic cell development and functions

Dendritic cells (DCs) are major regulators of adaptive immunity, as they are not only capable to induce efficient immune responses, but are also crucial to maintain peripheral tolerance and thereby inhibit autoimmune reactions. DCs bridge the innate and the adaptive immune system by presenting peptides of self and foreign antigens as peptide MHC complexes to T cells. These properties render DCs as interesting target cells for immunomodulatory therapies in cancer, but also autoimmune diseases. Several subsets of DCs with special properties and functions have been described. Recent achievements in understanding transcriptional programs on single cell level, together with the generation of new murine models targeting specific DC subsets, advanced our current understanding of DC development and function. Thus, DCs arise from precursor cells in the bone marrow with distinct progenitor cell populations splitting the monocyte populations and macrophage populations from the DC lineage, which upon lineage commitment can be separated into conventional cDC1, cDC2, and plasmacytoid DCs (pDCs). The DC populations harbor intrinsic programs enabling them to react for specific pathogens in dependency on the DC subset, and thereby orchestrate T cell immune responses. Similarities, but also varieties, between human and murine DC subpopulations are challenging, and will require further investigation of human specimens under consideration of the influence of the tissue micromilieu and DC subset localization in the future.

Vaginal Lactobacilli Induce Differentiation of Monocytic Precursors Toward Langerhans-like Cells: in Vitro Evidence

Lactobacilli have immunomodulatory mechanisms that affect the host cell immune system, leading to inhibition of HIV-1 transmission. Thus, lactobacilli as mucosal delivery vehicles for developing HIV-1 vaccines have attracted interest in recent years. Herein, we investigated the immunomodulatory effects of six strains of Lactobacillus naturally isolated from vaginal samples, including Lactobacillus crispatus (L. crispatus), L. fermentum, L. jensenii, L. gasseri, L. delbrueckii and L. johnsonii, on differentiation of monocytic precursors. L. crispatus, L. fermentum and L. delbrueckii could drive human monocytic cell line THP-1 cells to differentiate into dendritic-like cells according to the morphology. Moreover, L. crispatus increased costimulatory molecules including CD40, CD80 and CD86, and Langerhans cell specific C-type lectin receptors CD207, while L. fermentum decreased these molecules in THP-1 cells. Furthermore, L. crispatus promoted the differentiation of THP-1 cells with specific markers, phagocytic features, cytokine production ability and reduced the expression of receptors for HIV-1 entry of Langerhans cells. However, in the presence of L. fermentum, THP-1 cells did not show the above alterations. Moreover, similar effects of L. crispatus and L. fermentum were observed in CD14⁺ monocytes. These data suggested that L. crispatus facilitates the differentiation of monocytic precursors toward Langerhans-like cells in vitro. We further identified the cell wall components of Lactobacillus and found that peptidoglycans (PGNs), rather than bacteriocins, S-layer protein and lipoteichoic acid, were key contributors to the induction of CD207 expression. However, PGNs originating from Bacillus subtilis, E. coli JM109 and E. coli DH5α did not elevate CD207 expression, indicating that only PGN derived from Lactobacillus could enhance CD207 expression. Finally, the recognized receptors of L. crispatus (such as TLR2 and TLR6) and the upstream transcription factors (PU.1, TAL1, TIF1γ, and POLR2A) of CD207 were examined, and the expression of these molecules was enhanced in THP-1 cells following L. crispatus treatment. Thus, this study offers powerful evidence that vaginal lactobacilli modulate monocytic precursor differentiation into Langerhans-like cells probably via activating the TLR2/6-TFs-CD207 axis. These data provide clues for further investigation of the original occurrence, development and differentiation of Langerhans cells from monocytes.

The role of inhibitor of binding or differentiation 2 in the development and differentiation of immune cells

Inhibitor of binding or differentiation 2 (Id2), a member of helix-loop-helix (HLH) transcriptional factors, is recently reported as an important regulator of the development or differentiation of immune cells. It has been demonstrated that Id2 plays a critical role in the early lymphopoiesis. However, it has been discovered recently that Id2 displays new functions in different immune cells. In the adaptive immune cells, Id2 is required for determining T-cell subsets and B cells. In addition, Id2 is also involved in the development of innate immune cells, including dendritic cells (DCs), natural killer (NK) cells, and other innate lymphoid cells (ILCs). Here, we review the current reports about the role of Id2 in the development or differentiation of main immune cells.

Micro-environmental signals directing human epidermal Langerhans cell differentiation

Human Langerhans cells (LC) can be generated ex vivo from hematopoietic precursor cells in response to cytokines and cell-membrane associated ligands. These in vitro differentiation models provided mechanistic insights into the molecular and cellular pathways underlying the development of this unique, epithelia-associated dendritic cell subset. Notably, the human epidermal microenvironment is fully sufficient to induce LC differentiation from hematopoietic progenitors. Hence, dissecting the molecular characteristics of the human epithelial/epidermal LC niche, and testing defined ligands for their capacity to induce LC differentiation, led to a refined molecular model of LC lineage commitment. During epidermal ontogeny, spatially and temporally regulated availability of TGF-β family members cooperate with other keratinocyte-derived signals, such as E-cadherin and Notch ligands, for instructing LC differentiation. In this review, we discuss the signals known to instruct human hematopoietic progenitor cells and myelomonocytic cells to undergo LC lineage commitment. Additionally, the current methods for generation of large numbers of human LC-like cells ex vivo in defined serum-free media are discussed.

Langerhans Cells-Programmed by the Epidermis

Langerhans cells (LCs) reside in the epidermis as a dense network of immune system sentinels. These cells determine the appropriate adaptive immune response (inflammation or tolerance) by interpreting the microenvironmental context in which they encounter foreign substances. In a normal physiological, "non-dangerous" situation, LCs coordinate a continuous state of immune tolerance, preventing unnecessary and harmful immune activation. Conversely, when they sense a danger signal, for example during infection or when the physical integrity of skin has been compromised as a result of a trauma, they instruct T lymphocytes of the adaptive immune system to mount efficient effector responses. Recent advances investigating the molecular mechanisms underpinning the cross talk between LCs and the epidermal microenvironment reveal its importance for programming LC biology. This review summarizes the novel findings describing LC origin and function through the analysis of the transcriptomic programs and gene regulatory networks (GRNs). Review and meta-analysis of publicly available datasets clearly delineates LCs as distinct from both conventional dendritic cells (DCs) and macrophages, suggesting a primary role for the epidermal microenvironment in programming LC biology. This concept is further supported by the analysis of the effect of epidermal pro-inflammatory signals, regulating key GRNs in human and murine LCs. Applying whole transcriptome analyses and in silico analysis has advanced our understanding of how LCs receive, integrate, and process signals from the steady-state and diseased epidermis. Interestingly, in homeostasis and under immunological stress, the molecular network in LCs remains relatively stable, reflecting a key evolutionary need related to tissue localization. Importantly, to fulfill their key role in orchestrating antiviral adaptive immune responses, LC share specific transcriptomic modules with other DC types able to cross-present antigens to cytotoxic CD8⁺ T cells, pointing to a possible evolutionary convergence mechanism. With the development of more advanced technologies allowing delineation of the molecular networks at the level of chromatin organization, histone modifications, protein translation, and phosphorylation, future "omics" investigations will bring in-depth understanding of the complex molecular mechanisms underpinning human LC biology.

GM-CSF Monocyte-Derived Cells and Langerhans Cells As Part of the Dendritic Cell Family

Dendritic cells (DCs) and macrophages (Mph) share many characteristics as components of the innate immune system. The criteria to classify the multitude of subsets within the mononuclear phagocyte system are currently phenotype, ontogeny, transcription patterns, epigenetic adaptations, and function. More recently, ontogenetic, transcriptional, and proteomic research approaches uncovered major developmental differences between Flt3L-dependent conventional DCs as compared with Mphs and monocyte-derived DCs (MoDCs), the latter mainly generated in vitro from murine bone marrow-derived DCs (BM-DCs) or human CD14⁺ peripheral blood monocytes. Conversely, in vitro GM-CSF-dependent monocyte-derived Mphs largely resemble MoDCs whereas tissue-resident Mphs show a common embryonic origin from yolk sac and fetal liver with Langerhans cells (LCs). The novel ontogenetic findings opened discussions on the terminology of DCs versus Mphs. Here, we bring forward arguments to facilitate definitions of BM-DCs, MoDCs, and LCs. We propose a group model of terminology for all DC subsets that attempts to encompass both ontogeny and function.

Human skin dendritic cell fate is differentially regulated by the monocyte identity factor Kruppel-like factor 4 during steady state and inflammation

BACKGROUND

Langerhans cell (LC) networks play key roles in immunity and tolerance at body surfaces. LCs are established prenatally and can be replenished from blood monocytes. Unlike skin-resident dermal DCs (dDCs)/interstitial-type DCs and inflammatory dendritic epidermal cells appearing in dermatitis/eczema lesions, LCs lack key monocyte-affiliated markers. Inversely, LCs express various epithelial genes critical for their long-term peripheral tissue residency.

OBJECTIVE

Dendritic cells (DCs) are functionally involved in inflammatory diseases; however, the mechanisms remained poorly understood.

METHODS

In vitro differentiation models of human DCs, gene profiling, gene transduction, and immunohistology were used to identify molecules involved in DC subset specification.

RESULTS

Here we identified the monocyte/macrophage lineage identity transcription factor Kruppel-like factor 4 (KLF4) to be inhibited during LC differentiation from human blood monocytes. Conversely, KLF4 is maintained or induced during dermal DC and monocyte-derived dendritic cell/inflammatory dendritic epidermal cell differentiation. We showed that in monocytic cells KLF4 has to be repressed to allow their differentiation into LCs. Moreover, respective KLF4 levels in DC subsets positively correlate with proinflammatory characteristics. We identified epithelial Notch signaling to repress KLF4 in monocytes undergoing LC commitment. Loss of KLF4 in monocytes transcriptionally derepresses Runt-related transcription factor 3 in response to TGF-β1, thereby allowing LC differentiation marked by a low cytokine expression profile.

CONCLUSION

Monocyte differentiation into LCs depends on activation of Notch signaling and the concomitant loss of KLF4.

TGF-β1-induced transcription factor networks in Langerhans cell development and maintenance

Langerhans cells (LC) represent a specialized subset of evolutionarily conserved dendritic cells (DC) that populate stratified epithelial tissues, which are essential for the induction of skin and mucosal immunity and tolerance, including allergy. Transforming growth factor-β1 (TGF-β1) has been confirmed to be a predominant factor involved in LC development. Despite great advances in the understanding of LC ontogeny and diverse replenishment patterns, the underlying molecular mechanisms remain elusive. This review focuses on the recent discoveries in TGF-β1-mediated LC development and maintenance, with special attention to the involved transcription factors and related regulators.

Endogenous and tumour-derived microRNAs regulate cross-presentation in dendritic cells and consequently cytotoxic T cell function

Dendritic cells (DCs) are potent antigen presenting cells (APCs). They are also specialized in the induction of cytotoxic T lymphocyte mediated responses against extracellular antigens, including tumour-specific antigens, by presenting peptide-Major Histocompatibility Complex (MHC) I complexes to naïve CD8⁺ T cells in lymphoid tissues, a process called cross-presentation. Emerging evidence suggests that the efficiency of cross-presentation can be influenced by a unique set of microRNAs (miRNAs). Some are differentially expressed in the course of morphological and functional development of DCs while tumorigenic miRNAs (onco-miRs) can be delivered to and inserted into DCs via exosomes. The latter reprogram the miRNA repertoire of DCs, transforming them from effective APCs to negative modulators of immunity, ultimately aiding cancers to evade host immunity. On the other hand, endogenous microRNAs can influence cross-presentation either positively or negatively. In this review, we discuss the possible mechanisms by which specific miRNAs influence cross-presentation as well as the viability of manipulating the expression of miRNAs that regulate DC cross-presentation as a potential cancer immunotherapy intervention.

Nucleosome regulatory dynamics in response to TGFβ

Nucleosomes play important roles in a cell beyond their basal functionality in chromatin compaction. Their placement affects all steps in transcriptional regulation, from transcription factor (TF) binding to messenger ribonucleic acid (mRNA) synthesis. Careful profiling of their locations and dynamics in response to stimuli is important to further our understanding of transcriptional regulation by the state of chromatin. We measured nucleosome occupancy in human hepatic cells before and after treatment with transforming growth factor beta 1 (TGFβ1), using massively parallel sequencing. With a newly developed method, SuMMIt, for precise positioning of nucleosomes we inferred dynamics of the nucleosomal landscape. Distinct nucleosome positioning has previously been described at transcription start site and flanking TF binding sites. We found that the average pattern is present at very few sites and, in case of TF binding, the double peak surrounding the sites is just an artifact of averaging over many loci. We systematically searched for depleted nucleosomes in stimulated cells compared to unstimulated cells and identified 24 318 loci. Depending on genomic annotation, 44–78% of them were over-represented in binding motifs for TFs. Changes in binding affinity were verified for HNF4α by qPCR. Strikingly many of these loci were associated with expression changes, as measured by RNA sequencing.

Establishing and maintaining the Langerhans cell network

Langerhans cells (LCs) are the unique antigen-presenting cell of the epidermis. LCs have long been depicted in textbooks as the archetypical dendritic cell that alerts the immune system upon pathogen induced skin barrier breakage, however recent findings argue instead for a more tolerogenic function. While the LCs that populate the epidermis in steady-state arise from progenitors that seed the skin during embryogenesis, it is now apparent that a second pathway generating LCs from a bone marrow derived progenitor is active in inflammatory settings. This review emphasizes the determinants underpinning the establishment of the LC network in steady-state and under inflammatory conditions, as well as the transcriptional machinery governing their differentiation. The dual origin of LCs raises important questions about the functional differences between these subsets in balancing the epidermal immune response between immunity and tolerance.

Langerhans cells are generated by two distinct PU.1-dependent transcriptional networks

Langerhans cell homeostasis and differentiation depends on PU.1, the latter via regulation of TGF-β–dependent binding of PU.1 to the regulatory elements of RUNX3.

Langerhans cells (LCs) are the unique dendritic cells found in the epidermis. While a great deal of attention has focused on defining the developmental origins of LCs, reports addressing the transcriptional network ruling their differentiation remain sparse. We addressed the function of a group of key DC transcription factors—PU.1, ID2, IRF4, and IRF8—in the establishment of the LC network. We show that although steady-state LC homeostasis depends on PU.1 and ID2, the latter is dispensable for bone marrow–derived LCs. PU.1 controls LC differentiation by regulating the expression of the critical TGF-β responsive transcription factor RUNX3. PU.1 directly binds to the Runx3 regulatory elements in a TGF-β–dependent manner, whereas ectopic expression of RUNX3 rescued LC differentiation in the absence of PU.1 and promoted LC differentiation from PU.1-sufficient progenitors. These findings highlight the dual molecular network underlying LC differentiation, and show the central role of PU.1 in these processes.

Identification of bone morphogenetic protein 7 (BMP7) as an instructive factor for human epidermal Langerhans cell differentiation

Bone morphogenetic protein 7 (BMP7) promotes the differentiation of Langerhans cells in the epidermis during prenatal development.

Human Langerhans cell (LC) precursors populate the epidermis early during prenatal development and thereafter undergo massive proliferation. The prototypic antiproliferative cytokine TGF-β1 is required for LC differentiation from human CD34⁺ hematopoietic progenitor cells and blood monocytes in vitro. Similarly, TGF-β1 deficiency results in LC loss in vivo. However, immunohistology studies revealed that human LC niches in early prenatal epidermis and adult basal (germinal) keratinocyte layers lack detectable TGF-β1. Here we demonstrated that these LC niches express high levels of bone morphogenetic protein 7 (BMP7) and that Bmp7-deficient mice exhibit substantially diminished LC numbers, with the remaining cells appearing less dendritic. BMP7 induces LC differentiation and proliferation by activating the BMP type-I receptor ALK3 in the absence of canonical TGF-β1–ALK5 signaling. Conversely, TGF-β1–induced in vitro LC differentiation is mediated via ALK3; however, co-induction of ALK5 diminished TGF-β1–driven LC generation. Therefore, selective ALK3 signaling by BMP7 promotes high LC yields. Within epidermis, BMP7 shows an inverse expression pattern relative to TGF-β1, the latter induced in suprabasal layers and up-regulated in outer layers. We observed that TGF-β1 inhibits microbial activation of BMP7-generated LCs. Therefore, TGF-β1 in suprabasal/outer epidermal layers might inhibit LC activation, resulting in LC network maintenance.

Aging affects epidermal Langerhans cell development and function and alters their miRNA gene expression profile

Immunosenescence is a result of progressive decline in immune system function with advancing age. Epidermal Langerhans cells (LCs), belonging to the dendritic cell (DC) family, act as sentinels to play key roles in the skin immune responses. However, it has not been fully elucidated how aging affects development and function of LCs. Here, we systemically analyzed LC development and function during the aging process in C57BL/6J mice, and performed global microRNA (miRNA) gene expression profiles in aged and young LCs. We found that the frequency and maturation of epidermal LCs were significantly reduced in aged mice starting at 12 months of age, while the Langerin expression and ability to phagocytose Dextran in aged LCs were increased compared to LCs from < 6 month old mice. The migration of LCs to draining lymph nodes was comparable between aged and young mice. Functionally, aged LCs were impaired in their capacity to induce OVA-specific CD4⁺ and CD8⁺ T cell proliferation. Furthermore, the expression of miRNAs in aged epidermal LCs showed a distinct profile compared to young LCs. Most interestingly, aging-regulated miRNAs potentially target TGF-β-dependent and non- TGF-β-dependent signal pathways related to LCs. Overall, our data suggests that aging affects LCs development and function, and that age-regulated miRNAs may contribute to the LC developmental and functional changes in aging.

Two distinct types of Langerhans cells populate the skin during steady state and inflammation

Langerhans cells (LCs), the dendritic cells (DCs) in skin epidermis, possess an exceptional life cycle and developmental origin. Here we identified two types of LCs, short-term and long-term LCs, which transiently or stably reconstitute the LC compartment, respectively. Short-term LCs developed from Gr-1(hi) monocytes under inflammatory conditions and occurred independently of the transcription factor Id2. Long-term LCs arose from bone marrow in steady state and were critically dependent on Id2. Surface marker and gene expression analysis positioned short-term LCs close to Gr-1(hi) monocytes, which is indicative of their monocytic origin. We also show that LC reconstitution after UV light exposure occurs in two waves: an initial fast and transient wave of Gr-1(hi) monocyte-derived short-term LCs is followed by a second wave of steady-state precursor-derived long-term LCs. Our data demonstrate the presence of two types of LCs that develop through different pathways in inflammation and steady state.

Deletion of microRNA miR-223 increases Langerhans cell cross-presentation

Langerhans cells (LCs) are skin-residential dendritic cells that regulate skin immunity. MicroRNAs (miRNAs) are key regulators in the control of biological functions in a variety of cell types. Deletion of all miRNAs interrupts the homeostasis and function of epidermal LCs. However, the roles of individual miRNAs in regulating LC development and function are still completely unknown. MiRNA miR-223 is especially expressed in the myeloid compartment. Here, we reported that miR-223 is highly expressed in freshly isolated epidermal LCs, and tested whether miR-223 regulates LC development and function using miR-223 knockout (KO) mice. We found that the number, maturation, migration and phagocytic capacity of LCs were comparable between miR-223KO and wild-type mice. However, lack of miR-223 significantly increases LCs-mediated antigen-specific CD8(+) T cell proliferation in vivo and in vitro, while LCs from KO and WT mice showed comparable stimulation for antigen-specific CD4(+) T cells. Our data suggest that miR-223 negatively regulates LC cross-presentation, but may not be required for normal LC homeostasis and development.

Identification of Axl as a downstream effector of TGF-β1 during Langerhans cell differentiation and epidermal homeostasis

Transforming growth factor-β1 (TGF-β1) is a fundamental regulator of immune cell development and function. In this study, we investigated the effects of TGF-β1 on the differentiation of human Langerhans cells (LCs) and identified Axl as a key TGF-β1 effector. Axl belongs to the TAM (Tyro3, Axl, and Mer) receptor tyrosine kinase family, whose members function as inhibitors of innate inflammatory responses in dendritic cells and are essential to the prevention of lupus-like autoimmunity. We found that Axl expression is induced by TGF-β1 during LC differentiation and that LC precursors acquire Axl early during differentiation. We also describe prominent steady-state expression as well as inflammation-induced activation of Axl in human epidermal keratinocytes and LCs. TGF-β1-induced Axl enhances apoptotic cell (AC) uptake and blocks proinflammatory cytokine production. The antiinflammatory role of Axl in the skin is reflected in a marked impairment of the LC network preceding spontaneous skin inflammation in mutant mice that lack all three TAM receptors. Our findings highlight the importance of constitutive Axl expression to tolerogenic barrier immunity in the epidermis and define a mechanism by which TGF-β1 enables silent homeostatic clearing of ACs to maintain long-term self-tolerance.

The developing Xenopus limb as a model for studies on the balance between inflammation and regeneration

The roles of inflammation and immune cell reactivity triggered by amputation have only recently begun to be addressed in investigations of epimorphic regeneration, although studies of tissue repair in mammals clearly show the importance of the immune system in determining the quality of the repair process. Here, we first review inflammation-related work in non-mammalian systems of epimorphic regeneration which suggests that regeneration of an amputated appendage requires continuous modulation of the local immune response, from the first hours after amputation through the period of blastema patterning. We then present data on the effects of anti-inflammatory and proinflammatory agents on regeneration of larval Xenopus hindlimbs. Treatment with the glucocorticoid beclomethasone immediately after amputation inhibits regeneration in regeneration-complete stage 53 limbs. Other anti-inflammatory agents, including the inhibitors of cyclooxygenase-2 (COX-2) activity celecoxib and diclofenac, applied similarly to larvae amputated at stage 55, when the capacity for limb regeneration is normally being lost, restore regenerative capacity. This suggests that although injury-related events sensitive to glucocorticoids are necessary for regeneration, resolution of the inflammatory response may also be required to allow the complete regenerative response and normal blastema patterning. Conversely, if resolution of inflammation is prevented by local treatment of amputated limbs with beryllium, a strong immunoadjuvant, regeneration is inhibited, and gene expression data suggest that this inhibition results from a failure of normal blastema patterning. Both positive and negative effects of immune- or inflammation-related activities occur during anuran limb regeneration and this underscores the importance of considering immune cells in studies of epimorphic regeneration.

A critical role for endoglin in the emergence of blood during embryonic development

Much remains unknown about the signals that induce early mesoderm to initiate hematopoietic differentiation. Here, we show that endoglin (Eng), a receptor for the TGFβ superfamily, identifies all cells with hematopoietic fate in the early embryo. These arise in an Eng(+)Flk1(+) mesodermal precursor population at embryonic day 7.5 (E7.5), a cell fraction also endowed with endothelial potential. In Eng-knockout embryos, hematopoietic colony activity and numbers of CD71(+)Ter119(+) erythroid progenitors were severely reduced. This coincided with severely reduced expression of embryonic globin and key bone morphogenic protein (BMP) target genes, including the hematopoietic regulators Scl, Gata1, Gata2, and Msx-1. To interrogate molecular pathways active in the earliest hematopoietic progenitors, we applied transcriptional profiling to sorted cells from E7.5 embryos. Eng(+)Flk-1(+) progenitors coexpressed TGFβ and BMP receptors and target genes. Furthermore, Eng(+)Flk-1(+) cells presented high levels of phospho-SMAD1/5, indicating active TGFβ and/or BMP signaling. Remarkably, under hematopoietic serum-free culture conditions, hematopoietic outgrowth of Eng-expressing cells was dependent on the TGFβ superfamily ligands BMP4, BMP2, or TGF-β1. These data demonstrate that the E(+)F(+) fraction at E7.5 represents mesodermal cells competent to respond to TGFβ1, BMP4, or BMP2, shaping their hematopoietic development, and that Eng acts as a critical regulator in this process by modulating TGF/BMP signaling.

Regulation of dendritic cell development by GM-CSF: molecular control and implications for immune homeostasis and therapy

Dendritic cells (DCs) represent a small and heterogeneous fraction of the hematopoietic system, specialized in antigen capture, processing, and presentation. The different DC subsets act as sentinels throughout the body and perform a key role in the induction of immunogenic as well as tolerogenic immune responses. Because of their limited lifespan, continuous replenishment of DC is required. Whereas the importance of GM-CSF in regulating DC homeostasis has long been underestimated, this cytokine is currently considered a critical factor for DC development under both steady-state and inflammatory conditions. Regulation of cellular actions by GM-CSF depends on the activation of intracellular signaling modules, including JAK/STAT, MAPK, PI3K, and canonical NF-κB. By directing the activity of transcription factors and other cellular effector proteins, these pathways influence differentiation, survival and/or proliferation of uncommitted hematopoietic progenitors, and DC subset–specific precursors, thereby contributing to specific aspects of DC subset development. The specific intracellular events resulting from GM-CSF–induced signaling provide a molecular explanation for GM-CSF–dependent subset distribution as well as clues to the specific characteristics and functions of GM-CSF–differentiated DCs compared with DCs generated by fms-related tyrosine kinase 3 ligand. This knowledge can be used to identify therapeutic targets to improve GM-CSF–dependent DC-based strategies to regulate immunity.

Identification of TROP2 (TACSTD2), an EpCAM-like molecule, as a specific marker for TGF-β1-dependent human epidermal Langerhans cells

Langerin (CD207) expression is a hallmark of epidermal Langerhans cells (LCs); however, CD207(+) cells comprise several functional subsets. Murine studies showed that epidermal, but not dermal, CD207(+) cells require transforming growth factor-β 1 (TGF-β1) for development, whereas human data are lacking. Using gene profiling, we found that the surface molecule TROP2 (TACSTD2) is strongly and rapidly induced during TGF-β1-dependent LC commitment of human CD34(+) hematopoietic progenitor cells or monocytes. TROP2 is conserved between mouse and human, and shares substantial amino-acid identity with EpCAM, a marker for murine epidermal LCs. To our knowledge, neither TROP2 nor EpCAM expression has been analyzed in human dendritic cell (DC) subsets. We found that (i) all human epidermal LCs are TROP2(+)EpCAM(+); (ii) human dermis lacks CD207(+)EpCAM(-) or CD207(+)TROP2(-) DCs, i.e., equivalents of murine dermal CD207(+) DCs; and (iii) pulmonary CD207(+) cells are TROP2(-)EpCAM(-). Moreover, although EpCAM was broadly expressed by pulmonary and intestinal epithelial cells, as well as by bone marrow erythroid progenitor cells, these cells lacked TROP2. However, although TROP2 is expressed by human LCs as well as by human and murine keratinocytes, most murine LCs, except of a small subset, lacked TROP2. Therefore, TROP2 is a marker for human TGF-β1-dependent epidermal LCs.

Dendritic cells with lymphocyte-stimulating activity differentiate from human CD133 positive precursors

CD133 is a hallmark of primitive myeloid progenitors. We have addressed whether human cord blood cells selected for CD133 can generate dendritic cells, and Langerhans cells in particular, in conditions that promote that generation from CD34(+) progenitors. Transforming growth factor-β1 (TGF-β1) and anti-TGF-β1 antibody, respectively, were added in some experiments. With TGF-β, monocytoid cells were recognized after 7 days. Immunophenotypically immature dendritic cells were present at day 14. After 4 more days, the cells expressed CD54, CD80, CD83, and CD86 and were potent stimulators in mixed lymphocyte reaction; part of the cells expressed CD1a and langerin, but not Birbeck granules. Without TGF-β, only a small fraction of cells acquired a dendritic shape and expressed the maturation-related antigens, and lymphocytes were poorly stimulated. With anti-TGF-β, the cell growth was greatly hampered, CD54 and langerin were never expressed, and lymphocytes were stimulated weakly. In conclusion, CD133(+) progenitors can give rise in vitro, through definite steps, to mature, immunostimulatory dendritic cells with molecular features of Langerhans cells, although without Birbeck granules. Addition of TGF-β1 helps to stimulate cell growth and promotes the acquisition of mature immunophenotypical and functional features. Neither langerin nor Birbeck granules proved indispensable for lymphocyte stimulation.

Transforming growth factor-beta: recent advances on its role in immune tolerance

Transforming growth factor (TGF-β1) is a pleiotropic cytokine, secreted by immune and nonhematopoietic cells. TGF-β is involved in many different critical processes, such as embryonal development, cellular maturation and differentiation, wound healing, and immune regulation. It maintains immune homeostasis by acting as a potent immune suppressor through inhibition of proliferation, differentiation, activation, and effector function of immune cells. Paradoxically, depending on the context, it displays proinflammatory properties by being a potent chemoattractant for neutrophils and promoting inflammation. In addition, it does not only induce differentiation into the anti-inflammatory Treg cells, but also into the proinflammatory Th17 and Th9 cells and inhibits Th22 differentiation. TGF-β has been demonstrated to be involved in multiple pathologies. In infections, it protects against collateral damages caused by the immune system, but it also promotes immune evasion and chronic infections. In autoimmune diseases, a TGF-β dysfunction leads to the loss of tolerance to self-antigens. In cancer, TGF-β is a potent inhibitor of cell proliferation and acts as a tumor suppressor at the beginning of tumorogenesis. However, once the cells become resistant to TGF-β, it mainly supports tumor growth and metastasis by promoting immune evasion and angiogenesis. In asthma, it is assumed to promote allergen tolerance, but plays a detrimental role in irreversible remodeling of the airways. Despite the high numbers of TGF-β-targeted pathways, it is a promising drug target for treatment of autoimmunity, cancer, fibrosis, if cell specificity can be achieved.This review summarizes the progresses that have been accomplished on the understanding of TGF-β's signaling in the immune homeostasis and its role in pathogenesis.

TGF-beta1 accelerates dendritic cell differentiation from common dendritic cell progenitors and directs subset specification toward conventional dendritic cells

Dendritic cells (DCs) in lymphoid tissue comprise conventional DCs (cDCs) and plasmacytoid DCs (pDCs) that develop from common DC progenitors (CDPs). CDPs are Flt3(+)c-kit(int)M-CSFR(+) and reside in bone marrow. In this study, we describe a two-step culture system that recapitulates DC development from c-kit(hi)Flt3(-/lo) multipotent progenitors (MPPs) into CDPs and further into cDC and pDC subsets. MPPs and CDPs are amplified in vitro with Flt3 ligand, stem cell factor, hyper-IL-6, and insulin-like growth factor-1. The four-factor mixture readily induces self-renewal of MPPs and their progression into CDPs and has no self-renewal activity on CDPs. The amplified CDPs respond to all known DC poietins and generate all lymphoid tissue DCs in vivo and in vitro. Additionally, in vitro CDPs recapitulate the cell surface marker and gene expression profile of in vivo CDPs and possess a DC-primed transcription profile. TGF-β1 impacts on CDPs and directs their differentiation toward cDCs. Genome-wide gene expression profiling of TGF-β1-induced genes identified instructive transcription factors for cDC subset specification, such as IFN regulatory factor-4 and RelB. TGF-β1 also induced the transcription factor inhibitor of differentiation/DNA binding 2 that suppresses pDC development. Thus, TGF-β1 directs CDP differentiation into cDCs by inducing both cDC instructive factors and pDC inhibitory factors.

Langerhans cells and more: langerin-expressing dendritic cell subsets in the skin

Langerhans cells (LCs) are antigen-presenting dendritic cells (DCs) that reside in epithelia. The best studied example is the LC of the epidermis. By electron microscopy, their identifying feature is the unique rod- or tennis racket-shaped Birbeck granule. The phenotypic hallmark is their expression of the C-type lectin receptor langerin/CD207. Langerin, however, is also expressed on a recently discovered population of DC in the dermis and other tissues of the body. These 'dermal langerin(+) dendritic cells' are unrelated to LCs. The complex field of langerin-negative dermal DCs is not dealt with here. In this article, we briefly review the history, ontogeny, and homeostasis of LCs. More emphasis is laid on the discussion of functional properties in vivo. Novel models using genetically engineered mice are contributing tremendously to our understanding of the role of LCs in eliciting adaptive immune responses against pathogens or tumors and in inducing and maintaining tolerance against self antigens and innocuous substances in vivo. Also, innate effector functions are increasingly being recognized. Current activities in this area are reviewed, and possibilities for future exploitation of LC in medicine, e.g. for the improvement of vaccines, are contemplated.

Common clonal origin of an acute B-lymphoblastic leukemia and a Langerhans' cell sarcoma: evidence for hematopoietic plasticity

BACKGROUND

The hierarchical organization of hematopoiesis with unidirectional lineage determination has become a questionable tenet in view of the experimental evidence of reprogramming and transdifferentiation of lineage-determined cells. Clinical examples of hematopoietic lineage plasticity are rare. Here we report on a patient who presented with an acute B-lymphoblastic leukemia and developed a Langerhans' cell sarcoma 9 years later. We provide evidence that the second neoplasm is the result of transdifferentiation.

DESIGN AND METHODS

B-cell acute lymphoblastic leukemia was diagnosed in an 11-year old boy in 1996. Treatment according to the ALL-BFM-1995 protocol resulted in a complete remission. Nine years later, in 2005, Langerhans' cell sarcoma was diagnosed in a supraclavicular lymph node. Despite treatment with different chemotherapy protocols the patient had progressive disease. Finally, he received an allogeneic peripheral blood stem cell transplant and achieved a continuous remission. Molecular studies of IGH- and TCRG-gene rearrangements were performed with DNA from the Langerhans' cell sarcoma and the cryopreserved cells from the acute B-lymphoblastic leukemia. The expression of PAX5 and ID2 was analyzed with real-time reverse transcriptase polymerase chain reaction.

RESULTS

Identical IGH-rearrangements were demonstrated in the acute B-lymphoblastic leukemia and the Langerhans' cell sarcoma. The key factors required for B-cell and dendritic cell development, PAX5 and ID2, were differentially expressed, with a strong PAX5 signal in the acute B-lymphoblastic leukemia and only a weak expression in the Langerhans' cell sarcoma, whereas ID2 showed an opposite pattern.

CONCLUSIONS

The identical IGH-rearrangement in both neoplasms indicates transdifferentiation of the acute B-lymphoblastic leukemia into a Langerhans' cell sarcoma. Loss of PAX5 and the acquisition of ID2 suggest that these key factors are involved in the transdifferentiation from a B-cell phenotype into a Langerhans'/dendritic cell phenotype. (Clinical trial registration at: Deutsches KrebsStudienRegister, http://www.studien.de, study-ID:8).

miR-146a is differentially expressed by myeloid dendritic cell subsets and desensitizes cells to TLR2-dependent activation

Langerhans cells (LCs) in epithelia and interstitial dendritic cells (intDCs) in adjacent connective tissues represent two closely related myeloid-derived DC subsets that exert specialized functions in the immune system and are of clinical relevance for cell therapy. Both subsets arise from monocyte-committed intermediates in response to tissue-associated microenvironmental signals; however, molecular mechanisms underlying myeloid DC subset specification and function remain poorly defined. Using microarray profiling, we identified microRNA (miRNA) miR-146a to be constitutively expressed at higher levels in human LCs compared with intDCs. Moreover, miR-146a levels were low in monocytes and nondetectable in neutrophil granulocytes. Interestingly, constitutive high miR-146a expression in LCs is induced by the transcription factor PU.1 in response to TGF-beta1, a key microenvironmental signal for epidermal LC differentiation. We identified miR-146a as a regulator of monocyte and DC activation but not myeloid/DC subset differentiation. Ectopic miR-146a in monocytes and intDCs interfered with TLR2 downstream signaling and cytokine production, without affecting phenotypic DC maturation. Inversely, silencing of miR-146a in LCs enhanced TLR2-dependent NF-kappaB signaling. We therefore conclude that high constitutive miR-146a levels are induced by microenvironmental signals in the epidermis and might render LCs less susceptible to inappropriate activation by commensal bacterial TLR2 triggers at body surfaces.

Development of the prenatal cutaneous antigen-presenting cell network

The skin, and in particular the epidermis, is a physical barrier that protects the body from external threats and is critically involved in immune reactivity. Professional antigen-presenting cells, such as epidermal Langerhans cells and dermal dendritic cells, are gaining prominence as principal players orchestrating the decision between immunity and tolerance. A focus of research interest in recent years has been the investigation of these cells in mammalian prenatal skin. In this review, we will compare the recent progress in dissecting the phenotype and functional role of antigen-presenting cells in the developing human and mouse skin before birth and perinatally, and will discuss how this knowledge improves our understanding of the level of immunocompetence of the skin in utero.

Reciprocal role of GATA-1 and vitamin D receptor in human myeloid dendritic cell differentiation

Two major pathways of human myeloid dendritic cell (DC) subset differentiation have previously been delineated. Langerhans cells (LCs) reside in epithelia in the steady state, whereas monocytes can provide dendritic cells (DCs) on demand in response to inflammatory signals. Both DC subset pathways arise from shared CD14+ monocyte precursors, which in turn develop from myeloid committed progenitor cells. However, the underlying hematopoietic mechanisms still remain poorly defined. Here, we demonstrate that the vitamin D(3) receptor (VDR) is induced by transforming growth factor beta1 during LC lineage commitment and exerts a positive role during LC generation. In contrast, VDR is repressed during interleukin-4 (IL-4)-dependent monocyte-derived DC (moDC) differentiation. We identified GATA-1 as a repressor of VDR. GATA-1 is induced by IL-4 in moDCs. Forced inducible expression of GATA-1 mimics IL-4 in redirecting moDC differentiation and vice versa, GATA-1 knockdown arrests moDC differentiation at the monocyte stage. Moreover, ectopic GATA-1 expression stabilizes the moDC phenotype under monocyte-promoting conditions in the presence of vitamin D3 (VD3). In summary, human myeloid DC subset differentiation is inversely regulated by GATA-1 and VDR. GATA-1 mediates the repression of VDR and enables IL-4-dependent moDC differentiation. Conversely, VDR is induced downstream of transforming growth factor beta1 and is functionally involved in promoting LC differentiation.

Aryl hydrocarbon receptor activation inhibits in vitro differentiation of human monocytes and Langerhans dendritic cells

The transcription factor aryl hydrocarbon receptor (AhR) represents a promising therapeutic target in allergy and autoimmunity. AhR signaling induced by the newly described ligand VAF347 inhibits allergic lung inflammation as well as suppresses pancreatic islet allograft rejection. These effects are likely mediated via alterations in dendritic cell (DC) function. Moreover, VAF347 induces tolerogenic DCs. Langerhans cells (LCs) are immediate targets of exogenous AhR ligands at epithelial surfaces; how they respond to AhR ligands remained undefined. We studied AhR expression and function in human LCs and myelopoietic cell subsets using a lineage differentiation and gene transduction model of human CD34(+) hematopoietic progenitors. We found that AhR is highly regulated during myeloid subset differentiation. LCs expressed highest AhR levels followed by monocytes. Conversely, neutrophil granulocytes lacked AhR expression. AhR ligands including VAF347 arrested the differentiation of monocytes and LCs at an early precursor cell stage, whereas progenitor cell expansion or granulopoiesis remained unimpaired. AhR expression was coregulated with the transcription factor PU.1 during myeloid subset differentiation. VAF347 inhibited PU.1 induction during initial monocytic differentiation, and ectopic PU.1 restored monocyte and LC generation in the presence of this compound. AhR ligands failed to interfere with cytokine receptor signaling during LC differentiation and failed to impair LC activation/maturation. VAF347-mediated antiproliferative effect on precursors undergoing LC lineage differentiation occurred in a clinically applicable serum-free culture model and was not accompanied by apoptosis induction. In conclusion, AhR agonist signaling interferes with transcriptional processes leading to monocyte/DC lineage commitment of human myeloid progenitor cells.

Notch and wingless signaling cooperate in regulation of dendritic cell differentiation

Dendritic cell (DC) differentiation is regulated by stroma via a network of soluble and cell-bound factors. Notch is one of the major elements of this network. Its role in DC differentiation, however, is controversial. Here, we demonstrate that activation of Notch signaling in hematopoietic progenitor cells (HPCs) promoted differentiation of conventional DCs via activation of the canonical Wingless (Wnt) pathway. Inhibition of the Wnt pathway abrogated the effect of Notch on DC differentiation. The fact that activation of the Wnt pathway in Notch-1-deficient embryonic stem cells restored DC differentiation indicates that Wnt signaling is downstream of the Notch pathway in regulating DC differentiation. Notch signaling activated the Wnt pathway in HPCs via expression of multiple members of the Frizzled family of Wnt receptors, which was directly regulated by the CSL (RPB-Jkappa) transcription factor. Thus, these data suggest a model of DC differentiation via cooperation between Wnt and Notch pathways.

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.

Transcription factors in the control of dendritic cell life cycle

Dendritic cells (DCs) are potent antigen-presenting cells that guard all parts of the body. They have the unique ability to prime T cells and generate primary immune responses. Their journey beginning with the development from precursor cells and ending with their death is controlled by a group of transcription factors. Some of the transcription factors like PU.1 are involved in more than one stage of DC life. Other transcription factors including Ikaros and JAK3 are involved in the development of more than one cell type. For a long time, the cellular and molecular mechanisms underlying the development, differentiation, maturation, and other stages of DC life were not well understood. However, in recent years novel information has been published by many researchers to better understand the molecular mechanisms of the development and function of DCs in immunological diseases such as asthma, cancer, autoimmunity, and transplantation. This review will discuss the various transcription factors and signaling pathways involved in each stage of the life cycle of DCs.

TGF-beta combined with M-CSF and IL-4 induces generation of immune inhibitory cord blood dendritic cells capable of enhancing cytokine-induced ex vivo expansion of myeloid progenitors

Tolerogenic dendritic cells (DCs) may be valuable in transplantation for silencing immune reaction. Macrophage colony-stimulating factor (M-CSF)/IL-4 induces differentiation of cord blood (CB) monocytes into DCs (M-DCs) with tolerogenic phenotype/function. We assessed whether factors produced by tolerogenic DCs could modulate hematopoiesis. TGF-beta1 added to CB M-DC cultures induced bona fide DC morphology (TGF-M-DCs), similar to that of DCs generated with TGF-beta and granulocyte-macrophage colony-stimulating factor (GM-CSF)/IL-4 (TGF-GM-DCs). Of conditioned media (CM) produced from TGF-M-DCs, TGF-GM-DCs, M-DCs, and GM-DCs, TGF-M-DC CM was the only one that enhanced SCF, Flt3 ligand, and TPO expansion of myeloid progenitor cells ex vivo. This effect was blocked by neutralizing anti-M-CSF Ab, but protein analysis of CM suggested that M-CSF alone was not manifesting enhanced expansion of myeloid progenitors. LPS-stimulated TGF-M-DCs induced T-cell tolerance/anergy as effectively as M-DCs. TGF-M-DCs secreted significantly lower concentrations of progenitor cell inhibitory cytokines and were less potent in activating T cells than TGF-GM-DCs. Functional differences between TGF-M-DCs and TGF-GM-DCs included enhanced responses to LPS-induced ERK, JNK, and P38 activation in TGF-M-DCs and their immune suppressive-skewed cytokine release profiles. TGF-M-DCs appear unique among culture-generated DCs in their capability for silencing immunity while promoting expansion of myeloid progenitors, events that may be of therapeutic value.

Transforming growth factor β1 up-regulates interferon regulatory factor 8 during dendritic cell development

Langerhans cells (LC) represent the cutaneous contingent of dendritic cells (DC). Their development critically depends on transforming growth factor beta1 (TGF-beta1) as demonstrated by analysis of TGF-beta1(-/-) mice, which lack LC. Here we used a two-step culture system and transcriptional profiling by DNA microarrays to search for TGF-beta1 target genes in DC. The study identified interferon regulatory factor 8 (IRF-8) as a novel target gene of TGF-beta1 signaling in DC. TGF-beta1 effectively induced Smad2/3 phosphorylation and IRF-8 RNA and protein expression. Blocking the TGF-beta1/Smad pathway by ectopic expression of inhibitory Smad7 and by SB431542 inhibitor abolished TGF-beta1 induced up-regulation of IRF-8. Furthermore, TGF-beta1-dependent induction of IRF-8 occurred in the absence of protein biosynthesis, suggesting a direct action of TGF-beta1/Smad signaling on IRF-8 gene activity. TGF-beta1 also induced expression of the chemokine receptor CCR7 and enhanced DC migration towards CCR7 ligand ELC. DC of IRF-8(-/-) mice show reduced CCR7 expression and migratory activity, thereby implicating the TGF-beta1/Smad/IRF-8 signaling pathway in CCR7 regulation. Thus, this study identified a novel TGF-beta1/Smad/IRF-8 signaling pathway with an impact on DC phenotype and function.

Cells of cutaneous immunity in Xenopus: studies during larval development and limb regeneration

The anuran Xenopus laevis is an experimental model for vertebrate development, immunology, and regenerative biology. Using histochemistry and immunohistochemistry (IHC) we examined embryonic, larval, and postmetamorphic Xenopus skin for the presence of dendritic cells (DCs), Langerhans cells (LCs), and dendritic epidermal T cells (DETCs), all components of cutaneous immunity that have been implicated in skin repair and regeneration. Cells expressing three markers for dendritic and Langerhans cells (formalin-resistant ATPase activity, major histocompatibility complex [MHC] class II antigens, and vimentin) and having morphology like that of these cells first appeared during late embryonic stages, becoming abundant by prometamorphosis. Cells positive for these markers were also numerous in the wound epithelia of regenerating hindlimbs at both early and late larval stages. Cells tentatively identified as DETCs were found, beginning at early larval stages, using IHC with antibodies against heterologous CD3epsilon chain and T-cell receptor delta. Further characterization and work with the putative DCs, LCs, and DETCs demonstrated here will allow not only greater understanding of the amphibian immune system, but also further elucidation of regenerative growth and scarring.

Down-regulation of RXRalpha expression is essential for neutrophil development from granulocyte/monocyte progenitors

Neutrophil granulocytes (Gs) represent highly abundant and short-lived leukocytes that are constantly regenerated from a small pool of myeloid committed progenitors. Nuclear receptor (NR) family members are ligand-activated transcription factors that play key roles in cellular proliferation and differentiation processes including myelopoiesis. Retinoid X receptor alpha (RXRalpha) represents the predominant NR types I and II homo- and heterodimerization partner in myeloid cells. Here we show that human myeloid progenitors express RXRalpha protein at sustained high levels during macrophage colony-stimulating factor (M-CSF)-induced monopoiesis. In sharp contrast, RXRalpha is down-regulated during G-CSF-dependent late-stage neutrophil differentiation from myeloid progenitors. Down-regulation of RXRalpha is critically required for neutrophil development since ectopic RXRalpha inhibited granulopoiesis by impairing proliferation and differentiation. Moreover, ectopic RXRalpha was sufficient to redirect G-CSF-dependent granulocyte differentiation to the monocyte lineage and to promote M-CSF-induced monopoiesis. Functional genetic interference with RXRalpha signaling in hematopoietic progenitor/stem cells using a dominant-negative RXRalpha promoted the generation of late-stage granulocytes in human cultures in vitro and in reconstituted mice in vivo. Therefore, our data suggest that RXRalpha down-regulation is a critical requirement for the generation of neutrophil granulocytes.

A CD34(+) human cell line model of myeloid dendritic cell differentiation: evidence for a CD14(+)CD11b(+) Langerhans cell precursor

The study of early events in dendritic cell (DC) differentiation is hampered by the lack of homogeneous primary cell systems that allow the study of cytokine-driven, transitional DC differentiation steps. The CD34(+) acute myeloid leukemia cell line MUTZ-3 displays a unique ability to differentiate into interstitial DC (IDC) and Langerhans cells (LC) in a cytokine-dependent manner. Phenotypic characterization revealed MUTZ-3 to consist of three distinct subpopulations. Small CD34(+)CD14(-)CD11b(-) progenitors constitute the proliferative compartment of the cell line with the ability to differentiate through a CD34(-)CD14(-)CD11b(+) stage to ultimately give rise to a morphologically large, nonproliferating CD14(+)CD11b(hi) progeny. These CD14(+)CD11b(hi) cells were identified as common, immediate myeloid DC precursors with the ability to differentiate into LC and IDC, exhibiting characteristic and mutually exclusive expression of Langerin and DC-specific ICAM-grabbing nonintegrin, respectively. The identity of the MUTZ-3-derived LC subset was confirmed further by the presence of Birbeck granules. We conclude that the MUTZ-3 cell line provides a ready and continuous supply of common myeloid precursors, which should facilitate further study of the ontogeny of myeloid DC lineages.