The transcriptional repressor Gfi1 controls STAT3-dependent dendritic cell development and function

Overview of dendritic cells subsets and their involvement in immune-related pathological disease

Dendritic cells (DCs) are specialized antigen-presenting cells (APCs) in linking innate and adaptive immune responses. In addition to presenting antigens to T cells, DCs must also provide co-stimulatory signals along with cytokines for T cells to induce an appropriate cellular immune response. Tolerance is also established and maintained by DCs under homeostatic circumstances. There is remarkable phenotypic heterogeneity in DCs, each with different functional flexibility and specific expression of various markers. The three primary categories of DCs comprise conventional DCs (cDCs), plasmacytoid DCs (pDCs), and monocyte-derived DCs (moDCs). Langerhans cells (LCs) are another type of DCs, which are found in the skin's epidermal layer. DCs may be positioned or triggered inappropriately as a result of dysregulation of DC. This phenomenon can cause an imbalance in immune responses and even immune-related pathological disorders, i.e., autoimmune diseases and malignancies. Herein, by reviewing the ontogeny, biology, characteristics, and function of DCs subsets in immune system, we discuss the contribution of these cells in the mentioned immune-related disorders.

The TRIM37 variant rs57141087 contributes to triple-negative breast cancer outcomes in Black women

Triple-negative breast cancer (TNBC) disproportionately affects younger Black women, who show more aggressive phenotypes and poorer outcomes than women of other racial identities. While the impact of socioenvironmental inequities within and beyond health systems is well documented, the genetic influence in TNBC-associated racial disparities remains elusive. Here, we report that cancer-free breast tissue from Black women expresses TRIM37 at a significantly higher level relative to White women. A reporter-based screen for regulatory variants identifies a non-coding risk variant rs57141087 in the 5' gene upstream region of the TRIM37 locus with enhancer activity. Mechanistically, rs57141087 increases enhancer-promoter interactions through NRF1, resulting in stronger TRIM37 promoter activity. Phenotypically, high TRIM37 levels drive neoplastic transformations in immortalized breast epithelial cells. Finally, context-dependent TRIM37 expression reveals that early-stage TRIM37 levels affect the initiation and trajectory of breast cancer progression. Together, our results indicate a genotype-informed association of oncogenic TRIM37 with TNBC risk in Black women and implicate TRIM37 as a predictive biomarker to better identify patients at risk of aggressive TNBC.

Combinatorial depletions of G-protein coupled receptor kinases in immune cells identify pleiotropic and cell type-specific functions

G-protein coupled receptor kinases (GRKs) participate in the regulation of chemokine receptors by mediating receptor desensitization. They can be recruited to agonist-activated G-protein coupled receptors (GPCRs) and phosphorylate their intracellular parts, which eventually blocks signal propagation and often induces receptor internalization. However, there is growing evidence that GRKs can also control cellular functions beyond GPCR regulation. Immune cells commonly express two to four members of the GRK family (GRK2, GRK3, GRK5, GRK6) simultaneously, but we have very limited knowledge about their interplay in primary immune cells. In particular, we are missing comprehensive studies comparing the role of this GRK interplay for (a) multiple GPCRs within one leukocyte type, and (b) one specific GPCR between several immune cell subsets. To address this issue, we generated mouse models of single, combinatorial and complete GRK knockouts in four primary immune cell types (neutrophils, T cells, B cells and dendritic cells) and systematically addressed the functional consequences on GPCR-controlled cell migration and tissue localization. Our study shows that combinatorial depletions of GRKs have pleiotropic and cell-type specific effects in leukocytes, many of which could not be predicted. Neutrophils lacking all four GRK family members show increased chemotactic migration responses to a wide range of GPCR ligands, whereas combinatorial GRK depletions in other immune cell types lead to pro- and anti-migratory responses. Combined depletion of GRK2 and GRK6 in T cells and B cells shows distinct functional outcomes for (a) one GPCR type in different cell types, and (b) different GPCRs in one cell type. These GPCR-type and cell-type specific effects reflect in altered lymphocyte chemotaxis in vitro and localization in vivo. Lastly, we provide evidence that complete GRK deficiency impairs dendritic cell homeostasis, which unexpectedly results from defective dendritic cell differentiation and maturation in vitro and in vivo. Together, our findings demonstrate the complexity of GRK functions in immune cells, which go beyond GPCR desensitization in specific leukocyte types. Furthermore, they highlight the need for studying GRK functions in primary immune cells to address their specific roles in each leukocyte subset.

Unboxing dendritic cells: Tales of multi-faceted biology and function

Often referred to as the bridge between innate and adaptive immunity, dendritic cells (DCs) are professional antigen-presenting cells (APCs) that constitute a unique, yet complex cell system. Among other APCs, DCs display the unique property of inducing protective immune responses against invading microbes, or cancer cells, while safeguarding the proper homeostatic equilibrium of the immune system and maintaining self-tolerance. Unsurprisingly, DCs play a role in many diseases such as autoimmunity, allergy, infectious disease and cancer. This makes them attractive but challenging targets for therapeutics. Since their initial discovery, research and understanding of DC biology have flourished. We now recognize the presence of multiple subsets of DCs distributed across tissues. Recent studies of phenotype and gene expression at the single cell level have identified heterogeneity even within the same DC type, supporting the idea that DCs have evolved to greatly expand the flexibility of the immune system to react appropriately to a wide range of threats. This review is meant to serve as a quick and robust guide to understand the basic divisions of DC subsets and their role in the immune system. Between mice and humans, there are some differences in how these subsets are identified and function, and we will point out specific distinctions as necessary. Throughout the text, we are using both fundamental and therapeutic lens to describe overlaps and distinctions and what this could mean for future research and therapies.

Immunomodulatory Role and Therapeutic Potential of Non-Coding RNAs Mediated by Dendritic Cells in Autoimmune and Immune Tolerance-Related Diseases

Dendritic cells (DCs) are professional antigen-presenting cells that act as a bridge between innate immunity and adaptive immunity. After activation, DCs differentiate into subtypes with different functions, at which point they upregulate co-stimulatory molecules and produce various cytokines and chemokines. Activated DCs also process antigens for presentation to T cells and regulate the differentiation and function of T cells to modulate the immune state of the body. Non-coding RNAs, RNA transcripts that are unable to encode proteins, not only participate in the pathological mechanisms of autoimmune-related diseases but also regulate the function of immune cells in these diseases. Accumulating evidence suggests that dysregulation of non-coding RNAs contributes to DC differentiation, functions, and so on, consequently producing effects in various autoimmune diseases. In this review, we summarize the main non-coding RNAs (miRNAs, lncRNAs, circRNAs) that regulate DCs in pathological mechanisms and have tremendous potential to give rise to novel therapeutic targets and strategies for multiple autoimmune diseases and immune tolerance-related diseases.

Dendritic Cells Are the Intriguing Players in the Puzzle of Idiopathic Pulmonary Fibrosis Pathogenesis

Idiopathic pulmonary fibrosis (IPF) is the most devastating progressive interstitial lung disease that remains refractory to treatment. Pathogenesis of IPF relies on the aberrant cross-talk between injured alveolar cells and myofibroblasts, which ultimately leads to an aberrant fibrous reaction. The contribution of the immune system to IPF remains not fully explored. Recent evidence suggests that both innate and adaptive immune responses may participate in the fibrotic process. Dendritic cells (DCs) are the most potent professional antigen-presenting cells that bridge innate and adaptive immunity. Also, they exert a crucial role in the immune surveillance of the lung, where they are strategically placed in the airway epithelium and interstitium. Immature DCs accumulate in the IPF lung close to areas of epithelial hyperplasia and fibrosis. Conversely, mature DCs are concentrated in well-organized lymphoid follicles along with T and B cells and bronchoalveolar lavage of IPF patients. We have recently shown that all sub-types of peripheral blood DCs (including conventional and plasmacytoid DCs) are severely depleted in therapy naïve IPF patients. Also, the low frequency of conventional CD1c⁺ DCs is predictive of a worse prognosis. The purpose of this mini-review is to focus on the main evidence on DC involvement in IPF pathogenesis. Unanswered questions and opportunities for future research ranging from a better understanding of their contribution to diagnosis and prognosis to personalized DC-based therapies will be explored.

The transcription factors GFI1 and GFI1B as modulators of the innate and acquired immune response

GFI1 and GFI1B are small nuclear proteins of 45 and 37kDa, respectively, that have a simple two-domain structure: The first consists of a group of six c-terminal C₂H₂ zinc finger motifs that are almost identical in sequence and bind to very similar, specific DNA sites. The second is an N-terminal 20 amino acid SNAG domain that can bind to the pocket of the histone demethylase KDM1A (LSD1) near its active site. When bound to DNA, both proteins act as bridging factors that bring LSD1 and associated proteins into the vicinity of methylated substrates, in particular histone H3 or TP53. GFI1 can also bring methyl transferases such as PRMT1 together with its substrates that include the DNA repair proteins MRE11 and 53BP1, thereby enabling their methylation and activation. While GFI1B is expressed almost exclusively in the erythroid and megakaryocytic lineage, GFI1 has clear biological roles in the development and differentiation of lymphoid and myeloid immune cells. GFI1 is required for lymphoid/myeloid and monocyte/granulocyte lineage decision as well as the correct nuclear interpretation of a number of important immune-signaling pathways that are initiated by NOTCH1, interleukins such as IL2, IL4, IL5 or IL7, by the pre TCR or -BCR receptors during early lymphoid differentiation or by T and B cell receptors during activation of lymphoid cells. Myeloid cells also depend on GFI1 at both stages of early differentiation as well as later stages in the process of activation of macrophages through Toll-like receptors in response to pathogen-associated molecular patterns. The knowledge gathered on these factors over the last decades puts GFI1 and GFI1B at the center of many biological processes that are critical for both the innate and acquired immune system.

The Role of microRNAs in Organismal and Skin Aging

The aging process starts directly after birth and lasts for the entire lifespan; it manifests itself with a decline in an organism’s ability to adapt and is linked to the development of age-related diseases that eventually lead to premature death. This review aims to explore how microRNAs (miRNAs) are involved in skin functioning and aging. Recent evidence has suggested that miRNAs regulate all aspects of cutaneous biogenesis, functionality, and aging. It has been noted that some miRNAs were down-regulated in long-lived individuals, such as let-7, miR-17, and miR-34 (known as longevity-related miRNAs). They are conserved in humans and presumably promote lifespan prolongation; conversely, they are up-regulated in age-related diseases, like cancers. The analysis of the age-associated cutaneous miRNAs revealed the increased expression of miR-130, miR-138, and miR-181a/b in keratinocytes during replicative senescence. These miRNAs affected cell proliferation pathways via targeting the p63 and Sirtuin 1 mRNAs. Notably, miR-181a was also implicated in skin immunosenescence, represented by the Langerhans cells. Dermal fibroblasts also expressed increased the levels of the biomarkers of aging that affect telomere maintenance and all phases of the cellular life cycle, such as let-7, miR-23a-3p, 34a-5p, miR-125a, miR-181a-5p, and miR-221/222-3p. Among them, the miR-34 family, stimulated by ultraviolet B irradiation, deteriorates collagen in the extracellular matrix due to the activation of the matrix metalloproteinases and thereby potentiates wrinkle formation. In addition to the pro-aging effects of miRNAs, the plausible antiaging activity of miR-146a that antagonized the UVA-induced inhibition of proliferation and suppressed aging-related genes (e.g., p21WAF-1, p16, and p53) through targeting Smad4 has also been noticed. Nevertheless, the role of miRNAs in skin aging is still not fully elucidated and needs to be further discovered and explained.

Plasmacytoid dendritic cells in the eye

Plasmacytoid dendritic cells (pDCs) are a unique subpopulation of immune cells, distinct from classical dendritic cells. pDCs are generated in the bone marrow and following development, they typically home to secondary lymphoid tissues. While peripheral tissues are generally devoid of pDCs during steady state, few tissues, including the lung, kidney, vagina, and in particular ocular tissues harbor resident pDCs. pDCs were originally appreciated for their potential to produce large quantities of type I interferons in viral immunity. Subsequent studies have now unraveled their pivotal role in mediating immune responses, in particular in the induction of tolerance. In this review, we summarize our current knowledge on pDCs in ocular tissues in both mice and humans, in particular in the cornea, limbus, conjunctiva, choroid, retina, and lacrimal gland. Further, we will review our current understanding on the significance of pDCs in ameliorating inflammatory responses during herpes simplex virus keratitis, sterile inflammation, and corneal transplantation. Moreover, we describe their novel and pivotal neuroprotective role, their key function in preserving corneal angiogenic privilege, as well as their potential application as a cell-based therapy for ocular diseases.

Mouse Models and Tools for the in vivo Study of Neutrophils

Neutrophils are the most abundant leukocytes in human blood and critical actors of the immune system. Many neutrophil functions and facets of their activity in vivo were revealed by studying genetically modified mice or by tracking fluorescent neutrophils in animals using imaging approaches. Assessing the roles of neutrophils can be challenging, especially when exact molecular pathways are questioned or disease states are interrogated that alter normal neutrophil homeostasis. This review discusses the main in vivo models for the study of neutrophils, their advantages and limitations. The side-by-side comparison underlines the necessity to carefully choose the right model(s) to answer a given scientific question, and exhibit caveats that need to be taken into account when designing experimental procedures. Collectively, this review suggests that at least two models should be employed to legitimately conclude on neutrophil functions.

Origin and development of classical dendritic cells

Classical dendritic cells (cDCs) are mononuclear phagocytes of hematopoietic origin specialized in the induction and regulation of adaptive immunity. Initially defined by their unique T cell activation potential, it became quickly apparent that cDCs would be difficult to distinguish from other phagocyte lineages, by solely relying on marker-based approaches. Today, cDCs definition increasingly embed their unique ontogenetic features. A growing consensus defines cDCs on multiple criteria including: (1) dependency on the fms-like tyrosine kinase 3 ligand hematopoietic growth factor, (2) development from the common DC bone marrow progenitor, (3) constitutive expression of the transcription factor ZBTB46 and (4) the ability to induce, after adequate stimulation, the activation of naïve T lymphocytes. cDCs are a heterogeneous cell population that contains two main subsets, named type 1 and type 2 cDCs, arising from divergent ontogenetic pathways and populating multiple lymphoid and non-lymphoid tissues. Here, we present recent knowledge on the cellular and molecular pathways controlling the specification and commitment of cDC subsets from murine and human hematopoietic stem cells.

Molecular regulation of dendritic cell development and function in homeostasis, inflammation, and cancer

Dendritic cells (DCs) are the principal antigen-presenting cells of the immune system and play key roles in controlling immune tolerance and activation. As such, DCs are chief mediators of tumor immunity. DCs can regulate tolerogenic immune responses that facilitate unchecked tumor growth. Importantly, however, DCs also mediate immune-stimulatory activity that restrains tumor progression. For instance, emerging evidence indicates the cDC1 subset has important functions in delivering tumor antigens to lymph nodes and inducing antigen-specific lymphocyte responses to tumors. Moreover, DCs control specific therapeutic responses in cancer including those resulting from immune checkpoint blockade. DC generation and function is influenced profoundly by cytokines, as well as their intracellular signaling proteins including STAT transcription factors. Regardless, our understanding of DC regulation in the cytokine-rich tumor microenvironment is still developing and must be better defined to advance cancer treatment. Here, we review literature focused on the molecular control of DCs, with a particular emphasis on cytokine- and STAT-mediated DC regulation. In addition, we highlight recent studies that delineate the importance of DCs in anti-tumor immunity and immune therapy, with the overall goal of improving knowledge of tumor-associated factors and intrinsic DC signaling cascades that influence DC function in cancer.

Chromatin-Modifying Agent-Expanded Human Cord Blood Cells Display Reduced Allostimulatory Capacity

The limited number of hematopoietic stem cells (HSC) within a single unit of human cord blood currently limits its use as an alternate graft source. However, we have developed a strategy using 5-aza-2'-deoxycytidine (5azaD) and trichostatin A (TSA), which expands transplantable HSC 7- to 10-fold. In our current studies, we have assessed the allostimulatory capacity of the 5azaD/TSA-expanded grafts. The coexpression of immunophenotypic dendritic cell (DC) markers, such as HLA-DR/CD86 and HLA-DR/CD11c as determined by flow cytometry, and the allostimulatory capacity of 5azaD/TSA-expanded cells as determined by MLC were both significantly lower than control. It has been previously demonstrated that STAT3 is indispensable for the differentiation of DC from HSC. Real-time quantitative PCR analysis revealed that 5azaD/TSA-expanded cells expressed more STAT3 transcript than control while also expressing increased transcripts for STAT3 inhibitors including SHP1, p21, and GATA1. Western blot analysis indicates that chromatin-modifying agent-expanded grafts displayed a reduced ratio of p-STAT3 to total STAT3 than control cultures, which is likely indicative of STAT3 inactivity in 5azD/TSA-expanded grafts. Culturing 5azaD/TSA-expanded cord blood cells in extended cultures reveals that they are still capable of generating DC. Notably, STAT3 inactivity was transient because the transcript levels of STAT3 and its inhibitors, including SHP1, were comparable between 5azaD/TSA and control cultures following extended culture. Taken together, our studies indicate that the reduced allostimulatory capacity of 5azaD/TSA-expanded cells is likely because of reversible inhibition of STAT3-dependent DC differentiation. These results suggest that a graft composed of 5azaD/TSA-expanded cells possesses relatively less allostimulatory response but is still capable of generating DC in permissive conditions.

Human dendritic cell subsets: an update

Dendritic cells (DC) are a class of bone-marrow-derived cells arising from lympho-myeloid haematopoiesis that form an essential interface between the innate sensing of pathogens and the activation of adaptive immunity. This task requires a wide range of mechanisms and responses, which are divided between three major DC subsets: plasmacytoid DC (pDC), myeloid/conventional DC1 (cDC1) and myeloid/conventional DC2 (cDC2). Each DC subset develops under the control of a specific repertoire of transcription factors involving differential levels of IRF8 and IRF4 in collaboration with PU.1, ID2, E2-2, ZEB2, KLF4, IKZF1 and BATF3. DC haematopoiesis is conserved between mammalian species and is distinct from monocyte development. Although monocytes can differentiate into DC, especially during inflammation, most quiescent tissues contain significant resident populations of DC lineage cells. An extended range of surface markers facilitates the identification of specific DC subsets although it remains difficult to dissociate cDC2 from monocyte-derived DC in some settings. Recent studies based on an increasing level of resolution of phenotype and gene expression have identified pre-DC in human blood and heterogeneity among cDC2. These advances facilitate the integration of mouse and human immunology, support efforts to unravel human DC function in vivo and continue to present new translational opportunities to medicine.

Macrophages and osteoclasts stem from a bipotent progenitor downstream of a macrophage/osteoclast/dendritic cell progenitor

Monocytes/macrophages (MΦs), osteoclasts (OCs), and dendritic cells (DCs) are closely related cell types of high clinical significance, but the exact steps in their lineage commitment are unclear. In studies on MΦ and DC development, OC development is generally not addressed. Furthermore, findings on DC development are confusing, because monocytes can also differentiate into DC-like cells. To resolve these issues, we have examined the development of monocytes/MΦs, OCs, and DCs from common progenitors, using the homeostatic driver cytokines macrophage colony-stimulating factor, RANK ligand (L), and Flt3L. In mouse bone marrow, B220^-CD11b^low/-c-Kit⁺c-Fms⁺ cells could be dissected into a CD27⁺Flt3⁺ population that proved oligopotent for MΦ/OC/DC development (MODP) and a CD27^low/-Flt3^- population that proved bipotent for MΦ/OC development (MOP). Developmental potential and relationship of MODP and downstream MOP populations are demonstrated by differentiation cultures, functional analysis of MΦ/OC/DC offspring, and genome-wide messenger RNA expression analysis. A common DC progenitor (CDP) has been described as committed to plasmacytoid and conventional DC development. However, the human CDP proved identical to the MODP population, whereas the mouse CDP largely overlapped with the MODP population and was accordingly oligopotent for MΦ, OC, and DC development. The CX₃CR1⁺ MΦ/DC progenitor (MDP) population described in the mouse generated MΦs and OCs but not DCs. Thus, monocytes/MΦs, OCs, and DCs share a common progenitor that gives rise to a bipotent MΦ/OC progenitor, but a dedicated DC progenitor is currently undefined. The definition of these progenitor populations may serve diagnostics and interventions in diseases with pathogenic activity of MΦs, OCs, or DCs.

MiR-34a promotes DCs development and inhibits their function on T cell activation by targeting WNT1

MicroRNAs serve important functions in numerous biological processes. Whether microRNAs also act on dendritic cell (DC) differentiation and function remains unclear. In this study, both conventional DCs (cDCs) and plasmacytoid DCs (pDCs) were increased in miR-34a overexpressing bone marrow chimeric and transgenic (TG) mice. Further experiments showed that miR-34a promoted preDC differentiated into cDCs and pDCs without affecting the proliferation and apoptosis of DCs. Luciferase report assay and Western blot experiments demonstrated that WNT1 is the direct target of miR-34a in DCs. Interestingly, miR-34a overexpressing cDCs also produced a large amount of IL-17a and suppressed T cell activation because of the inhibition of TCF1 expression, thus increasing RORγT expression. Taken together, miR-34a promotes preDC to differentiate into cDCs and pDCs, as well as inhibits the function of cDCs on the activation of CD4+ T cells by producing IL-17a.

Development of conventional dendritic cells: from common bone marrow progenitors to multiple subsets in peripheral tissues

Our understanding of conventional dendritic cell (cDC) development and the functional specializations of distinct subsets in the peripheral tissues has increased greatly in recent years. Here, we review cDC development from the distinct progenitors in the bone marrow through to the distinct cDC subsets found in barrier tissues, providing an overview of the different subsets described in each location. In addition, we detail the transcription factors and local signals that have been proposed to control this developmental process. Importantly, despite these significant advances, numerous questions remain to be answered regarding cDC development. For example, it remains unclear whether the different subsets described, such as the CD103+CD11b+ and CD103-CD11b+ cDCs in the intestines, truly represent different populations or rather distinct developmental or activation stages. Furthermore, whether distinct progenitors exist for these cDC subsets remains to be determined. Thus in the last part of this review we discuss what we believe will be the main questions facing the field for the coming years.

The origin of DCs and capacity for immunologic tolerance in central and peripheral tissues

Dendritic cells (DCs) are specialized immune sentinels that play key role in maintaining immune homeostasis by efficiently regulating the delicate balance between protective immunity and tolerance to self. Although DCs respond to maturation signals present in the surrounding milieu, multiple layers of suppression also co-exist that reduce the infringement of tolerance against self-antigens. These tolerance inducing properties of DCs are governed by their origin and a range of other factors including distribution, cytokines, growth factors, and transcriptional programing, that collectively impart suppressive functions to these cells. DCs directing tolerance secrete anti-inflammatory cytokines and induce naïve T cells or B cells to differentiate into regulatory T cells (Tregs) or B cells. In this review, we provide a detailed outlook on the molecular mechanisms that induce functional specialization to govern central or peripheral tolerance. The tolerance-inducing nature of DCs can be exploited to overcome autoimmunity and rejection in graft transplantation.

The transcriptional repressor Gfi1 prevents lupus autoimmunity by restraining TLR7 signaling

The transcriptional repressor growth factor independence 1 (Gfi1) is important in myeloid and lymphoid differentiation. In the current study we evaluated the involvement of Gfi1 in systemic lupus erythematosus (SLE). We found that Genista mice, which carry a hypomorphic mutation in the gfi1 gene or Gfi1-deficient (Gfi1^-/- ) mice develop signs of spontaneous lupus autoimmunity, including increased serum levels of IgM and IgG2a, autoantibodies against RNA and DNA, glomerular immunodeposits and increased frequencies of plasmablasts, germinal center (GC) B cells and age-associated B cells (ABCs). On the contrary, Genista mice deprived of TLR7 did not show any of these phenotypes, suggesting that the observed lupus autoimmunity in Genista mice is TLR7-dependent. Moreover, Genista mice showed an increased activation of dendritic cells (DCs), B and T cells that was dependent on TLR7 for DCs and B cells, but not for T cells. Upon TLR7 or TLR4 stimulation Genista DCs produced increased amounts of TNF, IL-6 and IFN-β and showed increased NF-κB phosphorylation and IRF7 nuclear translocation, suggesting that Gfi1 controls the NF-κB and type I IFN signaling pathway downstream of TLRs. Our data reveal that Gfi1 plays a critical role in the prevention of spontaneous lupus autoimmunity by negatively regulating TLR7 signaling.

GATA2 regulates dendritic cell differentiation

Dendritic cells (DCs) are critical immune response regulators; however, the mechanism of DC differentiation is not fully understood. Heterozygous germ line GATA2 mutations induce GATA2-deficiency syndrome, characterized by monocytopenia, a predisposition to myelodysplasia/acute myeloid leukemia, and a profoundly reduced DC population, which is associated with increased susceptibility to viral infections, impaired phagocytosis, and decreased cytokine production. To define the role of GATA2 in DC differentiation and function, we studied Gata2 conditional knockout and haploinsufficient mice. Gata2 conditional deficiency significantly reduced the DC count, whereas Gata2 haploinsufficiency did not affect this population. GATA2 was required for the in vitro generation of DCs from Lin(-)Sca-1(+)Kit(+) cells, common myeloid-restricted progenitors, and common dendritic cell precursors, but not common lymphoid-restricted progenitors or granulocyte-macrophage progenitors, suggesting that GATA2 functions in the myeloid pathway of DC differentiation. Moreover, expression profiling demonstrated reduced expression of myeloid-related genes, including mafb, and increased expression of T-lymphocyte-related genes, including Gata3 and Tcf7, in Gata2-deficient DC progenitors. In addition, GATA2 was found to bind an enhancer element 190-kb downstream region of Gata3, and a reporter assay exhibited significantly reduced luciferase activity after adding this enhancer region to the Gata3 promoter, which was recovered by GATA sequence deletion within Gata3 +190. These results suggest that GATA2 plays an important role in cell-fate specification toward the myeloid vs T-lymphocyte lineage by regulating lineage-specific transcription factors in DC progenitors, thereby contributing to DC differentiation.

Transcriptional Control of Dendritic Cell Development

The dendritic cells (DCs) of the immune system function in innate and adaptive responses by directing activity of various effector cells rather than serving as effectors themselves. DCs and closely related myeloid lineages share expression of many surface receptors, presenting a challenge in distinguishing their unique in vivo functions. Recent work has taken advantage of unique transcriptional programs to identify and manipulate murine DCs in vivo. This work has assigned several nonredundant in vivo functions to distinct DC lineages, consisting of plasmacytoid DCs and several subsets of classical DCs that promote different immune effector modules in response to pathogens. In parallel, a correspondence between human and murine DC subsets has emerged, underlying structural similarities for the DC lineages between these species. Recent work has begun to unravel the transcriptional circuitry that controls the development and diversification of DCs from common progenitors in the bone marrow.

Transcriptional Regulation of Mononuclear Phagocyte Development

Mononuclear phagocytes (MP) are a quite unique subset of hematopoietic cells, which comprise dendritic cells (DC), monocytes as well as monocyte-derived and tissue-resident macrophages. These cells are extremely diverse with regard to their origin, their phenotype as well as their function. Developmentally, DC and monocytes are constantly replenished from a bone marrow hematopoietic progenitor. The ontogeny of macrophages is more complex and is temporally linked and specified by the organ where they reside, occurring early during embryonic or perinatal life. The functional heterogeneity of MPs is certainly a consequence of the tissue of residence and also reflects the diverse ontogeny of the subsets. In this review, we will highlight the developmental pathways of murine MP, with a particular emphasis on the transcriptional factors that regulate their development and function. Finally, we will discuss and point out open questions in the field.

Epigenetic program and transcription factor circuitry of dendritic cell development

Dendritic cells (DC) are professional antigen presenting cells that develop from hematopoietic stem cells through successive steps of lineage commitment and differentiation. Multipotent progenitors (MPP) are committed to DC restricted common DC progenitors (CDP), which differentiate into specific DC subsets, classical DC (cDC) and plasmacytoid DC (pDC). To determine epigenetic states and regulatory circuitries during DC differentiation, we measured consecutive changes of genome-wide gene expression, histone modification and transcription factor occupancy during the sequel MPP-CDP-cDC/pDC. Specific histone marks in CDP reveal a DC-primed epigenetic signature, which is maintained and reinforced during DC differentiation. Epigenetic marks and transcription factor PU.1 occupancy increasingly coincide upon DC differentiation. By integrating PU.1 occupancy and gene expression we devised a transcription factor regulatory circuitry for DC commitment and subset specification. The circuitry provides the transcription factor hierarchy that drives the sequel MPP-CDP-cDC/pDC, including Irf4, Irf8, Tcf4, Spib and Stat factors. The circuitry also includes feedback loops inferred for individual or multiple factors, which stabilize distinct stages of DC development and DC subsets. In summary, here we describe the basic regulatory circuitry of transcription factors that drives DC development.

From cytopenia to leukemia: the role of Gfi1 and Gfi1b in blood formation

The DNA-binding zinc finger transcription factors Gfi1 and Gfi1b were discovered more than 20 years ago and are recognized today as major regulators of both early hematopoiesis and hematopoietic stem cells. Both proteins function as transcriptional repressors by recruiting histone-modifying enzymes to promoters and enhancers of target genes. The establishment of Gfi1 and Gfi1b reporter mice made it possible to visualize their cell type-specific expression and to understand their function in hematopoietic lineages. We now know that Gfi1 is primarily important in myeloid and lymphoid differentiation, whereas Gfi1b is crucial for the generation of red blood cells and platelets. Several rare hematologic diseases are associated with acquired or inheritable mutations in the GFI1 and GFI1B genes. Certain patients with severe congenital neutropenia carry mutations in the GFI1 gene that lead to the disruption of the C-terminal zinc finger domains. Other mutations have been found in the GFI1B gene in families with inherited bleeding disorders. In addition, the Gfi1 locus is frequently found to be a proviral integration site in retrovirus-induced lymphomagenesis, and new, emerging data suggest a role of Gfi1 in human leukemia and lymphoma, underlining the role of both factors not only in normal hematopoiesis, but also in a wide spectrum of human blood diseases.

The multifaceted biology of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) are a unique DC subset that specializes in the production of type I interferons (IFNs). pDCs promote antiviral immune responses and have been implicated in the pathogenesis of autoimmune diseases that are characterized by a type I IFN signature. However, pDCs can also induce tolerogenic immune responses. In this Review, we summarize recent progress in the field of pDC biology, focusing on the molecular mechanisms that regulate the development and functions of pDCs, the pathways involved in their sensing of pathogens and endogenous nucleic acids, their functions at mucosal sites, and their roles in infection, autoimmunity and cancer.

The non-canonical Wnt pathway negatively regulates dendritic cell differentiation by inhibiting the expansion of Flt3(+) lymphocyte-primed multipotent precursors

The differentiation of dendritic cells (DC) is affected by the aging process. However, the molecular mechanisms responsible for the alteration of DC development in aged mice have not been clarified. Recently, Wnt5a was reported to be an important aging-related molecule in hematopoietic systems. Here, we hypothesized that the increased expression of Wnt5a in aged hematopoietic precursors led to deficient DC differentiation in aged mice. The percentages and cell numbers of plasmacytoid DC (pDC) and CD172a(-)CD8α(+)conventional DC (cDC) were decreased in aged mice compared to young mice. Further analysis indicated that the hematopoietic precursors that gave rise to DC, including Flt3(+) lymphoid-primed multipotent precursors (LMPP), common lymphoid progenitors (CLP) and common DC precursors (CDP), were all decreased in the bone marrow of aged mice. Overexpression of Wnt5a in hematopoietic precursors strongly affected the differentiation of cDC and pDC in vivo. Treatment of hematopoietic stem cells (HSC) with Wnt5a led to a significant decrease in the differentiation of the LMPP, CLP and CDP populations that was similar to the decrease observed in the bone marrow (BM) HSC of aged mice. Molecular studies demonstrated that Wnt5a negatively regulated the expression of an array of genes important for DC differentiation, including Flt3, Gfi-1, Ikaros, Bcl11a, and IL-7R, by activating the Wnt5a-Cdc42 pathway. Finally, we rejuvenated DC differentiation from aged precursors by blocking the non-canonical Wnt pathway. Our study identified the key roles of the non-canonical Wnt pathway in DC differentiation and DC aging.

Modulation of tolerogenic dendritic cells and autoimmunity

A key function of dendritic cells (DCs) is to induce either immune tolerance or immune activation. Many new DC subsets are being recognized, and it is now clear that each DC subset has a specialized function. For example, different DC subsets may express different cell surface molecules and respond differently to activation by secretion of a unique cytokine profile. Apart from intrinsic differences among DC subsets, various immune modulators in the microenvironment may influence DC function; inappropriate DC function is closely related to the development of immune disorders. The most exciting recent advance in DC biology is appreciation of human DC subsets. In this review, we discuss functionally different mouse and human DC subsets both in lymphoid organs and non-lymphoid organs, the molecules that regulate DC function, and the emerging understanding of the contribution of DCs to autoimmune diseases.

Ontogeny of myeloid cells

Granulocytes, monocytes, macrophages, and dendritic cells (DCs) represent a subgroup of leukocytes, collectively called myeloid cells. During the embryonic development of mammalians, myelopoiesis occurs in a stepwise fashion that begins in the yolk sac and ends up in the bone marrow (BM). During this process, these early monocyte progenitors colonize various organs such as the brain, liver, skin, and lungs and differentiate into resident macrophages that will self-maintain throughout life. DCs are constantly replenished from BM precursors but can also arise from monocytes in inflammatory conditions. In this review, we summarize the different types of myeloid cells and discuss new insights into their early origin and development in mice and humans from fetal to adult life. We specifically focus on the function of monocytes, macrophages, and DCs at these different developmental stages and on the intrinsic and environmental influences that may drive these adaptations.

Regulation of histamine synthesis and tryptase expression through transcription factors, growth factor independent 1 (Gfi1) and Gfi1b, in murine cultured mast cells

Mast cells are involved in various immunological responses, although it remains unknown how their terminal differentiation is regulated. We previously established a culture model that mimics the process of mast cell maturation in the cutaneous tissue and found that growth factor independent 1 (Gfi1) was up-regulated whereas its paralogue Gfi1b down-regulated. Here we investigated the roles of Gfi1 and Gfi1b in the process of mast cell maturation using a murine mast cell line, MC9. Gfi1 and Gfi1b cDNAs were stably expressed in MC9 cells using the recombinant lentivirus. Histamine synthesis was significantly induced by stem cell factor (SCF) alone, whereas tryptase expression was significantly augmented in the presence of both SCF and Swiss 3T3 cells. Since exogenously expressed Gfi1 and Gfi1b might affect their expression levels in MC9 cells, we investigated the relationship between the expression profiles of Gfi1/Gfi1b proteins and maturation indices, such as histamine synthesis and tryptase expression. The comparison suggested that histamine synthesis during the co-culture period was positively regulated by Gfi1b while augmented expression of tryptase was abolished by one-sided expression of Gfi1/Gfi1b. Our findings indicated the involvement of Gfi1 and Gfi1b in the process of murine mast cell maturation.

SPOCK3, a risk gene for adult ADHD and personality disorders

Attention-deficit/hyperactivity disorder (ADHD) is the most frequent psychiatric disorder in children, where it displays a global prevalence of 5 %. In up to 50 % of the cases, ADHD may persist into adulthood (aADHD), where it is often comorbid with personality disorders. Due to a potentially heritable nature of this comorbidity, we hypothesized that their genetic framework may contain common risk-modifying genes. SPOCK3, a poorly characterized, putatively Ca(2+)-binding extracellular heparan/chondroitin sulfate proteoglycan gene encoded by the human chromosomal region 4q32.3, was found to be associated with polymorphisms among the top ranks in a genome-wide association study (GWAS) on ADHD and a pooled GWAS on personality disorder (PD). We therefore genotyped 48 single nucleotide polymorphisms (SNPs) representative of the SPOCK3 gene region in 1,790 individuals (n aADHD = 624, n PD = 630, n controls = 536). In this analysis, we found two SNPs to be nominally associated with aADHD (rs7689440, rs897511) and four PD-associated SNPs (rs7689440, rs897511, rs17052671 and rs1485318); the latter even reached marginal significance after rigorous Bonferroni correction. Bioinformatics tools predicted a possible influence of rs1485318 on transcription factor binding, whereas the other candidate SNPs may have effects on alternative splicing. Our results suggest that SPOCK3 may modify the genetic risk for ADHD and PD; further studies are, however, needed to identify the underlying mechanisms.

Development and function of dendritic cell subsets

Classical dendritic cells (cDCs) form a critical interface between innate and adaptive immunity. As myeloid immune cell sentinels, cDCs are specialized in the sensing of pathogen challenges and cancer. They translate the latter for T cells into peptide form. Moreover, cDCs provide additional critical information on the original antigen context to trigger a diverse spectrum of appropriate protective responses. Here we review recent progress in our understanding of cDC subsets in mice. We will discuss cDC subset ontogeny and transcription factor dependencies, as well as emerging functional specializations within the cDC compartment in lymphoid and nonlymphoid tissues.

Transcriptional control of dendritic cell differentiation

Dendritic cells (DCs) are professional antigen presenting cells involved critically not only in provoking innate immune responses but also in establishing adaptive immune responses. Dendritic cells are heterogenous and divided into several subsets, including plasmactyoid DCs (pDCs) and several types of conventional DCs (cDCs), which show subset-specific functions. Plasmactyoid DCs are featured by their ability to produce large amounts of type I interferons (IFNs) in response to nucleic acid sensors, TLR7 and TLR9 and involved in anti-viral immunity and pathogenesis of certain autoimmune disorders such as psoriasis. Conventional DCs include the DC subsets with high crosspresentation activity, which contributes to anti-viral and anti-tumor immunity. These subsets are generated from hematopoietic stem cells (HSCs) via several intermediate progenitors and the development is regulated by the transcriptional mechanisms in which subset-specific transcription factors play major roles. We have recently found that an Ets family transcription factor, SPI-B, which is abundantly expressed in pDCs among DC subsets, plays critical roles in functions and late stage development of pDCs. SPI-B functions in cooperation with other transcription factors, especially, interferon regulatory factor (IRF) family members. Here we review the transcription factor-based molecular mechanisms for generation and functions of DCs, mainly by focusing on the roles of SPI-B and its relatives.

Gfi1 and gfi1b repress rag transcription in plasmacytoid dendritic cells in vitro

Growth factor independence genes (Gfi1 and Gfi1b) repress recombination activating genes (Rag) transcription in developing B lymphocytes. Because all blood lineages originate from hematopoietic stem cells (HSCs) and different lineage progenitors have been shown to share transcription factor networks prior to cell fate commitment, we hypothesized that GFI family proteins may also play a role in repressing Rag transcription or a global lymphoid transcriptional program in other blood lineages. We tested the level of Rag transcription in various blood cells when Gfi1 and Gfi1b were deleted, and observed an upregulation of Rag expression in plasmacytoid dendritic cells (pDCs). Using microarray analysis, we observed that Gfi1 and Gfi1b do not regulate a lymphoid or pDC-specific transcriptional program. This study establishes a role for Gfi1 and Gfi1b in Rag regulation in a non-B lineage cell type.

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.

Bcl11a controls Flt3 expression in early hematopoietic progenitors and is required for pDC development in vivo

Bcl11a is a transcription factor known to regulate lymphoid and erythroid development. Recent bioinformatic analysis of global gene expression patterns has suggested a role for Bcl11a in the development of dendritic cell (DC) lineages. We tested this hypothesis by analyzing the development of DC and other lineages in Bcl11a^−/− mice. We found that Bcl11a was required for expression of IL-7 receptor (IL-7R) and Flt3 in early hematopoietic progenitor cells. In addition, we found severely decreased numbers of plasmacytoid dendritic cells (pDCs) in Bcl11a^−/− fetal livers and in the bone marrow of Bcl11a^−/− fetal liver chimeras. Moreover, Bcl11a^−/− cells showed severely impaired in vitro development of Flt3L-derived pDCs and classical DCs (cDCs). In contrast, we found normal in vitro development of DCs from Bcl11a^−/− fetal liver cells treated with GM-CSF. These results suggest that the persistent cDC development observed in Bcl11a^−/− fetal liver chimeras reflects derivation from a Bcl11a- and Flt3-independent pathway in vivo.

Plasmacytoid dendritic cells: no longer an enigma and now key to transplant tolerance?

Plasmacytoid (p) dendritic cells (DC) are a specialized subset of DC whose primary role was initially defined by the production of type I interferons in response to viral infection. They are now known to also possess a repertoire of functions capable of determining T cell fate and activation. Under homeostatic conditions, non-lymphoid tissue-resident pDC play a critical role in the regulation of mucosal immunity, as well as the development of central and peripheral tolerance. Although these cells display a number of characteristics that differ from conventional DC, particularly altered costimulatory molecule expression and poor allostimulatory capacity when interacting with T cells, this phenotype favors the generation of alloantigen-specific regulatory CD4(+) or CD8(+) T cells critical to the development of graft tolerance. In this minireview, we discuss pDC ontogeny, functional biology and the emerging data that demonstrate the importance of pDC in the induction of tolerance, as well as recent studies that define mechanisms underlying pDC-mediated tolerance to both solid organ and haematopoietic stem cell transplants. We also highlight their use in clinical settings and the potential of pDC both as targets and cellular therapeutic agents to improve the outcome of organ transplantation.

Dendritic cell development: a choose-your-own-adventure story

Dendritic cells (DCs) are essential components of the immune system and contribute to immune responses by activating or tolerizing T cells. DCs comprise a heterogeneous mixture of subsets that are located throughout the body and possess distinct and specialized functions. Although numerous defined precursors from the bone marrow and spleen have been identified, emerging data in the field suggests many alternative routes of DC differentiation from precursors with multilineage potential. Here, we discuss how the combinatorial expression of transcription factors can promote one DC lineage over another as well as the integration of cytokine signaling in this process.

GRANULOCYTE INFILTRATION AND EXPRESSION OF THE PRO-ANGIOGENIC BV8 PROTEIN IN EXPERIMENTAL EL4 AND LEWIS LUNG CARCINOMA TUMORS

Although Vascular Endothelial Growth Factor (VEGF)-targeted therapies have shown efficacy in the treatment of certain advanced cancers, benefits to patients have been modest, which is attributed to tumor resistance to VEGF neutralization. Recent efforts to identify new targets to inhibit tumor angiogenesis have identified Bv8 (prokineticin 2), a myeloid cell-derived protein that promotes endothelial cell growth and tumor angiogenesis, but many mechanistic aspects of the pro-tumorigenic function of Bv8 are unclear. Here we demonstrate that CD11b+, Ly6C+, Ly6G+ granulocytes are the predominant cell source of Bv8 expression in bone marrow, spleen and in tumor tissues. Using granulocyte-deficient Growth factor independence-1 (Gfi1)-null mutant mice and normal littermates, we found that EL4 lymphoma tumors grow significantly larger in the granulocyte and Bv8-deficient mutant mice in comparison to the normal mice that display abundant tumor-associated granulocytes and Bv8 expression. Conversely, Lewis lung carcinoma (LLC-1) tumors grew to a significantly greater size in the normal mice in comparison to the Gfi1-null mice, but normal granulocyte tumor infiltration was modest. Quantitative analysis of tissue vascularization showed that EL4 and LLC-1 tumors from normal and Gfi1-mutant mice are similarly vascularized. These results confirm the critical contribution of the tumor microenvironment in determining the rate of tumor progression independently of tumor angiogenesis, and reveal some of the complexities of granulocyte and Bv8 functions in modulating tumor growth.

Transcriptional control of dendritic cell development

Dendritic cells (DCs) drive both adaptive and innate immunity. Recent findings support the notion that distinct subsets of classical DCs favor alternative modules of immunity, acting on innate lymphoid-like cells (ILCs) and T cells similarly to promote either ILC1/Th1/CTL- or ILC3/Th17-type responses. Coordination between DC subsets and their favored immune module might imply that the genetic programs for DC diversification preceded the emergence of recombination-activating gene-dependent adaptive immunity and operate initially in coordinating ILC repertoires for appropriate responses against pathogens. Consequently, understanding the molecular basis of DC developmental and diversification is important for an underlying appreciation of immune regulation. Currently, the basis for DC development into the recognized subsets/lineages is only partially understood, based on the requirements for several transcription factors including PU.1, Bcl11a, Irf8, E2-2, Id2, Irf4, Irf8, Batf3, and other BATF family members. This chapter will briefly review recent transcriptional aspects of DC development and function and then highlight some currently unresolved questions.

Re(de)fining the dendritic cell lineage

Dendritic cells (DCs) are essential mediators of innate and adaptive immune responses. Study of these critical cells has been complicated by their similarity to other hematopoietic lineages, particularly monocytes and macrophages. Progress has been made in three critical areas of DC biology: the characterization of lineage-restricted progenitors in the bone marrow, the identification of cytokines and transcription factors required during differentiation, and the development of genetic tools for the visualization and depletion of DCs in vivo. Collectively, these advances have clarified the nature of the DC lineage and have provided novel insights into their function during health and disease.

Snail/Gfi-1 (SNAG) family zinc finger proteins in transcription regulation, chromatin dynamics, cell signaling, development, and disease

The Snail/Gfi-1 (SNAG) family of zinc finger proteins is a group of transcriptional repressors that have been intensively studied in mammals. SNAG family members are similarly structured with an N-terminal SNAG repression domain and a C-terminal zinc finger DNA binding domain, however, the spectrum of target genes they regulate and the ranges of biological functions they govern vary widely between them. They play active roles in transcriptional regulation, formation of repressive chromatin structure, cellular signaling and developmental processes. They can also result in disease states due to deregulation. We have performed a thorough investigation of the relevant literature and present a comprehensive mini-review. Based on the available information, we also propose a mechanism by which SNAG family members may function.

A STATus report on DC development

DCs have a vital role in the immune system by recognizing exogenous or self-antigens and eliciting appropriate stimulatory or tolerogenic adaptive immune responses. DCs also contribute to human autoimmune disease and, when depleted, to immunodeficiency. Moreover, DCs are being explored for potential use in clinical therapies including cancer treatment. Thus, understanding the molecular mechanisms that regulate DCs is crucial to improving treatments for human immune disease and cancer. DCs constitute a heterogeneous population including plasmacytoid (pDC) and classic (cDC) subsets; however, the majority of DCs residing in lymphoid organs and peripheral tissues in steady state share common progenitor populations, originating with hematopoietic stem cells. Like other hematopoietic lineages, DCs require extracellular factors including cytokines, as well as intrinsic transcription factors, to control lineage specification, commitment, and maturation. Here, we review recent findings on the roles for cytokines and cytokine-activated STAT transcription factors in DC subset development. We also discuss how cytokines and STATs intersect with lineage-regulatory transcription factors and how insight into the molecular basis of human disease has revealed transcriptional regulators of DCs. Whereas this is an emerging area with much work remaining, we anticipate that knowledge gained by delineating cytokine and transcription factor mechanisms will enable a better understanding of DC subset diversity, and the potential to manipulate these important immune cells for human benefit.

Neutrophil depletion impairs natural killer cell maturation, function, and homeostasis

Neutropenia in mice and humans results in the generation of NK cells with an immature and hyporesponsive phenotype.

Natural killer (NK) cells are bone marrow (BM)–derived granular lymphocytes involved in immune defense against microbial infections and tumors. In an N-ethyl N-nitrosourea (ENU) mutagenesis strategy, we identified a mouse mutant with impaired NK cell reactivity both in vitro and in vivo. Dissection of this phenotype showed that mature neutrophils were required both in the BM and in the periphery for proper NK cell development. In mice lacking neutrophils, NK cells displayed hyperproliferation and poor survival and were blocked at an immature stage associated with hyporesponsiveness. The role of neutrophils as key regulators of NK cell functions was confirmed in patients with severe congenital neutropenia and autoimmune neutropenia. In addition to their direct antimicrobial activity, mature neutrophils are thus endowed with immunoregulatory functions that are conserved across species. These findings reveal novel types of cooperation between cells of the innate immune system and prompt examination of NK cell functional deficiency in patients suffering from neutropenia-associated diseases.

Transcriptional regulation of dendritic cell diversity

Dendritic cells (DCs) are specialized antigen presenting cells that are exquisitely adapted to sense pathogens and induce the development of adaptive immune responses. They form a complex network of phenotypically and functionally distinct subsets. Within this network, individual DC subsets display highly specific roles in local immunosurveillance, migration, and antigen presentation. This division of labor amongst DCs offers great potential to tune the immune response by harnessing subset-specific attributes of DCs in the clinical setting. Until recently, our understanding of DC subsets has been limited and paralleled by poor clinical translation and efficacy. We have now begun to unravel how different DC subsets develop within a complex multilayered system. These findings open up exciting possibilities for targeted manipulation of DC subsets. Furthermore, ground-breaking developments overcoming a major translational obstacle - identification of similar DC populations in mouse and man - now sets the stage for significant advances in the field. Here we explore the determinants that underpin cellular and transcriptional heterogeneity within the DC network, how these influence DC distribution and localization at steady-state, and the capacity of DCs to present antigens via direct or cross-presentation during pathogen infection.

Transcriptional programming of the dendritic cell network

Specialized subsets of dendritic cells (DCs) provide a crucial link between the innate and adaptive immune responses. The genetic programme that coordinates these distinct DC subsets is controlled by both cytokines and transcription factors. The initial steps in DC specification occur in the bone marrow and result in the generation of precursors committed to either the plasmacytoid or conventional DC pathways. DCs undergo further differentiation and lineage diversification in peripheral organs in response to local environmental cues. In this Review, we discuss new evidence regarding the coordination of the specification and commitment of precursor cells to different DC subsets and highlight the ensemble of transcription factors that control these processes.