ICSBP is essential for the development of mouse type I interferon-producing cells and for the generation and activation of CD8alpha(+) dendritic cells

Mechanisms overseeing myeloid-derived suppressor cell production in neoplastic disease

Perturbations in myeloid cell differentiation are common in neoplasia, culminating in immature populations known as myeloid-derived suppressor cells (MDSCs). MDSCs favor tumor progression due to their ability to suppress host immunity or promote invasion and metastasis. They are thought to originate from the bone marrow as a result of exposure to stromal- or circulating tumor-derived factors (TDFs). Although great interest has been placed on understanding how MDSCs function, less is known regarding how MDSCs develop at a transcriptional level. Our work explores the premise that MDSCs arise because cancer cells, through the production of certain TDFs, inhibit the expression of interferon regulatory factor-8 (IRF8) that is ordinarily essential for controlling fundamental properties of myeloid cell differentiation. Our interest in IRF8 has been based on the following rationale. First, it is well-recognized that IRF8 is a 'master regulator' of normal myelopoiesis, critical not only for producing monocytes, dendritic cells (DCs), and neutrophils, but also for controlling the balance of all three major myeloid cell types. This became quite evident in IRF8^-/- mice, whereby the loss of IRF8 leads to a disproportionate accumulation of neutrophils at the expense of monocytes and DCs. Second, we showed that such myeloid populations from IRF8^-/- mice exhibit similar characteristics to MDSCs from tumor-bearing mice. Third, in a reciprocal fashion, we showed that enforced expression of IRF8 in the myeloid system significantly mitigates tumor-induced MDSC accumulation and improves immunotherapy efficacy. Altogether, these observations support the hypothesis that IRF8 is an integral negative regulator of MDSC biology.

The role of cDC1s in vivo: CD8 T cell priming through cross-presentation

The cDC1 subset of classical dendritic cells is specialized for priming CD8 T cell responses through the process of cross-presentation. The molecular mechanisms of cross-presentation remain incompletely understood because of limited biochemical analysis of rare cDC1 cells, difficulty in their genetic manipulation, and reliance on in vitro systems based on monocyte- and bone-marrow-derived dendritic cells. This review will discuss cross-presentation from the perspective of studies with monocyte- or bone-marrow-derived dendritic cells while highlighting the need for future work examining cDC1 cells. We then discuss the role of cDC1s as a cellular platform to combine antigen processing for class I and class II MHC presentation to allow the integration of “help” from CD4 T cells during priming of CD8 T cell responses.

Dendritic cells as gatekeepers of tolerance

Dendritic cells (DC) are unique hematopoietic cells, linking innate and adaptive immune responses. In particular, they are considered as the most potent antigen presenting cells, governing both T cell immunity and tolerance. In view of their exceptional ability to present antigen and to interact with T cells, DC play distinct roles in shaping T cell development, differentiation and function. The outcome of the DC-T cell interaction is determined by the state of DC maturation, the type of DC subset, the cytokine microenvironment and the tissue location. Both regulatory T cells (Tregs) and DC are indispensable for maintaining central and peripheral tolerance. Over the past decade, accumulating data indicate that DC critically contribute to Treg differentiation and homeostasis.

Innate immune signaling and regulation in cancer immunotherapy

A pre-existing T cell-inflamed tumor microenvironment has prognostic utility and also can be predictive for response to contemporary cancer immunotherapies. The generation of a spontaneous T cell response against tumor-associated antigens depends on innate immune activation, which drives type I interferon (IFN) production. Recent work has revealed a major role for the STING pathway of cytosolic DNA sensing in this process. This cascade of events contributes to the activation of Batf3-lineage dendritic cells (DCs), which appear to be central to anti-tumor immunity. Non-T cell-inflamed tumors lack chemokines for Batf3 DC recruitment, have few Batf3 DCs, and lack a type I IFN gene signature, suggesting that failed innate immune activation may be the ultimate cause for lack of spontaneous T cell activation and accumulation. With this information in hand, new strategies for triggering innate immune activation and Batf3 DC recruitment are being developed, including novel STING agonists for de novo immune priming. Ultimately, the successful development of effective innate immune activators should expand the fraction of patients that can respond to immunotherapies, such as with checkpoint blockade antibodies.

A Druggable TCF4- and BRD4-Dependent Transcriptional Network Sustains Malignancy in Blastic Plasmacytoid Dendritic Cell Neoplasm

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive and largely incurable hematologic malignancy originating from plasmacytoid dendritic cells (pDCs). Using RNAi screening, we identified the E-box transcription factor TCF4 as a master regulator of the BPDCN oncogenic program. TCF4 served as a faithful diagnostic marker of BPDCN, and its downregulation caused the loss of the BPDCN-specific gene expression program and apoptosis. High-throughput drug screening revealed that bromodomain and extra-terminal domain inhibitors (BETis) induced BPDCN apoptosis, which was attributable to disruption of a BPDCN-specific transcriptional network controlled by TCF4-dependent super-enhancers. BETis retarded the growth of BPDCN xenografts, supporting their clinical evaluation in this recalcitrant malignancy.

Medullary thymic epithelial cells and CD8α+ dendritic cells coordinately regulate central tolerance but CD8α+ cells are dispensable for thymic regulatory T cell production

In the thymus, antigen presenting cells (APCs) namely, medullary thymic epithelial cells (mTECs) and thymic dendritic cells (tDCs) regulate T cell tolerance through elimination of autoreactive T cells and production of thymic T regulatory (tTreg) cells. How the different APCs in the thymus share the burden of tolerazing the emerging T cell repertoire remains unclear. For example, while mutations that inhibit mTEC development or function associate with peripheral autoimmunity, the role of tDCs in organ-specific autoimmunity and tTreg cell production remains controversial. In this report we used mice depleted of mTECs and/or CD8α⁺ DCs, to examine the contributions of these cell populations in thymic tolerance. We found that while mice depleted of CD8α⁺ DCs or mTECs were normal or developed liver inflammation respectively, combined depletion of mTECs and CD8α⁺ DCs resulted in overt peripheral autoimmunity. The autoimmune manifestations in mice depleted of both mTECs and CD8α⁺ cDCs associated with increased percentages of CD4⁺ and CD8⁺ T cells in the thymus. In contrast, while mTEC depletion resulted in reduced percentages of tTreg cells, no additional effect was observed when CD8α⁺ DCs were also depleted. These results reveal that: 1) mTECs and CD8α⁺ DCs cooperatively safeguard against peripheral autoimmunity through thymic T cell deletion; 2) CD8α⁺ DCs are dispensable for tTreg cell production, whereas mTECs play a non-redundant role in this process; 3) mTECs and CD8α⁺ DCs make unique contributions to tolerance induction that cannot be compensated for by other thymic APCs such as migratory SIRPα⁺ or plasmacytoid DCs.

The role of myeloid cells in cancer therapies

Recent clinical trials have demonstrated the ability to durably control cancer in some patients by manipulating T lymphocytes. These immunotherapies are revolutionizing cancer treatment but benefit only a minority of patients. It is thus a crucial time for clinicians, cancer scientists and immunologists to determine the next steps in shifting cancer treatment towards better cancer control. This Review describes recent advances in our understanding of tumour-associated myeloid cells. These cells remain less studied than T lymphocytes but have attracted particular attention because their presence in tumours is often linked to altered patient survival. Also, experimental studies indicate that myeloid cells modulate key cancer-associated activities, including immune evasion, and affect virtually all types of cancer therapy. Consequently, targeting myeloid cells could overcome limitations of current treatment options.

Classical dendritic cells are required for dietary antigen-mediated induction of peripheral T(reg) cells and tolerance

Oral tolerance prevents pathological inflammatory responses towards innocuous foreign antigens via peripheral regulatory T cells (pT_reg cells). However, whether a particular subset of antigen-presenting cells (APCs) is required during dietary antigen exposure to instruct naïve CD4⁺ T cells to differentiate into pT_reg cells has not been defined. Using myeloid lineage-specific APC depletion in mice, we found that monocyte-derived APCs are dispensable, while classical dendritic cells (cDCs) are critical for pT_reg cell induction and oral tolerance. CD11b⁻ cDCs from the gut-draining lymph nodes efficiently induced pT_reg cells, and conversely, loss of IRF8-dependent CD11b⁻ cDCs impaired their polarization, although oral tolerance remained intact. These data reveal the hierarchy of cDC subsets in pT_reg cell induction and their redundancy during oral tolerance development.

Immunity and Tolerance Induced by Intestinal Mucosal Dendritic Cells

Dendritic cells present in the digestive tract are constantly exposed to environmental antigens, commensal flora, and invading pathogens. Under steady-state conditions, these cells have high tolerogenic potential, triggering differentiation of regulatory T cells to protect the host from unwanted proinflammatory immune responses to innocuous antigens or commensals. On the other hand, these cells must discriminate between commensal flora and invading pathogens and mount powerful immune response against pathogens. A potential result of unbalanced tolerogenic versus proinflammatory responses mediated by dendritic cells is associated with chronic inflammatory conditions, such as Crohn's disease, ulcerative colitis, food allergies, and celiac disease. Herein, we review the dendritic cell population involved in mediating tolerance and immunity in mucosal surfaces, the progress in unveiling their development in vivo, and factors that can influence their functions.

Beta-catenin signaling drives differentiation and proinflammatory function of IRF8-dependent dendritic cells

Beta-catenin signaling has recently been tied to the emergence of tolerogenic dendritic cells (DCs). In this article, we demonstrate a novel role for beta-catenin in directing DC subset development through IFN regulatory factor 8 (IRF8) activation. We found that splenic DC precursors express beta-catenin, and DCs from mice with CD11c-specific constitutive beta-catenin activation upregulated IRF8 through targeting of the Irf8 promoter, leading to in vivo expansion of IRF8-dependent CD8a+, plasmacytoid, and CD103+ CD11b2 DCs. beta-catenin–stabilized CD8a+ DCs secreted elevated IL-12 upon in vitro microbial stimulation, and pharmacological beta-catenin inhibition blocked this response in wild-type cells. Upon infections with Toxoplasma gondii and vaccinia virus, mice with stabilized DC beta-catenin displayed abnormally high Th1 and CD8+ T lymphocyte responses, respectively. Collectively, these results reveal a novel and unexpected function for beta-catenin in programming DC differentiation toward subsets that orchestrate proinflammatory immunity to infection.

IRF4 and IRF8 Act in CD11c+ Cells To Regulate Terminal Differentiation of Lung Tissue Dendritic Cells

Dendritic cells (DCs) initiate immune responses in barrier tissues including lung and skin. Conventional DC (cDC) subsets, CD11b(-) (cDC1s) or CD11b(+) (cDC2s), arise via distinct networks of transcription factors involving IFN regulatory factor 4 (IRF4) and IRF8, and are specialized for unique functional responses. Using mice in which a conditional Irf4 or Irf8 allele is deleted in CD11c(+) cells, we determined whether IRF4 or IRF8 deficiency beginning in CD11c(+) cDC precursors (pre-cDCs) changed the homeostasis of mature DCs or pre-DCs in the lung, dermis, and spleen. CD11c-cre-Irf4(-/-) mice selectively lacked a lung-resident CD11c(hi)CD11b(+)SIRPα(+)CD24(+) DC subset, but not other lung CD11b(+) DCs or alveolar macrophages. Numbers of CD11b(+)CD4(+) splenic DCs, but not CD11b(+) dermal DCs, were reduced, indicating cDC2s in the lung and dermis develop via different pathways. Irf4 deficiency did not alter numbers of cDC1s. CD11c-cre-Irf8(-/-) mice lacked lung-resident CD103(+) DCs and splenic CD8α(+) DCs, yet harbored increased IRF4-dependent DCs. This correlated with a reduced number of Irf8(-/-) pre-cDCs, which contained elevated IRF4, suggesting that Irf8 deficiency diverts pre-cDC fate. Analyses of Irf4 and Irf8 haploinsufficient mice showed that, although one Irf4 allele was sufficient for lung cDC2 development, two functional Irf8 alleles were required for differentiation of lung cDC1s. Thus, IRF8 and IRF4 act in pre-cDCs to direct the terminal differentiation of cDC1 and cDC2 subsets in the lung and spleen. These data suggest that variation in IRF4 or IRF8 levels resulting from genetic polymorphisms or environmental cues will govern tissue DC numbers and, therefore, regulate the magnitude of DC functional responses.

Functional Specialization of Skin Dendritic Cell Subsets in Regulating T Cell Responses

Dendritic cells (DC) are a heterogeneous family of professional antigen-presenting cells classically recognized as most potent inducers of adaptive immune responses. In this respect, Langerhans cells have long been considered to be prototypic immunogenic DC in the skin. More recently this view has considerably changed. The generation of in vivo cell ablation and lineage tracing models revealed the complexity of the skin DC network and, in particular, established the existence of a number of phenotypically distinct Langerin⁺ and negative DC populations in the dermis. Moreover, by now we appreciate that DC also exert important regulatory functions and are required for the maintenance of tolerance toward harmless foreign and self-antigens. This review summarizes our current understanding of the skin-resident DC system in the mouse and discusses emerging concepts on the functional specialization of the different skin DC subsets in regulating T cell responses. Special consideration is given to antigen cross-presentation as well as immune reactions toward contact sensitizers, cutaneous pathogens, and tumors. These studies form the basis for the manipulation of the human counterparts of the murine DC subsets to promote immunity or tolerance for the treatment of human disease.

Predominant Role of Plasmacytoid Dendritic Cells in Stimulating Systemic Autoimmunity

Plasmacytoid dendritic cells (pDCs), which are prominent type I interferon (IFN-I)-producing immune cells, have been extensively implicated in systemic lupus erythematosus (SLE). However, whether they participate critically in lupus pathogenesis remains unknown. Recent studies using various genetic and cell type-specific ablation strategies have demonstrated that pDCs play a pivotal role in the development of autoantibodies and the progression of lupus under diverse experimental conditions. The findings of several investigations highlight a notion that pDCs operate critically at the early stage of lupus development. In particular, pDCs have a profound effect on B-cell activation and humoral autoimmunity in vivo. This deeper understanding of the vital role of pDCs in lupus pathogenesis supports the therapeutic targeting of the pDC-IFN-I pathway in SLE.

CD1a+ survivin+ dendritic cell infiltration in dermal lesions of systemic sclerosis

INTRODUCTION

Proto-oncogene survivin is a member of the inhibitor of apoptosis (IAP) family of proteins. The presence of serous antibodies against survivin in patients with systemic sclerosis has been previously reported; however, there are few reports regarding the pathophysiological relationship between survivin and systemic sclerosis. We herein investigated the expression and function of survivin in SSc patients.

METHODS

We performed immunohistochemistry analyses to determine the expression of XIAP, cIAP and survivin in skin lesions from patients with SSc and non-SSc. The expression levels of survivin in peripheral blood mononuclear cells (PBMCs) obtained from SSc patients and healthy controls were evaluated using RT-PCR and flow cytometry. Additionally, the function of survivin was verified with overexpression experiments using monocyte-derived dendritic cells (Mo-DCs).

RESULTS

The expression patterns of both XIAP and cIAP were similar, while only the survivin expression differed between the SSc and non-SSc skin lesions. Survivin-overexpressing cells were detected in the SSc dermis frequently. The positive rate of survivin in SSc dermis (64.3%, 9/14) was higher than that in non-SSc dermis (11.2%, 1/9). Furthermore, survivin+ cells expressed CD1a, one of the DC markers. Real-time PCR and FACS analyses revealed that the survivin-WT (wild type) expression levels in PBMCs, in particular CD14+ monocytes, from SSc patients were higher than that from healthy controls. Additionally, the overexpression experiments showed that survivin-WT-overexpressing CD1a+ Mo-DCs have the characteristics of promoting cell cycle progression and decreasing apoptotic cells.

CONCLUSIONS

These findings suggest that dermal survivin+ CD1a+ cell infiltration may be a potential biomarker of SSc skin lesions. PBMCs and monocytes from SSc patients also overexpressed survivin; therefore, dermal survivin+ DC may be derived from peripheral blood monocytes. Additionally, survivin may be involved in dermal CD1a+ DC proliferation through cell cycle activation and resistance to apoptosis. Survivin may be an important molecule for the pathogenesis of SSc.

A Hitchhiker's Guide to Myeloid Cell Subsets: Practical Implementation of a Novel Mononuclear Phagocyte Classification System

The classification of mononuclear phagocytes as either dendritic cells or macrophages has been mainly based on morphology, the expression of surface markers, and assumed functional specialization. We have recently proposed a novel classification system of mononuclear phagocytes based on their ontogeny. Here, we discuss the practical application of such a classification system through a number of prototypical examples we have encountered while hitchhiking from one subset to another, across species and between steady-state and inflammatory settings. Finally, we discuss the advantages and drawbacks of such a classification system and propose a number of improvements to move from theoretical concepts to concrete guidelines.

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.

Transcription factor IRF8 controls Th1-like regulatory T-cell function

Recent studies have suggested that regulatory T (Treg) cells comprise a heterogeneous population that regulates various aspects of the immune response, and that Treg cells use the factors that are expressed in their target cells to regulate them. We searched for factors that regulate Th1 response in Treg cells using a meta-analysis. In the process, we discovered that transcription factor interferon regulatory factor 8 (IRF8) was selectively expressed in Treg and Th1 cells. IRF8-deficient Treg cells showed defective expression of CXCR3 and aberrant expression of the Il4 and Il17 genes. Upon treatment with alpha galactosyl-C18-ceramide (αGal-C18-Cer), IRF8-deficient mice showed defective Treg cell recruitment in the liver. Eliciting Th1 immune response by anti-CD40 antibody injection in mice induced IRF8 expression in Treg cells. The expression of IRF8 was induced by Foxp3 in Treg cells. IRF8 had no effect on T-bet expression in Treg and vice versa. Thus, our results strongly suggest that IRF8 controls Th1 immune response in Treg cells independent of T-bet.

Human and Mouse Mononuclear Phagocyte Networks: A Tale of Two Species?

Dendritic cells (DCs), monocytes, and macrophages are a heterogeneous population of mononuclear phagocytes that are involved in antigen processing and presentation to initiate and regulate immune responses to pathogens, vaccines, tumor, and tolerance to self. In addition to their afferent sentinel function, DCs and macrophages are also critical as effectors and coordinators of inflammation and homeostasis in peripheral tissues. Harnessing DCs and macrophages for therapeutic purposes has major implications for infectious disease, vaccination, transplantation, tolerance induction, inflammation, and cancer immunotherapy. There has been a paradigm shift in our understanding of the developmental origin and function of the cellular constituents of the mononuclear phagocyte system. Significant progress has been made in tandem in both human and mouse mononuclear phagocyte biology. This progress has been accelerated by comparative biology analysis between mouse and human, which has proved to be an exceptionally fruitful strategy to harmonize findings across species. Such analyses have provided unexpected insights and facilitated productive reciprocal and iterative processes to inform our understanding of human and mouse mononuclear phagocytes. In this review, we discuss the strategies, power, and utility of comparative biology approaches to integrate recent advances in human and mouse mononuclear phagocyte biology and its potential to drive forward clinical translation of this knowledge. We also present a functional framework on the parallel organization of human and mouse mononuclear phagocyte networks.

Batf3 maintains autoactivation of Irf8 for commitment of a CD8α(+) conventional DC clonogenic progenitor

The transcription factors Batf3 and IRF8 are required for the development of CD8α(+) conventional dendritic cells (cDCs), but the basis for their actions has remained unclear. Here we identified two progenitor cells positive for the transcription factor Zbtb46 that separately generated CD8α(+) cDCs and CD4(+) cDCs and arose directly from the common DC progenitor (CDP). Irf8 expression in CDPs required prior autoactivation of Irf8 that was dependent on the transcription factor PU.1. Specification of the clonogenic progenitor of CD8α(+) cDCs (the pre-CD8 DC) required IRF8 but not Batf3. However, after specification of pre-CD8 DCs, autoactivation of Irf8 became Batf3 dependent at a CD8α(+) cDC-specific enhancer with multiple transcription factor AP1-IRF composite elements (AICEs) within the Irf8 superenhancer. CDPs from Batf3(-/-) mice that were specified toward development into pre-CD8 DCs failed to complete their development into CD8α(+) cDCs due to decay of Irf8 autoactivation and diverted to the CD4(+) cDC lineage.

A subset of human plasmacytoid dendritic cells expresses CD8α upon exposure to herpes simplex virus type 1

Classical and plasmacytoid dendritic cells (DC) play important roles in the defense against murine and human infections with herpes simplex virus (HSV). So far, CD8α expression has only been reported for murine DC. CD8α⁺ DC have prominent cross-presenting activities, which are enhanced by murine CD8α⁺ PDC. The human orthologue of murine CD8α⁺ DC, the CD141 (BDCA3)⁺ DC, mainly cross-present after TLR3 ligation. We report here the serendipitous finding that a subset of human PDC upregulates CD8α upon HSV-1 stimulation, as shown by gene array and flow cytometry analyses. CD8α, not CD8ß, was expressed upon exposure. Markers of activation, migration, and costimulation were upregulated on CD8α-expressing human PDC. In these cells, increased cytokine and chemokine levels were detected that enhance development and function of T, B, and NK cells, and recruit immature DC, monocytes, and Th1 cells, respectively. Altogether, human CD8α⁺ PDC exhibit a highly activated phenotype and appear to recruit other immune cells to the site of inflammation. Further studies will show whether CD8α-expressing PDC contribute to antigen cross-presentation, which may be important for immune defenses against HSV infections in vitro and in vivo.

Innate immune regulation by STAT-mediated transcriptional mechanisms

The term innate immunity typically refers to a quick but non-specific host defense response against invading pathogens. The innate immune system comprises particular immune cell populations, epithelial barriers, and numerous secretory mediators including cytokines, chemokines, and defense peptides. Innate immune cells are also now recognized to play important contributing roles in cancer and pathological inflammatory conditions. Innate immunity relies on rapid signal transduction elicited upon pathogen recognition via pattern recognition receptors (PRRs) and cell:cell communication conducted by soluble mediators, including cytokines. A majority of cytokines involved in innate immune signaling use a molecular cascade encompassing receptor-associated Jak protein tyrosine kinases and STAT (signal transducer and activator of transcription) transcriptional regulators. Here, we focus on roles for STAT proteins in three major innate immune subsets: neutrophils, macrophages, and dendritic cells (DCs). While knowledge in this area is only now emerging, understanding the molecular regulation of these cell types is necessary for developing new approaches to treat human disorders such as inflammatory conditions, autoimmunity, and cancer.

Fate mapping of dendritic cells

Dendritic cells (DCs) are a heterogeneous group of mononuclear phagocytes with versatile roles in immunity. They are classified predominantly based on phenotypic and functional properties, namely their stellate morphology, expression of the integrin CD11c, and major histocompatibility class II molecules, as well as their superior capacity to migrate to secondary lymphoid organs and stimulate naïve T cells. However, these attributes are not exclusive to DCs and often change within inflammatory or infectious environments. This led to debates over cell identification and questioned even the mere existence of DCs as distinct leukocyte lineage. Here, we review experimental approaches taken to fate map DCs and discuss how these have shaped our understanding of DC ontogeny and lineage affiliation. Considering the ontogenetic properties of DCs will help to overcome the inherent shortcomings of purely phenotypic- and function-based approaches to cell definition and will yield a more robust way of DC classification.

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.

EPCR-dependent PAR2 activation by the blood coagulation initiation complex regulates LPS-triggered interferon responses in mice

Infection and inflammation are invariably associated with activation of the blood coagulation mechanism, secondary to the inflammation-induced expression of the coagulation initiator tissue factor (TF) on innate immune cells. By investigating the role of cell-surface receptors for coagulation factors in mouse endotoxemia, we found that the protein C receptor (ProcR; EPCR) was required for the normal in vivo and in vitro induction of lipopolysaccharide (LPS)-regulated gene expression. In cultured bone marrow-derived myeloid cells and in monocytic RAW264.7 cells, the LPS-induced expression of functionally active TF, assembly of the ternary TF-VIIa-Xa initiation complex of blood coagulation, and the EPCR-dependent activation of protease-activated receptor 2 (PAR2) by the ternary TF-VIIa-Xa complex were required for the normal LPS induction of messenger RNAs encoding the TLR3/4 signaling adaptor protein Pellino-1 and the transcription factor interferon regulatory factor 8. In response to in vivo challenge with LPS, mice lacking EPCR or PAR2 failed to fully initiate an interferon-regulated gene expression program that included the Irf8 target genes Lif, Iigp1, Gbp2, Gbp3, and Gbp6. The inflammation-induced expression of TF and crosstalk with EPCR, PAR2, and TLR4 therefore appear necessary for the normal evolution of interferon-regulated host responses.

The interferon regulatory factors as novel potential targets in the treatment of cardiovascular diseases

The family of interferon regulatory factors (IRFs) consists of nine members (IRF1-IRF9) in mammals. They act as transcription factors for the interferons and thus exert essential regulatory functions in the immune system and in oncogenesis. Recent clinical and experimental studies have identified critically important roles of the IRFs in cardiovascular diseases, arising from their participation in divergent and overlapping molecular programmes beyond the immune response. Here we review the current knowledge of the regulatory effects and mechanisms of IRFs on the immune system. The role of IRFs and their potential molecular mechanisms as novel stress sensors and mediators of cardiovascular diseases are highlighted.

Mouse Conventional Dendritic Cells Can be Universally Classified Based on the Mutually Exclusive Expression of XCR1 and SIRPα

Since the identification of mouse dendritic cells (DC) in the early 70s, all attempts to consistently classify the identified functional DC subpopulations according to their surface molecule expression failed. In the absence of DC lineage markers, a great variety of non-congruent surface molecules were used instead. Recent advances in the understanding of the involvement of transcription factors in the differentiation of DC subpopulations, together with the identification of a lineage marker for cross-presenting DC, have now allowed to establish a consistent and unified DC classification in the mouse. We demonstrate in the present article that all conventional DC in the mouse can be universally subdivided into either XCR1⁺ (“cross-presenting”) DC or SIRPα⁺ DC, irrespective of their activation status. This advancement will greatly facilitate future work on the biology of mouse DC. We discuss this new classification in view of current DC classification systems in the mouse and the human.

A dual cis-regulatory code links IRF8 to constitutive and inducible gene expression in macrophages

The transcription factor IRF8 controls both developmental and inflammatory stimulus-inducible genes in macrophages. Mancino et al. found that IRF8 is recruited to distinct sets of DNA consensus sequences before and after lipopolysaccharide (LPS) stimulation. While constitutively expressed IRF8-dependent genes contained only sites mediating basal IRF8/PU.1 recruitment, inducible IRF8-dependent genes contained variable combinations of constitutive and inducible sites.

The transcription factor (TF) interferon regulatory factor 8 (IRF8) controls both developmental and inflammatory stimulus-inducible genes in macrophages, but the mechanisms underlying these two different functions are largely unknown. One possibility is that these different roles are linked to the ability of IRF8 to bind alternative DNA sequences. We found that IRF8 is recruited to distinct sets of DNA consensus sequences before and after lipopolysaccharide (LPS) stimulation. In resting cells, IRF8 was mainly bound to composite sites together with the master regulator of myeloid development PU.1. Basal IRF8–PU.1 binding maintained the expression of a broad panel of genes essential for macrophage functions (such as microbial recognition and response to purines) and contributed to basal expression of many LPS-inducible genes. After LPS stimulation, increased expression of IRF8, other IRFs, and AP-1 family TFs enabled IRF8 binding to thousands of additional regions containing low-affinity multimerized IRF sites and composite IRF–AP-1 sites, which were not premarked by PU.1 and did not contribute to the basal IRF8 cistrome. While constitutively expressed IRF8-dependent genes contained only sites mediating basal IRF8/PU.1 recruitment, inducible IRF8-dependent genes contained variable combinations of constitutive and inducible sites. Overall, these data show at the genome scale how the same TF can be linked to constitutive and inducible gene regulation via distinct combinations of alternative DNA-binding sites.

Intestinal dendritic cells in the regulation of mucosal immunity

The intestine presents a huge surface area to the outside environment, a property that is of critical importance for its key functions in nutrient digestion, absorption, and waste disposal. As such, the intestine is constantly exposed to dietary and microbial-derived foreign antigens, to which immune cells within the mucosa must suitably respond to maintain intestinal integrity, while also providing the ability to mount effective immune responses to potential pathogens. Dendritic cells (DCs) are sentinel immune cells that play a central role in the initiation and differentiation of adaptive immune responses. In the intestinal mucosa, DCs are located diffusely throughout the intestinal lamina propria, within gut-associated lymphoid tissues, including Peyer's patches and smaller lymphoid aggregates, as well as in intestinal-draining lymph nodes, including mesenteric lymph nodes. The recognition that dietary nutrients and microbial communities in the intestine influence both mucosal and systemic immune cell development and function as well as immune-mediated disease has led to an explosion of literature in mucosal immunology in recent years and a growing interest in the functionality of intestinal DCs. In the current review, we discuss recent findings from our group and others that have provided important insights regarding murine and human intestinal lamina propria DCs and highlighted marked developmental and functional heterogeneity within this compartment. A thorough understanding of the role these subsets play in the regulation of intestinal immune homeostasis and inflammation will help to define novel strategies for the treatment of intestinal pathologies and contribute to improved rational design of mucosal vaccines.

Differential requirement for irf8 in formation of embryonic and adult macrophages in zebrafish

Interferon regulatory factor 8 (Irf8) is critical for mammalian macrophage development and innate immunity, but its role in teleost myelopoiesis remains incompletely understood. In particular, genetic tools to analyze the role of Irf8 in zebrafish macrophage development at larval and adult stages are lacking. We generated irf8 null mutants in zebrafish using TALEN-mediated targeting. Our analysis defines different requirements for irf8 at different stages. irf8 is required for formation of all macrophages during primitive and transient definitive hematopoiesis, but not during adult-phase definitive hematopoiesis starting at 5-6 days postfertilization. At early stages, irf8 mutants have excess neutrophils and excess cell death in pu.1-expressing myeloid cells. Macrophage fates were recovered in irf8 mutants after wildtype irf8 expression in neutrophil and macrophage lineages, suggesting that irf8 regulates macrophage specification and survival. In juvenile irf8 mutant fish, mature macrophages are present, but at numbers significantly reduced compared to wildtype, indicating an ongoing requirement for irf8 after embryogenesis. As development progresses, tissue macrophages become apparent in zebrafish irf8 mutants, with the possible exception of microglia. Our study defines distinct requirement for irf8 in myelopoiesis before and after transition to the adult hematopoietic system.

Host genetic factors in susceptibility to mycobacterial disease

Primary immunodeficiencies (PID) are a group of rare inherited disorders that manifest as heightened susceptibility to infection, autoimmunity and/or malignancy. By exploring their genetic and cellular aetiology, we can learn much about the basis of pathogen-specific immunity in humans. This is exemplified by mycobacterial susceptibility, which occurs across several types of PID, either as an isolated problem or as part of a broader pattern of susceptibility to infection. These experiments of nature have contributed to our understanding of the central role of T cells in activating infected macrophages to eliminate phagosomal mycobacteria through mutually activating, cytokine-dependent interactions. In recent years, the discovery of novel forms of PID has emphasised the important role of dendritic cells and monocytes in mycobacterial defence in humans. Here, we provide a brief overview of these new disorders alongside other genetic causes of susceptibility to mycobacterial disease.

Tumor-induced myeloid dysfunction and its implications for cancer immunotherapy

Immune function relies on an appropriate balance of the lymphoid and myeloid responses. In the case of neoplasia, this balance is readily perturbed by the dramatic expansion of immature or dysfunctional myeloid cells accompanied by a reciprocal decline in the quantity/quality of the lymphoid response. In this review, we seek to: (1) define the nature of the atypical myelopoiesis observed in cancer patients and the impact of this perturbation on clinical outcomes; (2) examine the potential mechanisms underlying these clinical manifestations; and (3) explore potential strategies to restore normal myeloid cell differentiation to improve activation of the host antitumor immune response. We posit that fundamental alterations in myeloid homeostasis triggered by the neoplastic process represent critical checkpoints that govern therapeutic efficacy, as well as offer novel cellular-based biomarkers for tracking changes in disease status or relapse.

Transcription factor IRF8 plays a critical role in the development of murine basophils and mast cells

Basophils and mast cells play critical roles in host defense against pathogens and allergic disorders. However, the molecular mechanism by which these cells are generated is not completely understood. Here we demonstrate that interferon regulatory factor-8 (IRF8), a transcription factor essential for the development of several myeloid lineages, also regulates basophil and mast cell development. Irf8(-/-) mice displayed a severe reduction in basophil counts, which was accounted for by the absence of pre-basophil and mast cell progenitors (pre-BMPs). Although Irf8(-/-) mice retained peripheral tissue mast cells, remaining progenitors from Irf8(-/-) mice including granulocyte progenitors (GPs) were unable to efficiently generate either basophils or mast cells, indicating that IRF8 also contributes to the development of mast cells. IRF8 appeared to function at the GP stage, because IRF8 was expressed in GPs, but not in basophils, mast cells, and basophil/mast cell-restricted progenitor cells. Furthermore, we demonstrate that GATA2, a transcription factor known to promote basophil and mast cell differentiation, acts downstream of IRF8. These results shed light on the pathways and mechanism underlying the development of basophils and mast cells.

Interferon regulatory factor 8 (IRF8) interacts with the B cell lymphoma 6 (BCL6) corepressor BCOR

B cell lymphoma 6 (BCL6) corepressor (BCOR) was discovered as a BCL6-interacting corepressor, but little is known about its other biological activities in normal B cell development and function. Previously, we found that interferon regulatory factor 8 (IRF8), also known as interferon consensus sequence-binding protein, directly targets a large number of genes in germinal center B cells including BCL6. In this study, we screened potential binding partners of IRF8 using a retrovirus-based protein complementation assay screen in a mouse pre-B cell line. We found that IRF8 interacts directly with BCOR and that the α-helical region of IRF8 and the BCL6 binding domain of BCOR are required for this interaction. In addition, IRF8 protein interacts directly with BCL6. Using an siRNA-mediated IRF8 knockdown mouse B cell lymphoma cell line, we showed that IRF8 represses Bcor and enhances Bcl6 transcription. Taken together, these data suggest that a complex comprising BCOR-BCL6-IRF8 modulates BCL6-associated transcriptional regulation of germinal center B cell function.

Cancer-driven dynamics of immune cells in a microfluidic environment

Scope of the present work is to infer the migratory ability of leukocytes by stochastic processes in order to distinguish the spontaneous organization of immune cells against an insult (namely cancer). For this purpose, spleen cells from immunodeficient mice, selectively lacking the transcription factor IRF-8 (IRF-8 knockout; IRF-8 KO), or from immunocompetent animals (wild-type; WT), were allowed to interact, alternatively, with murine B16.F10 melanoma cells in an ad hoc microfluidic environment developed on a LabOnChip technology. In this setting, only WT spleen cells were able to establish physical interactions with melanoma cells. Conversely, IRF-8 KO immune cells exhibited poor dynamical reactivity towards the neoplastic cells. In the present study, we collected data on the motility of these two types of spleen cells and built a complete set of observables that recapitulate the biological complexity of the system in these experiments. With remarkable accuracy, we concluded that the IRF-8 KO cells performed pure uncorrelated random walks, while WT splenocytes were able to make singular drifted random walks that collapsed on a straight ballistic motion for the system as a whole, hence giving rise to a highly coordinate response. These results may provide a useful system to quantitatively analyse the real time cell-cell interactions and to foresee the behavior of immune cells with tumor cells at the tissue level.

IFN-gamma AU-rich element removal promotes chronic IFN-gamma expression and autoimmunity in mice

We generated a mouse model with a 162 nt AU-rich element (ARE) region deletion in the 3' untranslated region (3'UTR) of the interferon-gamma (IFN-γ) gene that results in chronic circulating serum IFN-γ levels. Mice homozygous for the ARE deletion (ARE-Del) (-/-) present both serologic and cellular abnormalities typical of patients with systemic lupus erythematosus (SLE). ARE-Del(-/-) mice display increased numbers of pDCs in bone marrow and spleen. Addition of IFN-γ to Flt3-ligand (Flt3L) treated in vitro bone marrow cultures results in a 2-fold increase in pDCs with concurrent increases in IRF8 expression. Marginal zone B (MZB) cells and marginal zone macrophages (MZMs) are absent in ARE-Del(-/-) mice. ARE-Del(+/-) mice retain both MZB cells and MZMs and develop no or mild autoimmunity. However, low dose clodronate treatment in ARE-Del(+/-) mice specifically eliminates MZMs and promotes anti-DNA antibody development and glomerulonephritis. Our findings demonstrate the consequences of a chronic IFN-γ milieu on B220(+) cell types and in particular the impact of MZB cell loss on MZM function in autoimmunity. Furthermore, similarities between disease states in ARE-Del(-/-) mice and SLE patients suggest that IFN-γ may not only be a product of SLE but may be critical for disease onset and progression.

The plasticity of inflammatory monocyte responses to the inflamed central nervous system

Over the last three decades it has become increasingly clear that monocytes, originally thought to have fixed, stereotypic responses to foreign stimuli, mediate exquisitely balanced protective and pathogenic roles in disease and immunity. This balance is crucial in core functional organs, such as the central nervous system (CNS), where minor changes in neuronal microenvironments and the production of immune factors can result in significant disease with fatal consequences or permanent neurological sequelae. Viral encephalitis and multiple sclerosis are examples of important human diseases in which the pathogenic contribution of monocytes recruited from the bone marrow plays a critical role in the clinical expression of disease, as they differentiate into macrophage or dendritic cells in the CNS to carry out effector functions. While antigen-specific lymphocyte populations are central to the adaptive immune response in both cases, in viral encephalitis a prominent macrophage infiltration may mediate immunopathological damage, seizure induction, and death. However, the autoimmune response to non-replicating, non-infectious, but abundant, self antigen has a different disease progression, associated with differentiation of significant numbers of infiltrating monocytes into dendritic cells in the CNS. Whilst a predominant presence of macrophages or dendritic cells in the inflamed CNS in viral encephalitis or multiple sclerosis is well described, the way in which the inflamed CNS mobilizes monocytes in the bone marrow to migrate to the CNS and the key drivers that lead to these specific differentiation pathways in vivo are not well understood. Here we review the current understanding of factors facilitating inflammatory monocyte generation, migration and entry into the brain, as well as their differentiation towards macrophages or dendritic cells in viral and autoimmune disease in relation to their respective disease outcomes.

Role and therapeutic value of dendritic cells in central nervous system autoimmunity

Dendritic cells (DCs) are professional antigen-presenting cells that control the generation of adaptive immunity. Consequently, DCs have a central role in the induction of protective immunity to pathogens and also in the pathogenic immune response responsible for the development and progression of autoimmune disorders. Thus the study of the molecular pathways that control DC development and function is likely to result in new strategies for the therapeutic manipulation of the immune response. In this review, we discuss the role and therapeutic value of DCs in autoimmune diseases, with a special focus on multiple sclerosis.

Dendritic cells, monocytes and macrophages: a unified nomenclature based on ontogeny

The mononuclear phagocyte system (MPS) has historically been categorized into monocytes, dendritic cells and macrophages on the basis of functional and phenotypical characteristics. However, considering that these characteristics are often overlapping, the distinction between and classification of these cell types has been challenging. In this Opinion article, we propose a unified nomenclature for the MPS. We suggest that these cells can be classified primarily by their ontogeny and secondarily by their location, function and phenotype. We believe that this system permits a more robust classification during both steady-state and inflammatory conditions, with the benefit of spanning different tissues and across species.