Batf3 and Id2 have a synergistic effect on Irf8-directed classical CD8α+ dendritic cell development

Concise review: The heterogenous roles of BATF3 in cancer oncogenesis and dendritic cells and T cells differentiation and function considering the importance of BATF3-dependent dendritic cells

The transcription factor, known as basic leucine zipper ATF-like 3 (BATF3), is a crucial contributor to the development of conventional type 1 dendritic cells (cDC1), which is definitely required for priming CD8 + T cell-mediated immunity against intracellular pathogens and malignancies. In this respect, BATF3-dependent cDC1 can bring about immunological tolerance, an autoimmune response, graft immunity, and defense against infectious agents such as viruses, microbes, parasites, and fungi. Moreover, the important function of cDC1 in stimulating CD8 + T cells creates an excellent opportunity to develop a highly effective target for vaccination against intracellular pathogens and diseases. BATF3 has been clarified to control the development of CD8α+ and CD103+ DCs. The presence of BATF3-dependent cDC1 in the tumor microenvironment (TME) reinforces immunosurveillance and improves immunotherapy approaches, which can be beneficial for cancer immunotherapy. Additionally, BATF3 acts as a transcriptional inhibitor of Treg development by decreasing the expression of the transcription factor FOXP3. However, when overexpressed in CD8 + T cells, it can enhance their survival and facilitate their transition to a memory state. BATF3 induces Th9 cell differentiation by binding to the IL-9 promoter through a BATF3/IRF4 complex. One of the latest research findings is the oncogenic function of BATF3, which has been approved and illustrated in several biological processes of proliferation and invasion.

Batf3-dependent orchestration of the robust Th1 responses and fungal control during cryptococcal infection, the role of cDC1

While type I conventional dendritic cells (cDC1s) are vital for generating adaptive immunity against intracellular pathogens and tumors, their role in defense against fungal pathogen Cryptococcus neoformans remains unclear. We investigated the role of the cDC1 subset in a fungus-restricting mouse model of cryptococcal infection. The cDC1 subset displayed a unique transcriptional signature with highly upregulated T-cell recruitment, polarization, and activation pathways compared to other DC subsets. Using Batf3-/- mice, which lack the cDC1 population, our results support that Batf3-dependent cDC1s are pivotal for the development of the effective immune response against cryptococcal infection, particularly within the lung and brain. Deficiency in Batf3 cDC1 led to diminished CD4 accumulation and decreased IFNγ production across multiple organs, supporting that cDC1s are a major driver of potent Th1 responses during cryptococcal infection. Consistently, mice lacking Batf3-cDC1 demonstrated markedly diminished fungicidal activity and weaker containment of the fungal pathogen. In conclusion, Batf3-dependent cDC1 can function as a linchpin in mounting Th1 response, ensuring effective fungal control during cryptococcal infection. Harnessing cDC1 pathways may present a promising strategy for interventions against this pathogen.IMPORTANCECryptococcus neoformans causes severe meningoencephalitis, accounting for an estimated 200,000 deaths each year. Central to mounting an effective defense against these infections is T-cell-mediated immunity, which is orchestrated by dendritic cells (DCs). The knowledge about the role of specific DC subsets in shaping anti-cryptococcal immunity is limited. Here, we demonstrate that Batf3 cDC1s are important drivers of protective Th1 CD4 T-cell responses required for clearance of cryptococcal infection. Deficiency of Batf3 cDC1 in the infected mice leads to significantly reduced Th1 response and exacerbated fungal growth to the point where depleting the remaining CD4 T cells no longer affects fungal burden. Unveiling this pivotal role of cDC1 in antifungal defense is likely to be important for the development of vaccines and therapies against life-threatening fungal pathogens.

TIM-3 increases the abundance of type-2 dendritic cells during Leishmania donovani infection by enhancing IL-10 production via STAT3

The outcome of the disease visceral leishmaniasis (VL), caused by Leishmania donovani (LD), largely relies on the relative dominance of host-protective type-1 T helper (Th1) cell response versus disease-promoting type-2 T helper (Th2) cell response. The Th1 and Th2 responses, in turn, are believed to be elicited by type-1 conventional dendritic cells (cDC1) and type-2 conventional DCs (cDC2), respectively. However, it is still unknown which DC subtype (cDC1 or cDC2) predominates during chronic LD infection and the molecular mechanism governing such occurrence. Here we report that in chronically infected mice, the splenic cDC1-cDC2 balance shifted toward the cDC2 subtype and that the receptor T cell immunoglobulin and mucin protein-3 (TIM-3) expressed by DCs played a key role in mediating this effect. Transfer of TIM-3-silenced DCs in fact prevented the predominance of the cDC2 subtype in mice with chronic LD infection. We also found that LD actually upregulated TIM-3 expression on DCs by triggering a TIM-3-mediated signaling pathway STAT3 (signal transducer and activator of transcription 3)→interleukin (IL)-10→c-Src→transcription factors Ets1, Ets2, USF1, and USF2. Notably, TIM-3 promoted STAT3 activation via a non-receptor tyrosine kinase Btk. Adoptive transfer experiments further demonstrated a critical role for STAT3-driven TIM-3 upregulation on DCs in increasing cDC2 abundance in chronically infected mice, which ultimately aided disease pathogenesis by augmenting Th2 responses. These findings document a new immunoregulatory mechanism contributing to disease pathology during LD infection and define TIM-3 as a key mediator of this process.

Methionine- and Choline-Deficient Diet Identifies an Essential Role for DNA Methylation in Plasmacytoid Dendritic Cell Biology

Diet plays an important role in lifestyle disorders associated with the disturbed immune system. During the study of methionine- and choline-deficient diet-induced nonalcoholic fatty liver disease, we observed a specific decrease in the plasmacytoid dendritic cell (pDC) fraction from murine spleens. While delineating the role for individual components, we identified that l-methionine supplementation correlates with representation of the pDC fraction. S-adenosylmethionine (SAM) is a key methyl donor, and we demonstrate that supplementation of methionine-deficient medium with SAM but not homocysteine reverses the defect in pDC development. l-Methionine has been implicated in maintenance of methylation status in the cell. Based on our observed effect of SAM and zebularine on DC subset development, we sought to clarify the role of DNA methylation in pDC biology. Whole-genome bisulfite sequencing analysis from the splenic DC subsets identified that pDCs display differentially hypermethylated regions in comparison with classical DC (cDC) subsets, whereas cDC1 and cDC2 exhibited comparable methylated regions, serving as a control in our study. We validated differentially methylated regions in the sorted pDC, CD8α⁺ cDC1, and CD4⁺ cDC2 subsets from spleens as well as FL-BMDC cultures. Upon analysis of genes linked with differentially methylated regions, we identified that differential DNA methylation is associated with the MAPK pathway such that its inhibition guides DC development toward the pDC subtype. Overall, our study identifies an important role for methionine in pDC biology.

Host T cell dedifferentiation effects drive HIV-1 latency stability

The development of therapies to eliminate the latent HIV-1 reservoir is hampered by our incomplete understanding of the biomolecular mechanism governing HIV-1 latency. To further complicate matters, recent single cell RNA-seq studies reported extensive heterogeneity between latently HIV-1-infected primary T cells, implying that latent HIV-1 infection can persist in greatly differing host cell environments. We here show that transcriptomic heterogeneity is also found between latently infected T cell lines, which allowed us to study the underlying mechanisms of intercell heterogeneity at high signal resolution. Latently infected T cells exhibited a de-differentiated phenotype, characterized by the loss of T cell-specific markers and gene regulation profiles reminiscent of hematopoietic stem cells (HSC). These changes had functional consequences. As reported for stem cells, latently HIV-1 infected T cells efficiently forced lentiviral superinfections into a latent state and favored glycolysis. As a result, metabolic reprogramming or cell re-differentiation destabilized latent infection. Guided by these findings, data-mining of single cell RNA-seq data of latently HIV-1 infected primary T cells from patients revealed the presence of similar dedifferentiation motifs. >20% of the highly detectable genes that were differentially regulated in latently infected cells were associated with hematopoietic lineage development (e.g. HUWE1, IRF4, PRDM1, BATF3, TOX, ID2, IKZF3, CDK6) or were hematopoietic markers (SRGN; hematopoietic proteoglycan core protein). The data add to evidence that the biomolecular phenotype of latently HIV-1 infected cells differs from normal T cells and strategies to address their differential phenotype need to be considered in the design of therapeutic cure interventions. IMPORTANCE HIV-1 persists in a latent reservoir in memory CD4 T cells for the lifetime of a patient. Understanding the biomolecular mechanisms used by the host cells to suppress viral expression will provide essential insights required to develop curative therapeutic interventions. Unfortunately, our current understanding of these control mechanisms is still limited. By studying gene expression profiles, we demonstrated that latently HIV-1-infected T cells have a de-differentiated T cell phenotype. Software-based data integration allowed for the identification of drug targets that would re-differentiate viral host cells and, in extension, destabilize latent HIV-1 infection events. The importance of the presented data lies within the clear demonstration that HIV-1 latency is a host cell phenomenon. As such, therapeutic strategies must first restore proper host cell functionality to accomplish efficient HIV-1 reactivation.

IRF8 and BATF3 interaction enhances the cDC1 specific Pfkfb3 gene expression

Dendritic cells (DCs) play central role in innate as well as adaptive immune responses regulated by diverse DC subtypes that vary in terms of surface markers, transcriptional profile and functional responses. Generation of DC diversity from progenitor stage is tightly regulated by complex molecular inter-play between transcription factors. We earlier demonstrated that Batf3 and Id2 expression have a synergistic effect on the Irf8 directed classical cDC1 development. In present study, Bi-molecular fluorescence complementation assay suggested that IRF8 interacts with BATF3, and ID2 may aid cDC1 development independently. Genome wide recruitment analysis of IRF8 and BATF3 from different DC subtypes led to identification of the overlapping regions of occupancy by these two transcription factors. Further analysis of overlapping peaks of IRF8 and BATF3 occupancy in promoter region within the cDC1 subtype specific transcriptional pattern identified a metabolically important Pfkfb3 gene. Among various immune cell types; splenic cDC1 subtype displayed enhanced expression of Pfkfb3. Analysis of Irf8^-/-, Irf8^R294C and Batf3^DCKO DC confirmed direct regulation of Pfkfb3 enhanced expression specifically in cDC1 subtype. Further we show that inhibition of PFKFB3 enzymatic activity by a chemical agent PFK15 led to reduction in cDC1 subtype in both in vitro FLDC cultures as well as in vivo mouse spleens. Together, our study identified the direct regulation of cDC1 specific enhanced expression of Pfkfb3 in glycolysis and cDC1 biology.

Mutation in Irf8 Gene (Irf8R294C ) Impairs Type I IFN-Mediated Antiviral Immune Response by Murine pDCs

Plasmacytoid dendritic cells (pDCs) are the key producers of type I interferons (IFNs), thus playing a central role in initiating antiviral immune response. Besides robust type I IFN production, pDCs also act as antigen presenting cells post immunogenic stimulation. Transcription factor Irf8 is indispensable for the development of both pDC and cDC1 subset. However, the mechanism underlying the differential regulation by IRF8 in cDC1- and pDC-specific genomic architecture of developmental pathways still remains to be fully elucidated. Previous studies indicated that the Irf8^R294C mutation specifically abrogates development of cDC1 without affecting that of pDC. In the present study using RNA-seq based approach, we have found that though the point mutation Irf8^R294C did not affect pDC development, it led to defective type I IFN production, thus resulting in inefficient antiviral response. This observation unraveled the distinctive roles of IRF8 in these two subpopulations—regulating the development of cDC1 whereas modulating the functionality of pDCs without affecting development. We have reported here that Irf8^R294C mutation also caused defect in production of ISGs as well as defective upregulation of costimulatory molecules in pDCs in response to NDV infection (or CpG stimulation). Through in vivo studies, we demonstrated that abrogation of type I IFN production was concomitant with reduced upregulation of costimulatory molecules in pDCs and increased NDV burden in IRF8^R294C mice in comparison with wild type, indicating inefficient viral clearance. Further, we have also shown that Irf8^R294C mutation abolished the activation of type I IFN promoter by IRF8, justifying the low level of type I IFN production. Taken together, our study signifies that the single point mutation in Irf8, Irf8^R294C severely compromised type I IFN-mediated immune response by murine pDCs, thereby causing impairment in antiviral immunity.

Lack of Interferon Regulatory Factor 8 Associated with Restricted IFN-gamma expression Augmented Japanese Encephalitis Virus Replication in the Mouse Brain

Interferon Regulatory Factor 8 (IRF8), a myeloid lineage transcription factor, emerges as an essential regulator for microglia activation. However, the precise role of IRF8 during Japanese encephalitis virus (JEV) infection in the brain remains elusive. Here we report that JEV infection enhances IRF8 expression in the infected mice brain. Comparative transcriptional profiling of whole-brain RNA analysis and validation by qRT-PCR reveals an impaired IFNγ and related gene expression in Irf8 knockout (Irf8^-/-) infected mice. Further, Ifnγ knockout (Ifnγ^-/-) mice exhibit a reduced level of Irf8. Both Ifnγ^-/- and Irf8^-/- mice exhibit significantly reduced levels of activated (CD11b⁺CD45^hi, CD11b⁺CD45^lo, Cd68, and CD86⁾ and infiltrating immune cells (Ly6C⁺, CD4, and CD8) in the infected brain as compared to WT mice. However, a higher level of granulocyte cells (Ly6G⁺) infiltration is evident in Irf8^-/- mice and the increased concentration of TNFα, IL6, MCP1 levels in the brain. Interestingly, neither Irf8^-/- nor Ifnγ^-/- has conferred protection against lethal JEV challenge to mice and exhibits augmentation in JEV replication in the brain. The gain of function of Irf8 by overexpressing functional IRF8 in an IRF8 deficient cell line attenuates viral replication and enhances IFNγ production. Overall, we summarise that in the murine model of JEV encephalitis, IRF8 modulation affects JEV replication. We also evidence that lack of Irf8 affects immune cells abundance in circulation and the infected brain leading to a reduction in IFNγ level and increased viral load in the brain. Importance Microglial cells, the resident macrophages in the brain, play a vital role in Japanese encephalitis virus (JEV) pathogenesis. The deregulated activity of microglia can be lethal for the brain. Therefore, it is crucial to understand the regulators that drive microglia's phenotype changes and induce inflammation in the brain. Interferon regulatory factor 8 (IRF8) is a myeloid lineage transcription factor involved in microglial activation. However, the impact of IRF8 modulation on JEV replication remains elusive. Moreover, the pathways regulated by IRF8 to initiate and amplify pathological neuroinflammation are not well understood. Here, we demonstrated the effect of IRF8 modulation on JEV replication, microglial activation, and immune cells infiltration in the brain.

Unexplored horizons of cDC1 in immunity and tolerance

Dendritic cells are a specialized subset of hematopoietic cells essential for mounting immunity against tumors and infectious disease as well as inducing tolerance for maintenance of homeostasis. DCs are equipped with number of immunoregulatory or stimulatory molecules that interact with other leukocytes to modulate their functions. Recent advances in DC biology identified a specific role for the conventional dendritic cell type 1 (cDC1) in eliciting cytotoxic CD8+ T cells essential for clearance of tumors and infected cells. The critical role of this subset in eliciting immune responses or inducing tolerance has largely been defined in mice whereas the biology of human cDC1 is poorly characterized owing to their extremely low frequency in tissues. A detailed characterization of the functions of many immunoregulatory and stimulatory molecules expressed by human cDC1 is critical for understanding their biology to exploit this subset for designing novel therapeutic modalities against cancer, infectious disease and autoimmune disorders.

Merocytic Dendritic Cells Compose a Conventional Dendritic Cell Subset with Low Metabolic Activity

Conventional dendritic cells (cDCs) are arguably the most potent APCs that induce the activation of naive T cells in response to pathogens. In addition, at steady-state, cDCs help maintain immune tolerance. Two subsets of cDCs have been extensively characterized, namely cDC1 and cDC2, each contributing differently to immune responses. Recently, another dendritic cell (DC) subset, termed merocytic DCs (mcDCs), was defined. In contrast to both cDC1 and cDC2, mcDCs reverse T cell anergy, properties that could be exploited to potentiate cancer treatments. Yet, whether mcDCs represent an unconventional DC or a cDC subset remains to be defined. In this article, we further characterize mcDCs and find that they bear true characteristics of cDC subsets. Indeed, as for cDCs, mcDCs express the cDC-restricted transcription factor Zbtb46 and display very potent APC activity. In addition, mcDC population dynamics parallels that of cDC1 and cDC2 in both reconstitution kinetic studies and parabiotic mice. We next investigated their relatedness to cDC1 and cDC2 and demonstrate that mcDCs are not dependent on cDC1-related Irf8 and Batf3 transcription factors, are dependent on Irf4, a cDC2-specific transcription factor, and express a unique transcriptomic signature. Finally, we find that cDC1, cDC2, and mcDCs all present with different metabolic phenotypes, in which mcDCs exhibit the lowest glucose uptake activity and mcDC survival is the least affected by glycolysis inhibition. Defining the properties of mcDCs in mice may help identify a functionally equivalent subset in humans leading to the development of innovative cancer immunotherapies.

Inflammatory Type 2 cDCs Acquire Features of cDC1s and Macrophages to Orchestrate Immunity to Respiratory Virus Infection

The phenotypic and functional dichotomy between IRF8⁺ type 1 and IRF4⁺ type 2 conventional dendritic cells (cDC1s and cDC2s, respectively) is well accepted; it is unknown how robust this dichotomy is under inflammatory conditions, when additionally monocyte-derived cells (MCs) become competent antigen-presenting cells (APCs). Using single-cell technologies in models of respiratory viral infection, we found that lung cDC2s acquired expression of the Fc receptor CD64 shared with MCs and of IRF8 shared with cDC1s. These inflammatory cDC2s (inf-cDC2s) were superior in inducing CD4⁺ T helper (Th) cell polarization while simultaneously presenting antigen to CD8⁺ T cells. When carefully separated from inf-cDC2s, MCs lacked APC function. Inf-cDC2s matured in response to cell-intrinsic Toll-like receptor and type 1 interferon receptor signaling, upregulated an IRF8-dependent maturation module, and acquired antigens via convalescent serum and Fc receptors. Because hybrid inf-cDC2s are easily confused with monocyte-derived cells, their existence could explain why APC functions have been attributed to MCs.

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Type I interferon drives differentiation of inf-cDC2s that closely resemble MCs

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Inf-cDC2s prime CD4⁺ and CD8⁺ T cells, whereas MCs lack APC function

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Inf-cDC2s internalize antibody-complexed antigen via Fc receptors

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IRF8 controls maturation gene module in inf-cDC2s

Insights from Patients with Dendritic Cell Immunodeficiency

Dendritic Cells (DCs), derived from haematopoietic stem cells, are critical to the dynamic and balanced functioning of the intact immune system and are of great interest as vehicles of immunotherapy. Genetically modified mouse models have proved powerful tools to map DC development and function in vivo but human studies have previously relied heavily on in vitro systems. Human dendritic cell immunodeficiency, resulting from single gene mutations, offers new opportunities to dissect the role of human DCs in vivo, determine the genetic requirements for their development and map their haematopoietic differentiation pathways. This review will summarise the clinical phenotypes of mutations in GATA2, IRF8 and IKZF1 genes which result in global or subset specific dendritic cell deficiencies, discuss the functional consequences of these cytopenias and how these syndromes have informed our knowledge of DC differentiation and human haematopoiesis.

RelB suppresses type I Interferon signaling in dendritic cells

Type I Interferon (IFN) signaling plays a critical role in dendritic cell (DC) development and functions. Inhibition of hyper type I IFN signaling promotes cDC2 subtype development. Relb is essential to development of cDC2 subtype and here we analyzed its effect on type I IFN signaling in DCs. We show that Relb suppresses the homeostatic type I IFN signaling in cDC2 cultures. TLR stimulation of FL-DCs led to RelB induction coinciding with fall in IFN signatures; conforming with the observation Relb expression reduced TLR stimulated IFN induction along with decrease in ISGs. Towards understanding mechanism, we show that effects of RelB are mediated by increased levels of IκBα. We demonstrate that RelB dampened antiviral responses by lowering ISG levels and the defect in cDC2 development in RelB null mice can be rescued in Ifnar1^-/- background. Overall, we propose a novel role of RelB as a negative regulator of the type I IFN signaling pathway; fine tuning development of cDC2 subtype.

Transcriptional control of dendritic cell development and functions

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

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.

Cell-autonomous FLT3L shedding via ADAM10 mediates conventional dendritic cell development in mouse spleen

Conventional dendritic cells (cDCs) derive from bone marrow (BM) precursors that undergo cascades of developmental programs to terminally differentiate in peripheral tissues. Pre-cDC1s and pre-cDC2s commit in the BM to each differentiate into CD8α⁺/CD103⁺ cDC1s and CD11b⁺ cDC2s, respectively. Although both cDCs rely on the cytokine FLT3L during development, mechanisms that ensure cDC accessibility to FLT3L have yet to be elucidated. Here, we generated mice that lacked a disintegrin and metalloproteinase (ADAM) 10 in DCs (Itgax-cre × Adam10-fl/fl; ADAM10^∆DC) and found that ADAM10 deletion markedly impacted splenic cDC2 development. Pre-cDC2s accumulated in the spleen with transcriptomic alterations that reflected their inability to differentiate and exhibited abrupt failure to survive as terminally differentiated cDC2s. Induced ADAM10 ablation also led to the reduction of terminally differentiated cDC2s, and restoration of Notch signaling, a major pathway downstream of ADAM10, only modestly rescued them. ADAM10^∆DC BM failed to generate cDC2s in BM chimeric mice with or without cotransferred ADAM10-sufficient BM, indicating that cDC2 development required cell-autonomous ADAM10. We determined cDC2s to be sources of soluble FLT3L, as supported by decreased serum FLT3L concentration and the retention of membrane-bound FLT3L on cDC2 surfaces in ADAM10^∆DC mice, and by demonstrating the release of soluble FLT3L by cDC2 in ex vivo culture supernatants. Through in vitro studies utilizing murine embryonic fibroblasts, we determined FLT3L to be a substrate for ADAM10. These data collectively reveal cDC2s as FLT3L sources and highlight a cell-autonomous mechanism that may enhance FLT3L accessibility for cDC2 development and survival.

Allograft Inflammatory Factor-1 Governs Hematopoietic Stem Cell Differentiation Into cDC1 and Monocyte-Derived Dendritic Cells Through IRF8 and RelB in vitro

The multistep differentiation process from hematopoietic stem cells through common myeloid progenitors into committed dendritic cell (DC) subsets remains to be fully addressed. These studies now show that Allograft Inflammatory Factor-1 (AIF1) is required for differentiation of classical DC type 1 (cDC1) subsets and monocyte-derived DC (Mo-DC). Phenotypic studies found that AIF1 expression increased in committed subsets differentiating from common myeloid progenitors (CMP). However, silencing AIF1 expression in hematopoietic stem progenitors restrained the capacity to differentiate into Mo-DC and cDC1 cell subsets under GM-CSF or Flt3-L stimuli conditions, respectively. This was further marked by restrained expression of IRF8, which is critical for development of Mo-DC and cDC1 subsets. As a result, absence of AIF1 restrained the cells at the Lin⁻CD117⁺FcγR⁻CD34⁺ CMP stage. Further biochemical studies revealed that abrogating AIF1 resulted in inhibition of the NFκB family member RelB expression and p38 MAPK phosphorylation during differentiation of Mo-DC. Lastly, protein binding studies identified that AIF1 interacts with protein kinase C (PKC) to influence downstream signaling pathways. Taken together, this is the first report showing a novel role of AIF1 as a calcium-responsive scaffold protein that supports IRF8 expression and interacts with PKC to drive NFκB-related RelB for successfully differentiating hematopoietic progenitor cells into cDC and Mo-DC subsets under Flt3-L and GM-CSF stimuli, respectively.

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

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

Ikaros family zinc finger 1 regulates dendritic cell development and function in humans

Ikaros family zinc finger 1 (IKZF1) is a haematopoietic transcription factor required for mammalian B-cell development. IKZF1 deficiency also reduces plasmacytoid dendritic cell (pDC) numbers in mice, but its effects on human DC development are unknown. Here we show that heterozygous mutation of IKZF1 in human decreases pDC numbers and expands conventional DC1 (cDC1). Lenalidomide, a drug that induces proteosomal degradation of IKZF1, also decreases pDC numbers in vivo, and reduces the ratio of pDC/cDC1 differentiated from progenitor cells in vitro in a dose-dependent manner. In addition, non-classical monocytes are reduced by IKZF1 deficiency in vivo. DC and monocytes from patients with IKZF1 deficiency or lenalidomide-treated cultures secrete less IFN-α, TNF and IL-12. These results indicate that human DC development and function are regulated by IKZF1, providing further insights into the consequences of IKZF1 mutation on immune function and the mechanism of immunomodulation by lenalidomide.

IKZF1 is a transcription factor known to regulate mammalian B-cell development. Here the authors show that IKZF1 is required for human pDC development and regulation of DC cytokine production in patients with IKZF1 haploinsufficiency, findings which are recapitulated in lenalidomide-induced IKZF1 deficiency.

Human dendritic cell immunodeficiencies

The critical functions of dendritic cells (DCs) in immunity and tolerance have been demonstrated in many animal models but their non-redundant roles in humans are more difficult to probe. Human primary immunodeficiency (PID), resulting from single gene mutations, may result in DC deficiency or dysfunction. This relatively recent recognition illuminates the in vivo role of human DCs and the pathophysiology of the associated clinical syndromes. In this review, the development and function of DCs as established in murine models and human in vitro systems, discussed. This forms the basis of predicting the effects of DC deficiency in vivo and understanding the consequences of specific mutations on DC development and function. DC deficiency syndromes are associated with heterozygous GATA2 mutation, bi-allelic and heterozygous IRF8 mutation and heterozygous IKZF1 mutation. The intricate involvement of DCs in the balance between immunity and tolerance is leading to increased recognition of their involvement in a number of other immunodeficiencies and autoimmune conditions. Owing to the precise control of transcription factor gene expression by super-enhancer elements, phenotypic anomalies are relatively commonly caused by heterozygous mutations.

Transcriptional patterns associated with BDCA3 expression on BDCA1+ myeloid dendritic cells

Myeloid dendritic cells, including BDCA3^hi DCs and BDCA1⁺ DCs (hereafter dubbed DC1 and DC2 for clarity), play a pivotal role in the induction and regulation of immune responses. Interestingly, a fraction of DC2 also express low to intermediate levels of BDCA3. It is unknown whether BDCA3⁺ DC2 also share other traits with DC1 that are absent in BDCA3^- DC2 and/or whether BDCA3 expression renders DC2 functionally distinct from their BDCA3-lacking counterparts. Here, we used expression analysis on a predefined set of immunology-related genes to determine divergence between BDCA3-positive and BDCA3-negative DC2 and their relation to bona fide BDCA3^hi DC1. Results showed that mRNA fingerprints of BDCA3⁺ DC2 and BDCA3^- DC2 are very similar, and clearly distinct from that of DC1. Differences in mRNA expression, however, were observed between BDCA3⁺ DC2 and BDCA3^- DC2 that pointed toward a more activated status of BDCA3⁺ DC2. In line with this, higher steady state maturation marker expression and TLR-induced maturation marker expression and inflammatory cytokine production by BDCA3⁺ DC2 were observed. This dataset provides insight into the relationship between myeloid DC populations and contributes to further understanding of DC immunobiology.

Interleukin-12 from CD103+ Batf3-Dependent Dendritic Cells Required for NK-Cell Suppression of Metastasis

Several host factors may affect the spread of cancer to distant organs; however, the intrinsic role of dendritic cells (DC) in controlling metastasis is poorly described. Here, we show in several tumor models that although the growth of primary tumors in Batf3-deficient mice, which lack cross-presenting DCs, was not different from primary tumors in wild-type (WT) control mice, Batf3-deficient mice had increased experimental and spontaneous metastasis and poorer survival. The increased metastasis was independent of CD4⁺ and CD8⁺ T lymphocytes, but required NK cells and IFNγ. Chimeric mice in which Batf3-dependent DCs uniformly lacked the capacity to produce IL12 had metastatic burdens similar to the Batf3-deficient mice, suggesting that Batf3⁺ DCs were the only cell type whose IL12 production was critical for controlling metastasis. We found that IL12-YFP reporter mice, whose lungs were injected with B16F10 melanoma, had increased numbers of IL12-expressing CD103⁺ DCs with enhanced CD86 expression. Bone-marrow-derived DCs from WT, but not Batf3-deficient, mice activated NK cells to produce IFNγ in an IL12-dependent manner and therapeutic injection of recombinant mouse IL12 decreased metastasis in both WT and Batf3-deficient mice. Analysis of TCGA datasets revealed an association between high expression of BATF3 and IRF8 and improved survival of breast cancer patients; BATF3 expression also significantly correlated with NK-cell receptor genes, IL12, and IFNG Collectively, our findings show that IL12 from CD103⁺ DCs is critical for NK cell-mediated control of tumor metastasis. Cancer Immunol Res; 5(12); 1098-108. ©2017 AACR.

Cutting Edge: ACVRL1 Signaling Augments CD8α+ Dendritic Cell Development

Dendritic cells (DCs) are a collection of different subtypes, each of which is characterized by specific surface markers, gene-expression patterns, and distinct functions. Members of the IFN regulatory factor family play critical roles in DC development and functions. Recently, Irf8 was shown to activate TGF-β signaling, which led to exacerbated neuroinflammation in the experimental autoimmune encephalomyelitis mouse model. We analyzed the effect of Irf8 on TGF-β/bone morphogenetic protein pathway-specific genes in DCs and identified Acvrl1, a type I TGF-β superfamily receptor, as a gene strongly induced by Irf8 expression. Among various DC subtypes, Acvrl1 is differentially expressed in CD8α(+) DCs. ACVRL1 signaling augmented Irf8-directed classical CD8α(+) DC development. Irf8 expression is essential for plasmacytoid DC and CD8α(+) DC development, and this study demonstrates that ACVRL1 signaling plays a pivotal role whereby it suppresses plasmacytoid DC development while enhancing that of CD8α(+) DCs, thus contributing to DC diversity development.

Comparative genomics analysis of mononuclear phagocyte subsets confirms homology between lymphoid tissue-resident and dermal XCR1(+) DCs in mouse and human and distinguishes them from Langerhans cells

Dendritic cells (DC) are mononuclear phagocytes which exhibit a branching (dendritic) morphology and excel at naïve T cell activation. DC encompass several subsets initially identified by their expression of cell surface molecules and later shown to possess distinct functions. DC subset differentiation is orchestrated by transcription factors, growth factors and cytokines. Identifying DC subsets is challenging as very few cell surface molecules are uniquely expressed on any one of these cell populations. There is no standard consensus to identify mononuclear phagocyte subsets; varying antigens are employed depending on the tissue and animal species studied and between laboratories. This has led to confusion in how to accurately define and classify DCs across tissues and between species. Here we report a comparative genomics strategy that enables universal definition of DC and other mononuclear phagocyte subsets across species. We performed a meta-analysis of several public datasets of human and mouse mononuclear phagocyte subsets isolated from blood, spleen, skin or cutaneous lymph nodes, including by using a novel and user friendly software, BubbleGUM, which generates and integrates gene signatures for high throughput gene set enrichment analysis. This analysis demonstrates the equivalence between human and mouse skin XCR1(+) DCs, and between mouse and human Langerhans cells.

Redefining Myeloid Cell Subsets in Murine Spleen

Spleen is known to contain multiple dendritic and myeloid cell subsets, distinguishable on the basis of phenotype, function and anatomical location. As a result of recent intensive flow cytometric analyses, splenic dendritic cell (DC) subsets are now better characterized than other myeloid subsets. In order to identify and fully characterize a novel splenic subset termed "L-DC" in relation to other myeloid cells, it was necessary to investigate myeloid subsets in more detail. In terms of cell surface phenotype, L-DC were initially characterized as a CD11b(hi)CD11c(lo)MHCII(-)Ly6C(-)Ly6G(-) subset in murine spleen. Their expression of CD43, lack of MHCII, and a low level of CD11c was shown to best differentiate L-DC by phenotype from conventional DC subsets. A complete analysis of all subsets in spleen led to the classification of CD11b(hi)CD11c(lo)MHCII(-)Ly6C(lo)Ly6G(-) cells as monocytes expressing CX3CR1, CD43 and CD115. Siglec-F expression was used to identify a specific eosinophil population, distinguishable from both Ly6C(lo) and Ly6C(hi) monocytes, and other DC subsets. L-DC were characterized as a clear subset of CD11b(hi)CD11c(lo)MHCII(-)Ly6C(-)Ly6G(-) cells, which are CD43(+), Siglec-F(-) and CD115(-). Changes in the prevalence of L-DC compared to other subsets in spleens of mutant mice confirmed the phenotypic distinction between L-DC, cDC and monocyte subsets. L-DC development in vivo was shown to occur independently of the BATF3 transcription factor that regulates cDC development, and also independently of the FLT3L and GM-CSF growth factors which drive cDC and monocyte development, so distinguishing L-DC from these commonly defined cell types.

Efficient vaccine against pandemic influenza: combining DNA vaccination and targeted delivery to MHC class II molecules

There are two major limitations to vaccine preparedness in the event of devastating influenza pandemics: the time needed to generate a vaccine and rapid generation of sufficient amounts. DNA vaccination could represent a solution to these problems, but efficacy needs to be enhanced. In a separate line of research, it has been established that targeting of vaccine molecules to antigen-presenting cells enhances immune responses. We have combined the two principles by constructing DNA vaccines that encode bivalent fusion proteins; these target hemagglutinin to MHC class II molecules on antigen-presenting cells. Such DNA vaccines rapidly induce hemagglutinin-specific antibodies and T cell responses in immunized mice. Responses are long-lasting and protect mice against challenge with influenza virus. In a pandemic situation, targeted DNA vaccines could be produced and tested within a month. The novel DNA vaccines could represent a solution to pandemic preparedness in the advent of novel influenza pandemics.

IFN regulatory factor 8 represses GM-CSF expression in T cells to affect myeloid cell lineage differentiation

During hematopoiesis, hematopoietic stem cells constantly differentiate into granulocytes and macrophages via a distinct differentiation program that is tightly controlled by myeloid lineage-specific transcription factors. Mice with a null mutation of IFN regulatory factor 8 (IRF8) accumulate CD11b(+)Gr1(+) myeloid cells that phenotypically and functionally resemble tumor-induced myeloid-derived suppressor cells (MDSCs), indicating an essential role of IRF8 in myeloid cell lineage differentiation. However, IRF8 is expressed in various types of immune cells, and whether IRF8 functions intrinsically or extrinsically in regulation of myeloid cell lineage differentiation is not fully understood. In this study, we report an intriguing finding that, although IRF8-deficient mice exhibit deregulated myeloid cell differentiation and resultant accumulation of CD11b(+)Gr1(+) MDSCs, surprisingly, mice with IRF8 deficiency only in myeloid cells exhibit no abnormal myeloid cell lineage differentiation. Instead, mice with IRF8 deficiency only in T cells exhibited deregulated myeloid cell differentiation and MDSC accumulation. We further demonstrated that IRF8-deficient T cells exhibit elevated GM-CSF expression and secretion. Treatment of mice with GM-CSF increased MDSC accumulation, and adoptive transfer of IRF8-deficient T cells, but not GM-CSF-deficient T cells, increased MDSC accumulation in the recipient chimeric mice. Moreover, overexpression of IRF8 decreased GM-CSF expression in T cells. Our data determine that, in addition to its intrinsic function as an apoptosis regulator in myeloid cells, IRF8 also acts extrinsically to repress GM-CSF expression in T cells to control myeloid cell lineage differentiation, revealing a novel mechanism that the adaptive immune component of the immune system regulates the innate immune cell myelopoiesis in vivo.

Plasmacytoid dendritic cells: development, functions, and role in atherosclerotic inflammation

Plasmacytoid dendritic cells (pDCs) are a specialized subset of DCs that links innate and adaptive immunity. They sense viral and bacterial pathogens and release high levels of Type I interferons (IFN-I) in response to infection. pDCs were shown to contribute to inflammatory responses in the steady state and in pathology. In atherosclerosis, pDCs are involved in priming vascular inflammation and atherogenesis through production of IFN-I and chemokines that attract inflammatory cells to inflamed sites. pDCs also contribute to the proinflammatory activation of effector T cells, cytotoxic T cells, and conventional DCs. However, tolerogenic populations of pDCs are found that suppress atherosclerosis-associated inflammation through down-regulation of function and proliferation of proinflammatory T cell subsets and induction of regulatory T cells with potent immunomodulatory properties. Notably, atheroprotective tolerogenic DCs could be induced by certain self-antigens or bacterial antigens that suggests for great therapeutic potential of these DCs for development of DC-based anti-atherogenic vaccines.

Border maneuvers: deployment of mucosal immune defenses against Toxoplasma gondii

Toxoplasma gondii is a highly prevalent protozoan pathogen that is transmitted through oral ingestion of infectious cysts. As such, mucosal immune defenses in the intestine constitute the first and arguably most important line of resistance against the parasite. The response to infection is now understood to involve complex three-way interactions between Toxoplasma, the mucosal immune system, and the host intestinal microbiota. Productive outcome of these interactions ensures resolution of infection in the intestinal mucosa. Nonsuccessful outcome may result in emergence of proinflammatory damage that can spell death for the host. Here, we discuss new advances in our understanding of the mechanisms underpinning these disparate outcomes, with particular reference to initiators, effectors, and regulators of mucosal immunity stimulated by Toxoplasma in the intestine.

Dendritic cell-targeted vaccines

Despite significant effort, the development of effective vaccines inducing strong and durable T-cell responses against intracellular pathogens and cancer cells has remained a challenge. The initiation of effector CD8⁺ T-cell responses requires the presentation of peptides derived from internalized antigen on class I major histocompatibility complex molecules by dendritic cells (DCs) in a process called cross-presentation. A current strategy to enhance the effectiveness of vaccination is to deliver antigens directly to DCs. This is done via selective targeting of antigen using monoclonal antibodies directed against endocytic receptors on the surface of the DCs. In this review, we will discuss considerations relevant to the design of such vaccines: the existence of DC subsets with specialized functions, the impact of the antigen intracellular trafficking on cross-presentation, and the influence of maturation signals received by DCs on the outcome of the immune response.