Transitional dendritic cells are distinct from conventional DC2 precursors and mediate proinflammatory antiviral responses.

High-dimensional approaches have revealed heterogeneity amongst dendritic cells (DCs), including a population of transitional DCs (tDCs) in mice and humans. However, the origin and relationship of tDCs to other DC subsets has been unclear. Here we show that tDCs are distinct from other well-characterized DCs and conventional DC precursors (pre-cDCs). We demonstrate that tDCs originate from bone marrow progenitors shared with plasmacytoid DCs (pDCs). In the periphery, tDCs contribute to the pool of ESAM+ type 2 DCs (DC2s), and these DC2s have pDC-related developmental features. Different from pre-cDCs, tDCs have less turnover, capture antigen, respond to stimuli and activate antigen-specific naïve T cells, all characteristics of differentiated DCs. Different from pDCs, viral sensing by tDCs results in IL-1ß secretion and fatal immune pathology in a murine coronavirus model. Our findings suggest that tDCs are a distinct pDC-related subset with a DC2 differentiation potential and unique proinflammatory function during viral infections.

Ablation of cDC2 development by triple mutations within the Zeb2 enhancer.

The divergence of the common dendritic cell progenitor1-3 (CDP) into the conventional type 1 and type 2 dendritic cell (cDC1 and cDC2, respectively) lineages4,5 is poorly understood. Some transcription factors act in the commitment of already specified progenitors-such as BATF3, which stabilizes Irf8 autoactivation at the +32 kb Irf8 enhancer4,6-but the mechanisms controlling the initial divergence of CDPs remain unknown. Here we report the transcriptional basis of CDP divergence and describe the first requirements for pre-cDC2 specification. Genetic epistasis analysis7 suggested that Nfil3 acts upstream of Id2, Batf3 and Zeb2 in cDC1 development but did not reveal its mechanism or targets. Analysis of newly generated NFIL3 reporter mice showed extremely transient NFIL3 expression during cDC1 specification. CUT&RUN and chromatin immunoprecipitation followed by sequencing identified endogenous NFIL3 binding in the -165 kb Zeb2 enhancer8 at three sites that also bind the CCAAT-enhancer-binding proteins C/EBPα and C/EBPß. In vivo mutational analysis using CRISPR-Cas9 targeting showed that these NFIL3-C/EBP sites are functionally redundant, with C/EBPs supporting and NFIL3 repressing Zeb2 expression at these sites. A triple mutation of all three NFIL3-C/EBP sites ablated Zeb2 expression in myeloid, but not lymphoid progenitors, causing the complete loss of pre-cDC2 specification and mature cDC2 development in vivo. These mice did not generate T helper 2 (TH2) cell responses against Heligmosomoides polygyrus infection, consistent with cDC2 supporting TH2 responses to helminths9-11. Thus, CDP divergence into cDC1 or cDC2 is controlled by competition between NFIL3 and C/EBPs at the -165 kb Zeb2 enhancer.

Human Conventional and Plasmacytoid Dendritic Cells Differ in Their Ability to Respond to Saccharomyces cerevisiae.

Saccharomyces cerevisiae is a commensal yeast colonizer of mucosal surfaces and an emerging opportunistic pathogen in the mucosa and bloodstream. The role of S. cerevisiae has been largely characterized in peripheral blood mononuclear cells and monocyte-derived dendritic cells, where yeast cells induce the production of inflammatory cytokines through the interaction with mannose receptors, chitin receptors, DC SIGN, and dectin1. However, the response of blood-circulating dendritic cells (DCs) to S. cerevisiae has never been investigated. Among blood DCs, conventional DCs (cDCs) are producers of inflammatory cytokines, while plasmacytoid DCs (pDCs) are a specialized population producing a large amount of interferon (IFN)-α, which is involved in the antiviral immune response. Here we report that both human DC subsets are able to sense S. cerevisiae. In particular, cDCs produce interleukin (IL)-6, express activation markers, and promotes T helper 17 cell polarization in response to yeasts, behaving similarly to monocyte-derived DCs as previously described. Interestingly, pDCs, not cDCs, sense fungal nucleic acids, leading to the generation of P1-pDCs (PD-L1+CD80-), a pDC subset characterized by the production of IFN-α and the induction of a Th profile producing IL-10. These results highlight a novel role of pDCs in response to S. cerevisiae that could be important for the regulation of the host microbiota-immune system balance and of anti-fungal immune response.

Upregulation of type 1 conventional dendritic cells implicates antigen cross-presentation in multisystem inflammatory syndrome.

BACKGROUND: Multisystem inflammatory syndrome in children (MIS-C) is an acute, febrile, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-associated syndrome, often with cardiohemodynamic dysfunction. Insight into mechanism of disease is still incomplete. OBJECTIVE: Our objective was to analyze immunologic features of MIS-C patients compared to febrile controls (FC). METHODS: MIS-C patients were defined by narrow criteria, including having evidence of cardiohemodynamic involvement and no macrophage activation syndrome. Samples were collected from 8 completely treatment-naive patients with MIS-C (SARS-CoV-2 serology positive), 3 patients with unclassified MIS-C-like disease (serology negative), 14 FC, and 5 MIS-C recovery (RCV). Three healthy controls (HCs) were used for comparisons of normal range. Using spectral flow cytometry, we assessed 36 parameters in antigen-presenting cells (APCs) and 29 in T cells. We used biaxial analysis and uniform manifold approximation and projection (UMAP). RESULTS: Significant elevations in cytokines including CXCL9, M-CSF, and IL-27 were found in MIS-C compared to FC. Classic monocytes and type 2 dendritic cells (DCs) were downregulated (decreased CD86, HLA-DR) versus HCs; however, type 1 DCs (CD11c+CD141+CLEC9A+) were highly activated in MIS-C patients versus FC, expressing higher levels of CD86, CD275, and atypical conventional DC markers such as CD64, CD115, and CX3CR1. CD169 and CD38 were upregulated in multiple monocyte subtypes. CD56dim/CD57-/KLRGhi/CD161+/CD38- natural killer (NK) cells were a unique subset in MIS-C versus FC without macrophage activation syndrome. CONCLUSION: Orchestrated by complex cytokine signaling, type 1 DC activation and NK dysregulation are key features in the pathophysiology of MIS-C. NK cell findings may suggest a relationship with macrophage activation syndrome, while type 1 DC upregulation implies a role for antigen cross-presentation.

Characterisation of dendritic cell frequency and phenotype in bovine afferent lymph reveals kinetic changes in costimulatory molecule expression.

The bovine afferent lymphatic cannulation model allows collection of large volumes of afferent lymph and provides an opportunity to study lymphatic cells trafficking from the periphery directly ex-vivo. The technique requires surgical intervention, but influence of the procedure or time post-surgery on cells trafficking in the lymph has not been well documented. Here, we measured the volume of lymph and number of cells/mL collected daily over a two week time-course. Animal to animal variability was demonstrated but no consistent changes in lymph volume or cell density were observed in relation to time post-cannulation. Cell populations (dendritic cells, αß T-cells, γδ T-cells and NK cells) were analysed by flow cytometry at 1, 3 and 10 days post-cannulation (DPC) and a reduced percentage of γδ T-cells in afferent lymph was observed at 1 DPC. In addition, cell surface molecule expression by afferent lymphatic dendritic cells (ALDC) was assessed due to the key role of these cells in initiating an adaptive immune response. Co-stimulatory molecules CD80 and CD86 were upregulated by CD172a+ve ALDC early in the time-course, suggesting that the cannulation procedure and duration of experiment may impact the activation state of DCs in the naïve host. This should be considered when analysing the response of these cells to vaccines or pathogens.

Participação dos neutrófilos durante a implantação embrionária no desenvolvimento da placenta em camundongos / Participation of neutrophils during embryonic implantation in the placental development in mice

A implantação do embrião na parede uterina é um processo complexo que consiste na interação do blastocisto com as células epiteliais do útero, e depende de diferentes tipos celulares do microambiente uterino. Embora a literatura mostre a participação de neutrófilos neste processo, os dados ainda são incipientes para proposição da função exata destas células nos períodos iniciais da gestação. Dados do nosso grupo de pesquisa mostraram que neutrófilos pró-angiogênicos induzem a tolerância gestacional, e que a depleção de neutrófilos durante as fases iniciais da gestação prejudica a implantação do blastocisto e a progressão da gestação. Com base nestes resultados, o presente estudo visou investigar se a depleção de neutrófilos na fase pré-receptiva da janela de implantação do blastocisto altera a morfologia placentária. Para tanto, foi utilizado o modelo de gestação alogênica, onde camundongos fêmeas C57BL/6, após cruzamento com machos Balb/C foram tratadas com anticorpo anti-Ly6G ou isotipo no dia 1,5 da gestação (24 horas após a detecção do plug vaginal) em dose suficiente para manter a depleção de neutrófilos circulantes por 48 horas (200µg/ 500µL; i.p). No final da gestação (dia 18,5), o sangue periférico foi coletado e, em seguida, os animais foram submetidos a laparotomia para retirada da placenta, a qual foi submetida à análise histológica. As análises dos leucócitos circulantes evidenciaram a efetividade do tratamento para depleção de neutrófilos periféricos. A análise histológica mostrou alterações significativas na morfologia da placenta nos animais tratados com anti-Ly6G. Foram detectadas a redução da zona juncional, de células trofoblásticas e de fatores angiogênicos, como fator de crescimento do endotélio vascular (VEGF), e das moléculas de adesão intracelular-1 (ICAM-1) e de plaqueta e endotélio (PECAM-1). Esses dados evidenciam a importância dos neutrófilos nos primeiros dias de gestação para o desenvolvimento da placenta

Blastocyst implantation is a complex process, consisting of the interaction between blastocyst and uterine epithelial cells. Also, it is well known that the implantation site resembles an inflammatory response, with a profusion of recruited immune cells into the endometrial stroma and lumen from the blood. The role of macrophages, natural killers, and dendritic cells have been extensively studied, however, the participation of neutrophils in this process remains unclear. Data from our research group showed that pro-angiogenic neutrophils induced gestation tolerance, also peripheral neutrophils depletion at the time of active placental development led to smaller embryo sizes and abnormal placentation in mice. In this context, the present study aimed to investigate whether pharmacological depletion of neutrophils in mice in the blastocyst implantation phase alters placental morphology. Therefore, C7/BL/6 female mice, after mating with Balb/C males, were treated with an anti-Ly6G antibody or isotype on day 1 of gestation (after detection of the vaginal plug) at a dose sufficient to maintain the depletion of circulating neutrophils for 48 hours (200 µg/500µL; i.p). At the end of the gestational day (day 18), peripheral blood was collected, and then the animals were submitted to laparotomy for the placenta removal and subsequent histological analysis. The analysis of circulating leukocytes from neutrophils depleted mice showed a reduction of peripheral neutrophils up to 48 hours after antibody injection. The histological analysis showed significant alterations in the placenta morphology of the animals treated with anti-Ly6G. The morphometric analyses showed a reduction in the size of neutrophils depleted placenta due to diminished junctional zone and reduction of trophoblast cells. Also, it was observed a reduction of vascular endothelial growth factors (VEGF), reduction of adhesion molecules intracell-1 (ICAM-1), and platelets and endothelium (PECAM-1) positive cells in the junctional zone. In conclusion, these data show the importance of neutrophils on the first days of pregnancy for the development of the placenta

Intraocular dendritic cells characterize HLA-B27-associated acute anterior uveitis.

Uveitis describes a heterogeneous group of inflammatory eye diseases characterized by infiltration of leukocytes into the uveal tissues. Uveitis associated with the HLA haplotype B27 (HLA-B27) is a common subtype of uveitis and a prototypical ocular immune-mediated disease. Local immune mechanisms driving human uveitis are poorly characterized mainly due to the limited available biomaterial and subsequent technical limitations. Here, we provide the first high-resolution characterization of intraocular leukocytes in HLA-B27-positive (n = 4) and -negative (n = 2) anterior uveitis and an infectious endophthalmitis control (n = 1) by combining single-cell RNA-sequencing with flow cytometry and protein analysis. Ocular cell infiltrates consisted primarily of lymphocytes in both subtypes of uveitis and of myeloid cells in infectious endophthalmitis. HLA-B27-positive uveitis exclusively featured a plasmacytoid and classical dendritic cell (cDC) infiltrate. Moreover, cDCs were central in predicted local cell-cell communication. This suggests a unique pattern of ocular leukocyte infiltration in HLA-B27-positive uveitis with relevance to DCs.

Uveitis is a form of inflammation in the eye. It can occur in response to infection, or when the immune system mistakenly attacks the eye, in what is known as autoimmune uveitis. In approximately 80 percent of cases, the front part of the eye is affected. During an inflammatory episode, the liquid inside the front part of the eye fills with immune cells, but the nature of these cells remains unknown. This is because uveitis is rare, and doctors cannot routinely take samples from inside the eyes of affected individuals to diagnose the disease. This lack of samples makes research into this disease challenging. There are two main groups of immune cells that could be responsible for uveitis: myeloid cells and lymphoid cells. Myeloid cells form the first line of immune defense against infection by non-specifically attacking and removing pathogens . Lymphoid cells form the second line of immune defense, attacking specific pathogens. Lymphoid cells also have long-term memory, meaning they can 'remember' previous infections and fight them more effectively. Lymphoid cells receive instructions from a type of myeloid cell called a dendritic cell about what to attack. Dendritic cells relay their instructions to lymphoid cells using molecules called human leukocyte antigens (HLA). Autoimmune uveitis affecting the front part of the eye is common in individuals with an HLA type called HLA-B27, suggesting that communication between dendritic and lymphoid cells plays an important role in this type of inflammation. To make the most of limited patient samples, Kasper et al. used single cell techniques to examine the immune cells from the fluid inside the eye. Six samples came from people with autoimmune uveitis, and one from a person with an eye infection. The infection sample contained mainly myeloid cells that might attack bacteria responsible for the infection. In contrast, the autoimmune uveitis samples contained mainly lymphoid cells. Of these samples, four were from individuals with the gene that codes for the HLA-B27 molecule. These samples had a unique pattern of immune cells, with more dendritic cells than the samples from individuals that did not have this gene. This study included only a small number of individuals, but it shows that analysing single immune cells from the eye is possible in uveitis. This snapshot could help researchers understand the local immune response in the eye, and find an optimal treatment.

Unraveling the Heterogeneity and Ontogeny of Dendritic Cells Using Single-Cell RNA Sequencing.

Dendritic cells (DCs) play essential roles in innate and adaptive immunity and show high heterogeneity and intricate ontogeny. Advances in high-throughput sequencing technologies, particularly single-cell RNA sequencing (scRNA-seq), have improved the understanding of DC subsets. In this review, we discuss in detail the remarkable perspectives in DC reclassification and ontogeny as revealed by scRNA-seq. Moreover, the heterogeneity and multifunction of DCs during diseases as determined by scRNA-seq are described. Finally, we provide insights into the challenges and future trends in scRNA-seq technologies and DC research.

Differential plasmacytoid dendritic cell phenotype and type I Interferon response in asymptomatic and severe COVID-19 infection.

SARS-CoV-2 fine-tunes the interferon (IFN)-induced antiviral responses, which play a key role in preventing coronavirus disease 2019 (COVID-19) progression. Indeed, critically ill patients show an impaired type I IFN response accompanied by elevated inflammatory cytokine and chemokine levels, responsible for cell and tissue damage and associated multi-organ failure. Here, the early interaction between SARS-CoV-2 and immune cells was investigated by interrogating an in vitro human peripheral blood mononuclear cell (PBMC)-based experimental model. We found that, even in absence of a productive viral replication, the virus mediates a vigorous TLR7/8-dependent production of both type I and III IFNs and inflammatory cytokines and chemokines, known to contribute to the cytokine storm observed in COVID-19. Interestingly, we observed how virus-induced type I IFN secreted by PBMC enhances anti-viral response in infected lung epithelial cells, thus, inhibiting viral replication. This type I IFN was released by plasmacytoid dendritic cells (pDC) via an ACE-2-indipendent but Neuropilin-1-dependent mechanism. Viral sensing regulates pDC phenotype by inducing cell surface expression of PD-L1 marker, a feature of type I IFN producing cells. Coherently to what observed in vitro, asymptomatic SARS-CoV-2 infected subjects displayed a similar pDC phenotype associated to a very high serum type I IFN level and induction of anti-viral IFN-stimulated genes in PBMC. Conversely, hospitalized patients with severe COVID-19 display very low frequency of circulating pDC with an inflammatory phenotype and high levels of chemokines and pro-inflammatory cytokines in serum. This study further shed light on the early events resulting from the interaction between SARS-CoV-2 and immune cells occurring in vitro and confirmed ex vivo. These observations can improve our understanding on the contribution of pDC/type I IFN axis in the regulation of the anti-viral state in asymptomatic and severe COVID-19 patients.

A genotype-phenotype screening system using conditionally immortalized immature dendritic cells.

Here, we describe a protocol for CRISPR/Cas9-mediated gene knockout in conditionally immortalized immature dendritic cells (DCs), which can be limitlessly expanded before differentiation. This facilitates the genetic screening of DC functions in vitro including assessment of phagocytosis, cytokine production, expression of co-stimulatory or co-inhibitory molecules, and antigen presentation, as well as evaluation of the capacity to elicit anticancer immune responses in vivo. Altogether, these approaches described in this protocol allow investigators to link the genotype of DCs to their phenotype. For complete details on the use and execution of this protocol, please refer to Le Naour et al. (2020).

Swine Dendritic Cell Response to Porcine Reproductive and Respiratory Syndrome Virus: An Update.

Dendritic cells (DCs) are the most potent antigen-presenting cells, unique to initiate and coordinate the adaptive immune response. In pigs, conventional DCs (cDCs), plasmacytoid DCs (pDCs), and monocyte-derived DCs (moDCs) have been described in blood and tissues. Different pathogens, such as viruses, could infect these cells, and in some cases, compromise their response. The understanding of the interaction between DCs and viruses is critical to comprehend viral immunopathological responses. Porcine reproductive and respiratory syndrome virus (PRRSV) is the most important respiratory pathogen in the global pig population. Different reports support the notion that PRRSV modulates pig immune response in addition to their genetic and antigenic variability. The interaction of PRRSV with DCs is a mostly unexplored area with conflicting results and lots of uncertainties. Among the scarce certainties, cDCs and pDCs are refractory to PRRSV infection in contrast to moDCs. Additionally, response of DCs to PRRSV can be different depending on the type of DCs and maybe is related to the virulence of the viral isolate. The precise impact of this virus-DC interaction upon the development of the specific immune response is not fully elucidated. The present review briefly summarizes and discusses the previous studies on the interaction of in vitro derived bone marrow (bm)- and moDCs, and in vivo isolated cDCs, pDCs, and moDCs with PRRSV1 and 2.

Development of an Inflammatory CD14+ Dendritic Cell Subset in Humanized Mice.

Humanized mouse models are attractive experimental models for analyzing the development and functions of human dendritic cells (DCs) in vivo. Although various types of DC subsets, including DC type 3 (DC3s), have been identified in humans, it remains unclear whether humanized mice can reproduce heterogeneous DC subsets. CD14, classically known as a monocyte/macrophage marker, is reported as an indicator of DC3s. We previously observed that some CD14+ myeloid cells expressed CD1c, a pan marker for bona fide conventional DC2 (cDC2s), in humanized mouse models in which human FLT3L and GM-CSF genes were transiently expressed using in vivo transfection (IVT). Here, we aimed to elucidate the identity of CD14+CD1c+ DC-like cells in humanized mouse models. We found that CD14+CD1c+ cells were phenotypically different from cDC2s; CD14+CD1c+ cells expressed CD163 but not CD5, whereas cDC2s expressed CD5 but not CD163. Furthermore, CD14+CD1c+ cells primed and polarized naïve CD4+ T cells toward IFN-γ+ Th1 cells more profoundly than cDC2s. Transcriptional analysis revealed that CD14+CD1c+ cells expressed several DC3-specific transcripts, such as CD163, S100A8, and S100A9, and were clearly segregated from cDC2s and monocytes. When lipopolysaccharide was administered to the humanized mice, the frequency of CD14+CD1c+ cells producing IL-6 and TNF-α was elevated, indicating a pro-inflammatory signature. Thus, humanized mice are able to sustain development of functional CD14+CD1c+ DCs, which are equivalent to DC3 subset observed in humans, and they could be useful for analyzing the development and function of DC3s in vivo.

Functions of Dendritic Cells and Its Association with Intestinal Diseases.

Dendritic cells (DCs), including conventional DCs (cDCs) and plasmacytoid DCs (pDCs), serve as the sentinel cells of the immune system and are responsible for presenting antigen information. Moreover, the role of DCs derived from monocytes (moDCs) in the development of inflammation has been emphasized. Several studies have shown that the function of DCs can be influenced by gut microbes including gut bacteria and viruses. Abnormal changes/reactions in intestinal DCs are potentially associated with diseases such as inflammatory bowel disease (IBD) and intestinal tumors, allowing DCs to be a new target for the treatment of these diseases. In this review, we summarized the physiological functions of DCs in the intestinal micro-environment, their regulatory relationship with intestinal microorganisms and their regulatory mechanism in intestinal diseases.

Heterogeneous populations from in vitro cultures of antigen presenting cells in pigs.

Dendritic cells (DCs) are the most potent antigen presenting cells (APCs). Because of the difficulty in obtaining these cells directly from tissues, different sources of DCs are frequently used for in vitro experimentation and many of their biological and functional characteristics were studied using these systems. Until recently, it was assumed that specific culture conditions polarized the differentiation of either DCs or macrophages (Macs); however, it was shown that some DC culture systems in other species generate heterogeneous cell populations that can be identified according to their CD11c and MHC class II (MHC-II) expression. Following this approach, porcine DCs were directly isolated from peripheral blood or differentiated in vitro by culturing bone marrow (BM) progenitor cells or blood monocytes treated with growth factors. Mostly homogeneous monocyte-derived DCs (MoDCs) were obtained with similar phenotype and phagocytic characteristics to that of blood DCs. On the contrary, BM-derived DC (BMDC) cultures generated two distinct heterogeneous populations identified as MHC-II+ and MHC-II++ cells. BMDCs MHC-II+ had similar phenotypic and phagocytic characteristics to those of MoDCs and blood DCs. However, BMDCs MHC-II++ population expressed a higher amount of surface markers and transcribed genes associated with Macs-lineage exhibiting a higher phagocytic capacity than all the other cells. Noteworthy, every cell system expressed different genetic signatures. These results will help interpreting and re-interpreting data obtained using in vitro systems.

CXCR4 signaling controls dendritic cell location and activation at steady state and in inflammation.

Dendritic cells (DCs) encompass several cell subsets that collaborate to initiate and regulate immune responses. Proper DC localization determines their function and requires the tightly controlled action of chemokine receptors. All DC subsets express CXCR4, but the genuine contribution of this receptor to their biology has been overlooked. We addressed this question using natural CXCR4 mutants resistant to CXCL12-induced desensitization and harboring a gain of function that cause the warts, hypogammaglobulinemia, infections, and myelokathexis (WHIM) syndrome (WS), a rare immunodeficiency associated with high susceptibility to the pathogenesis of human papillomavirus (HPV). We report a reduction in the number of circulating plasmacytoid DCs (pDCs) in WHIM patients, whereas that of conventional DCs is preserved. This pattern was reproduced in an original mouse model of WS, enabling us to show that the circulating pDC defect can be corrected upon CXCR4 blockade and that pDC differentiation and function are preserved, despite CXCR4 dysfunction. We further identified proper CXCR4 signaling as a critical checkpoint for Langerhans cell and DC migration from the skin to lymph nodes, with corollary alterations of their activation state and tissue inflammation in a model of HPV-induced dysplasia. Beyond providing new hypotheses to explain the susceptibility of WHIM patients to HPV pathogenesis, this study shows that proper CXCR4 signaling establishes a migration threshold that controls DC egress from CXCL12-containing environments and highlights the critical and subset-specific contribution of CXCR4 signal termination to DC biology.

Dendritic Cells Currently under the Spotlight; Classification and Subset Based upon New Markers.

Dendritic cells (DCs) are considered as a subset of mononuclear phagocytes that composed of multiple subsets with distinct phenotypic features. DCs play crucial roles in the initiation and modulation of immune responses to both allo- and auto-antigens during pathogenic settings, encompassing infectious diseases, cancer, autoimmunity, transplantation, as well as vaccination. DCs play a role in preventing autoimmunity via inducing tolerance to self-antigens. This review focus on the most common subsets of DCs in human. Owing to the low frequencies of DC cells in blood and tissues and also the lack of specific DC markers, studies of DCs have been greatly hindered. Human DCs arise by a dedicated pathway of lympho-myeloid hematopoiesis and give rise into specialized subtypes under the influence of transcription factors that are specific for each linage. In humans, the classification of DCs has been generally separated into the blood and cutaneous subsets, mainly because these parts are more comfortable to examine in humans.

Decoding the Heterogeneity of Human Dendritic Cell Subsets.

Dendritic cells (DCs) have been classified into distinct subsets based on phenotype and ontogeny. In the past few years, high throughput single-cell approaches have revealed further heterogeneity of human DCs, in particular at the transcriptomic level. Herein examined, are recent studies describing new human DC populations based on single-cell RNA-seq analysis, and a unified view of these emerging DC populations is presented. Also assessed are the features that define bona fide DC lineages, as opposed to cell states of the same lineage. Finally, where these newly described DC populations fit in the ontogeny-based classification of human DCs is examined.

Mucosal Dendritic Cell Subsets Control HIV-1's Viral Fitness.

Dendritic cell (DC) subsets are abundantly present in genital and intestinal mucosal tissue and are among the first innate immune cells that encounter human immunodeficiency virus type 1 (HIV-1) after sexual contact. Although DCs have specific characteristics that greatly enhance HIV-1 transmission, it is becoming evident that most DC subsets also have virus restriction mechanisms that exert selective pressure on the viruses during sexual transmission. In this review we discuss the current concepts of the immediate events following viral exposure at genital mucosal sites that lead to selection of specific HIV-1 variants called transmitted founder (TF) viruses. We highlight the importance of the TF HIV-1 phenotype and the role of different DC subsets in establishing infection. Understanding the biology of HIV-1 transmission will contribute to the design of novel treatment strategies preventing HIV-1 dissemination.

The ontogenetic path of human dendritic cells.

Dendritic cells (DCs) orchestrate adaptive immune responses. In healthy individuals, DCs are drivers and fine-tuners of T cell responses directed against invading pathogens or cancer cells. In parallel, DCs control autoreactive T cells, thereby maintaining T cell tolerance. Under various disease conditions, a disruption of this delicate balance can lead to chronic infections, tumor evasion, or autoimmunity. While great efforts have been made to unravel the origin and development of this powerful cell type in mice, only little is known about the ontogeny of human DCs. Here, we summarize the current understanding of the developmental path of DCs from hematopoietic stem cells to fully functional DCs in their local tissue environment and provide a template for the identification of DCs across various tissues.

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.