Classical dendritic cells as a unique immune cell lineage

Unmasking the NLRP3 inflammasome in dendritic cells as a potential therapeutic target for autoimmunity, cancer, and infectious conditions

Proper and functional immune response requires a complex interaction between innate and adaptive immune cells, which dendritic cells (DCs) are the primary actors in this coordination as professional antigen-presenting cells. DCs are armed with numerous pattern recognition receptors (PRRs) such as nucleotide-binding and oligomerization domain-like receptors (NLRs) like NLRP3, which influence the development of their activation state upon sensation of ligands. NLRP3 is a crucial component of the immune system for protection against tumors and infectious agents, because its activation leads to the assembly of inflammasomes that cause the formation of active caspase-1 and stimulate the maturation and release of proinflammatory cytokines. But, when NLRP3 becomes overactivated, it plays a pathogenic role in the progression of several autoimmune disorders. So, NLRP3 activation is strictly regulated by diverse signaling pathways that are mentioned in detail in this review. Furthermore, the role of NLRP3 in all of the diverse immune cells' subsets is briefly mentioned in this study because NLRP3 plays a pivotal role in modulating other immune cells which are accompanied by DCs' responses and subsequently influence differentiation of T cells to diverse T helper subsets and even impact on cytotoxic CD8+ T cells' responses. This review sheds light on the functional and therapeutic role of NLRP3 in DCs and its contribution to the occurrence and progression of autoimmune disorders, prevention of diverse tumors' development, and recognition and annihilation of various infectious agents. Furthermore, we highlight NLRP3 targeting potential for improving DC-based immunotherapeutic approaches, to be used for the benefit of patients suffering from these disorders.

Dendritic Cell-Based Immunotherapy: The Importance of Dendritic Cell Migration

Dendritic cells (DCs) are specialized antigen-presenting cells that are crucial for maintaining self-tolerance, initiating immune responses against pathogens, and patrolling body compartments. Despite promising aspects, DC-based immunotherapy faces challenges that include limited availability, immune escape in tumors, immunosuppression in the tumor microenvironment, and the need for effective combination therapies. A further limitation in DC-based immunotherapy is the low population of migratory DC (around 5%-10%) that migrate to lymph nodes (LNs) through afferent lymphatics depending on the LN draining site. By increasing the population of migratory DCs, DC-based immunotherapy could enhance immunotherapeutic effects on target diseases. This paper reviews the importance of DC migration and current research progress in the context of DC-based immunotherapy.

Immunomodulatory Role of Neuropeptides in the Cornea

The transparency of the cornea along with its dense sensory innervation and resident leukocyte populations make it an ideal tissue to study interactions between the nervous and immune systems. The cornea is the most densely innervated tissue of the body and possesses both immune and vascular privilege, in part due to its unique repertoire of resident immune cells. Corneal nerves produce various neuropeptides that have a wide range of functions on immune cells. As research in this area expands, further insights are made into the role of neuropeptides and their immunomodulatory functions in the healthy and diseased cornea. Much remains to be known regarding the details of neuropeptide signaling and how it contributes to pathophysiology, which is likely due to complex interactions among neuropeptides, receptor isoform-specific signaling events, and the inflammatory microenvironment in disease. However, progress in this area has led to an increase in studies that have begun modulating neuropeptide activity for the treatment of corneal diseases with promising results, necessitating the need for a comprehensive review of the literature. This review focuses on the role of neuropeptides in maintaining the homeostasis of the ocular surface, alterations in disease settings, and the possible therapeutic potential of targeting these systems.

Sepsis Inflammation Impairs the Generation of Functional Dendritic Cells by Targeting Their Progenitors

Background

Sepsis is a complex systemic immune dysfunction syndrome induced by infection. Sepsis has a high mortality rate, with most patients dying due to systemic organ failure or secondary infection. Dendritic cells (DCs) are professional antigen-presenting cells. Upon infection with microbes, DCs are activated to induce adaptive immune responses for controlling infection. DC generation and function are impaired during sepsis; however, the underlying mechanisms remain largely unknown.

Methods

Peripheral blood samples from sepsis patients were collected to examine DC subsets, DC progenitors, and apoptosis of DCs by flow cytometer. In vitro induction of DCs from hematopoietic stem/progenitor cells were established and a variety of sepsis-associated inflammatory mediators [e.g., interferon-gamma (IFN-γ), interleukin-1beta (IL-1β), tumor necrosis factor-alpha (TNF-α) and granulocyte-colony stimulating factor (G-CSF)] and Lipopolysaccharide (LPS) were determined for the impact on DC generation and function in vitro.

Results

Our results demonstrate that sepsis-induced systemic inflammation impairs the capacity of hematopoietic stem and progenitor cells (HSPCs) to produce DCs, including conventional DCs (cDCs) and plasmacytoid DCs (pDCs). We investigated peripheral blood (PB) samples from 34 pediatric patients on days 1 to 7 following diagnosis. Compared to healthy donors (n = 18), the sepsis patients exhibited a significantly fewer percentage and number of pDCs and cDCs, and a lower expression of antigen presenting molecule HLD-DR and co-stimulatory molecules (e.g., CD86) on the surface of DCs. This sepsis-induced DC impairment was associated with significantly increased apoptotic death of DCs and marked decreases of progenitor cells that give rise to DCs. Furthermore, we observed that among the tested sepsis-associated cytokines (e.g., IFN-γ, IL-1β, TNF-α, and G-CSF), G-CSF and IFN-γ impaired DC development from cultured HSPCs. G-CSF also markedly decreased the expression of HLA-DR on HSPC-derived DCs and their cytokine production, including IL-12 and IFN-β.

Conclusions

Collectively, these findings indicate that sepsis impairs the survival of functional DCs and their development from HSPCs. Strategies for improving DC reconstitution following sepsis may restore DC progenitors and their associated function.

Programmed Death Ligand 1-Expressing Classical Dendritic Cells MitigateHelicobacter-Induced Gastritis

BACKGROUND & AIMS

Helicobacter pylori has been reported to modulate local immune responses to colonize persistently in gastric mucosa. Although the induced expression of programmed cell death ligand 1 (PD-L1) has been suggested as an immune modulatory mechanism for persistent infection of H pylori, the main immune cells expressing PD-L1 and their functions in Helicobacter-induced gastritis still remain to be elucidated.

METHODS

The blockades of PD-L1 with antibody or PD-L1-deficient bone marrow transplantation were performed in Helicobacter-infected mice. The main immune cells expressing PD-L1 in Helicobacter-infected stomach were determined by flow cytometry and immunofluorescence staining. Helicobacter felis or H pylori-infected dendritic cell (DC)-deficient mouse models including Flt3-/-, Zbtb46-diphtheria toxin receptor, and BDCA2-diphtheria toxin receptor mice were analyzed for pathologic changes and colonization levels. Finally, the location of PD-L1-expressing DCs and the correlation with H pylori infection were analyzed in human gastric tissues using multiplexed immunohistochemistry.

RESULTS

Genetic or antibody-mediated blockade of PD-L1 aggravated Helicobacter-induced gastritis with mucosal metaplasia. Gastric classical DCs expressed considerably higher levels of PD-L1 than other immune cells and co-localized with T cells in gastritis lesions from Helicobacter-infected mice and human beings. H felis- or H pylori-infected Flt3-/- or classical DC-depleted mice showed aggravated gastritis with severe T-cell and neutrophil accumulation with low bacterial loads compared with that in control mice. Finally, PD-L1-expressing DCs were co-localized with T cells and showed a positive correlation with H pylori infection in human subjects.

CONCLUSIONS

The PD-1/PD-L1 pathway may be responsible for the immune modulatory function of gastric DCs that protects the gastric mucosa from Helicobacter-induced inflammation, but allows persistent Helicobacter colonization.

Melanoma-infiltrating dendritic cells: Limitations and opportunities of mouse models

The infiltration of melanoma lesions by dendritic cells (DCs) has been suggested to play a tumorigenic role due to the capacity of DCs to induce tumor tolerance and promote angiogenesis as well as metastasis. However, it has also been shown that tumor-infiltrating DCs (TIDCs) induce antitumor responses and hence may be targeted in cost-effective therapeutic approaches to obtain patient-specific DCs that present relevant tumor antigens, without the need for ex vivo DC expansion or tumor antigen identification. Unfortunately, little is known about the composition, nature and function of TIDCs found in human melanoma. The development of mouse melanoma models has greatly contributed to the molecular understanding of melanoma immunology in mice, but many questions on TIDCs remain unanswered. Here, we discuss current knowledge about melanoma TIDCs in various mouse models with regard to their translational potential and clinical relevance.

Novel concepts in plasmacytoid dendritic cell (pDC) development and differentiation

Plasmacytoid dendritic cells (pDCs) are an immune subset specialized in the production of Type I Interferons (IFNs). They are characterized by co-expression of myeloid and lymphoid markers. Their developmental origin has been studied since their discovery and the identification of a myeloid progenitor capable of generating all dendritic cell (DC) subsets, including pDCs, led to their classification within the myeloid compartment. However, recent findings challenge this hypothesis and provide evidence for a lymphoid origin for the majority of pDCs ^46-48. In this review we discuss and present the original myeloid and the newer lymphoid developmental trajectories of pDCs.

Dendritic Cell-Restricted Progenitors Contribute to Obesity-Associated Airway Inflammation via Adam17-p38 MAPK-Dependent Pathway

Proliferation of dendritic cell (DC)—restricted progenitor cells in bone marrow compartment is tightly regulated at steady state and responds to multiple tissue-specific triggers during disturbed homeostasis such as obesity. DCs in the lung stem from a rapidly dividing DC-restricted progenitor cells and are effective at generating adaptive immune responses in allergic airway inflammation. Precisely, how DC-restricted progenitor expansion and differentiation are influenced by airway inflammation to maintain constant supply of myeloid DCs is poorly understood. Here we show that a high fat diet (HFD) induces oxidative stress and accelerates the expansion of DC- restricted progenitor cells in bone marrow and correlates with persistent induction of p38 mitogen activated protein kinase (MAPK), which is blocked with a selective p38α/β MAPK inhibitor. Mice fed a HFD and sensitized to inhaled allergen house dust mite (HDM) led to alterations of DC- restricted progenitor cells that were characterized by increased expansion and seeding of lung DCs in airway inflammation. Mechanistically, we establish that the expansion induced by HFD dysregulates the expression of a disintegrin and metallopeptidase domain 17 (Adam17) and is required for p38 MAPK activation in DC-restricted progenitors. These results demonstrates that obesity produces persistent changes in DC precursors and that elevation of Adam17 expression is tightly coupled to p38 MAPK and is a key driver of proliferation. Altogether, these data provide phenotypic and mechanistic insight into dendritic cell supply chain in obesity-associated airway inflammation.

Myeloid and Plasmacytoid Dendritic Cells and Cancer - New Insights

Dendritic cells (DCs) use effective mechanisms to combat antigens and to bring about adaptive immune responses through their ability to stimulate näive T cells. At present, four major cell types are categorised as DCs: Classical or conventional (cDCs), Plasmacytoid (pDCs), Langerhans cells (LCs), and monocyte-derived DCs (Mo-DCs). It was suggested that pDCs, CD1c+ DCs and CD141+ DCs in humans are equivalent to mouse pDCs, CD11b+ DCs and CD8α+ DCs, respectively. Human CD141+ DCs compared to mouse CD8α+ DCs have remarkable functional and transcriptomic similarities. Characteristic markers, transcription factors, toll-like receptors, T helpers (Th) polarisation, cytokines, etc. of DCs are discussed in this review. Major histocompatibility complex (MHC) I and II antigen presentation, cross-presentation and Th polarisation are defined, and the dual role of DCs in the tumour is discussed. Human DCs are the main immune cells that orchestrate the immune response in the tumour microenvironment.

Pathological alteration and therapeutic implications of sepsis-induced immune cell apoptosis

Sepsis is a life-threatening organ dysfunction syndrome caused by dysregulated host response to infection that leads to uncontrolled inflammatory response followed by immunosuppression. However, despite the high mortality rate, no specific treatment modality or drugs with high efficacy is available for sepsis to date. Although improved treatment strategies have increased the survival rate during the initial state of excessive inflammatory response, recent trends in sepsis show that mortality occurs at a period of continuous immunosuppressive state in which patients succumb to secondary infections within a few weeks or months due to post-sepsis “immune paralysis.” Immune cell alteration induced by uncontrolled apoptosis has been considered a major cause of significant immunosuppression. Particularly, apoptosis of lymphocytes, including innate immune cells and adaptive immune cells, is associated with a higher risk of secondary infections and poor outcomes. Multiple postmortem studies have confirmed that sepsis-induced immune cell apoptosis occurs in all age groups, including neonates, pediatric, and adult patients, and it is considered to be a primary contributing factor to the immunosuppressive pathophysiology of sepsis. Therapeutic perspectives targeting apoptosis through various strategies could improve survival in sepsis. In this review article, we will focus on describing the major apoptosis process of immune cells with respect to physiologic and molecular mechanisms. Further, advances in apoptosis-targeted treatment modalities for sepsis will also be discussed.

Bone marrow dendritic cells regulate hematopoietic stem/progenitor cell trafficking

A resident population of dendritic cells (DCs) has been identified in murine bone marrow, but its contribution to the regulation of hematopoiesis and establishment of the stem cell niche is largely unknown. Here, we show that murine bone marrow DCs are perivascular and have a type 2 conventional DC (cDC2) immunophenotype. RNA expression analysis of sorted bone marrow DCs shows that expression of many chemokines and chemokine receptors is distinct from that observed in splenic cDC2s, suggesting that bone marrow DCs may represent a unique DC population. A similar population of DCs is present in human bone marrow. Ablation of conventional DCs (cDCs) results in hematopoietic stem/progenitor cell (HSPC) mobilization that is greater than that seen with ablation of bone marrow macrophages, and cDC ablation also synergizes with G-CSF to mobilize HSPCs. Ablation of cDCs is associated with an expansion of bone marrow endothelial cells and increased vascular permeability. CXCR2 expression in sinusoidal endothelial cells and the expression of two CXCR2 ligands, CXCL1 and CXCL2, in the bone marrow are markedly increased following cDC ablation. Treatment of endothelial cells in vitro with CXCL1 induces increased vascular permeability and HSPC transmigration. Finally, we show that HSPC mobilization after cDC ablation is attenuated in mice lacking CXCR2 expression. Collectively, these data suggest that bone marrow DCs play an important role in regulating HSPC trafficking, in part, through regulation of sinusoidal CXCR2 signaling and vascular permeability.

Dendritic cells in the regulation of immunity and inflammation

As potent antigen-presenting cells, dendritic cells (DCs) comprise the most heterogeneous cell population with significant cellular phenotypic and functional plasticity. They form a sentinel network to modulate immune responses, since intrinsic cellular mechanisms and complex external, environmental signals endow DCs with the distinct capacity to induce protective immunity or tolerance to self. Interactions between DCs and other cells of the immune system mediate this response. This interactive response depends on DC maturation status and subtype, as well as the microenvironment of the tissue location and DC-intrinsic regulators. Dysregulated DCs can initiate and perpetuate various immune disorders, which creates attractive therapeutic targets. In this review, we provide a detailed outlook on DC ontogeny and functional specialization. We highlight recent advances on the regulatory role that DCs play in immune responses, the putative molecular regulators that control DC functional responding and the contribution of DCs to inflammatory disease physiopathology.

Constructing cell lineages from single-cell transcriptomes

Advances in single-cell RNA-sequencing have helped reveal the previously underappreciated level of cellular heterogeneity present during cellular differentiation. A static snapshot of single-cell transcriptomes provides a good representation of the various stages of differentiation as differentiation is rarely synchronized between cells. Data from numerous single-cell analyses has suggested that cellular differentiation and development can be conceptualized as continuous processes. Consequently, computational algorithms have been developed to infer lineage relationships between cell types and construct developmental trajectories along which cells are re-ordered such that similarity between successive cell pairs is maximized. Here, we compare and contrast the existing computational methods, and illustrate how they may be applied to build mouse myeloid progenitor lineages from massively parallel RNA single-cell sequencing data.

Variation and significance of secretory immunoglobulin A, interleukin 6 and dendritic cells in oral cancer

The present study aimed to determine changes in the concentration of secretory immunoglobulin A (SIgA) and interleukin 6 (IL-6) in the saliva of patients with oral cancer, to evaluate the abnormal expression of cluster of differentiation (CD) 1a, CD83, CD80 and CD86 on dendritic cells (DCs) of oral cancer tissues and to discuss the interaction between SIgA, IL-6 and DCs in oral cancer. A total of 40 patients between 27 and 70 years of age, median age 52 years, with primary oral cancer were enrolled in the present study, and a group of 20 healthy male and female volunteers was used as the control group. The concentration of SIgA and IL-6 in the saliva of the preoperative patients was determined by ELISA. The expression levels of CD1a, CD83, CD80 and CD86 were detected by immunohistochemistry and flow cytometry, which was performed on histopathological sections from paraffin-embedded tumor and corresponding adjacent control tissues. The specimens were assessed using the semi-quantitative immunoreactive score (IRS). The concentration of SIgA in the saliva from patients with oral cancer decreased, whereas the IL-6 level significantly increased compared with the control subjects (P<0.05). In addition, the decrease of SIgA level and increase of IL-6 level exhibited a negative correlation (r=−0.543, P<0.05). According to the IRS score, the expression levels of CD1a, CD83, CD80 and CD86 in the cancer tissue were lower than the expression levels of the control group (P<0.05). Furthermore, the expression of CD80 and CD86 exhibited no correlation with histological grade or pathological type (P>0.05), but exhibited a negative correlation with clinical stage and lymph node metastasis (P<0.05). The concentration of SIgA and IL-6 in saliva may be used as an auxiliary diagnostic indicator for oral cancer. The detection of CD80 and CD86 expressed on DCs in oral cancer tissue may be useful for the diagnosis and evaluation of the prognosis of tumors. The present study hypothesized that the use of SIgA vaccines or IL-6 inhibitors may be useful for reversing the immune deficiency associated with DCs in oral cancer.

Elevated level of pro inflammatory cytokine and chemokine expression in chicken bone marrow and monocyte derived dendritic cells following LPS induced maturation

The study was designed to characterize and compare chicken bone marrow and peripheral blood monocyte derived dendritic cells (chBM-DC and chMoDC) and to evaluate inflammatory cytokine and chemokine alterations in response upon LPS stimulation. Typical morphology was observed in DCs from 48h of culture using recombinant chicken GM-CSF and IL-4. Maturation of DCs with LPS (1μg/ml) showed significant up regulation of mRNA of surface markers (CD40, CD80, CD83, CD86, MHC-II and DC-LAMP (CD208)), pro-inflammatory cytokines (IL-1β, IL-6, TNF-α (LITAF)), iNOS, chemokine CXCli2 and TLRs4 and 15. Basal level of TLR1 mRNA expression was higher followed by TLR15 in both DCs irrespective of their origin. Expression of iNOS and CXCLi2 mRNA in mature DCs of both origins were higher than other surface molecules and cytokines studied. Hence, its level of expression can also be used as an additional maturation marker for LPS induced chicken dendritic cell maturation along with CD83 and CD40. LPS matured DCs of both origins upregulated IL-12 and IFN-γ. Based on CD40 and CD83 mRNA expression, it was observed that LPS induced the maturation in both DCs, but chMoDCs responded better in expression of surface markers and inflammatory mediator genes.

Clusterin Modulates Allergic Airway Inflammation by Attenuating CCL20-Mediated Dendritic Cell Recruitment

Recruitment and activation of dendritic cells (DCs) in the lungs are critical for Th2 responses in asthma, and CCL20 secreted from bronchial epithelial cells (BECs) is known to influence the recruitment of DCs. Because asthma is a disease that is closely associated with oxidative stress, we hypothesized that clusterin, an oxidative stress regulatory molecule, may have a role in the development of allergic airway inflammation. The aim of this study was to examine whether clusterin regulates CCL20 production from the BECs and the subsequent DC recruitment in the lungs. To verify the idea, clusterin knockout (Clu(-/-)), clusterin heterogeneous (Clu(+/-)), and wild-type mice were exposed intranasally to house dust mite (HDM) extract to induce allergic airway inflammation. We found that the total number of immune cells in bronchoalveolar lavage fluid and the lung was increased in Clu(-/-) and Clu(+/-) mice. Of these immune cells, inflammatory DCs (CD11b(+)CD11c(+)) and Ly6C(high) monocyte populations in the lung were significantly increased, which was accompanied by increased levels of various chemokines, including CCL20 in bronchoalveolar lavage fluid, and increased oxidative stress markers in the lung. Moreover, HDM-stimulated human BECs with either up- or downregulated clusterin expression showed that CCL20 secretion was negatively associated with clusterin expression. Interestingly, clusterin also reduced the level of intracellular reactive oxygen species, which is related to induction of CCL20 expression after HDM stimulation. Thus, the antioxidant property of clusterin is suggested to regulate the expression of CCL20 in BECs and the subsequent recruitment of inflammatory DCs in the airway.

Transcriptional Regulation of Mononuclear Phagocyte Development

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

The role of lymphocytes in the development and treatment of alopecia areata

Alopecia areata (AA) development is associated with both innate and adaptive immune cell activation, migration to peri- and intra-follicular regions, and hair follicle disruption. Both CD4(+) and CD8(+) lymphocytes are abundant in AA lesions; however, CD8(+) cytotoxic T lymphocytes are more likely to enter inside hair follicles, circumstantially suggesting that they have a significant role to play in AA development. Several rodent models recapitulate important features of the human autoimmune disease and demonstrate that CD8(+) cytotoxic T lymphocytes are fundamentally required for AA induction and perpetuation. However, the initiating events, the self-antigens involved, and the molecular signaling pathways, all need further exploration. Studying CD8(+) cytotoxic T lymphocytes and their fate decisions in AA development may reveal new and improved treatment approaches.

The varieties of immunological experience: of pathogens, stress, and dendritic cells

In the 40 years since their discovery, dendritic cells (DCs) have been recognized as central players in immune regulation. DCs sense microbial stimuli through pathogen-recognition receptors (PRRs) and decode, integrate, and present information derived from such stimuli to T cells, thus stimulating immune responses. DCs can also regulate the quality of immune responses. Several functionally specialized subsets of DCs exist, but DCs also display functional plasticity in response to diverse stimuli. In addition to sensing pathogens via PRRs, emerging evidence suggests that DCs can also sense stress signals, such as amino acid starvation, through ancient stress and nutrient sensing pathways, to stimulate adaptive immunity. Here, I discuss these exciting advances in the context of a historic perspective on the discovery of DCs and their role in immune regulation. I conclude with a discussion of emerging areas in DC biology in the systems immunology era and suggest that the impact of DCs on immunity can be usefully contextualized in a hierarchy-of-organization model in which DCs, their receptors and signaling networks, cell-cell interactions, tissue microenvironment, and the host macroenvironment represent different levels of the hierarchy. Immunity or tolerance can then be represented as a complex function of each of these hierarchies.

Dendritic cells during Staphylococcus aureus infection: subsets and roles

Dendritic cells (DCs) are professional antigen-presenting cells (APCs) that play a crucial role in both innate and adaptive immune responses. DCs orient the immune responses by modulating the balance between protective immunity to pathogens and tolerance to self-antigens. Staphylococcus aureus (S. aureus) is a common member of human skin microbiota and can cause severe infections with significant morbidity and mortality. Protective immunity to pathogens by DCs is required for clearance of S. aureus. DCs sense the presence of the staphylococcal components using pattern recognition receptors (PRRs) and then orchestrate immune systems to resolve infections. This review summarizes the possible roles of DCs, in particular their Toll-like receptors (TLRs) involved in S. aureus infection and strategies by which the pathogen affects activation and function of DCs.

Sinusoidal immunity: macrophages at the lymphohematopoietic interface

Macrophages are widely distributed throughout the body, performing vital homeostatic and defense functions after local and systemic perturbation within tissues. In concert with closely related dendritic cells and other myeloid and lymphoid cells, which mediate the innate and adaptive immune response, macrophages determine the outcome of the inflammatory and repair processes that accompany sterile and infectious injury and microbial invasion. This article will describe and compare the role of specialized macrophage populations at two critical interfaces between the resident host lymphohematopoietic system and circulating blood and lymph, the carriers of cells, humoral components, microorganisms, and their products. Sinusoidal macrophages in the marginal zone of the spleen and subcapsular sinus and medulla of secondary lymph nodes contribute to the innate and adaptive responses of the host in health and disease. Although historically recognized as major constituents of the reticuloendothelial system, it has only recently become apparent that these specialized macrophages in close proximity to B and T lymphocytes play an indispensable role in recognition and responses to exogenous and endogenous ligands, thus shaping the nature and quality of immunity and inflammation. We review current understanding of these macrophages and identify gaps in our knowledge for further investigation.

A negative feedback loop of transcription factors specifies alternative dendritic cell chromatin States

During hematopoiesis, cells originating from the same stem cell reservoir differentiate into distinct cell types. The mechanisms enabling common progenitors to differentiate into alternative cell fates are not fully understood. Here, we identify cell-fate-determining transcription factors (TFs) governing dendritic cell (DC) development by annotating the enhancer landscapes of the DC lineage. Combining these analyses with detailed overexpression, knockdown, and ChIP-Seq studies, we show that Irf8 functions as a plasmacytoid DC epigenetic and fate-determining TF, regulating massive, cell-specific chromatin changes in thousands of pDC enhancers. Importantly, Irf8 forms a negative feedback loop with Cebpb, a monocyte-derived DC epigenetic fate-determining TF. We show that using this circuit logic, a pulse of TF expression can stably define epigenetic and transcriptional states, regardless of the microenvironment. More broadly, our study proposes a general paradigm that allows closely related cells with a similar set of signal-dependent factors to generate differential and persistent enhancer landscapes.

The role of dendritic cells in immunity against primary herpes simplex virus infections

Herpes simplex virus (HSV) is a DNA virus with tropism for infecting skin and mucosal epithelia during the lytic stages of its complex life cycle. The immune system has evolved a multitude of strategies to respond to primary HSV infections. These include rapid innate immune responses largely driven by pattern recognition systems and protective anti-viral immunity. Dendritic cells (DC) represent a versatile and heterogenic group of antigen presenting cells that are important for pathogen recognition at sites of infection and for priming of protective HSV-specific T cells. Here we will review the current knowledge on the role of DCs in the host immune response to primary HSV infection. We will discuss how DCs integrate viral cues into effective innate immune responses, will dissect how HSV infection of DCs interferes with their capacity to migrate from sites of infection to the draining lymph nodes and will outline how migratory DCs can make antigens available to lymph node resident DCs. The role of distinct DC subsets and their relevant contribution to antigen presentation on MHC class I and MHC class II molecules will be detailed in the context of T cell priming in the lymph node and the elicitation of effector function in infected tissues. An improved understanding of the fundamental mechanisms of how DCs recognize HSV, process and present its antigens to naïve and effector T cells will not only assist in the improvement of vaccine-based preventions of this important viral disease, but also serves as a paradigm to resolve basic immunological principles.

The porcine innate immune system: an update

Over the last few years, we have seen an increasing interest and demand for pigs in biomedical research. Domestic pigs (Sus scrofa domesticus) are closely related to humans in terms of their anatomy, genetics, and physiology, and often are the model of choice for the assessment of novel vaccines and therapeutics in a preclinical stage. However, the pig as a model has much more to offer, and can serve as a model for many biomedical applications including aging research, medical imaging, and pharmaceutical studies to name a few. In this review, we will provide an overview of the innate immune system in pigs, describe its anatomical and physiological key features, and discuss the key players involved. In particular, we compare the porcine innate immune system to that of humans, and emphasize on the importance of the pig as model for human disease.

The physiological role of DC-SIGN: a tale of mice and men

The innate immune receptor DC-SIGN (dendritic cell-specific intercellular adhesion molecule-3 grabbing non-integrin) was discovered over a decade ago and was initially identified as a pattern recognition receptor. In addition to its ability to recognize a broad range of pathogen-derived ligands and self-glycoproteins, DC-SIGN also mediates intercellular adhesion, as well as antigen uptake and signaling, which is a functional hallmark of dendritic cells (DCs). Most research on DC-SIGN has relied on in vitro studies. The in vivo function of DC-SIGN is difficult to address, in part because there are eight genetic homologs in mice with no clear DC-SIGN ortholog. Here, we summarize the functions attributed to DC-SIGN based on in vitro data and discuss the limitations of available mouse models to uncover the physiological role of this receptor in vivo.

The presence of CD209 expressing dendritic cells correlates with biofilm positivity in chronic rhinosinusitis with nasal polyposis

Biofilm-positive cases of chronic rhinosinusitis with nasal polyposis (CRSwNP) may form a separate clinical entity, which is characterized by high recurrence rates and resistance against different therapeutic strategies. This can be explained by a special immunologic phenotype. Biofilm existence has been supposed to correlate with increased amount of dendritic cells that are responsible for antigen presentation in CRSwNP. A total of 20 patients with CRSwNP undergoing endoscopic sinus surgery (ESS) were analyzed. The negative control group consisted of ten patients undergoing septoplasty without CRSwNP. Three series of individual nasal polyps and control specimens were processed to hematoxylin-eosin (HE) and Gram staining and to CD209-specific immunofluorescent assay, respectively. Biofilm was detected in 13 of 20 patients (65 %) with CRSwNP and in none of the ten negative controls. The subepithelial layer of biofilm-positive nasal polyps displayed a statistically significant (p < 0.001) increase in the numbers of CD209-expressing dendritic cells compared to biofilm-negative specimens. It was found that biofilm detectability showed strong correlation to the architecture of respiratory mucosa and to the dominant inflammatory cell type of the subepithelial layer. Persisting bacterial biofilms may affect the type of antigen presentation and consecutive immune reactions in the subepithelial layer of nasal mucosa. This phenomenon may result in different inflammatory pathways with specific cytokine profile compared to biofilm-negative cases. Co-existence of bacterial biofilms and dominant pattern of dendritic cells suggest a biofilm-associated immunologic phenotype in CRSwNP. This can explain the mucosal changes, functional disorders and therapy resistance featuring CRSwNP.

Transcriptional control of dendritic cell development

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