Streptococcus pyogenes activates human plasmacytoid and myeloid dendritic cells

Interactions between the NLRP3-Dependent IL-1β and the Type I Interferon Pathways in Human Plasmacytoid Dendritic Cells

Generally, a reciprocal antagonistic interaction exists between the antiviral type I interferon (IFN) and the antibacterial nucleotide-binding oligomerization domain (NOD)-like receptor pyrin domain containing 3 (NLRP3)-dependent IL-1β pathways that can significantly shape immune responses. Plasmacytoid dendritic cells (pDCs), as professional type I IFN-producing cells, are the major coordinators of antiviral immunity; however, their NLRP3-dependent IL-1β secretory pathway is poorly studied. Our aim was to determine the functional activity of the IL-1β pathway and its possible interaction with the type I IFN pathway in pDCs. We found that potent nuclear factor-kappa B (NF-κB) inducers promote higher levels of pro-IL-1β during priming compared to those activation signals, which mainly trigger interferon regulatory factor (IRF)-mediated type I IFN production. The generation of cleaved IL-1β requires certain secondary signals in pDCs and IFN-α or type I IFN-inducing viruses inhibit IL-1β production of pDCs, presumably by promoting the expression of various NLRP3 pathway inhibitors. In line with that, we detected significantly lower IL-1β production in pDCs of psoriasis patients with elevated IFN-α levels. Collectively, our results show that the NLRP3-dependent IL-1β secretory pathway is inducible in pDCs; however, it may only prevail under inflammatory conditions, in which the type I IFN pathway is not dominant.

In Vitro Priming of Human T Cells by Dendritic Cells Provides a Screening Tool for Candidate Vaccines for Burkholderia pseudomallei

Murine dendritic cells, when pulsed with heat-killed Burkholderia pseudomallei and used to immunise naïve mice, have previously been shown to induce protective immunity in vivo. We have now demonstrated the in vitro priming of naïve human T cells against heat-killed B. pseudomallei, by co-culture with syngeneic B. pseudomallei-pulsed dendritic cells. Additionally, we have enriched the DC fraction such that a study of the differential response induced by pulsed DCs of either myeloid or plasmacytoid lineage in syngeneic human T cells was achievable. Whilst both mDCs and pDCs were activated by pulsing, the mDCs contributed the major response to B. pseudomallei with the expression of the migration marker CCR7 and a significantly greater secretion of the proinflammatory TNFα and IL1β. When these DC factions were combined and used to prime syngeneic T cells, a significant proliferation was observed in the CD4+ fraction. Here, we have achieved human T cell priming in vitro with unadjuvanted B. pseudomallei, the causative organism of melioidosis, for which there is currently no approved vaccine. We propose that the approach we have taken could be used to screen for the human cellular response to candidate vaccines and formulations, in order to enhance the cell-mediated immunity required to protect against this intracellular pathogen and potentially more broadly against other, difficult-to-treat intracellular pathogens. To date, the polysaccharide capsule of B. pseudomallei, fused to a standard carrier protein, e.g., Crm, looks a likely vaccine candidate. Dendritic cells (DCs), providing, as they do, the first line of defence to infection, process and present microbial products to the immune system to direct downstream immune responses. Here, we have sought to use DCs ex vivo to identify immunogenic products from heat-killed B. pseudomallei. Using practical volumes of fresh human donor blood, we show that heat-killed B. pseudomallei activated and stimulated the expression of pro-inflammatory cytokines TNF-α, IL-1β and IL-6 from both myeloid and plasmacytoid DCs. Furthermore, B. pseudomallei-pulsed DCs cultured with naïve syngeneic T cells ex vivo, induced the activation and proliferation of the CD4+ T-cell population, which was identified by cell surface marker staining using flow cytometry. Thus, both DC subsets are important for driving primary T helper cell responses to B. pseudomallei in healthy individuals and have the potential to be used to identify immunogenic components of B. pseudomallei for future therapies and vaccines.

Characterization of genomic DNA of lactic acid bacteria for activation of plasmacytoid dendritic cells

Background

Lactococcus lactis strain Plasma (LC-Plasma) possesses strong stimulatory activity for plasmacytoid dendritic cells (pDCs) via the TLR9-Myd88 pathway. To reveal the effective lactic acid bacteria (LAB) genome structure for pDCs stimulatory activity, we performed in vitro screening, using randomly selected 200 bp DNA fragments from the LC-Plasma genome.

Results

We found that the CpG motif copy number in the fragments was positively and significantly correlated with pDCs stimulatory activity (R = 0.491, p < 0.01). However, the determination coefficient (R²) was 0.24, which means other factors affecte activity. We found that the G + C contents of the fragment showed a significant negative correlation with activity (R = − 0.474, p < 0.01). The correlation between pDCs stimulatory activity and the copy number of CpG motifs was greatly increased when DNA fragments were stratified by G + C contents.

We also performed bioinformatics analysis and a screening of LAB strains with high pDCs stimulatory activity. Species with a high copy number of CpG motifs in the low-G + C region of their genomes had higher probability of inducing high-pDCs stimulatory activity. L. lactis subsp. lactis, Leuconostoc mesenteroides, and Pediococcus pentosaceus were three typical examples of LAB that had high pDCs stimulatory activity.

Conclusions

Our data suggested that the G + C content of DNA is one of the critical factors for pDCs stimulatory activity by DNA fragments. Furthermore, we found that the copy number in the low-G + C regions strongly affected the pDCs stimulatory activity of whole cells of LAB strains. These results should be useful for the design of new DNA fragments containing CpG motifs. This study also demonstrated an in silico screening method for identifying bacterial species that are able to activate pDCs.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1458-y) contains supplementary material, which is available to authorized users.

Coxiella burnetii Induces Inflammatory Interferon-Like Signature in Plasmacytoid Dendritic Cells: A New Feature of Immune Response in Q Fever

Plasmacytoid dendritic cells (pDCs) play a major role in antiviral immunity via the production of type I interferons (IFNs). There is some evidence that pDCs interact with bacteria but it is not yet clear whether they are protective or contribute to bacterial pathogenicity. We wished to investigate whether Coxiella burnetii, the agent of Q fever, interacts with pDCs. The stimulation of pDCs with C. burnetii increased the expression of activation and migratory markers (CD86 and CCR7) as determined by flow cytometry and modulated gene expression program as revealed by a microarray approach. Indeed, genes encoding for pro-inflammatory cytokines, chemokines, and type I INF were up-regulated. The up-regulation of type I IFN was correlated with an increase in IFN-α release by C. burnetii-stimulated pDCs. We also investigated pDCs in patients with Q fever endocarditis. Using flow cytometry and a specific gating strategy, we found that the number of circulating pDCs was significantly lower in patients with Q fever endocarditis as compared to healthy donors. In addition, the remaining circulating pDCs expressed activation and migratory markers. As a whole, our study identified non-previously reported activation of pDCs by C. burnetii and their modulation during Q fever.

Plasmacytoid dendritic cell bactericidal activity against Burkholderia pseudomallei

Melioidosis sepsis, caused by Burkholderia pseudomallei, is associated with high mortality due to an overwhelming inflammatory response. Plasmacytoid dendritic cells (pDC) are potent producers of type I interferons (IFN). This study investigated whether pDC and type I IFN play a role during the early stages of B. pseudomallei infection. Human and murine pDC internalised and killed B. pseudomallei as efficiently as murine conventional DC (cDC). pDC derived from B. pseudomallei-susceptible (BALB/c) mice demonstrated poor intracellular killing and increased IFN-alpha compared to pDC derived from B. pseudomallei-resistant (C57BL/6) mice. This is the first evidence of pDC bactericidal activity against B. pseudomallei infection.

Plasmacytoid Dendritic Cells: Neglected Regulators of the Immune Response to Staphylococcus aureus

Plasmacytoid dendritic cells (pDC) are a rare subset of leukocytes equipped with Fcγ and Fcε receptors, which exert contrary effects on sensing of microbial nucleic acids by endosomal Toll-like receptors. In this article, we explain how pDC contribute to the immune response to Staphylococcus aureus. Under normal circumstances the pDC participates in the memory response to the pathogen: pDC activation is initiated by uptake of staphylococcal immune complexes with IgG or IgE. However, protein A-expressing S. aureus strains additionally trigger pDC activation in the absence of immunoglobulin. In this context, staphylococci exploit the pDC to induce antigen-independent differentiation of IL-10 producing plasmablasts, an elegant means to propagate immune evasion. We further discuss the role of type I interferons in infection with S. aureus and the implications of these findings for the development of immune based therapies and vaccination.

Plasmacytoid DCs are gentle guardians of tonsillar epithelium

The outcome of the interaction between plasmacytoid dendritic cells (pDCs) and bacteria has been very controversial: pDCs have been reported not to be activated by extracellular bacteria, to be activated but to only produce TNF-α and IL-6, or to be activated and produce IFN-α, the hallmark of pDC activation, but only if the bacteria have first been opsonized. In this issue of the European Journal of Immunology, Soumelis and colleagues [Eur. J. Immunol. 2013. 43: 1264-1273] unequivocally show that both blood and tonsillar pDCs are fully activated by bacteria and can produce IFN-α. They also show that pDCs are found in the stratified mucosal epithelium in human tonsils, and are "educated" by tonsillar epithelial cells not to release inflammatory cytokines, despite still being capable of activating T cells, albeit with no impact on T-cell polarization. Hence, pDCs can respond to bacteria but can be educated by epithelial cells to remain anergic to potential inflammatory signals. These findings support a mechanism by which intraepithelial pDCs, which are exposed to the microbiota colonizing the upper respiratory tract, remain capable of initiating immunity without overreacting to microbial stimulation.

Epithelial control of the human pDC response to extracellular bacteria

Plasmacytoid pre-dendritic cells (pDCs) are specialized in responding to nucleic acids, and link innate with adaptive immunity. Although the response of pDCs to viruses is well established, whether pDCs can respond to extracellular bacteria remains controversial. Here, we demonstrate that extracellular bacteria such as Neisseria meningitidis, Haemophilus influenzae, and Staphylococcus aureus activate pDCs to produce IFN-α, TNF-α, IL-6, and to upregulate CD86 expression. We observed that pDCs were present within tonsillar crypts and oro-nasopharyngeal epithelium, where they may contact extracellular bacteria, in situ. Tonsil epithelium-conditioned supernatants inhibited IFN-α, TNF-α, and IL-6 triggered by the direct contact of N. meningitidis or S. aureus with pDCs. However, pDC priming of naive T cells was not affected, suggesting that tonsil epithelium micro-environment limits local inflammation while preserving adaptive immunity in response to extracellular bacteria. Our results reveal an important and novel function of pDCs in the initiation of the mucosal innate and adaptive immunity to extracellular bacteria.

Production of interleukin-27 by human neutrophils regulates their function during bacterial infection

Septicemia is the most severe form of melioidosis caused by the Gram-negative bacterium, Burkholderia pseudomallei. Here, we show that levels of IL-27p28 transcript and protein were both significantly elevated in patients with sepsis, particularly melioidosis and in patients with unfavorable disease outcome. Moreover, human monocytes/macrophages and neutrophils were the major source of IL-27 during infection. The addition of exogenous IL-27 in vitro resulted in significantly increased bacterial survival, reduced B. pseudomallei-induced oxidative burst, and enhanced IL-1β and TNF-α production by purified neutrophils from healthy subjects. Finally, blockade of endogenous IL-27 in neutrophils using soluble IL-27 receptor antagonist prior to infection led to significantly reduced survival of bacteria and decreased IL-1β, but not TNF-α production. These results indicate a potential role for IL-27 in the suppression of anti-bacterial defense mechanisms that might contribute to disease severity in sepsis. The targeting of this cytokine may be beneficial in the management of human sepsis.

Kinetic and distinct distribution of conventional dendritic cells in the early phase of lipopolysaccharide-induced acute lung injury

Respiratory dendritic cells (DCs), especially conventional DCs (cDCs), are critically involved in the induction phase of the immune response in the respiratory system. However, little information concerning cDC kinetics in acute lung injury (ALI) is available. In this study, we have used a murine model of LPS-induced ALI to examine the kinetics and phenotype of respiratory, circulating and splenic cDCs. cDCs in the lung, blood, and spleen and the IL-6 level in the lung were detected at 6, 12 and 24 h after PBS or LPS challenge. In the ALI group, a rapid cDCs accumulation in the lung was observed, and there were highly significant correlations between the frequency of respiratory cDCs or the percentage of cDC expressing CD80 and the IL-6 concentration. However, the frequency of peripheral blood cDCs decreased rapidly in ALI mice, followed by a marked expansion. In addition, splenic cDCs only showed a transient expansion in ALI. cDCs within the blood, lungs and spleens had undergone a modest maturation in the ALI group. Our findings demonstrate that LPS-induced ALI provokes a dynamic and distinct distribution as well as phenotype changes in pulmonary, circulatory and splenic cDC populations. Lung cDCs may participate in the early inflammatory response to ALI.

Harnessing human plasmacytoid dendritic cells as professional APCs

The plasmacytoid dendritic cell (pDC) constitutes a unique DC subset that links the innate and adaptive arm of the immune system. Whereas the unique capability of pDCs to produce large amounts of type I IFNs in response to pathogen recognition is generally accepted, their antigen-presenting function is often neglected since most studies on antigen presentation are aimed at other DC subsets. Recently, pDCs were demonstrated capable to present antigen leading to protective tumor immunity. In this review, we discuss how pDCs could be exploited in the fight against cancer by analyzing their capacity to capture, process and (cross-) present antigen.

Differential interleukin-10 (IL-10) and IL-23 production by human blood monocytes and dendritic cells in response to commensal enteric bacteria

Human peripheral blood contains antigen-presenting cells (APC), including dendritic cells (DC) and monocytes, that may encounter microbes that have translocated from the intestine to the periphery in disease states like HIV-1 infection and inflammatory bowel disease. We investigated the response of DC and monocytes in peripheral blood mononuclear cells (PBMC) to a panel of representative commensal enteric bacteria, including Escherichia coli, Enterococcus sp., and Bacteroides fragilis. All three bacteria induced significant upregulation of the maturation and activation markers CD40 and CD83 on myeloid dendritic cells (mDC) and plasmacytoid dendritic cells (pDC). However, only mDC produced cytokines, including interleukin-10 (IL-10), IL-12p40/70, and tumor necrosis factor alpha (TNF-α), in response to bacterial stimulation. Cytokine profiles in whole PBMC differed depending on the stimulating bacterial species: B. fragilis induced production of IL-23, IL-12p70, and IL-10, whereas E. coli and Enterococcus induced an IL-10-predominant response. mDC and monocyte depletion experiments indicated that these cell types differentially produced IL-10 and IL-23 in response to E. coli and B. fragilis. Bacteroides thetaiotaomicron did not induce levels of IL-23 similar to those of B. fragilis, suggesting that B. fragilis may have unique proinflammatory properties among Bacteroides species. The addition of recombinant human IL-10 to PBMC cultures stimulated with commensal bacteria abrogated the IL-23 response, whereas blocking IL-10 significantly enhanced IL-23 production, suggesting that IL-10 controls the levels of IL-23 produced. These results indicate that blood mDC and monocytes respond differentially to innate stimulation with whole commensal bacteria and that IL-10 may play a role in controlling the proinflammatory response to translocated microbes.

Impairment of regulatory capacity of CD4+CD25+ regulatory T cells mediated by dendritic cell polarization and hyperthyroidism in Graves' disease

Graves' disease (GD) is one of the most common autoimmune diseases. The immune dysfunction in GD involves the generation of thyroid-stimulating hormone receptor (TSHR) autoantibodies that presumably arise consequent to interactions among dendritic cells (DCs), T cells, and regulatory T (Treg) cells. However, the immunological mechanisms of interactions between them that lead to the induction and regulation of this autoimmune disease are poorly defined. In this study, we investigated whether DCs are the main cause of the defective activity of Treg cells in GD patients. We found a significant decrease in the percentage of circulating CD4(+)CD25(+)FOXP3(+) Treg cells in untreated GD patients (uGD), which was negatively correlated with the concentration of TSHR autoantibodies. uGD-derived DCs were polarized to increase the number of plasmacytoid DCs (pDCs) and conferred the ability to abrogate the suppressive function of Treg cells through inducing apoptosis of CD4(+)CD25(+) Treg cells in an IFN-α-dependent manner, and elevated thyroid hormones further exacerbated the effect. The nucleotide UDP, which inhibits IFN-α secretion of pDCs through P2Y6 receptor signaling, restored the suppressive function of CD4(+)CD25(+) Treg cells. Collectively, uGD-derived DCs through pDC polarization and elevated thyroid hormones act in concert to impair the regulatory capacity of Treg cells, facilitating the production of TSHR autoantibodies in the pathogenesis of GD.

Human plasmacytoid dendritic cells phagocytose, process, and present exogenous particulate antigen

Plasmacytoid dendritic cells (pDCs) play a major role in shaping both innate and adaptive immune responses, mainly via their production of large amounts of type I IFNs. pDCs are considered to primarily present endogenous Ags and are thought not to participate in the uptake and presentation of Ags from the extracellular environment, in contrast to their myeloid counterparts, which efficiently endocytose extracellular particulates. In this study, we show that human pDCs are able to phagocytose and process particulate forms of Ag entrapped in poly(lactic-coglycolic acid) microparticles. Furthermore, pDCs were also able to sense TLR ligands (TLR-Ls) incorporated in these particles, resulting in rapid pDC activation and high IFN-alpha secretion. Combining a tetanus toxoid peptide and TLR-Ls (CpG C and R848) in these microparticles resulted in efficient pDC activation and concomitant Ag-specific T cell stimulation. Moreover, particulate Ag was phagocytosed and presented more efficiently than soluble Ag, indicating that microparticles can be exploited to facilitate efficient delivery of antigenic cargo and immunostimulatory molecules to pDCs. Together, our results show that in addition to their potency to stimulate innate immunity, pDCs can polarize adaptive immune responses against exogenous particulate Ag. These results may have important consequences for the development of new immunotherapeutic strategies exploiting Ag and TLR-Ls encapsulated in microparticles to target APC subsets.

Lactic acid bacteria as adjuvants for sublingual allergy vaccines

We compared immunomodulatory properties of 11 strains of lactic acid bacteria as well as their capacity to enhance sublingual immunotherapy efficacy in a murine asthma model. Two types of bacterial strains were identified, including: (i) potent inducers of IL-12p70 and IL-10 in dendritic cells, supporting IFN-gamma and IL-10 production in CD4+ T cells such as Lactobacillus helveticus; (ii) pure Th1 inducers such as L. casei. Sublingual administration in ovalbumin-sensitized mice of L. helveticus, but not L. casei, reduced airways hyperresponsiveness, bronchial inflammation and proliferation of specific T cells in cervical lymph nodes. Thus, probiotics acting as a Th1/possibly Treg, but not Th1 adjuvant, potentiate tolerance induction via the sublingual route.

Inhibition of dendritic cell maturation by group A Streptococcus

BACKGROUND

Exposure to group A Streptococcus (GAS) has been shown to induce maturation of dendritic cells (DC).

METHODS

To identify bacterial determinants that modulate DC activation in response to GAS infection, we analyzed the induction of maturation in human monocyte-derived DC following exposure to GAS clinical isolates.

RESULTS

Unexpectedly, only 6 of 24 GAS strains tested induced surface expression of major histocompatibility complex class II and costimulatory molecules CD80 and CD83 to levels consistent with DC maturation. Rather, the majority of the strains did not promote DC maturation, and many triggered DC apoptosis. GAS strains that failed to induce DC maturation were those that produced abundant hyaluronic acid (HA) capsular polysaccharide and/or large amounts of the cytotoxin streptolysin O (SLO). By use of isogenic mutants deficient in HA and/or SLO, we determined that GAS inhibits DC maturation through 2 distinct mechanisms: (1) inhibition of bacterial binding and/or phagocytosis by the HA capsule and (2) SLO-mediated induction of DC apoptosis by intracellular GAS.

CONCLUSIONS

These results demonstrate that GAS virulence factors modulate maturation and survival of human DC, effects that are likely to have a critical impact on activation of innate and adaptive immune responses to this important human pathogen.

The role of CD40 and CD154/CD40L in dendritic cells

In this review, we focus on the function of CD40-CD40L (CD154) interactions in the regulation of dendritic cell (DC)-T cell and DC-B cell crosstalk. In addition, we examine differences and similarities between the CD40 signaling pathway in DCs and other innate immune cell receptors, and how these pathways integrate DC functions. As research into DC vaccines and immunotherapies progresses, further understanding of CD40 and DC function will advance the applicability of DCs in immunotherapy for human diseases.

Critical immunological pathways are downregulated in APECED patient dendritic cells

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) is a monogenic autoimmune disease caused by mutations in the autoimmune regulator (AIRE) gene. AIRE functions as a transcriptional regulator, and it has a central role in the development of immunological tolerance. AIRE regulates the expression of ectopic antigens in epithelial cells of the thymic medulla and has been shown to participate in the development of peripheral tolerance. However, the mechanism of action of AIRE has remained elusive. To further investigate the role of AIRE in host immune functions, we studied the properties and transcript profiles in in vitro monocyte-differentiated dendritic cells (moDCs) obtained from APECED patients and healthy controls. AIRE-deficient monocytes showed typical DC morphology and expressed DC marker proteins cluster of differentiation 86 and human leukocyte antigen class II. APECED patient-derived moDCs were functionally impaired: the transcriptional response of cytokine genes to pathogens was drastically reduced. Interestingly, some changes were observable already at the immature DC stage. Pathway analyses of transcript profiles revealed that the expression of the components of the host cell signaling pathways involved in cell-cell signalling, innate immune responses, and cytokine activity were reduced in APECED moDCs. Our observations support a role for AIRE in peripheral tolerance and are the first ones to show that AIRE has a critical role in DC responses to microbial stimuli in humans.

Immune recognition of Streptococcus pyogenes by dendritic cells

Streptococcus pyogenes is one of the most frequent human pathogens. Recent studies have identified dendritic cells (DCs) as important contributors to host defense against S. pyogenes. The objective of this study was to identify the receptors involved in immune recognition of S. pyogenes by DCs. To determine whether Toll-like receptors (TLRs) were involved in DC sensing of S. pyogenes, we evaluated the response of bone marrow-derived DCs obtained from mice deficient in MyD88, an adapter molecule used by almost all TLRs, following S. pyogenes stimulation. Despite the fact that MyD88(-/-) DCs did not differ from wild-type DCs in the ability to internalize and kill S. pyogenes, the up-regulation of maturation markers, such as CD40, CD80, and CD86, and the production of inflammatory cytokines, such as interleukin-12 (IL-12), IL-6, and tumor necrosis factor alpha, were dramatically impaired in S. pyogenes-stimulated MyD88(-/-) DCs. These results suggest that signaling through TLRs is the principal pathway by which DCs sense S. pyogenes and become activated. Surprisingly, DCs deficient in signaling through each of the TLRs reported as potential receptors for gram-positive cell components, such as TLR1, TLR2, TLR4, TLR9, and TLR2/6, were not impaired in the secretion of proinflammatory cytokines and the up-regulation of costimulatory molecules after S. pyogenes stimulation. In conclusion, our results exclude a major involvement of a single TLR or the heterodimer TLR2/6 in S. pyogenes sensing by DCs and argue for a multimodal recognition in which a combination of several different TLR-mediated signals is essential for a rapid and effective response to the pathogen.