Bacterial recognition by TLR7 in the lysosomes of conventional dendritic cells

Toll-like receptor signaling pathways

Toll-like receptors (TLRs) play crucial roles in the innate immune system by recognizing pathogen-associated molecular patterns derived from various microbes. TLRs signal through the recruitment of specific adaptor molecules, leading to activation of the transcription factors NF-κB and IRFs, which dictate the outcome of innate immune responses. During the past decade, the precise mechanisms underlying TLR signaling have been clarified by various approaches involving genetic, biochemical, structural, cell biological, and bioinformatics studies. TLR signaling appears to be divergent and to play important roles in many aspects of the innate immune responses to given pathogens. In this review, we describe recent progress in our understanding of TLR signaling regulation and its contributions to host defense.

Role of Toll-like receptor 13 in innate immune recognition of group B streptococci

Murine Toll-like receptor 13 (TLR13), an endosomal receptor that is not present in humans, is activated by an unmethylated motif present in the large ribosomal subunit of bacterial RNA (23S rRNA). Little is known, however, of the impact of TLR13 on antibacterial host defenses. Here we examined the role of this receptor in the context of infection induced by the model pathogen group B streptococcus (GBS). To this end, we used bacterial strains masked from TLR13 recognition by virtue of constitutive expression of the ErmC methyltransferase, which results in dimethylation of the 23S rRNA motif at a critical adenine residue. We found that TLR13-mediated rRNA recognition was required for optimal induction of tumor necrosis factor alpha and nitrous oxide in dendritic cell and macrophage cultures stimulated with heat-killed bacteria or purified bacterial RNA. However, TLR13-dependent recognition was redundant when live bacteria were used as a stimulus. Moreover, masking bacterial rRNA from TLR13 recognition did not increase the ability of GBS to avoid host defenses and replicate in vivo. In contrast, increased susceptibility to infection was observed under conditions in which signaling by all endosomal TLRs was abolished, i.e., in mice with a loss-of-function mutation in the chaperone protein UNC93B1. Our data lend support to the conclusion that TLR13 participates in GBS recognition, although blockade of the function of this receptor can be compensated for by other endosomal TLRs. Lack of selective pressure by bacterial infections might explain the evolutionary loss of TLR13 in humans. However, further studies using different bacterial species are needed to prove this hypothesis.

The TIR-domain containing adaptor TRAM is required for TLR7 mediated RANTES production

Toll-like receptor 7 (TLR7) plays a vital role in the immune response to ssRNA viruses such as human rhinovirus (HRV) and Influenza, against which there are currently no treatments or vaccines with long term efficacy available. Clearly, a more comprehensive understanding of the TLR7 signaling axis will contribute to its molecular targeting. TRIF related adaptor molecule (TRAM) plays a vital role in TLR4 signaling by recruiting TRIF to TLR4, followed by endosomal trafficking of the complex and initiation of IRF3 dependent type I interferon production as well as NF-κB dependent pro-inflammatory cytokine production. Towards understanding the molecular mechanisms that regulate TLR7 functionality, we found that TRAM^−/− murine macrophages exhibited a transcriptional and translational impairment in TLR7 mediated RANTES, but not TNFα, production. Suppression of TRAM expression in human macrophages also resulted in an impairment in TLR7 mediated CCL5 and IFN-β, but not TNFα, gene induction. Furthermore, suppression of endogenous human TRAM expression in human macrophages significantly impaired RV16 induced CCL5 and IFNβ, but not TNFα gene induction. Additionally, TRAM-G2A dose-dependently inhibited TLR7 mediated activation of CCL5, IFNβ and IFNα reporter genes. TLR7-mediated phosphorylation and nuclear translocation of IRF3 was impaired in TRAM^−/− cells. Finally, co-immunoprecipitation studies indicated that TRAM physically interacts with MyD88 upon TLR7 stimulation, but not under basal conditions. Our results clearly demonstrate that TRAM plays a, hitherto unappreciated, role in TLR7 signaling through a novel signaling axis containing, but not limited to, MyD88, TRAM and IRF3 towards the activation of anti-viral immunity.

Essential role of interleukin-1 signaling in host defenses against group B streptococcus

Signal transduction via MyD88, an adaptor protein engaged by the Toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) family receptors, has a crucial role in host defenses against group B streptococcus (GBS). To examine the contribution of IL-1R signaling to MyD88-dependent host defenses, we analyzed GBS infection in type I IL-1R (IL-1RI)-deficient mice. Most of these animals displayed clinical signs of sepsis and neurological disease and died after a challenge with a bacterial dose that did not cause illness or death in any of the wild-type animals. Moreover, bacterial numbers in the blood and brains of the immunodefective mice were considerably increased. The ability of blood leukocytes or bone marrow-derived macrophages to kill GBS in vitro was not affected by a lack of IL-1RI. However, it was found in a newly developed model of GBS-induced peritoneal inflammation that IL-1 signaling selectively promoted the production of the chemokines KC and MIP-1α and neutrophil recruitment. Moreover, the secretion of KC and MIP-1α, but not tumor necrosis factor alpha, by peritoneal macrophages stimulated with GBS was significantly decreased in the absence of IL-1RI. Accordingly, the number of neutrophils in the blood and the concentration of myeloperoxidase, a neutrophil marker, in infected organs were severely reduced in the immunodefective mice during GBS disease, concomitantly with a reduction in tissue KC and MIP-1α levels. In conclusion, IL-1RI plays a crucial role in host defenses against GBS by inducing the high-level production of chemokines and the subsequent recruitment of neutrophilic polymorphonuclear leukocytes to infection sites.

Group B streptococcus (GBS) is a serious and frequent human pathogen. Experimental infection with this bacterium has been widely used to understand the mechanism whereby the body’s first line of defense, represented by cells and molecules of the innate immune system, fights infections. In both humans and mice, defective function of the adaptor molecule MyD88 has been associated with extreme susceptibility to infection by GBS and other extracellular bacteria. We show here that lack of signaling by interleukin-1 (IL-1) cytokines can largely, although not completely, explain the increased susceptibility to infection observed in the absence of MyD88 function. We show, in particular, that IL-1 signaling through the IL-1 receptor promotes the production of the leukocyte attractant chemokines KC and MIP-1α and recruitment of neutrophils to GBS infection sites, thereby enabling these leukocytes to clear the infection. Our findings indicate that stimulation of IL-1 signaling may be useful as an alternative therapeutic strategy to treat GBS infections.

The interleukin-1β/CXCL1/2/neutrophil axis mediates host protection against group B streptococcal infection

Previous studies have indicated that group B streptococcus (GBS), a frequent human pathogen, potently induces the release of interleukin-1β (IL-1β), an important mediator of inflammatory responses. Since little is known about the role of this cytokine in GBS disease, we analyzed the outcome of infection in IL-1β-deficient mice. These animals were markedly sensitive to GBS infection, with most of them dying under challenge conditions that caused no deaths in wild-type control mice. Lethality was due to the inability of the IL-1β-deficient mice to control local GBS replication and dissemination to target organs, such as the brain and the kidneys. Moreover, in a model of inflammation induced by the intraperitoneal injection of killed GBS, a lack of IL-1β was associated with selective impairment in the production of the neutrophil chemokines CXCL1 and CXCL2 and in neutrophil recruitment to the peritoneal cavity. Decreased blood neutrophil counts and impaired neutrophil recruitment to the brain and kidneys were also observed during GBS infection in IL-1β-deficient mice concomitantly with a reduction in CXCL1 and CXCL2 tissue levels. Notably, the hypersusceptibility to GBS infection observed in the immune-deficient animals was recapitulated by neutrophil depletion with anti-Gr1 antibodies. Collectively, our data identify a cytokine circuit that involves IL-1β-induced production of CXCL1 and CXCL2 and leads the recruitment of neutrophils to GBS infection sites. Moreover, our data point to an essential role of these cells in controlling the progression and outcome of GBS disease.

Toll-like receptor 7-mediated type I interferon signaling prevents cholestasis- and hepatotoxin-induced liver fibrosis

Toll-like receptor 7 (TLR7) signaling predominantly regulates production of type I interferons (IFNs), which has been suggested in clinical studies to be antifibrotic. However, the mechanistic role of the TLR7-type I IFN axis in liver fibrosis has not been elucidated. In the present study, liver fibrosis was induced in wild-type (WT), TLR7-deficient, and IFN-α/β receptor-1 (IFNAR1)-deficient mice and TLR7-mediated signaling was assessed in liver cells isolated from these mice. TLR7-deficient and IFNAR1-deficient mice were more susceptible to liver fibrosis than WT mice, indicating that TLR7-type I IFN signaling exerts a protective effect against liver fibrosis. Notably, the hepatic expression of interleukin-1 receptor antagonist (IL-1ra) was suppressed in TLR7- or IFNAR1-deficient mice compared with respective WT mice, and treatment with recombinant IL-1ra reduced liver fibrosis. In vivo activation of TLR7 significantly increased IFNa4 and IL-1ra expression in the liver. Interestingly, each cytokine had a different cellular source, showing that dendritic cells (DCs) are the responsible cell type for production of type I IFN, while Kupffer cells (KCs) mainly produce IL-1ra in response to type I IFN. Furthermore, TLR7 activation by R848 injection suppressed liver fibrosis and production of proinflammatory cytokines, and these effects were dependent on type I IFN signaling. Consistent with in vivo data, IFN-α significantly induced IL-1ra production in primary KCs.

CONCLUSION

TLR7 signaling activates DCs to produce type I IFN, which in turn induces antifibrogenic IL-1ra production in KCs. Thus, manipulation of the TLR7-type I IFN-IL-1ra axis may be a new therapeutic strategy for the treatment of liver fibrosis.

Innate immunity networks during infection with Borrelia burgdorferi

The recognition of Borrelia species represents a complex process in which multiple components of the immune system are involved. In this review, we summarize the interplay between the host innate system and Borrelia spp., from the recognition by pattern recognition receptors (PRRs) to the induction of a complex network of proinflammatory mediators. Several PRR families are crucial for recognition of Borrelia spp., including Toll-like receptors (TLRs) and Nucleotide Oligomerization Domain (NOD)-like receptors (NLRs). TLR-2 is crucial for the recognition of outer surface protein (Osp)A from Borrelia spp. and together with TLR8 mediates phagocytosis of the microorganism and production of type I interferons. Intracellular receptors such as TLR7, TLR8 and TLR9 on the one hand and the NLR receptor NOD2 on the other hand, represent the second major recognition system of Borrelia. PRR-dependent signals induce the release of pro-inflammatory cytokines such as interleukin-1 and T-helper-derived cytokines, which are thought to mediate the inflammation during Lyme disease. Understanding the regulation of host defense mechanisms against Borrelia has the potential to lead to the discovery of novel immunotherapeutic targets to improve the therapy against Lyme disease.

Induction of type I and type III interferons by Borrelia burgdorferi correlates with pathogenesis and requires linear plasmid 36

The capacity for Borrelia burgdorferi to cause disseminated infection in humans or mice is associated with the genotype of the infecting strain. The cytokine profiles elicited by B. burgdorferi clinical isolates of different genotype (ribosomal spacer type) groups were assessed in a human PBMC co-incubation model. RST1 isolates, which are more frequently associated with disseminated Lyme disease in humans and mice, induced significantly higher levels of IFN-α and IFN-λ1/IL29 relative to RST3 isolates, which are less frequently associated with disseminated infection. No differences in the protein concentrations of IFN-γ, IL-1β, IL-6, IL-8, IL-10 or TNF-α were observed between isolates of differing genotype. The ability of B. burgdorferi to induce type I and type III IFNs was completely dependent on the presence of linear plasmid (lp) 36. An lp36-deficient B. burgdorferi mutant adhered to, and was internalized by, PBMCs and specific dendritic cell (DC) subsets less efficiently than its isogenic B31 parent strain. The association defect with mDC1s and pDCs could be restored by complementation of the mutant with the complete lp36. The RST1 clinical isolates studied were found to contain a 2.5-kB region, located in the distal one-third of lp36, which was not present in any of the RST3 isolates tested. This divergent region of lp36 may encode one or more factors required for optimal spirochetal recognition and the production of type I and type III IFNs by human DCs, thus suggesting a potential role for DCs in the pathogenesis of B. burgdorferi infection.

Phagosomal TLR signaling upon Borrelia burgdorferi infection

Internalization and degradation of live Bb within phagosomal compartments of monocytes, macrophages and dendritic cells (DCs), allows for the release of lipoproteins, nucleic acids and other microbial products, triggering a broad and robust inflammatory response. Toll-like receptors (TLRs) are key players in the recognition of spirochetal ligands from whole viable organisms (i.e., vita-PAMPs). Herein we will review the role of endosomal TLRs in the response to the Lyme disease spirochete.

Borrelia burgdorferi RNA induces type I and III interferons via Toll-like receptor 7 and contributes to production of NF-κB-dependent cytokines

Borrelia burgdorferi elicits a potent cytokine response through activation of multiple signaling receptors on innate immune cells. Spirochetal lipoproteins initiate expression of NF-κB-dependent cytokines primarily via TLR2, whereas type I interferon (IFN) production is induced through the endosomal receptors TLR7 and TLR9 in human dendritic cells and TLR8 in monocytes. We demonstrate that DNA and RNA are the B. burgdorferi components that initiate a type I IFN response by human peripheral blood mononuclear cells (PBMCs). IFN-α protein and transcripts for IRF7, MX1, and OAS1 were induced by endosomal delivery of B. burgdorferi DNA, RNA, or whole-cell lysate, but not by lysate that had been treated with DNase and RNase. Induction of IFN-α and IFN-λ1, a type III IFN, by B. burgdorferi RNA or live spirochetes required TLR7-dependent signaling and correlated with significantly enhanced transcription and expression of IRF7 but not IRF3. Induction of type I and type III IFNs by B. burgdorferi RNA could be completely abrogated by a TLR7 inhibitor, IRS661. In addition to type I and type III IFNs, B. burgdorferi RNA contributed to the production of the NF-κB-dependent cytokines, IFN-γ, interleukin-10 (IL-10), IL-1β, IL-6, and tumor necrosis factor alpha (TNF-α), by human PBMCs. Collectively, these data indicate that TLR7-dependent recognition of RNA is pivotal for IFN-α and IFN-λ1 production by human PBMCs, and that RNA-initiated signaling contributes to full potentiation of the cytokine response generated during B. burgdorferi infection.

Identification of genes preferentially expressed by highly virulent piscine Streptococcus agalactiae upon interaction with macrophages

Streptococcus agalactiae, long recognized as a mammalian pathogen, is an emerging concern with regard to fish. In this study, we used a mouse model and in vitro cell infection to evaluate the pathogenetic characteristics of S. agalactiae GD201008-001, isolated from tilapia in China. This bacterium was found to be highly virulent and capable of inducing brain damage by migrating into the brain by crossing the blood–brain barrier (BBB). The phagocytosis assays indicated that this bacterium could be internalized by murine macrophages and survive intracellularly for more than 24 h, inducing injury to macrophages. Further, selective capture of transcribed sequences (SCOTS) was used to investigate microbial gene expression associated with intracellular survival. This positive cDNA selection technique identified 60 distinct genes that could be characterized into 6 functional categories. More than 50% of the differentially expressed genes were involved in metabolic adaptation. Some genes have previously been described as associated with virulence in other bacteria, and four showed no significant similarities to any other previously described genes. This study constitutes the first step in further gene expression analyses that will lead to a better understanding of the molecular mechanisms used by S. agalactiae to survive in macrophages and to cross the BBB.

Human TLR8 is activated upon recognition of Borrelia burgdorferi RNA in the phagosome of human monocytes

Phagocytosed Borrelia burgdorferi (Bb), the Lyme disease spirochete, induces a robust and complex innate immune response in human monocytes, in which TLR8 cooperates with TLR2 in the induction of NF-κB-mediated cytokine production, whereas TLR8 is solely responsible for transcription of IFN-β through IRF7. We now establish the role of Bb RNA in TLR8-mediated induction of IFN-β. First, using TLR2-transfected HEK.293 cells, which were unable to phagocytose intact Bb, we observed TLR2 activation by lipoprotein-rich borrelial lysates and TLR2 synthetic ligands but not in response to live spirochetes. Purified Bb RNA, but not borrelial DNA, triggered TLR8 activation. Neither of these 2 ligands induced activation of TLR7. Using purified human monocytes we then show that phagocytosed live Bb, as well as equivalent amounts of borrelial RNA delivered into the phagosome by polyethylenimine (PEI), induces transcription of IFN-β and secretion of TNF-α. The cytokine response to purified Bb RNA was markedly impaired in human monocytes naturally deficient in IRAK-4 and in cells with knockdown TLR8 expression by small interfering RNA. Using confocal microscopy we provide evidence that TLR8 colocalizes with internalized Bb RNA in both early (EEA1) and late endosomes (LAMP1). Live bacterial RNA staining indicates that spirochetal RNA does not transfer from the phagosome into the cytosol. Using fluorescent dextran particles we show that phagosomal integrity in Bb-infected monocytes is not affected. We demonstrate, for the first time, that Bb RNA is a TLR8 ligand in human monocytes and that transcription of IFN-β in response to the spirochete is induced from within the phagosomal vacuole through the TLR8-MyD88 pathway.

Regulation of NO synthesis, local inflammation, and innate immunity to pathogens by BET family proteins

Transcriptional activation of the Nos2 gene, encoding inducible nitric oxide synthase (iNOS), during infection or inflammation requires coordinate assembly of an initiation complex by the transcription factors NF-κB and type I interferon-activated ISGF3. Here we show that infection of macrophages with the intracellular bacterial pathogen Listeria monocytogenes caused binding of the BET proteins Brd2, Brd3, and, most prominently, Brd4 to the Nos2 promoter and that a profound reduction of Nos2 expression occurred in the presence of the BET inhibitor JQ1. RNA polymerase activity at the Nos2 gene was regulated through Brd-mediated C-terminal domain (CTD) phosphorylation at serine 5. Underscoring the critical importance of Brd for the regulation of immune responses, application of JQ1 reduced NO production in mice infected with L. monocytogenes, as well as innate resistance to L. monocytogenes and influenza virus. In a murine model of inflammatory disease, JQ1 treatment increased the colitogenic activity of dextran sodium sulfate (DSS). The data presented in our study suggest that BET protein inhibition in a clinical setting poses the risk of altering the innate immune response to infectious or inflammatory challenge.

The genetics of innate immunity sensors and human disease

Since their discovery, innate immunity microbial sensors have been increasingly studied and shown to play a critical role in innate responses to microbes in several experimental in vitro, ex vivo, and animal models. However, their role in the human response to infection in natural conditions has just started to be deciphered, by means of clinical studies of primary immunodeficiencies and epidemiological genetic studies. Here, we summarize the major findings concerning the genetic diversity of the various families of microbial sensors in humans, and of other molecules involved in the signaling pathways they trigger. Specifically, we review the genetic associations, revealed by both clinical and epidemiological genetics studies, of microbial sensors from five different families: Toll-like receptors, C-type lectin receptors, NOD-like receptors, RIG-I-like receptors, and cytosolic DNA sensors. In particular, we consider the relationships between variation at the genes encoding these molecules and susceptibility to and the severity of infectious diseases and other clinical conditions associated with immune dysfunction, including autoimmunity, inflammation, allergy, and cancer. Despite the fact that the genetic links between innate immunity sensors and human disorders remain still limited, human genetics studies are increasingly improving our understanding of the genuine functions of microbial sensors and downstream signaling molecules in the natural setting.

The NOD2 receptor does not play a major role in the pathogenesis of Group B Streptococcus in mice

Group B Streptococcus (GBS) capsular type III is an important agent of life-threatening invasive infections. It has been previously shown that encapsulated GBS is easily internalized by dendritic cells (DCs) and this internalization has an impact on cytokine production. The intracellular receptors or pathways underlying this response are not well understood. In this work, we investigated the role of NOD2 in the pathogenesis of GBS using a mouse model of infection. NOD2(-/-) mice showed similar levels of survival and bacteremia than control mice. Interestingly, ex vivo analysis of total spleen cells from infected animals showed that the absence of NOD2 results in reduced production of inflammatory cytokines. However this abridged inflammatory response does not seem to improve mouse survival. In conclusion, we demonstrated that NOD2 is not a crucial receptor to fight GBS infection and only partially contributes to the inflammatory response.

CD4+ T cell expression of MyD88 is essential for normal resolution of Chlamydia muridarum genital tract infection

Resolution of Chlamydia genital tract infection is delayed in the absence of MyD88. In these studies, we first used bone marrow chimeras to demonstrate a requirement for MyD88 expression by hematopoietic cells in the presence of a wild-type epithelium. Using mixed bone marrow chimeras we then determined that MyD88 expression was specifically required in the adaptive immune compartment. Furthermore, adoptive transfer experiments revealed that CD4(+) T cell expression of MyD88 was necessary for normal resolution of genital tract infection. This requirement was associated with a reduced ability of MyD88(-/-)CD4(+) T cells to accumulate in the draining lymph nodes and genital tract when exposed to the same inflammatory milieu as wild-type CD4(+) T cells. We also demonstrated that the impaired infection control we observed in the absence of MyD88 could not be recapitulated by deficiencies in TLR or IL-1R signaling. In vitro, we detected an increased frequency of apoptotic MyD88(-/-)CD4(+) T cells upon activation in the absence of exogenous ligands for receptors upstream of MyD88. These data reveal an intrinsic requirement for MyD88 in CD4(+) T cells during Chlamydia infection and indicate that the importance of MyD88 extends beyond innate immune responses by directly influencing adaptive immunity.

Nucleic acid scavenging polymers inhibit extracellular DNA-mediated innate immune activation without inhibiting anti-viral responses

Toll-like receptor (TLR) family members, 3, 7 and 9 are key components in initiation and progression of autoimmune disorders such as systemic lupus erythematosus (SLE). These TLRs are often referred to as nucleic acid-sensing TLRs based on their ability to recognize DNAs or RNAs produced by pathogens or damaged cells. During autoimmune disease progression these receptors recognize self nucleic acids as well as self nucleic acid-containing complexes and contribute to inflammatory cytokine production and subsequent enhancement of serum autoantibody levels. We have recently discovered that nucleic-acid scavenging polymers (NASPs) can neutralize the proinflammatory effects of nucleic acids. Here, we begin to explore what effects such NASPs have on normal immune function. We show that such NASPs can inhibit TLR activation without affecting nucleic acid-independent T cell activation. Moreover, we observe that stimulation of immune cells by encapsulated nucleic acids, such as those found in viral particles, is unaffected by NASPs. Thus NASPs only limit the activation of the immune system by accessible extra-cellular nucleic acid and do not engender non-specific immune suppression. These important findings suggest that NASPs represent a new approach toward anti-inflammatory drug development as these agents can potentially be utilized to block overt autoimmune disorders and inflammation while allowing normal immune responses to occur.

Early inhibition of IL-1β expression by IFN-γ is mediated by impaired binding of NF-κB to the IL-1β promoter but is independent of nitric oxide

The significance of bacterial RNA recognition for initiating innate immune responses against invading pathogens has only recently started to be elucidated. Bacterial RNA is an important trigger of inflammasome activation, resulting in caspase-1-dependent cleavage of pro-IL-1β into the active form. It was reported previously that prolonged treatment with IFN-γ can inhibit IL-1β production at the level of both transcription and Nlrp3 inflammasome activation in an NO-dependent manner. As a result of the delayed kinetics of NO generation after IFN-γ stimulation, these effects were only observed at later time points. We report that IFN-γ suppressed bacterial RNA and LPS induced IL-1β transcription in primary murine macrophages and dendritic cells by an additional, very rapid mechanism that was independent of NO. Costimulation with IFN-γ selectively attenuated binding of NF-κB p65 to the IL-1β promoter, thus representing a novel mechanism of IL-1β inhibition by IFN-γ. Transcriptional silencing was specific for IL-1β because expression of other proinflammatory cytokines, such as TNF, IL-6, and IL-12p40, was not affected. Furthermore, by suppressing IL-1β production, IFN-γ impaired differentiation of Th17 cells and production of neutrophil chemotactic factor CXCL1 in vitro. The findings provide evidence for a rapid immune-modulating effect of IFN-γ independent of NO.

RIG-I detects triphosphorylated RNA of Listeria monocytogenes during infection in non-immune cells

The innate immune system senses pathogens by pattern recognition receptors in different cell compartments. In the endosome, bacteria are generally recognized by TLRs; facultative intracellular bacteria such as Listeria, however, can escape the endosome. Once in the cytosol, they become accessible to cytosolic pattern recognition receptors, which recognize components of the bacterial cell wall, metabolites or bacterial nucleic acids and initiate an immune response in the host cell. Current knowledge has been focused on the type I IFN response to Listeria DNA or Listeria-derived second messenger c-di-AMP via the signaling adaptor STING. Our study focused on the recognition of Listeria RNA in the cytosol. With the aid of a novel labeling technique, we have been able to visualize immediate cytosolic delivery of Listeria RNA upon infection. Infection with Listeria as well as transfection of bacterial RNA induced a type-I-IFN response in human monocytes, epithelial cells or hepatocytes. However, in contrast to monocytes, the type-I-IFN response of epithelial cells and hepatocytes was not triggered by bacterial DNA, indicating a STING-independent Listeria recognition pathway. RIG-I and MAVS knock-down resulted in abolishment of the IFN response in epithelial cells, but the IFN response in monocytic cells remained unaffected. By contrast, knockdown of STING in monocytic cells reduced cytosolic Listeria-mediated type-I-IFN induction. Our results show that detection of Listeria RNA by RIG-I represents a non-redundant cytosolic immunorecognition pathway in non-immune cells lacking a functional STING dependent signaling pathway.

Age-dependent differences in systemic and cell-autonomous immunity to L. monocytogenes

Host defense against infection can broadly be categorized into systemic immunity and cell-autonomous immunity. Systemic immunity is crucial for all multicellular organisms, increasing in importance with increasing cellular complexity of the host. The systemic immune response to Listeria monocytogenes has been studied extensively in murine models; however, the clinical applicability of these findings to the human newborn remains incompletely understood. Furthermore, the ability to control infection at the level of an individual cell, known as “cell-autonomous immunity,” appears most relevant following infection with L. monocytogenes; as the main target, the monocyte is centrally important to innate as well as adaptive systemic immunity to listeriosis. We thus suggest that the overall increased risk to suffer and die from L. monocytogenes infection in the newborn period is a direct consequence of age-dependent differences in cell-autonomous immunity of the monocyte to L. monocytogenes. We here review what is known about age-dependent differences in systemic innate and adaptive as well as cell-autonomous immunity to infection with Listeria monocytogenes.

Recognition of pathogen-associated nucleic acids by endosomal nucleic acid-sensing toll-like receptors

Foreign nucleic acids, the essential signature molecules of invading pathogens that act as danger signals for host cells, are detected by endosomal nucleic acid-sensing toll-like receptors (TLRs) 3, 7, 8, 9, and 13. These TLRs have evolved to recognize ‘non-self’ nucleic acids within endosomal compartments and rapidly initiate innate immune responses to ensure host protection through induction of type I interferons, inflammatory cytokines, chemokines, and co-stimulatory molecules and maturation of immune cells. In this review, we highlight our understanding of the recognition of pathogen-associated nucleic acids and activation of corresponding signaling pathways through endosomal nucleic acid-sensing TLRs 3, 7, 8, 9, and 13 for an enormous diversity of pathogens, with particular emphasis on their compartmentalization, intracellular trafficking, proteolytic cleavage, autophagy, and regulatory programs.

A critical role for MAPK signalling pathways in the transcriptional regulation of toll like receptors

Toll-like Receptors (TLR) are phylogenetically conserved transmembrane proteins responsible for detection of pathogens and activation of immune responses in diverse animal species. The stimulation of TLR by pathogen-derived molecules leads to the production of pro-inflammatory mediators including cytokines and nitric oxide. Although TLR-induced events are critical for immune induction, uncontrolled inflammation can be life threatening and regulation is a critical feature of TLR biology. We used an avian macrophage cell line (HD11) to determine the relationship between TLR agonist-induced activation of inflammatory responses and the transcriptional regulation of TLR. Exposure of macrophages to specific TLR agonists induced upregulation of cytokine and nitric oxide pathways that were inhibited by blocking various components of the TLR signalling pathways. TLR activation also led to changes in the levels of mRNA encoding the TLR responsible for recognising the inducing agonist (cognate regulation) and cross-regulation of other TLR (non-cognate regulation). Interestingly, in most cases, regulation of TLR mRNA was independent of NFκB activity but dependent on one or more of the MAPK pathway components. Moreover, the relative importance of ERK, JNK and p38 was dependent upon both the stimulating agonist and the target TLR. These results provide a framework for understanding the complex pathways involved in transcriptional regulation of TLR, immune induction and inflammation. Manipulation of these pathways during vaccination or management of acute inflammatory disease may lead to improved clinical outcome or enhanced vaccine efficacy.

Innate immune function by Toll-like receptors: distinct responses in newborns and the elderly

Given the "inborn" nature of the innate immune system, it is surprising to find that innate immune function does in fact change with age. Similar patterns of distinct Toll-like-receptor-mediated immune responses come to light when one contrasts innate immune development at the beginning of life with that toward the end of life. Importantly, these developmental patterns of innate cytokine responses correlate with clinical patterns of susceptibility to disease: A heightened risk of suffering from excessive inflammation is often detected in prematurely born infants, disappears over the first few months of life, and reappears toward the end of life. In addition, risk periods for particular infections in early life reemerge in older adults. The near-mirror-image patterns that emerge in contrasts of early versus late innate immune ontogeny emphasize changes in host-environment interactions as the underlying molecular and teleologic drivers.

TLR7 triggering with polyuridylic acid promotes cross-presentation in CD8α+ conventional dendritic cells by enhancing antigen preservation and MHC class I antigen permanence on the dendritic cell surface

ssRNA can interact with dendritic cells (DCs) through binding to TLR7, inducing secretion of proinflammatory cytokines and type I IFN. Triggering TLR7 enhances cross-priming of CD8(+) T cells, which requires cross-presentation of exogenous Ag to DCs. However, how TLR triggering can affect Ag cross-presentation is still not clear. Using OVA as an Ag model, we observed that stimulation of TLR7 in DCs by polyuridylic acid (polyU), a synthetic ssRNA analog, generates a strong specific cytotoxic response in C57BL/6 mice. PolyU stimulate CD8α(+) DCs to cross-prime naive CD8(+) T cells in a type I IFN-dependent fashion. This enhanced cross-priming is accompanied by a higher density of OVA(256-264)/H-2K(b) complexes on CD8α(+) DCs treated with polyU, as well as by upregulation of costimulatory molecules and increased secretion of proinflammatory cytokines by DCs. Cross-priming of CD8(+) T cells by DCs treated with polyU requires proteasome and Ag translocation to cytosol through the Sec61 channel in DCs. The observed enhancement in OVA cross-presentation with polyU in DCs could be mediated by a limited Ag degradation in endophagosomal compartments and a higher permanence of OVA peptide/MHC class I complexes on DCs. These observations clearly reveal that key steps of Ag processing for cross-presentation can be modulated by TLR ligands, opening new avenues for understanding their mechanisms as adjuvants of the immune response.

Increased toll-like receptor signaling pathways characterize CD8+ cells in rapidly progressive SIV infection

Similar to HIV infection in humans, SIV infection in macaques induces progressive loss of immune cell components and function, resulting in immune deficiency in nearly all untreated infected subjects. In SIV-infected macaques, 25% of animals develop terminal AIDS within 6 months of infection. The factors responsible for the development of such rapid progression are unknown. We have previously found that defects in CD8+ T cells detectable from early infection correlate to rapid progression to simian AIDS. The transcriptional screening of molecular fingerprints on different steps along the activation/effector process of splenic CD8+ cells at termination revealed a distinction in rapid compared to regular progressors, which was characterized by a decrease in classic T cell receptor (TCR) components, and an increase in Toll-like receptor (TLR) and apoptotic pathways. A TLR pathway screening in lymphoid and myeloid cells from both the spleen and from the central nervous system of infected macaques revealed that the upregulation of TLR is not in the innate immune compartment, but rather in lymphoid cells that contain adaptive immune cells. Our findings suggest that opposing effects of TCR specific signaling and TLR engagement may drive the CD8 phenotypic failure that determines a rapid disease course in HIV infection.

Activation and regulation of interferon-β in immune responses

Interferons (IFNs) were discovered more than half a century ago, and extensive research has since identified multifarious roles for type I IFN in human immune responses. Here, we review the functions of IFN-β in innate and adaptive immunity. We also discuss the activation and influence of IFN-β on myeloid cell types, including monocytes and dendritic cells, as well as address the effects of IFN-β on T cells and B cells. Findings from our own laboratory, which explores the molecular mechanisms of IFN-β activation by LPS and viruses, as well as from other groups investigating the regulation of IFN-β by viral proteins and endogenous factors are described. The effects of post-translational modifications of the interferon regulatory factor (IRF)-3 on IFN-β induction are also highlighted. Many unanswered questions remain concerning the regulation of the type I IFN response in inflammation, especially the role of transcription factors in the modulation of inflammatory gene expression, and these questions will form the basis for exciting avenues of future research.

The role of innate immune signals in immunity to Brucella abortus

Innate immunity serves as the first line of defense against infectious agents such as intracellular bacteria. The innate immune platform includes Toll-like receptors (TLRs), retinoid acid-inducible gene-I-like receptors and other cytosolic nucleic acid sensors, nucleotide-binding and oligomerization domain-like receptors, adaptors, kinases and other signaling molecules that are required to elicit effective responses against different pathogens. Our research group has been using the Gram-negative bacteria Brucella abortus as a model of pathogen. We have demonstrated that B. abortus triggers MAPK and NF-κB signaling pathways in macrophages in a MyD88 and IRAK-4-dependent manner. Furthermore, we claimed that so far TLR9 is the most important single TLR during Brucella infection. The identification of host receptors that recognize pathogen-derived nucleic acids has revealed an essential role for nucleic acid sensing in the triggering of immunity to intracellular pathogens. Besides TLRs, herein we describe recent advances in NOD1, NOD2, and type I IFN receptors in innate immune pathways during B. abortus infection.

TLR8: the forgotten relative revindicated

The endosomal Toll-like receptors (TLRs) TLR3, TLR7, TLR8 and TLR9 are important in sensing foreign nucleic acids encountered by phagocytes. Because TLR8 was initially thought to be non-functional in mice, less is known about TLR8 than the genetically and functionally related TLR7. Originally associated with the recognition of single-stranded RNA of viral origin, there is now evidence that human TLR8 is also able to sense bacterial RNA released within phagosomal vacuoles, inducing the production of both nuclear factor (NF)-κB-dependent cytokines and type I interferons (IFNs), such as IFN-β. The functions of TLR8 extend beyond the recognition of foreign pathogens and include cross-talk with other endosomal TLRs, a process that may also have a role in the generation of autoimmunity.

Recognition of fungal RNA by TLR7 has a nonredundant role in host defense against experimental candidiasis

Despite convincing evidence for involvement of members of the Toll-like receptor (TLR) family in fungal recognition, little is known of the functional role of individual TLRs in antifungal defenses. We found here that TLR7 was partially required for the induction of IL-12 (IL-12p70) by Candida albicans or Saccharomyces cerevisiae. Moreover, the IL-12p70 response was completely abrogated in cells from 3d mice, which are unable to mobilize TLRs to endosomal compartments, as well as in cells from mice lacking either the TLR adaptor MyD88 or the IRF1 transcription factor. Notably, purified fungal RNA recapitulated IL-12p70 induction by whole yeast. Although RNA could also induce moderate TLR7-dependent IL-23 and tumor necrosis factor-alpha (TNF-α) secretion, TLR7 and other endosomal TLRs were redundant for IL-23 or TNF-α induction by whole fungi. Importantly, mice lacking TLR7 or IRF1 were hypersusceptible to systemic C. albicans infection. Our data suggest that IRF1 is downstream of a novel, nonredundant fungal recognition pathway that has RNA as a major target and requires phagosomal recruitment of intracellular TLRs. This pathway differs from those involved in IL-23 or TNF-α responses, which we show here to be independent from translocation of intracellular TLRs, phagocytosis, or phagosomal acidification.

Cutting edge: TLR13 is a receptor for bacterial RNA

Bacterial RNA (bRNA) can induce cytokine production in macrophages and dendritic cells (DCs) through a previously unidentified receptor. Gene expression analysis of murine DCs showed that bRNA induced gene regulation similar to that induced by stimulation of TLR7 with R848. Although TLR7 was dispensable for cytokine induction by bRNA, TLR-associated proteins MyD88 and UNC93B were required. TLR13 is an endosomal murine TLR that has been described to interact with UNC93B with, so far, no characterized ligand. Small interfering RNA against TLR13 reduced cytokine induction by bRNA in DCs. Moreover, Chinese hamster ovary cells transfected with TLR13, but not with TLR7 or 8, could activate NF-κB in response to bRNA or Streptococcus pyogenes in an RNA-specific manner. TLR7 antagonist IRS661 could, in addition, inhibit TLR13 signaling and reduced recognition of whole Gram-positive bacteria by DCs, also in the absence of TLR7. The results identify TLR13 as a receptor for bRNA.

MyD88 drives the IFN-β response to Lactobacillus acidophilus in dendritic cells through a mechanism involving IRF1, IRF3, and IRF7

Type I IFNs are induced by pathogens to protect the host from infection and boost the immune response. We have recently demonstrated that this IFN response is not restricted to pathogens, as the Gram-positive bacterium Lactobacillus acidophilus, a natural inhabitant of the intestine, induces high levels of IFN-β in dendritic cells. In the current study, we investigate the intracellular pathways involved in IFN-β upon stimulation of dendritic cells with L. acidophilus and reveal that this IFN-β induction requires phagosomal uptake and processing but bypasses the endosomal receptors TLR7 and TLR9. The IFN-β production is fully dependent on the TIR adapter molecule MyD88, partly dependent on IFN regulatory factor (IRF)1, but independent of the TIR domain-containing adapter inducing IFN-β MyD88 adapter-like, IRF and IRF7. However, our results suggest that IRF3 and IRF7 have complementary roles in IFN-β signaling. The IFN-β production is strongly impaired by inhibitors of spleen tyrosine kinase (Syk) and PI3K. Our results indicate that L. acidophilus induces IFN-β independently of the receptors typically used by bacteria, as it requires MyD88, Syk, and PI3K signaling and phagosomal processing to activate IRF1 and IRF3/IRF7 and thereby the release of IFN-β.

Innate immune cells in liver inflammation

Innate immune system is the first line of defence against invading pathogens that is critical for the overall survival of the host. Human liver is characterised by a dual blood supply, with 80% of blood entering through the portal vein carrying nutrients and bacterial endotoxin from the gastrointestinal tract. The liver is thus constantly exposed to antigenic loads. Therefore, pathogenic microorganism must be efficiently eliminated whilst harmless antigens derived from the gastrointestinal tract need to be tolerized in the liver. In order to achieve this, the liver innate immune system is equipped with multiple cellular components; monocytes, macrophages, granulocytes, natural killer cells, and dendritic cells which coordinate to exert tolerogenic environment at the same time detect, respond, and eliminate invading pathogens, infected or transformed self to mount immunity. This paper will discuss the innate immune cells that take part in human liver inflammation, and their roles in both resolution of inflammation and tissue repair.

TLR13 recognizes bacterial 23S rRNA devoid of erythromycin resistance-forming modification

Host protection from infection relies on the recognition of pathogens by innate pattern-recognition receptors such as Toll-like receptors (TLRs). Here, we show that the orphan receptor TLR13 in mice recognizes a conserved 23S ribosomal RNA (rRNA) sequence that is the binding site of macrolide, lincosamide, and streptogramin group (MLS) antibiotics (including erythromycin) in bacteria. Notably, 23S rRNA from clinical isolates of erythromycin-resistant Staphylococcus aureus and synthetic oligoribonucleotides carrying methylated adenosine or a guanosine mimicking a MLS resistance-causing modification failed to stimulate TLR13. Thus, our results reveal both a natural TLR13 ligand and specific mechanisms of antibiotic resistance as potent bacterial immune evasion strategy, avoiding recognition via TLR13.

Nucleic acid sensing at the interface between innate and adaptive immunity in vaccination

The demand is currently high for new vaccination strategies, particularly to help combat problematic intracellular pathogens, such as HIV and malarial parasites. In the past decade, the identification of host receptors that recognize pathogen-derived nucleic acids has revealed an essential role for nucleic acid sensing in the triggering of immunity to intracellular pathogens. This Review first addresses our current understanding of the nucleic acid-sensing immune machinery. We then explain how the study of nucleic acid-sensing mechanisms not only has revealed their central role in driving the responses mediated by many current vaccines, but is also revealing how they could be harnessed for the design of new vaccines.

Role of capsular polysaccharide in Group B Streptococccus interactions with dendritic cells

Group B Streptococcus (GBS) type III is an important agent of life-threatening invasive infections. Albeit the immune system plays a dual role in development and protection against disease, mechanisms leading to an efficient immune response against GBS remain obscure. Mouse bone marrow-derived dendritic cells (DCs) and primary spleen DCs were used to evaluate GBS capacity to modulate the functions of these important antigen-presenting cells. The role of capsular polysaccharide (CPS), one of the most important GBS virulence factors, in bacterial-DC interactions was evaluated by using a non-encapsulated mutant. Phagocytosis assays, confocal and electron microscopy showed that DCs efficiently internalize encapsulated GBS, but the latter possesses strong intracellular survival capacity. GBS devoid of CPS was internalized and killed at higher and faster rates than encapsulated GBS early after infection. Among several cytokines tested, GBS internalization was required for modulation of IL-12, IL-10 and CXCL10 pathways. In contrast, GBS induced DC expression of co-stimulatory molecules in a phagocytosis-independent manner. Finally, the production of pro-inflammatory and Th1 cytokines by GBS-stimulated DCs was differentially modulated by CPS expression, depending on DC origin. Our data suggest multiple mechanisms involved in GBS modulation of DC functions, which were selectively regulated by the presence of CPS.

Bacterial RNA mediates activation of caspase-1 and IL-1β release independently of TLRs 3, 7, 9 and TRIF but is dependent on UNC93B

Recognition of foreign nucleic acids is important for the induction of an innate immune response against invading pathogens. Although the pathways involved in sensing bacterial DNA and viral RNA are now well established, only limited knowledge is available on mechanisms underlying recognition of bacterial RNA. It has been reported that intracellular delivery of Escherichia coli RNA activates the Nlrp3 inflammasome, but whether this is a general property of bacterial RNA remains unclear as are the pathways involved in pro-IL-1β induction and caspase-1 activation by bacterial RNA. In this study, we report that bacterial RNA from both Gram-positive and Gram-negative bacteria induces activation of caspase-1 and secretion of IL-1β by murine dendritic cells and bone-marrow derived macrophages. Stimulation was independent of the presence of 5'-triphosphate termini and occurred with whole RNA preparations from bacteria but not from eukaryotes. Induction of pro-IL-1β as well as the priming for caspase-1 activation by bacterial RNA was dependent on UNC93B, an endoplasmic reticulum protein essential for delivery of TLRs to the endosome, whereas the established nucleic acid sensing endosomal TLRs 3, 7, and 9 were dispensable. Additionally, caspase-1 activation and IL-1β production by transfected bacterial RNA were absent in MyD88-deficient cells but independent of TRIF. Thus, our data indicate the presence of a yet unidentified intracellular nucleic acid receptor involved in bacterial RNA-induced inflammasome activation and release of IL-1β.

RNA mediated Toll-like receptor stimulation in health and disease

Besides their well known functions in storage and translation of information nucleic acids have emerged as a target of pattern recognition receptors that drive activation of innate immunity. Due to the paucity of building block monomers used in nucleic acids, discrimination of host and microbial nucleic acids as a means of self/foreign discrimination is a complicated task. Pattern recognition receptors rely on discrimination by sequence, structural features and spatial compartmentalization to differentiate microbial derived nucleic acids from host ones. Microbial nucleic acid detection is important for the sensing of infectious danger and initiating an immune response to microbial attack. Failures in the underlying recognitions systems can have severe consequences: thus, inefficient recognition of microbial nucleic acids may increase susceptibility to infectious diseases. On the other hand, excessive immune responses as a result of failed self/foreign discrimination are associated with autoimmune diseases. This review gives a general overview over the underlying concepts of nucleic acid sensing by Toll-like receptors. Within this general framework, we focus on bacterial RNA and synthetic RNA oligomers.

Sphingosine analogue AAL-R increases TLR7-mediated dendritic cell responses via p38 and type I IFN signaling pathways

Sphingosine analogues display immunosuppressive activities and thus have therapeutic potential in the treatment of autoimmune diseases. In this study, we investigated the effects of the sphingosine analogue AAL-R (FTY720 derivative) on dendritic cell (DC) response upon TLR stimulation. Unlike its known immunosuppressive activity, AAL-R increased TLR7-mediated DC responses by elevating the levels of MHC class I and costimulatory molecules and type I IFN expression and by enhancing the capacity of DCs to induce CD8(+) T cell proliferation. Importantly, the stimulatory activity of AAL-R was dependent on type I IFN signaling, as type I IFN receptor-deficient DCs failed to respond to AAL-R. Also, AAL-R activated p38 MAPK to increase type I IFN synthesis and TLR7-mediated DC maturation. These findings enhance our understanding of sphingosine regulation of the host immune system, in particular upon pathogenic infections.

Sialylation of Streptococcus suis serotype 2 is essential for capsule expression but is not responsible for the main capsular epitope

The capsular polysaccharide is a critical virulence factor of the swine and zoonotic pathogen Streptococcus suis serotype 2. The capsule of this bacterium is composed of five different sugars, including terminal sialic acid. To evaluate the role of sialic acid in the pathogenesis of the infection, the neuC gene, encoding for an enzyme essential for sialic acid biosynthesis, was inactivated in a highly virulent S. suis serotype 2 strain. Using transmission electron microscopy, it was shown that inactivation of neuC resulted in loss of expression of the whole capsule. Compared to the parent strain, the ΔneuC mutant strain was more phagocytosed by macrophages and was also severely impaired in virulence in a mouse infection model. Both native and desialylated S. suis serotype 2 purified capsular polysaccharides were recognized by a polyclonal anti-whole cell S. suis serotype 2 serum and a monospecific polyclonal anti-capsule serotype 2 serum. In contrast, only the native capsular polysaccharide was recognized by a monoclonal antibody specific for the sialic acid moiety of the serotype 2 capsule. Together, our results infer that sialylation of S. suis serotype 2 may be essential for capsule expression, but that this sugar is not the main epitope of this serotype.

Fisher scientific award lecture - the capsular polysaccharides of Group B Streptococcus and Streptococcus suis differently modulate bacterial interactions with dendritic cells

Infections with encapsulated bacteria cause serious clinical problems. Besides being poorly immunogenic, the bacterial capsular polysaccharide (CPS) cloaks antigenic proteins, allowing bacterial evasion of the host immune system. Despite the clinical significance of bacterial CPS and its suggested role in the pathogenesis of the infection, the mechanisms underlying innate and, critically, adaptive immune responses to encapsulated bacteria have not been fully elucidated. As such, we became interested in studying the CPS of two similar, but unique, streptococcal species: Group B Streptococcus (GBS) and Streptococcus suis . Both streptococci are well encapsulated, some capsular types are more virulent than others, and they can cause severe meningitis and septicemia. For both pathogens, the CPS is considered the major virulence factor. Finally, these two streptococci are the sole Gram-positive bacteria possessing sialic acid in their capsules. GBS type III is a leading cause of neonatal invasive infections. Streptococcus suis type 2 is an important swine and emerging zoonotic pathogen in humans. We recently characterized the S. suis type 2 CPS. It shares common structural elements with GBS, but sialic acid is α2,6-linked to galactose rather than α2,3-linked. Differential sialic acid expression by pathogens might result in modulation of immune cell activation and, consequently, may affect the immuno-pathogenesis of these bacterial infections. Here, we review and compare the interactions of these two sialylated encapsulated bacteria with dendritic cells, known as the most potent antigen-presenting cells linking innate and adaptive immunity. We further address differences between dendritic cells and professional phagocytes, such as macrophages and neutrophils, in their interplay with these encapsulated pathogens. Elucidation of the molecular and cellular basis of the impact of CPS composition on bacterial interactions with immune cells is critical for mechanistic understanding of anti-CPS responses. Knowledge generated will help to advance the development of novel, more effective anti-CPS vaccines and improved immunotherapies.