Immune Activation of Type I IFNs by Listeria monocytogenes Occurs Independently of TLR4, TLR2, and Receptor Interacting Protein 2 but Involves TANK-Binding Kinase 1

Intestinal infection triggers mitochondria-mediated α-synuclein pathology: relevance to Parkinson's disease

Parkinson's disease (PD) is a multifactorial neurodegenerative disease characterized by the loss of dopaminergic neurons in the midbrain. In the prodromal phase several autonomic symptoms including orthostatic hypotension and constipation are correlated with increased α-synuclein pathology in peripheral tissues. It is currently accepted that some idiopathic PD cases may start in the gut (body-first PD) with accumulation of pathological α-synuclein in enteric neurons that may subsequently propagate caudo-rostrally to the central nervous system. In addition to the already-established regulation of synaptic vesicle trafficking, α-synuclein also seems to play a role in neuronal innate immunity after infection. Our goal was to understand if seeding the gut with the foodborne pathogen Listeria monocytogenes by oral gavage would impact gut immunity and eventually the central nervous system. Our results demonstrate that L. monocytogenes infection induced oligomerization of α-synuclein in the ileum, along with a pronounced pro-inflammatory local and systemic response that ultimately culminated in neuronal mitochondria dysfunction. We propose that, having evolved from ancestral endosymbiotic bacteria, mitochondria may be directly targeted by virulence factors of intracellular pathogens, and that mitochondrial dysfunction and fragmentation resulting also from the activation of the innate immune system at the gut level, trigger innate immune responses in midbrain neurons, which include α-synuclein oligomerization and neuroinflammation, all of which hallmarks of PD.

Crosstalk Between Autophagy and the cGAS-STING Signaling Pathway in Type I Interferon Production

Innate immunity is the front-line defense against infectious microorganisms, including viruses and bacteria. Type I interferons are pleiotropic cytokines that perform antiviral, antiproliferative, and immunomodulatory functions in cells. The cGAS–STING pathway, comprising the main DNA sensor cyclic guanosine monophosphate/adenosine monophosphate synthase (cGAS) and stimulator of IFN genes (STING), is a major pathway that mediates immune reactions and is involved in the strong induction of type I IFN production, which can fight against microbial infections. Autophagy is an evolutionarily conserved degradation process that is required to maintain host health and facilitate capture and elimination of invading pathogens by the immune system. Mounting evidence indicates that autophagy plays an important role in cGAS–STING signaling pathway-mediated type I IFN production. This review briefly summarizes the research progress on how autophagy regulates the cGAS–STING pathway, regulating type I IFN production, with a particular focus on the crosstalk between autophagy and cGAS–STING signaling during infection by pathogenic microorganisms.

Human antimicrobial cytotoxic T lymphocytes, defined by NK receptors and antimicrobial proteins, kill intracellular bacteria

Human CD8⁺ cytotoxic T lymphocytes (CTLs) contribute to antimicrobial defense against intracellular pathogens through secretion of cytotoxic granule proteins granzyme B, perforin, and granulysin. However, CTLs are heterogeneous in the expression of these proteins, and the subset(s) responsible for antimicrobial activity is unclear. Studying human leprosy, we found that the subset of CTLs coexpressing all three cytotoxic molecules is increased in the resistant form of the disease, can be expanded by interleukin-15 (IL-15), and is differentiated from naïve CD8⁺ T cells by Langerhans cells. RNA sequencing analysis identified that these CTLs express a gene signature that includes an array of surface receptors typically expressed by natural killer (NK) cells. We determined that CD8⁺ CTLs expressing granzyme B, perforin, and granulysin, as well as the activating NK receptor NKG2C, represent a population of "antimicrobial CTLs" (amCTLs) capable of T cell receptor (TCR)-dependent and TCR-independent release of cytotoxic granule proteins that mediate antimicrobial activity.

Molecular basis of mycobacterial survival in macrophages

Macrophages play an essential role in the immune system by ingesting and degrading invading pathogens, initiating an inflammatory response and instructing adaptive immune cells, and resolving inflammation to restore homeostasis. More interesting is the fact that some bacteria have evolved to use macrophages as a natural habitat and tools of spread in the host, e.g., Mycobacterium tuberculosis (Mtb) and some non-tuberculous mycobacteria (NTM). Mtb is considered one of humanity's most successful pathogens and is the causal agent of tuberculosis, while NTMs cause opportunistic infections all of which are of significant public health concern. Here, we describe mechanisms by which intracellular pathogens, with an emphasis on mycobacteria, manipulate macrophage functions to circumvent killing and live inside these cells even under considerable immunological pressure. Such macrophage functions include the selective evasion or engagement of pattern recognition receptors, production of cytokines, reactive oxygen and nitrogen species, phagosome maturation, as well as other killing mechanisms like autophagy and cell death. A clear understanding of host responses elicited by a specific pathogen and strategies employed by the microbe to evade or exploit these is of significant importance for the development of effective vaccines and targeted immunotherapy against persistent intracellular infections like tuberculosis.

The Complement Anaphylatoxins C5a and C3a Suppress IFN-β Production in Response to Listeria monocytogenes by Inhibition of the Cyclic Dinucleotide-Activated Cytosolic Surveillance Pathway

Listeria monocytogenes is an intracellular Gram-positive bacterium that induces expression of type I IFNs (IFN-α/IFN-β) during infection. These cytokines are detrimental to the host during infection by priming leukocytes to undergo L. monocytogenes-mediated apoptosis. Our previous studies showed that C5aR1^-/- and C3aR^-/- mice are highly susceptible to L. monocytogenes infection as a result of increased IFN-β-mediated apoptosis of major leukocyte cell populations, including CD4⁺ and CD8⁺ T cells. However, the mechanisms by which C3a and C5a modulate IFN-β expression during L. monocytogenes infection were not examined in these initial investigations. Accordingly, we report in this article that C5a and C3a suppress IFN-β production in response to L. monocytogenes via cyclic di-AMP (c-di-AMP), a secondary messenger molecule of L. monocytogenes, in J774A.1 macrophage-like cells and in bone marrow-derived dendritic cells (BMDCs). Moreover, C5a and C3a suppress IFN-β production by acting through their respective receptors, because no inhibition was seen in C5aR1^-/- or C3aR^-/- BMDCs, respectively. C5a and C3a suppress IFN-β production in a manner that is dependent on Bruton's tyrosine kinase, p38 MAPK, and TANK-binding kinase 1 (TBK1), as demonstrated by the individual use of Bruton's tyrosine kinase, p38 MAPK, and TBK1 inhibitors. Pretreatment of cells with C5a and C3a reduced the expression of the IFN-β signaling molecules DDX41, STING, phosphorylated TBK1, and phosphorylated p38 MAPK in wild-type BMDCs following treatment with c-di-AMP. Collectively, these data demonstrate that C3a and C5a, via direct signaling through their specific receptors, suppress IFN-β expression by modulation of a distinct innate cytosolic surveillance pathway involving DDX41, STING, and other downstream molecular targets of L. monocytogenes-generated c-di-AMP.

The interferon response to bacterial and viral infections

Type I interferons (IFNs) were first described several decades ago as soluble factors that were capable of `interfering' with viral replication when added to infected cells. Type I IFNs have been shown to be induced by recognition of viral DNA and RNA via three distinct pathways: (i) a TRIFdependent pathway in macrophages via TLRs 3 and 4; (ii) a MyD88-dependent pathway in plasmacytoid dendritic cells (pDCs) via TLRs 7/8 and 9; and (iii) an intracellular recognition pathway utilizing the cytoplasmic receptors RIG-I/MDA5. Interestingly, these viral recognition pathways converge on TRAF3, which induces interferon through the activation of IRF3 or IRF7 by the TBK-1 and IKKi complexes. While type I IFN has been traditionally associated with antiviral responses, recent studies have demonstrated that many bacteria also induce type I interferon responses. The mechanisms of type I IFN induction and its role in host defense, however, are largely unclear. Studies with the Gram-positive intracellular bacterium Listeria monocytogenes indicated that it may trigger type I IFN induction through novel TLR-independent intracellular receptors and type I IFN may play a detrimental role to host response against listerial infection. In this article, we summarize some of these findings and discuss the functional differences of type I IFNs in bacterial and viral infections.

The Roles of Type I Interferon in Bacterial Infection

Type I interferons (IFNs) are pleiotropic cytokines well recognized for their role in the induction of a potent antiviral gene program essential for host defense against viruses. They also modulate innate and adaptive immune responses. However, the role of type I IFNs in host defense against bacterial infections is enigmatic. Depending on the bacterium, they exert seemingly opposite and capricious functions. In this review, we summarize the effect of type I IFNs on specific bacterial infections and highlight the effector mechanisms regulated by type I IFNs in an attempt to elucidate new avenues to understanding their role.

RNA sequencing provides exquisite insight into the manipulation of the alveolar macrophage by tubercle bacilli

Mycobacterium bovis, the agent of bovine tuberculosis, causes an estimated $3 billion annual losses to global agriculture due, in part, to the limitations of current diagnostics. Development of next-generation diagnostics requires a greater understanding of the interaction between the pathogen and the bovine host. Therefore, to explore the early response of the alveolar macrophage to infection, we report the first application of RNA-sequencing to define, in exquisite detail, the transcriptomes of M. bovis-infected and non-infected alveolar macrophages from ten calves at 2, 6, 24 and 48 hours post-infection. Differentially expressed sense genes were detected at these time points that revealed enrichment of innate immune signalling functions, and transcriptional suppression of host defence mechanisms (e.g., lysosome maturation). We also detected differentially expressed natural antisense transcripts, which may play a role in subverting innate immune mechanisms following infection. Furthermore, we report differential expression of novel bovine genes, some of which have immune-related functions based on orthology with human proteins. This is the first in-depth transcriptomics investigation of the alveolar macrophage response to the early stages of M. bovis infection and reveals complex patterns of gene expression and regulation that underlie the immunomodulatory mechanisms used by M. bovis to evade host defence mechanisms.

A Comprehensive Proteomics Analysis Reveals a Secretory Path- and Status-Dependent Signature of Exosomes Released from Tumor-Associated Macrophages

Exosomes are 30-120 nm-sized membrane vesicles of endocytic origin that are released into the extracellular environment and play roles in cell-cell communication. Tumor-associated macrophages (TAMs) are important constituents of the tumor microenvironment; thus, it is critical to study the features and complex biological functions of TAM-derived exosomes. Here, we constructed a TAM cell model from a mouse macrophage cell line, Ana-1, and performed comparative proteomics on exosomes, exosome-free media, and cells between TAMs and Ana-1. Proteomic analysis between exosome and exosome-free fractions indicated that the functions of exosome dominant proteins were mainly enriched in RNA processing and proteolysis. TAM status dramatically affected the abundances of 20S proteasome subunits and ribosomal proteins in their exosomes. The 20S proteasome activity assay strongly indicated that TAM exosomes possessed higher proteolytic activity. In addition, Ana-1- and TAM-derived exosomes have different RNA profiles, which may result from differential RNA processing proteins. Taken together, our comprehensive proteomics study provides novel views for understanding the complicated roles of macrophage-derived exosomes in the tumor microenvironment.

The human P-glycoprotein transporter enhances the type I interferon response to Listeria monocytogenes infection

Human multidrug efflux transporters are known for their ability to extrude antibiotics and toxic compounds out of cells, yet accumulating data indicate they have additional functions in diverse physiological processes not related to drug efflux. Here, we show that the human multidrug transporter P-glycoprotein (P-gp) (also named MDR1 and ABCB1) is transcriptionally induced in the monocytic cell line THP-1 upon infection with the human intracellular bacterial pathogen Listeria monocytogenes. Notably, we found that P-gp is important for full activation of the type I interferon response elicited against L. monocytogenes bacteria. Both inhibition of P-gp function by verapamil and inhibition of its transcription using mRNA silencing led to a reduction in the magnitude of the type I response in infected cells. This function of P-gp was specific to type I interferon cytokines elicited against cytosolic replicating bacteria and was not observed in response to cyclic di-AMP (c-di-AMP), a molecule that was shown to be secreted by L. monocytogenes during infection and to trigger type I interferons. Moreover, P-gp was not involved in activation of other proinflammatory cytokines, such as those triggered by vacuolar-restricted L. monocytogenes or lipopolysaccharide (LPS). Taken together, these findings demonstrate a role for P-gp in proper development of an innate immune response against intracellular pathogens, highlighting the complexity in employing therapeutic strategies that involve inhibition of multidrug resistance (MDR) efflux pumps.

The role of type I interferons in intestinal infection, homeostasis, and inflammation

Type I interferons are a widely expressed family of effector cytokines that promote innate antiviral and antibacterial immunity. Paradoxically, they can also suppress immune responses by driving production of anti-inflammatory cytokines, and dysregulation of these cytokines can contribute to host-mediated immunopathology and disease progression. Recent studies describe their anti-inflammatory role in intestinal inflammation and the locus containing IFNAR, a heterodimeric receptor for the type I interferons has been identified as a susceptibility region for human inflammatory bowel disease. This review focuses on the role of type I IFNs in the intestine in health and disease and their emerging role as immune modulators. Clear understanding of type I IFN-mediated immune responses may provide avenues for fine-tuning existing IFN treatment for infection and intestinal inflammation.

Type I interferons in bacterial infections: taming of myeloid cells and possible implications for autoimmunity

Type I interferons (IFNs) were first described for their ability to protect the host from viral infections and may also have beneficial effects under specific conditions within some bacterial infections. Yet, these pleiotropic cytokines are now known to exacerbate infections by numerous life-threatening bacteria, including the intracellular pathogens Listeria monocytogenes and Mycobacterium tuberculosis. The evidence that such detrimental effects occur during bacterial infections in both animals and humans argues for selective pressure. In this review, we summarize the evidence demonstrating a pro-bacterial role for type I IFNs and discuss possible mechanisms that have been proposed to explain such effects. The theme emerges that type I IFNs act to suppress myeloid cell immune responses. The evolutionary conservation of such anti-inflammatory effects, particularly in the context of infections, suggests they may be important for limiting chronic inflammation. Given the effectiveness of type I IFNs in treatment of certain autoimmune diseases, their production may also act to raise the threshold for activation of immune responses to self-antigens.

The bacterial pathogen Listeria monocytogenes and the interferon family: type I, type II and type III interferons

Interferons (IFNs) are secreted proteins of the cytokine family that regulate innate and adaptive immune responses to infection. Although the importance of IFNs in the antiviral response has long been appreciated, their role in bacterial infections is more complex and is currently a major focus of investigation. This review summarizes our current knowledge of the role of these cytokines in host defense against the bacterial pathogen Listeria monocytogenes and highlights recent discoveries on the molecular mechanisms evolved by this intracellular bacterium to subvert IFN responses.

Listeria monocytogenes MDR transporters are involved in LTA synthesis and triggering of innate immunity during infection

Multi-drug resistance (MDR) transporters are known eponymously for their ability to confer resistance to various antimicrobial drugs. However, it is likely that this is not their primary function and that MDR transporters evolved originally to play additional roles in bacterial physiology. In Listeria monocytogenes a set of MDR transporters was identified to mediate activation of innate immune responses during mammalian cell infection. This phenotype was shown to be dependent on c-di-AMP secretion, but the physiological processes underlying this phenomenon were not completely resolved. Here we describe a genetic approach taken to screen for L. monocytogenes genes or physiological pathways involved in MDR transporter-dependent triggering of the type I interferon response. We found that disruption of L. monocytogenes lipoteichoic acid (LTA) synthesis results in enhanced triggering of type I interferon responses in infected macrophage cells yet does not impact bacterial intracellular growth. This innate immune response required the MDR transporters and could be recapitulated by exposing macrophage cells to culture supernatants derived from LTA mutant bacteria. Notably, we found that the MDR transporters themselves are required for full production of LTA, an observation that links MDR transporters to LTA synthesis for the first time. In light of our findings, we propose that the MDR transporters play a role in regulating LTA synthesis, possibly via c-di-AMP efflux, a physiological function in cell wall maintenance that triggers the host innate immune system.

The transcriptional repressor BLIMP1 curbs host defenses by suppressing expression of the chemokine CCL8

The transcriptional repressor B lymphocyte-induced maturation protein 1 (BLIMP1) is a master regulator of B and T cell differentiation. To examine the role of BLIMP1 in innate immunity, we used a conditional knockout (CKO) of Blimp1 in myeloid cells and found that Blimp1 CKO mice were protected from lethal infection induced by Listeria monocytogenes. Transcriptome analysis of Blimp1 CKO macrophages identified the murine chemokine (C-C motif) ligand 8, CCL8, as a direct target of Blimp1-mediated transcriptional repression in these cells. BLIMP1-deficient macrophages expressed elevated levels of Ccl8, and consequently Blimp1 CKO mice had higher levels of circulating CCL8, resulting in increased neutrophils in the peripheral blood, promoting a more aggressive antibacterial response. Mice lacking the Ccl8 gene were more susceptible to L. monocytogenes infection than were wild-type mice. Although CCL8 failed to recruit neutrophils directly, it was chemotactic for γ/δ T cells, and CCL8-responsive γ/δ T cells were enriched for IL-17F. Finally, CCL8-mediated enhanced clearance of L. monocytogenes was dependent on γ/δ T cells. Collectively, these data reveal an important role for BLIMP1 in modulating host defenses by suppressing expression of the chemokine CCL8.

Therapeutic targeting of NOD1 receptors

The nucleotide-binding oligomerization domain 1 (NOD1) protein is an intracellular receptor for breakdown products of peptidoglycan (PGN), an essential bacterial cell wall component. NOD1 responds to γ-D-glutamyl-meso-diaminopimelic acid, which is an epitope unique to PGN structures from all Gram-negative bacteria and certain Gram-positive bacteria. Upon ligand recognition, NOD1 undergoes conformational changes and self-oligomerization mediated by the nucleotide-binding NACHT domains, followed by the recruitment and activation of the serine threonine kinase receptor-interacting protein 2 leading to the activation of NF-κB and MAPK pathways and induction of inflammatory genes. Much of our knowledge is derived from seminal studies using mice deficient in NOD1 and confirming an essential role for NOD1 in the host immune response against gastrointestinal and respiratory pathogens. In addition, recent studies have revealed a role for intracellular NOD1 receptors in the regulation of vascular inflammation and metabolism. This review will discuss our current understanding of intracellular NOD1 receptors in host immunity and chronic inflammatory disorders with a focus on cardiovascular diseases. Although therapeutic advances may have to wait until the complex interplay with pathogens, danger signals, other pattern recognition receptors and overlapping metabolic pathways is further unravelled, the steadily growing body of knowledge suggest that NOD1 antagonism might represent attractive candidate to reduce excessive inflammation associated to intestinal, cardiovascular and metabolic diseases.

Listeria monocytogenes multidrug resistance transporters and cyclic di-AMP, which contribute to type I interferon induction, play a role in cell wall stress

The intracellular bacterial pathogen Listeria monocytogenes activates a robust type I interferon response upon infection. This response is partially dependent on the multidrug resistance (MDR) transporter MdrM and relies on cyclic-di-AMP (c-di-AMP) secretion, yet the functions of MdrM and cyclic-di-AMP that lead to this response are unknown. Here we report that it is not MdrM alone but a cohort of MDR transporters that together contribute to type I interferon induction during infection. In a search for a physiological function of these transporters, we revealed that they play a role in cell wall stress responses. A mutant with deletion of four transporter genes (ΔmdrMTAC) was found to be sensitive to sublethal concentrations of vancomycin due to an inability to produce and shed peptidoglycan under this stress. Remarkably, c-di-AMP is involved in this phenotype, as overexpression of the c-di-AMP phosphodiesterase (PdeA) resulted in increased susceptibility of the ΔmdrMTAC mutant to vancomycin, whereas overexpression of the c-di-AMP diadenylate cyclase (DacA) reduced susceptibility to this drug. These observations suggest a physiological association between c-di-AMP and the MDR transporters and support the model that MDR transporters mediate c-di-AMP secretion to regulate peptidoglycan synthesis in response to cell wall stress.

Cytokines in the balance of protection and pathology during mycobacterial infections

The outcome of natural infections with pathogenic mycobacteria can range from early asymptomatic clearance through latent infection to clinical disease. Different host and pathogen-specific factors have been implicated in determining the outcome of these infections; however, it is clear that the interaction of mycobacteria with the innate and acquired components of the immune system plays a central role. Specifically, the recognition of mycobacterial components by innate immune cells through different pathogen recognition receptors (PPRs) induces a cytokine response that can promote early control of the infection. In fact, in the majority of individuals that come into contact with mycobacteria, this response is enough to control the infection. Among PRRs, Toll-like receptors (TLRs), Nucleotide Oligomerization Domain (NOD)-like receptors, and C-type lectins have all been implicated in recognition of mycobacteria and in the initiation of the cytokine response. Defining the mechanisms by which distinct mycobacterial components and their receptors stimulate the immune response is an area of intense research.

RIG-I detects infection with live Listeria by sensing secreted bacterial nucleic acids

Immunity against infection with Listeria monocytogenes is not achieved from innate immune stimulation by contact with killed but requires viable Listeria gaining access to the cytosol of infected cells. It has remained ill-defined how such immune sensing of live Listeria occurs. Here, we report that efficient cytosolic immune sensing requires access of nucleic acids derived from live Listeria to the cytoplasm of infected cells. We found that Listeria released nucleic acids and that such secreted bacterial RNA/DNA was recognized by the cytosolic sensors RIG-I, MDA5 and STING thereby triggering interferon β production. Secreted Listeria nucleic acids also caused RIG-I-dependent IL-1β-production and inflammasome activation. The signalling molecule CARD9 contributed to IL-1β production in response to secreted nucleic acids. In conclusion, cytosolic recognition of secreted bacterial nucleic acids by RIG-I provides a mechanistic explanation for efficient induction of immunity by live bacteria.

Listeria monocytogenes strain-specific impairment of the TetR regulator underlies the drastic increase in cyclic di-AMP secretion and beta interferon-inducing ability

Among a number of laboratory strains of Listeria monocytogenes used in experimental infection, strain LO28 is highly capable of inducing robust beta interferon (IFN-β) production in infected macrophages. In this study, we investigated the molecular mechanism of the IFN-β-inducing ability of LO28 by comparing it with that of strain EGD, a low-IFN-β-inducing strain. It was found that LO28 secretes a large amount of IFN-β-inducing factor, which turned out to be cyclic di-AMP. The secretion of cyclic di-AMP was dependent on MdrT, a multidrug resistance transporter, and LO28 exhibited a very high level of mdrT expression. The introduction of a null mutation into mdrT abolished the ability of LO28 to induce IFN-β production. Examination of genes responsible for the regulation of mdrT expression revealed a spontaneous 188-bp deletion in tetR of LO28. By constructing recombinant strains of LO28 and EGD in which tetR from each strain was replaced, it was confirmed that the distinct ability of LO28 is attributable mostly to tetR mutation. We concluded that the strong IFN-β-inducing ability of LO28 is due to a genetic defect in tetR resulting in the overexpression of mdrT and a concomitant increase in the secretion of cyclic di-AMP through MdrT.

Both TLR2 and TRIF contribute to interferon-β production during Listeria infection

Synthesis of interferon-β (IFN-β) is an innate response to cytoplasmic infection with bacterial pathogens. Our recent studies showed that Listeria monocytogenes limits immune detection and IFN-β synthesis via deacetylation of its peptidoglycan, which renders the bacterium resistant to lysozyme degradation. Here, we examined signaling requirements for the massive IFN-β production resulting from the infection of murine macrophages with a mutant strain of L. monocytogenes, ΔpgdA, which is unable to modify its peptidoglycan. We report the identification of unconventional signaling pathways to the IFN-β gene, requiring TLR2 and bacterial internalization. Induction of IFN-β was independent of the Mal/TIRAP adaptor protein but required TRIF and the transcription factors IRF3 and IRF7. These pathways were stimulated to a lesser degree by wild-type L. monocytogenes. They operated in both resident and inflammatory macrophages derived from the peritoneal cavity, but not in bone marrow-derived macrophages. The novelty of our findings thus lies in the first description of TLR2 and TRIF as two critical components leading to the induction of the IFN-β gene and in uncovering that individual macrophage populations adopt different strategies to link pathogen recognition signals to IFN-β gene expression.

Streptococcus pneumoniae stimulates a STING- and IFN regulatory factor 3-dependent type I IFN production in macrophages, which regulates RANTES production in macrophages, cocultured alveolar epithelial cells, and mouse lungs

Streptococcus pneumoniae is the leading cause of community-acquired pneumonia. In this study, we examine an innate immune recognition pathway that senses pneumococcal infection, triggers type I IFN production, and regulates RANTES production. We found that human and murine alveolar macrophages as well as murine bone marrow macrophages, but not alveolar epithelial cells, produced type I IFNs upon infection with S. pneumoniae. This response was dependent on the pore-forming toxin pneumolysin and appeared to be mediated by a cytosolic DNA-sensing pathway involving the adapter molecule STING and the transcription factor IFN regulatory factor 3. Indeed, DNA was present in the cytosol during pneumococcal infection as indicated by the activation of the AIM2 inflammasome, which is known to sense microbial DNA. Type I IFNs produced by S. pneumoniae-infected macrophages positively regulated gene expression and RANTES production in macrophages and cocultured alveolar epithelial cells in vitro. Moreover, type I IFNs controlled RANTES production during pneumococcal pneumonia in vivo. In conclusion, we identified an immune sensing pathway detecting S. pneumoniae that triggers a type I IFN response and positively regulates RANTES production.

Production of IFN-β during Listeria monocytogenes infection is restricted to monocyte/macrophage lineage

The family of type I interferons (IFN), which consists of several IFN-α and one IFN-β, are produced not only after stimulation by viruses, but also after infection with non-viral pathogens. In the course of bacterial infections, these cytokines could be beneficial or detrimental. IFN-β is the primary member of type I IFN that initiates a cascade of IFN-α production. Here we addressed the question which cells are responsible for IFN-β expression after infection with the intracellular pathogen Listeria monocytogenes by using a genetic approach. By means of newly established reporter mice, maximum of IFN-β expression was observed at 24 hours post infection in spleen and, surprisingly, 48 hours post infection in colonized cervical and inguinal lymph nodes. Colonization of lymph nodes was independent of the type I IFN signaling, as well as bacterial dose and strain. Using cell specific reporter function and conditional deletions we could define cells expressing LysM as the major IFN-β producers, with cells formerly defined as Tip-DCs being the highest. Neutrophilic granulocytes, dendritic cells and plasmacytoid dendritic cells did not significantly contribute to type I IFN production.

LIMP-2 links late phagosomal trafficking with the onset of the innate immune response to Listeria monocytogenes: a role in macrophage activation

The innate immune response to Listeria monocytogenes depends on phagosomal bacterial degradation by macrophages. Here, we describe the role of LIMP-2, a lysosomal type III transmembrane glycoprotein and scavenger-like protein, in Listeria phagocytosis. LIMP-2-deficient mice display a macrophage-related defect in Listeria innate immunity. They produce less acute phase pro-inflammatory cytokines/chemokines, MCP-1, TNF-α, and IL-6 but normal levels of IL-12, IL-10, and IFN-γ and a 25-fold increase in susceptibility to Listeria infection. This macrophage defect results in a low listericidal potential, poor response to TNF-α activation signals, impaired phago-lysosome transformation into antigen-processing compartments, and uncontrolled LM cytosolic growth that fails to induce normal levels of acute phase pro-inflammatory cytokines. LIMP-2 transfection of CHO cells confirmed that LIMP-2 participates in the degradation of Listeria within phagosomes, controls the late endosomal/lysosomal fusion machinery, and is linked to the activation of Rab5a. Therefore, the role of LIMP-2 appears to be connected to the TNF-α-dependent and early activation of Listeria macrophages through internal signals linking the regulation of late trafficking events with the onset of the innate Listeria immune response.

The N-ethyl-N-nitrosourea-induced Goldenticket mouse mutant reveals an essential function of Sting in the in vivo interferon response to Listeria monocytogenes and cyclic dinucleotides

Type I interferons (IFNs) are central regulators of the innate and adaptive immune responses to viral and bacterial infections. Type I IFNs are induced upon cytosolic detection of microbial nucleic acids, including DNA, RNA, and the bacterial second messenger cyclic-di-GMP (c-di-GMP). In addition, a recent study demonstrated that the intracellular bacterial pathogen Listeria monocytogenes stimulates a type I IFN response due to cytosolic detection of bacterially secreted c-di-AMP. The transmembrane signaling adaptor Sting (Tmem173, Mita, Mpys, Eris) has recently been implicated in the induction of type I IFNs in response to cytosolic DNA and/or RNA. However, the role of Sting in response to purified cyclic dinucleotides or during in vivo L. monocytogenes infection has not been addressed. In order to identify genes important in the innate immune response, we have been conducting a forward genetic mutagenesis screen in C57BL/6 mice using the mutagen N-ethyl-N-nitrosourea (ENU). Here we describe a novel mutant mouse strain, Goldenticket (Gt), that fails to produce type I IFNs upon L. monocytogenes infection. By genetic mapping and complementation experiments, we found that Gt mice harbor a single nucleotide variant (T596A) of Sting that functions as a null allele and fails to produce detectable protein. Analysis of macrophages isolated from Gt mice revealed that Sting is absolutely required for the type I interferon response to both c-di-GMP and c-di-AMP. Additionally, Sting is required for the response to c-di-GMP and L. monocytogenes in vivo. Our results provide new functions for Sting in the innate interferon response to pathogens.

Innate immune recognition and inflammasome activation in listeria monocytogenes infection

Listeria monocytogenes is an intracellular, Gram-positive bacterium that can cause life-threatening illness especially in immunocompromised individuals and newborns. The pathogen propagates within the cytosol of various host cells after escaping from the phagosomal compartment depending on the cytolysin listeriolysin O. While L. monocytogenes can manipulate the endocytic and many host-cell signaling cascades to its advantage, host cells are however capable of detecting Listeria infection at different cellular compartments by expressing innate immune receptors that trigger antibacterial defense pathways. These receptors include the Toll-like receptors, NOD-like receptors (NLRs), and cytosolic DNA sensors. Some NLRs as well as the DNA sensor AIM2 form multiprotein complexes called inflammasomes. Inflammasomes regulate caspase-1-dependent production of the key inflammatory cytokines IL-1β and IL-18 as well as pyroptotic cell death in L. monocytogenes-infected cells. This review describes the current knowledge about innate immune sensing and inflammasome activation in Listeria infection.

Lactobacillus acidophilus induces virus immune defence genes in murine dendritic cells by a Toll-like receptor-2-dependent mechanism

Lactobacilli are probiotics that, among other health-promoting effects, have been ascribed immunostimulating and virus-preventive properties. Certain Lactobacillus spp. have been shown to possess strong interleukin-12 (IL-12) -inducing properties. As IL-12 production depends on the up-regulation of type I interferons (IFNs), we hypothesized that the strong IL-12-inducing capacity of Lactobacillus acidophilus NCFM in murine bone-marrow-derived dendritic cells (DCs) is caused by an up-regulation of IFN-β, which subsequently induces IL-12 and the double-stranded RNA binding Toll-like receptor-3 (TLR-3). The expression of the genes encoding IFN-β, TLR-3, IL-12 and IL-10 in DCs upon stimulation with L. acidophilus NCFM was determined. Lactobacillus acidophilus NCFM induced a much stronger expression of Ifn-β, Il-12 and Il-10 compared with the synthetic double-stranded RNA ligand Poly I:C, whereas the levels of expressed Tlr-3 were similar. Whole genome microarray gene expression analysis revealed that other genes related to viral defence were significantly up-regulated and among the strongest induced genes in DCs stimulated with L. acidophilus NCFM. The ability to induce IFN-β was also detected in another L. acidophilus strain (X37), but was not a property of other probiotic strains tested, i.e. Bifidobacterium bifidum Z9 and Escherichia coli Nissle 1917. The IFN-β expression was markedly reduced in TLR-2(-/-) DCs, dependent on endocytosis, and the major cause of the induction of Il-12 and Tlr-3 in DCs stimulated with L. acidophilus NCFM. Collectively, our results reveal that certain lactobacilli trigger the expression of viral defence genes in DCs in a TLR-2 manner dependent on IFN-β.

Bifidobacterium bifidum actively changes the gene expression profile induced by Lactobacillus acidophilus in murine dendritic cells

Dendritic cells (DC) play a pivotal regulatory role in activation of both the innate as well as the adaptive immune system by responding to environmental microorganisms. We have previously shown that Lactobacillus acidophilus induces a strong production of the pro-inflammatory and Th1 polarizing cytokine IL-12 in DC, whereas bifidobacteria do not induce IL-12 but inhibit the IL-12 production induced by lactobacilli. In the present study, genome-wide microarrays were used to investigate the gene expression pattern of murine DC stimulated with Lactobacillus acidophilus NCFM and Bifidobacterium bifidum Z9. L. acidophilus NCFM strongly induced expression of interferon (IFN)-β, other virus defence genes, and cytokine and chemokine genes related to the innate and the adaptive immune response. By contrast, B. bifidum Z9 up-regulated genes encoding cytokines and chemokines related to the innate immune response. Moreover, B. bifidum Z9 inhibited the expression of the Th1-promoting genes induced by L. acidophilus NCFM and had an additive effect on genes of the innate immune response and Th2 skewing genes. The gene encoding Jun dimerization protein 2 (JDP2), a transcription factor regulating the activation of JNK, was one of the few genes only induced by B. bifidum Z9. Neutralization of IFN-β abrogated L. acidophilus NCFM-induced expression of Th1-skewing genes, and blocking of the JNK pathway completely inhibited the expression of IFN-β. Our results indicate that B. bifidum Z9 actively inhibits the expression of genes related to the adaptive immune system in murine dendritic cells and that JPD2 via blocking of IFN-β plays a central role in this regulatory mechanism.

Innate immune recognition in infectious and noninfectious diseases of the lung

Diseases of the respiratory tract are among the leading causes of death in the world population. Increasing evidence points to a key role of the innate immune system with its pattern recognition receptors (PRRs) in both infectious and noninfectious lung diseases, which include pneumonia, chronic obstructive pulmonary disease, acute lung injury, pneumoconioses, and asthma. PRRs are capable of sensing different microbes as well as endogenous molecules that are released after cell damage. This PRR engagement is the prerequisite for the initiation of immune responses to infections and tissue injuries which can be beneficial or detrimental to the host. PRRs include the Toll-like receptors, NOD-like receptors, RIG-I-like receptors, and cytosolic DNA sensors. The PRRs and their signaling pathways represent promising targets for prophylactic and therapeutic interventions in various lung diseases.

Essential role of mitochondrial antiviral signaling, IFN regulatory factor (IRF)3, and IRF7 in Chlamydophila pneumoniae-mediated IFN-beta response and control of bacterial replication in human endothelial cells

Chlamydophila pneumoniae infection of the vascular wall as well as activation of the transcription factor IFN regulatory factor (IRF)3 have been linked to development of chronic vascular lesions and atherosclerosis. The innate immune system detects invading pathogens by use of pattern recognition receptors, some of which are able to stimulate IRF3/7 activation and subsequent type I IFN production (e. g., IFN-beta). In this study, we show that infection of human endothelial cells with C. pneumoniae-induced production of IFN-beta, a cytokine that so far has been mainly associated with antiviral immunity. Moreover, C. pneumoniae infection led to IRF3 and IRF7 nuclear translocation in HUVECs and RNA interference experiments showed that IRF3 and IRF7 as well as the mitochondrial antiviral signaling (MAVS) were essential for IFN-beta induction. Finally, C. pneumoniae replication was enhanced in endothelial cells in which IRF3, IRF7, or MAVS expression was inhibited by small interfering RNA and attenuated by IFN-beta treatment. In conclusion, C. pneumoniae infection of endothelial cells activates an MAVS-, IRF3-, and IRF7-dependent signaling, which controls bacterial growth and might modulate development of vascular lesions.

NOD2, RIP2 and IRF5 play a critical role in the type I interferon response to Mycobacterium tuberculosis

While the recognition of microbial infection often occurs at the cell surface via Toll-like receptors, the cytosol of the cell is also under surveillance for microbial products that breach the cell membrane. An important outcome of cytosolic recognition is the induction of IFNalpha and IFNbeta, which are critical mediators of immunity against both bacteria and viruses. Like many intracellular pathogens, a significant fraction of the transcriptional response to Mycobacterium tuberculosis infection depends on these type I interferons, but the recognition pathways responsible remain elusive. In this work, we demonstrate that intraphagosomal M. tuberculosis stimulates the cytosolic Nod2 pathway that responds to bacterial peptidoglycan, and this event requires membrane damage that is actively inflicted by the bacterium. Unexpectedly, this recognition triggers the expression of type I interferons in a Tbk1- and Irf5-dependent manner. This response is only partially impaired by the loss of Irf3 and therefore, differs fundamentally from those stimulated by bacterial DNA, which depend entirely on this transcription factor. This difference appears to result from the unusual peptidoglycan produced by mycobacteria, which we show is a uniquely potent agonist of the Nod2/Rip2/Irf5 pathway. Thus, the Nod2 system is specialized to recognize bacteria that actively perturb host membranes and is remarkably sensitive to mycobacteria, perhaps reflecting the strong evolutionary pressure exerted by these pathogens on the mammalian immune system.

The major outer membrane protein of a periodontopathogen induces IFN-beta and IFN-stimulated genes in monocytes via lipid raft and TANK-binding kinase 1/IFN regulatory factor-3

Surface molecules of pathogens play an important role in stimulating host immune responses. Elucidation of the signaling pathways activated by critical surface molecules in host cells provides insight into the molecular pathogenesis resulting from bacteria-host interactions. MspTL is the most abundant outer membrane protein of Treponema lecithinolyticum, which is associated with periodontitis, and induces expression of a variety of proinflammatory factors. Although bacteria and bacterial components like LPS and flagellin are known to induce IFN-beta, induction by bacterial surface proteins has not been reported. In the present study, we investigated MspTL-mediated activation of signaling pathways stimulating up-regulation of IFN-beta and IFN-stimulated genes in a human monocytic cell line, THP-1 cells, and primary cultured human gingival fibroblasts. MspTL treatment of the cells induced IFN-beta and the IFN-stimulated genes IFN-gamma-inducible protein-10 (IP-10) and RANTES. A neutralizing anti-IFN-beta Ab significantly reduced the expression of IP-10 and RANTES, as well as STAT-1 activation, which was also induced by MspTL. Experiments using specific small interfering RNA showed that MspTL activated TANK-binding kinase 1 (TBK1), but not inducible IkappaB kinase (IKKi). MspTL also induced dimerization of IFN regulatory factor-3 (IRF-3) and translocation into the nucleus. The lipid rapid-disrupting agents methyl-beta-cyclodextrin, nystatin, and filipin inhibited the MspTL internalization and cellular responses, demonstrating that lipid raft activation was a prerequisite for MspTL cellular signaling. Our results demonstrate that MspTL, the major outer protein of T. lecithinolyticum, induced IFN-beta expression and subsequent up-regulation of IP-10 and RANTES via TBK1/IRF-3/STAT-1 signaling secondary to lipid raft activation.

Nonhematopoietic cells control the outcome of infection with Listeria monocytogenes in a nucleotide oligomerization domain 1-dependent manner

We analyzed the defensive role of the cytosolic innate recognition receptor nucleotide oligomerization domain 1 (NOD1) during infection with Listeria monocytogenes. Mice lacking NOD1 showed increased susceptibility to systemic intraperitoneal and intravenous infection with high or low doses of L. monocytogenes, as measured by the bacterial load and survival. NOD1 also controlled dissemination of L. monocytogenes into the brain. The increased susceptibility to reinfection of NOD1(-/-) mice was not associated with impaired triggering of listeria-specific T cells, and similar levels of costimulatory molecules or activation of dendritic cells was observed. Higher numbers of F480(+) Gr1(+) inflammatory monocytes and lower numbers of F480(-) Gr1(+) neutrophils were recruited into the peritoneum of infected WT mice than into the peritoneum of infected NOD1(-/-) mice. We determined that nonhematopoietic cells accounted for NOD1-mediated resistance to L. monocytogenes in bone marrow radiation chimeras. The levels of NOD1 mRNA in fibroblasts and bone marrow-derived macrophages (BMM) were upregulated after infection with L. monocytogenes or stimulation with different Toll-like receptor ligands. NOD1(-/-) BMM, astrocytes, and fibroblasts all showed enhanced intracellular growth of L monocytogenes compared to WT controls. Gamma interferon-mediated nitric oxide production and inhibition of L. monocytogenes growth were hampered in NOD1(-/-) BMM. Thus, NOD1 confers nonhematopoietic cell-mediated resistance to infection with L. monocytogenes and controls intracellular bacterial growth in different cell populations in vitro.

Listeria monocytogenes infection in the face of innate immunity

Pathogen recognition and induction of immune responses are important for efficient elimination of infection. However, pathogens such as Listeria monocytogenes employ strategies to evade or modulate these defences, thus creating a more favourable environment that ensures their survival and pathogenesis. New insights into these strategies, particularly those targeting innate immunity, have recently emerged. L. monocytogenes is initially detected at the cell surface or in phagosomes by toll-like receptor 2 and in the cytosol by nuclear oligodimerization domain (NOD)-like receptors (NOD1, NOD2) and NALP3 and Ipaf. It carries out N-deacetylation of peptidoglycan to avoid this detection by toll-like receptor 2 and NOD-like receptors. L. monocytogenes modulates transcription of host immunity genes through modification of histones and chromatin remodelling. Furthermore, L. monocytogenes has recently been shown to avoid autophagy and induce apoptosis in immune effector cells. In this review we discuss some of these strategies, which have provided new insights into the interaction between L. monocytogenes and the immune response at a crucial stage of infection.

TLR-independent type I interferon induction in response to an extracellular bacterial pathogen via intracellular recognition of its DNA

Type I interferon (IFN) is an important host defense cytokine against intracellular pathogens, mainly viruses. In assessing IFN production in response to group B streptococcus (GBS), we find that IFN-beta was produced by macrophages upon stimulation with both heat-killed and live GBS. Exposure of macrophages to heat-killed GBS activated a Toll-like receptor (TLR)-dependent pathway, whereas live GBS activated a TLR/NOD/RIG-like receptor (RLR)-independent pathway. This latter pathway required bacterial phagocytosis, proteolytic bacterial degradation, and phagolysosomal membrane destruction by GBS pore-forming toxins, leading to the release of bacterial DNA into the cytosol. GBS DNA in the cytosol induced IFN-beta production via a pathway dependent on the activation of the serine-threonine kinase TBK1 and phosphorylation of the transcription factor IRF3. Thus, activation of IFN-alpha/-beta production during infection with GBS, commonly considered an extracellular pathogen, appears to result from the interaction of GBS DNA with a putative intracellular DNA sensor or receptor.

Imaging Listeria monocytogenes infection in vivo

Listeria monocytogenes infection in mice is a highly prolific model of bacterial infection. Several in vivo imaging approaches have been used to study host cell dynamics in response to infection, including bioluminescence imaging, confocal microscopy and two-photon microscopy, The application of in vivo imaging to study transgenic mouse models is providing unprecedented opportunities to test specific molecular mechanistic theories about how the host immune response unfolds. In complementary studies, in vivo imaging can be performed using genetically engineered bacterial mutants to assess the impact of specific virulence factors in host cell invasion and pathogenesis. The purpose of this chapter is to provide a general rationale for why in vivo imaging is important, provide an overview of various techniques highlighting the strengths and weaknesses of each, and provide examples of how various imaging techniques have been used to study Listeria infection. Lastly, our goal is to make the reader aware of the tremendous potential these approaches hold for studying host-pathogen interactions.

KBMA Listeria monocytogenes is an effective vector for DC-mediated induction of antitumor immunity

Vaccine strategies that utilize human DCs to enhance antitumor immunity have yet to realize their full potential. Approaches that optimally target a spectrum of antigens to DCs are urgently needed. Here we report the development of a platform for loading DCs with antigen. It is based on killed but metabolically active (KBMA) recombinant Listeria monocytogenes and facilitates both antigen delivery and maturation of human DCs. Highly attenuated KBMA L. monocytogenes were engineered to express an epitope of the melanoma-associated antigen MelanA/Mart-1 that is recognized by human CD8+ T cells when presented by the MHC class I molecule HLA-A*0201. The engineered KBMA L. monocytogenes induced human DC upregulation of costimulatory molecules and secretion of pro-Th1 cytokines and type I interferons, leading to effective priming of Mart-1-specific human CD8+ T cells and lysis of patient-derived melanoma cells. KBMA L. monocytogenes expressing full-length NY-ESO-1 protein, another melanoma-associated antigen, delivered the antigen for presentation by MHC class I and class II molecules independent of the MHC haplotype of the DC donor. A mouse therapeutic tumor model was used to show that KBMA L. monocytogenes efficiently targeted APCs in vivo to induce protective antitumor responses. Together, our data demonstrate that KBMA L. monocytogenes may be a powerful platform that can both deliver recombinant antigen to DCs for presentation and provide a potent DC-maturation stimulus, making it a potential cancer vaccine candidate.

IFNbeta responses induced by intracellular bacteria or cytosolic DNA in different human cells do not require ZBP1 (DLM-1/DAI)

Intracellular bacteria and cytosolic stimulation with DNA activate type I IFN responses independently of Toll-like receptors, most Nod-like receptors and RIG-like receptors. A recent study suggested that ZBP1 (DLM-1/DAI) represents the long anticipated pattern recognition receptor which mediates IFNalpha/beta responses to cytosolic DNA in mice. Here we show that Legionella pneumophila infection, and intracellular challenge with poly(dA-dT), but not with poly(dG-dC), induced expression of IFNbeta, full-length hZBP1 and a prominent splice variant lacking the first Zalpha domain (hZBP1DeltaZalpha) in human cells. Overexpression of hZBP1 but not hZBP1DeltaZalpha slightly amplified poly(dA-dT)-stimulated IFNbeta reporter activation in HEK293 cells, but had no effect on IFNbeta and IL-8 production induced by bacteria or poly(dA-dT) in A549 cells. We found that mZBP1 siRNA impaired poly(dA-dT)-induced IFNbeta responses in mouse L929 fibroblasts at a later time point, while multiple hZBP1 siRNAs did not suppress IFNbeta or IL-8 expression induced by poly(dA-dT) or bacterial infection in human cells. In contrast, IRF3 siRNA strongly impaired the IFNbeta responses to poly(dA-dT) or bacterial infection. In conclusion, intracellular bacteria and cytosolic poly(dA-dT) activate IFNbeta responses in different human cells without requiring human ZBP1.

Novel functions of type I interferons revealed by infection studies with Listeria monocytogenes

Infection of cells and mice with Listeria monocytogenes stimulates production of type I interferons (IFN). These in turn sensitise the Listeria host to lethal sequelae of infection with these bacteria. Here, we summarise recent findings on the production and biological effects of type I IFN in the course of L. monocytogenes infection.

The DEAD-box helicase DDX3X is a critical component of the TANK-binding kinase 1-dependent innate immune response

TANK-binding kinase 1 (TBK1) is of central importance for the induction of type-I interferon (IFN) in response to pathogens. We identified the DEAD-box helicase DDX3X as an interaction partner of TBK1. TBK1 and DDX3X acted synergistically in their ability to stimulate the IFN promoter, whereas RNAi-mediated reduction of DDX3X expression led to an impairment of IFN production. Chromatin immunoprecipitation indicated that DDX3X is recruited to the IFN promoter upon infection with Listeria monocytogenes, suggesting a transcriptional mechanism of action. DDX3X was found to be a TBK1 substrate in vitro and in vivo. Phosphorylation-deficient mutants of DDX3X failed to synergize with TBK1 in their ability to stimulate the IFN promoter. Overall, our data imply that DDX3X is a critical effector of TBK1 that is necessary for type I IFN induction.

Macrophage proinflammatory response to Francisella tularensis live vaccine strain requires coordination of multiple signaling pathways

The macrophage proinflammatory response to Francisella tularensis (Ft) live vaccine strain (LVS) was shown previously to be TLR2 dependent. The observation that intracellular Ft LVS colocalizes with TLR2 and MyD88 inside macrophages suggested that Ft LVS might signal from within the phagosome. Macrophages infected with LVSDeltaiglC, a Ft LVS mutant that fails to escape from the phagosome, displayed greatly increased expression of a subset of TLR2-dependent, proinflammatory genes (e.g., Tnf) but decreased expression of others (e.g., Ifnb1). This latter subset was similarly mitigated in IFN-beta(-/-) macrophages indicating that while Ft LVS-induced TLR2 signaling is necessary, cytosolic sensing of Ft to induce IFN-beta is required for full induction of the macrophage proinflammatory response. Although LVSDeltaiglC greatly increased IL-1beta mRNA in wild-type macrophages, protein secretion was not observed. IL-1beta secretion was also diminished in Ft LVS-infected IFN-beta(-/-) macrophages. rIFN-beta failed to restore IL-1beta secretion in LVSDeltaiglC-infected macrophages, suggesting that signals in addition to IFN-beta are required for assembly of the inflammasome and activation of caspase-1. IFN-beta plays a central role in controlling the macrophage bacterial burden: bacterial recovery was greater in IFN-beta(-/-) than in wild-type macrophages and treatment of Ft LVS-infected macrophages with rIFN-beta or 5,6-dimethylxanthenone-4-acetic acid, a potent IFN-beta inducer, greatly decreased the intracellular Ft LVS burden. In toto, these observations support the hypothesis that the host inflammatory response to Ft LVS is complex and requires engagement of multiple signaling pathways downstream of TLR2 including production of IFN-beta via an unknown cytosolic sensor and activation of the inflammasome.

Prevention of allograft tolerance by bacterial infection with Listeria monocytogenes

Exposure to certain viruses and parasites has been shown to prevent the induction of transplantation tolerance in mice via the generation of cross-reactive memory T cell responses or the induction of bystander activation. Bacterial infections are common in the perioperative period of solid organ allograft recipients in the clinic, and correlations between bacterial infections and acute allograft rejection have been reported. However, whether bacterial infections at the time of transplantation have any effect on the generation of transplantation tolerance remains to be established. We used the Gram-positive intracellular bacterium Listeria monocytogenes (LM) as a model pathogen because its effects on immune responses are well described. Perioperative LM infection prevented cardiac and skin allograft acceptance induced by anti-CD154 and donor-specific transfusion in mice. LM-mediated rejection was not due to the generation of cross-reactive T cells and was largely independent of signaling via MyD88, an adaptor for most TLRs, IL-1, and IL-18. Instead, transplant rejection following LM infection was dependent on the expression of the phagosome-lysing pore former listeriolysin O and on type I IFN receptor signaling. Our results indicate that bacterial exposure at the time of transplantation can antagonize tolerogenic regimens by enhancing alloantigen-specific immune responses independently of the generation of cross-reactive memory T cells.

Distinct TLR- and NLR-mediated transcriptional responses to an intracellular pathogen

How the innate immune system tailors specific responses to diverse microbial infections is not well understood. Cells use a limited number of host receptors and signaling pathways to both discriminate among extracellular and intracellular microbes, and also to generate responses commensurate to each threat. Here, we have addressed these questions by using DNA microarrays to monitor the macrophage transcriptional response to the intracellular bacterial pathogen Listeria monocytogenes. By utilizing combinations of host and bacterial mutants, we have defined the host transcriptional responses to vacuolar and cytosolic bacteria. These compartment-specific host responses induced significantly different sets of target genes, despite activating similar transcription factors. Vacuolar signaling was entirely MyD88-dependent, and induced the transcription of pro-inflammatory cytokines. The IRF3-dependent cytosolic response induced a distinct set of target genes, including IFNβ. Many of these cytosolic response genes were induced by secreted cytokines, so we further identified those host genes induced independent of secondary signaling. The host response to cytosolic bacteria was reconstituted by the cytosolic delivery of L. monocytogenes genomic DNA, but we observed an amplification of this response by NOD2 signaling in response to MDP. Correspondingly, the induction of IFNβ was reduced in nod2^−/− macrophages during infection with either L. monocytogenes or Mycobacterium tuberculosis. Combinatorial control of IFNβ induction by recognition of both DNA and MDP may highlight a mechanism by which the innate immune system integrates the responses to multiple ligands presented in the cytosol by intracellular pathogens.

Macrophages are critical cells of the innate immune system, contributing to immediate and robust defense against microbial infections. Macrophages detect pathogens using host receptors located on the cell surface, in phagosomal vacuoles, and in the cytosol. While fundamental to innate immunity, it is not clear if these different receptors merely provide redundant mechanisms for sensing microbial infection, or if instead they induce distinct gene expression programs that may allow for threat-specific host responses. We addressed this question by dissecting the macrophage transcriptional responses to the model intracellular bacterial pathogen Listeria monocytogenes. Using genetic and genomic approaches, we found that the macrophage response to L. monocytogenes trapped in phagosomal compartments was distinct and separable from the response to live bacteria replicating in the host cytosol. The macrophage response to cytosolic bacteria was recapitulated by bacterial nucleic acid and cell wall fragments, and induced surprisingly few primary response genes. These findings highlight a mechanism by which the innate immune system may specifically sense intracellular bacteria, as the macrophage response to Mycobacterium tuberculosis was similarly regulated.

Type I IFN are host modulators of strain-specific Listeria monocytogenes virulence

Type I IFN (IFN-I) increase the sensitivity of cells and mice to lethal infection with Listeria monocytogenes. Therefore the amount of IFN-I produced during infection might be an important factor determining Listeria virulence. Two commonly used strains of L. monocytogenes, EGD and LO28, were identified as, respectively, low and high inducers of IFN-I synthesis in infected macrophages. Increased IFN-I production resulted from the stronger ability of the LO28 strain to trigger the IRF3 signalling pathway and correlated with an increased sensitization of macrophages to lethal infection. In contrast, stimulation of NFkappaB, MAPK, or inflammasome signalling by the LO28 and EGD strains did not differ significantly. The LO28 strain was more virulent in wild-type (wt) C57/BL6 mice than the EGD strain whereas both strains were similarly virulent in IFN-I receptor-deficient C57/BL6 mice. Together our data suggest that isolates of wt L. monocytogenes differ in their ability to trigger the IRF3 signalling pathway and IFN-I production, and that the amount of IFN-I produced during infection is an important determinant of Listeria virulence.

Irreversible loss of membrane-binding activity of Listeria-derived cytolysins in non-acidic conditions: a distinct difference from allied cytolysins produced by other Gram-positive bacteria

Listeriolysin O (LLO), a member of the cholesterol-dependent cytolysin (CDC) family, is a major virulence factor of Listeria monocytogenes and contributes to bacterial escape from intracellular killing of macrophages. LLO is activated under weakly acidic conditions; however, the molecular mechanism of this pH-dependent expression of cytolytic activity of LLO is poorly understood. In this study, CDCs including LLO, ivanolysin O (ILO), seeligeriolysin O (LSO), pneumolysin (PLY), streptolysin O (SLO) and perfringolysin O (PFO) were prepared as recombinant proteins and examined for their functional changes after treatment under various pH conditions. Haemolytic and membrane cholesterol-binding activities were not affected in PLY, SLO and PFO at any pH examined. By contrast, all the Listeria-derived cytolysins, LLO, ILO and LSO, were active only at an acidic pH and rapidly inactivated under neutral or alkaline conditions. Once inactivated, LLO could not be reactivated even by a downward pH shift. The hydrophobicity of LLO treated at neutral or alkaline pH was increased. These data suggested that the pH-dependent loss of cytolytic activity appeared to be due to irreversible structural changes of domain 4 that resulted in the loss of target membrane cholesterol binding.

Signaling to NF-kappaB by Toll-like receptors

Innate immunity is the first line of defense against invading pathogens. A family of Toll-like receptors (TLRs) acts as primary sensors that detect a wide variety of microbial components and elicit innate immune responses. All TLR signaling pathways culminate in activation of the transcription factor nuclear factor-kappaB (NF-kappaB), which controls the expression of an array of inflammatory cytokine genes. NF-kappaB activation requires the phosphorylation and degradation of inhibitory kappaB (IkappaB) proteins, which is triggered by two kinases, IkappaB kinase alpha (IKKalpha) and IKKbeta. In addition, several TLRs activate alternative pathways involving the IKK-related kinases TBK1 [TRAF family member-associated NF-kappaB activator (TANK) binding kinase-1] and IKKi, which elicit antiviral innate immune responses. Here, we review recent progress in our understanding of the role of NF-kappaB in TLR signaling pathways and discuss potential implications for molecular medicine.

Nod1 and Nod2 induce CCL5/RANTES through the NF-kappaB pathway

The Nod-like receptor proteins Nod1 and Nod2 participate in innate immune responses against bacteria through intracellular detection of peptidoglycan, a component of bacterial cell wall. Recent evidence has demonstrated that Nod1 stimulates the release of chemokines that attract neutrophils at the site of infection, such as CXCL8/IL-8 in humans, and CXCL1/keratinocyte-derived chemokine and CXCL2/MIP-2 in mice. We aimed to determine whether Nod proteins could trigger the release of CCL5/RANTES, a chemokine known to attract a number of immune cells, but not neutrophils. Our results demonstrate that activation of both Nod1 and Nod2 results in substantial secretion of CCL5 by murine macrophages. Moreover, in vivo, the intraperitoneal injection of murine Nod1 or Nod2 agonists resulted in a rapid secretion of CCL5 into the bloodstream. We also observed that Nod-dependent secretion of CCL5 did not correlate with the induction of the interferon-beta pathway, a major signaling cascade for the activation of CCL5 by viruses. In contrast, we identified a key role of the NF-kappaB pathway in Nod-dependent stimulation of the CCL5 promoter. Together, these results identify a novel target downstream of Nod1 and Nod2, which is likely to play a key role in orchestrating the global Nod-dependent immune defense during bacterial infections.

Type I IFN induction in response to Listeria monocytogenes in human macrophages: evidence for a differential activation of IFN regulatory factor 3 (IRF3)

Listeria monocytogenes is a prototypic bacterium for studying innate and adaptive cellular immunity as well as host defense. Using human monocyte-derived macrophages, we report that an infection with a wild-type strain, but not a listeriolysin O-deficient strain, of the Gram-positive bacterium L. monocytogenes induces expression of IFN-beta and a bioactive type I IFN response. Investigating the activation of signaling pathways in human macrophages after infection revealed that a wild-type strain and a hemolysin-deficient strain of L. monocytogenes activated the NF-kappaB pathway and induced a comparable TNF response. p38 MAPK and activating transcription factor 2 were phosphorylated following infection with either strain, and IFN-beta gene expression induced by wild-type L. monocytogenes was reduced when p38 was inhibited. However, neither IFN regulatory factor (IRF) 3 translocation to the nucleus nor posttranslational modifications and dimerizations were observed after L. monocytogenes infection. In contrast, vesicular stomatitis virus and LPS triggered IRF3 activation and signaling. When IRF3 was knocked down using small interfering RNA, a L. monocytogenes-induced IFN-beta response remained unaffected whereas a vesicular stomatitis virus-triggered response was reduced. Evidence against the possibility that IRF7 acts in place of IRF3 is provided. Thus, we show that wild-type L. monocytogenes induced an IFN-beta response in human macrophages and propose that this response involves p38 MAPK and activating transcription factor 2. Using various stimuli, we show that IRF3 is differentially activated during type I IFN responses in human macrophages.

Irf3 polymorphism alters induction of interferon beta in response to Listeria monocytogenes infection

Genetic makeup of the host plays a significant role in the course and outcome of infection. Inbred strains of mice display a wide range of sensitivities to Listeria monocytogenes infection and thus serve as a good model for analysis of the effect of genetic polymorphism. The outcome of L. monocytogenes infection in mice is influenced by the ability of this bacterium to induce expression of interferon beta mRNA, encoded in mouse by the Ifnb1 (interferon beta 1, fibroblast) gene. Mouse strains that lack components of the IFNβ signaling pathway are substantially more resistant to infection. We found that macrophages from the ByJ substrain of the common C57BL/6 inbred strain of mice are impaired in their ability to induce Ifnb1 expression in response to bacterial and viral infections. We mapped the locus that controls differential expression of Ifnb1 to a region on Chromosome 7 that includes interferon regulatory factor 3 (Irf3), which encodes a transcription factor responsible for early induction of Ifnb1 expression. In C57BL/6ByJ mice, Irf3 mRNA was inefficiently spliced, with a significant proportion of the transcripts retaining intron 5. Analysis of the Irf3 locus identified a single base-pair polymorphism and revealed that intron 5 of Irf3 is spliced by the atypical U12-type spliceosome. We found that the polymorphism disrupts a U12-type branchpoint and has a profound effect on the efficiency of splicing of Irf3. We demonstrate that a naturally occurring change in the splicing control element has a dramatic effect on the resistance to L. monocytogenes infection. Thus, the C57BL/6ByJ mouse strain serves as an example of how a mammalian host can counter bacterial virulence strategies by introducing subtle alteration of noncoding sequences.

Specific variances in an individual's DNA, known as genetic polymorphisms, can play a significant role in determining susceptibility to an infectious disease. To identify the genetic polymorphisms that are associated with resistance to the common human bacterial pathogen L. monocytogenes, we have carried out a series of genetic and molecular biology experiments using closely related strains of mice that are differentially susceptible to Listeria infection. Through this analysis, we have identified a spontaneous mutation in an intron of the Irf3 gene, which encodes a key transcription factor involved in innate immunity. This single nucleotide change affects the efficiency with which Irf3 mRNA is spliced, thus limiting the ability of bacteria to induce interferon beta expression in order to suppress innate immune defense. By analyzing this mutation, we found that processing of mouse Irf3 mRNA relies on an atypical U12 splicing mechanism that has been suggested to be a rate-limiting step in gene expression. Our findings not only provide an additional example of an important role of noncoding polymorphisms in control of gene function, but also demonstrate how such polymorphisms can fine tune innate immune response.