Type I interferon production enhances susceptibility to Listeria monocytogenes infection

Expression of the p60 autolysin enhances NK cell activation and is required for listeria monocytogenes expansion in IFN-gamma-responsive mice

Both peptidoglycan and muropeptides potently modulate inflammatory and innate immune responses. The secreted Listeria monocytogenes p60 autolysin digests peptidoglycan and promotes bacterial infection in vivo. Here, we report that p60 contributes to bacterial subversion of NK cell activation and innate IFN-gamma production. L. monocytogenes deficient for p60 (Deltap60) competed well for expansion in mice doubly deficient for IFNAR1 and IFN-gammaR1 or singly deficient for IFN-gammaR1, but not in wild-type, IFNAR1(-/-), or TLR2(-/-) mice. The restored competitiveness of p60-deficient bacteria suggested a specific role for p60 in bacterial subversion of IFN-gamma-mediated immune responses, since in vivo expansion of three other mutant L. monocytogenes strains (DeltaActA, DeltaNamA, and DeltaPlcB) was not complemented in IFN-gammaR1(-/-) mice. Bacterial expression of p60 was not required to induce socs1, socs3, and il10 expression in infected mouse bone marrow macrophages but did correlate with enhanced production of IL-6, IL-12p70, and most strikingly IFN-gamma. The primary source of p60-dependent innate IFN-gamma was NK cells, whereas bacterial p60 expression did not significantly alter innate IFN-gamma production by T cells. The mechanism for p60-dependent NK cell stimulation was also indirect, given that treatment with purified p60 protein failed to directly activate NK cells for IFN-gamma production. These data suggest that p60 may act on infected cells to indirectly enhance NK cell activation and increase innate IFN-gamma production, which presumably promotes early bacterial expansion through its immunoregulatory effects on bystander cells. Thus, the simultaneous induction of IFN-gamma production and factors that inhibit IFN-gamma signaling may be a common strategy for misdirection of early antibacterial immunity.

Type I IFN signaling is crucial for host resistance against different species of pathogenic bacteria

It is known that host cells can produce type I IFNs (IFN-alphabeta) after exposure to conserved bacterial products, but the functional consequences of such responses on the outcome of bacterial infections are incompletely understood. We show in this study that IFN-alphabeta signaling is crucial for host defenses against different bacteria, including group B streptococci (GBS), pneumococci, and Escherichia coli. In response to GBS challenge, most mice lacking either the IFN-alphabetaR or IFN-beta died from unrestrained bacteremia, whereas all wild-type controls survived. The effect of IFN-alphabetaR deficiency was marked, with mortality surpassing that seen in IFN-gammaR-deficient mice. Animals lacking both IFN-alphabetaR and IFN-gammaR displayed additive lethality, suggesting that the two IFN types have complementary and nonredundant roles in host defenses. Increased production of IFN-alphabeta was detected in macrophages after exposure to GBS. Moreover, in the absence of IFN-alphabeta signaling, a marked reduction in macrophage production of IFN-gamma, NO, and TNF-alpha was observed after stimulation with live bacteria or with purified LPS. Collectively, our data document a novel, fundamental function of IFN-alphabeta in boosting macrophage responses and host resistance against bacterial pathogens. These data may be useful to devise alternative strategies to treat bacterial infections.

The intestinal epithelial barrier: how to distinguish between the microbial flora and pathogens

The gastrointestinal tract is fundamental for the uptake of nutrients and fluids, but it also represents the greatest surface of the body in contact with the external environment and most human pathogens enter the body through the mucosal surface, especially in the intestine. The intestinal immune system protects the sterile core of the organism against invasion and systemic dissemination of both pathogens and limits for level penetration of commensal microorganisms. In addition, the human intestine is continually in contact with 10(14) commensal bacteria containing more than 500 different species. These commensal bacteria confer health benefits to their host by helping dietary digestion, development of gut immunity and preventing colonization by pathogens. To maintain integrity and normal function of intestine, a delicate equilibrium must be reached between the bacterial flora and intestinal immune system. This review discusses the recent advances in our understanding of how the mucosal intestinal barrier maintains a local homeostatic response to the resident intestinal bacteria, while protecting the host against enteric pathogens. In particular, the emerging function of Toll-like receptors (TLRs) and Nod-like receptors (NLRs) in controlling mucosal immunity will be presented.

Type I interferon response in the central nervous system

This review is dedicated to the influence of type I IFNs (also called IFN-alpha/beta) in the central nervous system (CNS). Studies in mice with type I IFN receptor or IFN-beta gene deficiency have highlighted the importance of the type I IFN system against CNS viral infections and non-viral autoimmune disorders. Direct antiviral effects of type I IFNs appear to be crucial in limiting early spread of a number of viruses in CNS tissues. Type I IFNs have also proved to be beneficial in autoimmune disorders like multiple sclerosis or experimental autoimmune encephalitis, probably through immunomodulatory effects. Increasing efforts are done to characterize IFN expression and response in the CNS: to identify type I IFN producing cells, to decipher pathways leading to type I IFN expression in those cells, and to identify responding cells. However, reversible and irreversible damages consecutive to chronic exposure of the CNS to type I IFNs underline the importance of a tightly regulated type I IFN homeostasis in this organ.

The Type I IFN Response to Infection with Mycobacterium tuberculosis Requires ESX-1-Mediated Secretion and Contributes to Pathogenesis

The ESX-1 secretion system is a major determinant of Mycobacterium tuberculosis virulence, although the pathogenic mechanisms resulting from ESX-1-mediated transport remain unclear. By global transcriptional profiling of tissues from mice infected with either wild-type or ESX-1 mutant bacilli, we found that host genes controlled by ESX-1 in vivo are predominantly IFN regulated. ESX-1-mediated secretion is required for the production of host type I IFNs during infection in vivo and in macrophages in vitro. The macrophage signaling pathway leading to the production of type I IFN required the host kinase TANK-binding kinase 1 and occurs independently of TLR signaling. Importantly, the induction of type I IFNs during M. tuberculosis infection is a pathogenic mechanism as mice lacking the type I IFNR were more restrictive for bacterial growth in the spleen than wild-type mice, although growth in the lung was unaffected. We propose that the ESX-1 secretion system secretes effectors into the cytosol of infected macrophages, thereby triggering the type I IFN response for the manipulation of host immunity.

The role of the activated macrophage in clearing Listeria monocytogenes infection

Macrophage activation often contributes to the strong immune response elicited upon infection. The ability of macrophages to become activated was discovered when sub-lethal primary infections of mice with the bacterium Listeria monocytogenes provided protection against secondary infections through non-humoral immunity. L. monocytogenes infect and propagate in macrophages by escaping the phagosome into the cytosol, where they avoid humoral immune mediators. Activated macrophages kill L. monocytogenes by blocking phagosomal escape. The timing of the antimicrobial activities within the phagosome is crucial to the outcome. In non-activated macrophages, bacterial factors generally prevail, and L. monocytogenes can escape from the vacuoles and grow within cytoplasm. Activated macrophages generate reactive oxygen or nitrogen intermediates early after bacterial uptake, which prevent the bacteria from escaping vacuoles into cytoplasm. The heterogeneity in the interactions between L. monocytogenes and the macrophage indicate a complex relationship between the host and the pathogen governed by chemistries that promote and inhibit escape from vacuoles. This review examines the mechanisms used by activated and non-activated macrophages to kill microbes, and how those mechanisms are employed against L. monocytogenes.

Blocking monoclonal antibodies specific for mouse IFN-alpha/beta receptor subunit 1 (IFNAR-1) from mice immunized by in vivo hydrodynamic transfection

Herein we report the generation of mouse monoclonal antibodies (mAbs) specific for the IFNAR-1 subunit of the mouse interferon-alpha/beta (IFN-alpha/beta) receptor (MAR1 mAbs) that block type I IFN receptor signaling and biologic response induction in vitro and in vivo. These mAbs were generated from Ifnar1 (/) mice immunized by in vivo hydrodynamic transfection with a plasmid encoding the extracellular domain (ECD) of murine IFNAR-1. All MAR1 mAbs bound native receptor expressed on cell surfaces and immunoprecipitated IFNAR-1 from solubilized cells, and two mAbs also detected IFNAR-1 by Western blot analysis. in vitro, the mAbs prevented ligand-induced intracellular signaling and induction of a variety of type I IFN-induced biologic responses but had no effect on IFN-gamma-induced responses. The most effective in vitro blocker, MAR1-5A3, also blocked type I IFN-induced antiviral, antimicrobial, and antitumor responses in vivo. We also explored whether murine IFNAR-1 surface expression required the presence of Tyk2. In contrast to Tyk2-deficient human cell lines, comparable IFNAR-1 expression was found on primary cells derived either from wild-type or Tyk2 (/) mice. These mAbs represent much needed tools to more clearly elucidate the biochemistry, cell biology, and physiologic function of the type I IFNs and their receptor in mediating host-protective immunity and immunopathology.

MxA-based recognition of viral illness in febrile children by a whole blood assay

Febrile children are often given antibiotics empirically and unnecessarily. MxA is a protein induced in peripheral lymphoid cells by type 1 interferons during active viral infection. The ability of a whole blood ELISA assay for MxA to identify children with viral illness was studied in 122 children who presented with acute onset fever and 52 age-matched healthy controls. The febrile children were divided into three groups according to their final diagnoses: etiologically diagnosed viral infection, clinically diagnosed viral infection, and bacterial infection. MxA levels in the bacterial infection group and controls were similar and low (90.9 +/- 69.7 and 76.9 +/- 63.2 ng/mL, respectively). In contrast, mean MxA levels in the two viral infection groups were higher than in both the bacterial and control groups (719.2 +/- 386.4 and 827.0 +/- 651.1, respectively). A receiver operating characteristic analysis showed that the area under the curve of the MxA level was greater than under the curves of both the white blood cell count and the C-reactive protein concentration. Whole blood assay of MxA is a clinically useful tool for diagnosing viral illness in febrile children and should help reduce use of unnecessary antibiotics.

Cytosolic Localization ofListeria monocytogenesTriggers an Early IFN-γ Response by CD8+T Cells That Correlates with Innate Resistance to Infection

IFN-gamma is critical for innate immunity against Listeria monocytogenes (L. monocytogenes), and it has long been thought that NK cells are the major source of IFN-gamma during the first few days of infection. However, it was recently shown that a significant number of CD44highCD8+ T cells also secrete IFN-gamma in an Ag-independent fashion within 16 h of infection with L. monocytogenes. In this report, we showed that infection with other intracellular pathogens did not trigger this early IFN-gamma response and that cytosolic localization of Listeria was required to induce rapid IFN-gamma production by CD44highCD8+ T cells. Infection of C57BL/6 mice with an Escherichia coli strain expressing listeriolysin O (LLO), a pore-forming toxin from L. monocytogenes, also resulted in rapid IFN-gamma expression by CD8+ T cells. These results suggest that LLO expression is essential for induction of the early IFN-gamma response, although it is not yet clear whether LLO plays a direct role in triggering a signal cascade that leads to cytokine production or whether it is required simply to release other bacterial product(s) into the host cell cytosol. Interestingly, mouse strains that displayed a rapid CD8+ T cell IFN-gamma response (C57BL/6, 129, and NZB) all had lower bacterial burdens in the liver 3 days postinfection compared with mouse strains that did not have an early CD8+ T cell IFN-gamma response (BALB/c, A/J, and SJL). These data suggest that participation of memory CD8+ T cells in the early immune response against L. monocytogenes correlates with innate host resistance to infection.

TLR-Dependent Induction of IFN-β Mediates Host Defense againstTrypanosoma cruzi

Host resistance to the intracellular protozoan parasite Trypanosoma cruzi depends on IFN-gamma production by T cells and NK cells. However, the involvement of innate immunity in host resistance to T. cruzi remains unclear. In the present study, we investigated host defense against T. cruzi by focusing on innate immunity. Macrophages and dendritic cells (DCs) from MyD88(-/-)TRIF(-/-) mice, in which TLR-dependent activation of innate immunity was abolished, were defective in the clearance of T. cruzi and showed impaired induction of IFN-beta during T. cruzi infection. Neutralization of IFN-beta in MyD88(-/-) macrophages led to enhanced T. cruzi growth. Cells from MyD88(-/-)IFNAR1(-/-) mice also showed impaired T. cruzi clearance. Furthermore, both MyD88(-/-)TRIF(-/-) and MyD88(-/-)IFNAR1(-/-) mice were highly susceptible to in vivo T. cruzi infection, highlighting the involvement of innate immune responses in T. cruzi infection. We further analyzed the molecular mechanisms for the IFN-beta-mediated antitrypanosomal innate immune responses. MyD88(-/-)TRIF(-/-) and MyD88(-/-)IFNAR1(-/-) macrophages and DCs exhibited defective induction of the GTPase IFN-inducible p47 (IRG47) after T. cruzi infection. RNA interference-mediated reduction of IRG47 expression in MyD88(-/-) macrophages resulted in increased intracellular growth of T. cruzi. These findings suggest that TLR-dependent expression of IFN-beta is involved in resistance to T. cruzi infection through the induction of IRG47.

Legionella pneumophila Induces IFNβ in Lung Epithelial Cells via IPS-1 and IRF3, Which Also Control Bacterial Replication

Legionella pneumophila, a Gram-negative facultative intracellular bacterium, causes severe pneumonia (Legionnaires' disease). Type I interferons (IFNs) were so far associated with antiviral immunity, but recent studies also indicated a role of these cytokines in immune responses against (intracellular) bacteria. Here we show that wild-type L. pneumophila and flagellin-deficient Legionella, but not L. pneumophila lacking a functional type IV secretion system Dot/Icm, or heat-inactivated Legionella induced IFNbeta expression in human lung epithelial cells. We found that factor (IRF)-3 and NF-kappaB-p65 translocated into the nucleus and bound to the IFNbeta gene enhancer after L. pneumophila infection of lung epithelial cells. RNA interference demonstrated that in addition to IRF3, the caspase recruitment domain (CARD)-containing adapter molecule IPS-1 (interferon-beta promoter stimulator 1) is crucial for L. pneumophila-induced IFNbeta expression, whereas other CARD-possessing molecules, such as RIG-I (retinoic acid-inducible protein I), MDA5 (melanoma differentiation-associated gene 5), Nod27 (nucleotide-binding oligomerization domain protein 27), and ASC (apoptosis-associated speck-like protein containing a CARD) seemed not to be involved. Finally, bacterial multiplication assays in small interfering RNA-treated cells indicated that IPS-1, IRF3, and IFNbeta were essential for the control of intracellular replication of L. pneumophila in lung epithelial cells. In conclusion, we demonstrated a critical role of IPS-1, IRF3, and IFNbeta in Legionella infection of lung epithelium.

Type I interferons in host defense

Type I interferons (IFNs) are a family of cytokines specialized to coordinate immunity to viruses and other intracellular infections. In the past several years, many of the receptors and signaling pathways that link pathogen detection to induction of type I IFNs have been identified and characterized. An integrated picture has emerged in which type I IFNs have essential functions in several seemingly disparate processes: they restrict viral spread by engaging machinery that ultimately cripples and kills infected cells, yet they are also positively linked to the activation and expansion of lymphocytes that are important for control of intracellular infections. These advances highlight the context-specific actions of type I IFNs and clarify the multiple points at which they are integrated into both innate and adaptive immunity.

Fifty years of interferon research: aiming at a moving target

Nearly half a century has passed since the first published description of interferons (IFNs). This commentary introduces the four accompanying review articles on type I IFN research and attempts to relate how the field of IFN research has been changing during its history.

IRFs: master regulators of signalling by Toll-like receptors and cytosolic pattern-recognition receptors

The interferon-regulatory factor (IRF) family of transcription factors was initially found to be involved in the induction of genes that encode type I interferons. IRFs have now been shown to have functionally diverse roles in the regulation of the immune system. Recently, the crucial involvement of IRFs in innate and adaptive immune responses has been gaining much attention, particularly with the discovery of their role in immunoregulation by Toll-like receptors and other pattern-recognition receptors.

Increased susceptibility to bacterial superinfection as a consequence of innate antiviral responses

The reason why severe localized or systemic virus infections enhance and aggravate bacterial superinfection is poorly understood. Here we show that virus-induced IFN type I caused apoptosis in bone marrow granulocytes, drastically reduced granulocyte infiltrates at the site of bacterial superinfection, caused up to 1,000-fold higher bacterial titers in solid organs, and increased disease susceptibility. The finding that the innate antiviral immune response reduces the antibacterial granulocyte defense offers an explanation for enhanced susceptibility to bacterial superinfection during viral disease.

Type I IFN Modulates Host Defense and Late Hyperinflammation in Septic Peritonitis

TLRs are considered important for the control of immune responses during endotoxic shock or polymicrobial sepsis. Signaling by TLRs may proceed through the adapter proteins MyD88 or TIR domain-containing adaptor inducinng IFN-beta. Both pathways can lead to the production of type I IFNs (IFN-alphabeta). In the present study, the role of the type I IFN pathway for host defense and immune pathology in sepsis was investigated using a model of mixed bacterial peritonitis. Systemic levels of IFN-alphabeta protein were markedly elevated during septic peritonitis. More detailed analyses revealed production of IFN-beta, but not IFN-alpha subtypes, and identified CD11b+ CD11c- macrophage-like cells as major producers of IFN-beta. The results further demonstrate that in IFN-alphabeta receptor I chain (IFNARI)-deficient mice, the early recruitment of neutrophils to the infected peritoneal cavity was augmented, most likely due to an increased local production of MCP-1 and leukotriene B4. In the absence of IFNARI, peritoneal neutrophils also exhibited enhanced production of reactive oxygen intermediates and elevated expression of Mac-1. Conversely, administration of recombinant IFN-beta resulted in reduced leukotriene B4 levels and decreased peritoneal neutrophil recruitment and activation. Analysis of the cytokine response to septic peritonitis revealed that IFNARI deficiency strongly attenuated late, but not early, hyperinflammation. In accordance with these findings, bacterial clearance and overall survival of IFNARI(-/-) mice were improved. Therefore, the present study reveals critical functions of the type I IFN pathway during severe mixed bacterial infections leading to sepsis. The results suggest that type I IFN exerts predominantly adverse effects under these conditions.

STAT1 regulates IFN-alpha beta- and IFN-gamma-dependent control of infection with Chlamydia pneumoniae by nonhemopoietic cells

STAT1 mediates signaling in response to IFN-alpha, -beta, and -gamma, cytokines required for protective immunity against several viral, bacterial, and eukaryotic pathogens. The protective role of STAT1 in the control of intranasal infection with the obligate intracellular bacterium Chlamydia pneumoniae was analyzed. IFN-gamma-/- or IFN-gamma receptor (R)-/- mice were highly susceptible to infection with C. pneumoniae. We found that STAT1-/- mice were even more susceptible to C. pneumoniae than IFN-gamma-/- or IFN-gammaR-/- mice. Phosphorylation of STAT1 was detected in the lungs of C. pneumoniae-infected wild-type, IFN-gammaR-/-, and IFN-alphabetaR-/- mice, but not in mice lacking both IFN-alphabetaR and IFN-gammaR. In line with this, IFN-alphabetaR-/-/IFN-gammaR-/- mice showed increased susceptibility to infection compared with IFN-gammaR-/- mice. However, C. pneumoniae-infected IFN-alphabetaR-/- or IFN regulatory factor 3-/- mice showed no increased susceptibility and similar IFN-gamma expression compared with wild-type mice. CD4+ or CD8+ cells released IFN-gamma in vivo and conferred protection against C. pneumoniae in a STAT1-independent manner. In contrast, STAT1 mediated a nonredundant protective role of nonhemopoietic cells but not of hemopoietic cells. Nonhemopoietic cells accounted for the expression of STAT1-mediated indoleamine 2, 3-dioxygenase and the p47 GTPase LRG-47, but not inducible NO synthase mRNA. In summary, we demonstrate that STAT1 mediates a cooperative effect of IFN-alphabeta and IFN-gamma on nonhemopoietic cells, resulting in protection against C. pneumoniae.

Twist mediates suppression of inflammation by type I IFNs and Axl

Type I interferons (IFNs) are pleiotropic cytokines with antiviral and immunomodulatory properties. The immunosuppressive actions of type I IFNs are poorly understood, but IFN-mediated suppression of TNFα production has been implicated in the regulation of inflammation and contributes to the effectiveness of type I IFNs in the treatment of certain autoimmune and inflammatory diseases. In this study, we investigated mechanisms by which type I IFNs suppress induction of TNFα production by immune complexes, Fc receptors, and Toll-like receptors. Suppression of TNFα production was mediated by induction and activation of the Axl receptor tyrosine kinase and downstream induction of Twist transcriptional repressors that bind to E box elements in the TNF promoter and suppress NF-κB–dependent transcription. Twist expression was activated by the Axl ligand Gas6 and by protein S and apoptotic cells. These results implicate Twist proteins in regulation of TNFα production by antiinflammatory factors and pathways, and provide a mechanism by which type I IFNs and Axl receptors suppress inflammatory cytokine production.

Differential expansion, activation and effector functions of conventional and plasmacytoid dendritic cells in mouse tissues transiently infected with Listeria monocytogenes

Dendritic cells (DC) are crucial in generating immunity to infection. Here we characterize changes in DC in terms of number, activation and effector functions, focusing on conventional DC (cDC) and plasmacytoid DC (pDC), in Listeria-infected mice. Kinetic studies showed a subset- and tissue-specific expansion of cDC and upregulation of CD80 and CD86 on splenic and mesenteric lymph node (MLN) cDC after intragastric infection. Expansion of pDC was more prolonged than cDC, and pDC upregulated CD86 and MHC-II, but not CD80, in both the spleen and MLN. cDC were an important source of IL-12 but not TNF-alpha during infection, while pDC made neither of these cytokines. Instead other CD11c(int) cells produced these cytokines. Using five-colour flow cytometry and double intracellular cytokine staining, we detected phenotypically similar CD11c(int)CD11b(+)Gr1(+) cells with distinct capacities to produce TNF-alpha/IL-12 or TNF-alpha/iNOS (inducible nitric oxide synthase) in Listeria-infected tissues. IL-12p70 was also produced by sorted CD11c(hi) and CD11c(int)CD11b(+)Gr1(+) cells. Furthermore, production of TNF-alpha, iNOS and IL-12 was differentially dependent on cellular localization of the bacteria. Cytosol-restricted bacteria induced TNF-alpha and iNOS-producing cells, albeit at lower frequency than wild-type bacteria. In contrast, IL-12 was induced only with wild-type bacteria. These data provide new insight into the relative abundance and function of distinct CD11c-expressing populations during the early stage of Listeria infection.

Natural killer cells as an initial defense against pathogens

Natural killer (NK) cells serve as a crucial first line of defense against tumors and a diverse range of pathogens. Recognition of infection by NK cells is accomplished by the activation of receptors on the NK cell surface, which initiate NK cell effector functions. Many of the receptors and ligands involved in NK cell antimicrobial activity have been identified, and we are beginning to appreciate how they function during infection. In addition, NK cells are activated by cytokines (e.g. interleukin 12 and type I interferons), which are products of activated macrophages and dendritic cells. In response to these activating stimuli, NK cells secrete cytokines and chemokines and lyse target cells. Recent studies have focused on the mechanisms by which NK cells recognize and respond to viruses, parasites and bacteria, and on the unique role of NK cells in innate immunity to infection.

Regulation of Apoptosis by Gram-Positive Bacteria: Mechanistic Diversity and Consequences for Immunity

Apoptosis, or programmed cell death (PCD), is an important physiological mechanism, through which the human immune system regulates homeostasis and responds to diverse forms of cellular damage. PCD may also be involved in immune counteraction to microbial infection. Over the past decade, the amount of research on bacteria-induced PCD has grown tremendously, and the implications of this mechanism on immunity are being elucidated. Some pathogenic bacteria actively trigger the suicide response in critical lineages of leukocytes that orchestrate both the innate and adaptive immune responses; other bacteria proactively prevent PCD to benefit their own survival and persistence. Currently, the microbial virulence factors, which represent the keys to unlocking the suicide response in host cells, are a primary focus of this field. In this review, we discuss these bacterial "apoptosis regulatory molecules" and the apoptotic events they either trigger or prevent, the host target cells of this regulatory activity, and the possible ramifications for immunity to infection. Gram-positive pathogens including Staphylococcus, Streptococcus, Bacillus, Listeria, and Clostridia species are discussed as important agents of human infection that modulate PCD pathways in eukaryotic cells.

Lymphocytes are detrimental during the early innate immune response against Listeria monocytogenes

Mice deficient in lymphocytes are more resistant than normal mice to Listeria monocytogenes infection during the early innate immune response. This paradox remains unresolved: lymphocytes are required for sterilizing immunity, but their presence during the early stage of the infection is not an asset and may even be detrimental. We found that lymphocyte-deficient mice, which showed limited apoptosis in infected organs, were resistant during the first four days of infection but became susceptible when engrafted with lymphocytes. Engraftment with lymphocytes from type I interferon receptor–deficient (IFN-αβR^−/−) mice, which had reduced apoptosis, did not confer increased susceptibility to infection, even when the phagocytes were IFN-αβR^+/+. The attenuation of innate immunity was due, in part, to the production of the antiinflammatory cytokine interleukin 10 by phagocytic cells after the apoptotic phase of the infection. Thus, immunodeficient mice were more resistant relative to normal mice because the latter went through a stage of lymphocyte apoptosis that was detrimental to the innate immune response. This is an example of a bacterial pathogen creating a cascade of events that leads to a permissive infective niche early during infection.

Recognition of cytosolic DNA activates an IRF3-dependent innate immune response

Nucleic acid recognition upon viral infection triggers type I interferon production. Viral RNA is detected by both endosomal, TLR-dependent and cytosolic, RIG-I/MDA5-dependent pathways. TLR9 is the only known sensor of foreign DNA; it is unknown whether innate immune recognition of DNA exists in the cytosol. Here we present evidence that cytosolic DNA activates a potent type I interferon response to the invasive bacterium Listeria monocytogenes. The noninvasive Legionella pneumophila triggers an identical response through its type IV secretion system. Activation of type I interferons by cytosolic DNA is TLR independent and requires IRF3 but occurs without detectable activation of NF-kappaB and MAP kinases. Microarray analyses reveal a unique but overlapping gene-expression program activated by cytosolic DNA compared to TLR9- and RIG-I/MDA5-dependent responses. These findings define an innate immune response to DNA linked to type I interferon production.

The wild-derived inbred mouse strain SPRET/Ei is resistant to LPS and defective in IFN-beta production

Although activation of Toll-like receptor 4 (TLR4)-positive cells is essential for eliminating Gram-negative bacteria, overactivation of these cells by the TLR4 ligand LPS initiates a systemic inflammatory reaction and shock. Here we demonstrate that SPRET/Ei mice, derived from Mus spretus, exhibit a dominant resistance against LPS-induced lethality. This resistance is mediated by bone marrow-derived cells. Macrophages from these mice exhibit normal signaling and gene expression responses that depend on the myeloid differentiation factor 88 adaptor protein, but they are impaired in IFN-beta production. The defect appears to be specific for IFN-beta, although the SPRET/Ei IFN-beta promoter is normal. In vivo IFN-beta induction by LPS or influenza virus is very low in SPRET/Ei mice, but IFN-beta-treatment restores the sensitivity to LPS, and IFN type 1 receptor-deficient mice are also resistant to LPS. Because of the defective induction of IFN-beta, these mice are completely resistant to Listeria monocytogenes and highly sensitive to Leishmania major infection. Stimulation of SPRET/Ei macrophages leads to rapid down-regulation of IFN type 1 receptor mRNA expression, which is reflected in poor induction of IFN-beta-dependent genes. This finding indicates that the resistance of SPRET/Ei mice to LPS is due to disruption of a positive-feedback loop that amplifies IFN-beta production. In contrast to TLR4-deficient mice, SPRET/Ei mice resist both LPS and sepsis induced with Klebsiella pneumoniae.

Innate (over)immunity and adaptive autoimmune disease

Autoimmune disease is characterized by clinical symptoms mediated by adaptive (T cell and B cell) immune reactions towards autoantigen-expressing tissue. Here we discuss that autoimmune disease is often preceded by autoreactivity, meaning the priming of autoantigen-specific immune cells without relevant tissue damage. Recent experimental evidence has demonstrated that both the induction of autoreactivity and the conversion into autoimmune disease is controlled by the activation of the nonspecific innate immune system. Also, the "inflammatory status" of the target organ critically influences the onset of overt autoimmune disease.

Th1 cytokines are essential for placental immunity to Listeria monocytogenes

The fetal allograft poses an immunological challenge: how is it protected while immunity to pathogens, particularly those that replicate in the placenta, is maintained? Several theories have been proposed to explain this fetal protection, including a pregnancy-based bias towards a Th2 rather than Th1 cytokine profile in order to avoid generating cytotoxic T cells that could threaten the fetus. Listeria monocytogenes preferentially replicates in the placenta and systemically requires a Th1 response for sterile eradication. In the placenta, the Th1 cytokines tumor necrosis factor alpha (TNF-alpha) and gamma interferon (IFN-gamma) are also synthesized in response to this pathogen, without fetal loss. Here we show, by using mice homozygous for null mutations in either the cytokine or cytokine receptor genes, a requirement for both TNF-alpha and IFN-gamma signaling for an effective placental immune response to L. monocytogenes. However, T cells were not recruited to the placenta. Genetic studies in which the fetal component of the placenta was genetically different from the mother indicated that both the production of and response to these cytokines were maternal. Despite the requirement for these cytokines, the early recruitment of neutrophils to the placenta was normal. Consequently, the bacterium appeared to be delayed in its colonization of this organ and did not fully gain hold until 72 h postinfection. These data show a requirement for Th1 cytokines during pregnancy for effective immunity and indicate that a bias away from Th1 cytokine synthesis is not a necessary prerequisite of pregnancy.

TLR agonists regulate PDGF-B production and cell proliferation through TGF-beta/type I IFN crosstalk

Transforming growth factor-beta (TGF-beta) and type I interferon (IFN) autocrine/paracrine loops are recognized as key mediators of signaling cascades that control a variety of cellular functions. Here, we describe a novel mechanism by which Toll-like receptor (TLR) agonists utilize these two autocrine/paracrine loops to differentially regulate the induction of PDGF-B, a growth factor implicated in a number of diseases ranging from tumor metastasis to glomerulonephritis. We demonstrate that CpG-specific induction of PDGF-B requires activation of Smads through TGFbeta1 autocrine/paracrine signaling. In contrast, polyinosinic:polycytidylic acid strongly represses CpG's as well as its own intrinsic ability to induce PDGF-B mRNA through type I IFN-mediated induction of Smad7, a negative regulator of Smad3/4. Furthermore, we have shown that this crosstalk mechanism translates into similar regulation of mesangial cell proliferation. Thus, our results demonstrate the importance of crosstalk between TGF-beta and type I IFNs in determining the specificity of TLR-mediated gene induction.

The regulation of the expression of inducible nitric oxide synthase by Src-family tyrosine kinases mediated through MyD88-independent signaling pathways of Toll-like receptor 4

Bacterial lipopolysaccharide (LPS) activates Toll-like receptor 4 (TLR4) leading to the expression of inflammatory gene products. Src-family tyrosine kinases (STKs) are known to be activated by LPS in monocytes/macrophages. Therefore, we determined the role of STKs in TLR4 signaling pathways and target gene expression in macrophages. The activation of NFkappaB, and p38 MAPK, and the expression of inducible nitric oxide synthase (iNOS) induced by LPS were not affected in macrophages deficient in three STKs (Lyn, Hck, and Fgr). These results suggest that the deletion of the three STKs among possibly nine STKs is not sufficient to abolish total activity of STKs possibly due to the functional redundancy of other STKs present in macrophages. However, two structurally unrelated pan-inhibitors of STKs, PP1 and SU6656, suppressed LPS-induced iNOS expression in MyD88-knockout as well as wild-type macrophages. The suppression of iNOS expression by the inhibitors was correlated with the downregulation of IFNbeta (a MyD88-independent gene) expression and subsequent decrease in STAT1 phosphorylation. Moreover, PP1 suppressed the expression of IFNbeta and iNOS induced by TRIF, a MyD88-independent adaptor of TLR4. PP1 suppressed STAT1 phosphorylation induced by LPS, but not by IFNbeta suggesting that STKs are involved in the primary downstream signaling pathways of TLR4, but not the secondary signaling pathways downstream of IFNbeta receptor. Together, these results demonstrate that STKs play a positive regulatory role in TLR4-mediated iNOS expression in a MyD88-independent (TRIF-dependent) manner. These results provide new insight in understanding the role of STKs in TLR4 signaling pathways and inflammatory target gene expression.

HypervirulentM. tuberculosisW/Beijing Strains Upregulate Type I IFNs and Increase Expression of Negative Regulators of the Jak-Stat Pathway

The role of type I interferons (IFNs) in the host response to bacterial infections is controversial. Here, we examined the role of IFN-alpha/beta in the murine response to infection with Mycobacterium tuberculosis, using wildtype mice, mice with impaired signaling through the type I IFN receptor (IFNAR), and mice treated to reduce levels of type I IFNs. In this study, we used virulent clinical isolates of M. tuberculosis, including HN878, W4, and CDC1551. Our results indicate that higher levels of type I IFNs are induced by the HN878 and W4 strains. Induction of type I IFNs was associated with lower levels of tumor necrosis factor-alpha (TNF-alpha) and interleukin- 12 (IL-12) and reduced T cell activation, and associated with decreased survival of the mice infected with HN878 or W4 relative to infection with CDC1551. Infection of mice with HN878 and W4 was also associated with relatively higher levels of mRNA for a number of negative regulators of the Jak-Stat signaling pathway, such as suppressors of cytokine signaling (SOCS) 1, 4, and 5, CD45, protein inhibitor of activated Stat1 (PIAS1), protein tyrosine phosphatase nonreceptor type 1 (Ptpn1), and protein tyrosine phosphatase nonreceptor type substrate 1 (Ptpns1). Taken together, these results suggest that increased type I IFNs may be deleterious for survival of M. tuberculosis-infected mice in association with reduced Th1 immunity.

The adjuvant activity of CpG DNA requires T-bet expression in dendritic cells

Treatment with synthetic oligodeoxynucleotides containing CpG motifs (CpG ODNs) is remarkably protective against otherwise lethal infection. Here, we describe an essential role for the transcription factor T-bet in mediating the protective function of CpG ODNs. Loss of T-bet in conventional CD11c(hi) dendritic cells (DCs) and in plasmacytoid DCs impaired production of IFNs. Strikingly, in contrast to Rag2-/- mice, Rag2-/- mice that also lacked T-bet (DKO) could not be rescued from lethal Listeria monocytogenes infection by prior treatment with CpG ODN. Rescue was achieved by adoptive transfer of CD11c(hi) DCs from WT, but not T-bet-/-, CpG ODN-treated donor mice. We conclude that T-bet in DCs is required for the adjuvant activity of CpG ODN in infection, revealing its vital role in innate immunity.

Listeria-infected myeloid dendritic cells produce IFN-beta, priming T cell activation

The intracellular bacterium Listeria monocytogenes infects dendritic cells (DC) and other APCs and induces potent cell-mediated protective immunity. However, heat-killed bacteria fail to do so. This study explored whether DC differentially respond to live and killed Listeria and how this affects T cell activation. To control for bacterial number, a replication-deficient strain, Lmdd, defective in D-alanine biosynthesis, was used. We found that DC internalize both live and heat-killed Lmdd and similarly up-regulate the expression of costimulatory molecules, a necessary step for T cell activation. However, only live Lmdd-infected DC stimulate T cells to express the early activation marker CD69 and enhance T cell activation upon TCR engagement. Infection with live, but not heat-killed, Lmdd induces myeloid DC to secrete copious amounts of IFN-beta, which requires bacterial cytosolic invasion. Exposure to high concentrations of IFN-beta sensitizes naive T cells for Ag-dependent activation.

Expression of Many Immunologically Important Genes inMycobacterium tuberculosis-Infected Macrophages Is Independent of Both TLR2 and TLR4 but Dependent on IFN-αβ Receptor and STAT1

Macrophages respond to several subcellular products of Mycobacterium tuberculosis (Mtb) through TLR2 or TLR4. However, primary mouse macrophages respond to viable, virulent Mtb by pathways largely independent of MyD88, the common adaptor molecule for TLRs. Using microarrays, quantitative PCR, and ELISA with gene-disrupted macrophages and mice, we now show that viable Mtb elicits the expression of inducible NO synthase, RANTES, IFN-inducible protein 10, immune-responsive gene 1, and many other key genes in macrophages substantially independently of TLR2, TLR4, their combination, or the TLR adaptors Toll-IL-1R domain-containing adapter protein and Toll-IL-1R domain-containing adapter inducing IFN-beta. Mice deficient in both TLR2 and TLR4 handle aerosol infection with viable Mtb as well as congenic controls. Viable Mtb also up-regulates inducible NO synthase, RANTES, IFN-inducible protein 10, and IRG1 in macrophages that lack mannose receptor, complement receptors 3 and 4, type A scavenger receptor, or CD40. These MyD88, TLR2/4-independent transcriptional responses require IFN-alphabetaR and STAT1, but not IFN-gamma. Conversely, those genes whose expression is MyD88 dependent do not depend on IFN-alphabetaR or STAT1. Transcriptional induction of TNF is TLR2/4, MyD88, STAT1, and IFN-alphabetaR independent, but TNF protein release requires the TLR2/4-MyD88 pathway. Thus, macrophages respond transcriptionally to viable Mtb through at least three pathways. TLR2 mediates the responses of a numerically minor set of genes that collectively do not appear to affect the course of infection in mice; regulation of TNF requires TLR2/4 for post-transcriptional control, but not for transcriptional induction; and many responding genes are regulated through an unknown, TLR2/4-independent pathway that may involve IFN-alphabetaR and STAT1.

The Yin and Yang of type I interferon activity in bacterial infection

Interferons (IFNs) are cytokines that are important for immune responses, particularly to intracellular pathogens. They are divided into two structurally and functionally distinct types that interact with different cell-surface receptors. Classically, type I IFNs are potent antiviral immunoregulators, whereas the type II IFN enhances antibacterial immunity. However, as outlined here, type I IFNs are also produced in response to infection with other pathogens, and an increasing body of work shows that type I IFNs have an important role in the host response to bacterial infection. Strikingly, their activity can be either favourable or detrimental, and can influence various immune effector mechanisms.

Ability of Orally Administered IFN-α-Containing Transgenic Potato Extracts to InhibitListeria monocytogenesInfection

Type I interferons (IFN-alpha/beta) were originally thought to be antiviral cytokines, but it has recently been reported that they also play an important role in potentiating innate and adaptive immune responses. Moreover, several studies have shown that the oral administration of type I IFN ameliorates various biologic activities. Here, we studied the ability of orally administered IFN-alpha to protect mice from systemic Listeria monocytogenes infection. Daily oral administration of purified natural IFN-alpha at a concentration of 1000 international units (IU)/20 microl reduced the bacterial burden in infected organs. We also examined the protective effect of IFN-alpha expressed in transgenic potato plants. A much lower concentration of IFN-alpha (20 IU/ 20 microl) in the plant extracts was almost as protective as much higher concentrations of purified natural IFN-alpha. Our observations indicate that transgenic cytokine-expressing plants can be used prophylactically as edible pharmaceuticals to enhance systemic defense responses in humans and animals.

Proteomic Characterization of the Angiogenesis Inhibitor SU6668 Reveals Multiple Impacts on Cellular Kinase Signaling

Knowledge about molecular drug action is critical for the development of protein kinase inhibitors for cancer therapy. Here, we establish a chemical proteomic approach to profile the anticancer drug SU6668, which was originally designed as a selective inhibitor of receptor tyrosine kinases involved in tumor vascularization. By employing immobilized SU6668 for the affinity capture of cellular drug targets in combination with mass spectrometry, we identified previously unknown targets of SU6668 including Aurora kinases and TANK-binding kinase 1. Importantly, a cell cycle block induced by SU6668 could be attributed to inhibition of Aurora kinase activity. Moreover, SU6668 potently suppressed antiviral and inflammatory responses by interfering with TANK-binding kinase 1-mediated signal transmission. These results show the potential of chemical proteomics to provide rationales for the development of potent kinase inhibitors, which combine rather unexpected biological modes of action by simultaneously targeting defined sets of both serine/threonine and tyrosine kinases involved in cancer progression.

Killed but metabolically active microbes: a new vaccine paradigm for eliciting effector T-cell responses and protective immunity

We developed a new class of vaccines, based on killed but metabolically active (KBMA) bacteria, that simultaneously takes advantage of the potency of live vaccines and the safety of killed vaccines. We removed genes required for nucleotide excision repair (uvrAB), rendering microbial-based vaccines exquisitely sensitive to photochemical inactivation with psoralen and long-wavelength ultraviolet light. Colony formation of the nucleotide excision repair mutants was blocked by infrequent, randomly distributed psoralen crosslinks, but the bacterial population was able to express its genes, synthesize and secrete proteins. Using the intracellular pathogen Listeria monocytogenes as a model platform, recombinant psoralen-inactivated Lm DeltauvrAB vaccines induced potent CD4(+) and CD8(+) T-cell responses and protected mice against virus challenge in an infectious disease model and provided therapeutic benefit in a mouse cancer model. Microbial KBMA vaccines used either as a recombinant vaccine platform or as a modified form of the pathogen itself may have broad use for the treatment of infectious disease and cancer.

The TLR7 Agonist Imiquimod Enhances the Anti-Melanoma Effects of a RecombinantListeria monocytogenesVaccine

Activation of innate immune cells through TLR triggers immunomodulating events that enhance cell-mediated immunity, raising the possibility that ligands to these receptors might act as adjuvants in conjunction with T cell activating vaccines. In this report, topical imiquimod, a synthetic TLR7 agonist, significantly enhanced the protective antitumor effects of a live, recombinant listeria vaccine against murine melanoma. This tumor protective effect was not dependent on direct application to the tumor and was associated with an increase in tumor-associated and splenic dendritic cells. Additionally, the combination of imiquimod treatment with prior vaccination led to development of localized vitiligo. These findings indicate that activation of the innate immune system with TLR ligands stimulates dendritic cell activity resulting in a bypass of peripheral tolerance and enhanced antitumor activity. The results of these studies have broad implications for future designs of immunotherapeutic vaccines against tumors and the treatment of metastatic melanoma.

Triggering of TLR3 by polyI:C in human corneal epithelial cells to induce inflammatory cytokines

Epithelial cells of the ocular surface are key in the first-line defense as a part of the mucosal immune system against pathogens. We investigated whether polyI:C induces the production by human corneal epithelial cells (HCEC) of pro-inflammatory cytokines and IFN-beta, and whether Toll-like receptor (TLR)-3 expression is amplified by polyI:C. TLR3 was expressed on the surface of HCEC. Stimulation with polyI:C elicited the elevated production and mRNA expression of IL-6 and IL-8 in HCEC. While polyI:C induced IFN-beta, far stronger than human fibroblasts, and TLR3 gene expression in HCEC, LPS stimulation did not. Similarly, polyI:C, but not LPS, induced the gene expression of IkappaBalpha and MAIL, members of the IkappaB family, in HCEC. The innate immune response of HCEC is distinct from that of immune-competent cells, and we suggest that this is indicative of the symbiotic relationship between corneal epithelium and microbes inhabiting the ocular surface.

Manipulation of innate immunity by bacterial pathogens

The past decade has witnessed tremendous growth in two related fields: innate immunity and microbial pathogenesis. Many pathogens have evolved mechanisms to infect their hosts in the face of a fully functional innate immune system, and there are numerous examples by which pathogens avoid recognition and/or suppress inflammation. In this review, I suggest that pathogens not only survive the innate immune response, but use it to promote their pathogenesis.

The host type I interferon response to viral and bacterial infections

Type I interferons (IFN) are well studied cytokines with anti-viral and immune-modulating functions. Type I IFNs are produced following viral infections, but until recently, the mechanisms of viral recognition leading to IFN production were largely unknown. Toll like receptors (TLRs) have emerged as key transducers of type I IFN during viral infections by recognizing various viral components. Furthermore, much progress has been made in defining the signaling pathways downstream of TLRs for type I IFN production. TLR7 and TLR9 have become apparent as universally important in inducing type I IFN during infection with most viruses, particularly by plasmacytoid dendritic cells. New intracellular viral pattern recognition receptors leading to type I IFN production have been identified. Many bacteria can also induce the up-regulation of these cytokines. Interestingly, recent studies have found a detrimental effect on host cells if type I IFN is produced during infection with the intracellular gram-positive bacterial pathogen, Listeria monocytogenes. This review will discuss the recent advances made in defining the signaling pathways leading to type I IFN production.

The role of type I interferons in non-viral infections

For a long time, the family of type I interferons (IFN-alpha/beta) has received little attention outside the fields of virology and tumor immunology. In recent years, IFN-alpha/beta regained the interest of immunologists, due to the phenotypic and functional characterization of IFN-alpha/beta-producing cells, the definition of novel immunomodulatory functions and signaling pathways of IFN-alpha/beta, and the observation that IFN-alpha/beta not only exerts antiviral effects but is also relevant for the pathogenesis or control of certain bacterial and protozoan infections. This review summarizes the current knowledge on the production and function of IFN-alpha/beta during non-viral infections in vitro and in vivo.

Type I interferons (alpha/beta) in immunity and autoimmunity

The significance of type I interferons (IFN-alpha/beta) in biology and medicine renders research on their activities continuously relevant to our understanding of normal and abnormal (auto) immune responses. This relevance is bolstered by discoveries that unambiguously establish IFN-alpha/beta, among the multitude of cytokines, as dominant in defining qualitative and quantitative characteristics of innate and adaptive immune processes. Recent advances elucidating the biology of these key cytokines include better definition of their complex signaling pathways, determination of their importance in modifying the effects of other cytokines, the role of Toll-like receptors in their induction, their major cellular producers, and their broad and diverse impact on both cellular and humoral immune responses. Consequently, the role of IFN-alpha/beta in the pathogenesis of autoimmunity remains at the forefront of scientific inquiry and has begun to illuminate the mechanisms by which these molecules promote or inhibit systemic and organ-specific autoimmune diseases.

IFN regulatory factor 3-dependent induction of type I IFNs by intracellular bacteria is mediated by a TLR- and Nod2-independent mechanism

Like viruses, intracellular bacteria stimulate their host cells to produce type I IFNs (IFN-alpha and IFN-beta). In our study, we investigated the signals and molecules relevant for the synthesis of and response to IFN by mouse macrophages infected with Listeria monocytogenes. We report that IFN-beta is the critical immediate-early IFN made during infection, because the synthesis of all other type I IFN, expression of a subset of infection-induced genes, and the biological response to type I IFN was lost upon IFN-beta deficiency. The induction of IFN-beta mRNA and the IFN-beta-dependent sensitization of macrophages to bacteria-induced death, in turn, was absolutely dependent upon the presence of the transcription factor IFN regulatory factor 3 (IRF3). IFN-beta synthesis and signal transduction occurred in macrophages deficient for TLR or their adaptors MyD88, TRIF, or TRAM. Expression of Nod2, a candidate receptor for intracellular bacteria, increased during infection, but the protein was not required for Listeria-induced signal transduction to the Ifn-beta gene. Based on our data, we propose that IRF3 is a convergence point for signals derived from structurally unrelated intracellular pathogens, and that L. monocytogenes stimulates a novel TLR- and Nod2-independent pathway to target IRF3 and the type I IFN genes.

Expression of interferon-inducible RNA adenosine deaminase ADAR1 during pathogen infection and mouse embryo development involves tissue-selective promoter utilization and alternative splicing

ADAR1 (adenosine deaminase acting on RNA) is widely expressed in adult mammals and has a critical role during embryogenesis. Two size forms of ADAR1 are known that possess adenosine-to-inosine editing activity: an interferon (IFN)-inducible approximately 150-kDa protein and a constitutively expressed N-terminally truncated approximately 110-kDa protein. We defined the structure of the 5'-flanking region of the mouse Adar1 gene, and we show here that mouse Adar1 transcripts possess alternative exon 1 structures (1A, 1B, and 1C) that initiate from unique promoters and are spliced to a common exon 2 junction. Exon 1A-containing transcripts encoding p150 were expressed in all tissues examined from adult mice (brain, cecum, heart, kidney, liver, lung, spleen, and Peyer's patches) and were elevated most significantly in liver but remained lowest in brain following oral infection with Salmonella. Exon 1B-containing RNA was most abundant in brain and was not increased in any tissue examined following infection. Exon 1C-containing RNA was very scarce. Exon 1A, but not exon 1B or 1C, expression was increased in fibroblast L cells treated with IFN, and a consensus ISRE element was present in the promoter driving exon 1A expression. Exon 1B, but not 1A, was detectable in embryonic day 10.5 embryos and was abundantly expressed in embryonic day 15 embryos. Furthermore, the ADAR1 p110 protein isoform was detected in embryonic tissue, whereas both p110 and the inducible p150 proteins were found in IFN-treated L cells. Finally, the presence of alternative exon 7a correlated with exon 1B-containing RNA, and alternative exon 7b correlated with exon 1A-containing RNA. These results establish that multiple promoters drive the expression of the Adar1 gene in adult mice, that the IFN inducible promoter and exon 1A-containing RNA are primarily responsible for the increased ADAR1 observed in Salmonella-infected mice, and that the constitutive exon 1B-containing transcript and encoded p110 protein product are abundantly expressed both in adult brain and during embryogenesis.

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

Type I IFNs are well established antiviral cytokines that have also been shown to be induced by bacteria. However, the signaling mechanisms regulating the activation of these cytokines during bacterial infections remain poorly defined. We show that although Gram-negative bacteria can activate the type I IFN pathway through TLR4, the intracellular Gram-positive bacterium Listeria monocytogenes (LM) can do so independently of TLR4 and TLR2. Furthermore, experiments using genetic mutants and chemical inhibitors suggest that LM-induced type I IFN activation occurs by an intracellular pathway involving the serine-threonine kinase TNFR-associated NF-kappaB kinase (TANK)-binding kinase 1 (TBK1). Interestingly, receptor-interacting protein 2, a component of the recently discovered nucleotide-binding oligomerization domain-dependent intracellular detection pathway, was not involved. Taken together, our data describe a novel signal transduction pathway involving TBK1 that is used by LM to activate type I IFNs. Additionally, we provide evidence that both the LM- and TLR-dependent pathways converge at TBK1 to activate type I IFNs, highlighting the central role of this molecule in modulating type I IFNs in host defense and disease.

Immune responses to Listeria monocytogenes

Listeria monocytogenes is a Gram-positive bacterium that is often used to study the mammalian immune response to infection because it is easy to culture, is relatively safe to work with and causes a highly predictable infection in laboratory mice. The broad application of this mouse model has resulted in a torrent of studies characterizing the contributions of different cytokines, receptors, adaptors and effector molecules to resistance against infection with Listeria monocytogenes. These studies, which are yielding one of the most comprehensive pictures of the 'battle' between host and microorganism, are reviewed here.

Mice lacking the type I interferon receptor are resistant to Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular pathogen that induces a cytosolic signaling cascade resulting in expression of interferon (IFN)-β. Although type I IFNs are critical in viral defense, their role in immunity to bacterial pathogens is much less clear. In this study, we addressed the role of type I IFNs by examining the infection of L. monocytogenes in BALB/c mice lacking the type I IFN receptor (IFN-α/βR^−/−). During the first 24 h of infection in vivo, IFN-α/βR^−/− and wild-type mice were similar in terms of L. monocytogenes survival. In addition, the intracellular fate of L. monocytogenes in macrophages cultured from IFN-α/βR^−/− and wild-type mice was indistinguishable. However, by 72 h after inoculation in vivo, IFN-α/βR^−/− mice were ∼1,000-fold more resistant to a high dose L. monocytogenes infection. Resistance was correlated with elevated levels of interleukin 12p70 in the blood and increased numbers of CD11b⁺ macrophages producing tumor necrosis factor α in the spleen of IFN-α/βR^−/− mice. The results of this study suggest that L. monocytogenes might be exploiting an innate antiviral response to promote its pathogenesis.