Synaptopodin is necessary for Shigella flexneri intercellular spread

For many intracellular pathogens, their virulence depends on an ability to spread between cells of an epithelial layer. For intercellular spread to occur, these pathogens deform the plasma membrane into a protrusion structure that is engulfed by the neighboring cell. Although the polymerization of actin is essential for spread, how these pathogens manipulate the actin cytoskeleton in a manner that enables protrusion formation is still incompletely understood. Here, we identify the mammalian actin binding protein synaptopodin as required for efficient intercellular spread. Using a model cytosolic pathogen, Shigella flexneri , we show that synaptopodin contributes to organization of actin around bacteria and increases the length of the actin tail at the posterior pole of the bacteria. We show that synaptopodin presence enables protrusions to form and to resolve at a greater rate, indicating that greater stability of the actin tail enables the bacteria to push against the membrane with greater force. We demonstrate that synaptopodin recruitment around bacteria requires the bacterial protein IcsA, and we show that this recruitment is further enhanced in a type 3 secretion system dependent manner. These data establish synaptopodin as required for intracellular bacteria to reprogram the actin cytoskeleton in a manner that enables efficient protrusion formation and enhance our understanding of the cellular function of synaptopodin.

Authors Summary

Intercellular spread is essential for many cytosolic dwelling pathogens during their infectious life cycle. Despite knowing the steps required for intercellular spread, relatively little is known about the host-pathogen interactions that enable these steps to occur. Here, we identify a requirement for the actin binding protein synaptopodin during intercellular spread by cytosolic bacteria. We show synaptopodin is necessary for the stability and recruitment of polymerized actin around bacteria. We also demonstrate synaptopodin is necessary to form plasma membrane structures known as protrusions that are necessary for the movement of these bacteria between cells. Thus, these findings implicate synaptopodin as an important actin-binding protein for the virulence of intracellular pathogens that require the actin cytoskeleton for their spread between cells.

Myeloid Cell CK2 Regulates Inflammation and Resistance to Bacterial Infection

Kinase activity plays an essential role in the regulation of immune cell defenses against pathogens. The protein kinase CK2 (formerly casein kinase II) is an evolutionarily conserved kinase with hundreds of identified substrates. CK2 is ubiquitously expressed in somatic and immune cells, but the roles of CK2 in regulation of immune cell function remain largely elusive. This reflects the essential role of CK2 in organismal development and limited prior work with conditional CK2 mutant murine models. Here, we generated mice with a conditional (floxed) allele of Csnk2a, which encodes the catalytic CK2α subunit of CK2. When crossed to Lyz2-cre mice, excision of Csnk2a sequence impaired CK2α expression in myeloid cells but failed to detectably alter myeloid cell development. By contrast, deficiency for CK2α increased inflammatory myeloid cell recruitment, activation, and resistance following systemic Listeria monocytogenes (Lm) infection. Results from mixed chimera experiments indicated that CK2α deficiency in only a subset of myeloid cells was not sufficient to reduce bacterial burdens. Nor did cell-intrinsic deficiency for CK2α suffice to alter accumulation or activation of monocytes and neutrophils in infected tissues. These data suggest that CK2α expression by Lyz2-expressing cells promotes inflammatory and anti-bacterial responses through effects in trans. Our results highlight previously undescribed suppressive effects of CK2 activity on inflammatory myeloid cell responses and illustrate that cell-extrinsic effects of CK2 can shape inflammatory and protective innate immune responses.

IL-10-producing NK cells exacerbate sublethal Streptococcus pneumoniae infection in the lung

Lung inflammation is tightly controlled to balance microbial clearance with the tissue damage that accompanies this response. Bacterial pathogens including Streptococcus pneumoniae (S. pneumoniae) modulate immune regulation by promoting secretion of the anti-inflammatory cytokine IL-10. The important cellular sources of IL-10 that impact protection against different bacterial infections are not well characterized. We find that S. pneumoniaeactivates IL-10 secretion from natural killer (NK) cells in the lung, which restrict host protection in a mouse model of sublethal infection. Direct transfer of wild-type NK cells into the lungs of IL-10-deficient mice drives bacterial expansion, identifying NK cells as a critical source of IL-10 promoting S. pneumoniae infection. The S. pneumoniae virulence protein Spr1875 was found to elicit NK cell IL-10 production in purified cells and in the lungs of live animals. These findings reveal therapeutic targets to combat bacterial-driven immune regulation in the lung.

Myeloid cells expressing CK2α modulates resistance to systemic Listeria monocytogenes infection

Reversable phosphorylation is a key regulatory mechanism of nearly all cellular processes. Kinase activity driving phosphorylation has implications in cell growth, survival, development, differentiation, and importantly, immune responses to pathogens. The protein kinase CK2 (casein kinase II) is a ubiquitously expressed and highly conserved haloenzyme with over 500 direct target substrates identified. Because of this promiscuous activity, CK2 is extensively involved in immune cell function, described in both innate and adaptive immune cells. However, functionality of CK2 is not clearly delineated, largely due to the lack of murine models available for study. Engineered mice deficient in either the catalytic α or α′, or the regulatory β subunits are either non-viable or non-fertile. Therefore, we have generated a conditional murine knockout model of CK2α by cre-mediated excision in lysozyme M (LysM) expressing cells. Progeny are viable and fertile, with no deficiencies of immune cell populations. Using this model system, we have identified a role of CK2α in negatively regulating monocyte and neutrophil infiltration and endosome acidification during the bacterial Listeria monocytogenes (Lm) infection, correlating with uncontrolled bacterial replication. Further, decreased inflammatory myeloid cells in infected tissues is a cell-intrinsic effect of CK2α. Future work is aimed to understand the mechanisms by which CK2α negatively regulates the accumulation of immune cells in infected tissues. Additionally, we now have a tool for investigating in vivo effects of CK2α that we can use to extend beyond these initial studies to evaluate its role in other bacterial/viral infections, autoimmunity and even cancer.

The role of NK cells during bacterial and viral/bacterial co-infection

Influenza virus is a large burden to the human population and a leading cause of death worldwide. Much of this high morbidity and mortality is due to co-infections with pulmonary bacterial pathogens, such as Streptococcus pneumoniae. Prior studies indicate that a primary influenza infection exacerbates secondary infection of S. pneumoniae in murine models. In part, this is due to increased bacterial burdens in the lung driven by pro- or anti-inflammatory cytokines such as IFNγ and IL-10. However, the cellular source(s) of these cytokines and their roles during infection is unclear. Previous work in our lab has shown that during systemic Listeria monocytogenes infection, NK cells are a critical source of early IFNγ with delayed IL-10 production and that IL-10 specifically facilitates pathogenesis. Here, we report that NK cells also increase susceptibility to pulmonary S. pneumoniae infection, as demonstrated by reduced bacterial burdens in organs following NK cell depletion. Using an in vitro BMDC/NK cell co-culture system, we further show that S. pneumoniae infection can directly elicit NK cell IL-10 production. Therefore, we hypothesize that NK cells play an important role during S. pneumoniae infection. To test whether NK cells also influence influenza/S. pneumoniae co-infection, we established a mouse model where S. pneumoniae bacterial burdens are increased in organs of co-infected mice. We are currently investigating how NK cells affect susceptibility to secondary S. pneumoniae infection and specifically the role of NK cell IL-10. Ultimately, this work has the potential to further define the mechanism(s) by which NK cells contribute to the control or pathogenesis of S. pneumoniae and influenza virus/S. pneumoniae co-infection.

Streptococcus pneumoniae coinfection dictates susceptibility to oral Listeria monocytogenes through suppressive Natural Killer (NK) cell activity

Coinfection with viral and bacterial pathogens is a major health concern and results in significant morbidity and mortality. Recent research has aimed to understand the complex interplay between multiple pathogens and the immune response during viral:bacterial coinfections. However, studies using bacterial-bacterial coinfections are lacking. Streptococcus pneumoniae (Spn) and Listeria monocytogenes (Lm) are both bacterial pathogens that are capable of causing severe infections in young, elderly, and immunocompromised populations. Here we show that coinfection with intratracheal (IT) Spn increases susceptibility of typically resistant mice to oral Lm infection. On day 3 post infection (p.i.), the presence of Spn increased oral Lm burdens in the spleen, liver, and lungs of coinfected C57/BL6 mice. In addition, while the anti-inflammatory cytokine Interleukin-10 (IL-10) is limited during oral Lm infection, Spn infection induced the production of IL-10 from Natural Killer (NK) cells, and that this correlated with an increase in Lm burdens during coinfection. To assess the importance of NK cell IL-10 in the absence of an ongoing infection, mice were inoculated with a recombinant Lm polypeptide, designated L1S, which stimulates NK cell IL-10 production. In the presence of L1S, the induction of NK cell IL-10 similarly increased systemic burdens of oral Lm. Lastly, BMMs stimulated with rIL-10 and infected with Lm have increased bacterial burdens when compared to untreated but infected controls. Overall, our data show that coinfection impairs host protection through the induction of NK cell production of IL-10 and dictates susceptibility to oral Lm infection.

Myeloid cell responsiveness to interferon-gamma is sufficient for initial resistance to Listeria monocytogenes

The type II interferon (IFNγ) promotes resistance to intracellular pathogens. Most immune and somatic cells also express the IFNγ receptor (IFNGR) and respond to IFNγ. While myeloid cell have been implicated as important targets of IFNγ, it remains unknown if IFNγ signaling to myeloid cell types suffices for resistance to infection. Here, we addressed this question by generating mice in which IFNGR1 is selectively expressed by myeloid cells. These “MSGR1” (myeloid selective IFNGR1) mice express an epitope-tagged Ifngr1 transgene (fGR1) from the myeloid-specific c-fms promoter in a background lacking endogenous Ifngr1. IFNGR staining was selectively observed on myeloid cells in the MSGR1 mice and correlated with responsiveness of these cells to IFNγ. During systemic infection by the bacterium Listeria monocytogenes, activation marker staining was comparable on monocytes from MSGR1 and control B6 mice. Bacterial burdens and survival were also equivalent in MSGR1 and wildtype B6 animals at a timepoint when B6.Ifngr1^−/− mice began to succumb. These data confirm that activation of inflammatory monocytes and neutrophils is a key mechanism by which IFNγ promotes innate anti-bacterial immunity and suggest that IFNγ targeting of myeloid cells is largely sufficient to mediate protection against systemic L. monocytogenes.

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Expression of IFNGR1 is restricted to monocytes and neutrophils in “MSGR1” (myeloid selective IFNGR1) mice.

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Myeloid cells from MSGR1 mice are responsive to IFNγ and show elevated activation compared to cells from B6.Ifngr1^−/− mice.

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MSGR1 myeloid cells respond to Listeria monocytogenes infection and promote early resistance.

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IFNγ stimulation of myeloid cells can thus protect against infection independent of effects on other hematopoietic and non-hematopoietic cell populations.

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Particularly in female mice, IFNγ stimulation of non-myeloid cells may also contribute to improved survival.

Selective Loss of Responsiveness to Exogenous but Not Endogenous Cyclic-Dinucleotides in Mice Expressing STING-R231H

Stimulator of interferon genes (STING) plays a central role in innate immune responses to viral and intracellular bacterial infections, and cellular damage. STING is a cytosolic sensor of cyclic dinucleotides (CDNs) including those produced by pathogenic bacteria and those arising endogenously as products of the DNA sensor cGAS (e.g., 2′3′ cGAMP). The two most common alternative allelic variants of STING in humans are STING-R71H-G230A-R293Q (STING-HAQ) and STING-R232H that are found in 20.4% and 13.7–17.6% of the population, respectively. To determine the biologic consequences of these genotypic variations, we generated knock-in mice containing the murine equivalents of each variant and studied their responsiveness to CDNs. Homozygous STING-HAQ (R71H-I229A-R292Q) and STING-R231H mice were found to be unresponsive to all exogenous CDNs tested (ci-di-GMP, ci-di-AMP, 3′3′ cGAMP and Rp,Rp-CDA). Responses of homozygous STING-HAQ mice to endogenous 2′3′ cGAMP was also greatly impaired. However, homozygous STING-R231H mice are fully responsive to 2′3′ cGAMP. Analysis of heterozygous mice revealed reduced responsiveness to exogenous and endogenous CDNs in mice carrying a single copy of STING-HAQ, while STING-R231H heterozygous mice exhibit reduced responsiveness to exogenous but not endogenous CDNs. These findings confirm and extend previous reports by demonstrating differing impact of allelic variation of STING on the ability to sense and respond to exogenous vs. endogenous CDNs. Finally, the STING-R231H variant mouse represents a useful tool with which to examine the relative contributions of STING sensing of exogenous and endogenous CDNs in the context of bacterial infections and CDN-based cancer immunotherapeutics.

Inflammation as a Modulator of Host Susceptibility to Pulmonary Influenza, Pneumococcal, and Co-Infections

Bacterial and viral pathogens are predominant causes of pulmonary infections and complications. Morbidity and mortality from these infections is increased in populations that include the elderly, infants, and individuals with genetic disorders such as Down syndrome. Immune senescence, concurrent infections, and other immune alterations occur in these susceptible populations, but the underlying mechanisms that dictate increased susceptibility to lung infections are not fully defined. Here, we review unique features of the lung as a mucosal epithelial tissue and aspects of inflammatory and immune responses in model pulmonary infections and co-infections by influenza virus and Streptococcus pneumoniae. In these models, lung inflammatory responses are a double-edged sword: recruitment of immune effectors is essential to eliminate bacteria and virus-infected cells, but inflammatory cytokines drive changes in the lung conducive to increased pathogen replication. Excessive accumulation of inflammatory cells also hinders lung function, possibly causing death of the host. Some animal studies have found that targeting host modulators of lung inflammatory responses has therapeutic or prophylactic effects in these infection and co-infection models. However, conflicting results from other studies suggest microbiota, sequence of colonization, or other unappreciated aspects of lung biology also play important roles in the outcome of infections. Regardless, a predisposition to excessive or aberrant inflammatory responses occurs in susceptible human populations. Hence, in appropriate contexts, modulation of inflammatory responses may prove effective for reducing the frequency or severity of pulmonary infections. However, there remain limitations in our understanding of how this might best be achieved—particularly in diverse human populations.

IFNγ receptor down-regulation facilitates Legionella survival in alveolar macrophages

Legionella pneumophila is an opportunistic human pathogen and causative agent of the acute pneumonia known as Legionnaire's disease. Upon inhalation, the bacteria replicate in alveolar macrophages (AM), within an intracellular vacuole termed the Legionella-containing vacuole. We recently found that, in vivo, IFNγ was required for optimal clearance of intracellular L. pneumophila by monocyte-derived cells (MC), but the cytokine did not appear to influence clearance by AM. Here, we report that during L. pneumophila lung infection, expression of the IFNγ receptor subunit 1 (IFNGR1) is down-regulated in AM and neutrophils, but not MC, offering a possible explanation for why AM are unable to effectively restrict L. pneumophila replication in vivo. To test this, we used mice that constitutively express IFNGR1 in AM and found that prevention of IFNGR1 down-regulation enhanced the ability of AM to restrict L. pneumophila intracellular replication. IFNGR1 down-regulation was independent of the type IV Dot/Icm secretion system of L. pneumophila indicating that bacterial effector proteins were not involved. In contrast to previous work, we found that signaling via type I IFN receptors was not required for IFNGR1 down-regulation in macrophages but rather that MyD88- or Trif- mediated NF-κB activation was required. This work has uncovered an alternative signaling pathway responsible for IFNGR1 down-regulation in macrophages during bacterial infection.

A Batf3/Nlrp3/IL-18 Axis Promotes Natural Killer Cell IL-10 Production during Listeria monocytogenes Infection

The bacterial pathogen Listeria monocytogenes (Lm) capitalizes on natural killer (NK) cell production of regulatory interleukin (IL)-10 to establish severe systemic infections. Here, we identify regulators of this IL-10 secretion. We show that IL-18 signals to NK cells license their ability to produce IL-10. IL-18 acts independent of IL-12 and STAT4, which co-stimulate IFNγ secretion. Dendritic cell (DC) expression of Nlrp3 is required for IL-18 release in response to the Lm p60 virulence protein. Therefore, mice lacking Nlrp3, Il18, or Il18R fail to accumulate serum IL-10 and are highly resistant to systemic Lm infection. We further show that cells expressing or dependent on Batf3 are required for IL-18-inducing IL-10 production observed in infected mice. These findings explain how Il18 and Batf3 promote susceptibility to bacterial infection and demonstrate the ability of Lm to exploit NLRP3 for the promotion of regulatory NK cell activity.

Ligand-induced IFNGR1 down-regulation calibrates myeloid cell IFNγ responsiveness

The type II IFN (IFNγ) enhances antimicrobial activity yet also drives expression of genes that amplify inflammatory responses. Hence, excessive IFNγ stimulation can be pathogenic. Here, we describe a previously unappreciated mechanism whereby IFNγ itself dampens myeloid cell activation. Staining of monocytes from Listeria monocytogenes-infected mice provided evidence of type I IFN-independent reductions in IFNGR1. IFNγ was subsequently found to reduce surface IFNGR1 on cultured murine myeloid cells and human CD14+ peripheral blood mononuclear cells. IFNγ-driven reductions in IFNGR1 were not explained by ligand-induced receptor internalization. Rather, IFNγ reduced macrophage Ifngr1 transcription by altering chromatin structure at putative Ifngr1 enhancer sites. This is a distinct mechanism from that used by type I IFNs. Ligand-induced reductions in IFNGR1 altered myeloid cell sensitivity to IFNγ, blunting activation of STAT1 and 3. Our data, thus, reveal a mechanism by which IFNGR1 abundance and myeloid cell sensitivity to IFNγ can be modulated in the absence of type I IFNs. Multiple mechanisms, thus, exist to calibrate macrophage IFNGR1 abundance, likely permitting the fine tuning of macrophage activation and inflammation.

NK Cell IL-10 Production Requires IL-15 and IL-10 Driven STAT3 Activation

Natural killer (NK) cells can produce IFNγ or IL-10 to regulate inflammation and immune responses but the factors driving NK cell IL-10 secretion are poorly-defined. Here, we identified NK cell-intrinsic STAT3 activation as vital for IL-10 production during both systemic Listeria monocytogenes (Lm) infection and following IL-15 cytokine/receptor complex (IL15C) treatment for experimental cerebral malaria (ECM). In both contexts, conditional Stat3 deficiency in NK cells abrogated production of IL-10. Initial NK cell STAT3 phosphorylation was driven by IL-15. During Lm infection, this required capture or presentation of IL-15 by NK cell IL-15Rα. Persistent STAT3 activation was required to drive measurable IL-10 secretion and required NK cell expression of IL-10Rα. Survival-promoting effects of IL-15C treatment in ECM were dependent on NK cell Stat3 while NK cell-intrinsic deficiency for Stat3, Il15ra, or Il10ra abrogated NK cell IL-10 production and increased resistance against Lm. NK cell Stat3 deficiency did not impact production of IFNγ, indicating the STAT3 activation initiated by IL-15 and amplified by IL-10 selectively drives the production of anti-inflammatory IL-10 by responding NK cells.

Ligand-induced IFNγ down regulation calibrates myeloid cell IFNγ responsiveness

Inflammation triggers production of type II IFN (IFNγ), which induces expression of gamma-activated genes (GAGs) that enhance antigen presentation and antimicrobial activity associated with M1-type macrophage activation. Negative regulation of these responses is important to prevent excessive macrophage activity and inflammation. We report here that stimulation with IFNγ transiently reduces myeloid cell IFNGR to raise the threshold for STAT1 activation in response to subsequent IFNγ. IFNγ production contributed to reduced monocyte surface IFNGR during the first day after systemic Listeria monocytogenes (Lm) infection. IFNγ treatment also reduced IFNGR staining on cultured bone marrow derived macrophages (BMDM) and dendritic cells (BMDCs), and human monocytes. These reductions in IFNGR were independent of type I IFNs, the primary trigger for IFNGR reductions later after Lm infection. Reductions in IFNGR correlated with reduced macrophage ifngr1 transcript abundance but this was independent of effects on the proximal Ifngr1 promoter. IFNGR down regulation following a brief exposure to IFNγ was further observed to transiently reduce myeloid cell sensitivity to subsequent IFNγ, measured as induction of phosphorylated (p)STAT1. Prolonged or elevated IFNγ could overcome the suppression of pSTAT1 activation. These data suggest transient IFNγ production reduces IFNGR to calibrate M1-type myeloid cell activation.

The gut microbiota in infants of obese mothers increases inflammation and susceptibility to NAFLD

Maternal obesity is associated with increased risk for offspring obesity and non-alcoholic fatty liver disease (NAFLD), but the causal drivers of this association are unclear. Early colonization of the infant gut by microbes plays a critical role in establishing immunity and metabolic function. Here, we compare germ-free mice colonized with stool microbes (MB) from 2-week-old infants born to obese (Inf-ObMB) or normal-weight (Inf-NWMB) mothers. Inf-ObMB-colonized mice demonstrate increased hepatic gene expression for endoplasmic reticulum stress and innate immunity together with histological signs of periportal inflammation, a histological pattern more commonly reported in pediatric cases of NAFLD. Inf-ObMB mice show increased intestinal permeability, reduced macrophage phagocytosis, and dampened cytokine production suggestive of impaired macrophage function. Furthermore, exposure to a Western-style diet in Inf-ObMB mice promotes excess weight gain and accelerates NAFLD. Overall, these results provide functional evidence supporting a causative role of maternal obesity-associated infant dysbiosis in childhood obesity and NAFLD.

Crosstalk between type I and II interferons in regulation of myeloid cell responses during bacterial infection

Type I and type II interferons (IFNα/β and IFNγ) are cytokines that play indispensable roles in directing myeloid cell activity during inflammatory and immune responses. Each IFN type binds a distinct receptor (IFNAR or IFNGR) to transduce signals that reshape gene expression and function of myeloid and other cell types. In the context of murine models and human bacterial infections, production of IFNγ generally promotes resistance while production of IFNα/β is associated with increased host susceptibility. Here, we review mechanisms of crosstalk between type I and II IFNs in myeloid cells and their impact on myeloid cell activation and anti-microbial function.

IL-18 from Batf3-dependent cells licenses natural killer cell IL-10 production during Listeria monocytogenes infection

Natural killer (NK) cells are innate lymphoid cells that regulate the immune response to infection through secretion of both pro- and anti-inflammatory cytokines. The pathogen Listeria monocytogenes (Lm) capitalizes on NK cell production of the anti-inflammatory cytokine interleukin (IL)-10 during establishment of severe infection. Lm-stimulated NK cell IL-10 limits immune cell recruitment and activation, allowing for bacterial expansion. Here, we report that IL-18 from dendritic cells (DCs) promotes this IL-10 production. IL-18 acts directly on NK cells to license IL-10 secretion that is independent of IL-12 and STAT4, which co-stimulate IFNγ secretion during Lm infection and can induce NK cell IL-10 in other contexts. DC release of IL-18 is driven by the Lm p60 virulence protein and requires DC expression of nlrp3 and batf3. Mice lacking nlrp3, il18, il18R, or batf3 fail to accumulate serum IL-10 and are highly resistant to systemic Lm infection. Our data thus show that during systemic infection Lm selectively targets Baft3-dependent cells to drive IL-18 release that licenses NK cell IL-10 production. Exploiting this previously-unappreciated pathway to promote IL-10 production enables Lm to dampen inflammatory and anti-microbial host responses.

Type I and II IFNs differentially regulate IFNGR1 to tune IFNγ responsiveness in myeloid cells

Interferons (IFNs) act on macrophages to shape immune responses against microbial infections and tumors. Type I and II IFNs signal to induce transcription of IFN-stimulated genes (ISGs). Binding of the type II IFN (IFNγ) to its receptor, IFNGR, induces ISGs that enhance macrophage inflammatory and anti-microbial activity critical for bacterial infection. Conversely, when type I IFNs bind their receptor (IFNAR), they reduce expression of myeloid cell surface IFNGR and dampen macrophage responsiveness to IFNγ. Macrophages stimulated with IFNγ also reduce cell surface IFNGR, but this was previously attributed to ligand-induced receptor internalization. Here, we confirm the ability of IFNγ stimulation to rapidly reduce cell surface IFNGR in cultured mouse and human macrophages. However, we find this ligand-induced reduction in IFNGR is associated with reduced ifngr1 transcript abundance. Unlike IFNβ, IFNγ did not impede de novo transcription from an ifngr1 promoter-driven reporter construct, suggesting IFNγ reduces ifngr1 transcript abundance via a distinct mechanism. BMDMs pulsed with IFNγ transiently became refractory to a secondary IFNγ treatment as measured by impaired STAT1 phosphorylation. As shown previously, type I IFNs cause a reduction in myeloid cell surface IFNGR1 seen 72 hpi with systemic L. monocytogenes (Lm), and this reduction was abrogated by myeloid cell expression of a transgenic IFNGR1 in fGR1 mice. In contrast, IFNγ was responsible for a reduction in IFNGR on myeloid cells early after systemic Lm infection (24 hip), and this was not abrogated in fGR1 mice. Together, our results demonstrate that type I and II IFNs act via distinct mechanisms to prevent hyper-activation of myeloid cells following exposure to IFNγ.

A Listeria-derived polypeptide promotes in vivo activation of NK cells for antitumor therapy

Immunotherapies have shown promise in treatment of cancer, but more potent and targeted therapies are needed. Natural killer (NK) cells are lymphocytes with innate ability to recognize and lyse tumor cells. When activated, they also produce type II interferon (IFNγ) to orchestrate the activity of other immune cells. Strategies to elicit NK cell activation in vivo have potential usefulness in anti-tumor immunotherapies. Here, we report on a strategy to stimulate NK cell activation and anti-tumor activity in mice with established B16.F10 murine melanomas. We and others previously observed that NK cells are rapidly activated during infection by pathogens such as the bacterium Listeria monocytogenes (Lm). A secreted Lm virulence protein, p60, and a fragment of p60 termed L1S were previously shown to stimulate innate immune responses and promote NK cell activation. We purified recombinant L1S and characterized its activity in cell culture studies. Recombinant L1S protein was also observed to promote accumulation and robust NK cell activation in the lungs when given via intratracheal instillation to control and tumor-bearing mice. Importantly, therapeutic administration of a single L1S dose was found to significantly reduce the number and area of "metastatic" tumor nodules on the lungs of mice with established B16.F10 murine melanomas. Depletion studies showed that these antitumor effects were dependent on NK cells and IFNγ. These data provide proof of concept that administration of a single immune-modulating microbial polypeptide can be used to therapeutically boost NK cell in vivo activation and promote anti-tumor responses.

Down regulation of macrophage IFNGR1 exacerbates systemic L. monocytogenes infection

Interferons (IFNs) target macrophages to regulate inflammation and resistance to microbial infections. The type II IFN (IFNγ) acts on a cell surface receptor (IFNGR) to promote gene expression that enhance macrophage inflammatory and anti-microbial activity. Type I IFNs can dampen macrophage responsiveness to IFNγ and are associated with increased susceptibility to numerous bacterial infections. The precise mechanisms responsible for these effects remain unclear. Type I IFNs silence macrophage ifngr1 transcription and thus reduce cell surface expression of IFNGR1. To test how these events might impact macrophage activation and host resistance during bacterial infection, we developed transgenic mice that express a functional FLAG-tagged IFNGR1 (fGR1) driven by a macrophage-specific promoter. Macrophages from fGR1 mice expressed physiologic levels of cell surface IFNGR1 at steady state and responded equivalently to WT C57Bl/6 macrophages when treated with IFNγ alone. However, fGR1 macrophages retained cell surface IFNGR1 and showed enhanced responsiveness to IFNγ in the presence of type I IFNs. When fGR1 mice were infected with the bacterium Listeria monocytogenes their resistance was significantly increased, despite normal type I and II IFN production. Enhanced resistance was dependent on IFNγ and associated with increased macrophage activation and antimicrobial function. These results argue that down regulation of myeloid cell IFNGR1 is an important mechanism by which type I IFNs suppress inflammatory and anti-bacterial functions of macrophages.

Altered Natural Killer Cell Function in HIV-Exposed Uninfected Infants

Objectives

HIV-exposed uninfected (HEU) infants have higher rates of severe and fatal infections compared with HIV-unexposed (HUU) infants, likely due to immune perturbations. We hypothesized that alterations in natural killer (NK) cell activity might occur in HEU infants and predispose them to severe infections.

Design

Case–control study using cryopreserved peripheral blood mononuclear cells (PBMCs) at birth and 6 months from HEU infants enrolled from 2002 to 2009 and HUU infants enrolled from 2011 to 2013.

Methods

NK cell phenotype and function were assessed by flow cytometry after 20-h incubation with and without K562 cells.

Results

The proportion of NK cells among PBMCs was lower at birth in 12 HEU vs. 22 HUU (1.68 vs. 10.30%, p < 0.0001) and at 6 months in 52 HEU vs. 72 HUU (3.09 vs. 4.65%, p = 0.0005). At birth, HEU NK cells demonstrated increased killing of K562 target cells (p < 0.0001) and increased expression of CD107a (21.65 vs. 12.70%, p = 0.047), but these differences resolved by 6 months. Stimulated HEU NK cells produced less interferon (IFN)γ at birth (0.77 vs. 2.64%, p = 0.008) and at 6 months (4.12 vs. 8.39%, p = 0.001), and showed reduced perforin staining at 6 months (66.95 vs. 77.30%, p = 0.0008). Analysis of cell culture supernatants indicated that lower NK cell activity in HEU was associated with reduced interleukin (IL)-12, IL-15, and IL-18. Addition of recombinant human IL-12 to stimulated HEU PBMCs restored IFNγ production to that seen in stimulated HUU cultures.

Conclusion

NK cell proportion, phenotype, and function are altered in HEU infants. NK cell cytotoxicity and degranulation are increased in HEU at birth, but HEU NK cells have reduced IFNγ and perforin production, suggesting an adequate initial response, but decreased functional reserve. NK cell function improved with addition of exogenous IL-12, implicating impaired production of IL-12 by accessory cells. Alterations in NK cell and accessory cell function may contribute to the increased susceptibility to infection in HEU infants.

Bacterial Manipulation of NK Cell Regulatory Activity Increases Susceptibility to Listeria monocytogenes Infection

Natural killer (NK) cells produce interferon (IFN)-γ and thus have been suggested to promote type I immunity during bacterial infections. Yet, Listeria monocytogenes (Lm) and some other pathogens encode proteins that cause increased NK cell activation. Here, we show that stimulation of NK cell activation increases susceptibility during Lm infection despite and independent from robust NK cell production of IFNγ. The increased susceptibility correlated with IL-10 production by responding NK cells. NK cells produced IL-10 as their IFNγ production waned and the Lm virulence protein p60 promoted induction of IL-10 production by mouse and human NK cells. NK cells consequently exerted regulatory effects to suppress accumulation and activation of inflammatory myeloid cells. Our results reveal new dimensions of the role played by NK cells during Lm infection and demonstrate the ability of this bacterial pathogen to exploit the induction of regulatory NK cell activity to increase host susceptibility.

Natural killer (NK) cells are an innate immune cell population known to promote antiviral immunity through cytolysis and production of cytokines. Yet, some pathogens encode proteins that cause increased NK cell activation. Here, using a model of systemic infection by the bacterial pathogen Listeria monocytogenes (Lm), we show that NK cell activation increases host susceptibility. Activated NK cells increased bacterial burdens in infected tissues despite their early production of the pro-inflammatory cytokine IFNγ. We found that the ability of NK cells to exacerbate infection was independent from their production of IFNγ and instead due to subsequent production of the anti-inflammatory cytokine IL-10. A single bacterial protein, p60, was sufficient to elicit NK cell production of both early IFNγ and delayed IL-10. IL-10-production by NK cells has been shown to occur in other systems, but our studies are first to show how this “regulatory” response impacts the course of a bacterial infection. We found that IL-10 producing NK cells suppress accumulation and activation of inflammatory myeloid cells. Our studies suggest that the exploitation of NK cell regulatory activity provides selective pressure for the evolution of pathogen proteins that promote NK cell activation.

A Breakthrough: Macrophage-Directed Cancer Immunotherapy

Successful immunotherapy of cancer is becoming a reality aided by the realization that macrophages play an important role in the growth or regression of tumors. Specifically, M2/repair-type macrophages predominate in human cancers and produce growth-promoting molecules that actively stimulate tumor growth in much the same way they help wounds heal. However, modulating M2/repair-type macrophages to M1/kill-type can slow or stop cancer growth. The effects involve direct activity of M1 kill-type as well as the ability of M1-type macrophages to stimulate Th1-type cytotoxic T cells and other effector cells. Macrophage responses can also predict cancer susceptibility; individuals with a high M1/kill to M2/repair ratio are less prone. That macrophages/innate immunity can be modulated to play a central role in directly or indirectly combating cancer is a breakthrough that seems likely to finally make successful immunotherapy of cancer a reality.

General parity between trio and pairwise breeding of laboratory mice in static caging

Changes made in the 8th edition of the Guide for the Care and Use of Laboratory Animals included new recommendations for the amount of space for breeding female mice. Adopting the new recommendations required, in essence, the elimination of trio breeding practices for all institutions. Both public opinion and published data did not readily support the new recommendations. In response, the National Jewish Health Institutional Animal Care and Use Committee established a program to directly compare the effects of breeding format on mouse pup survival and growth. Our study showed an overall parity between trio and pairwise breeding formats on the survival and growth of the litters, suggesting that the housing recommendations for breeding female mice as stated in the current Guide for the Care and Use of Laboratory Animals should be reconsidered.

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.

Rituximab as successful adjunct treatment in a patient with disseminated nontuberculous mycobacterial infection due to acquired anti-interferon-γ autoantibody

An acquired immune deficiency due to interferon gamma (IFN-γ) autoantibodies was diagnosed in a 78-year-old Japanese man with treatment-refractory disseminated nontuberculous mycobacterial infection. In addition to standard antimycobacterial therapy, he was successfully treated with rituximab to eliminate B cells and thereby the autoantibody. Subsequently, he obtained a sustained remission from infection.

Type I IFNs downregulate myeloid cell IFN-γ receptor by inducing recruitment of an early growth response 3/NGFI-A binding protein 1 complex that silences ifngr1 transcription

The ability of type I IFNs to increase susceptibility to certain bacterial infections correlates with downregulation of myeloid cell surface IFNGR, the receptor for the type II IFN (IFN-γ), and reduced myeloid cell responsiveness to IFN-γ. In this study, we show that the rapid reductions in mouse and human myeloid cell surface IFNGR1 expression that occur in response to type I IFN treatment reflect a rapid silencing of new ifngr1 transcription by repressive transcriptional regulators. Treatment of macrophages with IFN-β reduced cellular abundance of ifngr1 transcripts as rapidly and effectively as actinomycin D treatment. IFN-β treatment also significantly reduced the amounts of activated RNA polymerase II (pol II) and acetylated histones H3 and H4 at the ifngr1 promoter and the activity of an IFNGR1-luc reporter construct in macrophages. The suppression of IFNGR1-luc activity required an intact early growth response factor (Egr) binding site in the proximal ifngr1 promoter. Three Egr proteins and two Egr/NGFI-A binding (Nab) proteins were found to be expressed in bone macrophages, but only Egr3 and Nab1 were recruited to the ifngr1 promoter upon IFN-β stimulation. Knockdown of Nab1 in a macrophage cell line prevented downregulation of IFNGR1 and prevented the loss of acetylated histones from the ifngr1 promoter. These data suggest that type I IFN stimulation induces a rapid recruitment of a repressive Egr3/Nab1 complex that silences transcription from the ifngr1 promoter. This mechanism of gene silencing may contribute to the anti-inflammatory effects of type I IFNs.

Differential effects of type I and II interferons on myeloid cells and resistance to intracellular bacterial infections

The type I and II interferons (IFNs) play important roles in regulating immune responses during viral and bacterial infections and in the context of autoimmune and neoplastic diseases. These two IFN types bind to distinct cell surface receptors that are expressed by nearly all cells to trigger signal transduction events and elicit diverse cellular responses. In some cases, type I and II IFNs trigger similar cellular responses, while in other cases, the IFNs have unique or antagonistic effects on host cells. Negative regulators of IFN signaling also modulate cellular responses to the IFNs and play important roles in maintaining immunological homeostasis. In this review, we provide an overview of how IFNs stimulate cellular responses. We discuss the disparate effects of type I and II IFNs on host resistance to certain intracellular bacterial infections and provide an overview of models that have been proposed to account for these disparate effects. Mechanisms of antagonistic cross talk between type I and II IFNs are also introduced.

Early events regulating immunity and pathogenesis during Listeria monocytogenes infection

Listeria monocytogenes (Lm) is both a life-threatening pathogen of humans and a model organism that is widely used to dissect the mechanisms of innate and adaptive immune resistance to infection. Specific aspects of the immune response to systemic Lm infection can be protective, neutral, or in some cases deleterious. In this review, we seek to provide an overview of the early events during Lm infection that dictate or regulate host innate and adaptive immune responses. We highlight several recent developments that add to our understanding of the complex interplay between inflammatory responses, host susceptibility to infection, and the development of protective immunity.

Distinct licensing of IL-18 and IL-1β secretion in response to NLRP3 inflammasome activation

Inflammasome activation permits processing of interleukins (IL)-1β and 18 and elicits cell death (pyroptosis). Whether these responses are independently licensed or are “hard-wired” consequences of caspase-1 (casp1) activity has not been clear. Here, we show that that each of these responses is independently regulated following activation of NLRP3 inflammasomes by a “non-canonical” stimulus, the secreted Listeria monocytogenes (Lm) p60 protein. Primed murine dendritic cells (DCs) responded to p60 stimulation with reactive oxygen species (ROS) production and secretion of IL-1β and IL-18 but not pyroptosis. Inhibitors of ROS production inhibited secretion of IL-1β, but did not impair IL-18 secretion. Furthermore, DCs from caspase-11 (casp11)-deficient 129S6 mice failed to secrete IL-1β in response to p60 but were fully responsive for IL-18 secretion. These findings reveal that there are distinct licensing requirements for processing of IL-18 versus IL-1β by NLRP3 inflammasomes.

Nitric oxide increases susceptibility of Toll-like receptor-activated macrophages to spreading Listeria monocytogenes

Toll-like receptor (TLR) stimulation activates macrophages to resist intracellular pathogens. Yet, the intracellular bacterium Listeria monocytogenes (Lm) causes lethal infections in spite of innate immune cell activation. Lm uses direct cell-cell spread to disseminate within its host. Here, we have shown that TLR-activated macrophages killed cell-free Lm but failed to prevent infection by spreading Lm. Instead, TLR signals increased the efficiency of Lm spread from "donor" to "recipient" macrophages. This enhancement required nitric oxide (NO) production by nitric oxide synthase-2 (NOS2). NO increased Lm escape from secondary vacuoles in recipient cells and delayed maturation of phagosomes containing membrane-like particles that mimic Lm-containing pseudopods. NO also promoted Lm spread during systemic in vivo infection, as shown by the fact that inhibition of NOS2 with 1400W reduced spread-dependent Lm burdens in mouse livers. These findings reveal a mechanism by which pathogens capable of cell-cell spread can avoid the consequences of innate immune cell activation by TLR stimuli.

A LysM and SH3-domain containing region of the Listeria monocytogenes p60 protein stimulates accessory cells to promote activation of host NK cells

Listeria monocytogenes (Lm) infection induces rapid and robust activation of host natural killer (NK) cells. Here we define a region of the abundantly secreted Lm endopeptidase, p60, that potently but indirectly stimulates NK cell activation in vitro and in vivo. Lm expression of p60 resulted in increased IFNγ production by naïve NK cells co-cultured with treated dendritic cells (DCs). Moreover, recombinant p60 protein stimulated activation of naive NK cells when co-cultured with TLR or cytokine primed DCs in the absence of Lm. Intact p60 protein weakly digested bacterial peptidoglycan (PGN), but neither muropeptide recognition by RIP2 nor the catalytic activity of p60 was required for NK cell activation. Rather, the immune stimulating activity mapped to an N-terminal region of p60, termed L1S. Treatment of DCs with a recombinant L1S polypeptide stimulated them to activate naïve NK cells in a cell culture model. Further, L1S treatment activated NK cells in vivo and increased host resistance to infection with Francisella tularensis live vaccine strain (LVS). These studies demonstrate an immune stimulating function for a bacterial LysM domain-containing polypeptide and suggest that recombinant versions of L1S or other p60 derivatives can be used to promote NK cell activation in therapeutic contexts.

MPYS is required for IFN response factor 3 activation and type I IFN production in the response of cultured phagocytes to bacterial second messengers cyclic-di-AMP and cyclic-di-GMP

Cyclic-di-GMP and cyclic-di-AMP are second messengers produced by bacteria and influence bacterial cell survival, differentiation, colonization, biofilm formation, virulence, and bacteria-host interactions. In this study, we show that in both RAW264.7 macrophage cells and primary bone marrow-derived macrophages, the production of IFN-β and IL-6, but not TNF, in response to cyclic-di-AMP and cyclic-di-GMP requires MPYS (also known as STING, MITA, and TMEM173). Furthermore, expression of MPYS was required for IFN response factor 3 but not NF-κB activation in response to these bacterial metabolites. We also confirm that MPYS is required for type I IFN production by cultured macrophages infected with the intracellular pathogens Listeria monocytogenes and Francisella tularensis. However, during systemic infection with either pathogen, MPYS deficiency did not impact bacterial burdens in infected spleens. Serum IFN-β and IL-6 concentrations in the infected control and MPYS(-/-) mice were also similar at 24 h postinfection, suggesting that these pathogens stimulate MPYS-independent cytokine production during in vivo infection. Our findings indicate that bifurcating MPYS-dependent and -independent pathways mediate sensing of cytosolic bacterial infections.

Transient Receptor Potential Melastatin 2 (TRPM2) ion channel is required for innate immunity against Listeria monocytogenes

The generation of reactive oxygen species (ROS) is inherent to immune responses. ROS are crucially involved in host defense against pathogens by promoting bacterial killing, but also as signaling agents coordinating the production of cytokines. Transient Receptor Potential Melastatin 2 (TRPM2) is a Ca(2+)-permeable channel gated via binding of ADP-ribose, a metabolite formed under conditions of cellular exposure to ROS. Here, we show that TRPM2-deficient mice are extremely susceptible to infection with Listeria monocytogenes (Lm), exhibiting an inefficient innate immune response. In a comparison with IFNγR-deficient mice, TRPM2(-/-) mice shared similar features of uncontrolled bacterial replication and reduced levels of inducible (i)NOS-expressing monocytes, but had intact IFNγ responsiveness. In contrast, we found that levels of cytokines IL-12 and IFNγ were diminished in TRPM2(-/-) mice following Lm infection, which correlated with their reduced innate activation. Moreover, TRPM2(-/-) mice displayed a higher degree of susceptibility than IL-12-unresponsive mice, and supplementation with recombinant IFNγ was sufficient to reverse the unrestrained bacterial growth and ultimately the lethal phenotype of Lm-infected TRPM2(-/-) mice. The severity of listeriosis we observed in TRPM2(-/-) mice has not been reported for any other ion channel. These findings establish an unsuspected role for ADP-ribose and ROS-mediated cation flux for innate immunity, opening up unique possibilities for immunomodulatory intervention through TRPM2.

Non-surgical intratracheal instillation of mice with analysis of lungs and lung draining lymph nodes by flow cytometry

Phagocytic cells such as alveolar macrophages and lung dendritic cells (LDCs) continuously sample antigens from the alveolar spaces in the lungs. LDCs, in particular, are known to migrate to the lung draining lymph nodes (LDLNs) where they present inhaled antigens to T cells initiating an appropriate immune response to a variety of immunogens. To model interactions between the lungs and airborne antigens in mice, antigens can be administered intranasally, intratracheally or as aerosols. Delivery by each route involves distinct technical skills and limitations that need to be considered before designing an experiment. For example, intranasal and aerosolized exposure delivers antigens to both the lungs and the upper respiratory tract. Hence antigens can access the nasal associated lymphoid tissue (NALT), potentially complicating interpretation of the results. In addition, swallowing, sneezing and the breathing rate of the mouse may also lead to inconsistencies in the doses delivered. Although the involvement of the upper respiratory tract may be preferred for some studies, it can complicate experiments focusing on events specifically initiated in the lungs. In this setting, the intratracheal (i.t) route is preferable as it delivers test materials directly into the lungs and bypasses the NALT. Many i.t injection protocols involve either blind intubation of the trachea through the oral cavity or surgical exposure of the trachea to access the lungs. Herein, we describe a simple, consistent, non-surgical method for i.t instillation. The opening of the trachea is visualized using a laryngoscope and a bent gavage needle is then inserted directly into the trachea to deliver the innoculum. We also describe procedures for harvesting and processing of LDLNs and lungs for analysis of antigen trafficking by flow cytometry.

IL-15 and type I interferon are required for activation of tumoricidal NK cells by virus-infected dendritic cells

There is increasing evidence that natural killer (NK) cells play an important role in antitumor immunity following dendritic cell (DC) vaccination. Little is known, however, about the optimal stimulation of DCs that favors NK activation in tumor-bearing hosts. In this study, we demonstrate that treatment with toll-like receptor (TLR) ligands and infection with a mutant vesicular stomatitis virus (VSV-ΔM51) both induced DC maturation. Further, inoculation of these DCs led to robust NK-mediated protection against tumor challenge. Strikingly, only VSV-ΔM51-infected DCs were capable of suppressing the growth of established tumors, suggesting that additional signals provided by viral infection may be required to activate tumoricidal NK cells in tumor-bearing hosts. VSV-ΔM51 infection of DCs induced greater type I interferon (IFN I) production than TLR ligand treatment, and disruption of the IFN I pathway in DCs eliminated their ability to induce NK activation and tumor protection. However, further studies indicated that IFN I alone was not sufficient to activate NK cells, especially in the presence of a tumor, and DC-derived IL-15 was additionally required for tumoricidal NK activation. These results suggest that induction of IFN I by VSV-ΔM51 allows DCs to overcome tumor-associated immunosuppression and facilitate IL-15-mediated priming of tumoricidal NK cells. Thus, the mode of DC maturation should be carefully considered when designing DC-based cancer immunotherapies.

Cellular reactive oxygen species inhibit MPYS induction of IFNβ

Many inflammatory diseases, as well as infections, are accompanied by elevation in cellular levels of Reactive Oxygen Species (ROS). Here we report that MPYS, a.k.a. STING, which was recently shown to mediate activation of IFNβ expression during infection, is a ROS sensor. ROS induce intermolecular disulfide bonds formation in MPYS homodimer and inhibit MPYS IFNβ stimulatory activity. Cys-64, -148, -292, -309 and the potential C₈₈xxC₉₁ redox motif in MPYS are indispensable for IFNβ stimulation and IRF3 activation. Thus, our results identify a novel mechanism for ROS regulation of IFNβ stimulation.

Antagonistic crosstalk between type I and II interferons and increased host susceptibility to bacterial infections

Type I and II interferons (IFNs αβ and γ) have opposing effects on immune resistance to certain pathogenic bacteria. While IFNγ generally plays a protective role, IFNαβ exacerbates Listeria monocytogenes and Mycobacterium tuberculosis infections. Our findings provided evidence that this increased susceptibility reflects a novel antagonistic cross talk between IFNαβ and IFNγ. Macrophages infected with L. monocytogenes strains that induce IFNαβ production responded poorly to IFNγ, as measured by reduced phosphorylation of STAT1 and reduced IFNγ-dependent gene expression. The impaired responsiveness to IFNγ correlated with reduced expression of its receptor, IFNGR, by both infected and bystander macrophages. Down regulation of IFNGR was dependent on responsiveness to IFNγ and mimicked by recombinant IFNβ. Mice lacking responsiveness to IFNαβ (IFNAR1 (-/-)) retained high IFNGR expression, developed higher expression of MHC-II on macrophages and DCs, and were more resistant to systemic L. monocytogenes infection--but only in the presence of IFNγ. Thus, the ability of IFNαβ to down regulate IFNGR provides an explanation for its ability to reduce responsiveness to IFNγ and to increase host susceptibility to bacterial infection. It remains to be determined whether and how such antagonistic interferon crosstalk benefits the host.

Bacterial peptidoglycan degrading enzymes and their impact on host muropeptide detection

Peptidoglycan (PGN) is a major component of the bacterial cell envelope in both Gram-positive and Gram-negative bacteria. These muropeptides can be produced or modified by the activity of bacterial glycolytic and peptidolytic enzymes referred to as PGN hydrolases and autolysins. Some of these bacterial enzymes are crucial for bacterial pathogenicity and have been shown to modulate muropeptide release and/or host innate immune responses. The ability of muropeptides to modulate host responses is due to the fact that eukaryotes do not produce PGN and have instead evolved numerous strategies to detect intact PGN and PGN fragments (muropeptides). Here we review the structure of PGN and introduce the various bacterial enzymes known to degrade or modify bacterial PGN. Host factors involved in PGN and muropeptide detection are also briefly discussed, as are examples of how specific bacterial pathogens use PGN degradation and modification to subvert host innate immunity.

Activation of naive NK cells in response to Listeria monocytogenes requires IL-18 and contact with infected dendritic cells

The mechanisms for NK cell activation during infection by intracellular bacterial pathogens are not clearly defined. To dissect how Listeria monocytogenes infection elicits NK cell activation, we evaluated the requirements for activation of naive splenic NK cells by infected bone marrow-derived dendritic cells (BMDCs). We found that NK cell activation in this setting required infection of BMDCs by live wild type bacteria. NK cells were not activated when BMDCs were infected with a live hemolysin deficient (Deltahly) strain. Neutralization of IL-12, TNF-alpha, or caspase-1 each dramatically reduced NK cell IFN-gamma production in response to live wt L. monocytogenes infection. Addition of recombinant IL-18, but not IL-1beta, reversed the effects of caspase-1 inhibition. Recombinant IL-18 also restored NK cell activation by BMDCs infected with Deltahly L. monocytogenes, which produced IL-12 but not IL-18. IL-18 acted on NK cells because MyD88 expression was required in responding NK cells, but not infected BMDC. However, secreted cytokines were not sufficient for activation of naive NK cells by infected BMDCs. Rather, NK cell activation additionally required contact between infected BMDCs and NK cells. These data suggest that the activation of NK cells during L. monocytogenes infection requires both secreted cytokines and ligation of NK activating receptors during direct contact with infected DCs.

Induction of IFN-alphabeta enables Listeria monocytogenes to suppress macrophage activation by IFN-gamma

Production of type I interferon (IFN; IFN-alphabeta) increases host susceptibility to Listeria monocytogenes, whereas type II IFN (IFN-gamma) activates macrophages to resist infection. We show that these opposing immunological effects of IFN-alphabeta and IFN-gamma occur because of cross talk between the respective signaling pathways. We found that cultured macrophages infected with L. monocytogenes were refractory to IFN-gamma treatment as a result of down-regulation of the IFN-gamma receptor (IFNGR). The soluble factor responsible for these effects was identified as host IFN-alphabeta. Accordingly, macrophages and dendritic cells (DCs) showed reduced IFNGR1 expression and reduced responsiveness to IFN-gamma during systemic infection of IFN-alphabeta-responsive mice. Furthermore, the increased resistance of mice lacking the IFN-alphabeta receptor (IFNAR(-/-)) to L. monocytogenes correlated with increased expression of IFN-gamma-dependent activation markers by macrophages and DCs and was reversed by depletion of IFN-gamma. Thus, IFN-alphabeta produced in response to bacterial infection and other stimuli antagonizes the host response to IFN-gamma by down-regulating the IFNGR. Such cross talk permits prioritization of IFN-alphabeta-type immune responses and may contribute to the beneficial effects of IFN-beta in treatment of inflammatory diseases such as multiple sclerosis.

Feedback inhibition of macrophage responses to IFNγ during bacterial infection is mediated by IFNαβ (133.32)

We previously showed that increased IFNγ production by natural killer (NK) cells correlates with increased susceptibility to infection with Listeria monocytogenes. Here, we report that L. monocyogenes suppresses macrophage responsiveness to IFNγ and describe a mechanism contributing to this suppression. Macrophages infected with L. monocytogenes produced a soluble factor that reduced IFNGR1 transcripts and cell surface expression of both IFNGR1 and IFNGR2 proteins on infected and bystander macrophages and on certain other immune cells. We identify this factor as IFNβ or a substance induced by type I IFN, since (1) only L. monocytogenes strains that induce IFNβ caused IFNGR downregulation, (2) IFNGR downregulation was not seen in macrophages lacking a functional type I IFN receptor (IFNAR1-deficient mice), (3) IFNGR-downregulation was partially dependent on STAT1 and was induced by treatment of cells with recombinant IFNβ or IFNαβ -inducing toll like receptor (TLR) agonists. Our findings demonstrate that IFNβ induced during intracellular bacterial infection feeds back through IFNAR to inhibit IFNGR expression and IFNγ-dependent gene expression. These findings offer a common mechanistic explanation for the ability of IFNβ to enhance host susceptibility to intracellular bacterial infection and to suppress inflammation in the context of diseases such as multiple sclerosis.

Activation of cytosolic phospholipase A2alpha in resident peritoneal macrophages by Listeria monocytogenes involves listeriolysin O and TLR2

Eicosanoid production by macrophages is an early response to microbial infection that promotes acute inflammation. The intracellular pathogen Listeria monocytogenes stimulates arachidonic acid release and eicosanoid production from resident mouse peritoneal macrophages through activation of group IVA cytosolic phospholipase A2 (cPLA2alpha). The ability of wild type L. monocytogenes (WTLM) to stimulate arachidonic acid release is partially dependent on the virulence factor listeriolysin O; however, WTLM and L. monocytogenes lacking listeriolysin O (DeltahlyLM) induce similar levels of cyclooxygenase 2. Arachidonic acid release requires activation of MAPKs by WTLM and DeltahlyLM. The attenuated release of arachidonic acid that is observed in TLR2-/- and MyD88-/- macrophages infected with WTLM and DeltahlyLM correlates with diminished MAPK activation. WTLM but not DeltahlyLM increases intracellular calcium, which is implicated in regulation of cPLA2alpha. Prostaglandin E2, prostaglandin I2, and leukotriene C4 are produced by cPLA2alpha+/+ but not cPLA2alpha-/- macrophages in response to WTLM and DeltahlyLM. Tumor necrosis factor (TNF)-alpha production is significantly lower in cPLA2alpha+/+ than in cPLA2alpha-/- macrophages infected with WTLM and DeltahlyLM. Treatment of infected cPLA2alpha+/+ macrophages with the cyclooxygenase inhibitor indomethacin increases TNFalpha production to the level produced by cPLA2alpha-/- macrophages implicating prostaglandins in TNFalpha down-regulation. Therefore activation of cPLA2alpha in macrophages may impact immune responses to L. monocytogenes.

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.

Cytosolic expression of SecA2 is a prerequisite for long-term protective immunity

Induction of efficient adaptive T cell-mediated immunity against the intracellular bacterium Listeria monocytogenes requires its successful invasion of host cell cytosol. However, it is not clear whether its cytosolic escape and growth are sufficient to induce T cell-mediated clearance and protection upon secondary infection. To investigate this issue, we have searched for mutants that do not induce long-term protective immunity yet invade the cytosol of infected cells. We found that mice immunized with L. monocytogenes lacking the SecA2 ATPase, an auxiliary protein secretion system present in several Gram-positive pathogenic bacteria, mounted a robust cytolytic IFN-gamma-secreting CD8+ T cell response but were not protected against a secondary challenge with wild-type (wt) bacteria. Furthermore, CD8+ T cells from mice immunized with secA2- bacteria failed to transfer protection when injected into recipient mice demonstrating that they were unable to confer protection. Also, secA2- and wt L. monocytogenes spread to the same myeloid-derived cell types in vivo and SecA2 deficiency does not interfere with intracytosolic bacteria multiplication. Therefore, cytosol invasion is not sufficient for inducing secondary protective responses and induction of memory CD8+ T cells mediating long-term antibacterial protective immunity is dependent upon SecA2 expression inside the cytosol of host cells in vivo.

TRAIL-R as a negative regulator of innate immune cell responses

TRAIL receptor (TRAIL-R) signaling has been implicated in inducing apoptosis in tumor cells, but little is understood about its physiological function. Here, we report the generation and characterization of TRAIL-R(-/-) mice, which develop normal lymphocyte populations but possess enhanced innate immune responses. TRAIL-R(-/-) mice exhibited increased clearance of murine cytomegalovirus that correlated with increased levels of IL-12, IFN-alpha, and IFN-gamma. Stimulation of macrophages with Mycobacterium and Toll-like receptor (TLR)-2, -3, and -4, but not TLR9, ligands resulted in high levels of TRAIL upregulation and enhanced cytokine production in TRAIL-R(-/-) cells. The immediate-early TLR signaling events in TRAIL-R(-/-) macrophages and dendritic cells are normal, but I kappa B-alpha homeostatic regulation and NF-kappa B activity at later time points is perturbed. These data suggest that TRAIL-R negatively regulates innate immune responses.

Promiscuity of MHC class Ib-restricted T cell responses

Murine infection with the Gram-positive intracellular bacterium Listeria monocytogenes activates CD8(+) T cells that recognize bacterially derived N-formyl methionine peptides in the context of H2-M3 MHC class Ib molecules. Three peptides, fMIGWII, fMIVIL, and fMIVTLF, are targets of L. monocytogenes-specific CD8(+) T cells. To investigate epitope cross-recognition by H2-M3-restricted CD8(+) T cells, we deleted the sequence encoding fMIGWII from a virulent strain of L. monocytogenes. Infection with fMIGWII-deficient L. monocytogenes unexpectedly primed CD8(+) T cells that stain with fMIGWII/H2-M3 tetramers and lyse fMIGWII-coated target cells in vivo. Because the fMIGWII sequence is nonredundant, we speculated that other bacterially derived Ags are priming these responses. HPLC peptide fractionation of bacterial culture supernatants revealed several distinct L. monocytogenes-derived peptides that are recognized by fMIGWII-specific T cells. Our results demonstrate that the dominant H2-M3-restricted CD8(+) T cell population, although reactive with fMIGWII, is primed by other, non-fMIGWII peptides derived from L. monocytogenes. Although this degree of Ag receptor promiscuity is unusual for the adaptive immune system, it may be a more common feature of T cell responses restricted by nonpolymorphic MHC class Ib molecules.

SecA2-dependent secretion of autolytic enzymes promotes Listeria monocytogenes pathogenesis

Pathogenic bacteria secrete proteins that promote invasion of host tissues and resistance to immune responses. However, secretion mechanisms that contribute to the enormous morbidity and mortality of Gram-positive bacteria are largely undefined. An auxiliary protein secretion system (SecA2) has recently emerged in Listeria monocytogenes and eight other Gram-positive pathogens. Here, a proteomics approach identified seventeen SecA2-dependent secreted and surface proteins of L. monocytogenes, the two most abundant of which [the p60 and N-acetylmuramidase (NamA) autolysins] hydrolyze bacterial peptidoglycan (PGN) and contribute to host colonization. SecA2-deficient (DeltaSecA2) bacteria were rapidly cleared after systemic infection of murine hosts, and in cultured cells showed reduced cell-cell spread. p60 or NamA deficiencies (Deltap60 and DeltaNamA) caused intermediate reductions in bacterial virulence in vivo, yet showed no defect for infection of cultured cells. Restoration of virulence in Deltap60 bacteria required full-length p60 with an intact catalytic domain, suggesting that PGN hydrolysis by p60 is crucial for L. monocytogenes virulence. Coordinated PGN hydrolysis by p60 and NamA activities is predicted to generate a muramyl glycopeptide, glucosaminylmuramyl dipeptide (GMDP), which is known to modify host inflammatory responses. Thus, SecA2-dependent secretion may promote release of muramyl peptides that subvert host pattern recognition.