NOD2 contributes to the inflammatory responses of primary murine microglia and astrocytes to Staphylococcus aureus

Epigenetic Regulation of Leukocyte Inflammatory Mediator Production Dictates Staphylococcus aureus Craniotomy Infection Outcome

Staphylococcus aureus is a common cause of surgical-site infections, including those arising after craniotomy, which is performed to access the brain for the treatment of tumors, epilepsy, or hemorrhage. Craniotomy infection is characterized by complex spatial and temporal dynamics of leukocyte recruitment and microglial activation. We recently identified unique transcriptional profiles of these immune populations during S. aureus craniotomy infection. Epigenetic processes allow rapid and reversible control over gene transcription; however, little is known about how epigenetic pathways influence immunity to live S. aureus. An epigenetic compound library screen identified bromodomain and extraterminal domain-containing (BET) proteins and histone deacetylases (HDACs) as critical for regulating TNF, IL-6, IL-10, and CCL2 production by primary mouse microglia, macrophages, neutrophils, and granulocytic myeloid-derived suppressor cells in response to live S. aureus. Class I HDACs (c1HDACs) were increased in these cell types in vitro and in vivo during acute disease in a mouse model of S. aureus craniotomy infection. However, substantial reductions in c1HDACs were observed during chronic infection, highlighting temporal regulation and the importance of the tissue microenvironment for dictating c1HDAC expression. Microparticle delivery of HDAC and BET inhibitors in vivo caused widespread decreases in inflammatory mediator production, which significantly increased bacterial burden in the brain, galea, and bone flap. These findings identify histone acetylation as an important mechanism for regulating cytokine and chemokine production across diverse immune cell lineages that is critical for bacterial containment. Accordingly, aberrant epigenetic regulation may be important for promoting S. aureus persistence during craniotomy infection.

Host immune responses in the central nervous system during fungal infections

Fungal infections in the central nervous system (CNS) cause high morbidity and mortality. The frequency of CNS mycosis has increased over the last two decades as more individuals go through immunocompromised conditions for various reasons. Nevertheless, options for clinical interventions for CNS mycoses are still limited. Thus, there is an urgent need to understand the host-pathogen interaction mechanisms in CNS mycoses for developing novel treatments. Although the CNS has been regarded as an immune-privileged site, recent studies demonstrate the critical involvement of immune responses elicited by CNS-resident and CNS-infiltrated cells during fungal infections. In this review, we discuss mechanisms of fungal invasion in the CNS, fungal pathogen detection by CNS-resident cells (microglia, astrocytes, oligodendrocytes, neurons), roles of CNS-infiltrated leukocytes, and host immune responses. We consider that understanding host immune responses in the CNS is crucial for endeavors to develop treatments for CNS mycosis.

Immunopathogenesis of Craniotomy Infection and Niche-Specific Immune Responses to Biofilm

Bacterial infections in the central nervous system (CNS) can be life threatening and often impair neurological function. Biofilm infection is a complication following craniotomy, a neurosurgical procedure that involves the removal and replacement of a skull fragment (bone flap) to access the brain for surgical intervention. The incidence of infection following craniotomy ranges from 1% to 3% with approximately half caused by Staphylococcus aureus (S. aureus). These infections present a significant therapeutic challenge due to the antibiotic tolerance of biofilm and unique immune properties of the CNS. Previous studies have revealed a critical role for innate immune responses during S. aureus craniotomy infection. Experiments using knockout mouse models have highlighted the importance of the pattern recognition receptor Toll-like receptor 2 (TLR2) and its adaptor protein MyD88 for preventing S. aureus outgrowth during craniotomy biofilm infection. However, neither molecule affected bacterial burden in a mouse model of S. aureus brain abscess highlighting the distinctions between immune regulation of biofilm vs. planktonic infection in the CNS. Furthermore, the immune responses elicited during S. aureus craniotomy infection are distinct from biofilm infection in the periphery, emphasizing the critical role for niche-specific factors in dictating S. aureus biofilm-leukocyte crosstalk. In this review, we discuss the current knowledge concerning innate immunity to S. aureus craniotomy biofilm infection, compare this to S. aureus biofilm infection in the periphery, and discuss the importance of anatomical location in dictating how biofilm influences inflammatory responses and its impact on bacterial clearance.

Retinoic acid inducible gene-I mediated detection of bacterial nucleic acids in human microglial cells

Background

Bacterial meningitis and meningoencephalitis are associated with devastating neuroinflammation. We and others have demonstrated the importance of glial cells in the initiation of immune responses to pathogens invading the central nervous system (CNS). These cells use a variety of pattern recognition receptors (PRRs) to identify common pathogen motifs and the cytosolic sensor retinoic acid inducible gene-1 (RIG-I) is known to serve as a viral PRR and initiator of interferon (IFN) responses. Intriguingly, recent evidence indicates that RIG-I also has an important role in the detection of bacterial nucleic acids, but such a role has not been investigated in glia.

Methods

In this study, we have assessed whether primary or immortalized human and murine glia express RIG-I either constitutively or following stimulation with bacteria or their products by immunoblot analysis. We have used capture ELISAs and immunoblot analysis to assess human microglial interferon regulatory factor 3 (IRF3) activation and IFN production elicited by bacterial nucleic acids and novel engineered nucleic acid nanoparticles. Furthermore, we have utilized a pharmacological inhibitor of RIG-I signaling and siRNA-mediated knockdown approaches to assess the relative importance of RIG-I in such responses.

Results

We demonstrate that RIG-I is constitutively expressed by human and murine microglia and astrocytes, and is elevated following bacterial infection in a pathogen and cell type-specific manner. Additionally, surface and cytosolic PRR ligands are also sufficient to enhance RIG-I expression. Importantly, our data demonstrate that bacterial RNA and DNA both trigger RIG-I-dependent IRF3 phosphorylation and subsequent type I IFN production in human microglia. This ability has been confirmed using our nucleic acid nanoparticles where we demonstrate that both RNA- and DNA-based nanoparticles can stimulate RIG-I-dependent IFN responses in these cells.

Conclusions

The constitutive and bacteria-induced expression of RIG-I by human glia and its ability to mediate IFN responses to bacterial RNA and DNA and nucleic acid nanoparticles raises the intriguing possibility that RIG-I may be a potential target for therapeutic intervention during bacterial infections of the CNS, and that the use of engineered nucleic acid nanoparticles that engage this sensor might be a method to achieve this goal.

Immunity Against Bacterial Infection of the Central Nervous System: An Astrocyte Perspective

Bacterial infection of the central nervous system (CNS) is a severe and life-threatening condition with high mortality, and it may lead to permanent neurological deficits in survivors. Increasing evidence indicates that astrocytes, as the most abundant CNS glial cell population, regulate innate and adaptive immune responses in the CNS under pathological conditions in addition to their role in the maintenance of CNS homeostasis and neuronal function. Following antigen recognition, astrocytes participate in the initiation of innate immune responses, and prompt an adaptive immune response to recruit peripheral immune cells. Investigations have been conducted to understand the immunological role of astrocytes in CNS disease and injury, however, their part in bacterial infections of the CNS has not been fully evaluated. A better understanding will permit the identification of successful therapeutic targets for an improved prognosis and disease outcome.

Staphylococcus aureus modulation of innate immune responses through Toll-like (TLR), (NOD)-like (NLR) and C-type lectin (CLR) receptors

Early recognition of pathogens by the innate immune system is crucial for bacterial clearance. Many pattern recognition receptors (PRRs) such as Toll-like (TLRs) and (NOD)-like (NLRs) receptors have been implicated in initial sensing of bacterial components. The intracellular signaling cascades triggered by these receptors result in transcriptional upregulation of inflammatory pathways. Although this step is crucial for bacterial elimination, it is also associated with the potential for substantial immunopathology, which underscores the need for tight control of inflammatory responses. The leading human bacterial pathogen Staphylococcus aureus expresses over 100 virulence factors that exert numerous effects upon host cells. In this manner, the pathogen seeks to avoid host recognition or perturb PRR-induced innate immune responses to allow optimal survival in the host. These immune system interactions may result in enhanced bacterial proliferation but also provoke systemic cytokine responses associated with sepsis. This review summarizes recent findings on the various mechanisms applied by S. aureus to modulate or interfere with inflammatory responses through PRRs. Detailed understanding of these complex interactions can provide new insights toward future immune-stimulatory therapeutics against infection or immunomodulatory therapeutics to suppress or correct dysregulated inflammation.

NOD2-RIP2 contributes to the inflammatory responses of mice in vivo to Streptococcus pneumoniae

The objectives of this study were to investigate the role of the NOD2-RIP2 pathway in host responses to the gram-positive bacteria Streptococcus pneumoniae and to evaluate the effect of the RIP2 inhibitor gefitinib in a mouse model of S. pneumoniae meningitis. Mice were randomly divided into a normal saline control group (NS group), an S. pneumoniae meningitis group (SP group), a gefitinib group and a vehicle group. Animals in the NS group were sham-infected with sterile saline. Mice in the other three groups were inoculated with S. pneumoniae and left untreated, treated with gefitinib, or treated with vehicle. The results revealed upregulation of the mRNA and protein levels of NOD2, RIP2, NF-κB, TNF-α and IL-6 within the CNS of mice with S. pneumoniae meningitis. Moreover, mice in the meningitis group were severely ill and demonstrated a significant loss of neurons. Treatment with the RIP2 inhibitor gefitinib showed less intense activation of NF-kB, decreased TNF-α and IL-6, attenuated neuronal injury and improved illness. These results suggest that NOD2-RIP2 is involved in the host response to the gram-positive bacteria S. pneumoniae in the CNS and that the RIP2 inhibitor gefitinib protects the brain from damage caused by S. pneumoniae.

The bacterial peptidoglycan-sensing molecule Pglyrp2 modulates brain development and behavior

Recent studies have revealed that the gut microbiota modulates brain development and behavior, but the underlying mechanisms are still poorly understood. Here, we show that bacterial peptidoglycan (PGN) derived from the commensal gut microbiota can be translocated into the brain and sensed by specific pattern-recognition receptors (PRRs) of the innate immune system. Using expression-profiling techniques, we demonstrate that two families of PRRs that specifically detect PGN (that is, PGN-recognition proteins and NOD-like receptors), and the PGN transporter PepT1 are highly expressed in the developing brain during specific windows of postnatal development in both males and females. Moreover, we show that the expression of several PGN-sensing molecules and PepT1 in the developing striatum is sensitive to manipulations of the gut microbiota (that is, germ-free conditions and antibiotic treatment). Finally, we used the PGN-recognition protein 2 (Pglyrp2) knockout mice to examine the potential influence of PGN-sensing molecules on brain development and behavior. We demonstrate that the absence of Pglyrp2 leads to alterations in the expression of the autism risk gene c-Met, and sex-dependent changes in social behavior, similar to mice with manipulated microbiota. These findings suggest that the central activation of PRRs by microbial products could be one of the signaling pathways mediating the communication between the gut microbiota and the developing brain.

Analysis of nucleotide-binding oligomerization domain proteins in a murine model of pneumococcal meningitis

BACKGROUND

The innate immune system recognizes pathogens via its pattern recognition receptors. The objective of this study was to investigate the role of the nucleotide-binding oligomerization domain (NOD) proteins, a family of the novel bacterial pattern recognition receptors, in host responses to the gram-positive bacteria Streptococcus pneumoniae.

METHODS

Sprague-Dawley rats were infected via intracisternal injections of viable S. pneumoniae, and rats in the control group were injected with sterile saline. After infection, real-time PCR was performed to determine the presence of mRNAs encoding NOD1 and NOD2. Quantitative analyses of the NOD1, NOD2 and NF-kB proteins were also performed western blotting following challenge infections with viable S. pneumoniae. The TNF-α and IL-6 levels in brain homogenates were evaluated using enzyme-linked immunosorbent assays (ELISAs).

RESULTS

The results revealed up-regulations of the mRNA and protein levels of NOD2 within the CNS of rats with S. pneumoniae meningitis. Moreover, the activation of NF-κB in the brain tissues following infection with live S. pneumoniae was also significantly increased, which indicates that NOD2 mediated NF-κB activation in experimental pneumococcal meningitis. Similarly, TNF-α and IL-6 levels were increased in the brain following in vivo S. pneumoniae administration.

CONCLUSIONS

These results suggest that NOD2 is involved in the host response to the gram-positive bacteria S. pneumoniae in the CNS and that NOD2 might play an important role in the initiation and/or progression of CNS inflammation associated with pneumococcal meningitis.

Astrocytes produce IL-19 in response to bacterial challenge and are sensitive to the immunosuppressive effects of this IL-10 family member

There is growing appreciation that resident glial cells can initiate and/or regulate inflammation following trauma or infection in the central nervous system (CNS). We have previously demonstrated the ability of microglia and astrocytes to respond to bacterial pathogens or their products by rapid production of inflammatory mediators, followed by the production of the immunosuppressive cytokine interleukin (IL)−10. IL-19, another member of the IL-10 family of cytokines, has been studied in the context of a number of inflammatory conditions in the periphery and is known to modulate immune cell activity. In the present study, we demonstrate the constitutive and/or inducible expression of IL-19 and its cognate receptor subunits, IL-19Rα and IL-19Rβ (also known as IL-20R1 and IL-20R2, and IL-20RA and IL-20RB), in mouse brain tissue, and by primary murine and human astrocytes. We also provide evidence for the presence of a novel truncated IL-19Rα transcript variant in mouse brain tissue, but not glial cells, that shows reduced expression following bacterial infection. Importantly, IL-19R functionality in glia is indicated by the ability of IL-19 to regulate signaling component expression in these cells. Furthermore, while IL-19 itself had no effect on glial cytokine production, IL-19 treatment of bacterially infected or Toll-like receptor ligand stimulated astrocytes significantly attenuated pro-inflammatory cytokine production. The bacterially induced production of IL-19 by these resident CNS cells, the constitutive expression of its cognate receptor subunits, and the immunomodulatory effects of this cytokine, suggest a novel mechanism by which astrocytes can regulate CNS inflammation.

Intracellular Staphylococcus aureus-induced NF-κB activation and proinflammatory responses of P815 cells are mediated by NOD2

Staphylococcus aureus (S. aureus) is an important human pathogen which can cause a chronic condition with a high relapse rate despite the aggressive antimicrobial treatment. Recent studies showed that intracellular pattern recognition receptors (including NOD) in response to bacteria or bacterial products play a proinflammatory role by activating nuclear transcription factor-κB (NF-κB). But how NOD2 mediates the proinflammatory response to S. aureus in mast cells (MCs) is unclear. So, in this study, we attempted to examine the role of NOD2 in inflammatory responses of MCs to S. aureus. P815 cells (a mouse mast cell line) were cultured. Real-time PCR was used to detect the NOD2 mRNA expression in P815 cells during S. aureus infection. The siRNA against NOD2 gene was synthesized and transfected into S. aureus-infected P815 cells. By using the methods of ELISA and flow cytometry, the effects of NOD2 gene silencing on cell phagocytosis, cytokine secretion, NF-κB activation and cell apoptosis of the S. aureus-infected P815 cells were examined. It was found that S. aureus infection could increase the expression of NOD2 mRNA in P815 cells. NOD2 gene interference in P815 cells reduced the number of S. aureus engulfed by P815 cells, the level of cytokines and the activation of NF-κB. In addition, S. aureus could induce the apoptosis of P815 cells, but NOD2 gene silencing did not affect the cell apoptosis rate. Our data suggested that NOD2 plays a key role in pathogen recognition, signal transduction, and NF-κB activation in the inflammatory responses of MCs infected by S. aureus.

NLRs in immune privileged sites

Innate immune receptors such as the nucleotide-binding domain, leucine-rich repeat-containing (NBD-LRR) receptors, referred to as NLRs, are known to serve as a critical component of host defense. However, their participation in inflammatory responses within immune privileged sites such as the brain and eye is less understood. The potential importance of NLRs in regulation of inflammation within these particular sites is further underscored by their association with autoinflammatory disorders, wherein localized inflammation can occur within the brain or eye as neuroinflammation or uveitis, respectively. Many NLRs are expressed within the brain and eye and in this review, we discuss their roles in the inflammation of the central nervous system (CNS) and uveitis.

MyD88 is pivotal for immune recognition of Citrobacter koseri and astrocyte activation during CNS infection

Citrobacter koseri (C. koseri) is a Gram-negative bacterium that can cause a highly aggressive form of neonatal meningitis, which often progresses to establish multi-focal brain abscesses. The roles of Toll-like receptor 4 (TLR4) and its signaling adaptor MyD88 during CNS C. koseri infection have not yet been examined, which is important since recent evidence indicates that innate immune responses are tailored towards specific pathogen classes. Here TLR4 WT (C3H/FeJ) and TLR4 mutant (C3H/HeJ) mice as well as MyD88 KO animals were infected intracerebrally with live C. koseri, resulting in meningitis and ventriculitis with accompanying brain abscess formation. MyD88 KO mice were exquisitely sensitive to C. koseri, demonstrating enhanced mortality rates and significantly elevated bacterial burdens compared to WT animals. Interestingly, although early proinflammatory mediator release (i.e. 12 h) was MyD88-dependent, a role for MyD88-independent signaling was evident at 24 h, revealing a compensatory response to CNS C. koseri infection. In contrast, TLR4 did not significantly impact bacterial burdens or proinflammatory mediator production in response to C. koseri. Similar findings were obtained with primary astrocytes, where MyD88-dependent pathways were essential for chemokine release in response to intact C. koseri, whereas TLR4 was dispensable; implicating the involvement of alternative TLRs since highly enriched astrocytes did not produce IL-1 upon bacterial exposure, which also signals via MyD88. Collectively, these findings demonstrate the importance of MyD88-dependent mechanisms in eliciting maximal proinflammatory responses, astrocyte activation, and bacterial containment during CNS C. koseri infection, as well as a late-phase MyD88-independent signaling pathway for cytokine/chemokine production.

Nod2 suppresses Borrelia burgdorferi mediated murine Lyme arthritis and carditis through the induction of tolerance

The internalization of Borrelia burgdorferi, the causative agent of Lyme disease, by phagocytes is essential for an effective activation of the immune response to this pathogen. The intracellular, cytosolic receptor Nod2 has been shown to play varying roles in either enhancing or attenuating inflammation in response to different infectious agents. We examined the role of Nod2 in responses to B. burgdorferi. In vitro stimulation of Nod2 deficient bone marrow derived macrophages (BMDM) resulted in decreased induction of multiple cytokines, interferons and interferon regulated genes compared with wild-type cells. However, B. burgdorferi infection of Nod2 deficient mice resulted in increased rather than decreased arthritis and carditis compared to control mice. We explored multiple potential mechanisms for the paradoxical response in in vivo versus in vitro systems and found that prolonged stimulation with a Nod2 ligand, muramyl dipeptide (MDP), resulted in tolerance to stimulation by B. burgdorferi. This tolerance was lost with stimulation of Nod2 deficient cells that cannot respond to MDP. Cytokine patterns in the tolerance model closely paralleled cytokine profiles in infected Nod2 deficient mice. We propose a model where Nod2 has an enhancing role in activating inflammation in early infection, but moderates inflammation after prolonged exposure to the organism through induction of tolerance.

Comprehensive study of baicalin down-regulating NOD2 receptor expression of neurons with oxygen-glucose deprivation in vitro and cerebral ischemia-reperfusion in vivo

Cerebral ischemia-reperfusion can activate several transcription factors and lead to inflammatory reactions, which related to pattern recognition receptors with immune activating functions. NOD2 (nucleotide-binding oligomerization domain protein 2) is one of the receptors involved in innate immune response and is genetically associated with several inflammatory reactions. Since baicalin has the pharmacological effects of anti-inflammation and protection of brain from cerebral ischemia-reperfusion, we studied baicalin's effect on NOD2/TNFα in the cell of oxygen-glucose deprivation (OGD) in vitro and the mice of cerebral ischemia-reperfusion in vivo. The results showed that NOD2 and TNFα were up regulated in the cells with oxygen-glucose deprivation, not only in BV2 cells, but also in both of PC12 cells and primary neuron cells, which suggested NOD2 could express directly in neuron while OGD treatment. Baicalin (10 μg/ml) could effectively down regulate the expression of NOD2 and TNFα in both mRNA and protein levels. Meanwhile, baicalin (50 mg/kg, i.p.) could also down regulate the expression of NOD2 and TNFα in protein levels significantly, in which agreed with its effect in vitro study. These data demonstrated that targeting on NOD2 especially in neurons directly was possibly attributed to the neural-protective effect of baicalin in the injury of cerebral ischemia-reperfusion.