Toll-like receptor 4 and CD11b expressed on microglia coordinate eradication of Candida albicans cerebral mycosis

The fungal pathogen Candida albicans is linked to chronic brain diseases such as Alzheimer's disease (AD), but the molecular basis of brain anti-Candida immunity remains unknown. We show that C. albicans enters the mouse brain from the blood and induces two neuroimmune sensing mechanisms involving secreted aspartic proteinases (Saps) and candidalysin. Saps disrupt tight junction proteins of the blood-brain barrier (BBB) to permit fungal brain invasion. Saps also hydrolyze amyloid precursor protein (APP) into amyloid β (Aβ)-like peptides that bind to Toll-like receptor 4 (TLR4) and promote fungal killing in vitro while candidalysin engages the integrin CD11b (Mac-1) on microglia. Recognition of Aβ-like peptides and candidalysin promotes fungal clearance from the brain, and disruption of candidalysin recognition through CD11b markedly prolongs C. albicans cerebral mycosis. Thus, C. albicans is cleared from the brain through innate immune mechanisms involving Saps, Aβ, candidalysin, and CD11b.

Proteus mirabilis Urease: Unsuspected Non-Enzymatic Properties Relevant to Pathogenicity

Infection by Proteus mirabilis causes urinary stones and catheter incrustation due to ammonia formed by urease (PMU), one of its virulence factors. Non-enzymatic properties, such as pro-inflammatory and neurotoxic activities, were previously reported for distinct ureases, including that of the gastric pathogen Helicobacter pylori. Here, PMU was assayed on isolated cells to evaluate its non-enzymatic properties. Purified PMU (nanomolar range) was tested in human (platelets, HEK293 and SH-SY5Y) cells, and in murine microglia (BV-2). PMU promoted platelet aggregation. It did not affect cellular viability and no ammonia was detected in the cultures’ supernatants. PMU-treated HEK293 cells acquired a pro-inflammatory phenotype, producing reactive oxygen species (ROS) and cytokines IL-1β and TNF-α. SH-SY5Y cells stimulated with PMU showed high levels of intracellular Ca²⁺ and ROS production, but unlike BV-2 cells, SH-SY5Y did not synthesize TNF-α and IL-1β. Texas Red-labeled PMU was found in the cytoplasm and in the nucleus of all cell types. Bioinformatic analysis revealed two bipartite nuclear localization sequences in PMU. We have shown that PMU, besides urinary stone formation, can potentially contribute in other ways to pathogenesis. Our data suggest that PMU triggers pro-inflammatory effects and may affect cells beyond the renal system, indicating a possible role in extra-urinary diseases.

Human Commensal Prevotella histicola Ameliorates Disease as Effectively as Interferon-Beta in the Experimental Autoimmune Encephalomyelitis

Gut microbiota has emerged as an important environmental factor in the pathobiology of multiple sclerosis (MS), an inflammatory demyelinating disease of the central nervous system (CNS). Both genetic and environmental factors have been shown to play an important role in MS. Among genetic factors, the human leukocyte antigen (HLA) class II allele such as HLA-DR2, DR3, DR4, DQ6, and DQ8 show the association with the MS. We have previously used transgenic mice expressing MS susceptible HLA class II allele such as HLA-DR2, DR3, DQ6, and DQ8 to validate significance of HLA alleles in MS. Although environmental factors contribute to 2/3 of MS risk, less is known about them. Gut microbiota is emerging as an imporatnt environmental factor in MS pathogenesis. We and others have shown that MS patients have distinct gut microbiota compared to healthy control (HC) with a lower abundance of Prevotella. Additionally, the abundance of Prevotella increased in patients receiving disease-modifying therapies (DMTs) such as Copaxone and/or Interferon-beta (IFNβ). We have previously identified a specific strain of Prevotella (Prevotella histicola), which can suppress experimental autoimmune encephalomyelitis (EAE) disease in HLA-DR3.DQ8 transgenic mice. Since Interferon-β-1b [IFNβ (Betaseron)] is a major DMTs used in MS patients, we hypothesized that treatment with the combination of P. histicola and IFNβ would have an additive effect on the disease suppression. We observed that treatment with P. histicola suppressed disease as effectively as IFNβ. Surprisingly, the combination of P. histicola and IFNβ was not more effective than either treatment alone. P. histicola alone or in combination with IFNβ increased the frequency and number of CD4+FoxP3+ regulatory T cells in the gut-associated lymphoid tissue (GALT). Treatment with P. histicola alone, IFNβ alone, and in the combination decreased frequency of pro-inflammatory IFN-γ and IL17-producing CD4+ T cells in the CNS. Additionally, P. histicola alone or IFNβ alone or the combination treatments decreased CNS pathology, characterized by reduced microglia and astrocytic activation. In conclusion, our study indicates that the human gut commensal P. histicola can suppress disease as effectively as commonly used MS drug IFNβ and may provide an alternative treatment option for MS patients.

C9orf72 suppresses systemic and neural inflammation induced by gut bacteria

A hexanucleotide-repeat expansion in C9ORF72 is the most common genetic variant that contributes to amyotrophic lateral sclerosis and frontotemporal dementia1,2. The C9ORF72 mutation acts through gain- and loss-of-function mechanisms to induce pathways that are implicated in neural degeneration3-9. The expansion is transcribed into a long repetitive RNA, which negatively sequesters RNA-binding proteins5 before its non-canonical translation into neural-toxic dipeptide proteins3,4. The failure of RNA polymerase to read through the mutation also reduces the abundance of the endogenous C9ORF72 gene product, which functions in endolysosomal pathways and suppresses systemic and neural inflammation6-9. Notably, the effects of the repeat expansion act with incomplete penetrance in families with a high prevalence of amyotrophic lateral sclerosis or frontotemporal dementia, indicating that either genetic or environmental factors modify the risk of disease for each individual. Identifying disease modifiers is of considerable translational interest, as it could suggest strategies to diminish the risk of developing amyotrophic lateral sclerosis or frontotemporal dementia, or to slow progression. Here we report that an environment with reduced abundance of immune-stimulating bacteria10,11 protects C9orf72-mutant mice from premature mortality and significantly ameliorates their underlying systemic inflammation and autoimmunity. Consistent with C9orf72 functioning to prevent microbiota from inducing a pathological inflammatory response, we found that reducing the microbial burden in mutant mice with broad spectrum antibiotics-as well as transplanting gut microflora from a protective environment-attenuated inflammatory phenotypes, even after their onset. Our studies provide further evidence that the microbial composition of our gut has an important role in brain health and can interact in surprising ways with well-known genetic risk factors for disorders of the nervous system.

In vitro effects of commercial mouthwashes on several virulence traits of Candida albicans, viridans streptococci and Enterococcus faecalis colonizing the oral cavity

Oral microbiota consists of hundreds of different species of bacteria, fungi, protozoa and archaea, important for oral health. Oral mycoses, mostly affecting mucosae, are mainly caused by the opportunistic pathogen Candida albicans. They become relevant in denture-wearers elderly people, in diabetic patients, and in immunocompromised individuals. Differently, bacteria are responsible for other pathologies, such as dental caries, gingivitis and periodontitis, which affect even immune-competent individuals. An appropriate oral hygiene can avoid (or at least ameliorate) such pathologies: the regular and correct use of toothbrush, toothpaste and mouthwash helps prevent oral infections. Interestingly, little or no information is available on the effects (if any) of mouthwashes on the composition of oral microbiota in healthy individuals. Therefore, by means of in vitro models, we assessed the effects of alcohol-free commercial mouthwashes, with different composition (4 with chlorhexidine digluconate, 1 with fluoride, 1 with essential oils, 1 with cetylpyridinium chloride and 1 with triclosan), on several virulence traits of C. albicans, and a group of viridans streptococci, commonly colonizing the oral cavity. For the study here described, a reference strain of C. albicans and of streptococci isolates from pharyngeal swabs were used. Chlorhexidine digluconate- and cetylpyridinium chloride-containing mouthwashes were the most effective in impairing C. albicans capacity to adhere to both abiotic and biotic surfaces, to elicit proinflammatory cytokine secretion by oral epithelial cells and to escape intracellular killing by phagocytes. In addition, these same mouthwashes were effective in impairing biofilm formation by a group of viridans streptococci that, notoriously, cooperate with the cariogenic S. mutans, facilitating the establishment of biofilm by the latter. Differently, these mouthwashes were ineffective against other viridans streptococci that are natural competitors of S. mutans. Finally, by an in vitro model of mixed biofilm, we showed that mouthwashes-treated S. salivarius overall failed to impair C. albicans capacity to form a biofilm. In conclusion, the results described here suggest that chlorhexidine- and cetylpyridinium-containing mouthwashes may be effective in regulating microbial homeostasis of the oral cavity, by providing a positive balance for oral health. On the other side, chlorhexidine has several side effects that must be considered when prescribing mouthwashes containing this molecule.

Sex- and region-specific differences in microglia phenotype and characterization of the peripheral immune response following early-life infection in neonatal male and female rats

Early-life infection has been shown to have profound effects on the brain and behavior across the lifespan, a phenomenon termed "early-life programming". Indeed, many neuropsychiatric disorders begin or have their origins early in life and have been linked to early-life immune activation (e.g. autism, ADHD, and schizophrenia). Furthermore, many of these disorders show a robust sex bias, with males having a higher risk of developing early-onset neurodevelopmental disorders. The concept of early-life programming is now well established, however, it is still unclear how such effects are initiated and then maintained across time to produce such a phenomenon. To begin to address this question, we examined changes in microglia, the immune cells of the brain, and peripheral immune cells in the hours immediately following early-life infection in male and female rats. We found that males showed a significant decrease in BDNF expression and females showed a significant increase in IL-6 expression in the cerebellum following E.coli infection on postnatal day 4; however, for most cytokines examined in the brain and in the periphery we were unable to identify any sex differences in the immune response, at least at the time points examined. Instead, neonatal infection with E.coli increased the expression of a number of cytokines in the brain of both males and females similarly including TNF-α, IL-1β, and CD11b (a marker of microglia activation) in the hippocampus and, in the spleen, TNF-α and IL-1β. We also found that protein levels of GRO-KC, MIP-1a, MCP1, IP-10, TNF-α, and IL-10 were elevated 8-hours postinfection, but this response was resolved by 24-hours. Lastly, we found that males have more thin microglia than females on P5, however, neonatal infection had no effect on any of the microglia morphologies we examined. These data show that sex differences in the acute immune response to neonatal infection are likely gene, region, and even time dependent. Future research should consider these factors in order to develop a comprehensive understanding of the immune response in males and females as these changes are likely the initiating agents that lead to the long-term, and often sex-specific, effects of early-life infection.

Microbiota Signaling Pathways that Influence Neurologic Disease

Though seemingly distinct and autonomous, emerging evidence suggests there is a bidirectional interaction between the intestinal microbiota and the brain. This crosstalk may play a substantial role in neurologic diseases, including anxiety, depression, autism, multiple sclerosis, Parkinson's disease, and, potentially, Alzheimer's disease. Long hypothesized by Metchnikoff and others well over 100 years ago, investigations into the mind-microbe axis is now seeing a rapid resurgence of research. If specific pathways and mechanisms of interaction are understood, it could have broad therapeutic potential, as the microbiome is environmentally acquired and can be modified to promote health. This review will discuss immune, endocrine, and neural system pathways that interconnect the gut microbiota to central nervous system and discuss how these findings might be applied to neurologic disease.

Comparative morphology and phagocytic capacity of primary human adult microglia with time-lapse imaging

Microglia provide immune surveillance within the brain and spinal cord. Various microglial morphologies include ramified, amoeboid, and pseudopodic. The link between form and function is not clear, especially for human adult microglia which are limited in availability for study. Here, we examined primary human microglia isolated from normal-appearing white matter. Pseudopodic and amoeboid microglia were effective phagocytes, taking up E. coli bioparticles using ruffled cell membrane sheets and retrograde transport. Pseudopodic and amoeboid microglia were more effective phagocytes as compared to ramified microglia or monocyte-derived dendritic cells. Thus, amoeboid and pseudopodic microglia may both be effective as brain scavengers.

Retinal photoreceptor expresses toll-like receptors (TLRs) and elicits innate responses following TLR ligand and bacterial challenge

Toll-like receptors (TLRs) play an important role in host defense against microbial pathogens. Our previous studies have shown that TLRs are expressed on various retinal cells (Microglia and Müller glia) and orchestrate retinal innate responses in bacterial endophthalmitis. In this study, we used a well-characterized mouse cone photoreceptor cell line (661W); and demonstrated that these cells express all known TLRs. Although the stimulation of 661W cells with TLR ligands (Pam3Cys, PolyI:C, LPS, Flagellin, Poly DT, and ODN) did not alter TLR expression, downstream TLR-signaling pathways (NF-κB, p38, and ERK) are activated. Moreover, TLR-activated 661W cells secreted significant amounts of inflammatory mediators (IL-6, IL-1β, MIP-2, and KC) in their culture supernatant, as assessed by ELISA. A similar trend was observed in 661W cells challenged with live bacteria (Staphylococcus aureus). Interestingly, the neutralization of TLR2, a major receptor for S. aureus recognition, did not significantly attenuate bacterial-induced inflammatory mediators, suggesting the existence of TLR2-independent mechanisms in photoreceptor cells. Together, these results indicate that photoreceptors constitutively express functional TLRs and possess the ability to initiate innate responses following pathogen challenge, implicating their role in retinal innate immunity.

Systemic injection of low-dose lipopolysaccharide fails to break down the blood-brain barrier or activate the TLR4-MyD88 pathway in neonatal rat brain

We aimed to investigate whether peripheral low-dose lipopolysaccharide (LPS) induces the breakdown of the blood–brain barrier (BBB) and/or the activation of toll-like receptor 4 (TLR4) in the neonatal rat brain. Neonatal rats received intraperitoneal injections of low-dose LPS (0.3 mg/kg∙bw), and the BBB compromise was detected by Evans Blue extravasation and electron microscopy. Meanwhile, TLR4, adaptin myeloid differentiation factor 88 (MyD88), nuclear transcription factor kappa-B (NF-κB) p50 and tumor necrosis factor alpha (TNFα) in the neonatal rat brain were determined by quantitative real-time polymerase chain reaction (PCR) and Western Blot. Immunohistochemistry was used to determine the distribution and activation of microglia in the brain after LPS administration. It was demonstrated that Evans Blue extravasation was not observed in the brain parenchyma, and that tight junctions of cerebral endothelial cells remained intact after systemic injections of LPS in neonatal rats. Although intracerebroventricular injections of LPS activated microglia and up-regulated the expression of TLR4, MyD88, NF-κB p50 and TNFα in the neonatal rat brain, systemic LPS did not induce these responses. These findings indicate that while the neonatal rat brain responds to the direct intra-cerebral administration of LPS through robust TLR4 activation, systemic low-dose LPS does not induce the innate immune reaction or compromise the BBB in neonatal rats.

Vibrio vulnificus MO6-24/O lipopolysaccharide stimulates superoxide anion, thromboxane B₂, matrix metalloproteinase-9, cytokine and chemokine release by rat brain microglia in vitro

Although human exposure to Gram-negative Vibrio vulnificus (V. vulnificus) lipopolysaccharide (LPS) has been reported to result in septic shock, its impact on the central nervous system's innate immunity remains undetermined. The purpose of this study was to determine whether V. vulnificus MO6-24/O LPS might activate rat microglia in vitro and stimulate the release of superoxide anion (O₂⁻), a reactive oxygen species known to cause oxidative stress and neuronal injury in vivo. Brain microglia were isolated from neonatal rats, and then treated with either V. vulnificus MO6-24/O LPS or Escherichia coli O26:B6 LPS for 17 hours in vitro. O₂⁻ was determined by cytochrome C reduction, and matrix metalloproteinase-2 (MMP-2) and MMP-9 by gelatinase zymography. Generation of cytokines tumor necrosis factor alpha (TNF-α), interleukin-1 alpha (IL-1α), IL-6, and transforming growth factor-beta 1 (TGF-β1), chemokines macrophage inflammatory protein (MIP-1α)/chemokine (C-C motif) ligand 3 (CCL3), MIP-2/chemokine (C-X-C motif) ligand 2 (CXCL2), monocyte chemotactic protein-1 (MCP-1)/CCL2, and cytokine-induced neutrophil chemoattractant-2alpha/beta (CINC-2α/β)/CXCL3, and brain-derived neurotrophic factor (BDNF), were determined by specific immunoassays. Priming of rat microglia by V. vulnificus MO6-24/O LPS in vitro yielded a bell-shaped dose-response curve for PMA (phorbol 12-myristate 13-acetate)-stimulated O₂⁻ generation: (1) 0.1-1 ng/mL V. vulnificus LPS enhanced O₂⁻ generation significantly but with limited inflammatory mediator generation; (2) 10-100 ng/mL V. vulnificus LPS maximized O₂⁻ generation with concomitant release of thromboxane B2 (TXB2), matrix metalloproteinase-9 (MMP-9), and several cytokines and chemokines; (3) 1000-100,000 ng/mL V. vulnificus LPS, with the exception of TXB2, yielded both attenuated O₂⁻ production, and a progressive decrease in MMP-9, cytokines and chemokines investigated. Thus concentration-dependent treatment of neonatal brain microglia with V. vulnificus MO6-24/O LPS resulted in a significant rise in O₂⁻ production, followed by a progressive decrease in O₂⁻ release, with concomitant release of lactic dehydrogenase (LDH), and generation of TXB2, MMP-9, cytokines and chemokines. We hypothesize that the inflammatory mediators investigated may be cytotoxic to microglia in vitro, by an as yet undetermined autocrine mechanism. Although V. vulnificus LPS was less potent than E. coli LPS in vitro, inflammatory mediator release by the former was clearly more efficacious. Finally, we hypothesize that should V. vulnificus LPS gain entry into the CNS, it would be possible that microglia might become activated, resulting in high levels of O₂⁻ as well as neuroinflammatory TXB2, MMP-9, cytokines and chemokines.

Silencing of PrP C (prion protein) expression does not affect Brucella melitensis infection in human derived microglia cells

Cellular prion proteins (PrP(C)) are mainly expressed in the central nervous system where they have antioxidant effects and a role in the endocytosis of bacteria within cells. These proteins also have some crucial biological functions including roles in neurotransmission, signal transduction and programmed cell death. However, the role of prion proteins in neuronal Brucella infection, specifically in the interaction of the pathogen and the host cell is controversial. In the present study, the silencing of PrP(C) mRNA by small interfering RNA (siRNA) transfection was investigated in human microglia cells infected with Brucella melitensis. More than 70% of prion proteins were down-regulated in microglia by siRNA transfection and this caused a slight decrease in the cellular viability of the control cells. Silencing of PrP(C) suppressed the antioxidant systems, though it led to an up-regulation of pro-inflammatory cytokines such as IL-12 and TNF-α as demonstrated by qRT-PCR analysis. B. melitensis infection of prion protein-silenced cells led to increase host viability, but had no effect on bacterial phagocytosis. According to the present study, there is no significant effect of prion proteins on phagocytosis and intracellular killing of B. melitensis in microglia cells.

[Intestinal-brain axis. Neuronal and immune-inflammatory mechanisms of brain and intestine pathology]

Mutually directed connections between intestine and brain are implemented by endocrine, neural and immune systems and nonspecific natural immunity. Intestine micro flora as an active participant of intestine-brain axis not only influences intestine functions but also stimulates the development of CNS in perinatal period and interacts with higher nervous centers causing depression and cognitive disorders in pathology. A special role belongs to intestine microglia. Apart from mechanic (protective) and trophic functions for intestine neurons, glia implements neurotransmitter, immunologic, barrier and motoric functions in the intestine. An interconnection between intestine barrier function and hematoencephalic barrier regulation exists. Chronic endotoxinemia as a result of intestine barrier dysfunction forms sustained inflammation state in periventricular zone of the brain with consequent destabilization of hematoencephalic barriers and spread oF inflammation to other parts of the brain resulting in neurodegradation development.

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

We have recently demonstrated that microglia and astrocytes express nucleotide-binding oligomerization domain-2 (NOD2), a novel cytosolic pattern recognition receptor for bacterial motifs, and we have shown that this intracellular receptor is essential for glial responses to Gram-negative pathogens. Here, we demonstrate that intact Staphylococcus aureus, a major Gram-positive causative agent of brain abscesses, activates the transcription factor NF-kappaB and is a potent stimulus for inflammatory cytokine production in primary murine microglia and astrocytes. Interestingly, we demonstrate that NOD2 is essential for maximal glial responses to intact S. aureus, but not cellular lysates. As such, this data indicates that NOD2 plays an important role in initiating inflammatory mediator production by resident brain cells following S. aureus infection and we suggest that this cytosolic receptor acts in conjunction with cell surface pattern recognition receptors to elicit maximal glial responses.

Experimental studies of pneumococcal meningitis

This thesis summarizes experimental meningitis research conducted at Statens Serum Institut in collaboration with the Copenhagen HIV programme and the Danish Research Centre for Magnetic Resonance between 2001 and 2007. Previous experimental studies had shown that the host inflammatory response in invasive infections contributed significantly to an extremely poor outcome despite initiation of efficient antimicrobial chemotherapy. Consequently, we aimed to investigate and clarify how the course of disease in pneumococcal meningitis was modulated by local meningeal inflammation and concomitant systemic infection and inflammation. Experimental studies were based on the development of a rat model of pneumococcal meningitis, refined and optimized to closely resemble the human disease, mimicking disease severity, outcome, focal- and global brain injury and brain pathophysiology. These endpoints were evaluated by the development of a clinical score system, definition of outcomes and measurement of hearing loss by otoacoustic emission. The investigation of in-vitro and in-vivo brain pathology with histology and MRI revealed an injury pattern similar to that found clinically. Additionally, MRI enabled the study of parameters closely related to the cerebral pathophysiology of meningitis (brain oedema, blood brain barrier (BBB) permeability, focal brain injury and hydrocephalus). Modulation of the inflammatory host response was achieved by initiation of treatment prior to infection: 1) G-CSF treatment increased the peripheral availability of leukocytes, 2) Selectin blocker fucoidin attenuated meningeal leukocyte accumulation and 3) A serotype specific Ab augmented systemic pneumococcal phagocytosis. The studies revealed a dual role of the inflammatory response in pneumococcal meningitis. Whilst focal brain injury appeared to result from local meningeal infectious processes, clinical disease severity and outcome appeared determined by systemic infection. Furthermore systemic disease contributed significantly to BBB permeability and brain ventricle expansion. Ventricle expansion was also associated with clinical appearance. An augmented systemic host response limited pneumococcal bacteraemia and protected from fatal outcome, but did not reduce occurrence of focal brain injury. Thus, our findings suggest that meningitis sequelae arise from local disease complications whereas fatal outcome is accelerated by systemic infection. Understanding of the relationship and interplay between septicaemia, intracranial pressure, ventricle expansion and brain edema could help optimize the treatment of these disease complications by, for example, improved systemic infection control. New therapeutic approaches to improve survival and neurological outcome from pneumococcal meningitis may be achieved through identification of the pathogen factors that initiate and prolong extensive systemic and local inflammation. Investigation of genomic differences and protein expression between pneumococcal serotypes or between identical serotypes with different virulence are considered crucial to this progress. Future progress may also be achieved by disease prevention with pneumococcal vaccines. Randomized trials of treatment strategies including bacteriostatic agents, antioxidants or more specific anti-inflammatory agents are realistic possibilities in the near future.

Microglia are mediators of Borrelia burgdorferi-induced apoptosis in SH-SY5Y neuronal cells

Inflammation has long been implicated as a contributor to pathogenesis in many CNS illnesses, including Lyme neuroborreliosis. Borrelia burgdorferi is the spirochete that causes Lyme disease and it is known to potently induce the production of inflammatory mediators in a variety of cells. In experiments where B. burgdorferi was co-cultured in vitro with primary microglia, we observed robust expression and release of IL-6 and IL-8, CCL2 (MCP-1), CCL3 (MIP-1α), CCL4 (MIP-1β) and CCL5 (RANTES), but we detected no induction of microglial apoptosis. In contrast, SH-SY5Y (SY) neuroblastoma cells co-cultured with B. burgdorferi expressed negligible amounts of inflammatory mediators and also remained resistant to apoptosis. When SY cells were co-cultured with microglia and B. burgdorferi, significant neuronal apoptosis consistently occurred. Confocal microscopy imaging of these cell cultures stained for apoptosis and with cell type-specific markers confirmed that it was predominantly the SY cells that were dying. Microarray analysis demonstrated an intense microglia-mediated inflammatory response to B. burgdorferi including up-regulation in gene transcripts for TLR-2 and NFκβ. Surprisingly, a pathway that exhibited profound changes in regard to inflammatory signaling was triggering receptor expressed on myeloid cells-1 (TREM1). Significant transcript alterations in essential p53 pathway genes also occurred in SY cells cultured in the presence of microglia and B. burgdorferi, which indicated a shift from cell survival to preparation for apoptosis when compared to SY cells cultured in the presence of B. burgdorferi alone. Taken together, these findings indicate that B. burgdorferi is not directly toxic to SY cells; rather, these cells become distressed and die in the inflammatory surroundings generated by microglia through a bystander effect. If, as we hypothesized, neuronal apoptosis is the key pathogenic event in Lyme neuroborreliosis, then targeting microglial responses may be a significant therapeutic approach for the treatment of this form of Lyme disease.

Lyme disease, which is transmitted to humans through the bite of a tick, is currently the most frequently reported vector-borne illness in the northern hemisphere. Borrelia burgdorferi is the bacterium that causes Lyme disease and it is known to readily induce inflammation within a variety of infected tissues. Many of the neurological signs and symptoms that may affect patients with Lyme disease have been associated with B. burgdorferi-induced inflammation in the central nervous system (CNS). In this report we investigated which of the primary cell types residing in the CNS might be functioning to create the inflammatory environment that, in addition to helping clear the pathogen, could simultaneously be harming nearby neurons. We report findings that implicate microglia, a macrophage cell type in the CNS, as the key responders to infection with B. burgdorferi. We also present evidence indicating that this organism is not directly toxic to neurons; rather, a bystander effect is generated whereby the inflammatory surroundings created by microglia in response to B. burgdorferi may themselves be toxic to neuronal cells.

The antibiotics doxycycline and minocycline inhibit the inflammatory responses to the Lyme disease spirochete Borrelia burgdorferi

Tetracyclines moderate inflammatory responses of various etiologies. We hypothesized that tetracyclines, in addition to their antimicrobial function, could exert control over the inflammation elicited by Borrelia burgdorferi. To model systemic effects, we used the human monocytic cell line THP-1; to model effects in the central nervous system, we used rhesus monkey brain astrocytes and microglia. Cells were stimulated with live or sonicated B. burgdorferi or with the lipoprotein outer surface protein A in the presence of increasing concentrations of doxycycline or minocycline. Both antibiotics significantly reduced the production of tumor necrosis factor-alpha, interleukin (IL)-6, and IL-8 in a dose-dependent manner in all cell types. Microarray analyses of the effect of doxycycline on gene transcription in spirochete-stimulated monocytes revealed that the NFKB and CHUK (alias, IKKA) genes were down-regulated. Functionally, phosphorylation of IkappaBalpha and binding of NF-kappaB to target DNA were both reduced in these cells. Our results suggest that tetracyclines may have a dual therapeutic effect in Lyme disease.

NOD2 plays an important role in the inflammatory responses of microglia and astrocytes to bacterial CNS pathogens

While glial cells are recognized for their roles in maintaining neuronal function, there is growing appreciation that resident central nervous system (CNS) cells initiate and/or augment inflammation following trauma or infection. We have recently demonstrated that microglia and astrocytes constitutively express nucleotide-binding oligomerization domain-2 (NOD2), a member of the novel nucleotide-binding domain leucine-rich repeat region containing a family of proteins (NLR) that functions as an intracellular receptor for a minimal motif present in all bacterial peptidoglycans. In this study, we have confirmed the functional nature of NOD2 expression in astrocytes and microglia and begun to determine the relative contribution that this NLR makes in inflammatory CNS responses to clinically relevant bacterial pathogens. We demonstrate the increased association of NOD2 with its downstream effector molecule, Rip2 kinase, in primary cultures of murine microglia and astrocytes following exposure to bacterial antigens. We show that this cytosolic receptor underlies the ability of muramyl dipeptide to augment the production of inflammatory cytokines by glia following exposure to specific ligands for disparate Toll-like receptor homologues. In addition, we demonstrate that NOD2 is an important component in the in vitro inflammatory responses of resident glia to N. meningitidis and B. burgdorferi antigens. Finally, we have established that NOD2 is required, at least in part, for the astrogliosis, demyelination, behavioral changes, and elevated inflammatory cytokine levels observed following in vivo infection with these pathogens. As such, we have identified NOD2 as an important component in the generation of damaging CNS inflammation following bacterial infection.

Chlamydia pneumoniae infection of brain cells: An in vitro study

Inspired by the suggested associations between neurological diseases and infections, we determined the susceptibility of brain cells to Chlamydia pneumoniae (Cpn). Murine astrocyte (C8D1A), neuronal (NB41A3) and microglial (BV-2) cell lines were inoculated with Cpn. Infection was established by immunofluorescence and real-time PCR at various time points. Productive infection was assessed by transferring medium of infected cells to a detection layer. Finally, apoptosis and necrosis post-infection was determined. Our data demonstrate that the neuronal cell line is highly sensitive to Cpn, produces viable progeny and is prone to die after infection by necrosis. Cpn tropism was similar in an astrocyte cell line, apart from the higher production of extracellular Cpn and less pronounced necrosis. In contrast, the microglial cell line is highly resistant to Cpn as the immunohistochemical signs almost completely disappeared after 24 h. Nevertheless, significant Cpn DNA amounts could be detected, suggesting Cpn persistence. Low viable progeny and hardly any necrotic microglial cells were observed. Further research is warranted to determine whether these cell types show the same sensitivity to Cpn in an in vivo setting.

Noninfectious entry of HIV-1 into peripheral and brain macrophages mediated by the mannose receptor

Although protein receptors on the plasma membrane involved in the initial steps of productive HIV-1 infection have been well characterized, little is known about interactions between cellular carbohydrate receptors and HIV-1. Here, we report the involvement of a carbohydrate receptor, the macrophage mannose receptor (MR), and its role in supporting HIV-1 binding and entry. HIV-1 can enter the cytoplasm of human macrophages and microglia as well as murine macrophages by MR, although no subsequent viral replication was observed. Correspondingly, HIV-1 entry into Cos-7 cells after induction of expression of MR by transfection with MR-cDNA did not demonstrate viral replication. Our studies suggest that whereas MR may serve as a binding and an entry site, the MR-mediated pathway does not lead to productive HIV-1 infection. In addition, we report that recombinant HIV-1 gp120 blocks MR-mediated phagocytosis in human and murine alveolar macrophages and microglial cells. Therefore, characterization of the HIV-1 noninfectious MR-mediated phagocytic pathway may foster advances in HIV-1 vaccine design and an improved understanding of HIV-1/AIDS pathogenesis and host defenses.

Inflammatory responses following Chlamydia pneumoniae infection of glial cells

Recently, infections have been implicated in the pathogenesis of Alzheimer's disease. Apart from the direct effects of pathogens, it can be hypothesized that inflammatory mechanisms, such as the production of pro-inflammatory mediators by resident glia, may result in neurotoxicity. Here, we examined the inflammatory responses in murine microglial cell (MMC) and murine astrocyte cell (MAC) lines following infection with Chlamydia pneumoniae (Cpn), a pathogen that has recently been associated with Alzheimer's disease. Furthermore, we determined whether these inflammatory responses are sufficient to cause neuronal cell death in vitro. MMCs and MACs were infected with Cpn. Subsequently, various chemo- and cytokines were determined in the culture supernatant fluid of infected/control cells at different time points post-infection. Significantly higher levels of monocyte chemoattractant protein 1, interleukin (IL)-6, tumour necrosis factor (TNF)-alpha and IL-1beta were found in supernatant fluids of infected MMCs compared with controls. In contrast, in the supernatant fluid of infected MACs, only monocyte chemoattractant protein 1 and IL-6 displayed significantly higher levels compared with controls. Moreover, neurotoxicity was examined up to 72 h after transferring the conditioned supernatant fluid to a neuronal cell layer. No neuronal cell death was observed when supernatant fluids from infected/mock-treated MACs were transferred. However, when neurones were exposed to conditioned supernatant fluid from infected MMCs, a significant increase in cell death was observed compared with mock. Furthermore, adding neutralizing antibodies against IL-6 and TNF-alpha to that conditioned supernatant fluid prevented neuronal cell death by approximately 50%. In conclusion, these data suggest that Cpn infection results in a pro-inflammatory milieu, particularly by activating MMCs, that ultimately results in neurodegeneration with prominent roles for both IL-6 and TNF-alpha.

Adaptive response of microglial cells to in vitro infection by Candida albicans isolates with different genomic backgrounds

It has been recently demonstrated that Candida albicans isolates with distinct genomic backgrounds (namely, b and c genotypes) express different susceptibility to antifungal activity by human monocytes in vitro. We show here that, although comparable in their ability to undergo dimorphic transition and in susceptibility to phagocytosis by microglial cells, the b and c isolates show striking differences in terms of intracellular survival. Only the c genotype resists indeed to intracellular killing and eventually replicates inside microglial cells, that in turn respond to fungal infection, preferentially towards the c genotype, with nuclear factor-kappaB (NF-kappaB) activation and increased Mip1alpha production. These data indicate that C. albicans-microglial cell interaction is strictly dependent upon fungal genotype, strengthening the potential significance of genotyping as prognostic parameter in clinical infections by C. albicans.

Chlamydophila (Chlamydia) pneumoniae Infection of Human Astrocytes and Microglia in Culture Displays an Active, Rather than a Persistent, Phenotype

BACKGROUND

The intracellular pathogen Chlamydia pneumoniae can cause persistent infections during which its morphologic, molecular, and pathogenic characteristics differ importantly from those of active infection. This bacterium was identified within astrocytes and microglia in the brain of late-onset Alzheimer disease patients. We investigated whether infection of these two host cell types displays an active or persistent growth phenotype.

METHODS

The human astrocytoma and microglioma cell lines U-87 MG and CHME-5 (respectively) and the human epithelial cell line HEp-2 were infected by the standard method with C pneumoniae strain AR-39. Cultures were harvested at 24, 48, and 72 hours postinfection and subjected to analysis of inclusion morphology. DNA and RNA were prepared from portions of each infected culture sample and analyzed for relative chromosome accumulation and presence or absence of several specific bacterial mRNAs.

RESULTS

Astrocytes and microglial cells infected in vitro with C pneumoniae displayed inclusions that were indistinguishable from those characteristic of active infection of the standard HEp-2 host cell line. Real time polymerase chain reaction (PCR) showed that the relative accumulation of chlamydial chromosome over time during infection of these two cell lines also was virtually identical to that in actively infected HEp-2 cells. Reverse transcriptase PCR (RT-PCR) analyses showed that mRNA from ftsK, pyk, and other chlamydial genes whose expression is abrogated during persistent infection were easily identifiable in infected CHME-5 and U-87 MG cells.

CONCLUSIONS

In cultured human astrocytes and microglia, C pneumoniae displays an active, not a persistent, growth phenotype. This indicates normal passage through the developmental cycle with its probable concomitant destruction by lysis of some portion of host cells at the termination of that cycle.

A mechanism for neurodegeneration induced by group B streptococci through activation of the TLR2/MyD88 pathway in microglia

Group B Streptococcus (GBS) is a major cause of bacterial meningitis and neurological morbidity in newborn infants. The cellular and molecular mechanisms by which this common organism causes CNS injury are unknown. We show that both heat-inactivated whole GBS and a secreted proteinaceous factor from GBS (GBS-F) induce neuronal apoptosis via the activation of murine microglia through a TLR2-dependent and MyD88-dependent pathway in vitro. Microglia, astrocytes, and oligodendrocytes, but not neurons, express TLR2. GBS as well as GBS-F induce the synthesis of NO in microglia derived from wild-type but not TLR2(-/-) or MyD88(-/-) mice. Neuronal death in neuronal cultures complemented with wild-type microglia is NO-dependent. We show for the first time a TLR-mediated mechanism of neuronal injury induced by a clinically relevant bacterium. This study demonstrates a causal molecular relationship between infection with GBS, activation of the innate immune system in the CNS through TLR2, and neurodegeneration. We suggest that this process contributes substantially to the serious morbidity associated with neonatal GBS meningitis and may provide a potential therapeutic target.

Borrelia burgdorferi Induces TLR1 and TLR2 in human microglia and peripheral blood monocytes but differentially regulates HLA-class II expression

The spirochete Borrelia burgdorferi is the agent of Lyme disease, which causes central nervous system manifestations in up to 20% of patients. We investigated the response of human brain microglial cells, glial progenitors, neurons, astrocytes, as well as peripheral blood monocytes to stimulation with B. burgdorferi. We used oligoarrays to detect changes in the expression of genes important for shaping adaptive and innate immune responses. We found that stimulation with B. burgdorferi lysate increased the expression of Toll-like receptors (TLRs) 1 and 2 in all cell types except neurons. However, despite similarities in global gene profiles of monocytes and microglia, only microglial cells responded to the stimulation with a robust increase in HLA-DR, HLA-DQ, and also coexpressed CD11-c, a dendritic cell marker. In contrast, a large number of HLA-related molecules were repressed at both the RNA and the protein levels in stimulated monocytes, whereas secretion of IL-10 and TNF-alpha was strongly induced. These results show that signaling through TLR1/2 in response to B. burgdorferi can elicit opposite immunoregulatory effects in blood and in brain immune cells, which could play a role in the different susceptibility of these compartments to infection.

Central role for MyD88 in the responses of microglia to pathogen-associated molecular patterns

Microglia, the innate immune effector cells of the CNS parenchyma, express TLR that recognize conserved motifs of microorganisms referred to as pathogen-associated molecular patterns (PAMP). All TLRs identified to date, with the exception of TLR3, use a common adaptor protein, MyD88, to transduce activation signals. Recently, we reported that microglial activation in response to the Gram-positive bacterium Staphylococcus aureus was not completely attenuated following TLR2 ablation, suggesting the involvement of additional receptors. To assess the functional role of alternative TLRs in microglial responses to S. aureus and its cell wall product peptidoglycan as well as the Gram-negative PAMP LPS, we evaluated primary microglia from MyD88 knockout (KO) and wild-type mice. The induction of TNF-alpha, IL-12 p40, and MIP-2 (CXCL2) expression by S. aureus- and peptidoglycan-stimulated microglia was MyD88 dependent, as revealed by the complete inhibition of cytokine production in MyD88 KO cells. In addition, the expression of additional pattern recognition receptors, including TLR9, pentraxin-3, and lectin-like oxidized LDL receptor-1, was regulated, in part, via a MyD88-dependent manner as demonstrated by the attenuated expression of these receptors in MyD88 KO microglia. Microglial activation was only partially inhibited in LPS-stimulated MyD88 KO cells, suggesting the involvement of MyD88-independent pathways. Collectively, these findings reveal the complex mechanisms for microglia to respond to diverse bacterial pathogens, which occur via both MyD88-dependent and -independent pathways.

Chlamydia pneumoniae infection of microglial cells in vitro: a model of microbial infection for neurological disease

Chlamydia pneumoniae is the aetiological cause of a wide variety of chronic inflammatory diseases and may be associated with neurological disease. Microbiological and immunological aspects of the interaction between C. pneumoniae and the central nervous system (CNS) are not well understood because of the lack of a suitable infection model for neuronal studies. In the present study, an in vitro C. pneumoniae infection model was developed in the established microglial cell line EOC 20. Infection of the cells resulted in obvious induction of proinflammatory cytokines. The infection also selectively induced matrix metalloproteinase-9 (MMP-9) but not MMP-2. Moreover, beta interferon, which is known to modulate CNS disease, inhibited induction of MMP-9 following C. pneumoniae infection. These results support the view that C. pneumoniae infection may be associated with marked alteration of the ability of microglial cells to enhance cytokine production as well as induction of an MMP.

Interaction of leptospires with murine microglial cells

Leptospires, the agents of leptospirosis, exert tropism for the central nervous system, in the course of mammal infection. We evaluated the interaction between murine microglial cells and strains of pathogenic L. interrogans leptospires and non-pathogenic L. biflexa leptospires, mainly by flow cytometric assays. In the absence of opsonic conditions microglia are capable of ingesting--even quite slowly--the spirochetes and killing the non-pathogenic strain. The adhesion to microglia, which is quick and relevant for all the strains, does not involve the CR3 integrin receptor. These findings suggest that the murine microglia--in non opsonic conditions in vitro--do not effectively clear the pathogenic leptospires.

Innate immune response gene expression profiles of N9 microglia are pathogen-type specific

Glial cells, particularly microglia, are thought to play a pivotal role in initiating and guiding innate immune responses to CNS infections and in perpetuating inflammation and pathology in CNS diseases such as multiple sclerosis and Alzheimer's disease. We describe here the development and use of a new microarray designed to specifically profile transcript expression of innate immunity genes. Microarray analysis validated by quantitative PCR demonstrated an extensive range of pattern recognition receptor gene expression in resting N9 microglia, including Toll-like receptors, scavenger receptors and lectins. Stimulation with LPS or infection with virus modulated pattern recognition receptor, cytokine, chemokine and other innate immune transcripts in a distinct and stimulus-specific manner. This study demonstrates that a single glial cell phenotype has an innate capability to detect infection, determine its form and generate specific responses.

Mycobacterium tuberculosis-induced cytokine and chemokine expression by human microglia and astrocytes: effects of dexamethasone

Although corticosteroids are recommended as adjunctive therapy for tuberculous meningitis, the mechanism underlying their beneficial effect is poorly understood. In this study, human microglia and astrocytes were infected with Mycobacterium tuberculosis H37Rv, and cytokine and chemokine expression was examined with and without dexamethasone treatment. Microglia were the principal cells infected by tubercle bacilli, which elicited robust amounts of several cytokines and chemokines. Treatment with dexamethasone markedly suppressed production of these mediators. The results of this study support the concept that microglia play an important role in neuropathogenesis of tuberculosis and that dexamethasone could operate via modulation of the production of proinflammatory cytokines and chemokines by these brain macrophages.

Increased Expression of BDNF and Proliferation of Dentate Granule Cells After Bacterial Meningitis

Proliferation and differentiation of neural progenitor cells is increased after bacterial meningitis. To identify endogenous factors involved in neurogenesis, expression of brain-derived neurotrophic factor (BDNF), TrkB, nerve growth factor (NGF), and glial cell line-derived neurotrophic factor (GDNF) was investigated. C57BL/6 mice were infected by intracerebral injection of Streptococcus pneumoniae. Mice were killed 30 hours later or treated with ceftriaxone and killed 4 days after infection. Hippocampal BDNF mRNA levels were increased 2.4-fold 4 days after infection (p = 0.026). Similarly, BDNF protein levels in the hippocampal formation were higher in infected mice than in control animals (p = 0.0003). This was accompanied by an elevated proliferation of dentate granule cells (p = 0.0002). BDNF protein was located predominantly in the hippocampal CA3/4 area and the hilus of the dentate gyrus. The density of dentate granule cells expressing the BDNF receptor TrkB as well as mRNA levels of TrkB in the hippocampal formation were increased 4 days after infection (p = 0.027 and 0.0048, respectively). Conversely, NGF mRNA levels at 30 hours after infection were reduced by approximately 50% (p = 0.004). No significant changes in GDNF expression were observed. In conclusion, increased synthesis of BDNF and TrkB suggests a contribution of this neurotrophic factor to neurogenesis after bacterial meningitis.

Estradiol is required for a proper immune response to bacterial and viral pathogens in the female brain

Although the neuroprotective effects of estrogens are well recognized, the exact mechanisms involved in the ability of these sex steroids to protect the cerebral tissue still remain unclear. We tested in our study the hypothesis that estradiol (E(2)) modulates the innate immune response and expression of genes encoding proteins that a provide survival signal to neurons during infection. Mice received a single systemic or cerebral injection of LPS to trigger a robust but transient inflammatory reaction in the brain. The endotoxin increased transcriptional activation of genes encoding TLR2, TNF-alpha, and IL-12 in microglial cells. Expression of these transcripts was largely inhibited in the brain of ovariectomized mice at time 24 h postchallenge. E(2) replacement therapy totally rescued the ability of the endotoxin to trigger microglial cells and these permissive effects of E(2) are mediated via the estrogen receptor (ER)alpha. Indeed, ERalpha-deficient mice exhibited an inappropriate reaction to LPS when compared with ERbeta-deficient and wild-type mice. This defective innate immune response was also associated with a widespread viral replication and neurodegeneration in ovariectomized mice inoculated intranasally with HSV-2. These data provide evidence that interaction of E(2) with their nuclear ERalpha plays a critical role in the control of cytokines involved in the transfer from the innate to adaptive immunity. This transfer is deviant in mice lacking E(2), which allows pathogens to hide from immune surveillance and exacerbates neuronal damages during viral encephalitis.

The lack of Pneumococcal surface protein C (PspC) increases the susceptibility of Streptococcus pneumoniae to the killing by microglia

Microglial cells, the resident phagocytes in the brain, share many phenotypical and functional characteristics with peripheral macrophages, suggesting that they may participate in an innate immune response against microorganisms invading the central nervous system (CNS). In this study, we demonstrate that the microglial cells constitutively exhibit antibacterial activity in vitro against Streptococcus pneumoniae. By using a Pneumococcal surface protein C (PspC)-deleted strain and its wild-type counterpart, we found that the extent of such an activity is significantly influenced by the presence of a PspC molecule on the bacterial surface. The PspC- mutant FP20 is indeed more susceptible than the PspC+ strain HB565 to microglial killing. Interestingly, this phenomenon is observed when using a medium supplemented with heat-inactivated foetal bovine serum (FBS). Electron microscopy studies indicate that the microglial cells interact more efficiently with PspC- than with PspC+ pneumococci. Moreover, upon infection with the PspC- mutant, microglial cells produce levels of TNF-alpha, MIP-2, IL-10 and nitric oxide, significantly higher than those observed with PspC+ bacteria. These findings indicate that the lack of PspC significantly enhances the susceptibility of S. pneumoniae to both bactericidal activity and secretory response by the microglial cells, suggesting that this molecule may play an important role in the invasion of CNS by pneumococcus.

S. aureus-dependent microglial activation is selectively attenuated by the cyclopentenone prostaglandin 15-deoxy-Delta12,14- prostaglandin J2 (15d-PGJ2)

Microglial activation is a hallmark of brain abscess. The continual release of proinflammatory mediators by microglia following bacterial challenge may contribute, in part, to the destruction of surrounding normal tissue characteristic of brain abscess. Therefore, attenuating chronic microglial activation during the course of CNS bacterial infections may have therapeutic benefits. The purpose of this study was to evaluate the ability of the natural peroxisome proliferator-activated receptor (PPAR)-gamma agonist 15-deoxy-Delta12,14- prostaglandin J2 (15d-PGJ2) to modulate microglial activation in response to Staphylococcus aureus, one of the main etiologic agents of brain abscess in humans. 15d-PGJ2 was a potent inhibitor of proinflammatory cytokine (IL-1beta, TNF-alpha, IL-12 p40) and CC chemokine (MIP-1beta, MCP-1) production in primary microglia, but had no effect upon the expression of select CXC chemokines (MIP-2, KC). 15d-PGJ2 also selectively inhibited the S. aureus-dependent increase in microglial TLR2, CD14, MHC class II, and CD40 expression, whereas it had no effect on the co-stimulatory molecules CD80 and CD86. Microarray analysis revealed additional inflammatory mediators modulated by 15d-PGJ2 in primary microglia following S. aureus exposure, the majority of which were chemokines. These results suggest that suppressing microglial activation through the use of 15d-PGJ2 may lead to the sparing of damage to normal brain parenchyma that often results from brain abscess.

Inhibition of cytokines expression in human microglia infected by virulent and non-virulent mycobacteria

The pathogenesis of tuberculosis (TBC) meningitis is still unknown. As shown by previous studies, human microglia can be the target of mycobacteria, but no data are available about their cellular response to infection. Consequently, we studied the expression of tumor necrosis factor-alpha (TNF-alpha), interleukin-1 (IL-1) and IL-10 in human microglia pure cultures infected with the two variants of Mycobacterium avium (domed-opaque (SmD) and transparent (SmT)) and with Mycobacterium tuberculosis. Results showed that microglia was productively infected by mycobacteria which could grow inside the cells. Mycobacteria internalization was more rapid for M. avium, but M. tuberculosis infection turned out to be more efficient due to the incorporation of densely packed bacteria. TNF-alpha expression was not affected by M. avium, whereas an increase followed by a decrease was observed in M. tuberculosis. Both IL-1 and IL-10 cytokine expression was rapidly inhibited by infection with the more virulent bacteria, whereas the non-pathogenic one had almost no effect. Also, the expression of the co-stimulatory molecule CD137, a member of tumor necrosis factor receptor family, was affected by infection with virulent mycobacteria. Our results show that microglia response to mycobacterial infection is modulated in correlation with virulence, mainly toward inhibition of inflammatory response. This observation might be one of the mechanisms by which non-pathogenic mycobacteria are quickly eliminated, explaining one of the bases of virulence.

Substance P Augments Borrelia burgdorferi-Induced Prostaglandin E2 Production by Murine Microglia

Substance P is a ubiquitous CNS neuropeptide and has recently been demonstrated to augment immune cell function during inflammatory events. Central to the ability of substance P to modulate immune cell function is the interaction of substance P with the substance P neurokinin-1 receptor expressed by a variety of immune cells, including microglia. CNS involvement during Lyme disease can occur when Borrelia burgdorferi, the causative agent of Lyme disease, gains access to the CNS. In the present study, we demonstrate that substance P augments B. burgdorferi-induced expression of mRNA encoding COX-2 and subsequent secretion of PGE(2) by cultured, murine microglia. Furthermore, this effect is associated with the ability of substance P to enhance B. burgdorferi-induced NF-kappa B activation, as demonstrated by increased nuclear localization of the p65 (RelA) subunit of NF-kappa B in these cells. Interestingly, we demonstrate that substance P augments B. burgdorferi-induced expression of mRNA encoding two PGE(2) receptors, E-prostanoid receptor subtypes 2 and 4, as well as each receptor protein. In addition, these effects are mediated via interactions between substance P and its high affinity receptor, as evidenced by the absence of augmented PGE(2) synthesis in the presence of a specific neurokinin-1 receptor antagonist or in cells genetically deficient in the expression of these receptors. Taken together, the present demonstration that substance P can exacerbate B. burgdorferi-induced inflammatory responses in microglia in vitro may indicate a role for this neuropeptide in the development of CNS inflammation observed during human neuroborreliosis.

Phagocytosis of Escherichia coli by amoeboid microglial cells in the developing brain

Amoeboid microglial cells (AMC) in the corpus callosum of 1-day-old rats receiving a single intracerebral injection of Escherichia coli ( E. Coli) were examined at various time intervals following the injection. A large number of E. coli were internalized by the AMC at 1-3 h after the injection. However, no E. coli were identifiable at 1 day after the injection, but large phagosomes were observed in the cytoplasm of AMC. With time, the phagosomes in the AMC were reduced so that by 7 days the cells appeared comparable to the controls. Apoptotic or necrotic AMC were not encountered during the study period. This is consistent with the results of cell counts, which showed no significant change in the AMC population following E. coli injection compared with controls. The present results suggest that AMC are capable of removing live bacteria from their vicinity. Up-regulation of complement type 3 receptors and induction of major histocompatibility complex class II antigens were observed in the AMC at days 1-3 and 7 following E. coli administration. This may be related to their involvement in mediating endocytosis and their possible role in antigen presentation.

Fcγ receptor signaling in primary human microglia: differential roles of PI-3K and Ras/ERK MAPK pathways in phagocytosis and chemokine induction

Cryptococcus neoformans monoclonal antibody immune complex (IC) induces beta-chemokines and phagocytosis in primary human microglia via activation of Fc receptor for immunoglobulin G (FcgammaR). In this report, we investigated microglial FcgammaR signal-transduction pathways by using adenoviral-mediated gene transfer and specific inhibitors of cell-signaling pathways. We found that Src inhibitor PP2 and Syk inhibitor piceatannol inhibited phagocytosis, macrophage-inflammatory protein-1alpha (MIP-1alpha) release, as well as phosphorylation of extracellular-regulated kinase (ERK) and Akt, consistent with Src/Syk involvement early in FcgammaR signaling. Constitutively active mitogen-activated protein kinase kinase (MEK) induced MIP-1alpha, and Ras dominant-negative (DN) inhibited IC-induced ERK phosphorylation and MIP-1alpha production. These results suggest that the Ras/MEK/ERK pathway is necessary and sufficient in IC-induced MIP-1alpha expression. Neither Ras DN nor the MEK inhibitor U0126 inhibited phagocytosis. In contrast, phosphatidylinositol-3 kinase (PI-3K) inhibitors Wortmannin and LY294002 inhibited phagocytosis without affecting ERK phosphorylation or MIP-1alpha production. Conversely, Ras DN or U0126 did not affect Akt phosphorylation. Together, these results demonstrate distinct roles played by the PI-3K and Ras/MEK/ERK pathways in phagocytosis and MIP-1alpha induction, respectively. Our results demonstrating activation of functionally distinct pathways following microglial FcgammaR engagement may have implications for human central nervous system diseases.

Characterization of microglial responses to Staphylococcus aureus: effects on cytokine, costimulatory molecule, and Toll-like receptor expression

Microglia participate in innate immune responses in the central nervous system (CNS). This work demonstrates that microglia can recognize heat-inactivated Staphylococcus aureus and its cell wall product peptidoglycan (PGN) through the elaboration of numerous proinflammatory cytokines and chemokines. Microglia also exhibited S. aureus bactericidal activity. Microglia constitutively expressed low levels of Toll-like receptor 1 (TLR1), TLR2, TLR6, and CD14, all of which were enhanced following S. aureus exposure. Activation of resident microglia by S. aureus may serve to amplify the CNS antibacterial immune response through the release of cytokines, chemokines, and induction of bactericidal activity.

Fcgamma receptor I- and III-mediated macrophage inflammatory protein 1alpha induction in primary human and murine microglia

Microglial cell phagocytic receptors may play important roles in the pathogenesis and treatment of several neurological diseases. We studied microglial Fc receptor (FcR) activation with respect to the specific FcgammaR types involved and the downstream signaling events by using monoclonal antibody (MAb)-coated Cryptococcus neoformans immune complexes as the stimuli and macrophage inflammatory protein 1alpha (MIP-1alpha) production as the final outcome. C. neoformans complexed with murine immunoglobulin G (IgG) of gamma1, gamma2a, and gamma3, but not gamma2b isotype, was effective in inducing MIP-1alpha in human microglia. Since murine gamma2b binds to human FcgammaRII (but not FcgammaRI or FcgammaRIII), these results indicate that FcgammaRI and/or FcgammaRIII is involved in MIP-1alpha production. Consistent with this, an antibody that blocks FcgammaRII (IV.3) failed to inhibit MIP-1alpha production, while an antibody that blocks FcgammaRIII (3G8) did. An anti-C. neoformans MAb, 18B7 (IgG1), but not its F(ab')(2), induced extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase kinase phosphorylation, and MIP-1alpha release was suppressed by the ERK inhibitor U0126. C. neoformans plus 18B7 also induced degradation of I-kappaBalpha, and MIP-1alpha release was suppressed by the antioxidant NF-kappaB inhibitor pyrrolidine dithiocarbamate. To confirm the role of FcR more directly, we isolated microglia from wild-type and various FcR-deficient mice and then challenged them with C. neoformans plus 18B7. While FcgammaRII-deficient microglia showed little difference from the wild-type microglia, both FcgammaRI alpha-chain- and FcgammaRIII alpha-chain-deficient microglia produced less MIP-1alpha, and the common Fc gamma-chain-deficient microglia showed no MIP-1alpha release. Taken together, our results demonstrate a definitive role for FcgammaRI and FcgammaRIII in microglial chemokine induction and implicate ERK and NF-kappaB as the signaling components leading to MIP-1alpha expression. Our results delineate a new mechanism for microglial activation and may have implications for central nervous system inflammatory diseases.

Borrelia burgdorferi induces inflammatory mediator production by murine microglia

Lyme disease has been associated with damaging inflammation within the central nervous system. In the present study, we demonstrate that Borrelia burgdorferi is a significant stimulus for the production of IL-6, TNF-alpha, and PGE(2) by microglia. This effect is associated with induction of NF-kappaB, and increased expression of Toll-like receptor 2 and CD14, receptors known to underlie spirochete activation of other immune cell types. These studies identify microglia as a previously unappreciated source of inflammatory mediator production following challenge with B. burgdorferi. Such production may play an important role during the development of Lyme neuroborreliosis.

Apoptosis-inducing factor mediates microglial and neuronal apoptosis caused by pneumococcus

Streptococcus pneumoniae is the major cause of bacterial meningitis and it damages the hippocampus by inducing neuronal apoptosis. The blocking of caspases provides only partial protection in experimental meningitis, which suggests that there is an additional apoptotic pathway. A trigger of this pathway is the bacterium itself, as exposure of microglia or neurons to live pneumococci induces rapid apoptosis. In this study, apoptosis was not associated with the activation of caspases-1-10 and was not inhibited by z-VAD-fmk, a broad-spectrum caspase inhibitor. Rather, apoptosis was attributed to damage to mitochondria, which was followed by the release of apoptosis-inducing factor (AIF) from the mitochondria, large-scale DNA fragmentation, and hypodiploidy. Furthermore, intracytoplasmatic microinjection of AIF-specific antiserum markedly impaired pneumococcus-induced apoptosis. These findings indicate that AIF may play a central role in brain cell apoptosis and bacterial pathogenesis.

Infection of fetal feline brain cells in culture with Bartonella henselae

Bartonella henselae is known to cause central nervous system (CNS) disease in humans, and neurological signs have been observed in experimentally infected cats. However, the pathogenesis of CNS disease remains unclear. This study was undertaken to determine whether B. henselae infects feline fetal brain cells in vitro. Microglial-cell- and astrocyte-enriched cultures were inoculated with B. henselae. Giménez staining identified bacterial organisms within microglial cells by day 7 postinoculation. The viability of the intracellular bacteria was demonstrated by incubating cultures with gentamicin and plating cell lysate on agar. Electron microscopy identified intracellular organisms with characteristic Bartonella morphology but identified no ultrastructural abnormalities within infected microglial cells. No evidence of infection was seen in Bartonella-inoculated astrocyte cultures. These findings suggest a role for microglia in the pathogenesis of B. henselae-associated neurological disease.

Lesion associated expression of urokinase-type plasminogen activator receptor (uPAR, CD87) in human cerebral malaria

Blood-brain barrier disintegration and inflammatory cell recruitment are key processes in the pathogenesis of cerebral malaria (CM). Recent data provide convincing evidence that the serine protease urokinase-type plasminogen activator receptor (uPAR) is a key molecule in promoting cell adhesion and spreading. We have now analyzed expression of urokinase-type plasminogen activator receptor (uPAR, CD87), which is part of a cell surface associated proteolytic system, in brains of eight CM patients and seven neuropathologically unaltered and diseased controls by immunohistochemistry. Double labeling experiments with antibodies directed against CD68 (macrophages/microglial cells), myeloid-related protein (MRP8), and glial fibrillary acid protein (GFAP) confirmed the nature of uPAR expressing cells. We observed focal accumulation of uPAR expressing macrophages/microglial cells in Dürck's granulomas and adjacent to petechial hemorrhages, in astrocytes, and in endothelial cells. In contrast, focal uPAR expression in macrophages/microglial cells but not in astrocytes was found in microglial nodules of toxoplasmic encephalitis and in the cellular infiltrate of bacterial meningitis. Normal brains showed only faint uPAR expression in endothelial cells. We conclude from these data that lesion-associated uPAR expression at least in part contributes to blood-brain barrier alteration and immunologic dysfunction in CM patients.

Etiology of microglial nodules in brains of patients with acquired immunodeficiency syndrome

Microglial nodules associated with opportunistic and HIV-related lesions are frequently found in the brains of AIDS patients. However, in many cases, the causative agent is only presumptively suspected. We reviewed 199 brains of AIDS patients with micronodular lesions to clarify their etiology by immunohistochemistry (to Toxoplasma gondii, cytomegalovirus, herpes simplex virus I/II, varicella zoster virus and HIV-p24 core protein), PCR (for herpetic viruses and Mycobacterium tuberculosis) and electron microscopy. Productive HIV infection was observed in 110 cases (55.1%): 30 cases with Toxoplasma gondii encephalitis, 30 with cytomegalovirus encephalitis, eight with multiple cerebral diseases, while in the remaining 42 cases HIV was the only pathogenetic agent. Multinucleated giant cells (hallmark of HIV infection) were found in the MGNs of 85/110 cases with HIV-related lesions; the remaining 25 cases had only p24 positive cells but no multinucleated giant cells. In these latter cases the micronodular lesions had been initially attributed to the main opportunistic agent found in the brain, or defined as subacute encephalitis. Individual microglial nodules positive for an opportunistic pathogen were generally negative for HIV antigens. In 13 cases no opportunistic agent or HIV productive infection was found. In these cases, PCR and electron microscopy examination for HIV and other viral infections were negative. Our data suggest that HIV-immunohistochemistry should be used for the etiological diagnosis of micronodular lesions in AIDS brains, even in the presence of other pathogens. After extensive search, the etiology of the microglial nodules remains unknown in only a small percentage of cases.

Microglial activation by components of gram-positive and -negative bacteria: distinct and common routes to the induction of ion channels and cytokines

Gram-positive Streptococcus pneumoniae is the major pathogen causing lethal meningitis in adults. We used pneumococcal cell walls (PCW) to investigate microglial consequences of a bacterial challenge and to determine the role of serum in the activation process. PCW caused the characteristic induction of an outwardly rectifying K+ channel (IK+(OR)), together with a concomitant suppression of the constitutively expressed inward rectifier K+ current, and evoked the release of tumor necrosis factor-alpha (TNF alpha), interleukin-6 (IL-6), IL-12, KC, macrophage inflammatory protein (MIP) 1alpha and MIP-2. Serum presence strongly facilitated the PCW effects, similarly as observed for lipopolysaccharide (LPS) from gram-negative Escherichia coli. The inflammatory cytokine, interferon-gamma (IFNgamma) induced the same electrophysiological changes, but independent of serum. Recombinant LPS binding protein (LBP) could partially replace serum activity in LPS stimulations. In contrast, neither LBP nor an antibody-mediated blockade of the LPS receptor, CD14 had significant influences on PCW-inducible changes. Cell surface interactions and cofactor involvement in microglial activation by gram-positive bacteria are thus distinct from the mechanisms employed by LPS. Moreover, tyrphostin AG126, a protein kinase inhibitor that prevents activation of the mitogen-activated protein kinase, p42MAPK (ERK2), potently blocked the PCW-stimulated cytokine release while having only a limited effect on LPS-inducible cytokines. In contrast, AG126 did not influence IK+(OR) inductions. This indicates that PCW recruits more than 1 intracellular signaling pathway to trigger the various responses and that different bacterial agents signal through both common and individual routes during microglial activation.

Escherichia coli lipopolysaccharide potentiation and inhibition of rat neonatal microglia superoxide anion generation: correlation with prior lactic dehydrogenase, nitric oxide, tumor necrosis factor-alpha, thromboxane B2, and metalloprotease release

The effects of lipopolysaccharide (LPS) on the central nervous system, one of the first organs to be affected by sepsis, are still incompletely understood. Rat microglia (BMphi) constitute the main leukocyte-dependent source of reactive oxygen species in the central nervous system. The in vitro effect of LPS on agonist-stimulated superoxide (O2-) generation from BMphi appears controversial. Our purpose was to determine the time- and concentration-dependent effect of Escherichia coil LPS on phorbol-12 myristate 13-acetate-stimulated O2- generation from BMphi. Our results demonstrate that BMphi O2- generation in vitro peaked 17 h after stimulation of with .3 ng/mL LPS. Furthermore, stimulation of BMphi with LPS for 17 h resulted in the following concentration-dependent responses: .1-1 ng/mL LPS induced no prior mediator generation but potently enhanced subsequent phorbol-12 myristate 13-acetate-stimulated O2- generation; 3-10 ng/mL LPS caused nitric oxide, tumor necrosis factor-alpha (TNF-alpha), thromboxane B2 and matrix metalloproteinase-9 release although partially inhibiting ensuing phorbol-12 myristate 13-acetate-stimulated O2- generation; 30-100 ng/mL LPS, maximized nitric oxide, TNF-alpha, thromboxane B2, matrix metalloproteinase-9 generation with concomitant lactic dehydrogenase release although strongly deactivating successive phorbol-12 myristate 13-acetate-stimulated O2 production. Our in vitro studies suggest that enhanced release of these four mediators (nitric oxide, TNF-alpha, thromboxane B2, and matrix metalloproteinase-9) during stimulation of BMphi with LPS might play a critical role in the subsequent ability of BMphi to generate O2- in vivo. Potential clinical implications of our findings are suggested by the fact that LPS levels similar to the ones used in this study have been observed in cerebrospinal fluid both in Gram-negative meningitis and sepsis.

Differential effects of iron load on basal and interferon-gamma plus lipopolysaccharide enhance anticryptococcal activity by the murine microglial cell line BV-2

Here we evaluated the influence of intracellular iron levels on the constitutive and interferon (IFN)-gamma plus lipopolysaccharide (LPS) enhanced anticryptococcal activity by the murine microglial cell line BV-2. We demonstrated that iron loading via ferric nitrilotriacetate (FeNTA) resulted in a significant increase in the constitutive levels of anticryptococcal activity, while the enhancing effects by IFN-gamma plus LPS were prevented. Accordingly, a major increase was observed in the levels of thiobarbituric reactive substance (TBARS) produced upon iron loading under basal conditions, whereas IFN-gamma plus LPS treatment, that per se did not affect TBARS production, prevented by about 50% the enhancement otherwise occurring in response to iron loading. The potential involvement of multiple effector system and their relation to intracellular iron will be discussed.

Role of the capsule in microglial cell-Cryptococcus neoformans interaction: impairment of antifungal activity but not of secretory functions

Using two isogenic strains of Cryptococcus neoformans, we studied the influence of the capsule in C. neoformans microglial-cell interaction. We demonstrate that the acapsular mutant yeasts (CAP67) are more susceptible to phagocytosis and killing than encapsulated yeasts (B3501) by the murine microglial cells, BV-2. RT-PCR analysis showed that the pattern of gene transcripts for tumour necrosis factor alpha (TNF-a), interleukin (IL)-1beta, IL-6, IL-12p40 and granulocyte macrophage colony stimulating factor remains unchanged following BV-2 cell infection with CAP67 or B3501 yeasts. Moreover, no induction of TNF-alpha secretion occurs in BV-2 cells infected with either B3501 or CAP67 yeasts or exposed to glucuronoxylomannan (GXM) or galactoxylomannan (GalXM). Nevertheless, lipopolysaccharide-induced TNF-alpha secretion is downregulated by cell infection with B3501 or CAP67 yeasts or exposure to GXM or GalXM. Overall, by means of a continuous cell line, it appears that the C. neoformans capsule is detrimental to microglial cell antifungal activity, while no effect can be attributed to the capsule as trend of cytokine gene expression and TNF-alpha secretion.

Effect of cytokines on anticryptococcal activity of human microglial cells

The effect of selected cytokines on the antifungal activity of human microglia was studied with encapsulated and acapsular strains of Cryptococcus neoformans. None of the cytokines tested increased the fungistatic activity of microglia, suggesting that killing of cryptococci within the central nervous system is dependent on other host defense mechanisms.