Neuronal injury in bacterial meningitis: mechanisms and implications for therapy

LRRC25 Inhibits IFN-γ Secretion by Microglia to Negatively Regulate Anti-Tuberculosis Immunity in Mice

BACKGROUND

Leucine-rich repeat-containing protein-25 (LRRC25) can degrade the ISG15 gene in virus-infected cells and prevent overactivation of the type Ⅰ IFN pathway. However, the role of LRRC25 in bacterial infection is still unclear. In this pursuit, the present study aimed to explore the regulatory role and mechanism of LRRC25 in microglia infected with Mycobacterium tuberculosis in a mouse model.

METHODS

Q-PCR, WB, and cell immunofluorescence were employed to observe the change in LRRC25 in BV2 cells infected by H37Rv. Additionally, siRNA was designed to target the LRRC25 to inhibit its expression in BV2 cells. Flow cytometry and laser confocal imaging were used to observe the infection of BV2 cells after LRRC25 silencing. Q-PCR and ELISA were used to determine the changes in IFN-γ and ISG15 in the culture supernatant of each group.

RESULTS

Following H37Rv infection, it was observed that the expression of LRRC25 was upregulated. Upon silencing LRRC25, the proportion of BV2 cells infected by H37Rv decreased significantly. ELISA analysis showed that IFN-γ and ISG15 levels in cell culture supernatant decreased after H37Rv infection, while they significantly increased after LRRC25 silencing.

CONCLUSIONS

This study provides evidence that LRRC25 is the key negative regulator of microglial anti-Mtb immunity. It exerts its function by degrading free ISG15 and inhibiting the secretion of IFN-γ, thereby improving the anti-Mtb immunity of BV2 cells.

Septic encephalopathy in the elderly - biomarkers of potential clinical utility

Next to acute sickness behavior, septic encephalopathy is the most frequent involvement of the brain during infection. It is characterized by a cross-talk of pro-inflammatory cells across the blood-brain barrier, by microglial activation and leukocyte migration, but not by the entry of infecting organisms into the brain tissue. Septic encephalopathy is very frequent in older persons because of their limited cognitive reserve. The predominant clinical manifestation is delirium, whereas focal neurological signs and symptoms are absent. Electroencephalography is a very sensitive method to detect functional abnormalities, but these abnormalities are not specific for septic encephalopathy and of limited prognostic value. Routine cerebral imaging by computer tomography usually fails to visualize the subtle abnormalities produced by septic involvement of the brain. Magnetic resonance imaging is by far more sensitive to detect vasogenic edema, diffuse axonal injury or small ischemic lesions. Routine laboratory parameters most suitable to monitor sepsis, but not specific for septic encephalopathy, are C-reactive protein and procalcitonin. The additional measurement of interleukin (IL)-6, IL-8, IL-10 and tumor necrosis factor-α increases the accuracy to predict delirium and an unfavorable outcome. The most promising laboratory parameters to quantify neuronal and axonal injury caused by septic encephalopathy are neurofilament light chains (NfL) and S100B protein. Neuron-specific enolase (NSE) plasma concentrations are strongly influenced by hemolysis. We propose to determine NSE only in non-hemolytic plasma or serum samples for the estimation of outcome in septic encephalopathy.

Parainfectious cerebral vasculopathy complicating bacterial meningitis: Acute-short lived vasospasm followed by delayed-long lasting vasculitis

Bacterial meningitis is a serious, life-threatening infection of the meninges. Several radiological studies highlight prominent structural alterations occurring in the cerebral vasculature, leading to significant cerebrovascular consequences during bacterial meningitis. Beginning with reflexive arterial vasospasm , cerebrovascular disease during bacterial meningitis proceeds through a orderly sequence of arterial vasculitis with inflammatory cell infiltration, medial smooth muscle migration and proliferation, medial necrosis, adventitial fibrosis and eventual intimal stenosis. As such, this review focuses on changes occurring within cerebral arteries during disease progression, highlighting the various structural modifications occurring in the arterial vessels that contribute to disturbances in cerebral hemodynamics and, ultimately, cerebrovascular consequences during bacterial meningitis.

Adjunctive dexamethasone treatment in adults with listeria monocytogenes meningitis: a prospective nationwide cohort study

Background

A French cohort study described a detrimental effect of adjunctive dexamethasone treatment in listeria meningitis. Based on these results guidelines recommend not to use dexamethasone if L. monocytogenes is suspected or stop dexamethasone when the pathogen is detected. We studied clinical characteristics, treatment regimens and outcome of adults with Listeria monocytogenes meningitis in a nationwide cohort study on bacterial meningitis.

Methods

We prospectively assessed adults with community-acquired L. monocytogenes meningitis in the Netherlands between Jan 1, 2006, and July 1, 2022. We identified independent predictors for an unfavourable outcome (Glasgow Outcome Scale score 1 to 4) and mortality by logistic regression.

Findings

162 out of 2664 episodes (6%) of community-acquired bacterial meningitis episode were caused by L. monocytogenes in 162 patients. Adjunctive dexamethasone 10 mg QID was started with the first dose of antibiotics in 93 of 161 patients (58%) and continued for the full four days in 83 (52%) patients. Different doses, duration or timing of dexamethasone were recorded in 11 patients (7%) and 57 patients (35%) did not receive dexamethasone. The case fatality rate was 51 of 162 (31%) and an unfavourable outcome occurred in 91 of 162 patients (56%). Age and the standard regimen of adjunctive dexamethasone were independent predictors for an unfavourable outcome and mortality. The adjusted odds ratio of dexamethasone treatment for unfavourable outcome was 0.40 (95% confidence interval 0.19-0.81).

Interpretation

Adjunctive dexamethasone is associated with an improved outcome in patients with L. monocytogenes meningitis and should not be withheld if L. monocytogenes is suspected or detected as causative pathogen.

Funding

European Research Council and Netherlands Organisation for Health Research and Development.

Synergistic regulation of microglia differentiation by CD93 and integrin β1 in the rat pneumococcal meningitis model

Streptococcus pneumoniae is the main bacterial pathogen of meningitis worldwide, which has a high mortality rate and survivors are prone to central nervous system (CNS) sequelae. In this regard, microglia activation has been associated with injury to the CNS. The aim of this study was to investigate the relationship between CD93, integrin β1, and microglia activation. In the rat pneumococcal meningitis model, we found significant increases of CD93 and integrin β1 expression and differentiation of M1 phenotype microglia. Furthermore, we showed in vitro siRNA-mediated downregulation of CD93 and integrin β1 expression after infecting highly aggressive proliferating immortalized (HAPI) microglia cells with S. pneumoniae. We observed differentiation of S. pneumonia-infected HAPI microglia cells to the M1 phenotype and significant release of soluble CD93 (sCD93) and integrin β1 expression. Complement C1q and metalloproteinases promoted sCD93 release. We also showed that downregulation of CD93 significantly reduced differentiation to M1 microglia and increased differentiation to M2 microglia. However, addition of recombinant CD93 may regulate microglia differentiation to the M1 phenotype. Furthermore, the downregulation of integrin β1 resulted in downregulation of the CD93 protein. In conclusion, interaction between integrin β1 and CD93 promotes differentiation of microglia to the M1 phenotype, increases the release of pro-inflammatory factors, and leads to nervous system injury in pneumococcal meningitis.

Dysfunctional Glymphatic System with Disrupted Aquaporin 4 Expression Pattern on Astrocytes Causes Bacterial Product Accumulation in the CSF during Pneumococcal Meningitis

Pneumococcal meningitis, inflammation of the meninges due to an infection of the Central Nervous System caused by Streptococcus pneumoniae (the pneumococcus), is the most common form of community-acquired bacterial meningitis globally. Aquaporin 4 (AQP4) water channels on astrocytic end feet regulate the solute transport of the glymphatic system, facilitating the exchange of compounds between the brain parenchyma and the cerebrospinal fluid (CSF), which is important for the clearance of waste away from the brain. Wistar rats, subjected to either pneumococcal meningitis or artificial CSF (sham control), received Evans blue-albumin (EBA) intracisternally. Overall, the meningitis group presented a significant impairment of the glymphatic system by retaining the EBA in the CSF compartments compared to the uninfected sham group. Our results clearly showed that during pneumococcal meningitis, the glymphatic system does not function because of a detachment of the astrocytic end feet from the blood-brain barrier (BBB) vascular endothelium, which leads to misplacement of AQP4 with the consequent loss of the AQP4 water channel's functionality. IMPORTANCE The lack of solute drainage due to a dysfunctional glymphatic system leads to an increase of the neurotoxic bacterial material in the CSF compartments of the brain, ultimately leading to brain-wide neuroinflammation and neuronal damage with consequent impairment of neurological functions. The loss of function of the glymphatic system can therefore be a leading cause of the neurological sequelae developing post-bacterial meningitis.

Insights into the Anti-inflammatory and Antiviral Mechanisms of Resveratrol

Resveratrol is a naturally occurring stilbene phytoalexin phenolic compound, which has been extensively studied on its biological activity. It has been widely accepted that resveratrol possesses anti-inflammatory and antiviral activities. In this review, we summarize the anti-inflammatory dosages and mechanism and antiviral mechanism of resveratrol. Since viral infections are often accompanied by inflammation, we propose that the NF-κB signaling pathway is a key and common molecular mechanism of resveratrol to exert anti-inflammatory and antiviral effects. For future studies, we believe that resveratrol's anti-inflammatory and antiviral mechanisms can consider the upstream signaling molecules of the NF-κB signaling pathway. For resveratrol antivirus, future studies can be conducted on the interaction of resveratrol with key proteins or important enzymes of the virus. In addition, we also think that the clinical application of resveratrol is very important. In short, resveratrol is a promising anti-inflammatory and antiviral drug, and research on it needs to be expanded.

Metabolic Shifts as the Hallmark of Most Common Diseases: The Quest for the Underlying Unity

A hyper-specialization characterizes modern medicine with the consequence of classifying the various diseases of the body into unrelated categories. Such a broad diversification of medicine goes in the opposite direction of physics, which eagerly looks for unification. We argue that unification should also apply to medicine. In accordance with the second principle of thermodynamics, the cell must release its entropy either in the form of heat (catabolism) or biomass (anabolism). There is a decreased flow of entropy outside the body due to an age-related reduction in mitochondrial entropy yield resulting in increased release of entropy in the form of biomass. This shift toward anabolism has been known in oncology as Warburg-effect. The shift toward anabolism has been reported in most diseases. This quest for a single framework is reinforced by the fact that inflammation (also called the immune response) is involved in nearly every disease. This strongly suggests that despite their apparent disparity, there is an underlying unity in the diseases. This also offers guidelines for the repurposing of old drugs.

Neuronal death in pneumococcal meningitis is triggered by pneumolysin and RrgA interactions with β-actin

Neuronal damage is a major consequence of bacterial meningitis, but little is known about mechanisms of bacterial interaction with neurons leading to neuronal cell death. Streptococcus pneumoniae (pneumococcus) is a leading cause of bacterial meningitis and many survivors develop neurological sequelae after the acute infection has resolved, possibly due to neuronal damage. Here, we studied mechanisms for pneumococcal interactions with neurons. Using human primary neurons, pull-down experiments and mass spectrometry, we show that pneumococci interact with the cytoskeleton protein β-actin through the pilus-1 adhesin RrgA and the cytotoxin pneumolysin (Ply), thereby promoting adhesion and invasion of neurons, and neuronal death. Using our bacteremia-derived meningitis mouse model, we observe that RrgA- and Ply-expressing pneumococci co-localize with neuronal β-actin. Using purified proteins, we show that Ply, through its cholesterol-binding domain 4, interacts with the neuronal plasma membrane, thereby increasing the exposure on the outer surface of β-actin filaments, leading to more β-actin binding sites available for RrgA binding, and thus enhanced pneumococcal interactions with neurons. Pneumococcal infection promotes neuronal death possibly due to increased intracellular Ca2+ levels depending on presence of Ply, as well as on actin cytoskeleton disassembly. STED super-resolution microscopy showed disruption of β-actin filaments in neurons infected with pneumococci expressing RrgA and Ply. Finally, neuronal death caused by pneumococcal infection could be inhibited using antibodies against β-actin. The generated data potentially helps explaining mechanisms for why pneumococci frequently cause neurological sequelae.

The Potentials of Melatonin in the Prevention and Treatment of Bacterial Meningitis Disease

Bacterial meningitis (BM) is an acute infectious central nervous system (CNS) disease worldwide, occurring with 50% of the survivors left with a long-term serious sequela. Acute bacterial meningitis is more prevalent in resource-poor than resource-rich areas. The pathogenesis of BM involves complex mechanisms that are related to bacterial survival and multiplication in the bloodstream, increased permeability of blood-brain barrier (BBB), oxidative stress, and excessive inflammatory response in CNS. Considering drug-resistant bacteria increases the difficulty of meningitis treatment and the vaccine also has been limited to several serotypes, and the morbidity rate of BM still is very high. With recent development in neurology, there is promising progress for drug supplements of effectively preventing and treating BM. Several in vivo and in vitro studies have elaborated on understanding the significant mechanism of melatonin on BM. Melatonin is mainly secreted in the pineal gland and can cross the BBB. Melatonin and its metabolite have been reported as effective antioxidants and anti-inflammation, which are potentially useful as prevention and treatment therapy of BM. In bacterial meningitis, melatonin can play multiple protection effects in BM through various mechanisms, including immune response, antibacterial ability, the protection of BBB integrity, free radical scavenging, anti-inflammation, signaling pathways, and gut microbiome. This manuscript summarizes the major neuroprotective mechanisms of melatonin and explores the potential prevention and treatment approaches aimed at reducing morbidity and alleviating nerve injury of BM.

Oligodeoxynucleotides containing unmethylated cytosine-guanine motifs are effective immunostimulants against pneumococcal meningitis in the immunocompetent and neutropenic host

BACKGROUND

Bacterial meningitis is a fatal disease with a mortality up to 30% and neurological sequelae in one fourth of survivors. Available vaccines do not fully protect against this lethal disease. Here, we report the protective effect of synthetic oligodeoxynucleotides containing unmethylated cytosine-guanine motifs (CpG ODN) against the most frequent form of bacterial meningitis caused by Streptococcus pneumoniae.

METHODS

Three days prior to the induction of meningitis by intracerebral injection of S. pneumoniae D39, wild-type and Toll-like receptor (TLR9)^-/- mice received an intraperitoneal injection of 100 μg CpG ODN or vehicle. To render mice neutropenic, anti-Ly-6G monoclonal antibody was daily administrated starting 4 days before infection with a total of 7 injections. Kaplan-Meier survival analyses and bacteriological studies, in which mice were sacrificed 24 h and 36 h after infection, were performed.

RESULTS

Pre-treatment with 100 μg CpG ODN prolonged survival of immunocompetent and neutropenic wild-type mice but not of TLR9^-/- mice. There was a trend towards lower mortality in CpG ODN-treated immunocompetent and neutropenic wild-type mice. CpG ODN caused an increase of IL-12/IL-23p40 levels in the spleen and serum in uninfected animals. The effects of CpG ODN on bacterial concentrations and development of clinical symptoms were associated with an increased number of microglia in the CNS during the early phase of infection. Elevated concentrations of IL-12/IL-23p40 and MIP-1α correlated with lower bacterial concentrations in the blood and spleen during infection.

CONCLUSIONS

Pre-conditioning with CpG ODN strengthened the resistance of neutropenic and immunocompetent mice against S. pneumoniae meningitis in the presence of TLR9. Administration of CpG ODN decreased bacterial burden in the cerebellum and reduced the degree of bacteremia. Systemic administration of CpG ODN may help to prevent or slow the progression to sepsis of bacterial CNS infections in healthy and immunocompromised individuals even after direct inoculation of bacteria into the intracranial compartments, which can occur after sinusitis, mastoiditis, open head trauma, and surgery, including placement of an external ventricular drain.

Adjuvant Cannabinoid Receptor Type 2 Agonist Modulates the Polarization of Microglia Towards a Non-Inflammatory Phenotype in Experimental Pneumococcal Meningitis

Background

Microglia initiates and sustains the inflammatory reaction that drives the pathogenesis of pneumococcal meningitis. The expression of the G-protein cannabinoid receptor type 2 (CB2) in the brain is low, but is upregulated in glial cells during infection. Its activation down-regulates pro-inflammatory processes, driving microglia towards an anti-inflammatory phenotype. CB2 agonists are therefore therapeutic candidates in inflammatory conditions like pneumococcal meningitis. We evaluated the effects of JWH-133, a specific CB2 agonist on microglial cells, inflammation, and damage driven by S. pneumoniae in vitro and in experimental pneumococcal meningitis.

Materials/methods

Primary mixed glial cultures were stimulated with live or heat-inactivated S. pneumoniae, or lipopolysaccharide and treated with JWH-133 or vehicle. Nitric oxide and cytokines levels were measured in the supernatant. In vivo, pneumococcal meningitis was induced by intracisternal injection of live S. pneumoniae in 11 days old Wistar rats. Animals were treated with antibiotics (Ceftriaxone, 100 mg/kg, s.c. bid) and JWH-133 (1 mg/kg, i.p. daily) or vehicle (10% Ethanol in saline, 100 µl/25g body weight) at 18 h after infection. Brains were harvested at 24 and 42 h post infection (hpi) for histological assessment of hippocampal apoptosis and cortical damage and determination of cyto/chemokines in tissue homogenates. Microglia were characterized using Iba-1 immunostaining. Inflammation in brain homogenates was determined using membrane-based antibody arrays.

Results

In vitro, nitric oxide and cytokines levels were significantly lowered by JWH-133 treatment. In vivo, clinical parameters were not affected by the treatment. JWH-133 significantly lowered microglia activation assessed by quantification of cell process length and endpoints per microglia. Animals treated with JWH-133 demonstrated significantly lower parenchymal levels of chemokines (CINC-1, CINC-2α/β, and MIP-3α), TIMP-1, and IL-6 at 24 hpi, and CINC-1, MIP-1α, and IL-1α at 42 hpi. Quantitative analysis of brain damage did not reveal an effect of JWH-133.

Conclusions

JWH-133 attenuates microglial activation and downregulates the concentrations of pro-inflammatory mediators in pneumococcal infection in vitro and in vivo. However, we didn't observe a reduction in cortical or hippocampal injury. This data provides evidence that inhibition of microglia by adjuvant CB2 agonists therapy effectively downmodulates neuroinflammation but does not reduce brain damage in experimental pneumococcal meningitis.

Selective Loss of Brain-Derived Neurotrophic Factor Exacerbates Brain Injury by Enhancing Neuroinflammation in Experimental Streptococcus pneumoniae Meningitis

Streptococcus pneumoniae meningitis is a life-threatening bacterial infection of the central nervous system (CNS), and its unfavorable prognosis usually results from an intense inflammatory response. Recent studies have shown that brain-derived neurotrophic factor (BDNF) mediates anti-inflammatory and neuroprotective effects in CNS diseases; however, the distinct contribution of BDNF to pneumococcal meningitis (PM) remains unknown. In this study, we sought to investigate the effects of endogenous BDNF on the inflammatory response and brain damage in experimental PM. We used Camk2a-CreERT2 mice to delete Bdnf from the cerebral cortex and hippocampus, and meningitis was induced by intracisternal infection with S. pneumoniae. Clinical parameters were assessed during acute meningitis. At 24 h post-infection, histopathology, neutrophil granulocytes infiltration, and microglia/macrophage proliferation of brain tissues were evaluated. Additionally, cortical damage and hippocampal apoptosis were assessed using Nissl staining and terminal deoxynucleotidyl transferase dUTP-nick-end labeling (TUNEL), respectively. Pro-inflammatory cytokine levels were determined using real-time polymerase chain reaction (RT-PCR). Key molecules associated with the related signaling pathways were analyzed by RT-PCR and western blot. To investigate the role of microglia/macrophage in infected BDNF conditional knockout mice, GW2580 was used for microglia/macrophage depletion. Here, we, for the first time, found that BDNF conditional knockouts exhibited more profound clinical impairment, pathological severity, and neuron injury and enhanced microglia/macrophage proliferation than were observed in their littermate controls. Furthermore, the BDNF conditional knockouts showed an obviously increase in the expression of pro-inflammatory factors (Tnf-α, Il-1β, and Il-6). Mechanistically, loss of BDNF activated TLR2- and NOD2-mediated downstream nuclear factor kappa B (NF-κB) p65 and p38 mitogen-activated protein kinase (MAPK) pathways associated with S. pneumoniae infection. Furthermore, targeted depletion of microglia/macrophage population decreased the resistance of mice to PM with diminishing neuroinflammation in BDNF conditional knockouts. Our findings suggest that loss of BDNF may enhance the inflammatory response and contribute to brain injury during PM at least partially by modulating TLR2- and NOD2-mediated signaling pathways, thereby providing a potential therapeutic target for future interventions in bacterial meningitis pathologies.

Calculated parenteral initial therapy of bacterial infections: Bacterial meningitis

This is the thirteenth chapter of the guideline "Calculated initial parenteral treatment of bacterial infections in adults - update 2018" in the 2^nd updated version. The German guideline by the Paul-Ehrlich-Gesellschaft für Chemotherapie e.V. (PEG) has been translated to address an international audience. Bacterial meningitis is a life-threatening infectious disease with high mortality and disability rates requiring prompt initiation of antimicrobial treatment to lower these rates.

Fowlerstefin, a cysteine protease inhibitor of Naegleria fowleri, induces inflammatory responses in BV-2 microglial cells in vitro

Background

Naegleria fowleri is a free-living amoeba that causes an opportunistic fatal infection known as primary amoebic meningoencephalitis (PAM) in humans. Cysteine proteases produced by the amoeba may play critical roles in the pathogenesis of infection. In this study, a novel cysteine protease inhibitor of N. fowleri (fowlerstefin) was characterized to elucidate its biological function as an endogenous cysteine protease inhibitor of the parasite as well as a pathogenic molecule that induces immune responses in microglial cells.

Methods

Recombinant fowlerstefin was expressed in Escherichia coli. The inhibitory activity of fowlerstefin against several cysteine proteases, including human cathepsins B and L, papain and NfCPB-L, was analyzed. Fowlerstefin-induced pro-inflammatory response in BV-2 microglial cells was anayzed by cytokine array assay, reverse transcription polymerase chain reaction, and enzyme-linked immunosorbent assay.

Results

Fowlerstefin is a cysteine protease inhibitor with a monomeric structure, and belongs to the stefin family. Recombinant fowlerstefin effectively inhibited diverse cysteine proteases including cathepsin B-like cysteine proteases of N. fowleri (NfCPB-L), human cathepsins B and L, and papain. Expression of fowlerstefin in the amoeba was optimal during the trophozoite stage and gradually decreased in cysts. Fowlerstefin induced an inflammatory response in BV-2 microglial cells. Fowlerstefin induced the expression of several pro-inflammatory cytokines and chemokines including IL-6 and TNF in BV-2 microglial cells. Fowlerstefin-induced expression of IL-6 and TNF in BV-2 microglial cells was regulated by mitogen-activated protein kinase (MAPKs). The inflammatory response induced by fowlerstefin in BV-2 microglial cells was downregulated via inhibition of NF-κB and AP-1.

Conclusions

Fowlerstefin is a pathogenic molecule that stimulates BV-2 microglial cells to produce pro-inflammatory cytokines through NF-κB- and AP-1-dependent MAPK signaling pathways. Fowlerstefin-induced inflammatory cytokines exacerbate the inflammatory response in N. fowleri-infected areas and contribute to the pathogenesis of PAM.

Research progress on the basic helix-loop-helix transcription factors of Aspergillus species

Basic helix-loop-helix (bHLH) proteins belong to a superfamily of transcription factors, and they are widely distributed in eukaryotic organisms. Members of the bHLH protein family can form homodimers or heterodimers with themselves or other family members, and they often play bifunctional roles as activators and repressors to uniquely regulate the transcription of downstream target genes. The bHLH transcription factors are usually involved in developmental processes, including cellular proliferation and differentiation. Therefore, these transcription factors often play crucial roles in regulating growth, development, and differentiation in eukaryotes. Aspergillus species fungi are widely distributed in the environment, and they play important roles not only in the decomposition of organic matter as an important environmental microorganism but also in the fermentation and the food processing industry. Furthermore, some pathogenic fungi, such as Aspergillus flavus and Aspergillus fumigatus, affect the environment and human health in important ways. Recent research has shown that some Aspergillus bHLH proteins are significantly involved in the regulation of asexual and sexual reproduction, secondary metabolite production, carbohydrate metabolism, conidial and sclerotial production, among other processes. Here, we review the regulatory mechanisms and biological functions of the bHLH transcription factors of the Aspergillus genus to provide a theoretical reference for further study on the growth and development of Aspergillus and the functions of bHLHs.

Pathology of inflammatory diseases of the nervous system: Human disease versus animal models

Numerous recent studies have been performed to elucidate the function of microglia, macrophages, and astrocytes in inflammatory diseases of the central nervous system. Regarding myeloid cells a core pattern of activation has been identified, starting with the activation of resident homeostatic microglia followed by recruitment of blood borne myeloid cells. An initial state of proinflammatory activation is at later stages followed by a shift toward an‐anti‐inflammatory and repair promoting phenotype. Although this core pattern is similar between experimental models and inflammatory conditions in the human brain, there are important differences. Even in the normal human brain a preactivated microglia phenotype is evident, and there are disease specific and lesion stage specific differences in the contribution between resident and recruited myeloid cells and their lesion state specific activation profiles. Reasons for these findings reside in species related differences and in differential exposure to different environmental cues. Most importantly, however, experimental rodent studies on brain inflammation are mainly focused on autoimmune encephalomyelitis, while there is a very broad spectrum of human inflammatory diseases of the central nervous system, triggered and propagated by a variety of different immune mechanisms.

MMPs and ADAMs in neurological infectious diseases and multiple sclerosis

Metalloproteinases-such as matrix metalloproteinases (MMPs) and a disintegrin and metalloproteinases (ADAMs)-are involved in various diseases of the nervous system but also contribute to nervous system development, synaptic plasticity and neuroregeneration upon injury. MMPs and ADAMs proteolytically cleave many substrates including extracellular matrix components but also signaling molecules and receptors. During neuroinfectious disease with associated neuroinflammation, MMPs and ADAMs regulate blood-brain barrier breakdown, bacterial invasion, neutrophil infiltration and cytokine signaling. Specific and broad-spectrum inhibitors for MMPs and ADAMs have experimentally been shown to decrease neuroinflammation and brain damage in diseases with excessive neuroinflammation as a common denominator, such as pneumococcal meningitis and multiple sclerosis, thereby improving the disease outcome. Timing of metalloproteinase inhibition appears to be critical to effectively target the cascade of pathophysiological processes leading to brain damage without inhibiting the neuroregenerative effects of metalloproteinases. As the critical role of metalloproteinases in neuronal repair mechanisms and regeneration was only lately recognized, the original idea of chronic MMP inhibition needs to be conceptually revised. Recently accumulated research urges for a second chance of metalloproteinase inhibitors, which-when correctly applied and dosed-harbor the potential to improve the outcome of different neuroinflammatory diseases.

Metformin mediates neuroprotection and attenuates hearing loss in experimental pneumococcal meningitis

Background

Pneumococcal meningitis is associated with high risk of neurological sequelae such as cognitive impairment and hearing loss. These sequelae are due to parenchymal brain and inner ear damage primarily induced by the excessive inflammatory reaction in response to bacterial brain invasion. Metformin—a biguanide drug to treat diabetes mellitus type 2—was recently found to suppress neuroinflammation and induce neuroregeneration. This study evaluated the effect of metformin adjunctive to antibiotics on neuroinflammation, brain and inner ear damage, and neurofunctional outcome in experimental pediatric pneumococcal meningitis.

Methods

Eleven-day-old Wistar rats were infected intracisternally with 5.22 ± 1.27 × 10³ CFU Streptococcus pneumoniae and randomized for treatment with metformin (50 mg/kg, i.p., once daily for 3 weeks) plus ceftriaxone (100 mg/kg, i.p., bid, n = 61) or ceftriaxone monotherapy (n = 79). Cortical damage and hippocampal apoptosis were evaluated histomorphometrically 42 h post infection. Cerebrospinal fluid cytokine levels were analyzed during acute infection. Five weeks post infection, auditory brainstem responses were measured to determine hearing thresholds. Spiral ganglion neuron density and abundance of recently proliferated and integrated hippocampal granule neurons were assessed histologically. Additionally, the anti-inflammatory effect of metformin was studied in primary rat astroglial cells in vitro.

Results

Upon pneumococcal infection, metformin treatment significantly reduced levels of inflammatory cytokines and nitric oxide production in cerebrospinal fluid and in astroglial cell cultures in vitro (p < 0.05). Compared to animals receiving ceftriaxone monotherapy, adjunctive metformin significantly reduced cortical necrosis (p < 0.02) during acute infection and improved median click-induced hearing thresholds (60 dB vs. 100 dB, p < 0.002) 5 weeks after infection. Adjuvant metformin significantly improved pure tone hearing thresholds at all assessed frequencies compared to ceftriaxone monotherapy (p < 0.05) and protected from PM-induced spiral ganglion neuron loss in the inner ear (p < 0.05).

Conclusion

Adjuvant metformin reduces brain injury during pneumococcal meningitis by decreasing the excessive neuroinflammatory response. Furthermore, it protects spiral ganglion neurons in the inner ear and improves hearing impairments after experimental pneumococcal meningitis. These results identify adjuvant metformin as a promising therapeutic option to improve the outcome after pediatric pneumococcal meningitis.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1549-6) contains supplementary material, which is available to authorized users.

Combining Ceftriaxone with Doxycycline and Daptomycin Reduces Mortality, Neuroinflammation, Brain Damage, and Hearing Loss in Infant Rat Pneumococcal Meningitis

Despite appropriate antibiotic therapy, pneumococcal meningitis (PM) is associated with a case fatality rate of up to 30% in high-income countries. Survivors often suffer from severe lifelong disabilities. An excessive inflammatory reaction drives the pathophysiology, leading to brain damage and neurologic sequelae. We aimed to improve the outcome of experimental PM by simultaneously targeting different pathophysiological mechanisms with combined adjunctive therapies previously shown to be neuroprotective. In vitro, the anti-inflammatory effects of doxycycline and daptomycin were evaluated on primary rat astroglial cells stimulated with Streptococcus pneumoniae Eleven-day-old infant Wistar rats were infected intracisternally with S. pneumoniae and randomized for treatment with ceftriaxone or combination adjuvant therapy consisting of ceftriaxone, daptomycin, and doxycycline. During acute PM, combined-adjuvant therapy with ceftriaxone, daptomycin, and doxycycline increased the survival rate from 64.1% to 85.8% (P < 0.01) and alleviated weight loss compared to ceftriaxone monotherapy (P < 0.01). Levels of inflammatory cytokines were significantly reduced by combined-adjuvant therapy in vitro (P < 0.0001) and in cerebrospinal fluid in vivo (P < 0.05). In infected animals treated with combined adjunctive therapy, cortical damage was significantly reduced (P < 0.05), and animals showed a trend toward better hearing capacity 3 weeks after the infection (P = 0.089), an effect which was significant in mildly infected animals (48 decibels [dB] versus 67.22 dB; P < 0.05). These mildly infected animals showed significantly reduced cochlear fibrous occlusion (P < 0.01). By combining nonbacteriolytic daptomycin and anti-inflammatory doxycycline with ceftriaxone, the previously reported beneficial effects of the drugs were cumulated and identified the triple-antibiotic therapy as a promising therapeutic option for pediatric PM.

Prophylactic Palmitoylethanolamide Prolongs Survival and Decreases Detrimental Inflammation in Aged Mice With Bacterial Meningitis

Easy-to-achieve interventions to promote healthy longevity are desired to diminish the incidence and severity of infections, as well as associated disability upon recovery. The dietary supplement palmitoylethanolamide (PEA) exerts anti-inflammatory and neuroprotective properties. Here, we investigated the effect of prophylactic PEA on the early immune response, clinical course, and survival of old mice after intracerebral E. coli K1 infection. Nineteen-month-old wild type mice were treated intraperitoneally with two doses of either 0.1 mg PEA/kg in 250 μl vehicle solution (n = 19) or with 250 μl vehicle solution only as controls (n = 19), 12 h and 30 min prior to intracerebral E. coli K1 infection. The intraperitoneal route was chosen to reduce distress in mice and to ensure exact dosing. Survival time, bacterial loads in cerebellum, blood, spleen, liver, and microglia counts and activation scores in the brain were evaluated. We measured the levels of IL-1β, IL-6, MIP-1α, and CXCL1 in cerebellum and spleen, as well as of bioactive lipids in serum in PEA- and vehicle-treated animals 24 h after infection. In the absence of antibiotic therapy, the median survival time of PEA-pre-treated infected mice was prolonged by 18 h compared to mice of the vehicle-pre-treated infected group (P = 0.031). PEA prophylaxis delayed the onset of clinical symptoms (P = 0.037). This protective effect was associated with lower bacterial loads in the spleen, liver, and blood compared to those of vehicle-injected animals (P ≤ 0.037). PEA-pre-treated animals showed diminished levels of pro-inflammatory cytokines and chemokines in spleen 24 h after infection, as well as reduced serum concentrations of arachidonic acid and of one of its metabolites, 20-hydroxyeicosatetraenoic acid. In the brain, prophylactic PEA tended to reduce bacterial titers and attenuated microglial activation in aged infected animals (P = 0.042). Our findings suggest that prophylactic PEA can counteract infection associated detrimental responses in old animals. Accordingly, PEA treatment slowed the onset of infection symptoms and prolonged the survival of old infected mice. In a clinical setting, prophylactic administration of PEA might extend the potential therapeutic window where antibiotic therapy can be initiated to rescue elderly patients.

Characterization of Brain Dysfunction Induced by Bacterial Lipopeptides That Alter Neuronal Activity and Network in Rodent Brains

The immunopathological states of the brain induced by bacterial lipoproteins have been well characterized by using biochemical and histological assays. However, these studies have limitations in determining functional states of damaged brains involving aberrant synaptic activity and network, which makes it difficult to diagnose brain disorders during bacterial infection. To address this, we investigated the effect of Pam₃CSK₄ (PAM), a synthetic bacterial lipopeptide, on synaptic dysfunction of female mice brains and cultured neurons in parallel. Our functional brain imaging using PET with [¹⁸F]fluorodeoxyglucose and [¹⁸F] flumazenil revealed that the brain dysfunction induced by PAM is closely aligned to disruption of neurotransmitter-related neuronal activity and functional correlation in the region of the limbic system rather than to decrease of metabolic activity of neurons in the injection area. This finding was verified by in vivo tissue experiments that analyzed synaptic and dendritic alterations in the regions where PET imaging showed abnormal neuronal activity and network. Recording of synaptic activity also revealed that PAM reorganized synaptic distribution and decreased synaptic plasticity in hippocampus. Further study using in vitro neuron cultures demonstrated that PAM decreased the number of presynapses and the frequency of miniature EPSCs, which suggests PAM disrupts neuronal function by damaging presynapses exclusively. We also showed that PAM caused aggregation of synapses around dendrites, which may have caused no significant change in expression level of synaptic proteins, whereas synaptic number and function were impaired by PAM. Our findings could provide a useful guide for diagnosis and treatment of brain disorders specific to bacterial infection.SIGNIFICANCE STATEMENT It is challenging to diagnose brain disorders caused by bacterial infection because neural damage induced by bacterial products involves nonspecific neurological symptoms, which is rarely detected by laboratory tests with low spatiotemporal resolution. To better understand brain pathology, it is essential to detect functional abnormalities of brain over time. To this end, we investigated characteristic patterns of altered neuronal integrity and functional correlation between various regions in mice brains injected with bacterial lipopeptides using PET with a goal to apply new findings to diagnosis of brain disorder specific to bacterial infection. In addition, we analyzed altered synaptic density and function using both in vivo and in vitro experimental models to understand how bacterial lipopeptides impair brain function and network.

Combined effect of non-bacteriolytic antibiotic and inhibition of matrix metalloproteinases prevents brain injury and preserves learning, memory and hearing function in experimental paediatric pneumococcal meningitis

BACKGROUND

Pneumococcal meningitis is associated with high mortality and morbidity rates. Up to 50% of survivors show neurologic sequelae including hearing loss, cognitive impairments and learning disabilities, being particularly detrimental in affected infants and children where adjuvant therapy with dexamethasone has no proven beneficial effect. We evaluated the effect of concomitantly targeting specific pathophysiological mechanisms responsible for brain damage-i.e. matrix-metalloproteinase (MMP) activity and the exacerbated cerebral inflammation provoked through antibiotic-induced bacterial lysis. Here, we combined adjunctive therapies previously shown to be neuroprotective when used as single adjuvant therapies.

METHODS

Eleven-day-old Wistar rats were infected intracisternally with 6.44 ± 2.17 × 103 CFU Streptococcus pneumoniae and randomised for treatment with ceftriaxone combined with (a) single adjuvant therapy with daptomycin (n = 24), (b) single adjuvant therapy with Trocade (n = 24), (c) combined adjuvant therapy (n = 66) consisting of daptomycin and Trocade, or (d) ceftriaxone monotherapy (n = 42). Clinical parameters and inflammatory CSF cytokine levels were determined during acute meningitis. Cortical damage and hippocampal apoptosis were assessed 42 h after infection. Morris water maze and auditory brainstem responses were used to assess neurofunctional outcome 3 weeks after infection.

RESULTS

We found significantly reduced apoptosis in the hippocampal subgranular zone in infant rats receiving adjuvant Trocade (p < 0.01) or combined adjuvant therapy (p < 0.001). Cortical necrosis was significantly reduced in rats treated with adjuvant daptomycin (p < 0.05) or combined adjuvant therapy (p < 0.05) compared to ceftriaxone monotherapy. Six hours after treatment initiation, CSF cytokine levels were significantly reduced for TNF-α (p < 0.01), IL-1β (p < 0.01), IL-6 (p < 0.001) and IL-10 (p < 0.01) in animals receiving combined adjuvant intervention compared to ceftriaxone monotherapy. Importantly, combined adjuvant therapy significantly improved learning and memory performance in infected animals and reduced hearing loss (77.14 dB vs 60.92 dB, p < 0.05) by preserving low frequency hearing capacity, compared to ceftriaxone monotherapy.

CONCLUSION

Combined adjuvant therapy with the non-bacteriolytic antibiotic daptomycin and the MMP inhibitor Trocade integrates the neuroprotective effects of both single adjuvants in experimental paediatric pneumococcal meningitis by reducing neuroinflammation and brain damage, thereby improving neurofunctional outcome. This strategy represents a promising therapeutic option to improve the outcome of paediatric patients suffering from pneumococcal meningitis.

Diagnosis of Meningitis Caused by Pathogenic Microorganisms Using Magnetic Resonance Imaging: A Systematic Review

Introduction:

Bacterial meningitis is an acute infectious inflammation of the protective membranes covering the brain. Its early diagnosis is vital because of its high morbidity and mortality. It is mostly diagnosed by a gold standard diagnostic tool i.e. Cerebrospinal Fluid (CSF) analysis. However, it is sometimes difficult and or impossible to do this procedure and an alternative diagnostic tool is needed. Contrast enhanced magnetic resonance imaging can detect the pus or other changes in subarachnoid space. But our optimal aim is to use an imaging method without using contrast to be useable and available in more specific condition.

Methods:

This study aimed to survey the role of non-contrast Magnetic Resonance Imaging (MRI) in the diagnosis of the bacterial meningitis. MEDLINE/PubMed Central, Web of Science and Scopus were searched without time period and language limitation until March 2017. We found 6410 papers in our initial search. After assessing the content of the papers based on Cochrane library guidelines and inclusion/exclusion criteria, 6 relevant studies were included in the systematic review. All of included studies were observational studies.

Results:

MRI studies demonstrated that Fluid Attenuation Inversion Recovery (FLAIR) and Diffusion-Weighted Image (DWI) MR imaging among all MRI modalities can detect some abnormalities compatible with bacterial meningitis. FLAIR and DWI-MR imaging are potentially useful to diagnose bacterial meningitis and can be used in emergent condition in which bacterial meningitis is highly suspicious and the other diagnostic tools are not available or feasible.

Microglial Phenotypes and Functions in Multiple Sclerosis

Microglia are the resident immune cells that constantly survey the central nervous system. They can adapt to their environment and respond to injury or insult by altering their morphology, phenotype, and functions. It has long been debated whether microglial activation is detrimental or beneficial in multiple sclerosis (MS). Recently, the two opposing yet connected roles of microglial activation have been described with the aid of novel microglial markers, RNA profiling, and in vivo models. In this review, microglial phenotypes and functions in the context of MS will be discussed with evidence from both human pathological studies, in vitro and in vivo models. Microglial functional diversity-phagocytosis, antigen presentation, immunomodulation, support, and repair-will also be examined in detail. In addition, this review discusses the emerging evidence for microglia-related targets as biomarkers and therapeutic targets for MS.

Infrared spectroscopic characterization of monocytic microvesicles (microparticles) released upon lipopolysaccharide stimulation

Microvesicles (MVs) are involved in cell-cell interactions, including disease pathogenesis. Nondestructive Fourier-transform infrared (FTIR) spectra from MVs were assessed as a technique to provide new biochemical insights into a LPS-induced monocyte model of septic shock. FTIR spectroscopy provided a quick method to investigate relative differences in biomolecular content of different MV populations that was complementary to traditional semiquantitative omics approaches, with which it is difficult to provide information on relative changes between classes (proteins, lipids, nucleic acids, carbohydrates) or protein conformations. Time-dependent changes were detected in biomolecular contents of MVs and in the monocytes from which they were released. Differences in phosphatidylcholine and phosphatidylserine contents were observed in MVs released under stimulation, and higher relative concentrations of RNA and α-helical structured proteins were present in stimulated MVs compared with MVs from resting cells. FTIR spectra of stimulated monocytes displayed changes that were consistent with those observed in the corresponding MVs they released. LPS-stimulated monocytes had reduced concentrations of nucleic acids, α-helical structured proteins, and phosphatidylcholine compared with resting monocytes but had an increase in total lipids. FTIR spectra of MV biomolecular content will be important in shedding new light on the mechanisms of MVs and the different roles they play in physiology and disease pathogenesis.-Lee, J., Wen, B., Carter, E. A., Combes, V., Grau, G. E. R., Lay, P. A. Infrared spectroscopic characterization of monocytic microvesicles (microparticles) released upon lipopolysaccharide stimulation.

All- Trans-Retinoic Acid Augments the Histopathological Outcome of Neuroinflammation and Neurodegeneration in Lupus-Prone MRL/lpr Mice

Recently, we demonstrated that treatment with all- trans-retinoic acid (tRA) induced a paradoxical effect on immune activation during the development of autoimmune lupus. Here, we further describe its negative effects on mediating neuroinflammation and neurodegeneration. Female MRL/lpr mice were orally administered tRA or VARA (retinol mixed with 10% tRA) from 6 to 14 weeks of age. Both treatments had a significant effect on brain weight, which correlated with histopathological evidence of focal astrogliosis, meningitis, and ventriculitis. Infiltration of CD138- and Iba1-positve immune cells was observed in the third ventricle and meninges of treated mice that co-labeled with ICAM-1, indicating their inflammatory nature. Increased numbers of circulating plasma cells, autoantibodies, and total IgG were also apparent. IgG and C3 complement deposition in these brain regions were also prominent as was focal astrogliosis surrounding the ventricular lining and meninges. Using Fluoro-Jade staining, we further demonstrate that neuroinflammation was accompanied by neurodegeneration in the cortex of treated mice compared with vehicle controls. These findings indicate that vitamin A exposure exacerbates the immunogenic environment of the brain during the onset of systemic autoimmune disease. Vitamin A may therefore compromise the immuno-privileged nature of the central nervous system under a predisposed immunogenic environment.

The Early Adaptive Immune Response in the Pathophysiological Process of Pneumococcal Meningitis

BACKGROUND

 The adaptive immune system has been considered to play a minimal role in the early host response during bacterial meningitis.

METHODS

 We investigated the progression and outcome of pneumococcal meningitis in Rag1^-/- mice lacking functional B and T cells by assessing overall and symptom-free survival, bacteriological and histological studies, as well as flow cytometry and measurements of proinflammatory mediators.

RESULTS

 The intracerebral injection of S. pneumoniae D39 induced the recruitment of B and T cells (CD4⁺, γδ and natural killer) into the brain of wild-type mice. Mice with no functional B and T cells developed clinical symptoms and succumbed to the infection earlier than the wild-type group. In the CNS, Rag1^-/- mice showed lower levels of interleukin 1β, reduced microglial proliferation, and impaired granulocyte recruitment with an earlier spread of pneumococci into the bloodstream, compared with wild-type mice. Lack of B and T cells resulted in a severe impairment of bacterial clearance in blood, spleen, and liver and an exaggerated systemic inflammatory response.

CONCLUSIONS

 B and T cells are important effector cells delaying the spread of pneumococci from the brain to the systemic circulation and shaping the immune response, thereby prolonging the survival of the host in the absence of antibiotic treatment.

Effect of exposure to staphylococcus aureus, particulate matter, and their combination on the neurobehavioral function of mice

Neurotoxicity in Kunming mice caused by Staphylococcus aureus (S. aureus) and Particulate matter (PM) as individual matter and mixtures was studied in this paper. Male Kunming mice were instilled intratracheally with PM at doses of 0.2mg/mouse and S. aureus at doses of 5.08×10⁶ CFU/mouse as individual matter and mixtures two times at 5-day intervals. Morris water maze (MWM) test was performed during the exposure experiment. One day following the exposure experiment, the expression of neurotrophins, neurotransmitters, cholinergic system enzymes, oxidative damage levels, and pro-inflammatory cytokines (TNF-α, IL-1β) in the brain of mice were determined. Combined treatment of PM and S. aureus led to significant increment of escape latency at day 6, 8, and 10. Oxidative stress levels, and pro-inflammatory cytokines were affected significantly by S. aureus and PM as individual matter and mixtures. Meanwhile, Glu contents were increased significantly in S. aureus group, ChAT levels were decreased significantly in PM group, combined treatment of PM and S. aureus led to significant concentration reduction of AChE. Treatment of S. aureus or PM- S. aureus combination also led to significant concentration reduction of BDNF. Results showed that combined treatment of PM and S. aureus induced damage on physique and motor function, as well as impairment on learning and memory capacity of mice. Oxidative damage, abnormal metabolism of neurotransmitters and cholinergic system enzymes, and the alternation of neurotrophins and pro-inflammatory cytokines expression might be the possible mechanisms for PM - S. aureus -induced neurotoxicity.

Excretory and Secretory Proteins of Naegleria fowleri Induce Inflammatory Responses in BV-2 Microglial Cells

Naegleria fowleri, a free-living amoeba that is found in diverse environmental habitats, can cause a type of fulminating hemorrhagic meningoencephalitis, primary amoebic meningoencephalitis (PAM), in humans. The pathogenesis of PAM is not fully understood, but it is likely to be primarily caused by disruption of the host's nervous system via a direct phagocytic mechanism by the amoeba. Naegleria fowleri trophozoites are known to secrete diverse proteins that may indirectly contribute to the pathogenic function of the amoeba, but this factor is not clearly understood. In this study, we analyzed the inflammatory responses in BV-2 microglial cells induced by excretory and secretory proteins of N. fowleri (NfESP). Treatment of BV-2 cells with NfESP induced the expression of various cytokines and chemokines, including the proinflammatory cytokines IL-1α and TNF-α. NfESP-induced IL-1α and TNF-α expression in BV-2 cells were regulated by p38, JNK, and ERK MAPKs. NfESP-induced IL-1α and TNF-α production in BV-2 cells were effectively downregulated by inhibition of NF-kB and AP-1. These results collectively suggest that NfESP stimulates BV-2 cells to release IL-1α and TNF-α via NF-kB- and AP-1-dependent MAPK signaling pathways. The released cytokines may contribute to inflammatory responses in microglia and other cell types in the brain during N. fowleri infection.

Glucocorticoids: Protectors of the Brain during Innate Immune Responses

The innate immune response is a coordinated set of reactions involving cells of myeloid lineage and a network of signaling molecules. Such a response takes place in the CNS during trauma, stroke, spinal cord injury, and neurodegenerative diseases, suggesting that macrophages/microglia are the cells that perpetuate the progressive neuronal damage. However, there is accumulating evidence that these cells and their secreted proinflammatory molecules have more beneficial effects than detrimental consequences for the neuronal elements. Indeed, a timely controlled innate immune response may limit toxicity in swiftly eliminating foreign materials and debris that are known to interfere with recovery and regeneration. Each step of the immune cascade is under the tight control of stimulatory and inhibitory signals. Glucocorticoids (GCs) act as the critical negative feedback on all myeloid cells, including those present within the brain parenchyma. Because too little is like too much, both an inappropriate feedback of GCs on microglia and high circulating GC levels in stressed individuals have been associated with deleterious consequences for the brain. In this review, the authors discuss both sides of the story with a particular emphasis on the neuro-protective role of endogenous GCs during immune challenges and the problems in determining whether GCs can be a good therapy for the treatment of neuropathological conditions.

Anatomical site-specific contributions of pneumococcal virulence determinants

Streptococcus pneumoniae is an opportunistic pathogen globally associated with significant morbidity and mortality. It is capable of causing a wide range of diseases including sinusitis, conjunctivitis, otitis media, pneumonia, bacteraemia, sepsis, and meningitis. While its capsular polysaccharide is indispensible for invasive disease, and opsonising antibodies against the capsule are the basis for the current vaccines, a long history of biomedical research indicates that other components of this Gram-positive bacterium are also critical for virulence. Herein we review the contribution of pneumococcal virulence determinants to survival and persistence in the context of distinct anatomical sites. We discuss how these determinants allow the pneumococcus to evade mucociliary clearance during colonisation, establish lower respiratory tract infection, resist complement deposition and opsonophagocytosis in the bloodstream, and invade secondary tissues such as the central nervous system leading to meningitis. We do so in a manner that highlights both the critical role of the capsular polysaccharide and the accompanying and necessary protein determinants. Understanding the complex interplay between host and pathogen is necessary to find new ways to prevent pneumococcal infection. This review is an attempt to do so with consideration for the latest research findings.

Bacterial Peptidoglycan Traverses the Placenta to Induce Fetal Neuroproliferation and Aberrant Postnatal Behavior

Maternal infection during pregnancy is associated with adverse outcomes for the fetus, including postnatal cognitive disorders. However, the underlying mechanisms are obscure. We find that bacterial cell wall peptidoglycan (CW), a universal PAMP for TLR2, traverses the murine placenta into the developing fetal brain. In contrast to adults, CW-exposed fetal brains did not show any signs of inflammation or neuronal death. Instead, the neuronal transcription factor FoxG1 was induced, and neuroproliferation leading to a 50% greater density of neurons in the cortical plate was observed. Bacterial infection of pregnant dams, followed by antibiotic treatment, which releases CW, yielded the same result. Neuroproliferation required TLR2 and was recapitulated in vitro with fetal neuronal precursor cells and TLR2/6, but not TLR2/1, ligands. The fetal neuroproliferative response correlated with abnormal cognitive behavior in CW-exposed pups following birth. Thus, the bacterial CW-TLR2 signaling axis affects fetal neurodevelopment and may underlie postnatal cognitive disorders.

Host-pathogen interactions in bacterial meningitis

Bacterial meningitis is a devastating disease occurring worldwide with up to half of the survivors left with permanent neurological sequelae. Due to intrinsic properties of the meningeal pathogens and the host responses they induce, infection can cause relatively specific lesions and clinical syndromes that result from interference with the function of the affected nervous system tissue. Pathogenesis is based on complex host–pathogen interactions, some of which are specific for certain bacteria, whereas others are shared among different pathogens. In this review, we summarize the recent progress made in understanding the molecular and cellular events involved in these interactions. We focus on selected major pathogens, Streptococcus pneumonia, S. agalactiae (Group B Streptococcus), Neisseria meningitidis, and Escherichia coli K1, and also include a neglected zoonotic pathogen, Streptococcus suis. These neuroinvasive pathogens represent common themes of host–pathogen interactions, such as colonization and invasion of mucosal barriers, survival in the blood stream, entry into the central nervous system by translocation of the blood–brain and blood–cerebrospinal fluid barrier, and induction of meningeal inflammation, affecting pia mater, the arachnoid and subarachnoid spaces.

Microglia Function in the Normal Brain

The activation of microglia has been recognized for over a century by their morphological changes. Long slender microglia acquire a short sturdy ramified shape when activated. During the past 20 years, microglia have been accepted as an essential cellular component for understanding the pathogenic mechanism of many brain diseases, including neurodegenerative diseases. More recently, functional studies and imaging in mouse models indicate that microglia are active in the healthy central nervous system. It has become evident that microglia release several signal molecules that play key roles in the crosstalk among brain cells, i.e., astrocytes and oligodendrocytes with neurons, as well as with regulatory immune cells. Recent studies also reveal the heterogeneous nature of microglia diverse functions depending on development, previous exposure to stimulation events, brain region of residence, or pathological state. Subjects to approach by future research are still the unresolved questions regarding the conditions and mechanisms that render microglia protective, capable of preventing or reducing damage, or deleterious, capable of inducing or facilitating the progression of neuropathological diseases. This novel knowledge will certainly change our view on microglia as therapeutic target, shifting our goal from their general silencing to the generation of treatments able to change their activation pattern.

Patterns of Local and Systemic Cytokines in Bacterial Meningitis and its Relation with Severity and Long-Term Sequelae

Bacterial meningitis (BM) is a pyogenic infection present in the subarachnoid space, potentially fatal and frequently associated with neurological sequelae. During BM, cytokines (CTs) are locally produced. We sought to determine the CTs' clinical role as disease severity predictors in adults, which is not completely clear. Using a bead-based flow cytometric assay, levels of six CTs were determined in cerebrospinal fluid (CSF) and plasma from 18 adult BM patients and 19 uninfected controls. Long-term neurological sequelae were evaluated using the Glasgow Outcome Scale (GOS). All evaluated CTs were higher in CSF than in plasma, and the levels of CSF interleukin (IL)-6, IL-8, IL-10, IL-1β, and tumor necrosis factor-α and plasma IL-10 and IL-12p70 were significantly higher in patients with severe sepsis than with sepsis, suggesting an association with clinical severity. There was a strong negative correlation between CSF IL-6 and plasma IL-12p70 with GOS score, supporting the possible role of these CTs in the development of neurological long-term sequelae. These findings could be helpful to identify candidates to receive neuroprotective treatments and early physiotherapy schemes.

Interferon-γ from Brain Leukocytes Enhances Meningitis by Type 4 Streptococcus pneumoniae

Streptococcus pneumoniae is the leading cause of bacterial meningitis. Pneumococcal meningitis is a life-threatening disease with high rates of mortality and neurological sequelae. Immune targeting of S. pneumoniae is essential for clearance of infection; however, within the brain, the induced inflammatory response contributes to pathogenesis. In this study we investigate the local inflammatory response and the role of IFN-γ in a murine model of pneumococcal meningitis induced by intracranial injection of type 4 S. pneumoniae. Lymphoid and myeloid cell populations involved in meningitis, as well as cytokine gene expression, were investigated after infection. Animals were treated with a monoclonal antibody specific for murine IFN-γ to evaluate its role in animal survival. Intracranial inoculation of 3 × 10⁴ colony-forming units of type 4 strain TIGR4 caused 75% of mice to develop meningitis within 4 days. The amount of lymphocytes, NK cells, neutrophils, monocytes and macrophages in the brain increased 48 h post infection. IFN-γ mRNA levels were about 240-fold higher in brains of infected mice compared to controls. Pro-inflammatory cytokines such as IL-1β and TNF-α, and TLR2 were also upregulated. In vivo treatment with anti-IFN-γ antibody increased survival of infected mice. This study shows that IFN-γ produced during meningitis by type 4 S. pneumoniae enhances bacterial pathogenesis exerting a negative effect on the disease outcome.

Reparixin attenuates neuronal injury in experimental Klebsiella pneumoniae meningoencephalitis through dual effects on neuroprotection and neuroinflammation

AIMS

Bacterial meningitis causes high mortality and brain damage. The host immune response is associated with brain injury. Chemokine (C-X-C motif) (CXC) chemokines are neutrophil chemoattractants. This study focused on the beneficial effects of intracerebroventricular administration of reparixin, an inhibitor of chemokine (C-X-C motif) receptor (CXCR)1/2, to rats at 2 h following experimental Klebsiella pneumoniae meningoencephalitis.

METHODS

We used a previously established meningoencephalitis animal model in which Sprague-Dawley rats were infected by K. pneumoniae. Sham and infected animals were treated with vehicle or reparixin and sacrificed at various time points. Leukocyte infiltration into cerebrospinal fluid (CSF) and brain as well as gene and protein expression of chemokines and receptors, and neuronal apoptosis were examined. Primary cultures of neuron/glia were infected with K. pneumoniae as an in vitro model of meningoencephalitis.

RESULTS

Levels of chemokine (C-X-C motif) ligand (CXCL)2 in CSF time-dependently increased markedly as early as 2 h, and peaked at 8 h following infection and were much higher than those in serum collected simultaneously. Reparixin significantly reduced leukocyte infiltration into CSF and brain tissues, clinical illness, and brain cell apoptosis at 24 h. Reparixin reduced the elevated CSF concentrations of chemokines [CXCL1, CXCL2, chemokine (C-C motif) ligand (CCL)2 and CCL5] and proinflammatory cytokines. Reparixin also reduced the expression of mRNA of various chemokines, chemokine receptors and proinflammatory cytokines in infected brain tissues. Using primary cultures that are devoid of leukocytes, we further observed that reparixin attenuated the neuronal, but not microglial cell death after infection.

CONCLUSIONS

Reparixin not only reduces amplified inflammation, but also provides direct neuroprotective effects in K. pneumoniae meningoencephalitis.

Cerebral Ventriculitis Associated with Otogenic Meningoencephalitis in a Dog

A dog was evaluated for rapidly progressive mentation change, ataxia, and tetraparesis. The dog's neurological status deteriorated drastically. It became comatose with bilateral mydriasis, and the pupillary light reflex was absent. An anti-inflammatory dose of methylprednisolone was administered, and temporary stabilization of neurological status was achieved. MRI findings were suggestive of ventriculitis and meningoencephalitis originating from the left tympanic cavity. A gadolinium leakage phenomenon was noted, likely resulting from severe damage to the blood-cerebrospinal fluid barrier during the inflammatory process. Analysis of the cerebrospinal fluid and materials in the left tympanic cavity further confirmed the diagnosis. Following surgical and antibiotic treatment, the dog recovered well with only a mild residual head tilt. Seven months after surgery, the dog had a recurrent infection of the left tympanic cavity without intracranial involvement. A second surgery led to an uneventful recovery, and the dog was clinically normal except for a mild head tilt 3 yr after the initial presentation. This is the first report describing ventriculitis associated with otogenic meningoencephalitis in dogs and a gadolinium leakage phenomenon displayed on MRI. The long-term outcome of ventriculitis-complicated otogenic meningoencephalitis in dogs could be satisfied with prompt diagnosis and treatment.

Extending the viability of acute brain slices

The lifespan of an acute brain slice is approximately 6–12 hours, limiting potential experimentation time. We have designed a new recovery incubation system capable of extending their lifespan to more than 36 hours. This system controls the temperature of the incubated artificial cerebral spinal fluid (aCSF) while continuously passing the fluid through a UVC filtration system and simultaneously monitoring temperature and pH. The combination of controlled temperature and UVC filtering maintains bacteria levels in the lag phase and leads to the dramatic extension of the brain slice lifespan. Brain slice viability was validated through electrophysiological recordings as well as live/dead cell assays. This system benefits researchers by monitoring incubation conditions and standardizing this artificial environment. It further provides viable tissue for two experimental days, reducing the time spent preparing brain slices and the number of animals required for research.

TLR4-mediated brain inflammation halts neurogenesis: impact of hormonal replacement therapy

Experimental and epidemiological data show that the severity and the duration of brain inflammation are attenuated in females compared to males. This attenuated brain inflammation is ascribed to 17β-estradiol. However, several studies suggest that 17β-estradiol is also endowed with proinflammatory properties. The aim of the present study is to assess the effect of hormonal replacement therapies on lipopolysaccharide (LPS)-induced brain inflammation and its consequent effect on newly born neurons. Bilaterally ovariectomized rats received intrastriatal injection of LPS (250 ng/μl) and were subsequently given daily subcutaneous injections of either vehicle, 17β-estradiol (25 μg/kg) or 17β-estradiol and progesterone (5 mg/kg). Microglial activation and newly born neurons in the rostral migratory stream were monitored using double immunofluorescence. Nuclear factor κB (NFκB) signaling pathway and its target inflammatory proteins were assessed by either western blot [cyclooxygenase-2 (COX-2) and interleukin-6 (IL-6)] or enzyme-linked immunosorbent assay [tumor necrosis factor-α (TNF-α)]. LPS-induced activation of microglia, promoted NFκB signaling pathway and enhanced the production of proinflammatory proteins (TNF-α and COX-2). These proinflammatory responses were not attenuated by 17β-estradiol injection. Supplementation of 17β-estradiol with progesterone significantly dampened these proinflammatory processes. Interestingly, LPS-induced brain inflammation dampened the number of newly born neurons in the rostral migratory stream. Administration of combined 17β-estradiol and progesterone resulted in a significantly higher number of newly born neurons when compared to those seen in rats given either vehicle or 17β-estradiol alone. These data strongly suggest that combined 17β-estradiol and progesterone, and not 17β-estradiol alone, rescues neurogenesis from the deleterious effect of brain inflammation likely via the inhibition of the signaling pathways leading to the activation of proinflammatory genes.

Strategies to increase the activity of microglia as efficient protectors of the brain against infections

In healthy individuals, infections of the central nervous system (CNS) are comparatively rare. Based on the ability of microglial cells to phagocytose and kill pathogens and on clinical findings in immunocompromised patients with CNS infections, we hypothesize that an intact microglial function is crucial to protect the brain from infections. Phagocytosis of pathogens by microglial cells can be stimulated by agonists of receptors of the innate immune system. Enhancing this pathway to increase the resistance of the brain to infections entails the risk of inducing collateral damage to the nervous tissue. The diversity of microglial cells opens avenue to selectively stimulate sub-populations responsible for the defence against pathogens without stimulating sub-populations which are responsible for collateral damage to the nervous tissue. Palmitoylethanolamide (PEA), an endogenous lipid, increased phagocytosis of bacteria by microglial cells in vitro without a measurable proinflammatory effect. It was tested clinically apparently without severe side effects. Glatiramer acetate increased phagocytosis of latex beads by microglia and monocytes, and dimethyl fumarate enhanced elimination of human immunodeficiency virus from infected macrophages without inducing a release of proinflammatory compounds. Therefore, the discovery of compounds which stimulate the elimination of pathogens without collateral damage of neuronal structures appears an achievable goal. PEA and, with limitations, glatiramer acetate and dimethyl fumarate appear promising candidates.

Upregulation and lysosomal degradation of AQP4 in rat brains with bacterial meningitis

Brain edema is among the major complications in children with bacterial meningitis. Aquaporins are integral membrane pore proteins that form channels to regulate cellular water content. Aquaporin-4 (AQP4), which is enriched in parts of astrocytic membranes that are apposed to pial or perivascular basal laminae, is the predominant aquaporin in the central nervous system. Dystroglycan is among the proteins that are responsible for the site-specific anchorage of AQP4. To elucidate the role of AQP4 in the development of brain edema induced by meningitis, a model of bacterial meningitis was established by injecting group B β-hemolytic Streptococci into the cerebrospinal fluid of three-week-old rats. The brain water content increased in this model compared with that in the control group. The expression of AQP4 and dystroglycan was examined by Western blot and the degradation route of AQP4 was investigated by double immunofluorescence labeling. Western blot results showed that the expression of AQP4 and dystroglycan in rat brain increased in the meningitis model. Meanwhile, AQP4 was co-localized with the marker of lysosome in this model, indicating that the lysosome is involved in AQP4 degradation.

Vitamin d deficiency reduces the immune response, phagocytosis rate, and intracellular killing rate of microglial cells

Meningitis and meningoencephalitis caused by Escherichia coli are associated with high rates of mortality and neurological sequelae. A high prevalence of neurological disorders has been observed in geriatric populations at risk of hypovitaminosis D. Vitamin D has potent effects on human immunity, including induction of antimicrobial peptides (AMPs) and suppression of T-cell proliferation, but its influence on microglial cells is unknown. The purpose of the present study was to determine the effects of vitamin D deficiency on the phagocytosis rate, intracellular killing, and immune response of murine microglial cultures after stimulation with the Toll-like receptor (TLR) agonists tripalmitoyl-S-glyceryl-cysteine (TLR1/2), poly(I·C) (TLR3), lipopolysaccharide (TLR4), and CpG oligodeoxynucleotide (TLR9). Upon stimulation with high concentrations of TLR agonists, the release of tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) was decreased in vitamin D-deficient compared to that in vitamin D-sufficient microglial cultures. Phagocytosis of E. coli K1 after stimulation of microglial cells with high concentrations of TLR3, -4, and -9 agonists and intracellular killing of E. coli K1 after stimulation with high concentrations of all TLR agonists were lower in vitamin D-deficient microglial cells than in the respective control cells. Our observations suggest that vitamin D deficiency may impair the resistance of the brain against bacterial infections.

Interleukin-18 deficiency and its long-term behavioural and cognitive impacts in a murine model of pneumococcal meningitis

Pneumococcal meningitis often results in death or neurological sequelae, but the underlying pathogenetic mechanisms remain poorly understood. In C57BL/6J mice subjected to intracerebroventricular (icv) challenge with Streptococcus pneumoniae, the chemokine CCL2 and cytokines interferon-γ, interleukin (IL)-1β, IL-6 and tumour necrosis factor were prominently expressed in the brain during the acute phase of the disease. The upregulation of these immune mediators was markedly diminished in IL-18-deficient mice. Uninfected IL-18(-/-) mice exhibited decreases in anxiety phenotype and licking behaviour, and an increase in behavioural habituation, in an automated monitoring system (the IntelliCage). Without antibiotic intervention, a majority of IL-18(+/+) mice developed irreversible disease after icv S. pneumoniae but this was significantly improved by deleting IL-18 gene function. IL-18(+/+) mice cured of pneumococcal meningitis with four doses of ceftriaxone, initiated at 20 h post-inoculation, showed enduring sequelae. These included abnormal behavioural phenotypes featuring diurnal hypoactivity and nocturnal hyperactivity, light phobia and disrupted cognitive function. While the hyperactive phenotype was absent in the corresponding IL-18(-/-) survivors, cognitive impairments and behavioural deficits were still present. Overall, the results suggest that the high levels of cytokines and/or chemokines released after pneumococcal challenge provoked a series of pathological events, ultimately causing acute death. Furthermore, since only a subset of behavioural phenotypes were ameliorated in the pneumococcus-infected IL-18(-/-) mice, the pathological pathways causing mortality may be, at least in part, distinct from those leading to long-term neurological sequelae.

Reversal of Neuronal Atrophy: Role of Cellular Immunity in Neuroplasticity and Aging

Emerging evidence indicates that neuroimmunological changes in the brain can modify intrinsic brain processes that are involved in regulating neuroplasticity. Increasing evidence suggests that in some forms of motor neuron injury, many neurons do not die, but reside in an atrophic state for an extended period of time. In mice, facial motor neurons in the brain undergo a protracted period of degeneration or atrophy following resection of their peripheral axons. Reinjuring the proximal nerve stump of the chronically resected facial nerve stimulates a robust reversal of motor neuron atrophy which results in marked increases in both the number and size of injured motor neurons in the facial motor nucleus. In this brief review, we describe research from our lab which indicates that the reversal of atrophy in this injury model is dependent on normal cellular immunity. The role of T cells in this unique form of neuroplasticity following injury and in brain aging, are discussed. The potential role of yet undiscover intrinsic actions of recombination activating genes in the brain are considered. Further research using the facial nerve reinjury model could identify molecular signals involved in neuroplasticity, and lead to new ways to stimulate neuroregenerative processes in neurotrauma and other forms of brain insult and disease.

A novel automated test battery reveals enduring behavioural alterations and cognitive impairments in survivors of murine pneumococcal meningitis

Pneumococcal meningitis, caused by Streptococcus pneumoniae infection, is a major form of lethal bacterial meningitis. Survivors are predisposed to developing lifelong disabling sequelae, including cognitive impairment, psychological problems and motor deficits. In our experimental model, ventricular inoculation of 10(5) colony-forming units of S. pneumoniae type 3 caused 90% of mice to develop life-threatening meningitis within 48 h. Antibiotic treatment with ceftriaxone 20 h post infection reduced the incidence of severe meningitis to <10%. At the time of treatment, upregulation of pro-inflammatory cytokines was detected, including interleukin-1β, interleukin-6 and tumour necrosis factor. We evaluated the long-term behavioural and cognitive sequelae in control mice and those surviving meningitis using an automated system (the IntelliCage) in which mice perform a range of behavioural and spatial tasks to obtain water rewards from conditioning units in their home cage. Surviving mice showed a number of altered behaviours relative to controls, including (i) hypoexploration when first exposed to the IntelliCage, (ii) altered activity patterns (fewer visits to conditioning stations during the light phase and more in the dark phase), (iii) avoidance of light (a constant or flashing LED stimulus), (iv) impaired spatial learning (a complex patrolling task), and (v) impaired discrimination reversal learning. Overall these results suggest photophobia and weakened learning ability in post-meningitic mice, particularly on tasks engaging hippocampal and prefrontal neural substrates. This study also demonstrates a standardised and comprehensive battery of tests that can be readily used to investigate neurological sequelae in undisturbed mice residing in a complex home cage environment.