Targets for adjunctive therapy in pneumococcal meningitis

Pneumococcal Meningitis Induces Hearing Loss and Cochlear Ossification Modulated by Chemokine Receptors CX3CR1 and CCR2

PURPOSE

Pneumococcal meningitis is a major cause of hearing loss and permanent neurological impairment despite widely available antimicrobial therapies to control infection. Methods to improve hearing outcomes for those who survive bacterial meningitis remains elusive. We used a mouse model of pneumococcal meningitis to evaluate the impact of mononuclear phagocytes on hearing outcomes and cochlear ossification by altering the expression of CX3CR1 and CCR2 in these infected mice.

METHODS

We induced pneumococcal meningitis in approximately 500 C57Bl6 adult mice using live Streptococcus pneumoniae (serotype 3, 1 × 105 colony forming units (cfu) in 10 µl) injected directly into the cisterna magna of anesthetized mice and treated these mice with ceftriaxone daily until recovered. We evaluated hearing thresholds over time, characterized the cochlear inflammatory response, and quantified the amount of new bone formation during meningitis recovery. We used microcomputed tomography (microCT) scans to quantify cochlear volume loss caused by neo-ossification. We also performed perilymph sampling in live mice to assess the integrity of the blood-perilymph barrier during various time intervals after meningitis. We then evaluated the effect of CX3CR1 or CCR2 deletion in meningitis symptoms, hearing loss, macrophage/monocyte recruitment, neo-ossification, and blood labyrinth barrier function.

RESULTS

Sixty percent of mice with pneumococcal meningitis developed hearing loss. Cochlear fibrosis could be detected within 4 days of infection, and neo-ossification by 14 days. Loss of spiral ganglion neurons was common, and inner ear anatomy was distorted by scarring caused by new soft tissue and bone deposited within the scalae. The blood-perilymph barrier was disrupted at 3 days post infection (DPI) and was restored by seven DPI. Both CCR2 and CX3CR1 monocytes and macrophages were present in the cochlea in large numbers after infection. Neither chemokine receptor was necessary for the induction of hearing loss, cochlear fibrosis, ossification, or disruption of the blood-perilymph barrier. CCR2 knockout (KO) mice suffered the most severe hearing loss. CX3CR1 KO mice demonstrated an intermediate phenotype with greater susceptibility to hearing loss compared to control mice. Elimination of CX3CR1 mononuclear phagocytes during the first 2 weeks after meningitis in CX3CR1-DTR transgenic mice did not protect mice from any of the systemic or hearing sequelae of pneumococcal meningitis.

CONCLUSIONS

Pneumococcal meningitis can have devastating effects on cochlear structure and function, although not all mice experienced hearing loss or cochlear damage. Meningitis can result in rapid progression of hearing loss with fibrosis starting at four DPI and ossification within 2 weeks of infection detectable by light microscopy. The inflammatory response to bacterial meningitis is robust and can affect all three scalae. Our results suggest that CCR2 may assist in controlling infection and maintaining cochlear patency, as CCR2 knockout mice experienced more severe disease, more rapid hearing loss, and more advanced cochlear ossification after pneumococcal meningitis. CX3CR1 also may play an important role in the maintenance of cochlear patency.

Progressive cognitive impairment after recovery from neuroinvasive and non-neuroinvasive Listeria monocytogenes infection

Background

Neuro-cognitive impairment is a deleterious complication of bacterial infections that is difficult to treat or prevent. Listeria monocytogenes (Lm) is a neuroinvasive bacterial pathogen and commonly used model organism for studying immune responses to infection. Antibiotic-treated mice that survive systemic Lm infection have increased numbers of CD8⁺ and CD4⁺ T-lymphocytes in the brain that include tissue resident memory (T_RM) T cells, but post-infectious cognitive decline has not been demonstrated. We hypothesized that Lm infection would trigger cognitive decline in accord with increased numbers of recruited leukocytes.

Methods

Male C57BL/6J mice (age 8 wks) were injected with neuroinvasive Lm 10403s, non-neuroinvasive Δhly mutants, or sterile saline. All mice received antibiotics 2-16d post-injection (p.i.) and underwent cognitive testing 1 month (mo) or 4 mo p.i. using the Noldus PhenoTyper with Cognition Wall, a food reward-based discrimination procedure using automated home cage based observation and monitoring. After cognitive testing, brain leukocytes were quantified by flow cytometry.

Results

Changes suggesting cognitive decline were observed 1 mo p.i. in both groups of infected mice compared with uninfected controls, but were more widespread and significantly worse 4 mo p.i. and most notably after Lm 10403s. Impairments were observed in learning, extinction of prior learning and distance moved. Infection with Lm 10403s, but not Δhly Lm, significantly increased numbers of CD8⁺ and CD4⁺ T-lymphocytes, including populations expressing CD69 and T_RM cells, 1 mo p.i. Numbers of CD8⁺, CD69⁺CD8⁺ T-lymphocytes and CD8⁺ T_RM remained elevated at 4 mo p.i. but numbers of CD4⁺ cells returned to homeostatic levels. Higher numbers of brain CD8⁺ T-lymphocytes showed the strongest correlations with reduced cognitive performance.

Conclusions

Systemic infection by neuroinvasive as well as non-neuroinvasive Lm triggers a progressive decline in cognitive impairment. Notably, the deficits are more profound after neuroinvasive infection that triggers long-term retention of CD8⁺ T-lymphocytes in the brain, than after non-neuroinvasive infection, which does not lead to retained cells in the brain. These results support the conclusion that systemic infections, particularly those that lead to brain leukocytosis trigger a progressive decline in cognitive function and implicate CD8⁺ T-lymphocytes, including CD8⁺T_RM in the etiology of this impairment.

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.

Combined therapy with ceftriaxone and doxycycline does not improve the outcome of meningococcal meningitis in mice compared to ceftriaxone monotherapy

BACKGROUND

Meningococcal meningitis (MM) is a life-threatening disease associated with approximately 10% case fatality rates and neurological sequelae in 10-20% of the cases. Recently, we have shown that the matrix metalloproteinase (MMP) inhibitor BB-94 reduced brain injury in a mouse model of MM. The present study aimed to assess whether doxycycline (DOX), a tetracycline that showed a neuroprotective effect as adjuvant therapy in experimental pneumococcal meningitis (PM), would also exert a beneficial effect when given as adjunctive therapy to ceftriaxone (CRO) in experimental MM.

METHODS

BALB/c mice were infected by the intracisternal route with a group C Neisseria meningitidis strain. Eighteen h post infection (hpi), animals were randomised for treatment with CRO [100 mg/kg subcutaneously (s.c.)], CRO plus DOX (30 mg/kg s.c.) or saline (control s.c.). Antibiotic treatment was repeated 24 and 40 hpi. Mouse survival and clinical signs, bacterial counts in cerebella, brain damage, MMP-9 and cyto/chemokine levels were assessed 48 hpi.

RESULTS

Analysis of bacterial load in cerebella indicated that CRO and CRO + DOX were equally effective at controlling meningococcal replication. No differences in survival were observed between mice treated with CRO (94.4%) or CRO + DOX (95.5%), (p > 0.05). Treatment with CRO + DOX significantly diminished both the number of cerebral hemorrhages (p = 0.029) and the amount of MMP-9 in the brain (p = 0.046) compared to untreated controls, but not to CRO-treated animals (p > 0.05). Levels of inflammatory markers in the brain of mice that received CRO or CRO + DOX were not significantly different (p > 0.05). Overall, there were no significant differences in the parameters assessed between the groups treated with CRO alone or CRO + DOX.

CONCLUSIONS

Treatment with CRO + DOX showed similar bactericidal activity to CRO in vivo, suggesting no antagonist effect of DOX on CRO. Combined therapy significantly improved mouse survival and disease severity compared to untreated animals, but addition of DOX to CRO did not offer significant benefits over CRO monotherapy. In contrast to experimental PM, DOX has no adjunctive activity in experimental MM.

HIF-1α is involved in blood-brain barrier dysfunction and paracellular migration of bacteria in pneumococcal meningitis

Bacterial meningitis is a deadly disease most commonly caused by Streptococcus pneumoniae, leading to severe neurological sequelae including cerebral edema, seizures, stroke, and mortality when untreated. Meningitis is initiated by the transfer of S. pneumoniae from blood to the brain across the blood-cerebrospinal fluid barrier or the blood-brain barrier (BBB). The underlying mechanisms are still poorly understood. Current treatment strategies include adjuvant dexamethasone for inflammation and cerebral edema, followed by antibiotics. The success of dexamethasone is however inconclusive, necessitating new therapies for controlling edema, the primary reason for neurological complications. Since we have previously shown a general activation of hypoxia inducible factor (HIF-1α) in bacterial infections, we hypothesized that HIF-1α, via induction of vascular endothelial growth factor (VEGF) is involved in transmigration of pathogens across the BBB. In human, murine meningitis brain samples, HIF-1α activation was observed by immunohistochemistry. S. pneumoniae infection in brain endothelial cells (EC) resulted in in vitro upregulation of HIF-1α/VEGF (Western blotting/qRT-PCR) associated with increased paracellular permeability (fluorometry, impedance measurements). This was supported by bacterial localization at cell-cell junctions in vitro and in vivo in brain ECs from mouse and humans (confocal, super-resolution, electron microscopy, live-cell imaging). Hematogenously infected mice showed increased permeability, S. pneumoniae deposition in the brain, along with upregulation of genes in the HIF-1α/VEGF pathway (RNA sequencing of brain microvessels). Inhibition of HIF-1α with echinomycin, siRNA in bEnd5 cells or using primary brain ECs from HIF-1α knock-out mice revealed reduced endothelial permeability and transmigration of S. pneumoniae. Therapeutic rescue using the HIF-1α inhibitor echinomycin resulted in increased survival and improvement of BBB function in S. pneumoniae-infected mice. We thus demonstrate paracellular migration of bacteria across BBB and a critical role for HIF-1α/VEGF therein and hence propose targeting this pathway to prevent BBB dysfunction and ensuing brain damage in infections.

Cerebral Metabolic Changes Related to Oxidative Metabolism in a Model of Bacterial Meningitis Induced by Lipopolysaccharide

BACKGROUND

Cerebral mitochondrial dysfunction is prominent in the pathophysiology of severe bacterial meningitis. In the present study, we hypothesize that the metabolic changes seen after intracisternal lipopolysaccharide (LPS) injection in a piglet model of meningitis is compatible with mitochondrial dysfunction and resembles the metabolic patterns seen in patients with bacterial meningitis.

METHODS

Eight pigs received LPS injection in cisterna magna, and four pigs received NaCl in cisterna magna as a control. Biochemical variables related to energy metabolism were monitored by intracerebral microdialysis technique and included interstitial glucose, lactate, pyruvate, glutamate, and glycerol. The intracranial pressure (ICP) and brain tissue oxygen tension (PbtO₂) were also monitored along with physiological variables including mean arterial pressure, blood glucose, lactate, and partial pressure of O₂ and CO₂. Pigs were monitored for 60 min at baseline and 240 min after LPS/NaCl injection.

RESULTS

After LPS injection, a significant increase in cerebral lactate/pyruvate ratio (LPR) compared to control group was registered (p = 0.01). This increase was due to a significant increased lactate with stable and normal values of pyruvate. No significant change in PbtO₂ or ICP was registered. No changes in physiological variables were observed.

CONCLUSIONS

The metabolic changes after intracisternal LPS injection is compatible with disturbance in the oxidative metabolism and partly due to mitochondrial dysfunction with increasing cerebral LPR due to increased lactate and normal pyruvate, PbtO₂, and ICP. The metabolic pattern resembles the one observed in patients with bacterial meningitis. Metabolic monitoring in these patients is feasible to monitor for cerebral metabolic derangements otherwise missed by conventional intensive care monitoring.

Endocervical and Neutrophil Lipoxygenases Coordinate Neutrophil Transepithelial Migration to Neisseria gonorrhoeae

Background

Infection with Neisseria gonorrhoeae (GC) is characterized by robust neutrophil influx that is insufficient to clear the bacteria. Sustained neutrophilic inflammation contributes to serious clinical sequelae that particularly affect women, including pelvic inflammatory disease and infertility.

Methods

We established a 3-component system using GC, End1 polarized human endocervical cells, and primary human neutrophils to investigate neutrophil transepithelial migration following infection.

Results

Neutrophil migration across endocervical monolayers increased with the infectious dose and required GC-epithelial cell contact. Epithelial protein kinase C, cytosolic phospholipase A2, 12R-lipoxygenase (LOX), and eLOX3 hepoxilin synthase were required for neutrophil transmigration to GC, and migration was abrogated by blocking the MRP2 efflux pump and by adding recombinant soluble epoxide hydrolase. These results are all consistent with epithelial cell production of the neutrophil chemoattractant hepoxilin A3 (HXA3). Neutrophil transmigration was also accompanied by increasing apical concentrations of leukotriene B4 (LTB4). Neutrophil 5-lipoxygenase and active BLT1 receptor were required for apical LTB4 and neutrophil migration.

Conclusions

Our data support a model in which GC-endocervical cell contact infection stimulates HXA3 production, driving neutrophil migration that is amplified by neutrophil-derived LTB4. Therapeutic targeting of these pathways could limit inflammation and deleterious clinical sequelae in women with gonorrhea.

The Role of Microglia in Bacterial Meningitis: Inflammatory Response, Experimental Models and New Neuroprotective Therapeutic Strategies

Microglia have a pivotal role in the pathophysiology of bacterial meningitis. The goal of this review is to provide an overview on how microglia respond to bacterial pathogens targeting the brain, how the interplay between microglia and bacteria can be studied experimentally, and possible ways to use gained knowledge to identify novel preventive and therapeutic strategies. We discuss the dual role of microglia in disease development, the beneficial functions crucial for bacterial clearing, and the destructive properties through triggering neuroinflammation, characterized by cytokine and chemokine release which leads to leukocyte trafficking through the brain vascular endothelium and breakdown of the blood-brain barrier integrity. Due to intrinsic complexity of microglia and up until recently lack of specific markers, the study of microglial response to bacterial pathogens is challenging. New experimental models and techniques open up possibilities to accelerate progress in the field. We review existing models and discuss possibilities and limitations. Finally, we summarize recent findings where bacterial virulence factors are identified to be important for the microglial response, and how manipulation of evoked responses could be used for therapeutic or preventive purposes. Among promising approaches are: modulations of microglia phenotype switching toward anti-inflammatory and phagocytic functions, the use of non-bacterolytic antimicrobials, preventing release of bacterial components into the neural milieu and consequential amplification of immune activation, and protection of the blood-brain barrier integrity.

Blood‒Brain Barrier Pathology and CNS Outcomes in Streptococcus pneumoniae Meningitis

Streptococcus pneumoniae is a major meningitis-causing pathogen globally, bringing about significant morbidity and mortality, as well as long-term neurological sequelae in almost half of the survivors. Subsequent to nasopharyngeal colonisation and systemic invasion, translocation across the blood‒brain barrier (BBB) by S. pneumoniae is a crucial early step in the pathogenesis of meningitis. The BBB, which normally protects the central nervous system (CNS) from deleterious molecules within the circulation, becomes dysfunctional in S. pneumoniae invasion due to the effects of pneumococcal toxins and a heightened host inflammatory environment of cytokines, chemokines and reactive oxygen species intracranially. The bacteria‒host interplay within the CNS likely determines not only the degree of BBB pathological changes, but also host survival and the extent of neurological damage. This review explores the relationship between S. pneumoniae bacteria and the host inflammatory response, with an emphasis on the BBB and its roles in CNS protection, as well as both the acute and long-term pathogenesis of meningitis.

Effect of Hemoadsorption for Cytokine Removal in Pneumococcal and Meningococcal Sepsis

Bacterial meningitis and septicemia are invasive bacterial diseases, representing a significant cause of morbidity and mortality worldwide. Both conditions are characterized by an impressive inflammatory response, resulting rapidly in cerebral edema, infarction, hydrocephalus, and septic shock with multiple organ failure. Despite advances in critical care, outcome and prognosis remain critical. Available adjunctive treatments to control the inflammatory response have shown encouraging results in the evolution of patients with sepsis and systemic inflammation, but meningococcal or pneumococcal infection has not been investigated. We herein report five patients with similar critical pathological conditions, characterized by pneumococcal or meningococcal sepsis and treated with hemoadsorption for cytokine removal. All patients showed a progressive stabilization in hemodynamics along with a rapid and marked reduction of catecholamine dosages, a stabilization in metabolic disorders, and less-than-expected loss of extremities. Therapy proved to be safe and well tolerated. From this first experience, extracorporeal cytokine removal seems to be a valid and safe therapy in the management of meningococcal and pneumococcal diseases and may contribute to the patient stabilization and prevention of severe sequelae. Further studies are required to confirm efficacy in a larger context.

Adult Patients with Pneumococcal Meningitis at a Neurosurgical Neurologic Center: Different Predisposing Conditions?

BACKGROUND

In previous studies of pneumococcal meningitis in adults within general hospitals or national cohorts, the most common predisposing conditions were otitis media, sinusitis, pneumonia, immunosuppression, alcoholism, and diabetes. The epidemiology of pneumococcal meningitis is changing because of the use of vaccines in childhood, and antibiotic resistance has increased.

METHODS

We retrospectively reviewed the cases of patients with diagnoses of pneumococcal meningitis proved by an inflammatory cerebrospinal fluid (CSF) with a positive culture, treated during a period of 14 years at an adult neurosurgical neurologic referral center. Our aim was to define their clinical course, predisposing conditions, antimicrobial susceptibilities, and outcome.

RESULTS

We reviewed the cases of 30 patients, 17 men and 13 women, with a mean age of 36.7 years. Fourteen patients (46.6%) had previous neurosurgery, 12 patients (40%) had CSF fistula, 8 had remote head trauma, and 8 also presented recurrent meningitis. Resistance to ceftriaxone or vancomycin was less than 5%, and penicillin resistance was 53%. Eight patients (26.7%) had died. An increased risk of death was associated with coma at admission, septic shock, upper gastrointestinal bleeding, mechanical ventilation, thrombocytopenia, and a low CSF opening pressure.

CONCLUSIONS

We conclude that patients with pneumococcal meningitis treated at neurosurgical neurologic centers have different predisposing conditions with severe disease and high mortality, thus prompting us to recommend aggressive pneumococcal vaccination in patients with CSF leaks and severe head trauma. Prospective studies to identify which neurosurgical patients may benefit in the long term from a pneumococcal vaccine are urgently needed.

Brain-derived neurotrophic factor is regulated via MyD88/NF-κB signaling in experimental Streptococcus pneumoniae meningitis

Streptococcus pneumoniae meningitis is an intractable disease of the central nervous system (CNS). Brain-derived neurotrophic factor (BDNF) is a member of the neurotrophic family and found to participate in the immune inflammatory response. In this study, we investigated if activation of the classical inflammatory signaling pathway, myeloid differentiation factor 88 (MyD88)/nuclear factor-kappa B (NF-κB), regulates BDNF expression in experimental S. pneumoniae meningitis. MyD88 knockout (myd88-/-) mice and wild-type littermates were infected intracisternally with S. pneumoniae suspension. Twenty-four hours after inoculation, histopathology of brains was evaluated. Cytokine and chemokine in brains and spleens was analyzed using ELISA. NF-κB activation was evaluated using EMSA. Cortical and hippocampal BDNF was assessed using RT-PCR and ELISA, respectively. BDNF promoter activity was evaluated using ChIP-PCR. myd88-/- mice showed an obviously weakened inflammatory host response. This diminished inflammation was consistent with worse clinical parameters, neuron injury, and apoptosis. Deficiency in MyD88 was associated with decreased BDNF expression. Furthermore, we identified a valid κB-binding site in the BDNF promoter, consistent with activation of NF-κB induced by inflammation. To sum up, MyD88/NF-κB signaling has a crucial role in up-regulating BDNF, which might provide potential therapeutic targets for S. pneumoniae meningitis.

Psoriasin has divergent effects on the innate immune responses of murine glial cells

Antimicrobial peptides are an important part of the innate immune defense in the central nervous system (CNS). The expression of the antimicrobial peptides psoriasin (S100A7) is up-regulated during bacterial meningitis. However, the exact mechanisms induced by psoriasin to modulate glial cell activity are not yet fully understood. Our hypothesis is that psoriasin induced pro- and anti-inflammatory signaling pathways as well as regenerative factors to contribute in total to a balanced immune response. Therefore, we used psoriasin-stimulated glial cells and analyzed the translocation of the pro-inflammatory transcription factor nuclear factor 'kappa-light-chain-enhancer' of activated B-cells (NFκB) in murine glial cells and the expression of pro- and anti-inflammatory mediators by real time RT-PCR, ELISA technique, and western blotting. Furthermore, the relationship between psoriasin and the antioxidative stress transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) was investigated. Stimulation with psoriasin not only enhanced NFκB translocation and increased the expression of the pro-inflammatory cytokines, interleukin-6 (IL-6) and tumor necrosis factor-α (TNF- α) but also neurotrophin expression. Evidence for functional interactions between psoriasin and Nrf2 were detected in the form of increased antioxidant response element (ARE) activity and induction of Nrf2/ARE-dependent heme oxygenase 1 (HO-1) expression in psoriasin-treated microglia and astrocytes. The results illustrate the ability of psoriasin to induce immunological functions in glia cells where psoriasin exerts divergent effects on the innate immune response.

Antibody-induced neutrophil depletion prior to the onset of pneumococcal meningitis influences long-term neurological complications in mice

During pneumococcal meningitis, clearance of bacteria by recruited neutrophils is crucial for host protection. However, these innate immune mechanisms are often insufficient and treatment with antibiotics is necessary to prevent death. Despite this antibiotic treatment, approximately half of all survivors suffer lifelong neurological problems. There is growing evidence indicating the harmful effects of neutrophils on CNS integrity. Therefore, the present study investigated the roles of neutrophils in the acute inflammatory response and the resulting long-term neuropsychological effects in murine pneumococcal meningitis. Long-term behavioural and cognitive functions in mice were measured using an automated IntelliCage system. Neutrophil depletion with antibody 1A8 as adjunctive therapy was shown to remarkably impair survival in meningitic C57BL/6J mice despite antibiotic (ceftriaxone) treatment. This was accompanied by increased bacterial load in the cerebrospinal fluid (CSF) and an increase in IL-1β, but decrease in TNF, within the CSF at 20h after bacterial inoculation. In the longer term, the surviving neutrophil-depleted post-meningitic (PM) mice displayed reduced diurnal hypolocomotion compared to PM mice treated with an isotype antibody. However, they showed nocturnal hyperactivity, and greater learning impairment in a patrolling task that is believed to depend upon an intact hippocampus. The data thus demonstrate two important mechanisms: 1. Neutrophil extravasation into the CNS during pneumococcal meningitis influences the pro-inflammatory response and is central to control of the bacterial load, an increase in which may lead to death. 2. Neutrophil-mediated changes in the acute inflammatory response modulate the neuropsychological sequelae in mice that survive pneumococcal meningitis.

Exogenous BDNF increases neurogenesis in the hippocampus in experimental Streptococcus pneumoniae meningitis

BACKGROUND

Despite the effective use of antibiotics, occurrences of mortality and neurological sequelae following Streptococcus pneumoniae meningitis remain high.

METHODS

We investigated the neurogenesis of endogenous neural stem cells (NSCs) after inoculation with exogenous brain-derived neurotrophic factor (BDNF) in the hippocampus dentate gyrus following experimental S. pneumoniae meningitis using a double-labeling immunofluorescence analysis with 5-bromo-2'-deoxyuridine (BrdU), Nestin, DCX and NeuN.

RESULTS

Our results showed that 7days after inoculation, the number of BrdU & Nestin co-labeled cells increased in the hippocampus in meningitis rats compared with control rats (p<0.05). But the number of DCX-positive cells decreased in the dentate gyrus of infected rats treated with saline (p<0.05). However, these cell numbers returned to close to normal-control levels in infected rats treated with BDNF (p>0.05). After treatment with exogenous BDNF, the number of BrdU & Nestin co-labeled cells increased in the hippocampus in both the meningitis rats and normal control rats (p<0.05), but this increase was more significant in the former (p<0.05). We found that the percentage of BrdU & DCX/BrdU co-labeled cells increased in infected rats treated with BDNF both 7days and 14days after inoculation in a greater proportion compared to other groups (p<0.05). No significant differences were found in the percentage of BrdU & NeuN/BrdU 28days after inoculation among all of the groups (p>0.05).

CONCLUSION

Our findings suggest that S. pneumoniae meningitis activates the proliferation of endogenous NSCs, but impairs their early differentiation. Administration of exogenous BDNF might improve the neurogenesis of endogenous NSCs in the hippocampus and may provide a promising therapy after bacterial meningitis.

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.

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.