Variation of 46 Innate Immune Genes Evaluated for their Contribution in Pneumococcal Meningitis Susceptibility and Outcome.

Pneumococcal meningitis is the most common and severe form of bacterial meningitis. Early recognition of the pathogen and subsequent innate immune response play a vital role in disease susceptibility and outcome. Genetic variations in innate immune genes can alter the immune response and influence susceptibility and outcome of meningitis disease. Here we conducted a sequencing study of coding regions from 46 innate immune genes in 435 pneumococcal meningitis patients and 416 controls, to determine the role of genetic variation on pneumococcal meningitis susceptibility and disease outcome. Strongest signals for susceptibility were rs56078309 CXCL1 (p=4.8e-04) and rs2008521 in CARD8 (p=6.1e-04). For meningitis outcome the rs2067085 in NOD2 (p=5.1e-04) and rs4251552 of IRAK4 were the strongest associations with unfavorable outcome (p=6.7e-04). Haplotype analysis showed a haplotype block, determined by IRAK4 rs4251552, significantly associated with unfavorable outcome (p=0.004). Cytokine measurements from cerebrospinal fluid showed that with the IRAK4 rs4251552 G risk allele had higher levels of IL-6 compared to individuals with A/A genotype (p=0.04). We show that genetic variation within exons and flanking regions of 46 innate immunity genes does not yield significant association with pneumococcal meningitis. The strongest identified signal IRAK4 does imply a potential role of genetic variation in pneumococcal meningitis.

V-akt murine thymoma viral oncogene homolog 3 (AKT3) contributes to poor disease outcome in humans and mice with pneumococcal meningitis.

Pneumococcal meningitis is the most common and severe form of bacterial meningitis. Fatality rates are substantial, and long-term sequelae develop in about half of survivors. Here, we have performed a prospective nationwide genetic association study using the Human Exome BeadChip and identified gene variants in encoding dynactin 4 (DCTN4), retinoic acid early transcript 1E (RAET1E), and V-akt murine thymoma viral oncogene homolog 3 (AKT3) to be associated with unfavourable outcome in patients with pneumococcal meningitis. No clinical replication cohort is available, so we validated the role of one of these targets, AKT3, in a pneumococcal meningitis mouse model. Akt3 deficient mice had worse survival and increased histopathology scores for parenchymal damage (infiltration) and vascular infiltration (large meningeal artery inflammation) but similar bacterial loads, cytokine responses, compared to wild-type mice. We found no differences in cerebrospinal fluid cytokine levels between patients with risk or non-risk alleles. Patients with the risk genotype (rs10157763, AA) presented with low scores on the Glasgow Coma Scale and high rate of epileptic seizures. Thus, our results show that AKT3 influences outcome of pneumococcal meningitis.

CCAAT/enhancer-binding protein δ (C/EBPδ) aggravates inflammation and bacterial dissemination during pneumococcal meningitis.

BACKGROUND: The prognosis of bacterial meningitis largely depends on the severity of the inflammatory response. The transcription factor CAAT/enhancer-binding protein δ (C/EBPδ) plays a key role in the regulation of the inflammatory response during bacterial infections. Consequently, we assessed the role of C/EBPδ during experimental meningitis. METHODS: Wild-type and C/EBPδ-deficient mice (C/EBPδ(-/-)) were intracisternally infected with Streptococcus pneumoniae and sacrificed after 6 or 30 h, or followed in a survival study. RESULTS: In comparison to wild-type mice, C/EBPδ(-/-) mice showed decreased bacterial loads at the primary site of infection and decreased bacterial dissemination to lung and spleen 30 h after inoculation. Expression levels of the inflammatory mediators IL-10 and KC were lower in C/EBPδ(-/-) brain homogenates, whereas IL-6, TNF-α, IL-1ß, and MIP-2 levels were not significantly different between the two genotypes. Moreover, C/EBPδ(-/-) mice demonstrated an attenuated systemic response as reflected by lower IL-10, IL-6, KC, and MIP-2 plasma levels. No differences in clinical symptoms or in survival were observed between wild-type and C/EBPδ(-/-) mice. CONCLUSION: C/EBPδ in the brain drives the inflammatory response and contributes to bacterial dissemination during pneumococcal meningitis. C/EBPδ does, however, not affect clinical parameters of the disease and does not confer a survival benefit.

Leukocyte attraction by CCL20 and its receptor CCR6 in humans and mice with pneumococcal meningitis.

We previously identified CCL20 as an early chemokine in the cerebrospinal fluid (CSF) of patients with pneumococcal meningitis but its functional relevance was unknown. Here we studied the role of CCL20 and its receptor CCR6 in pneumococcal meningitis. In a prospective nationwide study, CCL20 levels were significantly elevated in the CSF of patients with pneumococcal meningitis and correlated with CSF leukocyte counts. CCR6-deficient mice with pneumococcal meningitis and WT mice with pneumococcal meningitis treated with anti-CCL20 antibodies both had reduced CSF white blood cell counts. The reduction in CSF pleocytosis was also accompanied by an increase in brain bacterial titers. Additional in vitro experiments showed direct chemoattractant activity of CCL20 for granulocytes. In summary, our results identify the CCL20-CCR6 axis as an essential component of the innate immune defense against pneumococcal meningitis, controlling granulocyte recruitment.

Cerebrospinal fluid complement activation in patients with pneumococcal and meningococcal meningitis.

BACKGROUND: Recent research into the treatment of bacterial meningitis has examined the innate immune system, specifically the complement system, as a potential target for adjuvant therapy. However, the effects of blocking the complement system may be pathogen dependent. METHODS: We measured cerebrospinal fluid (CSF) levels of complement components C1q, C3a, iC3b, C5a, sC5b-9, CFH and MBL in 310 patients with pneumococcal and meningococcal meningitis from a prospective nationwide cohort study. The CSF complement component levels were successfully determined for between 289 (93%) and 307 (99%) patients, depending on available volumes of stored CSF. RESULTS: Complement factors C1q and MBL as well as common complement pathway factors C3a, iC3b, C5a, sC5b-9 and complement regulator CFH were all elevated in patients with bacterial meningitis as compared to the controls. CSF levels of complement components C5a and sC5b-9 were higher in patients with pneumococcal meningitis compared to those with meningococcal meningitis. After correction for age, immunocompromised state and level of consciousness, the CSF concentrations of C5a and sC5b-9 remained different between causative microorganisms (P = 0.006 and P = 0.016 respectively). In pneumococcal meningitis high C5a and C5b-9 levels are associated with the occurrence of systemic complications, unfavorable outcome and death, whereas an inverse relationship between C5b-9 levels and mortality is observed in meningococcal meningitis. CONCLUSIONS: Our study shows striking variations in complement activation depending on the pathogen responsible for the bacterial meningitis. In pneumococcal meningitis, high CSF complement levels were a strong indicator of disease severity and mortality, however in meningococcal meningitis, an inverse relationship between sC5b-9 and mortality was observed.

Streptococcus pneumoniae arginine synthesis genes promote growth and virulence in pneumococcal meningitis.

Streptococcus pneumoniae (pneumococcus) is a major human pathogen causing pneumonia, sepsis and bacterial meningitis. Using a clinical phenotype based approach with bacterial whole-genome sequencing we identified pneumococcal arginine biosynthesis genes to be associated with outcome in patients with pneumococcal meningitis. Pneumococci harboring these genes show increased growth in human blood and cerebrospinal fluid (CSF). Mouse models of meningitis and pneumonia showed that pneumococcal strains without arginine biosynthesis genes were attenuated in growth or cleared, from lung, blood and CSF. Thus, S. pneumoniae arginine synthesis genes promote growth and virulence in invasive pneumococcal disease.

Cerebrospinal fluid inflammatory markers in patients with Listeria monocytogenes meningitis.

BACKGROUND: Listeria monocytogenes meningitis is the third most common cause of bacterial meningitis and is associated with high rates of mortality and unfavorable outcome. METHODS: We analyzed 101 cytokines, chemokines and complement factors in CSF of adult patients with Listeria meningitis included in a prospective cohort study and compared these biomarkers between Listeria meningitis patients and negative controls, and between Listeria meningitis patients with a favorable and an unfavorable outcome. RESULTS: CSF was available from 26 of 62 (42%) Listeria meningitis patients and 19 negative controls. Fifteen (58%) Listeria meningitis patients had an unfavorable outcome. In Listeria meningitis CSF levels of 51 biomarkers were significantly elevated compared to negative controls after Bonferroni correction. The 11 most significantly elevated (P < .01) biomarkers of unfavorable outcome in Listeria meningitis were markers of T-cell activation (sIL-2Rα, sCD40L and IL-1), interferon-related (IFN-α2, IL-18, CX3CL1, CCL20), markers of complement activation (C3a), and endothelial growth factor related (VEGF, CXCL7). CONCLUSIONS: Our data suggest that T-cell activation, complement activation, interferon- and endothelial growth factor production are important in the immune response to Listeria meningitis, and thereby influence outcome. GENERAL SIGNIFICANCE: Our study provides target pathways for further studies in the pathophysiology of Listeria meningitis.

Inflammasome activation mediates inflammation and outcome in humans and mice with pneumococcal meningitis.

BACKGROUND: Inflammasomes are multi-protein intracellular signaling complexes that have recently been hypothesized to play a role in the regulation of the inflammation response. We studied associations between inflammasome-associated cytokines IL-1ß and IL-18 in cerebrospinal fluid (CSF) of patients with bacterial meningitis and clinical outcome, and pneumococcal serotype. In a murine model of pneumococcal meningitis we examined the pathophysiological roles of two inflammasome proteins, NLRP3 (Nod-like receptor protein-3) and adaptor protein ASC (apoptosis-associated speck-like protein). METHODS: In a nationwide prospective cohort study, CSF cytokine levels were measured and related to clinical outcome and pneumococcal serotype. In a murine model of pneumococcal meningitis using Streptococcus pneumoniae serotype 3, we examined bacterial titers, cytokine profiles and brain histology at 6 and 30 hours after inoculation in wild-type (WT), Asc and Nlrp3 deficient mice. RESULTS: In patients with bacterial meningitis, CSF levels of inflammasome associated cytokines IL-1ß and IL-18 were related to complications, and unfavorable disease outcome. CSF levels of IL-1ß were associated with pneumococcal serotype (p<0.001). In our animal model, Asc and Nlrp3 deficient mice had decreased systemic inflammatory responses and bacterial outgrowth as compared to WT mice. Differences between Asc⁻/⁻ and WT mice appeared sooner after bacterial inoculation and were more widespread (lower pro-inflammatory cytokine levels in both blood and brain homogenate) than in Nlrp3⁻/⁻ mice. Nlrp3 deficiency was associated with an increase of cerebral neutrophil infiltration and cerebral hemorrhages when compared to WT controls. CONCLUSIONS: Our results implicate an important role for inflammasome proteins NLRP3 and ASC in the regulation of the systemic inflammatory response and the development of cerebral damage during pneumococcal meningitis, which may dependent on the pneumococcal serotype.

Genetic variation in inflammasome genes is associated with outcome in bacterial meningitis.

Bacterial meningitis is a severe and deadly disease, most commonly caused by Streptococcus pneumoniae. Disease outcome has been related to severity of the inflammatory response in the subarachnoid space. Inflammasomes are intracellular signaling complexes contributing to this inflammatory response. The role of genetic variation in inflammasome genes in bacterial meningitis is largely unknown. In a prospective nationwide cohort of patients with pneumococcal meningitis, we performed a genetic association study and found that single-nucleotide polymorphisms in the inflammasome genes CARD8 (rs2043211) and NLRP1 (rs11621270) are associated with poor disease outcome. Levels of the inflammasome associated cytokines interleukin (IL)-1ß and IL-18 in cerebrospinal fluid also correlated with clinical outcome, but were not associated with the CARD8 and NLRP1 polymorphisms. Our results implicate an important role of genetic variation in inflammasome genes in the regulation of inflammatory response and clinical outcome in patients with bacterial meningitis.

Common polymorphisms in the complement system and susceptiblity to bacterial meningitis.

OBJECTIVE: Risk factors for susceptibility to bacterial meningitis have been identified, but basic causes of inter-individual differences in susceptibility are largely unknown. METHODS: To determine the effect of genetic variation in the complement system on susceptibility to bacterial meningitis we performed a prospective nationwide genetic association study in patients with community-acquired bacterial meningitis. We genotyped 17 common SNPs (minor allele frequencies >5%) in genes coding for complement components and evaluated functional consequences by measuring complement levels in the cerebrospinal fluid. RESULTS: From March 2006 to June 2009 we included 636 adults with community-acquired bacterial meningitis. DNA was available for 439 patients and 302 controls. Rs1047286 (Pro314Leu) in complement component 3 was associated with reduced susceptibility to bacterial meningitis after correction for multiple testing: the protective Leu/Leu genotype was found in 5 of 435 patients (1%) compared to 15 of 302 controls (5%; odds ratio [OR] 4.50, 95% confidence interval [CI] 1.62-12.50, p = 0.0017). Rs1047286 is in strong linkage disequilibrium with Rs2230199 (C3 Arg102Gly), of which the Arg/Arg genotype was associated with higher CSF levels of C3 and lower levels of C5a and terminal complement complex (TCC; soluble C5b-9), indicating decreased consumption of C3 and less activation of the complement system. Rs1047286 was associated with susceptibility albeit not significantly after Bonferroni correction (OR 1.37, 95% CI 1.01-1.87; p = 0.04). CONCLUSIONS: This study shows an association between a common single nucleotide polymorphism in C3 and susceptibility for community-acquired bacterial meningitis.

Characterization of a pneumococcal meningitis mouse model.

BACKGROUND: S. pneumoniae is the most common causative agent of meningitis, and is associated with high morbidity and mortality. We aimed to develop an integrated and representative pneumococcal meningitis mouse model resembling the human situation. METHODS: Adult mice (C57BL/6) were inoculated in the cisterna magna with increasing doses of S. pneumoniae serotype 3 colony forming units (CFU; n = 24, 104, 105, 106 and 107 CFU) and survival studies were performed. Cerebrospinal fluid (CSF), brain, blood, spleen, and lungs were collected. Subsequently, mice were inoculated with 104 CFU S. pneumoniae serotype 3 and sacrificed at 6 (n = 6) and 30 hours (n = 6). Outcome parameters were bacterial outgrowth, clinical score, and cytokine and chemokine levels (using Luminex®) in CSF, blood and brain. Meningeal inflammation, neutrophil infiltration, parenchymal and subarachnoidal hemorrhages, microglial activation and hippocampal apoptosis were assessed in histopathological studies. RESULTS: Lower doses of bacteria delayed onset of illness and time of death (median survival CFU 104, 56 hrs; 105, 38 hrs, 106, 28 hrs. 107, 24 hrs). Bacterial titers in brain and CSF were similar in all mice at the end-stage of disease independent of inoculation dose, though bacterial outgrowth in the systemic compartment was less at lower inoculation doses. At 30 hours after inoculation with 104 CFU of S. pneumoniae, blood levels of KC, IL6, MIP-2 and IFN- γ were elevated, as were brain homogenate levels of KC, MIP-2, IL-6, IL-1ß and RANTES. Brain histology uniformly showed meningeal inflammation at 6 hours, and, neutrophil infiltration, microglial activation, and hippocampal apoptosis at 30 hours. Parenchymal and subarachnoidal and cortical hemorrhages were seen in 5 of 6 and 3 of 6 mice at 6 and 30 hours, respectively. CONCLUSION: We have developed and validated a murine model of pneumococcal meningitis.

Pathogenesis and pathophysiology of pneumococcal meningitis.

Pneumococcal meningitis continues to be associated with high rates of mortality and long-term neurological sequelae. The most common route of infection starts by nasopharyngeal colonization by Streptococcus pneumoniae, which must avoid mucosal entrapment and evade the host immune system after local activation. During invasive disease, pneumococcal epithelial adhesion is followed by bloodstream invasion and activation of the complement and coagulation systems. The release of inflammatory mediators facilitates pneumococcal crossing of the blood-brain barrier into the brain, where the bacteria multiply freely and trigger activation of circulating antigen-presenting cells and resident microglial cells. The resulting massive inflammation leads to further neutrophil recruitment and inflammation, resulting in the well-known features of bacterial meningitis, including cerebrospinal fluid pleocytosis, cochlear damage, cerebral edema, hydrocephalus, and cerebrovascular complications. Experimental animal models continue to further our understanding of the pathophysiology of pneumococcal meningitis and provide the platform for the development of new adjuvant treatments and antimicrobial therapy. This review discusses the most recent views on the pathophysiology of pneumococcal meningitis, as well as potential targets for (adjunctive) therapy.

Macrophage migration inhibitory factor, infection, the brain, and corticosteroids.

Bacterial meningitis is a complex disorder in which injury is caused, in part, by the causative organism and, in part, by the host's own inflammatory response. Macrophage migration inhibitory factor (MIF) is a pro-inflammatory cytokine and a neuro-endocrine mediator that might play a role in pneumococcal meningitis. Here, we discuss the role of MIF in infection, the brain, and corticosteroids and conclude that experimental meningitis studies have to determine whether MIF is a potential target for adjunctive therapy in pneumococcal meningitis.