During acute bacterial infections such as meningitis, neutrophils enter the tissue where they combat the infection before they undergo apoptosis and are taken up by macrophages. Neutrophils show pro-inflammatory activity and may contribute to tissue damage. In pneumococcal meningitis, neuronal damage despite adequate chemotherapy is a frequent clinical finding. This damage may be due to excessive neutrophil activity. We here show that transgenic expression of Bcl-2 in haematopoietic cells blocks the resolution of inflammation following antibiotic therapy in a mouse model of pneumococcal meningitis. The persistence of neutrophil brain infiltrates was accompanied by high levels of IL-1 beta and G-CSF as well as reduced levels of anti-inflammatory TGF-beta. Significantly, Bcl-2-transgenic mice developed more severe disease that was dependent on neutrophils, characterized by pronounced vasogenic edema, vasculitis, brain haemorrhages and higher clinical scores. In vitro analysis of neutrophils demonstrated that apoptosis inhibition completely preserves neutrophil effector function and prevents internalization by macrophages. The inhibitor of cyclin-dependent kinases, roscovitine induced apoptosis in neutrophils in vitro and in vivo. In wild type mice treated with antibiotics, roscovitine significantly improved the resolution of the inflammation after pneumococcal infection and accelerated recovery. These results indicate that apoptosis is essential to turn off activated neutrophils and show that inflammatory activity and disease severity in a pyogenic infection can be modulated by targeting the apoptotic pathway in neutrophils.

Myeloid cells, including neutrophils and macrophages, play important roles in innate immune defense against acute bacterial infections. Myeloid Src family kinases (SFKs) p59/61(hck) (Hck), p58(c-fgr) (Fgr), and p53/56(lyn) (Lyn) are known to control integrin beta(2) signal transduction and FcgammaR-mediated phagocytosis in leukocytes. In this study, we show that leukocyte recruitment into the cerebrospinal fluid space and bacterial clearance is hampered in mice deficient in all three myeloid SFKs (hck(-/-)fgr(-/-)lyn(-/-)) during pneumococcal meningitis. As a result, the hck(-/-)fgr(-/-)lyn(-/-) mice developed increased intracranial pressure and a worse clinical outcome (increased neurologic deficits and mortality) compared with wild-type mice. Impaired bacterial killing was associated with a lack of phagocytosis and superoxide production in triple knockout neutrophils. Moreover, in hck(-/-)fgr(-/-)lyn(-/-) neutrophils, phosphorylation of p40(phox) was absent in response to pneumococcal stimulation, indicating a defect in NAPDH oxidase activation. Mice lacking the complement receptor 3 (CR3; CD11b/CD18), which belongs to the beta(2)-integrin family, also displayed impaired host defense against pneumococci, along with defective neutrophil superoxide production, but cerebrospinal fluid pleocytosis was normal. Cerebral expression of cytokines and chemokines was not decreased in both mouse strains, indicating that CR3 and myeloid SFKs are dispensable for the production of inflammatory mediators. Thus, our study demonstrates the pivotal role of myeloid SFKs and CR3 in mounting an effective defense against CNS infection with Streptococcus pneumonia by regulating phagocytosis and NADPH oxidase-dependent superoxide production. These data support the role of SFKs as critical mediators of CR3 signal transduction in host defense.

Hearing loss is one of the most common sequelae in survivors of pneumococcal meningitis, affecting up to 26% of them. Here, we established the first mouse model of meningitis-associated hearing loss and investigated the role played by the Toll-like receptor-associated adapter molecule MyD88(+) C57BL/6 mice were infected intracisternally by Streptococcus pneumoniae. By use of audiometry and histological analysis, cochleae were assessed in uninfected control mice during the acute stage and after recovery. MyD88-deficient mice were analyzed 24 h after infection. Wild-type mice lost hearing capacity to a significant degree, which was accompanied by a granulocytic cochlear inflammation. After recovery, hearing loss was still evident, and spiral ganglion neuronal loss, hair cell damage, and fibrocytic occlusion of the cochlea were observed. In contrast, mice lacking MyD88 developed significantly less hearing loss and had diminished cochlear inflammation. Our results strongly suggest a proinflammatory role for MyD88 in the initiation of the inflammatory response during pneumococcal meningitis-associated labyrinthitis.

To study pathogenic mechanisms of Lyme meningoradiculitis, dorsal root ganglia (DRG) cells and two neuronal cell lines (B50, SH-SY5Y) were incubated with Borrelia garinii, the Borrelia species most frequently isolated from CSF of Lyme neuroborreliosis patients in Europe. We demonstrated that (I) OspA-positive B. garinii adhere to neuronal cells, (II) Borrelia adhesion can be blocked by a monoclonal antibody against OspA, (III) preincubation with proteoglycans interferes with the adhesion process and (IV) rOspA directly binds to the proteoglycans. This indicates that both OspA and the cell bound proteoglycans are involved in the attachment of B. garinii to neuronal cells.

In this study, we investigated cytokine expression during experimental pneumococcal meningitis. Mice were intracisternally infected with Streptococcus pneumoniae and treated with ceftriaxone starting at 24h after infection. At different time points before and after antibiotic therapy, the cytokine expression pattern was determined in mouse brains using protein arrays. Underlining the power of this method, the meningitis-relevant cytokines interleukin-1β (IL-1β), IL-6, KC, macrophage inflammatory protein-2 (MIP-2), and monocyte chemoattractant protein-1 (MCP-1/CCL2) were markedly elevated in infected animals. Newly identified proteins during the acute stage of the disease (until 30h after infection) included lymphotactin (XCL-1), MIP-1γ (CCL9) and MCP-5 (CCL12), cytokine responsive gene- 2 (CRG-2/CXCL10) and CXCL16, and insulin-like growth factor binding protein 3 (IGFBP3). During later stages, an induction of T-cell activation-3 (TCA-3/CCL1), platelet factor-4 (PF-4/CXCL4) and stromal derived factor-1α (SDF-1α/CXCL13), and IL-4 was observed. The validity of this method was supported by an additional ELISA analysis of the expression profile of CXCL16 and IGFBP3, which was identical to that observed by protein array. In conclusion, the use of protein array technology led to an extension of the current picture of protein expression in pneumococcal meningitis. Most important, new factors that might play a role in pneumococcal meningitis were identified.

Increased vascular permeability causing vasogenic brain edema is characteristic for many acute neurological diseases such as stroke, brain trauma, and meningitis. Src family kinases, especially c-Src, play an important role in regulating blood-brain barrier permeability in response to VEGF, but also mediate leukocyte function and cytokine signalling. Here we demonstrate that pharmacological inhibition of Src or c-Src deficiency does not influence cerebrospinal fluid (CSF) pleocytosis, brain edema formation, and bacterial outgrowth during experimental pneumococcal meningitis despite the increased cerebral expression of inflammatory chemokines, such as IL-6, CCL-9, CXCL-1, CXCL-2 and G-CSF as determined by protein array analysis. In contrast, inhibition of Src significantly reduced brain edema formation, lesion volume, and clinical worsening in cold-induced brain injury without decreasing cytokine/chemokine expression. While brain trauma was associated with increased cerebral VEGF formation, VEGF levels significantly declined during pneumococcal meningitis. Therefore, we conclude that in brain trauma blood-brain barrier tightness is regulated by the VEGF/Src pathway whereas c-Src does not influence brain edema formation and leukocyte function during bacterial meningitis.

Bacterial meningitis is a medical emergency and is optimally managed in an intensive care environment. Despite the use of antibiotics, the prognosis of this disease is poor because of central nervous system complications such as brain edema formation, cerebrovascular alterations, intracranial hemorrhage, and hydrocephalus. Effective adjunctive therapies are still missing. Experimental studies with animal models have provided new insights into the pathophysiology during the acute phase of bacterial meningitis. In recent years, knockout mice have become a powerful tool to investigate the role of particular genes and have also been applied in bacterial meningitis research. The use of these mice offered new insights into the role of different cytokines, proteases, and oxidants involved in the inflammatory cascade. Translating this knowledge into new therapies will provide new treatment strategies for this serious disease in the future.

To evaluate the potential role of the uPAR/uPA/PAI-1 system in HIV-induced blood–brain-barrier (BBB) disruption, CSF uPA-dependent plasminogen activation (PdPA) was analyzed by casein zymography, and CSF protein levels of all three molecules were measured by ELISA. CSF uPAR, but not uPA, PAI-1, or PdPA levels was significantly increased in neurologically compromised HIV+ patients. Only individual patients with severe AIDS dementia complex had increased levels of uPA (but not PAI-1) which fell upon initiation of antiretroviral therapy. The levels of all three molecules did not correlate with the CSF to serum albumin ratio suggesting not an important role in HIV-induced BBB disruption.

Tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA) have been suggested to play an important role in inflammatory diseases. Increased levels of tPA, uPA, uPA receptor ( uPAR), and their inhibitor, plasminogen activator inhibitor (PAI)-1, have been found in the cerebrospinal fluid (CSF) of patients with bacterial meningitis. Here, we show that expression of tPA, uPA, uPAR, PAI-1, and PAI-2 is upregulated during experimental pneumococcal meningitis. In uPAR-deficient mice, CSF pleocytosis was significantly attenuated 24 h after infection, compared with that in infected wild-type (wt) mice. Lack of uPAR did not influence blood-brain barrier permeability, intracranial pressure, expression of chemokines (keratinocyte-derived cytokine and macrophage inflammatory protein-2), bacterial killing, or clinical outcome. No differences in pathophysiological alterations were observed in tPA-deficient mice, compared with those in infected wt mice. These results indicate that uPAR participates in the recruitment of leukocytes to the CSF space during pneumoccal meningitis.

Fas (CD95) and Fas ligand (FasL, CD95L) have been implicated to be involved in the acute inflammatory response by attracting neutrophils and regulating their survival. Increased levels of soluble Fas and FasL are found in cerebrospinal fluid (CSF) samples of patients with bacterial meningitis but not in controls. Functional FasL (gld)- or Fas (lpr)-deficient mice were used to assess their role in the pathophysiology of pneumococcal meningitis. Induction of meningitis in wild-type (WT) mice caused an increase in CSF white blood cell (WBC) count, intracranial pressure (ICP), and vessel permeability, paralleled by a worse clinical status at 24 h. The inflammatory response was accompanied by elevated levels of IL-1β, MMP-2, and MMP-9 in the brain. Neither gld- nor lpr-mice showed significant differences in the above-mentioned pneumococci-induced pathophysiological alterations. These results indicate that Fas and FasL are not essential in the regulation of the acute inflammatory response during pneumococcal meningitis.

Vascular endothelial growth factor (VEGF) is a potent angiogenic and mitogenic peptide, which also induces several mediators that may play a role in HIV induced CNS damage. VEGF levels were determined in cerebrospinal fluid (CSF) and serum samples from patients with (n = 8) and without (n = 19) directly HIV associated CNS disorders and HIV negative control patients (n = 18). VEGF serum but not CSF levels were significantly increased in HIV infected patients with (381.1 (78.9) pg/ml) HIV associated CNS diseases compared with those without (120.8 (13.1) pg/ml) and HIV negative control patients (133.1(14.8) pg/ml). Serum samples from patients with untreated HIV associated encephalopathy (HIVE, n = 3) contained the highest VEGF levels (583.9 (71.5) pg/ml). In two patients VEGF serum levels were reduced during antiretroviral therapy. However, regardless of effective viral suppression, patients with HIVE still had higher levels compared with HIV infected patients without HIVE. A relevant increase of serum VEGF was not observed in patients without HIVE though high HI viral load. We conclude that HIVE is associated with increased serum VEGF levels. Further studies are warranted to elucidate the role of VEGF in HIVE.

Symptomatic aseptic, chemical meningitis is a rare complication of myelography. Its acute clinical course and standard laboratory findings are indistinguishable from those of bacterial meningitis. The authors present a case of severe postmyelographic chemical meningitis and compare CSF and serum inflammatory markers to a group of seven patients with proven bacterial meningitis. As in viral meningitis, procalcitonin might be able to discriminate between bacterial and chemical causes of CNS inflammation.

Despite the use of antibiotics, the prognosis of bacterial meningitis is still poor due to central nervous system (CNS) complications, such as brain edema formation, cerebrovascular alterations, and intracranial hemorrhage. Experimental studies with animal models have given new insights into its pathophysiology during the acute phase of the disease. In recent years, genetically engineered mice have become a powerful tool in investigating the role of particular genes by targeted deletion and have also been applied in bacterial meningitis research. By using knockout mice, new knowledge of the roles of the different cytokines, proteases, and oxidants involved in the inflammatory cascade has emerged. In the future, temporal and cell type-specific control of gene expression will provide even more information on the impact of a particular gene on meningitis-induced brain damage.

Interleukin (IL)-6 is a multifunctional cytokine with diverse actions and has been implicated in the pathophysiology of many neurological and inflammatory disorders. In this study, we investigated the role of IL-6 in pneumococcal meningitis. Cerebral infection in wild-type (WT) mice caused an increase in vascular permeability and intracranial pressure (ICP), which were significantly reduced in IL-6(-/-) mice. In contrast, meningitis in IL-6(-/-) mice was associated with a significant increase in CSF white blood cell count compared with infected WT mice, indicating an enhanced inflammatory response. Analysis of mRNA expression in the brain showed an increase in tumour necrosis factor (TNF)-alpha, IL-1beta, and macrophage inflammatory protein 2 (MIP-2) levels, but decreased expression of granulocyte-macrophage colony-stimulating factor in infected IL-6(-/-) mice compared with infected WT controls. Similar results were obtained when rats challenged with pneumococci were systemically treated with neutralizing anti-IL-6 antibodies, resulting in an increased pleocytosis but at the same time a reduction of vascular permeability, brain oedema formation, and ICP, which was not accompanied by a downregulation of matrix metalloproteinases. Our data indicate that IL-6 plays an important anti-inflammatory role in bacterial meningitis by reducing leukocyte infiltration but contributes to the rise in intracranial pressure by increasing blood-brain barrier (BBB) permeability. These findings suggest that the migration of leukocytes across the BBB and the increase in vascular permeability are two independent processes during bacterial meningitis.

The prognosis of bacterial (purulent) meningitis depends on early diagnosis and prompt initiation of antibiotic therapy. However, despite the improvement of antimicrobial therapy during the last decades, mortality rate and sequelae due to bacterial meningitis remain unacceptably high. For example, the mortality rate of meningitis due to Streptococcus pneumoniae, the organism most often responsible for bacterial meningitis in adults, has remained relatively unchanged during the last decades and is still as high as 20-30%. The unfavourable courses of bacterial meningitis are often due to intracranial complications including brain oedema, cerebrovascular arterial or venous involvement, and hydrocephalus. These complications usually lead to an increased intracranial pressure. Patients with bacterial meningitis may require adjunctive therapy including administration of hyperosmolar agents, hyperventilation or ventricular drainage. Anticoagulation with dose-adjusted intravenous heparin should be considered in patients with meningitis-associated septic venous sinus thrombosis proven by MRI, MR angiography or cerebral angiography. Recently, a prospective, randomised, multicentre, double-blind trial (performed by de Gans and van de Beek) in adults with acute bacterial meningitis showed a beneficial effect of dexamethasone (10 mg was administered 15 to 20 minutes before or with the first dose of antibiotic and was given every 6 hours for 4 days) compared with placebo. Treatment with dexamethasone was associated with a significant reduction in the risk of an unfavourable outcome, and a significant reduction in mortality. Among the patients with pneumococcal meningitis, 14% of the dexamethasone group died, as compared with 34% of the placebo group. Gastrointestinal bleeding occurred in 2 patients of the dexamethasone group and in 5 patients of the placebo group. Based on these data the use of dexamethasone is recommended in adult patients with suspected bacterial meningitis, i.e. in patients with clinical suspicion plus cloudy cerebrospinal fluid, detection of bacteria in the cerebrospinal fluid (CSF) by microscopy of a Gram's stained smear or a CSF cell count of more than 1000 cells/μl.