The pro-inflammatory cytokine interferon-gamma is an important driver of neuropathology and behavioural sequelae in experimental pneumococcal meningitis.

Interferon-gamma is known to play a complex modulatory role in immune defence during microbial infections. Its actions in pneumococcal meningitis, however, remain ill-defined. Here, a pathological role for IFN-γ was demonstrated using a murine model of pneumococcal meningitis, in that C57BL/6J mice deficient in this pro-inflammatory cytokine (IFN-γ(-/-)) showed less severe acute and long-term neuropathology following intracerebral challenge with Streptococcus pneumoniae. The absence of IFN-γ significantly lengthened the survival of mice that otherwise would have developed fatal clinical signs within two days of CNS infection. Compared to their wild-type counterparts, IFN-γ(-/-) mice showed a diminished inflammatory response (attenuated levels of pro-inflammatory cytokines in the cerebrospinal fluid) and milder brain pathologies (less BBB permeability to protein and brain haemorrhage) during the acute phase of disease. Following a full regime of antibiotic treatment, we found substantial brain injuries in the wild-type mice 10days after infection. IFN-γ(-/-) mice, however, showed decreased neuronal damage in both hippocampus and cortex. In the longer term (≈10weeks p.i.), the wild-type mice that had survived meningitis due to antibiotic treatment had neurobehavioural abnormalities including diurnal hypoactivity, nocturnal hyperactivity and impaired performance in a discrimination reversal task. IFN-γ(-/-) mice, concomitantly tested in the automated IntelliCage platform, had reduced behavioural and cognitive disorders compared to wild-type mice. Both IFN-γ(-/-) and wild-type survivors of pneumococcal meningitis showed impaired working memory in the IntelliCage-based complex patrolling task. These observations indicate an association between IFN-γ-driven acute brain pathology and the long-term neurological sequelae resulting from pneumococcal meningitis.

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)-1ß, 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.

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-1ß, 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.