Meningeal macrophages reflect lymphocytic choriomeningitis virus pathogenic phenotypes

Where are we? The anatomy of the murine cortical meninges revisited for intravital imaging, immunology, and clearance of waste from the brain

Rapid progress is being made in understanding the roles of the cerebral meninges in the maintenance of normal brain function, in immune surveillance, and as a site of disease. Most basic research on the meninges and the neural brain is now done on mice, major attractions being the availability of reporter mice with fluorescent cells, and of a huge range of antibodies useful for immunocytochemistry and the characterization of isolated cells. In addition, two-photon microscopy through the unperforated calvaria allows intravital imaging of the undisturbed meninges with sub-micron resolution. The anatomy of the dorsal meninges of the mouse (and, indeed, of all mammals) differs considerably from that shown in many published diagrams: over cortical convexities, the outer layer, the dura, is usually thicker than the inner layer, the leptomeninx, and both layers are richly vascularized and innervated, and communicate with the lymphatic system. A membrane barrier separates them and, in disease, inflammation can be localized to one layer or the other, so experimentalists must be able to identify the compartment they are studying. Here, we present current knowledge of the functional anatomy of the meninges, particularly as it appears in intravital imaging, and review their role as a gateway between the brain, blood, and lymphatics, drawing on information that is scattered among works on different pathologies.

The clinical and laboratory diagnosis of acute meningitis and acute encephalitis

INTRODUCTION

Acute bacterial meningitis (ABM) is a life-threatening infectious disease requiring prompt antimicrobial therapy. ABM must be differentiated from systemic disorders with CNS manifestations that may mimic ABM. ABM should also be differentiated from acute meningoencephalitis (AME) and acute viral encephalitis (AVE). Nonviral causes of AME are treatable. Among the causes of AVE, Herpes simplex encephalitis (HSE) is treatable. This article reviews the clinical diagnostic approach to ABM, AME and AVE.

AREAS COVERED

The differential diagnostic (DDx) approach to ABM, AME and AVE is based on clinical and laboratory findings. A specific pathogen diagnosis is based on serum/cerebrospinal fluid (CSF) tests. This overview presents the diagnostic approach to ABM, AME and AVE in normal hosts (excluding brain abscesses and chronic CNS infections).

EXPERT OPINION

It is time critical to diagnose ABM and begin empiric antimicrobial therapy based on the known/most likely pathogen. The diagnosis of ABM depends on clinical features and the CSF profile. The CSF Gram stain and CSF lactic acid (LA) levels provide the most rapid, reliable and cost-effective tests to diagnose ABM. CSF LA levels are also the best way to diagnose partially treated acute bacterial meningitis (PTABM). In those cases of AME/AVE due to viruses with a CSF profile mimicking ABM, for example, HSE, unelevated CSF LA levels rapidly/reliably rule out ABM as a diagnostic possibility.

Tumor necrosis factor alpha in cerebrospinal fluid during bacterial, but not viral, meningitis. Evaluation in murine model infections and in patients

To evaluate the potential role of cachectin/TNF-alpha in the pathogenesis of bacterial and viral meningitis, concentrations and kinetics of TNF-alpha were determined in cerebrospinal fluid (CSF). After intracerebral, but not systemic, infection with Listeria monocytogenes in mice, TNF-alpha was detected as early as 3 h after infection reaching maximum titers after 24 h. However, TNF-alpha was not found in serum during the course of Listeria infection. In contrast to bacterial meningitis, no TNF-alpha was detected at any time in CSF of mice suffering from severe lymphocytic choriomeningitis induced by intracerebral infection with lymphocytic choriomeningitis virus. This difference is striking since both model infections led to a massive infiltration of polymorphonuclear and mononuclear leukocytes into the meninges and CSF. The results found for the two model infections were paralleled by findings in humans; CSF from three out of three patients with bacterial meningitis examined during the first day of hospitalization showed significant levels of TNF-alpha; none of the CSF obtained later than 3 d after hospitalization was positive. In addition, similarly to what was found in mice with viral meningitis, zero out of seven patients with viral meningitis had detectable TNF-alpha in CSF.