Neuronal injury in bacterial meningitis

Neuronal injury in meningitis is not a single event and several mechanisms are interacting with each other ultimately leading to the death of neurons. As previously discussed, cells functionally involved during meningitis include glial cells (astrocytes and microglia), meningeal macrophages, endothelial cells of the intracerebral vessels, and later also immunocompetent cells (monocytes, lymphocytes) recruited from the systemic circulation.

The main factors triggering inflammation are the bacterial components released into the CSF, that are recognised by glial cells and involve innate immune functions stimulating Toll-like receptors (TLRs) (Kielian et al. 2002; Bowman et al. 2003; Esen et al. 2004) and inducing translocation of nuclear factor κB (NF-κB), the activation of mitogen activated protein kinases (MAPK) and the transcription of genes encoding inflammatory mediators (Akira and Hemmi 2003) (discussed in detail in further sections). This initial inflammatory response from the brain parenchyma - and in particular astrocytes and microglia - seems to be an important factor contributing in neuronal damage. As demonstrated in some in vitro studies, heat-inactivated pneumococci injure both neurons co-cultured with glial cells (Kim and Tauber 1996) and neurons in organotypic hippocampal cultures (Schmidt et al. 2001; von Mering et al. 2001). Neuronal damage can be induced by pneumococcal lipoteichoic acid, peptidoglycan and bacterial DNA (Schmidt et al. 2001; Kim and Tauber 1996).

Evidence from in vivo studies shows that the cortical brain damage in bacterial meningitis has morphological features of necrosis. The principal elements of necrosis are mitochondrial energy depletion and non-caspase proteolytic cascades (serine proteases, calpains, cathepsins). Several clinical and experimental studies indicated that ischemia is an important contributor to this injury (Forderreuther et al. 1992; Koedel et al. 1995; Nau and Bruck 2002).

The mechanisms of ischemic or hypoxic lesions in meningitis are 1) vasculitis, vasospasm and obstruction of cerebral arteries, 2) generalised cerebral oedema, 3) impairment of cerebral autoregulation, leading to hypoxic lesions during hypotension. The pro-inflammatory mediators released by activated glial cells are largely responsible for these events.

On the other hand there are evidences of apoptotic cell death in several regions of the brain in the course of meningitis (Braun et al. 2002; Mitchell et al. 2004). Apoptosis can involve both the caspase-dependent and the caspase-independent pathway, and both pathways have been implicated in neuronal cell death induced by Streptococcus pneumoniae. Several caspases were demonstrated to be involved in experimental pneumococcal meningitis (von Mering et al. 2001; Gianinazzi et al. 2003).

One of the mechanisms involved in neuronal damage during meningitis is oxidative and nitrosative stress. Reactive oxygen species (superoxide, hydrogen peroxide, hydroxyl radical) and nitrogen species (nitric oxide, peroxynitrite) are produced by stimulated glial cells, macrophages and granulocytes: 1) after challenge with bacterial components and cytokines, 2) as a consequence of hypoxia and ischemia, or produced directly by bacteria (Hirst et al.

2000).

During bacterial meningitis, superoxide generation has been detected cytochemically in meningeal and ventricular inflammatory cells, as well as along penetrating cortical vessels (Leib et al. 1996a). The participation of ROS in injury is supported by the ability of a wide variety of inhibitors to ameliorate the course of inflammatory damage. Antioxidants such as superoxide dismutase, N-acetyl-L-cysteine and catalase reduce brain oedema, intracranial pressure and CSF leukocytosis, and attenuate the increase in regional cerebral blood flow in early pneumococcal meningitis (Pfister et al. 1992; Koedel and Pfister 1997).

Moreover, nitric oxide can be induced in most resident brain cells and invading immune cells in response to bacterial products. The release of NO can be stimulated directly by the bacterial components or by the pro-inflammatory mediators (e.g. TNF-α, IFN-γ, IL-1β) (Kong et al. 2000). Nitric oxide (NO) and superoxide radicals (O2^-) react rapidly to form the peroxynitrite anion (ONOO^-), which decomposes and forms the strong oxidants, hydroxyl radical and nitrogen dioxide. The significant role of NO in the pathophysiology of bacterial meningitis has been shown for group B streptococci, S. pneumoniae, H. influenzae and E. Coli (Bernatowicz et al. 1995; Koedel and Pfister 1999). The CSF concentration of nitrite, a stable end product of NO metabolism and indicator of NO production, is increased in bacterial meningitis (Uysal et al. 1999). N-nitro-L-arginine, an inhibitor of NO synthase, inhibits the increase of regional cerebral blood flow and intracranial pressure in the early phase of experimental pneumococcal meningitis (Koedel et al. 1995). Nevertheless, despite the indications of the damaging role of NO, the clinical application of NO inhibitors remains to be established.

Another mechanism that triggers neuronal cell death is excitotoxicity. The excessive release of glutamate causes membrane depolarisation, Ca²⁺ influx and energy failure, which leads to the release of cytochrome c from mitochondria. There are evidences that excitotoxicity plays a role in meningitis. Glutamate concentrations in the interstitial space of the brain and in the CSF are increased in animals and humans suffering from meningitis (Guerra-Romero et al. 1993; Spranger et al. 1996). In the infant rat model of bacterial meningitis the glutamate antagonist kynurenic acid was moderately neuroprotective (Leib et

al. 1996b). It has been shown that the pro-inflammatory TNF-α, which is rapidly produced by microglia or astrocytes during experimental meningitis or after injection of bacterial cell wall components, can promote the release of glutamate from microglia that subsequently leads to excitotoxicity (Chao and Hu 1994).

TNF-α can also induce apoptosis through caspase activation. As mentioned previously, the binding of TNF-α to the TNF-R (p55) leads to the cleavage and activation of caspase-8 and subsequently caspase-3. Von Mering et al. (2001) in a model of experimental meningitis demonstrated that the activation of caspases was impaired in TNF-α deficient mice, suggesting the role of this pathway in neuronal apoptosis. TNF-α was also shown to be in part responsible for hippocampal damage in meningitis (Bogdan et al. 1997).

The above-mentioned mechanisms that lead to neuronal cell death in bacterial meningitis are summarized in Fig. 5.

Fig. 5. Hypothetical cascade of events leading to neuronal cell death in bacterial meningitis (adopted from Nau and Bruck 2002)