Chronic meningeal inflammation as a cause of cortical grey matter pathology in multiple sclerosis

Abstract

Subpial demyelination in cerebral cortical grey matter is associated with clinical progression in multiple sclerosis and is suggested to result from diffusion of pro-inflammatory cytokines from areas of meningeal inflammation into the cortex.

In order to test this hypothesis we have developed an animal model of subpial demyelination driven by meningeal inflammation, involving delivery of cytokines into the subarachnoid space (SAS). Dark Agouti rats were immunised with a subclinical dose (10µg) of recombinant mouse myelin oligodendrocyte glycoprotein followed 21-24 days later by injection of TNF (1.25-5µg) and IFN-γ (75-300ng) into the SAS of the sagittal sulcus. The presence of the cytokines in the SAS resulted in acute demyelination and inflammation followed by resolution of pathology. This supports the hypothesis that cytotoxic/pro-inflammatory molecules diffuse from areas of meningeal inflammation into the underlying cortex resulting in microglial activation and subpial demyelination. Increasing the doses of TNF and IFN-γ resulted in increased extent, but not duration, of pathology due to the acute presence of the cytokines. We conclude that a chronic inflammatory milieu in the CSF/meningeal compartment is required to achieve chronic microglial activation and subpial demyelination and neuronal loss.

In order to achieve the chronic presence of the cytokines in the SAS a high titre VSV-G-pseudotyped lentiviral vector carrying the enhanced green fluorescent protein (eGFP) gene under control of the CMV promoter was tested. It induced extensive and long-term, up to 12 weeks, eGFP expression in the sagittal sulcus in the absence of long-term microglial activation in naïve animals. Expression was localised to astrocytes, leptomeningeal cells and a small number of pyramidal neurons. The vector did not induce non-specific demyelination and inflammation in animals immunised with a subclinical dose of rmMOG. In order to achieve more localised expression, the vector was injected with collagen hydrogel. The hydrogel delayed eGFP expression but increased its spread along the anteroposterior axis. The distribution and duration of expression appeared optimal for achieving the chronic presence of TNF and IFN-γ in the CSF/meningeal compartment required to develop this novel model, if expression at the injection site could be increased using the hydrogel, which requires optimisation. We propose that the chronic presence of the cytokines will result in chronic meningeal inflammation and cortical grey matter pathology, allowing evaluation of the role of cytotoxic/pro-inflammatory molecules.

The identity of several of the cytotoxic/pro-inflammatory molecules suggested to diffuse from areas of meningeal inflammation were also identified in post-mortem MS meninges using PCR arrays. Expression of CXCL13, IL5RA, IFNG and CXCL9 were increased, and that of CXCL1 decreased, in MS patients, consistent with an inflammatory milieu in the CSF/meningeal compartment and suggesting that these molecules may represent novel therapeutic targets for modulating meningeal inflammation and cortical pathology.