IFN-gamma signaling in the central nervous system controls the course of experimental autoimmune encephalomyelitis independently of the localization and composition of inflammatory foci

Background: Murine experimental autoimmune encephalomyelitis (EAE), a model for multiple sclerosis, presents typically as ascending paralysis. However, in mice in which interferon-gamma (IFN gamma) signaling is disrupted by genetic deletion, limb paralysis is accompanied by atypical deficits, including head tilt, postural imbalance, and circling, consistent with cerebellar/vestibular dysfunction. This was previously attributed to intense cerebellar and brainstem infiltration by peripheral immune cells and formation of neutrophil-rich foci within the CNS. However, the exact mechanism by which IFN gamma signaling prohibits the development of vestibular deficits, and whether the distribution and composition of inflammatory foci within the CNS affects the course of atypical EAE remains elusive. Methods: We induced EAE in IFN gamma-/- mice and bone marrow chimeric mice in which IFN gamma R is not expressed in the CNS but is intact in the periphery (IFN gamma(RKO)-K-CNS) and vice versa (IFN gamma(RKO)-K-peri). Blood-brain barrier permeability was determined by Evans blue intravenous administration at disease onset. Populations of immune cell subsets in the periphery and the CNS were quantified by flow cytometry. CNS tissues isolated at various time points after EAE induction, were analyzed by immunohistochemistry for composition of inflammatory foci and patterns of axonal degeneration. Results: Incidence and severity of atypical EAE were more pronounced in IFN gamma(RKO)-K-CNS as compared to IFN gamma(RKO)-K-peri mice. Contrary to what we anticipated, cerebella/brainstems of IFN gamma(RKO)-K-CNS mice were only minimally infiltrated, while the same areas of IFN gamma(RKO)-K-peri mice were extensively populated by peripheral immune cells. Furthermore, the CNS of IFN gamma(RKO)-K-peri mice was characterized by persistent neutrophil-rich foci as compared to IFN gamma(RKO)-K-CNS. Immunohistochemical analysis of the CNS of IFN gamma-/- and IFN gamma R chimeric mice revealed that IFN gamma protective actions are exerted through microglial STAT1. Conclusions: Alterations in distribution and composition of CNS inflammatory foci are not sufficient for the onset of atypical EAE. IFN gamma dictates the course of neuroinflammatory disorders mainly through actions exerted within the CNS. This study provides strong evidence that link microglial STAT1 inactivation to vestibular dysfunction.