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 (IFNγ) 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 IFNγ 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 IFNγ-/- mice and bone marrow chimeric mice in which IFNγR is not expressed in the CNS but is intact in the periphery (IFNγR(CNS)KO) and vice versa (IFNγR(peri)KO). 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 IFNγR(CNS)KO as compared to IFNγR(peri)KO mice. Contrary to what we anticipated, cerebella/brainstems of IFNγR(CNS)KO mice were only minimally infiltrated, while the same areas of IFNγR(peri)KO mice were extensively populated by peripheral immune cells. Furthermore, the CNS of IFNγR(peri)KO mice was characterized by persistent neutrophil-rich foci as compared to IFNγR(CNS)KO. Immunohistochemical analysis of the CNS of IFNγ-/- and IFNγR chimeric mice revealed that IFNγ 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. IFNγ 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.