Inhibition of reactive astrocytosis in established experimental autoimmune encephalomyelitis favors infiltration by myeloid cells over T cells and enhances severity of disease.

Reactive astrocytosis, involving activation, hypertrophy, and proliferation of astrocytes, is a characteristic response to inflammation or injury of the central nervous system. We have investigated whether inhibition of reactive astrocytosis influences established experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. We made use of transgenic mice, which express herpes simplex virus-derived thymidine kinase under control of a glial fibrillary acidic protein promotor (GFAP HSV-TK mice). Treatment of these mice with ganciclovir leads to inhibition of reactive astrocytosis. When GFAP HSV-TK mice were treated for seven days following onset of EAE with ganciclovir, disease severity increased. Although aquaporin-4 staining on astrocyte endfeet at the glia limitans remained equally detectable, GFAP immunoreactivity and mRNA expression in CNS were reduced by this treatment. Ganciclovir-treated GFAP HSV-TK mice with EAE had a 78% increase in the total number of infiltrating myeloid cells (mainly macrophages), whereas we did not find an increase in infiltrating T cells, using quantitative flow cytometry. Per cell expression of mRNA for the macrophage-associated molecules TNFα, MMP-12 and TIMP-1 was elevated in spinal cord of GFAP HSV-TK mice treated with ganciclovir. Relative expression of CD3ε was downregulated, and expression levels of IFNγ, IL-4, IL-10, IL-17, and Foxp3 were not significantly changed. mRNA expression of CCL2 was upregulated, and CXL10 was downregulated. Thus, inhibition of reactive astrocytosis after initiation of EAE leads to increased macrophage, but not T cell, infiltration, and enhanced severity of EAE. This emphasizes the role of astrocytes in controlling leukocyte infiltration in neuroinflammation.