The protective role of cytotoxic T cells and interferon against coronavirus invasion of the brain

Abstract

MHV-A59 causes focal acute encephalitis, acute hepatitis, and chronic demyelination while MHV-2 causes acute hepatitis and no brain involvement. The difference in organ tropism between these two closely related MHVs is not related to the ability of these viruses to grow in brain cells since both viruses grow equally well in primary glial cell cultures derived from neonatal mouse brains. We postulated therefore that the ability of the virus to stimulate certain host immunological factors may be important for protection of the brain against invasion and replication of the virus. In this study we performed preliminary experiments to investigate the potential role of two host factors in protection of the brain against MHV invasion: cytotoxic T cells and interferon. Four week old β2M ((-/-)) mice, lacking β2 microglobulin, MHC class 1 expression and functional cytotoxic CD8+ T cells were inoculated intracerebrally (IC) with MHV-2 and analyzed at various intervals post infection for histopathology and vital titers in organs. Histology revealed both acute hepatitis and acute encephalitis. Acute encephalitis was observed in periventricular areas. Mononuclear lymphocytic infiltration involved the choroid plexus, the ependyma and in the surrounding brain parenchyma. There was no involvement of other areas of the brain including areas that are typically involved in A59 infection of C57B1/6 mice. By contrast, C57B1/6 mice infected with MHV-2 showed no involvement of the brain parenchyma and only slight inflammation of the choroid plexus was present. High titers of infectious virus was detected by plaque assay in both brains and livers of β2M ((-/-)) mice infected with MHV-2 in contrast to only liver titers in C57B1/6 mice infected with a similar dose of MHV-2. Polyclonal rabbit-anti mouse IFN α/β or anti IFN β (Lee Biomolecular Research LAb.) was given to groups of 4-week-old C57B1/6 mice at a dose of 10,000 U per one I.P. treatment, 24 hours prior to I.C. inoculation of 1LD50 of MHV-2 or MHV-A59. At various intervals post inoculation virus tilers from brains and livers were determined by plaque assay, and the histopathology of all the internal organs was analyzed by H and E staining. Treatment with preimmune serum from the same rabbit was used as control with no effect on disease outcome in either one of the viruses. While IFN antibodies had little or no effect on the outcome of disease in MHV-A59 infection, mice treated with either anti IFN α/β or anti IFN β had high titers of virus recovered from the brain and histopathological evidence of acute meningoencephalitis. Thus cytotoxic T cells and interferon may have a protective role against brain invasion of the virus in MHV-2 infection in mice.