Howe, Charles L

Neuromyelitis optica (NMO)-immunoglobulin G (IgG) is a clinically validated serum biomarker that distinguishes relapsing central nervous system (CNS) inflammatory demyelinating disorders related to NMO from multiple sclerosis. This autoantibody targets astrocytic aquaporin-4 (AQP4) water channels. Clinical, radiological, and immunopathological data suggest that NMO-IgG might be pathogenic. Characteristic CNS lesions exhibit selective depletion of AQP4, with and without associated myelin loss; focal vasculocentric deposits of IgG, IgM, and complement; prominent edema; and inflammation. The effect of NMO-IgG on astrocytes has not been studied. In this study, we demonstrate that exposure to NMO patient serum and active complement compromises the membrane integrity of CNS-derived astrocytes. Without complement, astrocytic membranes remain intact, but AQP4 is endocytosed with concomitant loss of Na+-dependent glutamate transport and loss of the excitatory amino acid transporter 2 (EAAT2) . Our data suggest that EAAT2 and AQP4 exist in astrocytic membranes as a macromolecular complex. Transport-competent EAAT2 protein is up-regulated in differentiating astrocyte progenitors and in nonneural cells expressing AQP4 transgenically. Marked reduction of EAAT2 in AQP4-deficient regions of NMO patient spinal cord lesions supports our immunocytochemical and immunoprecipitation data. Thus, binding of NMO-IgG to astrocytic AQP4 initiates several potentially neuropathogenic mechanisms: complement activation, AQP4 and EAAT2 down-regulation, and disruption of glutamate homeostasis.

ABSTRACTWe evaluated the participatory role of human HLA-DR molecules in control of virus from the central nervous system and in the development of subsequent spinal cord demyelination. The experiments utilized intracranial infection with Theiler's murine encephalomyelitis virus (TMEV), a picornavirus that, in some strains of mice, results in primary demyelination. We studied DR2 and DR3 transgenic mice that were bred onto a combined class I-deficient mouse (beta-2 microglobulin deficient; β2m0) and class II-deficient mouse (Aβ0) of theH-2bbackground. Aβ0.β2m0mice infected with TMEV died within 18 days of infection. These mice showed severe encephalomyelitis due to rapid replication of virus genome. In contrast, transgenic mice with insertion of a single human class II major histocompatibility complex (MHC) gene (DR2 or DR3) survived the acute infection. DR2 and DR3 mice controlled virus infection by 45 days and did not develop spinal cord demyelination. Levels of virus RNA were reduced in HLA-DR transgenic mice compared to Aβ0.β2m0mice. Virus-neutralizing antibody responses did not explain why DR mice survived the infection and controlled virus replication. However, DR mice showed an increase in gamma interferon and interleukin-2 transcripts in the brain, which were associated with protection. The findings support the hypothesis that the expression of a single human class II MHC molecule can, by itself, influence the control of an intracerebral pathogen in a host without a competent class I MHC immune response. The mechanism of protection appears to be the result of cytokines released by CD4+T cells.

ABSTRACT Epidemiological studies suggest that an environmental factor (possibly a virus) acquired early in life may trigger multiple sclerosis (MS). The virus may remain dormant in the central nervous system but then becomes activated in adulthood. All existing models of MS are characterized by inflammation or demyelination that follows days after virus infection or antigen inoculation. While investigating the role of CD4 + T cell responses following Theiler's virus infection in mice deficient in STAT4 or STAT6, we discovered a model in which virus infection was followed by demyelination after a very prolonged incubation period. STAT4−/− mice were resistant to demyelination for 180 days after infection, but developed severe demyelination after this time point. Inflammatory cells and up‐regulation of Class I and Class II MHC antigens characterized these lesions. Virus antigen was partially controlled during the early chronic phase of the infection even though viral RNA levels remained high throughout infection. Demyelination correlated with the appearance of virus antigen expression. Bone marrow reconstitution experiments indicated that the mechanism of the late onset demyelination was the result of the STAT4−/− immune system. Thus, virus infection of STAT4−/− mice results in a model that may allow for dissection of the immune events predisposing to late‐onset demyelination in MS.

ABSTRACTWe evaluated the role of gamma interferon (IFN-γ) in protecting neurons from virus-induced injury following central nervous system infection. IFN-γ−/−and IFN-γ+/+mice of the resistant major histocompatibility complex (MHC)H-2bhaplotype and intracerebrally infected with Theiler's murine encephalomyelitis virus (TMEV) cleared virus infection from anterior horn cell neurons. IFN-γ+/+H-2bmice also cleared virus from the spinal cord white matter, whereas IFN-γ−/−H-2bmice developed viral persistence in glial cells of the white matter and exhibited associated spinal cord demyelination. In contrast, infection of IFN-γ−/−mice of the susceptibleH-2qhaplotype resulted in frequent deaths and severe neurologic deficits within 16 days of infection compared to the results obtained for controls. Morphologic analysis demonstrated severe injury to spinal cord neurons in IFN-γ−/−H-2qmice during early infection. More virus RNA was detected in the brain and spinal cord of IFN-γ−/−H-2qmice than in those of control mice at 14 and 21 days after TMEV infection. Virus antigen was localized predominantly to anterior horn cells in infected IFN-γ−/−H-2qmice. IFN-γ deletion did not affect the humoral response directed against the virus. However, the level of expression of CD4, CD8, class I MHC, or class II MHC in the central nervous system of IFN-γ−/−H-2qmice was lower than those in IFN-γ+/+H-2qmice. Finally, in vitro analysis of virus-induced death in NSC34 cells and spinal motor neurons showed that IFN-γ exerted a neuroprotective effect in the absence of other aspects of the immune response. These data support the hypothesis that IFN-γ plays a critical role in protecting spinal cord neurons from persistent infection and death.