Viral encephalitis is a serious and important human and animal health problem, as
exemplified by West Nile encephalitis and HIV-related dementia. There is a need
for better understanding of the pathogenesis of virus encephalitis. Experimentally,
Semliki Forest virus (SFV) is an excellent mouse model of viral encephalitis and
virus-induced demyelination. SFV is an alphavirus of the Togaviridae. In mice it is
neuroinvasive and neurotropic. Following infection of susceptible mouse strains,
clearance of infectious brain virus is coincident with inflammatory cell infiltration
and is followed by demyelination. There is some existing evidence to suggest that
CNS demyelination observed following SFV infection has an immune aetiology and
based on transient depletion studies that CD8+ T-cells are likely to be the main
effectors of this.
This thesis examines the role of acquired immune responses in SFV encephalitis.
The role of components of the acquired immune response in mediating clearance of
infectious virus and virus RNA and in the pathogenesis of demyelination was
examined using mice with genetic deletions affecting components of acquired
immune responses. This included mice deficient in CD4⁺ and CD8⁺ T-cell responses
and in specific T-cell mediators IFNγ, perforin and Fas. Infectious virus was
determined by plaque assay, the presence of virus RNA by quantitative, real-time
PCR and the neuropathology by standard histopathology and immunohistochemistry.
Analysis of inflammatory infiltrates in the SFV infected CNS demonstrated a rapid
influx of macrophages and NK cells and a >40-fold increase in T-lymphocytes,
predominantly CD8+ cells. Mice lacking CD8+ T-cells showed no difference in their
ability to clear infectious virus from the brain, but had a slower clearance of virus
RNA. Adoptive transfer of CD8+ T-cells to SFV infected SCID mice demonstrated
that CD8+ T-cells mediated the demyelinating lesions. Mice lacking CD4⁺ T-cells
were unable to generate good antibody responses and were unable to clear infectious
virus. Transfer of anti-SFV hyperimmune (HI) serum to SFV infected SCID mice
lowered virus RNA to levels comparable to those in immunocompetent (BALB/c)
mice. However, antibody alone was not sufficient to eliminate virus RNA and
infectious virus reappeared once antibody levels dropped.
IFNγR⁻/⁻ mice were found to have slower clearance of virus RNA compared to wildtype mice but IFNγ was not necessary for the development of demyelinating lesions.
A protective role for IFNγ was demonstrated in SFV infection; recombinant IFNγ
transiently protected SFV infected IFNa/ßR⁻/⁻ mice. Neither perforin nor Fas was
necessary for clearance of infectious virus or viral RNA. SFV infected Fas knock-out mice had increased CNS demyelination.
In summary this thesis demonstrates that in SFV infection, CD8⁺ T-cells are the main
component of the CNS inflammatory response; CD8⁺ T-cells mediate the lesions of
demyelination; clearance of infectious virus is mediated by antibody but antibody
alone is insufficient to clear all virus RNA; CD8⁺ T-cells and the IFN-γ system
contribute to the elimination of virus RNA. It is likely that both antibody and CD8⁺
T-cells are required to eliminate SFV infection.