Semliki Forest virus-induced apoptosis in glial cells

Multiple sclerosis (MS) is the most common neurological disease affecting young adults. The incidence rates in Scotland are among the highest in the world. Virus infection may have a precipitating role, or exacerbate symptoms. That viruses can produce inflammatory central nervous system (CNS) demyelinating disease has been well established from study of several natural and experimental infections; one model system is Semliki Forest virus (SFV) infection of mice.SFV infection of BALB/c mice infected at less than 12 days of age (P12) causes fulminant encephalitis, characterised by apoptotic death of neuronal cell populations. In contrast, virus-infected neurons in mice older than P14 survive. In immunocompetent animals lesions of inflammatory demyelination develop. In immunocompromised animals (for example nu/nu mice) virus persists but no lesions of demyelination develop. SFV had previously been observed to trigger apoptosis in numerous proliferating cell lines and in immature differentiating neuronal cell cultures. One explanation for the dichotomy in the outcome of neuronal infection is that levels of key pro- or anti-apoptotic proteins differ as a function of maturational state.Oligodendrocytes are infected by SFV but little is known about the outcome of infection in these cells. The absence of demyelination in immunocompromised animals suggests these cells can survive infection. The aim of this thesis is to determine the fate of oligodendrocytes upon SFV infection, to ascertain the nature of any cell death and, if observed, to determine whether this is dependent on cell maturation state. Three experimental systems are used; glial cell lines, mixed glial cell cultures, and tissue sections from infected adult BALB/c and nu/nu mice.Two differentiation-inducible oligodendrocyte-type-2 astrocyte (0-2A) progenitor cell lines, SS5 and BC30, were utilised. 0-2A progenitor cells died rapidly of apoptosis as determined by DAPI labelling, DNA fragmentation and morphological features such as cell blistering and cell blebbing. Differentiation delayed virus induced cell death between 12 and 24 hours but did not prevent it: again cell death occurred by apoptosis.The outcome of SFV infection of oligodendroglia derived from mixed primary cultures was dependent on the degree of cellular homogeneity. Immunocytochemical studies revealed that immature 0-2A progenitors died rapidly, as revealed by double staining for caspase-3 and early 0-2A cell markers. The susceptibility of mature oligodendrocytes was found to be dependent on cellular environment. Mature oligodendrocytes were equally susceptible to virus-induced apoptosis as immature progenitors when cultured in isolation i.e. as enriched purified populations. On the contrary, cultures of mature oligodendrocytes maintained in the presence of other glial cells, or in contact with extracellular matrix molecules present in vivo, survived infection for the period under study: two weeks.The fate of the mature oligodendrocyte was further investigated in the corpus callosum in vivo using sections from SFV-infected adult BALB/c and nu/nu mice. Dual immunolabelling for virus and apoptosis (TUNEL staining) revealed an active period of cell death between PID 4 and 6. Dual iv immunofluorescence for 2'/-3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) and SFV or caspase-3 visualised by confocal microscopy indicated that although mature oligodendrocytes were infected with SFV they did not die by apoptosis.In conclusion, this study presents evidence that isolated enriched cultures of mature oligodendrocytes are susceptible to apoptosis as are their immature counterparts. Paradoxically, mature oligodendrocytes studied in vivo or maintained in the presence of other glial cells factors in vitro, did not undergo virus-induced apoptosis, unlike their immature relatives