Variability in the immune response to Theiler's Murine Encephalomyelitis Virus in different strains of mice

Theiler's Murine Encephalomyelitis Virus (TMEV) has been a favoured model for Multiple Sclerosis (MS) since 1975 when Lipton first reported that infection with TMEV caused a biphasic Central Nervous System (CNS) disease leading to demyelination. TMEV is a picornavirus belonging to the cardiovirus genus and is a natural enteric pathogen of mice which can occasionally initiate a chronic persistent infection of the CNS. This depends on the strain and dose of virus and the strain, age and sex of the mouse. Intracerebral infection of all mouse strains with the avirulent BeAn strain of TMEV results in an acute encephalomyelitis which in susceptible mouse strains, is followed by a persistent CNS infection with lesions of inflammatory demyelination or in resistant mouse strains eradication of the virus. On the other hand i.e. infection with the neurovirulent GDVII strain of Theiler's virus results in a fulminant encephalitis in mice of all genetic backgrounds.

The main aim of this study was to determine the cytokine and immunoglobulin profiles elicited in different mouse strains during the acute phase of infection. mRNA transcript levels for numerous cytokines were studied in the brains and spinal cords in Balb/c (resistant), CBA (intermediately susceptibility) and SJL/J (susceptible) mice, during the acute phase of disease, using the technique of RNase protection assay (RPA). The RPA included analysis of transcripts for TNFp, TNFa, TGFp, IFNy, IL-la, ILip, IL-2, IL-4, IL-5, IL-6, IL-7, IL-10, IL-13, IL-12p40 and IL-12p35. There were similarities between the strains in the levels of pro-inflammatory cytokines expressed including TNFa, TNFp and ILla. However, there were several fundamental differences between the strains including the inability of susceptible SJL/J mice to express IL-ip in the brain and the spinal cord when compared to Balb/c and CBA mice. SJL/J mice had an increase in expression of IL-4 and IL-10 and a decrease in expression of IL-2 and IFNy when compared to Balb/c and CBA mice. Expression of pro-inflammatory, anti¬ inflammatory, Thl and Th2 type cytokines correlated with the increase in levels of cellular infiltrates (CD3+, CD4+, CD8+ and F4/80+) in the CNS. Anti¬ viral immunoglobulin isotypes were also different in the three mouse strains studied. All strains produced similar levels of IgM however, Balb/c mice had significantly increased levels of IgGl and IgG2a compared to CBA and SJL/J mice during the acute phase of disease.

This study also investigated TMEV persistence in CBA (intermediately susceptible) mice and the cytokine and anti-viral immunoglobulin isotypes associated with persistence. Virus persisted for >60 days in 50% of infected CBA mice, as determined by RT-PCR. Animals in which virus persisted had significantly increased RNA transcripts in the CNS for TNFa, IL-12p35 and IL-12p40. Persistently infected animals also had increased levels of anti-viral IgGl, IgG2a and IgG2b when compared to animals which had cleared the virus.

The importance of interferons a/p and y were investigated. Virus spread extensively throughout the white matter regions of the brains in IFNa/pR°/° mice (constructed on a genetically resistant background (FI-2b)), during the acute phase of infection, indicating the importance of IFNa/p in preventing infection of oligodendrocytes. Infection of IFNyR°/° mice (also on a genetically resistant background) resulted in viral persistence and increased levels of anti-viral IgM, IgGl, IgG2a and IgG2b, demonstrating IFNy is essential for viral clearance. Perforin is a functional effector molecule in CTL killing, therefore, its role(s) during the acute and chronic phase of Theiler's virus infection was investigated to ascertain its importance. Studies in perforin knockout mice (also on a resistant genetic background) demonstrated that perforin is essential to control viral infection during the acute phase of infection, and is an absolute requirement for viral clearance.

Infection with GDVII resulted in high levels of virus replication in the brains and spinal cords of infected mice. Levels of TNFa, IL-la, IL-2, IFNy and IL- 12p40 increase throughout infection in the brains of infected animals, and TNFa, IL-2 and IL-12p40 increase in the spinal cords. High virus titres, and an increase in the above pro-inflammatory cytokines correlated with an increase in levels of programmed cell death in CNS tissues.

Infection of neonatal mice with the BeAn strain results in 100% mortality, with increased virus titres in the CNS. Expression of TNFp, TNFa, IL-4, IL-la and IL-6 increased throughout the course of infection of neonatal mice. TNFa has been implicated in the phenomenon of death by shock. Therefore, TNFa may have important implications in the pathogenesis of Theiler's virus infection in neonates.