Production and role of cytokines in the CNS of mice with acute viral encephalomyelitis

Semliki Forest Virus (SFV) causes a more severe acute encephalomyelitis in B6 than in SJL mice despite similar T cell proliferation and antibody responses in these two strains. To determine the immunological mechanisms that may contribute to this difference, CNS tissues from SFV-infected B6 and SJL mice were analyzed for viral replication, inflammatory responses and cytokine production, by semiquantitative reverse transcriptase-PCR and immunohistochemistry. Although initially similar on day 2 p.i., SFV replicated to higher viral titers in B6 than SJL mice on days 4 and 7 p.i. Infectious virus was cleared from both strains by day 10 p.i. There were no differences in numbers of CD4⁺, CD8⁺ or MHC class I and II⁺ inflammatory cells at any time point. Higher levels of IL-4 mRNA, lower levels of TNF-α, IL-6, IL-1β and IL-2 mRNAs and lower IL-2⁺ and IFN-γ⁺ cells were found in B6. These findings suggest that despite comparable immune responses, different patterns of cytokine production correlated with higher levels of virus in the brains and more severe clinical disease in B6, and more efficient clearance of virus and less severe disease in SJL mice.