Expression of Allograft Inflammatory Factor-1 and Haeme Oxygenase-1 in Brains of Rats Infected with the Neurotropic Borna Disease Virus

Experimental infection of Lewis rats with Borna disease virus (BDV) causes an immune-mediated nonpurulent meningoencephalitis. Viral persistence in the central nervous system is accompanied by mononuclear infiltrates, activated monocytic/microglial cells and reactive astrocytes. The immune-mediated process was further characterized by expression analysis of allograft inflammatory factor-1 (AIF-1), a novel marker of monocyte/microglial activation and of glial fibrillary acid protein (GFAP) between day 3 and day 50 post infection (p.i.). Potential neuroprotective effects of these cells were studied by the induction of haeme oxygenase-1 (HO-1), a defensive molecule against oxidative stress in various brain insults. In BDV-infected rat brains, mononuclear infiltrates and AIF-1 expression increased up to day 28 p.i. During early time points p.i., AIF-1 expression was mainly found in inflammatory lesions and adjacent brain parenchyma. Already 24 days p.i., a widespread upregulation of AIF-1 was observed which declined only moderately beyond day 28 p.i. HO-1 induction was maximal between days 18 and 28 p.i. Increased amounts of GFAP-positive astrocytes were present beyond 24 days p.i. Viral antigen expression increased simultaneously to the inflammatory reaction and persisted up to 50 days p.i. Widespread upregulation of AIF-1 indicates an early, long-lasting microglial activation, which might be involved in the immunesurveillance of the immune-mediated inflammatory events. The early peak of HO-1 most likely represents a neuroprotective, anti-inflammatory response by invading monocytes, microglial cells and astrocytes during the formation of encephalitic lesions and acute viral replication.