MEK-specific inhibitor U0126 blocks spread of Borna disease virus in cultured cells.

Borna disease virus (BDV) is a highly neurotropic virus that causes Borna disease, a virus-induced immune-mediated encephalomyelitis, in a variety of warm-blooded animals. Recent studies reported that BDV can be detected in patients with psychiatric disorders. BDV is noncytopathic, replicates in the nucleus of infected cells, and spreads intraaxonally in vivo. Upon infection of susceptible cultured cells, virus can be detected in foci. Little is known about the cellular components required for BDV replication. Here, we show that the cellular Raf/MEK/ERK signaling cascade is activated upon infection with BDV. In the presence of the MEK-specific inhibitor U0126, cells get infected with BDV; however, there is a block in virus spread to neighboring cells. The effect of the inhibitor on virus spread was still observed when the compound was added 2 h postinfection but not if treatment was initiated as late as 4 h after infection. Our results provide new insights into the BDV-host cell interaction and show that virus infection can be controlled with drugs interfering with a cellular signaling pathway. Since concentrations of the MEK inhibitor required to block BDV focus formation are not toxic for the host cells, our finding may be important with respect to antiviral drug development.

Inhibition of Borna disease virus replication by ribavirin.

The guanosine analogue ribavirin was tested for antiviral activity in two neural cell lines, human oligodendrocytes and rat glia, against Borna disease virus (BDV) strains V and He/80. Ribavirin treatment resulted in lower levels of virus and viral transcripts within 12 h. Addition of guanosine but not adenosine resulted in a profound reduction of the ribavirin effect. Ribavirin appears to be an effective antiviral agent for treatment of BDV infection in vitro. A likely mechanism for its activity is reduction of the intracellular GTP pool, resulting in inhibition of transcription and capping of BDV mRNAs.

Inhibition of Borna disease virus multiplication by interferon: cell line differences in susceptibility.

It has previously been reported that de novo infection of primary rabbit brain cells with Borna disease virus (BDV) can be blocked with interferon-alpha/beta (IFN), whereas this cytokine has no inhibitory effect on BDV in persistently infected rat lung cells [v. Rheinbaben et al., J. Gen. Virol. (1985) 66: 2,777-2,780]. It remained unclear, however, whether these results indicated that IFN exclusively targets early steps of the BDV replication cycle or whether they simply reflected cell line differences. We now show that BDV replication was effectively inhibited by IFN in both acutely and persistently infected monkey Vero cells. By contrast, IFN had no clear protective effect on either de novo or persistent BDV infections of rat C6 glioblastoma cells. IFN protected C6 cells from the cytopathic effects of vesicular stomatitis virus, excluding the possibility that these cells are devoid of a functional IFN system. In primary rat fibroblasts and in a human oligodendroglial cell line, IFN induced an efficient antiviral state against BDV. These results indicate that BDV is highly susceptible to the antiviral effect of IFN in some cell lines, while others seem to lack undefined components of the IFN system which mediate protection against BDV.

Amantadine does not have antiviral activity against Borna disease virus.

We have investigated the antiviral activity of amantadine (AD) against Borna disease virus (BDV) in several culture cell systems. We present evidence that AD, in the range 5 to 10 microM, does not have antiviral activity against BDV. Treatment of BDV infected cells with AD for six days caused neither a reduction in the number of infected cells, nor a decrease in steady state levels of BDV RNA or proteins. Moreover, treatment of cells with AD prior infection did not affect BDV multiplication, whereas influenza A virus yield was less than 1% with respect to that obtained in untreated control cells.

Intrinsic responses to Borna disease virus infection of the central nervous system.

Immune cells invading the central nervous system (CNS) in response to Borna disease virus (BDV) antigens are central to the pathogenesis of Borna disease (BD). We speculate that the response of the resident cells of the brain to infection may be involved in the sensitization and recruitment of these inflammatory cells. To separate the responses of resident cells from those of cells infiltrating from the periphery, we used dexamethasone to inhibit inflammatory reactions in BD. Treatment with dexamethasone prevented the development of clinical signs of BD, and the brains of treated animals showed no neuropathological lesions and a virtual absence of markers of inflammation, cell infiltration, or activation normally seen in the CNS of BDV-infected rats. In contrast, treatment with dexamethasone exacerbated the expression of BDV RNA, which was paralleled by a similarly elevated expression of mRNAs for egr-1, c-fos, and c-jun. Furthermore, dexamethasone failed to inhibit the increase in expression of mRNAs for tumor necrosis factor alpha, macrophage inflammatory protein 1 beta, interleukin 6, and mob-1, which occurs in the CNS of animals infected with BDV. Our findings suggest that these genes, encoding transcription factors, chemokines, and proinflammatory cytokines, might be directly activated in CNS resident cells by BDV. This result supports the hypothesis that the initial phase of the inflammatory response to BDV infection in the brain may be dependent upon virus-induced activation of CNS resident cells.

N-glycosylated protein(s) are important for the infectivity of Borna disease virus (BDV).

Tunicamycin inhibited the production of infectious Borna disease virus (BDV) and glycosidase treatment eliminated the infectivity of cell-free virus. A glycoprotein of approximately 17 kDa, found in association with infectious virus, was identified by Concanavalin A binding.