Although the action of interferons (IFNs) has been extensively studied in vitro, limited information is available on the spatial and temporal activation pattern of IFN-induced genes in vivo. We created BAC transgenic mice expressing firefly luciferase under transcriptional control of the Mx2 gene promoter. Expression of the reporter with regard to onset and kinetics of induction parallels that of Mx2 and is thus a hallmark for the host response. Substantial constitutive expression of the reporter gene was observed in the liver and most other tissues of transgenic mice, whereas this expression was strongly reduced in animals lacking functional type I IFN receptors. As expected, the reporter gene was induced not only in response to type I (␣ and ␤) and type III () IFNs but also in response to a variety of IFN inducers such as double-stranded RNA, lipopolysaccharide (LPS), and viruses. In vivo IFN subtypes show clear differences with respect to their kinetics of action and to their spatial activation pattern: while the type I IFN response was strong in liver, spleen, and kidney, type III IFN reactivity was most prominent in organs with mucosal surfaces. Infection of reporter mice with virus strains that differ in their pathogenicity shows that the IFN response is significantly altered in the strength of IFN action at sites which are not primarily infected as well as by the onset and duration of gene induction.

The type I interferon (IFN) response represents one of the first lines of defense against influenza virus infections. In this study, we assessed the protective potential of exogenous IFN-␣ against seasonal and highly pathogenic influenza viruses in ferrets. Intranasal treatment with IFN-␣ several hours before infection with the H1N1 influenza A virus strain A/USSR/90/77 reduced viral titers in nasal washes at least 100-fold compared to mock-treated controls. IFN-treated animals developed only mild and transient respiratory symptoms, and the characteristic fever peak seen in mock-treated ferrets 2 days after infection was not observed. Repeated application of IFN-␣ substantially increased the protective effect of the cytokine treatment. IFN-␣ did not increase survival after infection with the highly pathogenic H5N1 avian influenza A virus strain A/Vietnam/ 1203/2004. However, viral titers in nasal washes were significantly reduced at days 1 and 3 postinfection. Our study shows that intranasal application of IFN-␣ can protect ferrets from seasonal influenza viruses, which replicate mainly in the upper respiratory tract, but not from highly pathogenic influenza viruses, which also disseminate to the lung. Based on these results, a more intensive evaluation of IFN-␣ as an emergency drug against pandemic influenza A is warranted.

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We previously showed that mouse adaptation of cDNA-derived Borna disease virus (BDV) strain He/80 FR was associated exclusively with mutations in the viral polymerase complex. Interestingly, independent mouse adaptation of non-recombinant He/80 was correlated with different alterations in the polymerase and mutations in the viral glycoprotein. We used reverse genetics to demonstrate that changes in the polymerase which improve enzymatic activity represent the decisive host range mutations. The glycoprotein mutations did not confer replication competence in mice, although they slightly improved viral performance if combined with polymerase mutations. Our findings suggest that the viral polymerase restricts the host range of BDV.Borna disease virus (BDV) is a non-segmented negativestrand RNA virus (Briese et al., 1994) which belongs to the order Mononegavirales (de la Torre, 1994;Schneemann et al., 1995). BDV naturally infects the central nervous system (CNS) of a wide range of mammalian species and is the causative agent of Borna disease (Rott & Becht, 1995), an immune-mediated neurological disorder mainly affecting horses and sheep (Ludwig et al., 1988). Successful experimental infection of a variety of warm-blooded animals has been reported (Rott & Becht, 1995; Staeheli et al., 2000). Nevertheless, most tissue culture-adapted laboratory strains of BDV do not readily infect the CNS of mice, but can be adapted to mice by serial passage. Recently, we successfully adapted molecularly cloned BDV to the mouse and showed that adaptive mutations in the L polymerase (L 1116 R and N 1398 D) as well as in the polymerase cofactor P (R 66 K) contributed to replication competence of BDV in mice (Ackermann et al., 2007). In a previous study, Nishino et al. (2002) identified four different amino acid changes in a derivative of BDV strain He/80, designated CRNP5, that unlike its parent propagates efficiently in the brain of mice. Other amino acid changes originally reported to be present in CRNP5 (Nishino et al., 2002) could not be confirmed if the sequence was compared with those of BDV strain He/80 FR (data not shown). The remaining amino acid changes in CRNP5 affected the viral glycoprotein G (F 458 S and Y 480 H) and the polymerase L (K 1417 R and G 1686 R).To determine the relative contributions of these four mutations, we used reverse genetics technology established by our group (Martin et al., 2006;Schneider et al., 2005).We generated full-length cDNAs carrying either the amino acid substitutions F 458 S and Y 480 H in G, the substitutions K 1417 R and G 1686 R in L or all four substitutions in combination and recovered the corresponding viruses. The resulting recombinant viruses were designated BDV-G SH , BDV-L RR and BDV-G SH /L RR , respectively (Fig. 1). To further analyse the individual contributions of the mutations at amino acid positions 1417 and 1686 in L, we created fulllength cDNAs carrying these mutations individually. The corresponding viruses were recovered and were designated BDV-L 1417 R and BDV-L 1686 ...

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