- Volume 81, Issue 11, 2000

To study the biological relevance of using bovine herpesvirus-1 (BHV-1) as a vector for expressing cytokines, a BHV-1 virus that expressed bovine interferon-γ (IFN-γ) was constructed. This recombinant virus (BHV-1/IFNγ) was then used to infect the natural host in a respiratory disease model. In vitro characterization of the recombinant interferon-γ confirmed that the cytokine expressed in BHV-1-infected cells was biologically active. The in vivo effects of the recombinant IFN-γ were then analysed during a primary infection and after reactivation of a latent infection. During the primary infection, similar body temperature, clinical responses and virus shedding were observed for calves infected with either recombinant BHV-1/IFNγ or parental gC−/LacZ+ virus. An analysis of cellular and humoral responses did not reveal any significant immunomodulation by BHV-1/IFNγ during the primary infection. The stability and activity of recombinant IFN-γ was also analysed following the establishment of a latent infection. The presence of recombinant IFN-γ did not significantly alter virus shedding following reactivation. The isolation of reactivated BHV-1/IFNγ virus confirmed that a functional IFN-γ gene was retained during latency. Thus, herpesviruses may provide virus vectors that retain functional genes during latency and recrudescence.

In this study, four inhibitor of apoptosis genes (iaps) in the genome of Epiphyas postvittana nucleopolyhedrovirus (EppoMNPV) that are homologous to iap-1, iap-2, iap-3 and iap-4 genes of other baculoviruses have been identified. All four iap genes were sequenced and the iap-1 and iap-2 genes were shown to be functional inhibitors of apoptosis. The iap-1, iap-2 and iap-3 genes contain two baculovirus apoptosis inhibitor repeat motifs and a C3HC4 RING finger-like motif. The activity of the iap genes was tested by transient expression in Spodoptera frugiperda (Sf-21) cells treated with the apoptosis-inducing agents actinomycin D, cycloheximide, anisomycin, tumour necrosis factor-α and UV light. The iap-2 gene prevented apoptosis induced by all agents tested, indicating activity towards a conserved component(s) of multiple apoptotic pathways. However, the iap-2 gene was unable to function in the absence of a gene immediately upstream of iap-2 that has homology to the orf69 gene of Autographa californica MNPV. The use of a CMV promoter rescued the apoptosis inhibition activity of the iap-2 gene, indicating that the upstream orf69 homologue is associated with expression of iap-2. The iap-1 gene was able to delay the onset of apoptosis caused by all of the induction agents tested but, unlike iap-2, was unable to prevent the development of an apoptotic response upon prolonged exposure of cells to the apoptosis induction agents. No anti-apoptotic activity was observed for the iap-3 and iap-4 genes of EppoMNPV.

Tomato ringspot nepovirus RNA-1-encoded polyprotein (P1) contains the domains for the putative NTP-binding protein, VPg, 3C-like protease and a putative RNA-dependent RNA polymerase in its C-terminal region. The N-terminal region of P1, with a coding capacity for a protein (or a precursor) of 67 kDa, has not been characterized. Using partial cDNA clones, it is shown that the 3C-like protease can process the N-terminal region of P1 at a novel cleavage site in vitro, allowing the release of two proteins, X1 (located at the N terminus of P1) and X2 (located immediately upstream of the NTB domain). P1 precursors in which the protease was inactive or absent were not cleaved by exogenously added protease, suggesting that P1 processing was predominantly in cis. Results from site-directed mutagenesis of putative cleavage sites suggest that dipeptides Q423/G and Q620/G are the X1-X2 and X2-NTB cleavage sites, respectively. The putative X1 protein contains a previously identified alanine-rich sequence which is present in nepoviruses but not in the related comoviruses. The putative X2 protein contains a region with similarity to the comovirus 32 kDa protease co-factor (the only mature protein released from the N terminus of comovirus P1 polyproteins) and to the corresponding region of other nepovirus P1 polyproteins. These results raise the possibility that the presence of two distinct protein domains in the N-terminal part of the P1 polyprotein may be a common feature of nepoviruses.

Transmissible spongiform encephalopathies are often propagated by extracerebral inoculation. The mechanism of spread from peripheral portals of entry to the central nervous system (neuroinvasion) is complex: while lymphatic organs typically show early accumulation of prions, and B-cells and follicular dendritic cells are required for efficient neuroinvasion, actual entry into the central nervous system occurs probably via peripheral nerves and may utilize a PrPC-dependent mechanism. This study shows that transgenic mice overexpressing PrPC undergo rapid and efficient neuroinvasion upon intranerval and footpad inoculation of prions. These mice exhibited deposition of the pathological isoform of the prion protein (PrPSc) and infectivity in specific portions of the central and peripheral sensory pathways, but almost no splenic PrPSc accumulation. In contrast, wild-type mice always accumulated splenic PrPSc, and had widespread deposition of PrPSc throughout the central nervous system even when prions were injected directly into the sciatic nerve. These results indicate that a lympho-neural sequence of spread occurs in wild-type mice even upon intranerval inoculation, while overexpression of PrPC leads to substantial predilection of intranerval over lymphoreticular spread. The rate of transport of infectivity in peripheral nerves was ca. 0·7 mm per day, and prion infectivity titres of sciatic nerves were much higher in tga20 than in wild-type mice, suggesting that overexpression of PrPC modulates the capacity for intranerval transport.