Prolonged nasal shedding and viraemia of cytopathogenic bovine virus diarrhoea virus in experimental late-onset mucosal disease

The Molecular Biology of Pestiviruses

Pestiviruses are among the economically most important pathogens of livestock. The biology of these viruses is characterized by unique and interesting features that are both crucial for their success as pathogens and challenging from a scientific point of view. Elucidation of these features at the molecular level has made striking progress during recent years. The analyses revealed that major aspects of pestivirus biology show significant similarity to the biology of human hepatitis C virus (HCV). The detailed molecular analyses conducted for pestiviruses and HCV supported and complemented each other during the last three decades resulting in elucidation of the functions of viral proteins and RNA elements in replication and virus-host interaction. For pestiviruses, the analyses also helped to shed light on the molecular basis of persistent infection, a special strategy these viruses have evolved to be maintained within their host population. The results of these investigations are summarized in this chapter.

Differential activation of interferon regulatory factors-3 and -7 by non-cytopathogenic and cytopathogenic bovine viral diarrhoea virus

Non-cytopathogenic bovine viral diarrhoea virus (ncpBVDV) has previously been shown to inhibit the function of interferon regulatory factor-3 in cultured cells [J. Virol. 76 (2002) 8979]. In this study, we show that, like ncpBVDV, when cells were previously exposed to cytopathogenic BVDV (cpBVDV) the appearance of an IRF-3-DNA complex from nuclear extracts that can be induced by heterologous virus infection was not observed. Infection of cells with ncpBVDV or cpBVDV resulted in neither the translocation of IRF-7 from the cytoplasm to the nucleus of infected cells, nor an inhibition of its nuclear translocation in cells super-infected by Semliki Forest Virus. We conclude that cpBVDV and ncpBVDV both share the ability to inhibit the full function of IRF-3 but neither stimulate or block the nuclear uptake of IRF-7.

Inhibition of beta interferon transcription by noncytopathogenic bovine viral diarrhea virus is through an interferon regulatory factor 3-dependent mechanism

The induction and inhibition of the interferon (IFN) response and apoptosis by bovine viral diarrhea virus (BVDV) has been examined. Here we show that prior infection of cells by noncytopathogenic BVDV (ncp BVDV) fails to block transcriptional responses to alpha/beta IFN. In contrast, ncp BVDV-infected cells fail to produce IFN-alpha/beta or MxA in response to double-stranded RNA (dsRNA) or infection with a heterologous virus (Semliki Forest virus [SFV]). ncp BVDV preinfection is unable to block cp BVDV- or SFV-induced apoptosis. The effects of ncp BVDV infection on the transcription factors controlling the IFN-beta induction pathway have been analyzed. The transcription factor NF-kappa B was not activated following ncp BVDV infection, but ncp BVDV infection was not able to block the activation of NF-kappa B by either SFV or tumor necrosis factor alpha. Furthermore, ncp BVDV infection did not result in the activation of stress kinases (JNK1 and JNK2) or the phosphorylation of transcription factors ATF-2 and c-Jun; again, ncp BVDV infection was not able to block their activation by SFV. Interferon regulatory factor 3 (IRF-3) was shown to be translocated to the nuclei of infected cells in response to ncp BVDV, although DNA-binding of IRF-3 was not seen in nuclear extracts. In contrast, an IRF-3-DNA complex was observed in nuclear extracts from cells infected with SFV, but the appearance of this complex was blocked when cells were previously exposed to ncp BVDV. We conclude that the inhibition of IFN induction by this pestivirus involves a block to IRF-3 function, and we speculate that this may be a key characteristic for the survival of pestiviruses in nature.

Antigen-presenting cells from calves persistently infected with bovine viral diarrhoea virus, a member of the Flaviviridae, are not compromised in their ability to present viral antigen

The aim of this study was to assess whether the infection of antigen-presenting cells (APC) in vivo, evident in calves persistently infected (PI) with bovine viral diarrhoea virus (BVDV), compromised their ability to stimulate virus-specific T cell responses. Major histocompatibility complex (MHC) molecule-identical cattle were identified from the inbred family at the Institute for Animal Health. One was PI and immunotolerant to BVDV. Virus was not isolated from the remaining calves, which were classified as BVDV-immune or BVDV-naïve depending on the presence or absence of BVDV-specific antibodies in sera. Two-colour flow-cytometric analysis of PBMC from the PI calf showed that 40% of CD14(+) monocytes were infected in vivo. Monocytes from the PI calf (PI monocytes) were used as naturally infected ex vivo APC with CD4(+) or CD8(+) T cells isolated from the BVDV-naïve or BVDV-immune animals. PI monocytes stimulated proliferative responses with CD4(+) and CD8(+) T cells from BVDV-immune animals, but not from BVDV-naïve calves. This provided evidence for the presence of virus-specific CD4(+) and CD8(+) memory T cells after acute infection and indicated that ex vivo monocytes from PI, immunotolerant calves stimulated both MHC class I- and MHC class II-restricted T cell responses to BVDV. Additionally, naturally infected ex vivo monocytes cultured in vitro for 3 days stimulated effective T cell responses to the virus with which they were infected.

Pathogenesis of primary respiratory disease induced by isolates from a new genetic cluster of bovine viral diarrhea virus type I

The pathogenesis of infection induced by cytopathogenic isolates from the newly identified genetic cluster Id of bovine viral diarrhea virus (BVDV) type I was studied in two experimental infections of previously seronegative, immunocompetent calves. Experiment 1 focused on the evaluation of clinical patterns, viremia, and serological responses. All infected calves in this experiment developed respiratory symptoms and seroconverted to BVDV positivity. Contact calves also contracted a respiratory tract infection following exposure to infected animals. Viremia was demonstrated between postinfection days 2 and 17, and the virus was detected in organ specimens of all but one each of the infected and contact calves. In experiment 2, the distribution of BVDV in various tissues of calves euthanized at defined days postinfection was studied. In two of these calves recurrent shedding of BVDV in nasal secretions was shown. BVDV was detected in various tissues of all infected calves throughout the experiment and also following seroconversion and the clearance of BVDV from the circulatory system. Despite the widespread distribution of the virus in various organs, significant tissue damage was found mainly in respiratory tract and lymphoid tissues. These experiments revealed that viruses from cluster Id of BVDV are able to induce primary respiratory disease in previously seronegative, immunocompetent calves. Contact transmission and virus recurrence, contrary to observations from acute experimental infections with noncytopathogenic BVDV, are likely to reflect differences in biological features of these cytopathogenic isolates. Virus shedding and its presence in tissues following peripheral clearance and in the presence of antibodies may have implications in the diagnosis, pathogenesis, and epidemiology of BVDV-induced syndromes in cattle.

Single amino acid differences are sufficient for CD4(+) T-cell recognition of a heterologous virus by cattle persistently infected with bovine viral diarrhea virus

Cattle that are persistently infected (PI) with one strain of bovine viral diarrhea virus (BVDV) can resolve infection with a second, antigenically heterologous strain but not the homologous strain. Since CD4(+) T cells are thought to be critical for the resolution of acute BVDV infection (Howard et al., 1992, Vet. Immunol. Immunopathol. 32, 303-314), we have examined the recognition of a heterologous virus (NADL) by CD4(+) T cells from Pe515-PI animals. The immune response of non-PI control cattle challenged with NADL or Pe515ncp was strain cross-reactive, whereas Pe515-PI animals responded to NADL only. The immune repertoire of both groups included NS3, which differs by approximately 1% (9/683) amino acids between these two viruses. Lymphoproliferative responses to proteins and synthetic peptides corresponding to three nonconservative differences in NS3 demonstrated that CD4(+) T cells from non-PI control animals responded well to proteins but poorly to the peptides from both viruses. In contrast, PI animals were responsive to heterologous proteins and peptides but nonresponsive to the homologous equivalents. A single amino acid difference between the two sequences was sufficient to allow responsiveness.