Differential Transcription of Ovine Herpesvirus 2 Genes in Lymphocytes from Reservoir and Susceptible Species

Abstract

Ovine herpesvirus 2 (OvHV-2) is a lymphotropic gammaherpesvirus that asymptomatically infects most sheep, but causes malignant catarrhal fever in cattle, bison, pigs and deer. There is no permissive cell culture system but OvHV-2-infected T lymphocytes can be cultured from diseased animals. We showed that the OvHV-2 genome was in a circular conformation in sheep peripheral blood mononuclear cells and that the latency-associated ORF73 was transcribed, while expression of the productive cycle genes ORF9 (DNA polymerase) and ORF50 (R-transactivator) was barely detectable, suggestive of latency. Doxorubicin treatment of these cells induced the appearance of linear viral DNA and transcription of productive cycle genes along with several viral unique genes. In contrast, cultured T cells from diseased cattle and rabbits contained a mixture of circular and linear genome configurations indicative of a mixture of latently- and productively-infected cells. Most of the OvHV-2 unique genes were transcribed in these cells but ORF50 expression was only seen after doxorubicin treatment indicating a 'leaky' latent pattern of gene expression. 5-azacytidine treatment increased the proportion of circular DNA and inhibited the expression of most of the OvHV-2 unique genes except Ov2.5 (vIL-10) and Ov4.5 (Bcl-2 homologue) in the cattle cell line. These studies provide key insights into the differences in OvHV-2 gene expression in cells from reservoir and susceptible species and, for the first time, an in vitro system for studying the latent and productive phases of the OvHV-2 virus life cycle.

Malignant Catarrhal Fever of Cattle Is Associated with Low Abundance of IL-2 Transcript and a Predominantly Latent Profile of Ovine Herpesvirus 2 Gene Expression

“…Moreover, OvHV-2 is rarely, if at all, naturally transmitted from one cattle to another and it has been difficult to recover infectivity at all from cattle with MCF [43], [44], [45]. Conversely, a mixture of OvHV-2 transcripts has been detected in cultured peripheral T-lymphocytes from cattle with MCF [29]. Those authors concluded that, at least in the periphery, latently infected cells may co-exist with cells harboring productively replicating OvHV-2.…”

“…However, the presence of viral DNA in each single dysregulated cell, which contributes to the disease picture, has been a matter of debate. Some authors believe that only a fraction of lymphocytes is infected, whereas others argue that the frequency of virus positive cells in vivo is being underestimated due to the lack of sensitive methods for detection [6], [24], [25], [29], [30].…”

Host gene expression changes in cattle infected with Alcelaphine herpesvirus 1

“…Thus the virus may be limited to some form of latency or to an aberrant pattern of gene expression that is abortive for virus propagation. Analysis of LGL from cattle and rabbits infected with OvHV-2 showed that these cells contained circular (latent) genomes and did not express known lytic cycle transcripts, although some viral gene expression was detected (Thonur et al, 2006). The conflicting evidence of recent studies may also suggest that virus gene expression patterns can vary depending on host species and cell type infected.…”

Ovine Herpesvirus 2 Glycoproteins B, H, and L Are Sufficient for, and Viral Glycoprotein Ov8 Can Enhance, Cell-Cell Membrane Fusion

“…Alternatively, translation of Ov8 from an mRNA, without splicing, to the first in-frame stop codon at nucleotides 83683 to 83685 would give rise to a protein of 704 aa with a molecular mass of 74 kDa, 17 potential N-glycosylation sites, and no transmembrane domain. Although transcripts from the Ov8 open reading frame have been detected in OvHV-2-infected lymphoblastoid cells (30), no studies have looked at the encoded protein(s).…”