Ovine herpesvirus 2 encodes a previously unrecognized protein, pOv8.25, that targets mitochondria and triggers apoptotic cell death

Malignant catarrhal fever (MCF) is a rare but frequently lethal disease of certain cloven-hoofed animals. At least 10 different viruses, all members of the macavirus genus in the subfamily gammaherpesvirinae, are known as causative agents of MCF. Among these, Ovine herpesvirus 2 (OvHV-2) is the most frequent and economically most important MCF agent. Phenotypically, MCF is characterized by severe lymphocytic arteritis-periarteritis, which leads to the accumulation of activated lymphocytes, accompanied by apoptosis and necrosis in a broad range of tissues. However, a viral factor that might be responsible for the tissue damage has not yet been identified. We have studied a seemingly intergenic locus on the OvHV-2 genome, which had previously been shown to be transcriptionally highly active in MCF-affected tissue. We identified by 5'- and 3' RACE a conserved, double-spliced transcript that encoded for a 9.9 kDa hydrophobic protein. The newly detected gene, Ov8.25, and its splicing pattern was conserved among OvHV-2 strains from different origins. Upon transient expression of synthetic variants of this gene in various cell types, including bovine lymphocytes, the protein (pOv8.25) was shown to targeting the mitochondria, followed by caspase-dependent apoptosis and necrosis. Notably, a deletion mutant of the same protein lost these abilities. Finally, we detected pOv8.25 in brain-infiltrating lymphocytes of cattle with MCF. Thus, the cell-death causing properties of pOv8.25 in affected cells may be involved in the emergence of typical MCF-associated apoptosis and necrosis. Thus, we have identified a novel OvHV-2 protein, which might contribute to the phenotype of MCF-related lesions.IMPORTANCEOvine herpesvirus 2 (OvHV-2) circulates among sheep without causing disease. However, upon transmission to cattle, the same virus instigates a frequently lethal disease, Malignant catarrhal fever (MCF). While cause of death and pathogenesis of tissue lesions are still poorly understood, MCF is characterized by accumulation of lymphocytes in various tissues, associated with vasculitis and cell death. As infectious virus is hardly present in those lesions, the cause of cell death cannot simply be explained by viral replication. The significance of our research is in identifying and characterizing a previously overlooked gene of OvHV-2 (Ov8.25), which is highly expressed in animals with MCF. Its encoded protein targets mitochondria, causing apoptosis and necrosis, thus, contributing to understand the source and nature of cell death. As the corresponding genetic locus is also active in the context of MCF due to a different Macavirus, we may have detected a common denominator of the disease phenotype