The A2 gene of alcelaphine herpesvirus-1 is a transcriptional regulator affecting cytotoxicity in virus-infected T cells but is not required for malignant catarrhal fever induction in rabbits

•The A2 gene product of alcelaphine herpesvirus-1 is a basic leucine zipper protein.•Deletion of the A2 gene in the virus genome delays virus growth in culture and also the onset of malignant catarrhal fever in rabbits.•The A2 gene product is involved in pathways affecting host T cell receptor phenotype, signalling, cell cycle and apoptosis.•The A2 gene product is involved in regulating host T cell cytotoxicity. This is important as T cell cytotoxicity is thought to be responsible for disease induction in MCF.

Alcelaphine herpesvirus-1 (AlHV-1) causes malignant catarrhal fever (MCF). The A2 gene of AlHV-1 is a member of the bZIP transcription factor family. We wished to determine whether A2 is a virulence gene or not and whether it is involved in pathogenesis by interference with host transcription pathways. An A2 gene knockout (A2ΔAlHV-1) virus, revertant (A2revAlHV-1) virus, and wild-type virus (wtAlHV-1) were used to infect three groups of rabbits. A2ΔAlHV-1-infected rabbits succumbed to MCF, albeit with a delayed onset compared to the control groups, so A2 is not a critical virulence factor. Differential gene transcription analysis by RNAseq and qRT-PCR validation of a selection of these was performed in infected large granular lymphocyte (LGL) T cells obtained in culture from the MCF-affected animals. A2 was involved in the transcriptional regulation of immunological, cell cycle and apoptosis pathways. In particular, there was a bias towards γδ T cell receptor (TCR) expression and downregulation of αβ TCR. TCR signalling, apoptosis, cell cycle, IFN-γ and NFAT pathways were affected. Of particular interest was partial inhibition of the cytotoxicity-associated pathways involving perforin and the granzymes A and B in the A2ΔAlHV-1-infected LGLs compared to controls. In functional assays, A2ΔAlHV-1-infected LGLs were significantly less cytotoxic than wtAlHV-1- and A2revAlHV-1-infected LGLs using rabbit corneal epithelial cells (SIRC) as targets. This implies that A2 is involved in a pathway enhancing the expression of LGL cytotoxicity. This is important as virus-infected T cell cytotoxicity in vivo has been suggested as a potential mechanism of disease induction in MCF.