A7 gene expression is essential for alcelaphine herpesvirus 1-driven CD8+ T cell expansion and typical malignant catarrhal fever

Alcelaphine herpesvirus 1 (AlHV-1) persists in wildebeest asymptomatically but induces malignant catarrhal fever (MCF), a fatal lymphoproliferative disease upon transmission to several ruminants, including cattle. Our recent data have demonstrated that AlHV-1 latently infects CD8+ T-cells resulting in their proliferation and MCF lesions. How AlHV-1 infects CD8+ T-cells and induce their proliferation remain however unknown. To better understand MCF pathogenesis, we first used Illumina high-throughput sequencing to obtain the full genomic sequence of the highly passaged attenuated strain WC11. Sequencing data revealed a genomic sequence very similar to the pathogenic strain C500 with only four ORFs that were not conserved in WC11, including full deletion of the gene A7. A7 is a positional homolog of Epstein-Barr virus (EBV) BZLF2 encoding the C-type lectin-like glycoprotein gp42. Gp42 is expressed in the envelope of EBV virions and mediates entry into B cells. Likewise, A7 has been predicted to encode a glycoprotein containing a C-type lectin-like domain. The absence of A7 in the high passaged attenuated strain could participate in the attenuation of WC11 through a defect of virus entry into target cells. Hence, we used the C500 BAC clone to generate an A7STOP recombinant strain and observed that a lack of A7 expression resulted in significant increased viral growth in fibroblasts in vitro. Next, we infected rabbits intranasally with the A7STOP virus. While the A7STOP virus induced hyperthermia with a significant delay compared to the WT virus, neither enlargement of lymphoid organs nor typical expansion of CD8+ T cells could be detected in the absence of A7 expression. Interestingly, viral DNA loads and antibody responses were significantly reduced in lymphoid tissues of A7STOP infected animals. Finally, the infiltrations of lymphoid cells typically observed in WT-infected animals were severely reduced in absence of A7. In conclusion, these findings demonstrated that the lack of A7 significantly alters the development of MCF, likely through deviating CD8+ T cell tropism.