Cis-acting inhibition of MHC class I-restricted epitope presentation by Alcelaphine herpesvirus 1 genome maintenance protein

Abstract

γ-Herpesviruses persist as latent episomes in actively dividing lymphocytes. Their consequent need to express a viral genome maintenance protein (GMP) during latency presents a potential immune target. However, the GMPs from several γ-herpesviruses have evolved related strategies to limit their own MHC class I epitope presentation to cytotoxic T lymphocytes (CTLs). Alcelaphine herpesvirus 1 (AlHV-1) is a γ-herpesvirus that persists asymptomatically in its natural host, the wildebeest. However, AlHV-1 transmission to a large number of susceptible ruminants, including cattle, results in the development of a lethal lymphoproliferative disease named malignant catarrhal fever (MCF). We recently observed that the AlHV-1 GMP-homologue encoded by ORF73 is highly expressed during MCF and that the impairment of its expression renders AlHV-1 unable to induce MCF. With its 1300 aa, AlHV-1 ORF73 is the largest γ-herpesvirus GMP described to date and contains a large acidic internal repeat region that could be involved in the cis-acting CTL evasion mechanism. Here, we sought to determine the CTL evasion properties of AlHV-1 ORF73. We first performed bioinformatic analyses to characterize the protein domains. Then, we used an in vitro assay to demonstrate that ORF73 severely limits the presentation at the cell surface of an MHC class I-restricted epitope linked to ORF73 in cis. These results suggest that AlHV-1 has developed mechanisms to evade cytotoxic anti-viral response during latency. The exact mechanisms explaining the presentation defect remain to be deciphered as well as the role of the cis-acting CTL evasion mechanism of ORF73 in the pathogenesis of MCF