Guinea Pig Cytomegalovirus Protective T Cell Antigen GP83 Is a Functional pp65 Homolog for Innate Immune Evasion and Pentamer-Dependent Virus Tropism

Abstract

ABSTRACT The guinea pig is the only small animal model for congenital cytomegalovirus (CMV) but requires species-specific guinea pig cytomegalovirus (GPCMV). Tegument protein GP83 is the presumed homolog of HCMV pp65, but gene duplication in the UL82-UL84 homolog locus in various animal CMVs made it unclear if GP83 was a functional homolog. A GP83 null deletion mutant GPCMV (GP83dPC+) generated in the backdrop of glycoprotein pentamer complex (PC)-positive virus, required for nonfibroblast infection, had normal growth kinetics on fibroblasts but was highly impaired on epithelial and trophoblast cells. GP83dPC+ virus was highly sensitive to type I interferon (IFN-I), suggesting that GP83 had an innate immune evasion function. GP83 interacted with cellular DNA sensors guinea pig IFI16 and cGAS, indicating a role in the cGAS/STING pathway. Ectopically expressed GP83 in trophoblast cells restored GP83dPC+ virus growth. Additionally, mutant virus growth was restored in epithelial cells by expression of bovine viral diarrhea virus (BVDV) Npro protein targeting IRF3 as part of the cGAS/STING pathway, or alternatively, by expression of the fibroblast cell receptor platelet-derived growth factor receptor alpha (PDGFRA). HCMV pp65 is a T cell target antigen, and a recombinant adenovirus encoding GP83 was evaluated as a vaccine. In GPCMV challenge studies, vaccinated animals had various levels of protection against wild-type virus, with a protective response against the 22122 prototype strain but little protection against a novel clinical strain of GPCMV (TAMYC), despite 100% identity in GP83 protein sequences. Overall, GP83 is a functional pp65 homolog with novel importance for epithelial cell infection, but a GP83 T cell response provides limited vaccine efficacy. IMPORTANCE Congenital CMV (cCMV) is a leading cause of cognitive impairment and deafness in newborns, and a vaccine is a high priority. The guinea pig is the only small animal model for cCMV but requires guinea pig cytomegalovirus (GPCMV). The translational impact of GPCMV research is potentially reduced if the virus does not encode functional HCMV homolog proteins. This study demonstrates that tegument protein GP83 (pp65 homolog) is involved in innate immune evasion and highly important for infection of nonfibroblast cells via the viral glycoprotein pentamer complex (PC)-dependent endocytic entry pathway. The PC pathway is highly significant for virus dissemination and disease in the host, including cCMV. A GP83 candidate adenovirus (Ad) vaccine strategy in animals induced a cell-mediated response but failed to provide cross-strain protection against a novel clinical strain of GPCMV. Results suggest that the pp65 antigen provides very limited efficacy as a stand-alone vaccine, especially in cross-strain protection.