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Cytomegalovirus is a genus of herpesviruses, members of which share a long history of co-evolution with their primate hosts including Great Apes, Old and New World monkeys. These viruses are ubiquitous within their host populations and establish lifelong infection in most individuals. Although asymptomatic in healthy individuals, infection poses a significant risk to individuals with a weakened or underdeveloped immune system. The genome of human cytomegalovirus is the largest among human-infecting viruses, and comprises at least fifteen separate gene families, which may have arisen by gene duplication. Within human cytomegalovirus, the RL11 gene family is the largest. RL11 genes are non-essential in vitro but have immune evasion roles that are likely critical to persistence in vivo. These genes demonstrate an extreme level of inter-species and intra-strain sequence diversity, that makes it challenging to deduce the evolutionary relationships within this gene family. Understanding the evolutionary relationships of these genes, especially accurate ortholog identification, is essential for reconstructing ancestral genomes, deciphering gene repertoire and order, and enabling reliable functional analyses across the Cytomegalovirus species, thereby offering insights into evolutionary processes, genetic diversity, and the functional significance of genes. In this work, we combined in silico genome screening with sequence-based and structure-guided phylogenetic analysis to reconstruct the evolutionary history of the RL11 gene family. We confirmed that RL11 genes are unique to cytomegaloviruses of Old World monkeys and Great Apes, showing that this gene family was formed by multiple early duplication events and later lineage-specific losses. We identified four main clades of RL11 genes and showed that their expansions were mainly lineage-specific and happened independently in cytomegaloviruses of Great Apes, African Old World monkeys and Asian Old World monkeys. We also identified groups of orthologous genes across the Cytomegalovirus tree showing that some human cytomegalovirus-specific RL11 genes emerged before the divergence of human and chimpanzee cytomegaloviruses but were subsequently lost in the latter. The extensive and dynamic species-specific evolution of this gene family suggests their functions target elements of host immunity that have similarly co-evolved during speciation.
Cytomegalovirus is a genus of herpesviruses, members of which share a long history of co-evolution with their primate hosts including Great Apes, Old and New World monkeys. These viruses are ubiquitous within their host populations and establish lifelong infection in most individuals. Although asymptomatic in healthy individuals, infection poses a significant risk to individuals with a weakened or underdeveloped immune system. The genome of human cytomegalovirus is the largest among human-infecting viruses, and comprises at least fifteen separate gene families, which may have arisen by gene duplication. Within human cytomegalovirus, the RL11 gene family is the largest. RL11 genes are non-essential in vitro but have immune evasion roles that are likely critical to persistence in vivo. These genes demonstrate an extreme level of inter-species and intra-strain sequence diversity, that makes it challenging to deduce the evolutionary relationships within this gene family. Understanding the evolutionary relationships of these genes, especially accurate ortholog identification, is essential for reconstructing ancestral genomes, deciphering gene repertoire and order, and enabling reliable functional analyses across the Cytomegalovirus species, thereby offering insights into evolutionary processes, genetic diversity, and the functional significance of genes. In this work, we combined in silico genome screening with sequence-based and structure-guided phylogenetic analysis to reconstruct the evolutionary history of the RL11 gene family. We confirmed that RL11 genes are unique to cytomegaloviruses of Old World monkeys and Great Apes, showing that this gene family was formed by multiple early duplication events and later lineage-specific losses. We identified four main clades of RL11 genes and showed that their expansions were mainly lineage-specific and happened independently in cytomegaloviruses of Great Apes, African Old World monkeys and Asian Old World monkeys. We also identified groups of orthologous genes across the Cytomegalovirus tree showing that some human cytomegalovirus-specific RL11 genes emerged before the divergence of human and chimpanzee cytomegaloviruses but were subsequently lost in the latter. The extensive and dynamic species-specific evolution of this gene family suggests their functions target elements of host immunity that have similarly co-evolved during speciation.
The signal sequences of the human cytomegalovirus (CMV) UL40 protein and its rhesus CMV (RhCMV) counterpart, Rh67, contain a peptide (VMAPRT[L/V][F/I/L/V]L, VL9) that is presented by major histocompatibility complex (MHC) antigen E (MHC-E). The CMV VL9 peptides replace VL9 peptides derived from classical MHC (Ia) signal sequences, which are lost when CMV disrupts antigen processing and presentation and MHC Ia expression. This allows infected cells to maintain MHC-E surface expression and escape killing by Natural Killer cells. We demonstrate that processing of the Rh67 VL9 peptide mirrors that of UL40, despite the lack of sequence conservation between the two proteins. Processing of both VL9 peptides is dependent on cleavage of their signal sequences by the host protease signal peptide peptidase. As previously shown for UL40, up-regulation of MHC-E expression by Rh67 requires only its signal sequence, with sequences upstream of VL9 critical for conferring independence from TAP, the transporter associated with antigen processing. Our results also suggest that the mature UL40 and Rh67 proteins contribute to CMV immune evasion by decreasing surface expression of MHC Ia. Unexpectedly, while the Rh67 VL9 peptide is resistant to the effects of Rh67, UL40 can partially counteract the up-regulation of MHC-E expression mediated by its own VL9 peptide. This suggests differences in the mechanisms by which the two VL9 peptides up-regulate MHC-E, and further work will be required to determine if any such differences have implications for translating a RhCMV-vectored simian immunodeficiency virus (SIV) vaccine to HIV-1 using human CMV as a vector. IMPORTANCE The protective immune response induced by a rhesus cytomegalovirus (RhCMV)-vectored simian immunodeficiency virus (SIV) vaccine in rhesus macaques depends on the presence of the viral Rh67 gene in the vaccine. The Rh67 protein contains a peptide that allows the RhCMV-infected cells to maintain expression of major histocompatibility complex (MHC) antigen E at the cell surface. We show that production of this peptide, referred to as “VL9,” mirrors that of the equivalent peptide present in the human cytomegalovirus (CMV) protein UL40, despite the little sequence similarity between the two CMV proteins. We also show that the mature UL40 and Rh67 proteins, which have no previously described function, also contribute to CMV immune evasion by reducing cell surface expression of MHC proteins important for the immune system to detect infected cells. Despite these similarities, our work also reveals possible differences between Rh67 and UL40, and these may have implications for the use of human CMV as the vector for a potential HIV-1 vaccine.
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