Cytomegalovirus microRNAs Facilitate Persistent Virus Infection in Salivary Glands

Abstract

Micro (mi)RNAs are small non-coding RNAs that regulate the expression of their targets' messenger RNAs through both translational inhibition and regulation of target RNA stability. Recently, a number of viruses, particularly of the herpesvirus family, have been shown to express their own miRNAs to control both viral and cellular transcripts. Although some targets of viral miRNAs are known, their function in a physiologically relevant infection remains to be elucidated. As such, no in vivo phenotype of a viral miRNA knock-out mutant has been described so far. Here, we report on the first functional phenotype of a miRNA knock-out virus in vivo. During subacute infection of a mutant mouse cytomegalovirus lacking two viral miRNAs, virus production is selectively reduced in salivary glands, an organ essential for virus persistence and horizontal transmission. This phenotype depends on several parameters including viral load and mouse genetic background, and is abolished by combined but not single depletion of natural killer (NK) and CD4+ T cells. Together, our results point towards a miRNA-based immunoevasion mechanism important for long-term virus persistence.

Virus Progeny of Murine Cytomegalovirus Bacterial Artificial Chromosome pSM3fr Show Reduced Growth in Salivary Glands due to a Fixed Mutation of MCK-2

“…Only divergency which was found in at least two of the four sequences and which lead to changes in the amino acid composition of viral proteins was considered. In total, 22 positions of sequence divergency to the published MCMV Smith sequence could be found, affecting 15 ORFs, most of them in accordance with previously published data (3,5,9,16,25,41,46). Many sequence deviations of the pSM3fr sequence reported by us and others (41) presumably are sequence failures of the deposited MCMV Smith strain sequence.…”

“…This was also shown by other groups (10,12,15,21,32) but was never addressed, although it stood in marked contrast to the high salivary gland titers observed when BALB/c mice were infected with MCMV strains not cloned as BACs (17,22,28,39). To find out whether this phenotype of the pSM3fr-derived virus was due to genomic differences between the MCMV Smith strain and the pSM3fr genome, we sequenced the complete genomes of pSM3fr, two independent pSM3fr-derived mutants (pSM3fr-⌬m157-⌬M27 and pSM3fr-⌬loxP) and a virus stock derived from pSM3fr-⌬m157-⌬M27.…”

“…Since then, it has been used by several groups to study MCMV biology in vitro and in vivo. Mutations were introduced in pSM3fr, and the properties of reconstituted mutants were compared to those of the parental virus reconstituted from pSM3fr (named C3X or MW97.01) (6,15,19,32,35,49).…”

Divergent MicroRNA Targetomes of Closely Related Circulating Strains of a Polyomavirus

“…Although the mechanisms are poorly defined, SV40 and other polyomaviruses establish long-term persistent infections in vivo. Clearly, negatively regulating early gene transcripts could be a way to enforce persistence, akin to the role that has been proposed for some herpesviral miRNAs (1,87,88). Indeed, a recent report from Brokema and Imperiale on the BK human polyomavirus is consistent with this notion (86).…”

“…In this model, viral miRNA targeting of the early transcripts and select host transcripts plays a role in promoting and/or reinforcing persistent infection. This could occur analogous to some herpesviral miRNAs, in which host targets are associated with increased cell viability, immune evasion, and the indirect negative regulation of viral lytic genes (1,(87)(88)(89)(90)(91)(92)(93)(94). Clearly, such models await future testing in relevant persistent models of polyomavirus infection.…”