Human cytomegalovirus (HCMV) can be found in the majority of humans. Although typically asymptomatic in healthy individuals, it can be life-threatening in immunocompromised individuals and newborn infants. To reproduce, a virus has to infect a host cell. As such, viruses have evolved many mechanisms of counteracting the host defense system. Generally, cytomegaloviruses (CMVs) are restricted to replicate within a particular host species, due in part to co-evolutionary adaptions over millions of years that have honed the ability of these viruses to utilize dependency factors and to evade restriction factors encoded by their specific host species. One of the most rapidly evolving host cell restriction factors is protein kinase R (PKR), which is activated in the presence of double-stranded RNA that accumulates in the event of most viral infections, and inhibits translation initiation, and viral replication. To bypass the PKR pathway and continue replication, many viruses have evolved factors that can antagonize PKR such as TRS1 in human CMV (HCMV).
Viruses that have lost their ability to interact with factors in different species will therefore be unable to be propagated in new hosts. For example, HCMV is unable to replicate in other species because HCMV TRS1 is unable to bind to PKR within cells from other species. However, rhesus macaque CMV (RhCMV), is able to cross species barriers and replicate in human as well as rhesus cells. TRS1 from RhCMV partially inhibits human PKR, which allows for some replication in human fibroblast cells. This opportunity to understand variations in how RhCMV interacts with its host species presents a valuable model for studying HCMV pathogenesis and vaccine development. In a recent publication in the Journal of Virology, Dr. Adam Geballe and members of his laboratory in the Human Biology Division and collaborators at Oregon Health and Sciences University explored the molecular mechanisms that allowed this uncommon cross-species replication to happen.
The authors report that the RhCMV antagonist of PKR, rTRS1 is essential for RhCMV replication in human cells by preventing human PKR from phosphorylating the translation initiation factor eIF2α, thereby allowing continued translation and viral replication. They also demonstrate that although rTRS1 is necessary for RhCMV replication, it is not sufficient to rescue HCMV replication when lacking its own PKR antagonists in human fibroblasts. Conversely, overexpression of rTRS1 in human fibroblasts, enabled HCMV to replicate, suggesting that increased levels or early expression of a weak antagonist can overcome a resistant restriction factor like human PKR. These results underscore a strategy viruses may adopt to overcome resistant host restriction factors so as to cross species barriers to replicate in a new host.
When examining potential mechanisms that might allow RhCMV to replicate in human cells Dr. Geballe and his lab found that the amounts and kinetics of double-stranded RNA production was not a factor. Instead, RhCMV expresses rTRS1 at levels two to three times higher in human cells than those of the HCMV-encoded PKR antagonists during HCMV infection. These data suggest, that even a slight increase in expression of this weak PKR antagonist is sufficient to overcome PKR to enable RhCMV replication in human cells.
In conclusion, Dr. Geballe stated “We are finding remarkable variation in the specific mechanisms by which cytomegaloviruses counteract the PKR host defense system in different primates. Our new paper shows one strategy - early and abundant expression of an intrinsically weak antagonist of human PKR - that allows rhesus cytomegalovirus to replicate in human cells. We are eager to understand how other nonhuman primate CMVs have evolved along different trajectories to gain a more complete perspective on how CMVs have adapted to combat changes in host genes and to understand how these adaptations might enable the virus to jump species barriers.”
Figure provided by Dr. Stephanie Child
Child SJ, Hickson SE, Bayer A, Malouli D, Fruh K, and Geballe AP. 2017. Antagonism of the protein kinase R pathway in human cells by rhesus cytomegalovirus. Journal of Virology. doi:10.1128/JVI.01793-17
Funding was provided by the National Institutes of Health.