Antagonizing the SAMD9 pathway is key to myxoma virus host shut-off and immune evasion

Poxviruses have life cycles exclusively in the cytoplasm. However, these viruses can have profound impact to host transcription. One possible mechanism is through viral manipulation of host protein synthesis and such ability is critical for viral immune evasion. Many mammalian poxviruses encode more than one viral protein to interact with the host SAMD9 protein. In myxoma virus (MYXV), a rabbit specific poxvirus and non-pathogenic for other species, viral M062 protein is the lone inhibitor to SAMD9 with broad species specificity and loss of M062R in viral genome (Δ M062R mutant) leads to profound infection defect. We previously found Δ M062R remodeled transcriptomic landscape in monocytes/macrophages that is associated with the crosstalk between the SAMD9 pathway and cGAS/STING/IRF3 DNA sensing pathway. In this study we completed the characterization of Δ M062R infection. We observed that although this replication-defective virus preserved intact early protein synthesis, it failed to conduct host shutoff. Despite a defect in viral DNA replication, Δ M062R infection retained intact intermediate protein synthesis comparable to the wildtype virus. Using time course dual RNAseq analyses we found that the overall viral gene transcription profile was mostly indistinguishable from that of the wildtype MYXV. However, the slightly attenuated late RNA synthesis along with the block at viral protein synthesis led to its infection defect. Infection by Δ M062R in macrophages potentiated the antiviral responses to new danger signals. We provided an initial characterization of such a state in which host antiviral protein synthesis may be promoted leading to the immunological consequence.

Poxviruses utilize multi-faceted strategies to evade and manipulate host immunity. Through targeted gene deletion, we generated useful tools of mutant poxviruses to investigate specific crosstalk between host defense mechanisms. Through studying MYXV M062 protein function, we previously identified SAMD9 as one host target of poxvirus C7L superfamily, in which family M062R is one member. However, what kind of cellular outcome caused by Δ M062R infection remained unknown. The infection defect of Δ M062R caused the induction of host inflammation program is likely due to the activation of the host pathway governed by SAMD9. Because little is known about SAMD9 cellular function and the pathways it regulates, which are important for cellular homeostasis and immune regulation, this study on Δ M062R induced effect in host cells will provide new insight on how SAMD9 affects cellular protein synthesis and immunological responses.