Identification of an Antiviral Signaling Variant Demonstrates Immune Regulation Through Alternative Translation

Abstract

Innate immune signaling pathways initiate host defenses against viral pathogens. Receptors specific for viral nucleic acids activate these pathways culminating in cell-to-cell communication and/or cell death. In mammals, this cell- to-cell communication is achieved through the production of interferons and pro- inflammatory cytokines, which activate antiviral defenses in uninfected neighboring cells and instruct adaptive immune responses. The production of these signaling molecules is essential for the defense against viral infection, but must also be tightly regulated to prevent unnecessary inflammation. As an antiviral defense, cell death is also an effective mechanism to limit viral replication and spread but comes with the cost of tissue damage and inflammation. Therefore, regulating these antiviral responses is critical for controlling the spread of infection as well as preventing unnecessary pathologies related to excessive signaling. Hundreds of genes are involved in controlling these immune responses and a wide variety of mechanisms are utilized to regulate them. One mechanism to regulate gene function is the generation of protein variants through alternative translation. While polycistronic transcripts are a common feature of bacterial and viral gene expression, the process of alternative translation as a means to regulate gene function is not a feature generally attributed to mammalian mRNA. This dissertation describes a novel regulator of antiviral signaling that is generated through alternative translation. Expression of the transcript encoding the antiviral adaptor protein, MAVS, results in the production of two proteins: the full-length MAVS adaptor and a truncated variant, miniMAVS. Production of these proteins is in part regulated by cis-acting elements that control translation initiation. Production of miniMAVS regulates antiviral signaling by limiting interferon production induced by full-length MAVS, whereas both MAVS variants positively regulate cell death. To identify other examples of alternative translation in mammalian cells a genome-wide ribosomal profiling technique was used to generate a candidate list of antiviral truncation variants. This dissertation therefore demonstrates that protein variants generated through alternative translation of polycistronic mRNAs can be an effective mechanism for immune regulation and may be more common than previously understood.