Evasion of MDA5-Mediated Innate Immunity by Paramyxoviruses

Abstract

The innate immune sensor MDA5, a RIG-I-like receptor (RLR), is critical for the detection of viral nucleic acid, eliciting an antiviral immune response. Aberrant immune activation can be detrimental to the host; therefore, RLR activity is strictly regulated. In the uninfected cell, MDA5 is constitutively phosphorylated at S88, preventing antiviral signaling. Upon sensing of viral RNA, MDA5 is activated via dephosphorylation by the phosphatases PP1a/c. Dephosphorylation of MDA5 allows interaction with the mitochondrial adaptor protein MAVS, inducing downstream signaling, leading to the production of antiviral cytokines including type-I interferons (IFNs). Many viruses have evolved sophisticated mechanisms to avoid detection by RLRs. Here, we present a novel evasion mechanism of paramyxoviruses to escape the MDA5-induced innate immune response: inhibition of its key regulators, PP1a and PP1c. The V proteins of measles virus (MV) and multiple other paramyxoviruses interact with PP1a/c, preventing MDA5 S88 dephosphorylation and subsequent innate immune signaling. In Chapter 2, we identify a conventional PP1-binding motif in the unique C-terminal region of the MV V protein which mediates this interaction. Mutation of this motif abrogates PP1 binding and MDA5 antagonism without effecting other known activities of the V protein such as STAT inhibition. To determine the physiological relevance of the V-PP1 interaction for MDA5 antagonism, we generated a recombinant MV carrying a PP1-binding deficient V protein. This mutant virus no longer suppressed MDA5 dephosphorylation by PP1, resulting in increased expression of IFN and IFN-stimulated genes (ISGs) and impaired replication in lung epithelial and dendritic cells compared to the parental virus. In Chapter 3, we expand our understanding of paramyxovirus antagonism of MDA5 by examining the virus-specificity of inhibition of S88 dephosphorylation. We found that the ability to interact with PP1 and inhibit MDA5 S88 dephosphorylation is shared by multiple viruses, including mumps (MuV), Nipah, and Hendra viruses. We mapped the PP1 interaction to a minimal binding region in the MuV-V C-terminal domain. This region contains a putative PP1-binding motif which is not conserved between viruses. The importance of this putative motif requires additional investigation. Together, our findings reveal PP1 antagonism as a novel immune evasion strategy of paramyxoviruses.