MDA5

Abstract: Persistent infections of the skin with the human papillomavirus of genus beta (β-HPV) in immunocompetent individuals are asymptomatic, but in immunosuppressed patients, β-HPV infections exhibit much higher viral loads on the skin and are associated with an increased risk of skin cancer. Unlike with HPV16, a high-risk α-HPV, the impact of β-HPV early genes on the innate immune sensing of viral nucleic acids has not been studied. Here, we used primary skin keratinocytes and U2OS cells expressing HPV8 or distinct HPV8 early genes and well-defined ligands of the nucleic-acid-sensing receptors RIG-I, MDA5, TLR3, and STING to analyze a potential functional interaction. We found that primary skin keratinocytes and U2OS cells expressed RIG-I, MDA5, TLR3, and STING, but not TLR7, TLR8, or TLR9. While HPV16-E6 downregulated the expression of RIG-I, MDA5, TLR3, and STING and, in conjunction with HPV16-E7, effectively suppressed type I IFN in response to MDA5 activation, the presence of HPV8 early genes showed little effect on the expression of these immune receptors, except for HPV8-E2, which was associated with an elevated expression of TLR3. Nevertheless, whole HPV8 genome expression, as well as the selective expression of HPV8-E1 or HPV8-E2, was found to suppress MDA5-induced type I IFN and the proinflammatory cytokine IL-6. Furthermore, RNA isolated from HPV8-E2 expressing primary human keratinocytes, but not control cells, stimulated a type I IFN response in peripheral blood mononuclear cells, indicating that the expression of HPV8-E2 in keratinocytes leads to the formation of stimulatory RNA ligands that require the active suppression of immune recognition. These results identify HPV8-E1 and HPV8-E2 as viral proteins that are responsible for the immune escape of β-HPV from the innate recognition of viral nucleic acids, a mechanism that may be necessary for establishing persistent β-HPV infections.

Abstract: Foot-and-mouth disease virus (FMDV) is a single-stranded, positive-sense RNA virus containing at least 13 proteins. Many of these proteins show immune modulation capabilities. As a non-structural protein of the FMDV, 2B is involved in the rearrangement of the host cell membranes and the disruption of the host secretory pathway as a viroporin. Previous studies have also shown that FMDV 2B plays a role in the modulation of host type-I interferon (IFN) responses through the inhibition of expression of RIG-I and MDA5, key cytosolic sensors of the type-I IFN signaling. However, the exact molecular mechanism is poorly understood. Here, we demonstrated that FMDV 2B modulates host IFN signal pathway by the degradation of RIG-I and MDA5. FMDV 2B targeted the RIG-I for ubiquitination and proteasomal degradation by recruiting E3 ubiquitin ligase ring finger protein 125 (RNF125) and also targeted MDA5 for apoptosis-induced caspase-3- and caspase-8-dependent degradation. Ultimately, FMDV 2B significantly inhibited RNA virus-induced IFN-β production. Importantly, we identified that the C-terminal amino acids 126-154 of FMDV 2B are essential for 2B-mediated degradation of the RIG-I and MDA5. Collectively, these results provide a clearer understanding of the specific molecular mechanisms used by FMDV 2B to inhibit the IFN responses and a rational approach to virus attenuation for future vaccine development.