Viral Infection: An Evolving Insight into the Signal Transduction Pathways Responsible for the Innate Immune Response

Figure 2

Viral proteins inhibit nucleic acid receptors of the intracytosolic innate immune response. Viruses inhibit each of the two signals that initiate the inflammasome activation process. The first signal—IL-1β and/or IL-18 binding and activation of the TLR/IL-1β receptor pathway—is inhibited by soluble IL-1β and IL-18 (from VACV); downstream, inhibitors of signaling to NF-κB (from VACV or HCV) repeatedly target this important antiviral pathway that optimally requires NF-κB translocation leading to the production of pro-IL-1β and pro-IL-18. Second, the inflammasome processes these pro-IL-1β and pro-IL-18 proteins via caspase-1 that is itself processed upon clustering mediated at the NLRP3 inflammasome upon detection of intracytosolic pathogens. This leads to IL-1β and IL-18 production and release that activates the IL-1β/IL-18 pathway in an autocrine manner, as well as the innate and adaptive immune response. Inflammasome activation is inhibited by myxoma virus M013, measles viruses V protein, and KSHV vNLR. Finally signaling to IRF3 by intracytosolic DNA or RNA is inhibited at the level of MAVS by HCV’s NS3/4A and at the level of TBK1 by VACV C6 and N1 (Figure 1).