Evasion of innate immune responses to poliovirus: A genetic analysis

The rate of protein secretion in host cells is inhibited during infection with several different picornaviruses, with consequences likely to have significant effects on viral growth, spread, and pathogenesis. This Sin + (secretion inhibition+) phenotype has been documented for poliovirus, foot-and-mouth disease virus and coxsackievirus B3 and can lead to reduced cell-surface expression of MHC class I and TNF receptor as well as reduced extracellular secretion of induced cytokines such as IL-6, IL-8 and interferon-beta. The inhibition of protein secretion is global, affecting the movement of all tested cargo proteins through the cellular secretion apparatus. To test the physiological significance of the Sin + phenotype in animal models, Sin- mutant viruses were needed that fail to inhibit host protein secretion and also exhibit robust growth properties.;In this work a new Sin- poliovirus mutant with wild-like growth and spread is characterized. This virus was previously identified in a FACS-based screen to select virus-infected cells that nevertheless expressed newly synthesized surface proteins. The new Sin - virus was found to contain coding changes in non-structural proteins 2A (N32D) and 2C (E253G). In this virus, the 2C mutation is responsible for the Sin- phenotype and the 2A mutation suppresses a resulting growth defect by increasing the rate of viral spread. The increased rate of viral spread is the direct result of increased caspase-activation and death in cells infected with poliovirus carrying the 2A-N32D mutation. We hypothesize that a wild-type function of poliovirus 2A is to protect infected cells from caspase-induced cell death, and that this function is lost in the presence of the N32D mutation. Of the three genetic mechanisms of extragenic suppression, the 2A mutation was shown to suppress the 2C-E253G mutant phenotype by bypass suppression, not interactional or informational.;To ask whether inhibition of host protein secretion facilitated growth in a mammalian host, mice were infected with wild-type poliovirus, the newly characterized Sin mutant, or a previously described Sin - mutant, 3A-2. Both Sin- polioviruses were found to have significant growth defects in mouse muscle that were not apparent in tissue culture. Natural killer cells and TLR-3 signaling were found to have no effect on this growth defect. However, the growth defect disappeared in mice lacking a type I interferon response. These data are consistent with the hypothesis that the main function of host protein secretion inhibition by poliovirus in this mouse model is evasion of the type I interferon response.