The eIF2α Kinase Heme-Regulated Inhibitor Protects the Host from Infection by Regulating Intracellular Pathogen Trafficking

The host employs both cell-autonomous and system-level responses to limit pathogen replication in the initial stages of infection. Previously, we reported that the eukaryotic initiation factor 2 alpha (eIF2 alpha) kinases heme-regulated inhibitor (HRI) and protein kinase R (PKR) control distinct cellular and immune-related activities in response to diverse bacterial pathogens. Specifically for Listeria monocytogenes, there was reduced translocation of the pathogen to the cytosolic compartment in HRI-deficient cells and consequently reduced loading of pathogen-derived antigens on major histocompatibility complex class I (MHC-I) complexes. Here we show that Hri(-/-) mice, as well as wild-type mice treated with an HRI inhibitor, are more susceptible to listeriosis. In the first few hours of L. monocytogenes infection, there was much greater pathogen proliferation in the liver of Hri(-/-) mice than in the liver of Hri(-/-) mice. Further, there was a rapid increase of serum interleukin-6 (IL-6) levels in Hri(-/-) mice in the first few hours of infection whereas the increase in IL-6 levels in Hri(-/-) mice was notably delayed. Consistent with these in vivo findings, the rate of listeriolysin O (LLO)-dependent pathogen efflux from infected Hri(-/-) macrophages and fibroblasts was significantly higher than the rate seen with infected Hri(-/-) cells. Treatment of cells with an eIF2 alpha kinase activator enhanced both the HRI-dependent and PKR-dependent infection phenotypes, further indicating the pharmacologically malleability of this signaling pathway. Collectively, these results suggest that HRI mediates the cellular confinement and killing of virulent L. monocytogenes in addition to promoting a system-level cytokine response and that both are required to limit pathogen replication during the first few hours of infection.