Immune priming is defined as enhanced protection upon secondary exposure to a pathogen. Such enhanced resistance after prior exposure has been demonstrated for a number of insect species including the red flour beetle, Tribolium castaneum. In testing this phenomenon, the majority of studies have focused on introducing the pathogen into the insect's hemocoel via septic wounding through the cuticle. Although such septic injury can occur in nature, many pathogens enter their hosts via the oral route, i.e. by ingestion. Bacillus thuringiensis bacteria are well-known insect pathogens that infect their host orally. We found that T. castaneum larvae showed increased survival after oral exposure to B. thuringiensis, when they had been orally primed with filter-sterilized media in which spores of B. thuringiensis had been raised. Such priming was achieved only with a naturally pathogenic strain of B. thuringiensis and a strain that was made pathogenic by transfer of plasmids. Moreover, primed larvae were smaller in size 24 h after priming and had a longer developmental time, indicating that investment in such a response comes at a cost. However, the increased survival in primed larvae was not caused by larval size differences upon challenge.

Manipulation of the MHC-I presentation pathway, and thus limiting MHC-I cell surface expression, is used by many viruses to evade immune recognition. In particular, downregulation of MHC-I molecules at the cell surface can reduce the ability of CD8 + T cells to recognize viral peptides presented by MHC-I molecules and thereby delay viral clearance by CD8 + T cells. To date, MHC-I downregulation by influenza viruses has not been reported. Given that influenza virus infections are a global health concern and that CD8 + T cells play an important role in promoting influenza virus clearance and recovery from influenza disease, we investigated whether influenza A and B viruses (IAV, IBV) downregulated MHC-I as a novel mechanism to evade cellular immunity. Here, we showed that infection of several cell types, including epithelial A549 cells, with a panel of IAV and IBV viruses downregulated the surface MHC-I expression on IAV/IBV-infected cells during the late stages of influenza virus infection in vitro . This observation was consistent across a panel of class I-reduced (C1R) cell lines expressing 14 different HLA-A or -B alleles and a panel of 721.221 cell lines expressing 11 HLA-C alleles. Interestingly, IBV infection caused more pronounced reduction in surface MHC-I expression compared to IAV. Importantly, the two viruses utilized two distinct mechanisms for MHC-I downregulation. Our data demonstrated that while IAV caused a global loss of MHC-I within influenza-infected cells, IBV infection resulted in the preferential loss of MHC-I molecules from the cell surface, consequent of delayed MHC-I trafficking to the cell surface, resulting from retaining MHC-I intracellularly during IBV infection. Overall, our study suggests that influenza viruses across both IAV and IBV subtypes have the potential to downregulate MHC-I surface expression levels. Our findings provide new insights into the host-pathogen interaction of influenza A and B viruses and inform the design of novel vaccine strategies against influenza viruses.

The integrated stress response (ISR) is a cellular response system activated upon different types of stresses, including viral infection, to restore cellular homeostasis. However, many viruses manipulate this response for their own advantage. In this study, we investigated the association between murine norovirus (MNV) infection and the ISR and demonstrate that MNV regulates the ISR by activating and recruiting key ISR host factors. We observed that during MNV infection, there is a progressive increase in phosphorylated eukaryotic initiation factor 2α (p-eIF2α), resulting in the suppression of host translation, and yet MNV translation still progresses under these conditions. Interestingly, the shutoff of host translation also impacts the translation of key signaling cytokines such as beta interferon, interleukin-6, and tumor necrosis factor alpha. Our subsequent analyses revealed that the phosphorylation of eIF2α was mediated via protein kinase R (PKR), but further investigation revealed that PKR activation, phosphorylation of eIF2α, and translational arrest were uncoupled during infection. We further observed that stress granules (SGs) are not induced during MNV infection and that MNV can restrict SG nucleation and formation. We observed that MNV recruited the key SG nucleating protein G3BP1 to its replication sites and intriguingly the silencing of G3BP1 negatively impacts MNV replication. Thus, it appears that MNV utilizes G3BP1 to enhance replication but equally to prevent SG formation, suggesting an anti-MNV property of SGs. Overall, this study highlights MNV manipulation of SGs, PKR, and translational control to regulate cytokine translation and to promote viral replication. IMPORTANCE Viruses hijack host machinery and regulate cellular homeostasis to actively replicate their genome, propagate, and cause disease. In retaliation, cells possess various defense mechanisms to detect, destroy, and clear infecting viruses, as well as signal to neighboring cells to inform them of the imminent threat. In this study, we demonstrate that the murine norovirus (MNV) infection stalls host protein translation and the production of antiviral and proinflammatory cytokines. However, virus replication and protein translation still ensue. We show that MNV further prevents the formation of cytoplasmic RNA granules, called stress granules (SGs), by recruiting the key host protein G3BP1 to the MNV replication complex, a recruitment that is crucial to establishing and maintaining virus replication. Thus, MNV promotes immune evasion of the virus by altering protein translation. Together, this evasion strategy delays innate immune responses to MNV infection and accelerates disease onset.