Listeria monocytogenes induces an interferon-enhanced activation of the integrated stress response that is detrimental for resolution of infection in mice

Nitrate-mediated luminal expansion of Salmonella Typhimurium is dependent on the ER stress protein CHOP

Salmonella Typhimurium is an enteric pathogen that employs a variety of mechanisms to exploit inflammation resulting in expansion in the intestinal tract, but host factors that contribute to or counteract the luminal expansion are not well-defined. Endoplasmic reticulum (ER) stress induces inflammation and plays an important role in the pathogenesis of infectious diseases. However, little is known about the contribution of ER stress-induced inflammation during Salmonella pathogenesis. Here, we demonstrate that the ER stress markers Hspa5 and Xbp1 are induced in the colon of S. Typhimurium infected mice, but the pro-apoptotic transcription factor Ddit3, that encodes for the protein CHOP, is significantly downregulated. S. Typhimurium-infected mice deficient for CHOP displayed a significant decrease in inflammation, colonization, dissemination, and pathology compared to littermate control mice. Preceding the differences in S. Typhimurium colonization, a significant decrease in Nos2 gene and iNOS protein expression was observed. Deletion of Chop decreased the bioavailability of nitrate in the colon leading to reduced fitness advantage of wild type S. Typhimurium over a napA narZ narG mutant strain (deficient in nitrate respiration). CD11b+ myeloid cells, but not intestinal epithelial cells, produced iNOS resulting in nitrate bioavailability for S. Typhimurium to expand in the intestinal tract in a CHOP-dependent manner. Altogether our work demonstrates that the host protein CHOP facilitates iNOS expression in CD11b+ cells thereby contributing to luminal expansion of S. Typhimurium via nitrate respiration.

The ion channel CALHM6 controls bacterial infection-induced cellular cross-talk at the immunological synapse

Membrane ion channels of the calcium homeostasis modulator (CALHM) family promote cell-cell crosstalk at neuronal synapses via ATP release, where ATP acts as a neurotransmitter. CALHM6, the only CALHM highly expressed in immune cells, has been linked to the induction of natural killer (NK) cell anti-tumour activity. However, its mechanism of action and broader functions in the immune system remain unclear. Here, we generated Calhm6-/- mice and report that CALHM6 is important for the regulation of the early innate control of Listeria monocytogenes infection in vivo. We find that CALHM6 is upregulated in macrophages by pathogen-derived signals and that it relocates from the intracellular compartment to the macrophage-NK cell synapse, facilitating ATP release and controlling the kinetics of NK cell activation. Anti-inflammatory cytokines terminate CALHM6 expression. CALHM6 forms an ion channel when expressed in the plasma membrane of Xenopus oocytes, where channel opening is controlled by a conserved acidic residue, E119. In mammalian cells, CALHM6 is localised to intracellular compartments. Our results contribute to the understanding of neurotransmitter-like signal exchange between immune cells that fine-tunes the timing of innate immune responses.

Protein Kinase R in Bacterial Infections: Friend or Foe?

The global antimicrobial resistance crisis poses a significant threat to humankind in the coming decades. Challenges associated with the development of novel antibiotics underscore the urgent need to develop alternative treatment strategies to combat bacterial infections. Host-directed therapy is a promising new therapeutic strategy that aims to boost the host immune response to bacteria rather than target the pathogen itself, thereby circumventing the development of antibiotic resistance. However, host-directed therapy depends on the identification of druggable host targets or proteins with key functions in antibacterial defense. Protein Kinase R (PKR) is a well-characterized human kinase with established roles in cancer, metabolic disorders, neurodegeneration, and antiviral defense. However, its role in antibacterial defense has been surprisingly underappreciated. Although the canonical role of PKR is to inhibit protein translation during viral infection, this kinase senses and responds to multiple types of cellular stress by regulating cell-signaling pathways involved in inflammation, cell death, and autophagy - mechanisms that are all critical for a protective host response against bacterial pathogens. Indeed, there is accumulating evidence to demonstrate that PKR contributes significantly to the immune response to a variety of bacterial pathogens. Importantly, there are existing pharmacological modulators of PKR that are well-tolerated in animals, indicating that PKR is a feasible target for host-directed therapy. In this review, we provide an overview of immune cell functions regulated by PKR and summarize the current knowledge on the role and functions of PKR in bacterial infections. We also review the non-canonical activators of PKR and speculate on the potential mechanisms that trigger activation of PKR during bacterial infection. Finally, we provide an overview of existing pharmacological modulators of PKR that could be explored as novel treatment strategies for bacterial infections.

SRC-3 deficiency protects host from Listeria monocytogenes infection through increasing ROS production and decreasing lymphocyte apoptosis

Listeria monocytogenes is the third major cause of death among food poisoning. Our previous studies have demonstrated that steroid receptor coactivator 3 (SRC-3) plays a critical protective role in host defense against extracellular bacterial pathogens such as Escherichia coli and Citrobacter rodentium. However, its role involved in intracellular bacterial pathogen infection remains unclear. Herein, we found that SRC-3-/- mice are more resistant to L. monocytogenes infection after tail intravenous injection with L. monocytogenes compared with wild-type mice. After infecting with L. monocytogenes, SRC-3-/- mice exhibited decreased mortality rate, decreased bacterial load, less body weight loss, less proinflammatory cytokines and less severe tissue damage compared with wild-type mice. SRC-3-/- mice produced more ROS and decreased L. monocytogenes-induced lymphocyte apoptosis. Mechanically, SRC-3-/- mice displayed decreased expressions of negative regulator of ROS (NRROS) and interferon (IFN)-β and its target genes such as Daxx, Mx1 and TRAIL associated with apoptosis. Taken together, SRC-3 deficiency can protect host from L. monocytogenes infection through increasing ROS production and decreasing lymphocyte apoptosis via affecting the expressions of NRROS and IFN-β.

Proteomics reveals the potential mechanism of Mrps35 controlling Listeria monocytogenes intracellular proliferation in macrophages

Macrophages are sentinels in the organism which can resist and destroy various bacteria through direct phagocytosis. Here, we reported that expression level of mitochondrial ribosomal protein S35 (Mrps35) continued to decrease over infection time after Listeria monocytogenes (L. monocytogenes) infected macrophages. Our results indicated that knockdown Mrps35 increased the load of L. monocytogenes in macrophages. This result supported that Mrps35 played the crucial roles in L. monocytogenes infection. Moreover, we performed the comprehensive proteomics to analyze the differentially expressed protein of wild type and Mrps35 Knockdown Raw264.7 cells by L. monocytogenes infection over 6 h. Based on the results of mass spectrometry, we presented a wide variety of hypotheses about the mechanism of Mrps35 controlling the L. monocytogenes intracellular proliferation. Among them, experiments confirmed that Mrps35 and 60S ribosomal protein L22-like 1 (Rpl22l1) were a functional correlation or potentially a compensatory mechanism during L. monocytogenes infection. This study provided new insights into understanding that L. monocytogenes infection changed the basic synthesis or metabolism-related proteins of host cells.

Arthropods Under Pressure: Stress Responses and Immunity at the Pathogen-Vector Interface

Understanding what influences the ability of some arthropods to harbor and transmit pathogens may be key for controlling the spread of vector-borne diseases. Arthropod immunity has a central role in dictating vector competence for pathogen acquisition and transmission. Microbial infection elicits immune responses and imparts stress on the host by causing physical damage and nutrient deprivation, which triggers evolutionarily conserved stress response pathways aimed at restoring cellular homeostasis. Recent studies increasingly recognize that eukaryotic stress responses and innate immunity are closely intertwined. Herein, we describe two well-characterized and evolutionarily conserved mechanisms, the Unfolded Protein Response (UPR) and the Integrated Stress Response (ISR), and examine evidence that these stress responses impact immune signaling. We then describe how multiple pathogens, including vector-borne microbes, interface with stress responses in mammals. Owing to the well-conserved nature of the UPR and ISR, we speculate that similar mechanisms may be occurring in arthropod vectors and ultimately impacting vector competence. We conclude this Perspective by positing that novel insights into vector competence will emerge when considering that stress-signaling pathways may be influencing the arthropod immune network.

Impact of Type I Interferons on Susceptibility to Bacterial Pathogens

Interferons (IFNs) are a broad class of cytokines that have multifaceted roles. Type I IFNs have variable effects when it comes to host susceptibility to bacterial infections, that is, the resulting outcomes can be either protective or deleterious. The mechanisms identified to date have been wide and varied between pathogens. In this review, we discuss recent literature that provides new insights into the mechanisms of how type I IFN signaling exerts its effects on the outcome of infection from the host's point of view.

The integrated stress response mediates necrosis in murine Mycobacterium tuberculosis granulomas

The mechanism by which only some individuals infected with Mycobacterium tuberculosis develop necrotic granulomas with progressive disease while others form controlled granulomas that contain the infection remains poorly defined. Mice carrying the sst1-suscepible (sst1S) genotype develop necrotic inflammatory lung lesions, similar to human tuberculosis (TB) granulomas, which are linked to macrophage dysfunction, while their congenic counterpart (B6) mice do not. In this study we report that (a) sst1S macrophages developed aberrant, biphasic responses to TNF characterized by superinduction of stress and type I interferon pathways after prolonged TNF stimulation; (b) the late-stage TNF response was driven via a JNK/IFN-β/protein kinase R (PKR) circuit; and (c) induced the integrated stress response (ISR) via PKR-mediated eIF2α phosphorylation and the subsequent hyperinduction of ATF3 and ISR-target genes Chac1, Trib3, and Ddit4. The administration of ISRIB, a small-molecule inhibitor of the ISR, blocked the development of necrosis in lung granulomas of M. tuberculosis-infected sst1S mice and concomitantly reduced the bacterial burden. Hence, induction of the ISR and the locked-in state of escalating stress driven by the type I IFN pathway in sst1S macrophages play a causal role in the development of necrosis in TB granulomas. Interruption of the aberrant stress response with inhibitors such as ISRIB may offer novel host-directed therapy strategies.

MiR-26a targets EphA2 to resist intracellular Listeria monocytogenes in macrophages

At infection sites, macrophages are sentinels that resist and destroy various pathogens, through direct phagocytosis. In macrophages, microRNAs play a variety of crucial roles, the most striking of which is the regulation of the ability of the host cell to resist infection. However, the underlying mechanisms associated with the anti-infection effects mediated by microRNAs remain largely unknown. Here, we demonstrated that miR-26a is downregulated during infection by Listeria monocytogenes (Lm). In miR-26a overexpressing mice, the Lm bacterial burden of liver and spleen decreased significantly within 72 h of infection, compared with that in control mice. Subsequently, RNA sequencing (RNA-seq) data suggested that miR-26a may attenuate the survival of Lm by targeting the Ephrin receptor tyrosine kinase A2 (EphA2). The knockdown of EphA2 in RAW264.7 macrophage cells resulted in decreased intracellular Lm burden. Taken together, these findings validate EphA2 as a target of miR-26a and provide a mechanism through which Lm may survive within macrophages by altering host miRNA expression.

Strength of tonic T cell receptor signaling instructs T follicular helper cell-fate decisions

T follicular helper (T_FH) cells are critical in adaptive immune responses to pathogens and vaccines; however, what drives the initiation of their developmental program remains unclear. Studies suggest that a T cell antigen receptor (TCR)-dependent mechanism may be responsible for the earliest T_FH cell-fate decision, but a critical aspect of the TCR has been overlooked: tonic TCR signaling. We hypothesized that tonic signaling influences early T_FH cell development. Here, two murine TCR-transgenic CD4⁺ T cells, LLO56 and LLO118, which recognize the same antigenic peptide presented on major histocompatibility complex molecules but experience disparate strengths of tonic signaling, revealed low tonic signaling promotes T_FH cell differentiation. Polyclonal T cells paralleled these findings, with naive Nur77 expression distinguishing T_FH cell potential. Two mouse lines were also generated to both increase and decrease tonic signaling strength, directly establishing an inverse relationship between tonic signaling strength and T_FH cell development. Our findings elucidate a central role for tonic TCR signaling in early T_FH cell-lineage decisions.

Inhibitory effects of JQ1 on listeria monocytogenes-induced acute liver injury by blocking BRD4/RIPK1 axis

Listeria monocytogenes (LM) is a facultative intracellular bacterium that causes septicemia-associated acute hepatic injury. However, the pathogenesis of this process is still unclear, and there is still a lack of effective therapeutic strategy for the treatment of LM-induced liver injury. In this study, we attempted to explore the effects of necroptosis on bacterial-septicemia-associated hepatic disease and to explore the contribution of JQ1, a selective BRD4 inhibitor, to the suppression of necroptosis and inhibition of LM-triggered hepatic injury. The results indicated that hepatic BRD4 was primarily stimulated by LM both in vitro and in vivo, along with significantly up-regulated expression of receptor-interacting protein kinase (RIPK)-1, RIPK3, and p-mixed lineage kinase-like (MLKL), showing the elevated necroptosis. However, JQ1 treatment and RIPK1 knockout were found to significantly alleviate LM-induced acute liver injury. Histological alterations and cell death in hepatic samples in LM-infected mice were also alleviated by JQ1 administration or RIPK1 deletion. However, JQ1-improved hepatic injury by LM was abrogated by RIPK1 over-expression, suggesting that the protective effects of JQ1 took place mainly in an RIPK1-dependent manner. In addition, LM-evoked inflammatory response in liver tissues were also alleviated by JQ1, which was similar to the findings observed in mice lacking RIPK1. The anti-inflammatory effects of JQ1 were diminished by RIPK1 over-expression in LM-infected mice. Finally, both in vivo and in vitro experiments suggested that JQ1 dramatically improved hepatic mitochondrial dysfunction in LM-injected mice, but this effect was abolished by RIPK1 over-expression. In conclusion, these results indicated that suppressing BRD4 by JQ1 could ameliorate LM-associated liver injury by suppressing necroptosis, inflammation, and mitochondrial dysfunction by inhibiting RIPK1.

Integrated Stress Responses to Bacterial Pathogenesis Patterns

Activation of an appropriate innate immune response to bacterial infection is critical to limit microbial spread and generate cytokines and chemokines to instruct appropriate adaptive immune responses. Recognition of bacteria or bacterial products by pattern recognition molecules is crucial to initiate this response. However, it is increasingly clear that the context in which this recognition occurs can dictate the quality of the response and determine the outcome of an infection. The cross talk established between host and pathogen results in profound alterations on cellular homeostasis triggering specific cellular stress responses. In particular, the highly conserved integrated stress response (ISR) has been shown to shape the host response to bacterial pathogens by sensing cellular insults resulting from infection and modulating transcription of key genes, translation of new proteins and cell autonomous antimicrobial mechanisms such as autophagy. Here, we review the growing body of evidence demonstrating a role for the ISR as an integral part of the innate immune response to bacterial pathogens.