Listeria monocytogenes: The Impact of Cell Death on Infection and Immunity

Reference genes for gene expression profiling in mouse models of Listeria monocytogenes infection

RT-qPCR dissects transcription-based processes but requires reference genes (RGs) for data normalization. This study prospected RGs for mouse macrophages (pMØ) and spleen infected with Listeria monocytogenes. The pMØ were infected in vitro with L. monocytogenes or vehicle for 4 h. Mice were injected with L. monocytogenes (or vehicle) and euthanized 24 h post-injection. The RGs came from a multispecies primer set, from the literature or designed here. The RG ranking relied on GeNorm, NormFinder, BestKeeper, Delta-CT and RefFinder. B2m-H3f3a-Ppia were the most stable RGs for pMØ, albeit RG indexes fine-tuned estimations of cytokine relative expression. Actβ-Ubc-Ppia were the best RGs for spleen but modestly impacted the cytokine relative expression. Hence, mouse models of L. monocytogenes require context-specific RGs for RT-qPCR, thus reinforcing its paramount contribution to accurate gene expression profiling.

Eosinophils promote CD8+ T cell memory generation to potentiate anti-bacterial immunity

Memory CD8+ T cell generation is crucial for pathogen elimination and effective vaccination against infection. The cellular and molecular circuitry that underlies the generation of memory CD8+ T cells remains elusive. Eosinophils can modulate inflammatory allergic responses and interact with lymphocytes to regulate their functions in immune defense. Here we report that eosinophils are required for the generation of memory CD8+ T cells by inhibiting CD8+ T cell apoptosis. Eosinophil-deficient mice display significantly impaired memory CD8+ T cell response and weakened resistance against Listeria monocytogenes (L.m.) infection. Mechanistically, eosinophils secrete interleukin-4 (IL-4) to inhibit JNK/Caspase-3 dependent apoptosis of CD8+ T cells upon L.m. infection in vitro. Furthermore, active eosinophils are recruited into the spleen and secrete more IL-4 to suppress CD8+ T cell apoptosis during early stage of L.m. infection in vivo. Adoptive transfer of wild-type (WT) eosinophils but not IL-4-deficient eosinophils into eosinophil-deficient mice could rescue the impaired CD8+ T cell memory responses. Together, our findings suggest that eosinophil-derived IL-4 promotes the generation of CD8+ T cell memory and enhances immune defense against L.m. infection. Our study reveals a new adjuvant role of eosinophils in memory T cell generation and provides clues for enhancing the vaccine potency via targeting eosinophils and related cytokines.

The relationship between Listeria infections and host immune responses: Listeriolysin O as a potential target

Listeria monocytogenes (Lm), a foodborne bacterium, can infect people and has a high fatality rate in immunocompromised individuals. Listeriolysin O (LLO), the primary virulence factor of Lm, is critical in regulating the pathogenicity of Lm. This review concludes that LLO may either directly or indirectly activate a number of host cell viral pathophysiology processes, such as apoptosis, pyroptosis, autophagy, necrosis and necroptosis. We describe the invasion of host cells by Lm and the subsequent removal of Lm by CD8 T cells and CD4 T cells upon receipt of the LLO epitopes from major histocompatibility complex class I (MHC-I) and major histocompatibility complex class II (MHC-II). The development of several LLO-based vaccines that make use of the pore-forming capabilities of LLO and the immune response of the host cells is then described. Finally, we conclude by outlining the several natural substances that have been shown to alter the three-dimensional conformation of LLO by binding to particular amino acid residues of LLO, which reduces LLO pathogenicity and may be a possible pharmacological treatment for Lm.

Apoptotic signaling clears engineered Salmonella in an organ-specific manner

Pyroptosis and apoptosis are two forms of regulated cell death that can defend against intracellular infection. When a cell fails to complete pyroptosis, backup pathways will initiate apoptosis. Here, we investigated the utility of apoptosis compared to pyroptosis in defense against an intracellular bacterial infection. We previously engineered Salmonella enterica serovar Typhimurium to persistently express flagellin, and thereby activate NLRC4 during systemic infection in mice. The resulting pyroptosis clears this flagellin-engineered strain. We now show that infection of caspase-1 or gasdermin D deficient macrophages by this flagellin-engineered S. Typhimurium induces apoptosis in vitro. Additionally, we engineered S. Typhimurium to translocate the pro-apoptotic BH3 domain of BID, which also triggers apoptosis in macrophages in vitro. During mouse infection, the apoptotic pathway successfully cleared these engineered S. Typhimurium from the intestinal niche but failed to clear the bacteria from the myeloid niche in the spleen or lymph nodes. In contrast, the pyroptotic pathway was beneficial in defense of both niches. To clear an infection, cells may have specific tasks that they must complete before they die; different modes of cell death could initiate these 'bucket lists' in either convergent or divergent ways.

An N-acetyltransferase required for ESAT-6 N-terminal acetylation and virulence in Mycobacterium marinum

IMPORTANCE

N-terminal acetylation is a protein modification that broadly impacts basic cellular function and disease in higher organisms. Although bacterial proteins are N-terminally acetylated, little is understood how N-terminal acetylation impacts bacterial physiology and pathogenesis. Mycobacterial pathogens cause acute and chronic disease in humans and in animals. Approximately 15% of mycobacterial proteins are N-terminally acetylated, but the responsible enzymes are largely unknown. We identified a conserved mycobacterial protein required for the N-terminal acetylation of 23 mycobacterial proteins including the EsxA virulence factor. Loss of this enzyme from M. marinum reduced macrophage killing and spread of M. marinum to new host cells. Defining the acetyltransferases responsible for the N-terminal protein acetylation of essential virulence factors could lead to new targets for therapeutics against mycobacteria.

Apoptotic signaling clears engineered Salmonella in an organ-specific manner

Pyroptosis and apoptosis are two forms of regulated cell death that can defend against intracellular infection. Although pyroptosis and apoptosis have distinct signaling pathways, when a cell fails to complete pyroptosis, backup pathways will initiate apoptosis. Here, we investigated the utility of apoptosis compared to pyroptosis in defense against an intracellular bacterial infection. We previously engineered Salmonella enterica serovar Typhimurium to persistently express flagellin, and thereby activate NLRC4 during systemic infection in mice. The resulting pyroptosis clears this flagellin-engineered strain. We now show that infection of caspase-1 or gasdermin D deficient macrophages by this flagellin-engineered S. Typhimurium induces apoptosis in vitro. Additionally, we also now engineer S. Typhimurium to translocate the pro-apoptotic BH3 domain of BID, which also triggers apoptosis in macrophages in vitro. In both engineered strains, apoptosis occurred somewhat slower than pyroptosis. During mouse infection, the apoptotic pathway successfully cleared these engineered S. Typhimurium from the intestinal niche, but failed to clear the bacteria in the myeloid niche in the spleen or lymph nodes. In contrast, the pyroptotic pathway was beneficial in defense of both niches. In order to clear an infection, distinct cell types may have specific tasks that they must complete before they die. In some cells, either apoptotic or pyroptotic signaling may initiate the same tasks, whereas in other cell types these modes of cell death may lead to different tasks that may not be identical in defense against infection. We recently suggested that such diverse tasks can be considered as different cellular 'bucket lists' to be accomplished before a cell dies.

Gold Glyconanoparticles Combined with 91–99 Peptide of the Bacterial Toxin, Listeriolysin O, Are Efficient Immunotherapies in Experimental Bladder Tumors

Simple Summary

We propose a novel type of immunotherapy for bladder cancer using gold nanoparticles bound to a peptide of a bacterial toxin with anti-tumor capacities as listeriolysin O called Listeria nanovaccines. Here, we present the pre-clinical experiments on a mice model of bladder cancer and blood cells of patients with bladder cancer using these Listeria nanovaccines that activate the immune system, block the tumor immunosuppression environment, and reduce the tumor size. The impact of Listeria nanovaccines on the field of immunotherapies for solid tumors can be extended to other solid tumors containing lymphocyte infiltration. Therefore, we propose Listeria nanovaccines as immunotherapy for tumors such as melanoma, urothelial bladder carcinoma, non-small cell lung carcinoma, and glioblastoma.

Abstract

This study presents proof of concept assays to validate gold nanoparticles loaded with the bacterial peptide 91–99 of the listeriolysin O toxin (GNP-LLO_91–99 nanovaccines) as immunotherapy for bladder tumors. GNP-LLO_91–99 nanovaccines showed adjuvant abilities as they induce maturation and activation of monocyte-derived dendritic cells (MoDCs) to functional antigen-presenting cells in healthy donors and patients with melanoma or bladder cancer (BC), promoting a Th1 cytokine pattern. GNP-LLO_91–99 nanovaccines were also efficient dendritic cell inducers of immunogenic tumor death using different bladder and melanoma tumor cell lines. The establishment of a pre-clinical mice model of subcutaneous BC confirmed that a single dose of GNP-LLO_91–99 nanovaccines reduced tumor burden 4.7-fold and stimulated systemic Th1-type immune responses. Proof of concept assays validated GNP-LLO_91–99 nanovaccines as immunotherapy by comparison to anti-CTLA-4 or anti-PD-1 antibodies. In fact, GNP-LLO_91–99 nanovaccines increased percentages of CD4⁺ and CD8⁺ T cells, B cells, and functional antigen-presenting DCs in tumor-infiltrated lymphocytes, while they reduced the levels of myeloid-derived suppressor cells (MDSC) and suppressor T cells (T_reg). We conclude that GNP-LLO_91–99 nanovaccines can work as monotherapies or combinatory immunotherapies with anti-CTLA-4 or anti-PD-1 antibodies for solid tumors with high T cell infiltration, such as bladder cancer or melanoma.

Listeria monocytogenes Inoculation Impedes the Development of Brain Pathology in Experimental Cerebral Malaria by Inhibition of Parasitemia

Cerebral malaria (CM) is a serious central nervous system dysfunction caused by Plasmodium falciparum infection. In this study, we investigated the effect of Listeria monocytogenes (Lm) inoculation on experimental cerebral malaria (ECM) using Plasmodium berghei ANKA (PbA)-infected C57BL/6 mice. Live Lm inoculation inhibited the parasitemia and alleviated ECM symptoms. The protective effect against ECM symptoms was connected with improved brain pathology manifested as a less-damaged blood-brain barrier, decreased parasite sequestration, and milder local inflammation. Meanwhile, Lm inoculation decreased expression of cell adhesion molecules (ICAM-1 and VCAM-1) and accumulation of pathogenic CD8⁺ T cells in the brain. In keeping with the suppression of parasitemia, there was an upregulation of IFN-γ, IL-12, MCP-1, and NO expression in the spleen by Lm inoculation upon PbA infection. Early treatment with exogenous IFN-γ exhibited a similar effect to Lm inoculation on PbA infection. Taken together, Lm inoculation impedes the development of brain pathology in ECM, and early systemic IFN-γ production may play a critical role in these protective effects.

Listeria monocytogenes isolates from Cornu aspersum snails: Whole genome-based characterization and host-pathogen interactions in a snail infection model

Even though Listeria monocytogenes is an extensive-studied foodborne pathogen, genome analysis of isolates from snails that may represent a reservoir of L. monocytogenes are still scarce. Here, we use whole-genome sequencing (WGS) to assess the genomic diversity of hypervirulent, virulent and non-virulent phenotypes of 15 L. monocytogenes isolated from snails to unveil their survival, virulence, and host-pathogen mechanisms of interactions in a snail infection model. Most of isolates (66.7%) were characterized as multidrug resistant (MDR) and belonged to clonal complexes (CCs) which are strongly associated with cases of human infection. All isolates contained intact genes associated with invasion and infection while hypervirulent isolates are adapted to host environment, possessing genes which are involved in teichoic acid biosynthesis, peptidoglycan modification and biofilm formation, correlating with their tolerance to haemolymph plasma phenotype and biofilm formation ability. A snail infection model showed that hypervirulent isolates triggered programmed host cell death pathway by increasing up to 30% the circulating apoptotic hemocytes in combination with induced nitrate production and reactive oxygen species (ROS) generation in snails' haemolymph. In contrast, the administration of the non-virulent strain which possesses a truncated mogR gene that regulates flagellar motility gene expression led only to an increase of necrotic non-apoptotic cells. Overall, this study provides significant insights into the genetic diversity of L. monocytogenes from snails, the genomic features of them linked to their hypervirulent/non-virulent phenotype, and the mechanisms of host-pathogen interactions.

Different Shades of Listeria monocytogenes: Strain, Serotype, and Lineage-Based Variability in Virulence and Stress Tolerance Profiles

Listeria monocytogenes is a public health and food safety challenge due to its virulence and natural stress resistance phenotypes. The variable distribution of L. monocytogenes molecular subtypes with respect to food products and processing environments and among human and animal clinical listeriosis cases is observed. Sixty-two clinical and food-associated L. monocytogenes isolates were examined through phenome and genome analysis. Virulence assessed using a zebrafish infection model revealed serotype and genotype-specific differences in pathogenicity. Strains of genetic lineage I serotype 4b and multilocus sequence type clonal complexes CC1, CC2, CC4, and CC6 grew and survived better and were more virulent than serotype 1/2a and 1/2c lineage II, CC8, and CC9 strains. Hemolysis, phospholipase activity, and lysozyme tolerance profiles were associated with the differences observed in virulence. Osmotic stress resistance evaluation revealed serotype 4b lineage I CC2 and CC4 strains as more osmotolerant, whereas serotype 1/2c lineage II CC9 strains were more osmo-sensitive than others. Variable tolerance to the widely used quaternary ammonium compound benzalkonium chloride (BC) was observed. Some outbreak and sporadic clinical case associated strains demonstrated BC tolerance, which might have contributed to their survival and transition in the food-processing environment facilitating food product contamination and ultimately outbreaks or sporadic listeriosis cases. Genome comparison uncovered various moderate differences in virulence and stress associated genes between the strains indicating that these differences in addition to gene expression regulation variations might largely be responsible for the observed virulence and stress sensitivity phenotypic differences. Overall, our study uncovered strain and genotype-dependent variation in virulence and stress resilience among clinical and food-associated L. monocytogenes isolates with potential public health risk implications. The extensive genome and phenotypic data generated provide a basis for developing improved Listeria control strategies and policies.

Phagocytes produce prostaglandin E2 in response to cytosolic Listeria monocytogenes

Listeria monocytogenes is an intracellular bacterium that elicits robust CD8+ T-cell responses. Despite the ongoing development of L. monocytogenes-based platforms as cancer vaccines, our understanding of how L. monocytogenes drives robust CD8+ T-cell responses remains incomplete. One overarching hypothesis is that activation of cytosolic innate pathways is critical for immunity, as strains of L. monocytogenes that are unable to access the cytosol fail to elicit robust CD8+ T-cell responses and in fact inhibit optimal T-cell priming. Counterintuitively, however, activation of known cytosolic pathways, such as the inflammasome and type I IFN, lead to impaired immunity. Conversely, production of prostaglandin E2 (PGE2) downstream of cyclooxygenase-2 (COX-2) is essential for optimal L. monocytogenes T-cell priming. Here, we demonstrate that vacuole-constrained L. monocytogenes elicit reduced PGE2 production compared to wild-type strains in macrophages and dendritic cells ex vivo. In vivo, infection with wild-type L. monocytogenes leads to 10-fold increases in PGE2 production early during infection whereas vacuole-constrained strains fail to induce PGE2 over mock-immunized controls. Mice deficient in COX-2 specifically in Lyz2+ or CD11c+ cells produce less PGE2, suggesting these cell subsets contribute to PGE2 levels in vivo, while depletion of phagocytes with clodronate abolishes PGE2 production completely. Taken together, this work demonstrates that optimal PGE2 production by phagocytes depends on L. monocytogenes access to the cytosol, suggesting that one reason cytosolic access is required to prime CD8+ T-cell responses may be to facilitate production of PGE2.

Listeria monocytogenes infection rewires host metabolism with regulatory input from type I interferons

Listeria monocytogenes (L. monocytogenes) is a food-borne bacterial pathogen. Innate immunity to L. monocytogenes is profoundly affected by type I interferons (IFN-I). Here we investigated host metabolism in L. monocytogenes-infected mice and its potential control by IFN-I. Accordingly, we used animals lacking either the IFN-I receptor (IFNAR) or IRF9, a subunit of ISGF3, the master regulator of IFN-I-induced genes. Transcriptomes and metabolite profiles showed that L. monocytogenes infection induces metabolic rewiring of the liver. This affects various metabolic pathways including fatty acid (FA) metabolism and oxidative phosphorylation and is partially dependent on IFN-I signaling. Livers and macrophages from Ifnar1-/- mice employ increased glutaminolysis in an IRF9-independent manner, possibly to readjust TCA metabolite levels due to reduced FA oxidation. Moreover, FA oxidation inhibition provides protection from L. monocytogenes infection, explaining part of the protection of Irf9-/- and Ifnar1-/- mice. Our findings define a role of IFN-I in metabolic regulation during L. monocytogenes infection. Metabolic differences between Irf9-/- and Ifnar1-/- mice may underlie the different susceptibility of these mice against lethal infection with L. monocytogenes.

Why is Listeria monocytogenes such a potent inducer of CD8+ T-cells?

Listeria monocytogenes is a rapidly growing, Gram-positive, facultative intracellular pathogen that has been used for over 5 decades as a model to study basic aspects of infection and immunity. In a murine intravenous infection model, immunisation with a sublethal infection of L. monocytogenes initially leads to rapid intracellular multiplication followed by clearance of the bacteria and ultimately culminates in the development of long-lived cell-mediated immunity (CMI) mediated by antigen-specific CD8+ cytotoxic T-cells. Importantly, effective immunisation requires live, replicating bacteria. In this review, we summarise the cell and immunobiology of L. monocytogenes infection and discuss aspects of its pathogenesis that we suspect lead to robust CMI. We suggest five specific features of L. monocytogenes infection that positively impact the development of CMI: (a) the bacteria have a predilection for professional antigen-presenting cells; (b) the bacteria escape from phagosomes, grow, and secrete antigens into the host cell cytosol; (c) bacterial-secreted proteins enter the major histocompatibility complex (MHC) class I pathway of antigen processing and presentation; (d) the bacteria do not induce rapid host cell death; and (e) cytosolic bacteria induce a cytokine response that favours CMI. Collectively, these features make L. monocytogenes an attractive vaccine vector for both infectious disease applications and cancer immunotherapy.

Absence of TNF Leads to Alternative Activation in Peritoneal Macrophages in Experimental Listeria Monocytogenes Infection

Macrophages are crucial effectors of innate immunity against the pathogenic bacterium Listeria monocytogenes. The pro-inflammatory cytokine tumour necrosis factor-α (TNF) has been shown to be crucial for resistance to L. monocytogenes and mice deficient in TNF signalling succumb quickly after infection. However, the mechanisms underlying TNF-mediated defence against L. monocytogenes infection have not been completely elucidated. Here, we demonstrate that TNF concurrently functions to support a pro-inflammatory M1 phenotype while actively blocking macrophage polarization to the M2 phenotype. Compared to WT mice, peritoneal macrophages in TNF-deficient mice inoculated with L. monocytogenes respond with M2 polarization by upregulating Arg1. Consistently, TNF blockade in vitro resulted in M2 polarization in peritoneal macrophages during L. monocytogenes infection. Additionally, TNF promotes the transition from M2 to M1 polarization in peritoneal macrophages. Further investigation of peritoneal macrophage polarization suggested the NF-κB pathway is involved in the TNF-dependent M2 to M1 shift. Conversely, treatment of peritoneal macrophage with a PPARγ agonist blunted the expression of M1 genes induced by TNF and reduced NF-κB signalling pathway activation. Competing signalling mechanisms therefore play an essential role in the ability of peritoneal macrophage to resolve L. monocytogenes infections with TNF playing an essential role in driving M1 polarization.Abbreviations: LPM: large peritoneal macrophage; SPM: small peritoneal macrophage; LLO: listeriolysin O; iNOS: inducible nitric oxide synthase; DCs: dendritic cells.

Antilisterial Potential of Lactic Acid Bacteria in Eliminating Listeria monocytogenes in Host and Ready-to-Eat Food Application

Listeriosis is a severe food borne disease with a mortality rate of up to 30% caused by pathogenic Listeria monocytogenes via the production of several virulence factors including listeriolysin O (LLO), transcriptional activator (PrfA), actin (Act), internalin (Int), etc. It is a foodborne disease predominantly causing infections through consumption of contaminated food and is often associated with ready-to-eat food (RTE) and dairy products. Common medication for listeriosis such as antibiotics might cause an eagle effect and antibiotic resistance if it is overused. Therefore, exploration of the use of lactic acid bacteria (LAB) with probiotic characteristics and multiple antimicrobial properties is increasingly getting attention for their capability to treat listeriosis, vaccine development, and hurdle technologies. The antilisterial gene, a gene coding to produce antimicrobial peptide (AMP), one of the inhibitory substances found in LAB, is one of the potential key factors in listeriosis treatment, coupled with the vast array of functions and strategies; this review summarizes the various strategies by LAB against L. monocytogenes and the prospect in development of a ‘generally regarded as safe’ LAB for treatment of listeriosis.

PANoptosis in microbial infection

The immune system has evolved multiple mechanisms to restrict microbial infections and regulate inflammatory responses. Without appropriate regulation, infection-induced inflammatory pathology can be deadly. The innate immune system recognizes the microbial molecules conserved in many pathogens and engages a rapid response by producing inflammatory mediators and activating programmed cell death pathways, including pyroptosis, apoptosis, and necroptosis. Activation of pattern recognition receptors, in combination with inflammatory cytokine-induced signaling through death domain-containing receptors, initiates a highly interconnected cell death process called PANoptosis (pyroptosis, apoptosis, necroptosis). Broadly speaking, PANoptosis is critical for restricting a wide range of pathogens (including bacteria, viruses, fungi, and parasites), which we describe in this review. We propose that re-examining the role of cell death and inflammatory cytokines through the lens of PANoptosis will advance our understanding of host-pathogen evolution and may reveal new treatment strategies for controlling a wide range of infectious diseases.

Secondary metabolites and anti-microbial/anti-oxidant profiles in Ocimum spp.: Role of soil physico-chemical characteristics as eliciting factors

Ocimum has long been used as a medicinal plant, although little information is available about its bioactive ingredients, and the influence of soil properties on modulation of secondary metabolites in Ocimum has yet to be ascertained. In this study, we present a thorough survey of all potential metabolic compounds in O. sanctum and O. basilicum. In both species, certain compounds (e.g., quercetin, kaempferol, catechin, and S-adenosyl homocysteine) were detected coincidently. In the case of O. basilicum, other vital phenolic acids (e.g., ursolic, vanilic, coumaric, and syringic acids) were identified. The aqueous extracts (AEs) of Ocimum recorded decrease of 6-94% in the proliferation of pathogenic bacteria (e.g., Listeria monocytogenes, Staphylococcus sp., Salmonella sp., and Bacillus sp.). The AEs also showed effective antioxidant activity by reducing free radicals by a factor of 1.04-1.13. Root-zone soil samples of both Ocimum spp. were collected from strategic locations with varying levels of key soil attributes (e.g., soil organic carbon (SOC), microbial biomass carbon (MBC), urease, and phosphatase). At high levels of SOC, MBC, and soil enzymes, the bioactivity of Ocimum spp. was observed to be promoted, especially with respect to secondary metabolite expression, anti-pathogenic activity, and anti-oxidant properties. As such, the findings of strong correlations between secondary metabolite concentrations and bioactivity attributes in Ocimum suggest the potent role of soil quality in eliciting the production of secondary metabolite in association with bioactivity in Ocimum spp.

PPARα exacerbates necroptosis, leading to increased mortality in postinfluenza bacterial superinfection

Patients infected with influenza are at high risk of secondary bacterial infection, which is a major proximate cause of morbidity and mortality. We have shown that in mice, prior infection with influenza results in increased inflammation and mortality upon Staphylococcus aureus infection, recapitulating the human disease. Lipidomic profiling of the lungs of superinfected mice revealed an increase in CYP450 metabolites during lethal superinfection. These lipids are endogenous ligands for the nuclear receptor PPARα, and we demonstrate that Ppara ^-/- mice are less susceptible to superinfection than wild-type mice. PPARα is an inhibitor of NFκB activation, and transcriptional profiling of cells isolated by bronchoalveolar lavage confirmed that influenza infection inhibits NFκB, thereby dampening proinflammatory and prosurvival signals. Furthermore, network analysis indicated an increase in necrotic cell death in the lungs of superinfected mice compared to mice infected with S. aureus alone. Consistent with this, we observed reduced NFκB-mediated inflammation and cell survival signaling in cells isolated from the lungs of superinfected mice. The kinase RIPK3 is required to induce necrotic cell death and is strongly induced in cells isolated from the lungs of superinfected mice compared to mice infected with S. aureus alone. Genetic and pharmacological perturbations demonstrated that PPARα mediates RIPK3-dependent necroptosis and that this pathway plays a central role in mortality following superinfection. Thus, we have identified a molecular circuit in which infection with influenza induces CYP450 metabolites that activate PPARα, leading to increased necrotic cell death in the lung which correlates with the excess mortality observed in superinfection.

Listeria monocytogenes cancer vaccines: bridging innate and adaptive immunity

Purpose of the Review

Immunotherapy has emerged as a promising cancer treatment, however success in only select clinical indications underscores the need for novel approaches. Recently Listeria monocytogenes-based vaccines have been developed to drive tumor specific T-cell responses. Here, we discuss recent preclinical studies using L. monocytogenes vaccines, innate immune pathways that influence T-cell priming, and new vaccine strategies in clinical trials.

Recent Findings

Recent studies indicate that in addition to inducing antigen specific T-cell responses, L. monocytogenes vaccines remodel the TME. In addition, several innate immune pathways influence adaptive immune responses to L. monocytogenes and modulating these pathways holds promise to enhance anti-tumor T-cell responses.

Summary

The interplay between innate and adaptive immune responses to L. monocytogenes is poorly understood. Understanding these interactions will facilitate the design of better anti-cancer vaccines and improved use of combination therapies.

Organization of the Skin Immune System and Compartmentalized Immune Responses in Infectious Diseases

The skin is an organ harboring several types of immune cells that participate in innate and adaptive immune responses. The immune system of the skin comprises both skin cells and professional immune cells that together constitute what is designated skin-associated lymphoid tissue (SALT). In this review, I extensively discuss the organization of SALT and the mechanisms involved in its responses to infectious diseases of the skin and mucosa. The nature of these SALT responses, and the cellular mediators involved, often determines the clinical course of such infections. I list and describe the components of innate immunity, such as the roles of the keratinocyte barrier and of inflammatory and natural killer cells. I also examine the mechanisms involved in adaptive immune responses, with emphasis on new cytokine profiles, and the role of cell death phenomena in host-pathogen interactions and control of the immune responses to infectious agents. Finally, I highlight the importance of studying SALT in order to better understand host-pathogen relationships involving the skin and detail future directions in the immunological investigation of this organ, especially in light of recent findings regarding the skin immune system.

Multicombination Approach Suppresses Listeria monocytogenes-Induced Septicemia-Associated Acute Hepatic Failure: The Role of iRhom2 Signaling

The mortality rate of acute liver failure significantly increases due to fatal septicemia. Inactive rhomboid protein 2 (iRhom2) is an essential regulator of shedding TNF-α by trafficking with TNF-α converting enzyme (TACE). Fisetin, a flavonoid present in various fruits and plants, possesses anti-oxidative stress and anti-inflammatory activities. Here, multi-combination nanoparticles Fe@Au conjugated with fisetin, iRhom2 small interfering RNA (siRNA), and TNF-α inhibitor (FN) are prepared to examine their effects on fatal septicemia-associated hepatic failure induced by Listeria monocytogenes (LM) in mice and to reveal the underlying mechanisms. After LM infection, upregulation of glutamic-oxalacetic transaminease, glutamic-pyruvic transaminase, alkaline phosphatase, TNF-α, malondialdehyde, H₂ O₂ , and O₂^- is observedcompared to FN-treated mice. The iRhom2/TACE/TNF-α signals are enhanced in vivo and in vitro, resulting in oxidative stress, which is especially associated with the activation of kupffer cells and other macrophages. Decrease in Nrf2 activation and increase of inflammation-associated regulators are also noted in vivo and in vitro. Furthermore, overexpression of TNF-α derived from macrophages aggravates hepatic failure. Inversely, the processes above are restored by FN nanoparticles through the regulation of the iRhom2/TACE/TNF-α axis and Nrf2 activation. These findings suggest that FN may be a potential approach to protect against bacterial septicemia-related diseases by targeting iRhom2.

Attenuation of Listeria monocytogenes Virulence by Cannabis sativa L. Essential Oil

Anti-virulence strategies are being explored as a novel approach to combat pathogens. Such strategies include inhibition of surface adhesion, tissue invasion, toxin production, and/or interference with the gene regulation of other virulence traits. Listeria monocytogenes, the causative agent of listeriosis, is a facultative intracellular food pathogen characterized by a wide distribution in the environment. Its ability to persist within biofilms and to develop resistance to sanitizers is the cause of significant problems in food processing plants and of steep costs for the food industry. In humans, the treatment of listeriosis is hampered by the intracellular location of listeriae and the poor intracellular penetration of some antibiotics. Eleven L. monocytogenes isolates from patients who were diagnosed with invasive listeriosis in Italy in 2014-2016 were studied. This in vitro and in vivo study explored the antibacterial and anti-virulence properties of a steam-distilled essential oil of Cannabis sativa L., which is being intensively investigated for its high content in powerful bioactive phytochemicals. Susceptibility experiments demonstrated a moderate bactericidal activity of the essential oil (Minimum Bactericidal Concentration > 2048 μg/mL). Assessment of the effects of sublethal concentrations of the essential oil on L. monocytogenes virulence traits demonstrated a significant action on motility. Listeriae were non-motile after exposure to the essential oil. Light and scanning electron microscopy documented aggregates of listeriae with the flagella trapped inside the cluster. Real-time RT-PCR experiments showed downregulation of flagellar motility genes and of the regulatory gene prfA. The ability to form biofilm and to invade Caco-2 cells was also significantly reduced. Galleria mellonella larvae infected with L. monocytogenes grown in presence of sublethal concentrations of the essential oil showed much higher survival rates compared with controls, suggesting that the extract inhibited tissue invasion. Food contamination with L. monocytogenes is a major concern for the food industry, particularly for plants making ready-to-eat and processed food. The present work provides a baseline in the study of the anti-virulence properties of the C. sativa essential oil against L. monocytogenes. Further studies are needed to understand if it could be used as an alternative agent for the control of L. monocytogenes in food processing plants.