Dissociation of innate immune responses in microglia infected with Listeria monocytogenes

Divergent host-pathogen interactions in neurolisteriosis: cytosolic replication vs. phagosomal dormancy of Listeria monocytogenes in CNS macrophages

Bacterial infections of the central nervous system (CNS) pose a significant threat to public health, especially with the growing challenge of antimicrobial resistance. Among these, Listeria monocytogenes (Lm) stands out as a key pathogen, responsible for often fatal neurolisteriosis in humans and cattle. Emerging evidence highlights the distinct roles played by microglia, the resident macrophages of the CNS, and infiltrating monocyte-derived macrophages (MDM) during neuroinflammation. Using bovine models, we investigated the interactions between these two macrophage populations and Lm during infection. Our results show that Lm thrives in the cytosol of microglia, driving productive infection and facilitating bacterial spread. In contrast, MDM effectively sequesters Lm within the phagolysosomal system, limiting its replication and inducing a viable but non-culturable (VBNC) state without completely eliminating the pathogen. Listeriolysin O contributes to the dichotomy of Lm fate, determining whether Lm escapes into the cytosol or transitions to the VBNC state. These findings underscore the complexity of Lm-host dynamics in neurolisteriosis, emphasizing the distinct yet complementary roles of microglia and MDM in shaping CNS infection. By elucidating these mechanisms, our study offers new perspectives on the neurolisteriosis pathogenesis and opens avenues for innovative therapeutic approaches to combat bacterial neuroinfections.

Kinetics, thresholds, and a comparison of mechanisms underlying systemic infection by Listeria monocytogenes

Studies on the system-scale pathogenesis of Listeria monocytogenes infection have classically focused on its ability to colonize in the intestines following an exposure event. However, despite this, many of the most dangerous complications arising from L. monocytogenes infection are observed days, weeks, or months after exposure, resulting indirectly from bacteria escaping this intestinal colonization hub and invading other organs. Over time, findings of various individual phenomena observed during systemic infection have accumulated, including a shift away from the principal route of intestinal dissemination, delays in bacterial colonization of the central nervous system, differing bacterial flux rates across organs, and multi-stability of bacterial population levels. To further our quantitative understanding of foodborne bacterial infection dynamics, a compartmental model of systemic infection that synthesizes these findings is proposed. Under parameterization to infection in BALB/c mice, the model is used to show a substantial decrease in bacterial populations resulting from dissemination through the mesenteric lymph nodes, as compared to the portal vein, when controlling for the number of bacteria passing through each route. Due to the compartmental nature of this model, we anticipate that this result may be paralleled in other microbes which make use of these pathways to escape the intestinal environment. Additionally, we predict thresholds for intestinal dissemination along each of these routes, which must be surpassed to induce systemic infection, and describe how these thresholds change over time. Supplementarily, logistic curves are fitted to synthetic data as a means of robustly quantifying the dose-response relationship beyond the intestinal barrier.

Bovine neutrophil chemotaxis to Listeria monocytogenes in neurolisteriosis depends on microglia-released rather than bacterial factors

BACKGROUND

Listeria monocytogenes (Lm) is a bacterial pathogen of major concern for humans and ruminants due to its neuroinvasive potential and its ability to cause deadly encephalitis (neurolisteriosis). On one hand, polymorphonuclear neutrophils (PMN) are key players in the defense against Lm, but on the other hand intracerebral infiltration with PMN is associated with significant neural tissue damage. Lm-PMN interactions in neurolisteriosis are poorly investigated, and factors inducing PMN chemotaxis to infectious foci containing Lm in the central nervous system (CNS) remain unidentified.

METHODS

In this study, we assessed bovine PMN chemotaxis towards Lm and supernatants of infected endogenous brain cell populations in ex vivo chemotaxis assays, to identify chemotactic stimuli for PMN chemotaxis towards Lm in the brain. In addition, microglial secretion of IL-8 was assessed both ex vivo and in situ.

RESULTS

Our data show that neither Lm cell wall components nor intact bacteria elicit chemotaxis of bovine PMN ex vivo. Moreover, astrocytes and neural cells fail to induce bovine PMN chemotaxis upon infection. In contrast, supernatant from Lm infected microglia readily induced chemotaxis of bovine PMN. Microglial expression and secretion of IL-8 was identified during early Lm infection in vitro and in situ, although IL-8 blocking with a specific antibody could not abrogate PMN chemotaxis towards Lm infected microglial supernatant.

CONCLUSIONS

These data provide evidence that host-derived rather than bacterial factors trigger PMN chemotaxis to bacterial foci in the CNS, that microglia have a primary role as initiators of bovine PMN chemotaxis into the brain during neurolisteriosis and that blockade of these factors could be a therapeutic target to limit intrathecal PMN chemotaxis and PMN associated damage in neurolisteriosis.

NF-κB/ROS and ERK pathways regulate NLRP3 inflammasome activation in Listeria monocytogenes infected BV2 microglia cells

Listeria monocytogenes is a food-borne pathogen responsible for neurolisteriosis, which is potentially lethal in immunocompromised individuals. Microglia are the main target cells for L. monocytogenes in central nervous system (CNS). However, the precise mechanisms by which they trigger neuroinflammatory processes remain unknown. The BV2 microglial cell line and a murine model of L. monocytogenes infection were used for experiments in this study. Listeria monocytogenes induced pyroptosis and nucleotide binding and oligomerization, leucine-rich repeat, pyrin domain-containing 3 (NLRP3) inflammasome activation in BV2. Pharmacological inhibition of the NLRP3 inflammasome attenuated L. monocytogenes-induced pyroptosis. Moreover, inhibition of nuclear factor kappa-B (NF-κB) and extracellular regulated protein kinases (ERK) pathways induced a decrease in caspase1 activation and mature IL-1β-17 secretion. Our collective findings support critical involvement of the NLRP3 inflammasome in L. monocytogenes-induced neuroinflammation and, to an extent, ROS production. In addition, ERK and NF-κB signaling play an important role in activation of the NLRP3 inflammasome, both in vitro and in vivo.

Innate immune responses to Listeria in vivo

Listeria monocytogenes (Lm) is a foodborne bacterial pathogen that causes listeriosis, a severe infection that manifests as bacteremia and meningo-encephalitis mostly in immunocompromised individuals, and maternal-fetal infection. A critical pathogenic determinant of Lm relies on its ability to actively cross the intestinal barrier, disseminate systemically and cross the blood-brain and placental barriers. Here we illustrate how Lm both evades innate immunity, favoring its dissemination in host tissues, and triggers innate immune defenses that participate to its control.

Innate and Adaptive Immune Responses during Listeria monocytogenes Infection

It could be argued that we understand the immune response to infection with Listeria monocytogenes better than the immunity elicited by any other bacteria. L. monocytogenes are Gram-positive bacteria that are genetically tractable and easy to cultivate in vitro, and the mouse model of intravenous (i.v.) inoculation is highly reproducible. For these reasons, immunologists frequently use the mouse model of systemic listeriosis to dissect the mechanisms used by mammalian hosts to recognize and respond to infection. This article provides an overview of what we have learned over the past few decades and is divided into three sections: "Innate Immunity" describes how the host initially detects the presence of L. monocytogenes and characterizes the soluble and cellular responses that occur during the first few days postinfection; "Adaptive Immunity" discusses the exquisitely specific T cell response that mediates complete clearance of infection and immunological memory; "Use of Attenuated Listeria as a Vaccine Vector" highlights the ways that investigators have exploited our extensive knowledge of anti-Listeria immunity to develop cancer therapeutics.

The olfactory epithelium as a port of entry in neonatal neurolisteriosis

Bacterial infections of the central nervous system (CNS) remain a major cause of mortality in the neonatal population. Commonly used parenteral infection models, however, do not reflect the early course of the disease leaving this critical step of the pathogenesis largely unexplored. Here, we analyzed nasal exposure of 1-day-old newborn mice to Listeria monocytogenes (Lm). We found that nasal, but not intragastric administration, led to early CNS infection in neonate mice. In particular, upon bacterial invasion of the olfactory epithelium, Lm subsequently spread along the sensory neurons entering the brain tissue at the cribriform plate and causing a significant influx of monocytes and neutrophils. CNS infection required listeriolysin for penetration of the olfactory epithelium and ActA, a mediator of intracellular mobility, for translocation into the brain tissue. Taken together, we propose an alternative port of entry and route of infection for neonatal neurolisteriosis and present a novel infection model to mimic the clinical features of late-onset disease in human neonates.

Listeria monocytogenes causes meningitis in newborns. Here, Pägelow et al. present a mouse model of neonatal cerebral listeriosis, and show that nasal inoculation, but not intragastric administration, leads to early brain infection in the absence of bacteraemia during the neonatal period.

GNP-GAPDH1-22 nanovaccines prevent neonatal listeriosis by blocking microglial apoptosis and bacterial dissemination

Clinical cases of neonatal listeriosis are associated with brain disease and fetal loss due to complications in early or late pregnancy, which suggests that microglial function is altered. This is believed to be the first study to link microglial apoptosis with neonatal listeriosis and listeriosis-associated brain disease, and to propose a new nanovaccine formulation that reverses all effects of listeriosis and confers Listeria monocytogenes (LM)-specific immunity. We examined clinical cases of neonatal listeriosis in 2013–2015 and defined two useful prognostic immune biomarkers to design listeriosis vaccines: high anti-GAPDH_1-22 titres and tumor necrosis factor (TNF)/interleukin (IL)-6 ratios. Therefore, we developed a nanovaccine with gold glyco-nanoparticles conjugated to LM peptide 1-22 of GAPDH (Lmo2459), GNP-GAPDH_1-22 nanovaccinesformulated with a pro-inflammatory Toll-like receptor 2/4-targeted adjuvant. Neonates born to non-vaccinated pregnant mice with listeriosis, showed brain and vascular diseases and significant microglial dysfunction by induction of TNF-α-mediated apoptosis. This programmed TNF-mediated suicide explains LM dissemination in brains and livers and blocks production of early pro-inflammatory cytokines such as IL-1β and interferon-α/β. In contrast, neonates born to GNP-GAPDH_1–22-vaccinated mothers before LM infection, did not develop listeriosis or brain diseases and had functional microglia. In nanovaccinated mothers, immune responses shifted towards Th1/IL-12 pro-inflammatory cytokine profiles and high production of anti-GAPDH_1–22 antibodies, suggesting good induction of LM-specific memory.

Pregnancy Vaccination with Gold Glyco-Nanoparticles Carrying Listeria monocytogenes Peptides Protects against Listeriosis and Brain- and Cutaneous-Associated Morbidities

Listeriosis is a fatal infection for fetuses and newborns with two clinical main morbidities in the neonatal period, meningitis and diffused cutaneous lesions. In this study, we vaccinated pregnant females with two gold glyconanoparticles (GNP) loaded with two peptides, listeriolysin peptide 91-99 (LLO_91-99) or glyceraldehyde-3-phosphate dehydrogenase 1-22 peptide (GAPDH_1-22). Neonates born to vaccinated mothers were free of bacteria and healthy, while non-vaccinated mice presented clear brain affections and cutaneous diminishment of melanocytes. Therefore, these nanoparticle vaccines are effective measures to offer pregnant mothers at high risk of listeriosis interesting therapies that cross the placenta.

Novel nanoparticle vaccines for Listeriosis

In recent years, nanomedicine has transformed many areas of traditional medicine, and enabled fresh insights into the prevention of previously difficult to treat diseases. An example of the transformative power of nanomedicine is a recent nano-vaccine against listeriosis, a serious bacterial infection affecting not only pregnant women and their neonates, but also immune-compromised patients with neoplastic or chronic autoimmune diseases. There is a major unmet need for an effective and safe vaccine against listeriosis, with the challenge that an effective vaccine needs to generate protective T cell immunity, a hitherto difficult to achieve objective. Now utilizing a gold nanoparticle antigen delivery approach together with a novel polysaccharide nanoparticulate adjuvant, an effective T-cell vaccine has been developed that provides robust protection in animal models of listeriosis, raising the hope that one day this nanovaccine technology may protect immune-compromised humans against this serious opportunistic infection.

A novel therapy for melanoma developed in mice: transformation of melanoma into dendritic cells with Listeria monocytogenes

Listeria monocytogenes is a gram-positive bacteria and human pathogen widely used in cancer immunotherapy because of its capacity to induce a specific cytotoxic T cell response in tumours. This bacterial pathogen strongly induces innate and specific immunity with the potential to overcome tumour induced tolerance and weak immunogenicity. Here, we propose a Listeria based vaccination for melanoma based in its tropism for these tumour cells and its ability to transform in vitro and in vivo melanoma cells into matured and activated dendritic cells with competent microbicidal and antigen processing abilities. This Listeria based vaccination using low doses of the pathogen caused melanoma regression by apoptosis as well as bacterial clearance. Vaccination efficacy is LLO dependent and implies the reduction of LLO-specific CD4⁺ T cell responses, strong stimulation of innate pro-inflammatory immune cells and a prevalence of LLO-specific CD8⁺ T cells involved in tumour regression and Listeria elimination. These results support the use of low doses of pathogenic Listeria as safe melanoma therapeutic vaccines that do not require antibiotics for bacterial removal.

A trip in the "New Microbiology" with the bacterial pathogen Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen causing an opportunistic disease called listeriosis. This bacterium invades and replicates in most cell types, due to its multiple strategies to exploit host molecular mechanisms. Research aiming at unravelling Listeria invasion and intracellular lifestyle has led to a number of key discoveries in infection biology, cell biology and also microbiology. In this review, we report on our most recent advances in understanding the intimate crosstalk between the bacterium and its host, resulting from in-depth studies performed over the past five years. We specifically highlight new concepts in RNA-based regulation in bacteria and discuss important findings in cell biology, including a new role for clathrin and an atypical mitochondrial fragmentation mechanism. We also illustrate the notion that bacterial infection regulates host gene expression at the chromatin level, contributing to an emerging field called patho-epigenetics. This review corresponds to the lecture given by one of us (P.C.) on the occasion of the 2014 FEBS|EMBO Woman in Science Award.

Cellular vaccines in listeriosis: role of the Listeria antigen GAPDH

The use of live Listeria-based vaccines carries serious difficulties when administrated to immunocompromised individuals. However, cellular carriers have the advantage of inducing multivalent innate immunity as well as cell-mediated immune responses, constituting novel and secure vaccine strategies in listeriosis. Here, we compare the protective efficacy of dendritic cells (DCs) and macrophages and their safety. We examined the immune response of these vaccine vectors using two Listeria antigens, listeriolysin O (LLO) and glyceraldehyde-3-phosphate-dehydrogenase (GAPDH), and several epitopes such as the LLO peptides, LLO_189−201 and LLO₉₁₋₉₉ and the GAPDH peptide, GAPDH₁₋₂₂. We discarded macrophages as safe vaccine vectors because they show anti-Listeria protection but also high cytotoxicity. DCs loaded with GAPDH₁₋₂₂ peptide conferred higher protection and security against listeriosis than the widely explored LLO₉₁₋₉₉ peptide. Anti-Listeria protection was related to the changes in DC maturation caused by these epitopes, with high production of interleukin-12 as well as significant levels of other Th1 cytokines such as monocyte chemotactic protein-1, tumor necrosis factor-α, and interferon-γ, and with the induction of GAPDH₁₋₂₂-specific CD4⁺ and CD8⁺ immune responses. This is believed to be the first study to explore the use of a novel GAPDH antigen as a potential DC-based vaccine candidate for listeriosis, whose efficiency appears to highlight the relevance of vaccine designs containing multiple CD4⁺ and CD8⁺ epitopes.