Dendritic cells inhibit the progression of Listeria monocytogenes intracellular infection by retaining bacteria in major histocompatibility complex class II-rich phagosomes and by limiting cytosolic growth

Exogenous NADH promotes the bactericidal effect of aminoglycoside antibiotics against Edwardsiella tarda

The global surge in multidrug-resistant bacteria owing to antibiotic misuse and overuse poses considerable risks to human and animal health. With existing antibiotics losing their effectiveness and the protracted process of developing new antibiotics, urgent alternatives are imperative to curb disease spread. Notably, improving the bactericidal effect of antibiotics by using non-antibiotic substances has emerged as a viable strategy. Although reduced nicotinamide adenine dinucleotide (NADH) may play a crucial role in regulating bacterial resistance, studies examining how the change of metabolic profile and bacterial resistance following by exogenous administration are scarce. Therefore, this study aimed to elucidate the metabolic changes that occur in Edwardsiella tarda (E. tarda), which exhibits resistance to various antibiotics, following the exogenous addition of NADH using metabolomics. The effects of these alterations on the bactericidal activity of neomycin were investigated. NADH enhanced the effectiveness of aminoglycoside antibiotics against E. tarda ATCC15947, achieving bacterial eradication at low doses. Metabolomic analysis revealed that NADH reprogrammed the ATCC15947 metabolic profile by promoting purine metabolism and energy metabolism, yielding increased adenosine triphosphate (ATP) levels. Increased ATP levels played a crucial role in enhancing the bactericidal effects of neomycin. Moreover, exogenous NADH promoted the bactericidal efficacy of tetracyclines and chloramphenicols. NADH in combination with neomycin was effective against other clinically resistant bacteria, including Aeromonas hydrophila, Vibrio parahaemolyticus, methicillin-resistant Staphylococcus aureus, and Listeria monocytogenes. These results may facilitate the development of effective approaches for preventing and managing E. tarda-induced infections and multidrug resistance in aquaculture and clinical settings.

Production of IFN-γ by splenic dendritic cells during innate immune responses against Francisella tularensis LVS depends on MyD88, but not TLR2, TLR4, or TLR9

Production of IFN-γ is a key innate immune mechanism that limits replication of intracellular bacteria such as Francisella tularensis (Ft) until adaptive immune responses develop. Previously, we demonstrated that the host cell types responsible for IFN-γ production in response to murine Francisella infection include not only natural killer (NK) and T cells, but also a variety of myeloid cells. However, production of IFN-γ by mouse dendritic cells (DC) is controversial. Here, we directly demonstrated substantial production of IFN-γ by DC, as well as hybrid NK-DC, from LVS-infected wild type C57BL/6 or Rag1 knockout mice. We demonstrated that the numbers of conventional DC producing IFN-γ increased progressively over the course of 8 days of LVS infection. In contrast, the numbers of conventional NK cells producing IFN-γ, which represented about 40% of non-B/T IFN-γ-producing cells, peaked at day 4 after LVS infection and declined thereafter. This pattern was similar to that of hybrid NK-DC. To further confirm IFN-γ production by infected cells, DC and neutrophils were sorted from naïve and LVS-infected mice and analyzed for gene expression. Quantification of LVS by PCR revealed the presence of Ft DNA not only in macrophages, but also in highly purified, IFN-γ producing DC and neutrophils. Finally, production of IFN-γ by infected DC was confirmed by immunohistochemistry and confocal microscopy. Notably, IFN-γ production patterns similar to those in wild type mice were observed in cells derived from LVS-infected TLR2, TLR4, and TLR2xTLR9 knockout (KO) mice, but not from MyD88 KO mice. Taken together, these studies demonstrate the pivotal roles of DC and MyD88 in IFN-γ production and in initiating innate immune responses to this intracellular bacterium.

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.

IL-10-Dependent Crosstalk between Murine Marginal Zone B Cells, Macrophages, and CD8α+ Dendritic Cells Promotes Listeria monocytogenes Infection

Type 1 CD8α⁺ conventional dendritic cells (cDC1s) are required for CD8⁺ T cell priming but, paradoxically, promote splenic Listeria monocytogenes infection. Using mice with impaired cDC2 function, we ruled out a role for cDC2s in this process and instead discovered an interleukin-10 (IL-10)-dependent cellular crosstalk in the marginal zone (MZ) that promoted bacterial infection. Mice lacking the guanine nucleotide exchange factor DOCK8 or CD19 lost IL-10-producing MZ B cells and were resistant to Listeria. IL-10 increased intracellular Listeria in cDC1s indirectly by reducing inducible nitric oxide synthase expression early after infection and increasing intracellular Listeria in MZ metallophilic macrophages (MMMs). These MMMs trans-infected cDC1s, which, in turn, transported Listeria into the white pulp to prime CD8⁺ T cells. However, this also facilitated bacterial expansion. Therefore, IL-10-mediated crosstalk between B cells, macrophages, and cDC1s in the MZ promotes both Listeria infection and CD8⁺ T cell activation.

Development of endocytosis, degradative activity, and antigen processing capacity during GM-CSF driven differentiation of murine bone marrow

Dendritic cells (DC) are sentinels of the immune system, alerting and enlisting T cells to clear pathogenic threats. As such, numerous studies have demonstrated their effective uptake and proteolytic activities coupled with antigen processing and presentation functions. Yet, less is known about how these cellular mechanisms change and develop as myeloid cells progress from progenitor cells to more differentiated cell types such as DC. Thus, our study comparatively examined these functions at different stages of myeloid cell development driven by the GM-CSF. To measure these activities at different stages of development, GM-CSF driven bone marrow cells were sorted based on expression of Ly6C, CD115, and CD11c. This strategy enables isolation of cells representing five distinct myeloid cell types: Common Myeloid Progenitor (CMP), Granulocyte/Macrophage Progenitor (GMP), monocytes, monocyte-derived Macrophage/monocyte-derived Dendritic cell Precursors (moMac/moDP), and monocyte-derived DC (moDC). We observed significant differences in the uptake capacity, proteolysis, and antigen processing and presentation functions between these myeloid cell populations. CMP showed minimal uptake capacity with no detectable antigen processing and presenting function. The GMP population showed higher uptake capacity, modest proteolytic activity, and little T cell stimulatory function. In the monocyte population, the uptake capacity reached its peak, yet this cell type had minimal antigen processing and presentation function. Finally, moMac/moDP and moDC had a modestly decreased uptake capacity, high degradative capacity and strong antigen processing and presentation functions. These insights into when antigen processing and presentation function develop in myeloid cells during GM-CSF driven differentiation are crucial to the development of vaccines, allowing targeting of the most qualified cells as an ideal vaccine vehicles.

Monocytes Are the Predominant Cell Type Associated with Listeria monocytogenes in the Gut, but They Do Not Serve as an Intracellular Growth Niche

After foodborne transmission of the facultative intracellular bacterial pathogen Listeria monocytogenes, most of the bacterial burden in the gut is extracellular. However, we previously demonstrated that intracellular replication in an as yet unidentified cell type was essential for dissemination and systemic spread of L. monocytogenes In this article, we show that the vast majority of cell-associated L. monocytogenes in the gut were adhered to Ly6C^hi monocytes, a cell type that inefficiently internalized L. monocytogenes With bone marrow-derived in vitro cultures, high multiplicity of infection or the use of opsonized bacteria enhanced uptake of L. monocytogenes in CD64^- monocytes, but very few bacteria reached the cell cytosol. Surprisingly, monocytes that had upregulated CD64 expression in transition toward becoming macrophages fully supported intracellular growth of L. monocytogenes In contrast, inflammatory monocytes that had increased CD64 expression in the bone marrow of BALB/c/By/J mice prior to L. monocytogenes exposure in the gut did not support L. monocytogenes growth. Thus, contrary to the perception that L. monocytogenes can infect virtually all cell types, neither naive nor inflammatory Ly6C^hi monocytes served as a productive intracellular growth niche for L. monocytogenes. These results have broad implications for innate immune recognition of L. monocytogenes in the gut and highlight the need for additional studies on the interaction of extracellular, adherent L. monocytogenes with the unique subsets of myeloid-derived inflammatory cells that infiltrate sites of infection.

The lectin Siglec-G inhibits dendritic cell cross-presentation by impairing MHC class I-peptide complex formation

CD8α(+) dendritic cells (DCs) are specialized at cross-presenting extracellular antigens on major histocompatibility complex (MHC) class I molecules to initiate cytotoxic T lymphocyte (CTL) responses; however, details of the mechanisms that regulate cross-presentation remain unknown. We found lower expression of the lectin family member Siglec-G in CD8α(+) DCs, and Siglec-G deficient (Siglecg(-/-)) mice generated more antigen-specific CTLs to inhibit intracellular bacterial infection and tumor growth. MHC class I-peptide complexes were more abundant on Siglecg(-/-) CD8α(+) DCs than on Siglecg(+/+) CD8α(+) DCs. Mechanistically, phagosome-expressed Siglec-G recruited the phosphatase SHP-1, which dephosphorylated the NADPH oxidase component p47(phox) and inhibited the activation of NOX2 on phagosomes. This resulted in excessive hydrolysis of exogenous antigens, which led to diminished formation of MHC class I-peptide complexes for cross-presentation. Therefore, Siglec-G inhibited DC cross-presentation by impairing such complex formation, and our results add insight into the regulation of cross-presentation in adaptive immunity.

The influence of infectious factors on dendritic cell apoptosis

Pathogens can have a negative influence on dendritic cells (DCs), causing their apoptosis, which prevents active presentation of foreign antigens. It results in a state of immunosuppression which makes the body susceptible to secondary infections. Infected immature DCs have lower expression of co-stimulatory and adhesion molecules, reduced ability to secrete cytokines and an inhibited maturation process and are incapable of effective antigen presentation and activation of T-lymphocytes. In some cases, the ability of DCs to undergo rapid apoptosis is important for the body defense, which is probably because of DCs' ability to cross-present and cooperate with other cells. Apoptotic bodies released from the infected DCs are phagocytosed by other DCs, which then stimulate the effector cells and present antigens more efficiently than infected cells. The aim of this article is to review how the DCs respond to viral and bacterial factors and which biochemical mechanisms are responsible for their apoptosis.

Fascin confers resistance to Listeria infection in dendritic cells

Ag-presenting dendritic cells (DCs) must survive bacterial infection to present Ag information to naive T cells. The greater ability of DCs' host defense is evident from the report that DCs are more resistant to Listeria monocytogenes than macrophages. However, the molecular mechanism of this resistance is unclear. We found that Listeria replicate more slowly in wild-type DCs compared with fascin1 knockout DCs. This finding is significant because fascin1, an actin-bundling protein, is specifically and greatly induced upon maturation of dendritic cells, but not other blood cells, including macrophages and neutrophils. Infection by Listeria makes phagosomes more acidic in wild-type DCs than in fascin1 knockout DCs, suggesting that fascin1 facilitates phagolysosomal fusion for killing of phagocytosed Listeria. We further found that fascin1 binds to LC3, an autophagosome marker, both in vivo and in vitro. Listeria are associated with LC3 to a greater extent in wild-type DCs than in fascin1 knockout DCs, suggesting that fascin1 facilitates autophagy for eradication of cytoplasmic Listeria. Taken together, our results suggest that fascin1 plays critical roles in the survival of DCs during Listeria infection, allowing DCs to function in innate and adaptive immunity.

Regulatory B cell (B10 Cell) expansion during Listeria infection governs innate and cellular immune responses in mice

Pathogens use numerous methods to subvert host immune responses, including the modulation of host IL-10 production by diverse cell types. However, the B cell sources of IL-10 and their overall influence on innate and cellular immune responses have not been well characterized during infections. Using Listeria as a model pathogen, infection drove the acute expansion of a small subset of regulatory B cells (B10 cells) that potently suppress inflammation and autoimmunity through the production of IL-10. Unexpectedly, spleen bacteria loads were 92-97% lower in B10 cell-deficient CD19(-/-) mice, in mice depleted of mature B cells, and in mice treated with CD22 mAb to preferentially deplete B10 cells before infection. By contrast, the adoptive transfer of wild-type B10 cells reduced bacterial clearance by 38-fold in CD19(-/-) mice through IL-10-dependent pathways. B10 cell depletion using CD22 mAb significantly enhanced macrophage phagocytosis of Listeria and their production of IFN-γ, TNF-α, and NO ex vivo. Accelerated bacteria clearance following B10 cell depletion significantly reduced Ag-specific CD4(+) T cell proliferation and cytokine production, but did not alter CD8(+) T cell responses. B10 cell regulatory function during innate immune responses was nonetheless dependent on cognate interactions with CD4(+) T cells because B10 cells deficient in IL-10, MHC-II, or IL-21R expression did not influence Listeria clearance. Thus, Listeria manipulates immune responses through a strategy of immune evasion that involves the preferential expansion of endogenous B10 cells that regulate the magnitude and duration of both innate and cellular immune responses.

The timing of IFNβ production affects early innate responses to Listeria monocytogenes and determines the overall outcome of lethal infection

Dendritic cells (DCs) and natural killer (NK) cells are essential components of the innate immunity and play a crucial role in the first phase of host defense against infections and tumors. Listeria monocytogenes (Lm) is an intracellular pathogen that colonizes the cytosol of eukaryotic cells. Recent findings have shown Lm specifically in splenic CD8a⁺ DCs shortly after intravenous infection. We examined gene expression profiles of mouse DCs exposed to Lm to elucidate the molecular mechanisms underlying DCs interaction with Lm. Using a functional genomics approach, we found that Lm infection induced a cluster of late response genes including type I IFNs and interferon responsive genes (IRGs) in DCs. Type I INFs were produced at the maximal level only at 24 h post infection indicating that the regulation of IFNs in the context of Lm infection is delayed compared to the rapid response observed with viral pathogens. We showed that during Lm infection, IFNγ production and cytotoxic activity were severely impaired in NK cells compared to E. coli infection. These defects were restored by providing an exogenous source of IFNβ during the initial phase of bacterial challenge. Moreover, when treated with IFNβ during early infection, NK cells were able to reduce bacterial titer in the spleen and significantly improve survival of infected mice. These findings show that the timing of IFNβ production is fundamental to the efficient control of the bacterium during the early innate phase of Lm infection.

Phagosomes induced by cytokines function as anti-Listeria vaccines: novel role for functional compartmentalization of STAT-1 protein and cathepsin-D

Phagosomes are critical compartments for innate immunity. However, their role in the protection against murine listeriosis has not been examined. We describe here that listericidal phago-receptosomes are induced by the function of IFN-γ or IL-6 as centralized compartments for innate and adaptive immunity because they are able to confer protection against murine listeriosis. These phago-receptosomes elicited LLO(91-99)/CD8(+)- and LLO(189-201)/CD4(+)-specific immune responses and recruited mature dendritic cells to the vaccination sites controlled by T cells. Moreover, they present exceptional features as efficient vaccine vectors. First, they compartmentalize a novel listericidal STAT-1-mediated signaling pathway that confines multiple innate immune components to the same environment. Second, they show features of MHC class II antigen-loading competent compartments for cathepsin-D-mediated LLO processing. Third, murine cathepsin-D deficiencies fail to develop protective immunity after vaccination with listericidal phago-receptosomes induced by IFN-γ or IL-6. Therefore, it appears that the connection of STAT-1 and cathepsin-D in a single compartment is relevant for protection against listeriosis.

The intact structural form of LLO in endosomes cannot protect against listeriosis

LLO is the major immuno-dominant antigen in listeriosis and is also required for protective immunity. Two forms of LLO can be observed in endosomal membranes, a LLO intact form and a Ctsd-processed LLO(1-491) form. Endosomes obtained from resting macrophages contained only LLO intact forms, while endosomes obtained from IFN-activated macrophages contained both forms. Both types of endosomes elicited LLO(90-91)/CD8(+) and LLO(189-201)/CD4(+) specific immune responses. However, only endosomes containing the Ctsd-processed LLO(1-491) form showed significant CD4(+) and CD8(+) T cell responses similar to LM infected bone marrow derived macrophages and characteristic of protective Listeria immunity. Moreover, endosomes with intact LLO could not confer protection as vaccine carriers against murine listeriosis. While endosomes with Ctsd-processed LLO(1-491) form showed a moderate ability, slightly lower than high efficiency vaccine vectors as MØ infected with LM. These studies argue that all cell-free membrane vesicles might serve as valid vaccine carriers against infectious agents. Exclusively those cell-free vesicles MIIC competent for LLO processing are protective vaccines vectors since they recruit significant numbers of mature dendritic cells to the vaccination sites and contain a LLO(1-491) form that might be accessible for MHC class I and class II antigen presentation.

Invasive extravillous trophoblasts restrict intracellular growth and spread of Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular bacterial pathogen that can infect the placenta, a chimeric organ made of maternal and fetal cells. Extravillous trophoblasts (EVT) are specialized fetal cells that invade the uterine implantation site, where they come into direct contact with maternal cells. We have shown previously that EVT are the preferred site of initial placental infection. In this report, we infected primary human EVT with L. monocytogenes. EVT eliminated ∼80% of intracellular bacteria over 24-hours. Bacteria were unable to escape into the cytoplasm and remained confined to vacuolar compartments that became acidified and co-localized with LAMP1, consistent with bacterial degradation in lysosomes. In human placental organ cultures bacterial vacuolar escape rates differed between specific trophoblast subpopulations. The most invasive EVT—those that would be in direct contact with maternal cells in vivo—had lower escape rates than trophoblasts that were surrounded by fetal cells and tissues. Our results suggest that EVT present a bottleneck in the spread of L. monocytogenes from mother to fetus by inhibiting vacuolar escape, and thus intracellular bacterial growth. However, if L. monocytogenes is able to spread beyond EVT it can find a more hospitable environment. Our results elucidate a novel aspect of the maternal-fetal barrier.

Infection of the placenta and fetus is an important cause of pregnancy complications and fetal and neonatal morbidity and mortality. Listeria monocytogenes is an intracellular bacterial pathogen that causes pregnancy-related infections in humans. The pathogenesis of listeriosis during pregnancy is poorly understood. We have previously shown that transmission of L. monocytogenes from maternal cells and tissues to fetal cells occurs in the uterine implantation site, and that a small subpopulation of specialized fetal cells called extravillous trophoblasts are the preferred initial site of infection. Here we use primary human placental organ and cell culture systems to characterize the intracellular fate of L. monocytogenes in extravillous trophoblasts. We found that these cells entrap bacteria in vacuolar compartments where they are degraded and therefore reduce bacterial dissemination into deeper structures of the placenta. Our study provides new insights into the nature of the maternal-fetal barrier. Extravillous trophoblasts that are accessible to infection with intracellular pathogens from infected maternal cells have host defense mechanisms that constitute a bottleneck in maternal-fetal transmission.