Listeriolysin o is strongly immunogenic independently of its cytotoxic activity

Decoupling immunomodulatory properties from lipid binding in the α-pore-forming toxin Sticholysin II

Sticholysin II (StII), a pore-forming toxin from the marine anemone Stichodactyla helianthus, enhances an antigen-specific cytotoxic T lymphocyte (CTL) response when co-encapsulated in liposomes with a model antigen. This capacity does not depend exclusively on its pore-forming activity and is partially supported by its ability to activate Toll-like receptor 4 (TLR4) in dendritic cells, presumably by interacting with this receptor or by triggering signaling cascades upon binding to lipid membrane. In order to investigate whether the lipid binding capacity of StII is required for immunomodulation, we designed a mutant in which the aromatic amino acids from the interfacial binding site Trp110, Tyr111 and Trp114 were substituted by Ala. In the present work, we demonstrated that StII3A keeps the secondary structure composition and global folding of StII, while it loses its lipid binding and permeabilization abilities. Despite this, StII3A upregulates dendritic cells maturation markers, enhances an antigen-specific effector CD8+ T cells response and confers antitumor protection in a preventive scenario in C57BL/6 mice. Our results indicate that a mechanism independent of its lipid binding ability is involved in the immunomodulatory capacity of StII, pointing to StII3A as a promising candidate to improve the reliability of the Sts-based vaccine platform.

Story of Pore-Forming Proteins from Deadly Disease-Causing Agents to Modern Applications with Evolutionary Significance

Animal venoms are a complex mixture of highly specialized toxic molecules. Among them, pore-forming proteins (PFPs) or toxins (PFTs) are one of the major disease-causing toxic elements. The ability of the PFPs in defense and toxicity through pore formation on the host cell surface makes them unique among the toxin proteins. These features made them attractive for academic and research purposes for years in the areas of microbiology as well as structural biology. All the PFPs share a common mechanism of action for the attack of host cells and pore formation in which the selected pore-forming motifs of the host cell membrane-bound protein molecules drive to the lipid bilayer of the cell membrane and eventually produces water-filled pores. But surprisingly their sequence similarity is very poor. Their existence can be seen both in a soluble state and also in transmembrane complexes in the cell membrane. PFPs are prevalent toxic factors that are predominately produced by all kingdoms of life such as virulence bacteria, nematodes, fungi, protozoan parasites, frogs, plants, and also from higher organisms. Nowadays, multiple approaches to applications of PFPs have been conducted by researchers both in basic as well as applied biological research. Although PFPs are very devastating for human health nowadays researchers have been successful in making these toxic proteins into therapeutics through the preparation of immunotoxins. We have discussed the structural, and functional mechanism of action, evolutionary significance through dendrogram, domain organization, and practical applications for various approaches. This review aims to emphasize the PFTs to summarize toxic proteins together for basic knowledge as well as to highlight the current challenges, and literature gap along with the perspective of promising biotechnological applications for their future research.

Get out or die trying: Peptide- and protein-based endosomal escape of RNA therapeutics

RNA therapeutics offer great potential to transform the biomedical landscape, encompassing the treatment of hereditary conditions and the development of better vaccines. However, the delivery of RNAs into the cell is hampered, among others, by poor endosomal escape. This major hurdle is often tackled using special lipids, polymers, or protein-based delivery vectors. In this review, we will focus on the most prominent peptide- and protein-based endosomal escape strategies with focus on RNA drugs. We discuss cell penetrating peptides, which are still incorporated into novel transfection systems today to promote endosomal escape. However, direct evidence for enhanced endosomal escape by the action of such peptides is missing and their transfection efficiency, even in permissive cell culture conditions, is rather low. Endosomal escape by the help of pore forming proteins or phospholipases, on the other hand, allowed to generate more efficient transfection systems. These are, however, often hampered by considerable toxicity and immunogenicity. We conclude that the perfect enhancer of endosomal escape has yet to be devised. To increase the chances of success, any new transfection system should be tested under relevant conditions and guided by assays that allow direct quantification of endosomal escape.

Recent advances in microbial toxin-related strategies to combat cancer

It is a major concern to treat cancer successfully, due to the distinctive pathophysiology of cancer cells and the gradual manifestation of resistance. Specific action, adverse effects and development of resistance has prompted the urgent requirement of exploring alternative anti-tumour treatment therapies. The naturally derived microbial toxins as a therapy against cancer cells are a promisingly new dimension. Various important microbial toxins such as Diphtheria toxin, Vibrio cholera toxin, Aflatoxin, Patulin, Cryptophycin-55, Chlorella are derived from several bacterial, fungal and algal species. These agents act on different biotargets such as inhibition of protein synthesis, reduction in cell growth, regulation of cell cycle and many cellular processes. Bacterial toxins produce actions primarily by targeting protein moieties and some immunomodulation and few acts through DNA. Fungal toxins appear to have more DNA damaging activity and affect the cell cycle. Algal toxins produce alteration in mitochondrial phosphorylation. In conclusion, microbial toxins and their metabolites appear to have a great potential to provide a promising option for the treatment and management to combat cancer.

Recombinant Listeria ivanovii strain expressing listeriolysin O in place of ivanolysin O might be a potential antigen carrier for vaccine construction

Listeria monocytogenes (LM) induces efficient and specific T-cell immune responses in the host. Listeriolysin O (LLO) is the main virulence protein of LM. LLO helps LM escape from the lysosome. However, the pronounced pathogenicity of LM limits its practical application as a live bacterial vector. Listeria ivanovii (LI) also displays intracellular parasitic abilities, cell to cell transfer, and other LM properties, with an elevated biosafety relative to LM. We have confirmed that LI can be used as a viable bacterial vaccine vector. However, we have also observed in vivo that LI vector vaccine candidates survive in the immune organ (spleen) for a shorter time compared with the survival time of LM and elicit weaker immune responses compared with LM. Studies have confirmed that hemolysin correlates with some important biological properties of Listeria, including cell invasion, intracellular proliferation, and the ability to induce immune responses. We speculated that the weaker immunogenicity of LI compared to LM may be related to the function of ivanolysin O (ILO). Here, we established a hemolysin gene deletion strain, LIΔilo, and a modified strain, LIΔilo:hly, whose ilo was replaced by hly. The hemolysin-modified strain was attenuated; however, it led to significantly improved invasive and proliferative activities of antigen-presenting cells, including those of RAW 264.7 macrophages, compared with the effects of LI. Mice immunized twice with LIΔilo:hly showed higher cytokine levels and better challenge protection rates than LI-immunized mice. This is the first description in Listeria carrier vaccine research of the modification of LI hemolysin to obtain a better vaccine carrier than LI. The recombinant strain LIΔilo:hly showed good biosafety and immunogenicity, and thus appears to be a good vector strain for vaccine development.

Listeria toxin promotes phosphorylation of the inflammasome adaptor ASC through Lyn and Syk to exacerbate pathogen expansion

Inflammasome activation exacerbates infectious disease caused by pathogens such as Listeria monocytogenes, Staphylococcus aureus, and severe acute respiratory syndrome coronavirus 2. Although these pathogens activate host inflammasomes to regulate pathogen expansion, the mechanisms by which pathogen toxins contribute to inflammasome activation remain poorly understood. Here we show that activation of inflammasomes by Listeria infection is promoted by amino acid residue T223 of listeriolysin O (LLO) independently of its pore-forming activity. LLO T223 is critical for phosphorylation of the inflammasome adaptor ASC at amino acid residue Y144 through Lyn-Syk signaling, which is essential for ASC oligomerization. Notably, a Listeria mutant expressing LLO T223A is impaired in inducing ASC phosphorylation and inflammasome activation. Furthermore, the virulence of LLO T223A mutant is markedly attenuated in vivo due to impaired ability to activate the inflammasome. Our results reveal a function of a pathogen toxin that exacerbates infection by promoting phosphorylation of ASC.

Sticholysin II shows similar immunostimulatory properties to LLO stimulating dendritic cells and MHC-I restricted T cell responses of heterologous antigen

Induction of CD8⁺ T cell responses against tumor cells and intracellular pathogens is an important goal of modern vaccinology. One approach of translational interest is the use of liposomes encapsulating pore-forming proteins (PFPs), such as Listeriolysin O (LLO), which has shown efficacy at priming strong and sustained CD8⁺ T cell responses. Recently, we have demonstrated that Sticholysin II (StII), a PFP from the sea anemone Stichodactyla helianthus, co-encapsulated into liposomes with ovalbumin (OVA) was able to stimulate, antigen presenting cells, antigen-specific CD8⁺ T cells and anti-tumor activity in mice. In the present study, we aimed to compare StII and LLO in terms of their abilities to stimulate dendritic cells and to induce major histocompatibility complex (MHC) class I restricted T cell responses against OVA. Interestingly, StII exhibited similar abilities to LLO in vitro of inducing dendritic cells maturation, as measured by increased expression of CD40, CD80, CD86 and MHC-class II molecules, and of stimulating OVA cross-presentation to a CD8⁺ T cell line. Remarkably, using an ex vivo Enzyme-Linked ImmunoSpot Assay (ELISPOT) to monitor gamma interferon (INF-γ) producing effector memory CD8⁺ T cells, liposomal formulations containing either StII or LLO induced comparable frequencies of OVA-specific INF-γ producing CD8⁺ T cells in mice that were sustained in time. However, StII-containing liposomes stimulated antigen-specific memory CD8⁺ T cells with a higher potential to secrete IFN-γ than liposomes encapsulating LLO. This StII immunostimulatory property further supports its use for the rational design of T cell vaccines against cancers and intracellular pathogens. In summary, this study indicates that StII has immunostimulatory properties similar to LLO, despite being evolutionarily distant PFPs.

Mitofusin 2 in Macrophages Links Mitochondrial ROS Production, Cytokine Release, Phagocytosis, Autophagy, and Bactericidal Activity

Mitofusin 2 (Mfn2) plays a major role in mitochondrial fusion and in the maintenance of mitochondria-endoplasmic reticulum contact sites. Given that macrophages play a major role in inflammation, we studied the contribution of Mfn2 to the activity of these cells. Pro-inflammatory stimuli such as lipopolysaccharide (LPS) induced Mfn2 expression. The use of the Mfn2 and Mfn1 myeloid-conditional knockout (KO) mouse models reveals that Mfn2 but not Mfn1 is required for the adaptation of mitochondrial respiration to stress conditions and for the production of reactive oxygen species (ROS) upon pro-inflammatory activation. Mfn2 deficiency specifically impairs the production of pro-inflammatory cytokines and nitric oxide. In addition, the lack of Mfn2 but not Mfn1 is associated with dysfunctional autophagy, apoptosis, phagocytosis, and antigen processing. Mfn2^{floxed;CreLysM} mice fail to be protected from Listeria, Mycobacterium tuberculosis, or LPS endotoxemia. These results reveal an unexpected contribution of Mfn2 to ROS production and inflammation in macrophages.

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.

Interaction of Macrophages and Cholesterol-Dependent Cytolysins: The Impact on Immune Response and Cellular Survival

Cholesterol-dependent cytolysins (CDCs) are key virulence factors involved in many lethal bacterial infections, including pneumonia, necrotizing soft tissue infections, bacterial meningitis, and miscarriage. Host responses to these diseases involve myeloid cells, especially macrophages. Macrophages use several systems to detect and respond to cholesterol-dependent cytolysins, including membrane repair, mitogen-activated protein (MAP) kinase signaling, phagocytosis, cytokine production, and activation of the adaptive immune system. However, CDCs also promote immune evasion by silencing and/or destroying myeloid cells. While there are many common themes between the various CDCs, each CDC also possesses specific features to optimally benefit the pathogen producing it. This review highlights host responses to CDC pathogenesis with a focus on macrophages. Due to their robust plasticity, macrophages play key roles in the outcome of bacterial infections. Understanding the unique features and differences within the common theme of CDCs bolsters new tools for research and therapy.

Listeriolysin O immunogenetic adjuvant enhanced potency of hepatitis C virus NS3 DNA vaccine

Hepatitis C virus (HCV) is a major health problem all over the world. Among HCV proteins, nonstructural protein 3 (NS3) is one of the most promising target for anti-HCV therapy and a candidate for vaccine design. DNA vaccine is an efficient approach to stimulate antigen-specific immunity but the main problem with that is less immunogenic efficiency in comparison with traditional vaccines. Several approaches have been applied to enhance the immunogenicity of DNA. Recently, bacteria-derived substances are considered as one of the most attractive adjuvants for vaccines, which among them, Listeriolysin O (LLO) of Listeria monocytogenes is a toxin with an extremely immunogenic feature. We investigated detoxified form of LLO gene as genetic adjuvant to modulate NS3 DNA vaccine potency. Immunogenic truncated NS3 gene sequence of HCV (1095-1380aa) and detoxified LLO gene region (5-441aa) were amplified by PCR and cloned into the pcDNA3.1 plasmid separately. The expression of recombinant proteins (pc-NS3, pLLO) was confirmed in HEK293T cell line by western blotting. BALB/c mice models received three doses of different formula of plasmids in two-week intervals and two weeks after the final immunization, the immune responses were evaluated by specific total antibody level, lymphocyte proliferation, cytotoxicity, and cytokine levels assays. To evaluate in vivo cytotoxic activity, tumor challenge was performed. The recombinant plasmids were successfully expressed in mammalian cell line, and coadministration of pc-NS3 with pLLO induced the highest titer of total IgG against NS3 antigen compared with other controls. Determination of IgG subclasses confirmed the efficient increase in mixed responses with Th1 dominancy. Furthermore, significant levels of cytokines (p < .05) and lymphocyte proliferation responses (p < .05) indicated the superiority of this regimen. The findings may have important implication for LLO gene application as genetic adjuvant in immune response against HCV.

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.

High-throughput Measurement of Plasma Membrane Resealing Efficiency in Mammalian Cells

In their physiological environment, mammalian cells are often subjected to mechanical and biochemical stresses that result in plasma membrane damage. In response to these damages, complex molecular machineries rapidly reseal the plasma membrane to restore its barrier function and maintain cell survival. Despite 60 years of research in this field, we still lack a thorough understanding of the cell resealing machinery. With the goal of identifying cellular components that control plasma membrane resealing or drugs that can improve resealing, we have developed a fluorescence-based high-throughput assay that measures the plasma membrane resealing efficiency in mammalian cells cultured in microplates. As a model system for plasma membrane damage, cells are exposed to the bacterial pore-forming toxin listeriolysin O (LLO), which forms large 30-50 nm diameter proteinaceous pores in cholesterol-containing membranes. The use of a temperature-controlled multi-mode microplate reader allows for rapid and sensitive spectrofluorometric measurements in combination with brightfield and fluorescence microscopy imaging of living cells. Kinetic analysis of the fluorescence intensity emitted by a membrane impermeant nucleic acid-binding fluorochrome reflects the extent of membrane wounding and resealing at the cell population level, allowing for the calculation of the cell resealing efficiency. Fluorescence microscopy imaging allows for the enumeration of cells, which constitutively express a fluorescent chimera of the nuclear protein histone 2B, in each well of the microplate to account for potential variations in their number and allows for eventual identification of distinct cell populations. This high-throughput assay is a powerful tool expected to expand our understanding of membrane repair mechanisms via screening for host genes or exogenously added compounds that control plasma membrane resealing.

Pharmacological Targeting of Pore-Forming Toxins as Adjunctive Therapy for Invasive Bacterial Infection

For many of the most important human bacterial infections, invasive disease severity is fueled by the cell damaging and pro-inflammatory effects of secreted pore-forming toxins (PFTs). Isogenic PFT-knockout mutants, e.g., Staphylococcus aureus lacking α-toxin or Streptococcus pneumoniae deficient in pneumolysin, show attenuation in animal infection models. This knowledge has inspired multi-model investigations of strategies to neutralize PFTs or counteract their toxicity as a novel pharmacological approach to ameliorate disease pathogenesis in clinical disease. Promising examples of small molecule, antibody or nanotherapeutic drug candidates that directly bind and neutralize PFTs, block their oligomerization or membrane receptor interactions, plug establishment membrane pores, or boost host cell resiliency to withstand PFT action have emerged. The present review highlights these new concepts, with a special focus on β-PFTs produced by leading invasive human Gram-positive bacterial pathogens. Such anti-virulence therapies could be applied as an adjunctive therapy to antibiotic-sensitive and -resistant strains alike, and further could be free of deleterious effects that deplete the normal microflora.

Pre-clinical development of Listeria-based nanovaccines as immunotherapies for solid tumours: insights from melanoma

Gold glyconanoparticles loaded with the listeriolysin O peptide 91-99 (GNP-LLO91-99), a bacterial peptide with anti-metastatic properties, are vaccine delivery platforms facilitating immune cell targeting and increasing antigen loading. Here, we present proof of concept analyses for the consideration of GNP-LLO91-99 nanovaccines as a novel immunotherapy for cutaneous melanoma. Studies using mouse models of subcutaneous melanoma indicated that GNP-LLO91-99 nanovaccines recruite and modulate dendritic cell (DC) function within the tumour, alter tumour immunotolerance inducing melanoma-specific cytotoxic T cells, cause complete remission and improve survival. GNP-LLO91-99 nanovaccines showed superior tumour regression and survival benefits, when combined with anti-PD-1 or anti-CTLA-4 checkpoint inhibitors, resulting in an improvement in the efficacy of these immunotherapies. Studies on monocyte-derived DCs from patients with stage IA, IB or IIIB melanoma confirmed the ability of GNP-LLO91-99 nanovaccines to complement the action of checkpoint inhibitors, by not only reducing the expression of cell-death markers on DCs, but also potentiating DC antigen-presentation. We propose that GNP-LLO91-99 nanovaccines function as immune stimulators and immune effectors and serve as safe cancer therapies, alone or in combination with other immunotherapies.

Recombinant Listeria promotes tumor rejection by CD8+ T cell-dependent remodeling of the tumor microenvironment

Agents that remodel the tumor microenvironment (TME), prime functional tumor-specific T cells, and block inhibitory signaling pathways are essential components of effective immunotherapy. We are evaluating live-attenuated, double-deleted Listeria monocytogenes expressing tumor antigens (LADD-Ag) in the clinic. Here we show in numerous mouse models that while treatment with nonrecombinant LADD induced some changes in the TME, no antitumor efficacy was observed, even when combined with immune checkpoint blockade. In contrast, LADD-Ag promoted tumor rejection by priming tumor-specific KLRG1⁺PD1^loCD62L^- CD8⁺ T cells. These IFNγ-producing effector CD8⁺ T cells infiltrated the tumor and converted the tumor from an immunosuppressive to an inflamed microenvironment that was characterized by a decrease in regulatory T cells (Treg) levels, a proinflammatory cytokine milieu, and the shift of M2 macrophages to an inducible nitric oxide synthase (iNOS)⁺CD206^- M1 phenotype. Remarkably, these LADD-Ag-induced tumor-specific T cells persisted for more than 2 months after primary tumor challenge and rapidly controlled secondary tumor challenge. Our results indicate that the striking antitumor efficacy observed in mice with LADD-based immunotherapy stems from TME remodeling which is a direct consequence of eliciting potent, systemic tumor-specific CD8⁺ T cells.

Listeriolysin O: from bazooka to Swiss army knife

Listeria monocytogenes (Lm) is a Gram-positive facultative intracellular pathogen. Infections in humans can lead to listeriosis, a systemic disease with a high mortality rate. One important mechanism of Lm dissemination involves cell-to-cell spread after bacteria have entered the cytosol of host cells. Listeriolysin O (LLO; encoded by the hly gene) is a virulence factor present in Lm that plays a central role in the cell-to-cell spread process. LLO is a member of the cholesterol-dependent cytolysin (CDC) family of toxins that were initially thought to promote disease largely by inducing cell death and tissue destruction-essentially acting like a 'bazooka'. This view was supported by structural studies showing CDCs can form large pores in membranes. However, it is now appreciated that LLO has many subtle activities during Lm infection of host cells, and many of these likely do not involve large pores, but rather small membrane perforations. It is also appreciated that membrane repair pathways of host cells play a major role in limiting membrane damage by LLO and other toxins. LLO is now thought to represent a 'Swiss army knife', a versatile tool that allows Lm to induce many membrane alterations and cellular responses that promote bacterial dissemination during infection.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

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

Type I interferons (IFNs) induce a detrimental response during Listeria monocytogenes (L. monocytogenes) infection. We were interested in identifying mechanisms linking IFN signaling to negative host responses against L. monocytogenes infection. Herein, we found that infection of myeloid cells with L. monocytogenes led to a coordinated induction of type I IFNs and activation of the integrated stress response (ISR). Infected cells did not induce Xbp1 splicing or BiP upregulation, indicating that the unfolded protein response was not triggered. CHOP (Ddit3) gene expression was upregulated during the ISR activation induced by L. monocytogenes. Myeloid cells deficient in either type I IFN signaling or PKR activation had less upregulation of CHOP following infection. CHOP‐deficient mice showed lower expression of innate immune cytokines and were more resistant than wild‐type counterparts following L. monocytogenes infection. These findings indicate that L. monocytogenes infection induces type I IFNs, which activate the ISR through PKR, which contributes to a detrimental outcome in the infected host.

Evaluation of the safety and adjuvant effect of a detoxified listeriolysin O mutant on the humoral response to dengue virus antigens

Listeriolysin O (LLO) has been proposed as a potential carrier or adjuvant molecule in the vaccination field. However, the cytotoxic and pro-apoptotic effects of LLO are the major limitations for this purpose. Here, we have performed a preclinical safety evaluation and characterized a new potential adjuvant application for a non-cytolytic LLO mutant (dtLLO) to enhance and modulate the immune response against the envelope (E) protein from dengue virus. In addition, we have studied the adjuvant effects of dtLLO on human immune cells and the role of membrane cholesterol for the binding and proinflammatory property of the toxoid. Our in-vivo results in the murine model confirmed that dtLLO is a safer molecule than wild-type LLO (wtLLO), with a significantly increased survival rate for mice challenged with dtLLO compared with mice challenged with wtLLO (P < 0·001). Histopathological analysis showed non-toxic effects in key target organs such as brain, heart, liver, spleen, kidney and lung after challenge with dtLLO. In vitro, dtLLO retained the capacity of binding to plasma membrane cholesterol on the surface of murine and human immune cells. Immunization of 6-8-week-old female BALB/c mice with a combination of dtLLO mixed with E protein elicited a robust specific humoral response with isotype diversification of immunoglobulin (Ig)G antibodies (IgG1 and IgG2a). Finally, we demonstrated that cholesterol and lipid raft integrity are required to induce a proinflammatory response by human cells. Taken together, these findings support a potential use of the dtLLO mutant as a safe and effective adjuvant molecule in vaccination.

Liposomal Coencapsulation of Doxorubicin with Listeriolysin O Increases Potency via Subcellular Targeting

Liposomal doxorubicin is a clinically important drug formulation indicated for the treatment of several different forms of cancer. For doxorubicin to exert a therapeutic effect, it must gain access to the nucleus. However, a large proportion of the liposomal doxorubicin dose fails to work because it is sequestered within endolysosomal organelles following endocytosis of the liposomes due to the phenomenon of ion trapping. Listeriolysin O (LLO) is a pore-forming protein that can provide a mechanism for endosomal escape. The present study demonstrates that liposomal coencapsulation of doxorubicin with LLO enables a significantly larger percentage of the dose to colocalize with the nucleus compared to liposomes containing doxorubicin alone. The change in intracellular distribution resulted in a significantly more potent formulation of liposomal doxorubicin as demonstrated in both the ovarian carcinoma cell line A2780 and its doxorubicin-resistant derivative A2780ADR.

The pancreas anatomy conditions the origin and properties of resident macrophages

Calderon et al. define the origin, turnover, and functional characteristics of pancreatic macrophages at both the exocrine and endocrine sites under noninflammatory conditions.

We examine the features, origin, turnover, and gene expression of pancreatic macrophages under steady state. The data distinguish macrophages within distinct intrapancreatic microenvironments and suggest how macrophage phenotype is imprinted by the local milieu. Macrophages in islets of Langerhans and in the interacinar stroma are distinct in origin and phenotypic properties. In islets, macrophages are the only myeloid cells: they derive from definitive hematopoiesis, exchange to a minimum with blood cells, have a low level of self-replication, and depend on CSF-1. They express Il1b and Tnfa transcripts, indicating classical activation, M1, under steady state. The interacinar stroma contains two macrophage subsets. One is derived from primitive hematopoiesis, with no interchange by blood cells and alternative, M2, activation profile, whereas the second is derived from definitive hematopoiesis and exchanges with circulating myeloid cells but also shows an alternative activation profile. Complete replacement of islet and stromal macrophages by donor stem cells occurred after lethal irradiation with identical profiles as observed under steady state. The extraordinary plasticity of macrophages within the pancreatic organ and the distinct features imprinted by their anatomical localization sets the base for examining these cells in pathological conditions.

Listeriolysin O mediates cytotoxicity against human brain microvascular endothelial cells

Penetration of the brain microvascular endothelial layer is one of the routes Listeria monocytogenes use to breach the blood-brain barrier. Because host factors in the blood severely limit direct invasion of human brain microvascular endothelial cells (HBMECs) by L. monocytogenes, alternative mechanisms might be used by this bacterium to penetrate the endothelial cell layer. In this study, we evaluated the cytotoxicity of proteins secreted by L. monocytogenes against HBEMCs using a live/dead staining method. Interestingly, the integrity of the plasma membrane of HBMECs was impaired by proteins secreted by the EGD wild-type strain but not proteins secreted by the isogenic ΔprfA strain. Therefore, we investigated the cytotoxicity of proteins secreted by several isogenic mutant strains (ΔplcA, Δmpl and Δhly) incapable of producing the prfA-regulated bacterial products PlcA, Mpl and LLO, respectively. Results from both fluorescent microscopy and flow cytometry analyses showed that proteins secreted by the Δhly strain were not cytotoxic to HBMECs, whereas those secreted by the ΔplcA and Δmpl strains were cytotoxic. These results suggest that LLO-mediated cytotoxicity against brain microvascular endothelial cells enables L. monocytogenes to effectively penetrate the brain microvascular endothelial layer.

Plasticity of listeriolysin O pores and its regulation by pH and unique histidine [corrected]

Pore formation of cellular membranes is an ancient mechanism of bacterial pathogenesis that allows efficient damaging of target cells. Several mechanisms have been described, however, relatively little is known about the assembly and properties of pores. Listeriolysin O (LLO) is a pH-regulated cholesterol-dependent cytolysin from the intracellular pathogen Listeria monocytogenes, which forms transmembrane β-barrel pores. Here we report that the assembly of LLO pores is rapid and efficient irrespective of pH. While pore diameters at the membrane surface are comparable at either pH 5.5 or 7.4, the distribution of pore conductances is significantly pH-dependent. This is directed by the unique residue H311, which is also important for the conformational stability of the LLO monomer and the rate of pore formation. The functional pores exhibit variations in height profiles and can reconfigure significantly by merging to other full pores or arcs. Our results indicate significant plasticity of large β-barrel pores, controlled by environmental cues like pH.

Biological effects of listeriolysin O: implications for vaccination

Listeriolysin O (LLO) is a thiol-activated cholesterol-dependent pore-forming toxin and the major virulence factor of Listeria monocytogenes (LM). Extensive research in recent years has revealed that LLO exerts a wide array of biological activities, during the infection by LM or by itself as recombinant antigen. The spectrum of biological activities induced by LLO includes cytotoxicity, apoptosis induction, endoplasmic reticulum stress response, modulation of gene expression, intracellular calcium oscillations, and proinflammatory activity. In addition, LLO is a highly immunogenic toxin and the major target for innate and adaptive immune responses in different animal models and humans. Recently, the crystal structure of LLO has been published in detail. Here, we review the structure-function relationship for this fascinating microbial molecule, highlighting the potential uses of LLO in the fields of biomedicine and biotechnology, particularly in vaccination.

Is the exoproteome important for bacterial pathogenesis? Lessons learned from interstrain exoprotein diversity in Listeria monocytogenes grown at different temperatures

Bacterial exoproteomes vary in composition and quantity among species and within each species, depending on the environmental conditions to which the cells are exposed. This article critically reviews the literature available on exoproteins synthesized by the foodborne pathogenic bacterium Listeria monocytogenes grown at different temperatures. The main challenges posed for exoproteome analyses and the strategies that are being used to overcome these constraints are discussed. Over thirty exoproteins from L. monocytogenes are considered, and the multifunctionality of some of them is discussed. Thus, at the host temperature of 37°C, good examples are provided by Lmo0443, a potential marker for low virulence, and by the virulence factors internalin C (InlC) and listeriolysin O (LLO). Based on the reported LLO-induced mucin exocytosis, a model is proposed for the involvement of extracellular LLO in optimizing the conditions for InlC intervention in the invasion of intestinal epithelial cells. At lower growth temperatures, exoproteins such as flagellin (FlaA) and oligopeptide permease (OppA) may explain the persistence of particular strains in the food industry environment, eventually allowing the development of new tools to eradicate L. monocytogenes, a major concern for public health.

Embryonic and adult-derived resident cardiac macrophages are maintained through distinct mechanisms at steady state and during inflammation

Cardiac macrophages are crucial for tissue repair after cardiac injury but are not well characterized. Here we identify four populations of cardiac macrophages. At steady state, resident macrophages were primarily maintained through local proliferation. However, after macrophage depletion or during cardiac inflammation, Ly6c(hi) monocytes contributed to all four macrophage populations, whereas resident macrophages also expanded numerically through proliferation. Genetic fate mapping revealed that yolk-sac and fetal monocyte progenitors gave rise to the majority of cardiac macrophages, and the heart was among a minority of organs in which substantial numbers of yolk-sac macrophages persisted in adulthood. CCR2 expression and dependence distinguished cardiac macrophages of adult monocyte versus embryonic origin. Transcriptional and functional data revealed that monocyte-derived macrophages coordinate cardiac inflammation, while playing redundant but lesser roles in antigen sampling and efferocytosis. These data highlight the presence of multiple cardiac macrophage subsets, with different functions, origins, and strategies to regulate compartment size.

Apolipoprotein E mediation of neuro-inflammation in a murine model of multiple sclerosis

Apolipoprotein E (ApoE) functions as a ligand in receptor-mediated endocytosis of lipoprotein particles and has been demonstrated to play a role in antigen presentation. To explore the contribution of ApoE during autoimmune central nervous system (CNS) demyelination, we examined the clinical, cellular immune function, and pathologic consequences of experimental autoimmune encephalomyelitis (EAE) induction in ApoE knockout (ApoE(-/-)) mice. We observed reduced clinical severity of EAE in ApoE(-/-) mice in comparison to WT mice that was concomitant with an early reduction of dendritic cells (DCs) followed by a reduction of additional innate cells in the spinal cord at the peak of disease without any differences in axonal damage. While T cell priming was enhanced in ApoE(-/-) mice, reduced severity of EAE was also observed in ApoE(-/-) recipients of encephalitogenic wild type T cells. Expression of ApoE during EAE was elevated within the CNS of wild type mice, particularly by innate cells such as DCs. Overall, ApoE promotes clinical EAE, likely by mediation of inflammation localized within the CNS.

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.

Targeted cytolysins synergistically potentiate cytoplasmic delivery of gelonin immunotoxin

Targeted endocytic uptake is a first step toward tissue-specific cytoplasmic macromolecular delivery; however, inefficient escape from the endolysosomal compartment makes this generally impractical at present. We report here a targeted cytolysin approach that dramatically potentiates endosomal release of an independently targeted potent gelonin immunotoxin. Fibronectin domains engineered for affinity to EGF receptor or carcinoembryonic antigen were fused to the plant toxin gelonin or bacterial pore-forming cytolysins. These fusion proteins display synergistic activity in both antigen-specific cytotoxicity in vitro, enhancing potency by several orders of magnitude, and in tumor growth inhibition in vivo. In addition, the number of internalized gelonin molecules required to induce apoptosis is reduced from approximately 5 × 10(6) to less than 10(3). Targeted potentiation shows promise for enhancing cytoplasmic delivery of other macromolecular payloads such as DNA, siRNA, and miRNA.

Cutting edge: Conditional MHC class II expression reveals a limited role for B cell antigen presentation in primary and secondary CD4 T cell responses

The activation, differentiation, and subsequent effector functions of CD4 T cells depend on interactions with a multitude of MHC class II (MHCII)-expressing APCs. To evaluate the individual contribution of various APCs to CD4 T cell function, we have designed a new murine tool for selective in vivo expression of MHCII in subsets of APCs. Conditional expression of MHCII in B cells was achieved using a cre-loxP approach. After i.v. or s.c. priming, partial proliferation and activation of CD4 T cells was observed in mice expressing MHCII only by B cells. Restricting MHCII expression to B cells constrained secondary CD4 T cell responses in vivo, as demonstrated in a CD4 T cell-dependent model of autoimmunity, experimental autoimmune encephalomyelitis. These results highlight the limitations of B cell Ag presentation during initiation and propagation of CD4 T cell function in vivo using a novel system to study individual APCs by the conditional expression of MHCII.

Role of pore-forming toxins in bacterial infectious diseases

Pore-forming toxins (PFTs) are the most common bacterial cytotoxic proteins and are required for virulence in a large number of important pathogens, including Streptococcus pneumoniae, group A and B streptococci, Staphylococcus aureus, Escherichia coli, and Mycobacterium tuberculosis. PFTs generally disrupt host cell membranes, but they can have additional effects independent of pore formation. Substantial effort has been devoted to understanding the molecular mechanisms underlying the functions of certain model PFTs. Likewise, specific host pathways mediating survival and immune responses in the face of toxin-mediated cellular damage have been delineated. However, less is known about the overall functions of PFTs during infection in vivo. This review focuses on common themes in the area of PFT biology, with an emphasis on studies addressing the roles of PFTs in in vivo and ex vivo models of colonization or infection. Common functions of PFTs include disruption of epithelial barrier function and evasion of host immune responses, which contribute to bacterial growth and spreading. The widespread nature of PFTs make this group of toxins an attractive target for the development of new virulence-targeted therapies that may have broad activity against human pathogens.

Listeriolysin O as a strong immunogenic molecule for the development of new anti-tumor vaccines

The pore-forming toxin listeriolysin O (LLO), which is produced by Listeria monocytogenes, mediates bacterial phagosomal escape and facilitates bacterial multiplication during infection. This toxin has recently gained attention because of its confirmed role in the controlled and specific modulation of the immune response. Currently, cancer immunotherapies are focused on conquering the immune tolerance induced by poorly immunogenic tumor antigens and eliciting strong, lasting immunological memory. An effective way to achieve these goals is the co-administration of potent immunomodulatory adjuvant components with vaccine vectors. LLO, a toxin that belongs to the family of cholesterol-dependent cytolysins (CDCs), exhibits potent cell type-non-specific toxicity and is a source of dominant CD4(+) and CD8(+) T cell epitopes. According to recent research, in addition to its effective cytotoxicity as a cancer immunotherapeutic drug, the non-specific adjuvant property of LLO makes it promising for the development of efficacious anti-tumor vaccines.

More than a pore: the cellular response to cholesterol-dependent cytolysins

Targeted disruption of the plasma membrane is a ubiquitous form of attack used in all three domains of life. Many bacteria secrete pore-forming proteins during infection with broad implications for pathogenesis. The cholesterol-dependent cytolysins (CDC) are a family of pore-forming toxins expressed predominately by Gram-positive bacterial pathogens. The structure and assembly of some of these oligomeric toxins on the host membrane have been described, but how the targeted cell responds to intoxication by the CDCs is not as clearly understood. Many CDCs induce lysis of their target cell and can activate apoptotic cascades to promote cell death. However, the extent to which intoxication causes cell death is both CDC- and host cell-dependent, and at lower concentrations of toxin, survival of intoxicated host cells is well documented. Additionally, the effect of CDCs can be seen beyond the plasma membrane, and it is becoming increasingly clear that these toxins are potent regulators of signaling and immunity, beyond their role in intoxication. In this review, we discuss the cellular response to CDC intoxication with emphasis on the effects of pore formation on the host cell plasma membrane and subcellular organelles and whether subsequent cellular responses contribute to the survival of the affected cell.

Listeriolysin O: the Swiss army knife of Listeria

Listeriolysin O (LLO) is a toxin produced by Listeria monocytogenes, an opportunistic bacterial pathogen responsible for the disease listeriosis. This disease starts with the ingestion of contaminated foods and mainly affects immunocompromised individuals, newborns, and pregnant women. In the laboratory, L. monocytogenes is used as a model organism to study processes such as cell invasion, intracellular survival, and cell-to-cell spreading, as this Gram-positive bacterium has evolved elaborate molecular strategies to subvert host cell functions. LLO is a major virulence factor originally shown to be crucial for bacterial escape from the internalization vacuole after entry into cells. However, recent studies are revisiting the role of LLO during infection and are revealing new insights into the action of LLO, in particular before bacterial entry. These latest findings along with their impact on the infectious process will be discussed.