A Combined Use of Autolysin p60 and Listeriolysin O Antigens Induces High Protective Immune Responses Against Listeria monocytogenes Infection

Complete genome of the Listeria monocytogenes strain AUF, used as a live listeriosis veterinary vaccine

Listeria monocytogenes (Lm) is a highly pathogenic bacterium that can cause listeriosis, a relatively rare food-borne infectious disease that affects farm, domestic, wild animals and humans as well. The infected livestock is the frequent sources of Lm. Vaccination is one of the methods of controlling listeriosis in target farm animals to prevent Lm-associated food contamination. Here we report the complete sequence of the Lm strain AUF attenuated from a fully-virulent Lm strain by ultraviolet irradiation, successfully used since the 1960s as a live whole-cell veterinary vaccine. The de novo assembled genome consists of a circular chromosome of 2,942,932 bp length, including more than 2,800 CDSs, 17 pseudogenes, 5 antibiotic resistance genes, and 56/92 virulence genes. Two wild Lm strains, the EGD and the 10403S that is also used in cancer Immunotherapy, were the closest homologs for the Lm strain AUF. Although all three strains belonged to different sequence types (ST), namely ST12, ST85, and ST1538, they were placed in the same genetic lineage II, CC7.

Immunopeptidomics-based design of mRNA vaccine formulations against Listeria monocytogenes

Listeria monocytogenes is a foodborne intracellular bacterial pathogen leading to human listeriosis. Despite a high mortality rate and increasing antibiotic resistance no clinically approved vaccine against Listeria is available. Attenuated Listeria strains offer protection and are tested as antitumor vaccine vectors, but would benefit from a better knowledge on immunodominant vector antigens. To identify novel antigens, we screen for Listeria peptides presented on the surface of infected human cell lines by mass spectrometry-based immunopeptidomics. In between more than 15,000 human self-peptides, we detect 68 Listeria immunopeptides from 42 different bacterial proteins, including several known antigens. Peptides presented on different cell lines are often derived from the same bacterial surface proteins, classifying these antigens as potential vaccine candidates. Encoding these highly presented antigens in lipid nanoparticle mRNA vaccine formulations results in specific CD8+ T-cell responses and induces protection in vaccination challenge experiments in mice. Our results can serve as a starting point for the development of a clinical mRNA vaccine against Listeria and aid to improve attenuated Listeria vaccines and vectors, demonstrating the power of immunopeptidomics for next-generation bacterial vaccine development.

Evaluation of an attenuated Listeria monocytogenes as a vaccine vector to control Helicobacter pylori infection

The increasing resistance of Helicobacter pylori (H. pylori) to antibiotics has limited the efficacy of antibiotic therapy in the treatment of H. pylori-associated gastric diseases. The vaccine as an alternative method is becoming a safe and effective way to address this problem. In previous studies, live vector vaccines have proved to be effective in controlling H. pylori infection. Attenuated Listeria monocytogenes (L. monocytogenes) is a potential candidate vector applied in clinical trials, which can deliver foreign antigens and induce a broad immune response. To further explore the effectiveness of L. monocytogenes as a vaccine vector against H. pylori, attenuated L. monocytogenes-based vaccine EGDeΔactA/inlB(EGDeAB)-MECU was constructed to secrete a multi-epitope chimeric antigen (MECU) containing multiple B cell epitopes from H. pylori antigens. EGDeAB-MECU could secrete MECU stably. After immunized by gavage and intravenous injection, both EGDeAB and EGDeAB-MECU could significantly decrease gastric H. pylori colonization and induce a high level of specific antibodies against H. pylori. In conclusion, attenuated L. monocytogenes had an immunotherapeutic effect on H. pylori-infected mice, indicating its further development as a promising candidate vaccine vector for the H. pylori vaccine.

Listeria monocytogenes: review of pathogenesis and virulence determinants-targeted immunological assays

Listeria monocytogenes is one of the most invasive foodborne pathogens and is responsible for numerous outbreaks worldwide. Most of the methods to detect this bacterium in food require selective enrichment using traditional bacterial culture techniques that can be time-consuming and labour-intensive. Moreover, molecular methods are expensive and need specific technical knowledge. In contrast, immunological approaches are faster, simpler, and user-friendly alternatives and have been developed for the detection of L. monocytogenes in food, environmental, and clinical samples. These techniques are dependent on the constitutive expression of L. monocytogenes antigens and the specificity of the antibodies used. Here, updated knowledge on pathogenesis and the key immunogenic virulence determinants of L. monocytogenes that are used for the generation of monoclonal and polyclonal antibodies for the serological assay development are summarised. In addition, immunological approaches based on enzyme-linked immunosorbent assay, immunofluorescence, lateral flow immunochromatographic assays, and immunosensors with relevant improvements are highlighted. Though the sensitivity and specificity of the assays were improved significantly, methods still face many challenges that require further validation before use.

A Cross-Protective Vaccine Against 4b and 1/2b Listeria monocytogenes

Listeria monocytogenes (Lm) is a foodborne zoonotic pathogen that causes listeriosis with a mortality rate of 20-30%. Serovar 4b and 1/2b isolates account for most of listeriosis outbreaks, however, no listeriosis vaccine is available for either prophylactic or therapeutic use. Here, we developed a triple-virulence-genes deletion vaccine strain, and evaluated its safety, immunogenicity, and cross-protective efficiency. The virulence of NTSNΔactA/plcB/orfX was reduced 794-folds compared with the parental strain. Additionally, it was completely eliminated in mice at day 7 post infection and no obvious pathological changes were observed in the organs of mice after prime-boost immunization for 23 days. These results proved that the safety of the Lm vaccine strain remarkably increased. More importantly, the NTSNΔactA/plcB/orfX strain stimulated higher anti-Listeriolysin O (LLO) antibodies, induced significantly higher expression of IFN-γ, TNF-α, IL-17, and IL-6 than the control group, and afforded 100% protection against serovar 4b and 1/2b challenges. Taken together, our research demonstrates that the triple-genes-deletion vaccine has high safety, can elicit strong Th1 type immune response, and affords efficient cross-protection against two serovar Lm strains. It is a promising vaccine for prevention of listeriosis.

A listeriolysin O subunit vaccine is protective against Listeria monocytogenes

Listeria monocytogenes is a facultative intracellular pathogen responsible for the life-threatening disease listeriosis. The pore-forming toxin listeriolysin O (LLO) is a critical virulence factor that plays a major role in the L. monocytogenes intracellular lifecycle and is indispensable for pathogenesis. LLO is also a dominant antigen for T cells involved in sterilizing immunity and it was proposed that LLO acts as a T cell adjuvant. In this work, we generated a novel full-length LLO toxoid (LLO^T) in which the cholesterol-recognition motif, a threonine-leucine pair located at the tip of the LLO C-terminal domain, was substituted with two glycine residues. We showed that LLO^T lost its ability to bind cholesterol and to form pores. Importantly, LLO^T retained binding to the surface of epithelial cells and macrophages, suggesting that it could efficiently be captured by antigen-presenting cells. We then determined if LLO^T can be used as an antigen and adjuvant to protect mice from L. monocytogenes infection. Mice were immunized with LLO^T alone or together with cholera toxin or Alum as adjuvants. We found that mice immunized with LLO^T alone or in combination with the Th2-inducing adjuvant Alum were not protected against L. monocytogenes. On the other hand, mice immunized with LLO^T along with the experimental adjuvant cholera toxin, were protected against L. monocytogenes, as evidenced by a significant decrease in bacterial burden in the liver and spleen three days post-infection. This immunization regimen elicited mixed Th1, Th2, and Th17 responses, as well as the generation of LLO-neutralizing antibodies. Further, we identified T cells as being required for immunization-induced reductions in bacterial burden, whereas B cells were dispensable in our model of non-pregnant young mice. Overall, this work establishes that LLO^T is a promising vaccine antigen for the induction of protective immunity against L. monocytogenes by subunit vaccines containing Th1-driving adjuvants.

Characterization of the pathogenesis and immune response to Listeria monocytogenes strains isolated from a sustained national outbreak

Listeria monocytogenes is an intracellular pathogen responsible for listeriosis, a foodborne disease that can lead to life-threatening meningitis. The 2011 L. monocytogenes cantaloupe outbreak was among the deadliest foodborne outbreaks in the United States. We conducted in vitro and in vivo infection analyses to determine whether strains LS741 and LS743, two clinical isolates from the cantaloupe outbreak, differ significantly from the common laboratory strain 10403S. We showed that LS741 and LS743 exhibited increased virulence, characterized by higher colonization of the brain and other organs in mice. Assessment of cellular immune responses to known CD8+ T cell antigens was comparable between all strains. However, pre-existing immunity to 10403S did not confer protection in the brain against challenge with LS741. These studies provide insights into the pathogenesis of clinical isolates linked to the 2011 cantaloupe outbreak and also indicate that currently utilized laboratory strains are imperfect models for studying L. monocytogenes pathogenesis.

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.

Domain function dissection and catalytic properties of Listeria monocytogenes p60 protein with bacteriolytic activity

The major extracellular protein p60 of Listeria monocytogenes (Lm-p60) is an autolysin that can hydrolyze the peptidoglycan of bacterial cell wall and has been shown to be required for L. monocytogenes virulence. The predicted three-dimensional structure of Lm-p60 showed that Lm-p60 could be split into two independent structural domains at the amino acid residue 270. Conserved motif analysis showed that V30, D207, S395, and H444 are the key amino acid residues of the corresponding motifs. However, not only the actual functions of these two domains but also the catalytic properties of Lm-p60 are unclear. We try to express recombinant Lm-p60 and identify the functions of two domains by residue substitution (V30A, D207A, S395A, and H444A) and peptide truncation. The C-terminal domain was identified as catalytic element and N-terminal domain as substrate recognition and binding element. Either N-terminal domain truncation or C-terminal domain truncation presents corresponding biological activity. The catalytic activity of Lm-p60 with a malfunctioned substrate-binding domain was decreased, while the substrate binding was not affected by a mulfunctioned catalytic domain. With turbidimetric method, we determined the optimal conditions for the bacteriolytic activity of Lm-p60 against Micrococcus lysodeikficus. The assay for the effect of Lm-p60 on the bacteriolytic activity of lysozyme revealed that the combined use of Lm-p60 protein with lysozyme showed a strong synergistic effect on the bacteriolytic activity.

Listeriosis in animals, its public health significance (food-borne zoonosis) and advances in diagnosis and control: a comprehensive review

Listeriosis is an infectious and fatal disease of animals, birds, fish, crustaceans and humans. It is an important food-borne zoonosis caused by Listeria monocytogenes, an intracellular pathogen with unique potential to spread from cell to cell, thereby crossing blood-brain, intestinal and placental barriers. The organism possesses a pile of virulence factors that help to infect the host and evade from host immune machinery. Though disease occurrence is sporadic throughout the world, it can result in severe damage during an outbreak. Listeriosis is characterized by septicaemia, encephalitis, meningitis, meningoencephalitis, abortion, stillbirth, perinatal infections and gastroenteritis with the incubation period varying with the form of infection. L. monocytogenes has been isolated worldwide from humans, animals, poultry, environmental sources like soil, river, decaying plants, and food sources like milk, meat and their products, seafood and vegetables. Since appropriate vaccines are not available and infection is mainly transmitted through foods in humans and animals, hygienic practices can prevent its spread. The present review describes etiology, epidemiology, transmission, clinical signs, post-mortem lesions, pathogenesis, public health significance, and advances in diagnosis, vaccines and treatment of this disease. Special attention has been given to novel as well as prospective emerging therapies that include bacteriophage and cytokine therapy, avian egg yolk antibodies and herbal therapy. Various vaccines, including advances in recombinant and DNA vaccines and their modes of eliciting immune response, are also discussed. Due focus has also been given regarding appropriate prevention and control strategies to be adapted for better management of this zoonotic disease.

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.