The inactivated and ISA 61 VG adjuvanted vaccine enhances protection against cross-serotype Listeria monocytogenes

Listeriosis is a zoonotic disease caused by Listeria monocytogenes (Lm), posing a significant threat to the breeding industry and public health. Ruminant livestock are particularly susceptible to Lm,  thus effective strategies are needed for controlling ovine listeriosis. In this study, we developed two inactivated vaccines and evaluated their efficacy against Lm infection in murine and ovine models. We inactivated the Lm serotype 4h XYSN strain and adjuvanted it with water-in-oil ISA 61 VG (61 VG-AIV) or aluminum (Al-AIV). Pathological observations confirmed the safety of both vaccines in mice and sheep. The immunological assays demonstrated that, compared with the Al-AIV, the 61 VG-AIV induced higher levels of Lm-specific antibodies and proinflammatory cytokines, suggesting that the ISA 61 VG adjuvant has superior immunostimulatory effects compared with the alum adjuvant. 61 VG-AIV elicited greater immunoprotection than Al-AIV (83.4% vs. 50%) against serotype 4h Lm strain challenge in mice. Additionally, 61 VG-AIV afforded cross-protection against challenges with serotypes 1/2a, 1/2b, and 4b Lm strains. Importantly, high immunoprotection in sheep was conferred by the 61 VG-AIV group (83.4%). Taken together, our findings demonstrate that the ISA 61 VG adjuvant contributes to enhancing the humoral and cellular immune responses of inactivated Lm, and 61 VG-AIV is a promising vaccine candidate for the prevention and control of animal listeriosis. This research lays a solid foundation for its application in veterinary medicine.

Current Progress in the Development of mRNA Vaccines Against Bacterial Infections

Bacterial infections have accompanied humanity for centuries. The discovery of the first antibiotics and the subsequent golden era of their discovery temporarily shifted the balance in this confrontation to the side of humans. Nevertheless, the excessive and improper use of antibacterial drugs and the evolution of bacteria has gotten the better of humans again. Therefore, today, the search for new antibacterial drugs or the development of alternative approaches to the prevention and treatment of bacterial infections is relevant and topical again. Vaccination is one of the most effective strategies for the prevention of bacterial infections. The success of new-generation vaccines, such as mRNA vaccines, in the fight against viral infections has prompted many researchers to design mRNA vaccines against bacterial infections. Nevertheless, the biology of bacteria and their interactions with the host's immunity are much more complex compared to viruses. In this review, we discuss structural features and key mechanisms of evasion of an immune response for nine species of bacterial pathogens against which mRNA vaccines have been developed and tested in animals. We focus on the results of experiments involving the application of mRNA vaccines against various bacterial pathogens in animal models and discuss possible options for improving the vaccines' effectiveness. This is one of the first comprehensive reviews of the use of mRNA vaccines against bacterial infections in vivo to improve our knowledge.

Signals behind Listeria monocytogenes virulence mechanisms

The tight and coordinated regulation of virulence gene expression is crucial to ensure the survival and persistence of bacterial pathogens in different contexts within their hosts. Considering this, bacteria do not express virulence factors homogenously in time and space, either due to their associated fitness cost or to their detrimental effect at specific infection stages. To efficiently infect and persist into their hosts, bacteria have thus to monitor environmental cues or chemical cell-to-cell signaling mechanisms that allow their transition from the external environment to the host, and therefore adjust gene expression levels, intrinsic biological activities, and appropriate behaviors. Listeria monocytogenes (Lm), a major Gram-positive facultative intracellular pathogen, stands out for its adaptability and capacity to thrive in a wide range of environments. Because of that, Lm presents itself as a significant concern in food safety and public health, that can lead to potentially life-threatening infections in humans. A deeper understanding of the intricate bacterial virulence mechanisms and the signals that control them provide valuable insights into the dynamic interplay between Lm and the host. Therefore, this review addresses the role of some crucial signals behind Lm pathogenic virulence mechanisms and explores how the ability to assimilate and interpret these signals is fundamental for pathogenesis, identifying potential targets for innovative antimicrobial strategies.

A live attenuated DIVA vaccine affords protection against Listeria monocytogenes challenge in sheep

Listeria monocytogenes (Lm) is a deadly foodborne pathogen that comprises 14 serotypes, among which, serotype 4b Lm is the primary cause of listeriosis outbreaks in humans and animals. Here, we evaluated the safety, immunogenicity, and protective efficacy of a serotype 4b vaccine candidate Lm NTSNΔactA/plcB/orfX in sheep. The infection dynamics, clinical features, and pathological observation verified that the triple genes deletion strain has adequate safety for sheep. Moreover, NTSNΔactA/plcB/orfX significantly stimulated humoral immune response and 78% protection against lethal wild-type strain challenge. Notably, the attenuated vaccine could differentiate infected and vaccinated animals (DIVA) via serology determination of the antibody against listeriolysin O (LLO, encoded by hly) and phosphatidylinositol-specific phospholipase C (PI-PLC, encoded by plcB). These data suggest that the serotype 4b vaccine candidate has high efficacy, safety, and DIVA characteristics, and may be used to prevent Lm infection in sheep, which provides a theoretical basis for its future application in livestock and poultry breeding.

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.

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.

Live-Attenuated Bacterial Vectors for Delivery of Mucosal Vaccines, DNA Vaccines, and Cancer Immunotherapy

Vaccines save millions of lives each year from various life-threatening infectious diseases, and there are more than 20 vaccines currently licensed for human use worldwide. Moreover, in recent decades immunotherapy has become the mainstream therapy, which highlights the tremendous potential of immune response mediators, including vaccines for prevention and treatment of various forms of cancer. However, despite the tremendous advances in microbiology and immunology, there are several vaccine preventable diseases which still lack effective vaccines. Classically, weakened forms (attenuated) of pathogenic microbes were used as vaccines. Although the attenuated microbes induce effective immune response, a significant risk of reversion to pathogenic forms remains. While in the twenty-first century, with the advent of genetic engineering, microbes can be tailored with desired properties.

In this review, I have focused on the use of genetically modified bacteria for the delivery of vaccine antigens. More specifically, the live-attenuated bacteria, derived from pathogenic bacteria, possess many features that make them highly suitable vectors for the delivery of vaccine antigens. Bacteria can theoretically express any heterologous gene or can deliver mammalian expression vectors harboring vaccine antigens (DNA vaccines). These properties of live-attenuated microbes are being harnessed to make vaccines against several infectious and noninfectious diseases. In this regard, I have described the desired features of live-attenuated bacterial vectors and the mechanisms of immune responses manifested by live-attenuated bacterial vectors. Interestingly anaerobic bacteria are naturally attracted to tumors, which make them suitable vehicles to deliver tumor-associated antigens thus I have discussed important studies investigating the role of bacterial vectors in immunotherapy. Finally, I have provided important discussion on novel approaches for improvement and tailoring of live-attenuated bacterial vectors for the generation of desired immune responses.

Listeria monocytogenes Relies on the Heme-Regulated Transporter hrtAB to Resist Heme Toxicity and Uses Heme as a Signal to Induce Transcription of lmo1634, Encoding Listeria Adhesion Protein

For pathogenic bacteria, host-derived heme represents an important metabolic cofactor and a source for iron. However, high levels of heme are toxic to bacteria. We have previously shown that excess heme has a growth-inhibitory effect on the Gram-positive foodborne pathogen Listeria monocytogenes, and we have learned that the LhrC1-5 family of small RNAs, together with the two-component system (TCS) LisRK, play a role in the adaptation of L. monocytogenes to heme stress conditions. However, a broader knowledge on how this pathogen responds to heme toxicity is still lacking. Here, we analyzed the global transcriptomic response of L. monocytogenes to heme stress. We found that the response of L. monocytogenes to excess heme is multifaceted, involving various strategies acting to minimize the toxic effects of heme. For example, heme exposure triggers the SOS response that deals with DNA damage. In parallel, L. monocytogenes shuts down the transcription of genes involved in heme/iron uptake and utilization. Furthermore, heme stress resulted in a massive increase in the transcription of a putative heme detoxification system, hrtAB, which is highly conserved in Gram-positive bacteria. As expected, we found that the TCS HssRS is required for heme-mediated induction of hrtAB and that a functional heme efflux system is essential for L. monocytogenes to resist heme toxicity. Curiously, the most highly up-regulated gene upon heme stress was lmo1634, encoding the Listeria adhesion protein, LAP, which acts to promote the translocation of L. monocytogenes across the intestinal barrier. Additionally, LAP is predicted to act as a bifunctional acetaldehyde-CoA/alcohol dehydrogenase. Surprisingly, a mutant lacking lmo1634 grows well under heme stress conditions, showing that LAP is not required for L. monocytogenes to resist heme toxicity. Likewise, a functional ResDE TCS, which contributes to heme-mediated expression of lmo1634, is not required for the adaptation of L. monocytogenes to heme stress conditions. Collectively, this study provides novel insights into the strategies employed by L. monocytogenes to resist heme toxicity. Our findings indicate that L. monocytogenes is using heme as a host-derived signaling molecule to control the expression of its virulence genes, as exemplified by lmo1634.

Ferrous iron efflux systems in bacteria

Bacteria require iron for growth, with only a few reported exceptions. In many environments, iron is a limiting nutrient for growth and high affinity uptake systems play a central role in iron homeostasis. However, iron can also be detrimental to cells when it is present in excess, particularly under aerobic conditions where its participation in Fenton chemistry generates highly reactive hydroxyl radicals. Recent results have revealed a critical role for iron efflux transporters in protecting bacteria from iron intoxication. Systems that efflux iron are widely distributed amongst bacteria and fall into several categories: P1B-type ATPases, cation diffusion facilitator (CDF) proteins, major facilitator superfamily (MFS) proteins, and membrane bound ferritin-like proteins. Here, we review the emerging role of iron export in both iron homeostasis and as part of the adaptive response to oxidative stress.

Bioengineered and biohybrid bacteria-based systems for drug delivery

The use of bacterial cells as agents of medical therapy has a long history. Research that was ignited over a century ago with the accidental infection of cancer patients has matured into a platform technology that offers the promise of opening up new potential frontiers in medical treatment. Bacterial cells exhibit unique characteristics that make them well-suited as smart drug delivery agents. Our ability to genetically manipulate the molecular machinery of these cells enables the customization of their therapeutic action as well as its precise tuning and spatio-temporal control, allowing for the design of unique, complex therapeutic functions, unmatched by current drug delivery systems. Early results have been promising, but there are still many important challenges that must be addressed. We present a review of promises and challenges of employing bioengineered bacteria in drug delivery systems and introduce the biohybrid design concept as a new additional paradigm in bacteria-based drug delivery.

The Listeria monocytogenes Fur-regulated virulence protein FrvA is an Fe(II) efflux P1B4 -type ATPase

Listeria monocytogenes FrvA (Lmo0641) is critical for virulence in the mouse model and is an ortholog of the Bacillus subtilis Fur- and PerR-regulated Fe(II) efflux P1B4 -type ATPase PfeT. Previously, FrvA was suggested to protect against heme toxicity. Here, we demonstrate that an frvA mutant is sensitive to iron intoxication, but not to other metals. Expression of frvA is induced by high iron and this induction requires Fur. FrvA functions in vitro as a divalent cation specific ATPase most strongly activated by ferrous iron. When expressed in B. subtilis, FrvA increases resistance to iron both in wild-type and in a pfeT null strain. FrvA is a high affinity Fe(II) exporter and its induction imposes severe iron limitation in B. subtilis resulting in derepression of both Fur- and PerR-regulated genes. FrvA also recognizes Co(II) and Zn(II) as substrates and can complement B. subtilis strains defective in the endogenous export systems for these cations. Building on these results, we conclude that FrvA functions in the efflux of Fe(II), and not heme during listerial infection.

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.

An update on the transport and metabolism of iron in Listeria monocytogenes: the role of proteins involved in pathogenicity

Listeria monocytogenes is a Gram-positive bacterium that causes a rare but severe human disease with high mortality rate. The microorganism is widespread in the natural environment where it shows a saprophytic lifestyle. In the human body it infects many different cell types, where it lives intracellularly, however it may also temporarily live extracellularly. The ability to survive and grow in such diverse niches suggests that this bacterium has a wide range of mechanisms for both the acquisition of various sources of iron and effective management of this microelement. In this review, data about the mechanisms of transport, metabolism and regulation of iron, including recent findings in these areas, are summarized with focus on the importance of these mechanisms for the virulence of L. monocytogenes. These data indicate the key role of haem transport and maintenance of intracellular iron homeostasis for the pathogenesis of L. monocytogenes. Furthermore, some of the proteins involved in iron homeostasis like Fri and FrvA seem to deserve special attention due to their potential use in the development of new therapeutic antilisterial strategies.

Listeria monocytogenes: a promising vehicle for neonatal vaccination

Vaccination as a medical intervention has proven capable of greatly reducing the suffering from childhood infectious disease. However, newborns and infants in particular are age groups for whom adequate vaccine-mediated protection is still largely lacking. With the challenges that the neonatal immune system faces and the required highest level of stringency for safety, designing vaccines for early life in general and the newborn in particular poses great difficulty. Nevertheless, recent advances in our understanding of neonatal immunity and its responses to vaccines and adjuvants suggest that neonatal vaccination is a task fully within reach. Among the most promising developments in neonatal vaccination is the use of Listeria monocytogenes (Lm) as a delivery platform. In this review, we will outline key properties of Lm that make it such an ideal neonatal and early life vaccine vehicle, and also discuss potential constraints of Lm as a vaccine delivery platform.

Vaccination studies: detection of a Listeria monocytogenes-specific T cell immune response using the ELISPOT technique

During systemic infection by Listeria monocytogenes the host develops a robust T cell-mediated immune response against the major immunodominant antigens of the pathogen. The enzyme-linked immuno-spot (ELISPOT) test is an accurate and reproducible means of measuring the extent of this T cell response. Here we describe a detailed ELISPOT protocol for measuring an epitope-specific CD8+ T cell-mediated immune response in mice vaccinated with low doses of L. monocytogenes. The basic approach can be easily adapted for the analysis of other vaccination regimes and target epitopes.