Heat-Labile Enterotoxins as Adjuvants or Anti-Inflammatory Agents

Current Progress in the Science of Novel Adjuvant Nano-Vaccine-Induced Protective Immune Responses

Vaccinations are vital as they protect us from various illness-causing agents. Despite all the advancements in vaccine-related research, developing improved and safer vaccines against devastating infectious diseases including Ebola, tuberculosis and acquired immune deficiency syndrome (AIDS) remains a significant challenge. In addition, some of the current human vaccines can cause adverse reactions in some individuals, which limits their use for massive vaccination program. Therefore, it is necessary to design optimal vaccine candidates that can elicit appropriate immune responses but do not induce side effects. Subunit vaccines are relatively safe for the vaccination of humans, but they are unable to trigger an optimal protective immune response without an adjuvant. Although different types of adjuvants have been used for the formulation of vaccines to fight pathogens that have high antigenic diversity, due to the toxicity and safety issues associated with human-specific adjuvants, there are only a few adjuvants that have been approved for the formulation of human vaccines. Recently, nanoparticles (NPs) have gain specific attention and are commonly used as adjuvants for vaccine development as well as for drug delivery due to their excellent immune modulation properties. This review will focus on the current state of adjuvants in vaccine development, the mechanisms of human-compatible adjuvants and future research directions. We hope this review will provide valuable information to discovery novel adjuvants and drug delivery systems for developing novel vaccines and treatments.

SARS-CoV-2: Immunity, Challenges with Current Vaccines, and a Novel Perspective on Mucosal Vaccines

The global rollout of COVID-19 vaccines has played a critical role in reducing pandemic spread, disease severity, hospitalizations, and deaths. However, the first-generation vaccines failed to block severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and transmission, partially due to the limited induction of mucosal immunity, leading to the continuous emergence of variants of concern (VOC) and breakthrough infections. To meet the challenges from VOC, limited durability, and lack of mucosal immune response of first-generation vaccines, novel approaches are being investigated. Herein, we have discussed the current knowledge pertaining to natural and vaccine-induced immunity, and the role of the mucosal immune response in controlling SARS-CoV2 infection. We have also presented the current status of the novel approaches aimed at eliciting both mucosal and systemic immunity. Finally, we have presented a novel adjuvant-free approach to elicit effective mucosal immunity against SARS-CoV-2, which lacks the safety concerns associated with live-attenuated vaccine platforms.

Mucosal vaccination: onward and upward

INTRODUCTION

The unique mucosal immune system allows the generation of robust protective immune responses at the front line of pathogen encounters. The needle-free delivery route and cold chain-free logistic requirements also provide additional advantages in ease and economy. However, the development of mucosal vaccines faces several challenges, and only a handful of mucosal vaccines are currently licensed. These vaccines are all in the form of live attenuated or inactivated whole organisms, whereas no subunit-based mucosal vaccine is available.

AREAS COVERED

The selection of antigen, delivery vehicle, route and adjuvants for mucosal vaccination are highly important. This is particularly crucial for subunit vaccines, as they often fail to elicit strong immune responses. Emerging research is providing new insights into the biological and immunological uniqueness of mucosal tissues. However, many aspects of the mucosal immunology still await to be investigated.

EXPERT OPINION

This article provides an overview of the current understanding of mucosal vaccination and discusses the remaining knowledge gaps. We emphasize that because of the potential benefits mucosal vaccines can bring from the biomedical, social and economic standpoints, the unmet goal to achieve mucosal vaccine success is worth the effort.

Treating allergies via skin - Recent advances in cutaneous allergen immunotherapy

Subcutaneous allergen immunotherapy has been practiced clinically for decades to treat airborne allergies. Recently, the cutaneous route, which exploits the immunocompetence of the skin has received attention, which is evident from attempts to use it to treat peanut allergy. Delivery of allergens into the skin is inherently impeded by the barrier imposed by stratum corneum, the top layer of the skin. While the stratum corneum barrier must be overcome for efficient allergen delivery, excessive disruption of this layer can predispose to development of allergic inflammation. Thus, the most desirable allergen delivery approach must provide a balance between the level of skin disruption and the amount of allergen delivered. Such an approach should aim to achieve high allergen delivery efficiency across various skin types independent of age and ethnicity, and optimize variables such as safety profile, allergen dosage, treatment frequency, application time and patient compliance. The ability to precisely quantify the amount of allergen being delivered into the skin is crucial since it can allow for allergen dose optimization and can promote consistency and reproducibility in treatment response. In this work we review prominent cutaneous delivery approaches, and offer a perspective on further improvisation in cutaneous allergen-specific immunotherapy.

Intranasal Immunization with Zika Virus Envelope Domain III-Flagellin Fusion Protein Elicits Systemic and Mucosal Immune Responses and Protection against Subcutaneous and Intravaginal Virus Challenges

Zika virus (ZIKV) infections in humans are mainly transmitted by the mosquito vectors, but human-to-human sexual transmission is also another important route. Developing a ZIKV mucosal vaccine that can elicit both systemic and mucosal immune responses is of particular interest. In this study, we constructed a recombinant ZIKV envelope DIII (ZDIII) protein genetically fused with Salmonella typhimurium flagellin (FliC-ZDIII) as a novel mucosal antigen for intranasal immunization. The results indicated that the FliC-ZDIII fusion proteins formulated with E. coli heat-labile enterotoxin B subunit (LTIIb-B5) adjuvant greatly increased the ZDIII-specific IgG, IgA, and neutralizing titers in sera, and the ZDIII-specific IgA titers in bronchoalveolar lavage and vaginal fluids. Protective immunity was further assessed by subcutaneous and intravaginal ZIKV challenges. The second-generation FliCΔD3-2ZDIII was shown to result in a reduced titer of anti-FliC IgG antibodies in sera and still retained the same levels of serum IgG, IgA, and neutralizing antibodies and mucosal IgA antibodies without compromising the vaccine antigenicity. Therefore, intranasal immunization with FliCΔD3-2ZDIII fusion proteins formulated with LTIIb-B5 adjuvant elicited the greatest protective immunity against subcutaneous and intravaginal ZIKV challenges. Our findings indicated that the combination of FliCΔD3-2ZDIII fusion proteins and LTIIb-B5 adjuvant for intranasal immunization can be used for developing ZIKV mucosal vaccines.

Attacking the Intruder at the Gate: Prospects of Mucosal Anti SARS-CoV-2 Vaccines

The sudden outbreak of the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) pandemic in December 2019 caused crises and health emergencies worldwide. The rapid spread of the virus created an urgent need for the development of an effective vaccine and mass immunization to achieve herd immunity. Efforts of scientific teams at universities and pharmaceutical companies around the world allowed for the development of various types of preparations and made it possible to start the vaccination process. However, it appears that the developed vaccines are not effective enough and do not guarantee long-lasting immunity, especially for new variants of SARS-CoV-2. Considering this problem, it is promising to focus on developing a Coronavirus Disease 2019 (COVID-19) mucosal vaccine. Such a preparation applied directly to the mucous membranes of the upper respiratory tract might provide an immune barrier at the primary point of virus entry into the human body while inducing systemic immunity. A number of such preparations against SARS-CoV-2 are already in various phases of preclinical and clinical trials, and several of them are very close to being accepted for general use, constituting a milestone toward pandemic containment.

Adjuvants: Engineering Protective Immune Responses in Human and Veterinary Vaccines

Adjuvants are key components of many vaccines, used to enhance the level and breadth of the immune response to a target antigen, thereby enhancing protection from the associated disease. In recent years, advances in our understanding of the innate and adaptive immune systems have allowed for the development of a number of novel adjuvants with differing mechanisms of action. Herein, we review adjuvants currently approved for human and veterinary use, describing their use and proposed mechanisms of action. In addition, we will discuss additional promising adjuvants currently undergoing preclinical and/or clinical testing.

Intranasal vaccine: Factors to consider in research and development

Most existing vaccines for human use are administered by needle-based injection. Administering vaccines needle-free intranasally has numerous advantages over by needle-based injection, but there are only a few intranasal vaccines that are currently approved for human use, and all of them are live attenuated influenza virus vaccines. Clearly, there are immunological as well as non-immunological challenges that prevent vaccine developers from choosing the intranasal route of administration. We reviewed current approved intranasal vaccines and pipelines and described the target of intranasal vaccines, i.e. nose and lymphoid tissues in the nasal cavity. We then analyzed factors unique to intranasal vaccines that need to be considered when researching and developing new intranasal vaccines. We concluded that while the choice of vaccine formulations, mucoadhesives, mucosal and epithelial permeation enhancers, and ligands that target M-cells are important, safe and effective intranasal mucosal vaccine adjuvants are needed to successfully develop an intranasal vaccine that is not based on live-attenuated viruses or bacteria. Moreover, more effective intranasal vaccine application devices that can efficiently target a vaccine to lymphoid tissues in the nasal cavity as well as preclinical animal models that can better predict intranasal vaccine performance in clinical trials are needed to increase the success rate of intranasal vaccines in clinical trials.

Immunomodulatory regulation by heat-labile enterotoxins and potential therapeutic applications

Introduction: Heat-labile enterotoxins (HLTs) and their cognate ganglioside receptors have been extensively studied because of their therapeutic potential. Gangliosides play arole in modulating effector cells of the immune system, and HLTs provide a novel means for stimulating ganglioside-mediated responses in immunocompetent cells.Areas covered: To evaluate the mechanisms of HLT adjuvanticity, a systemic literature review was performed using relevant keyword searches of the PubMed database, accessing literature published as recently as late 2020. Since HLTs bind to specific ganglioside receptors on immunocytes, they can act as regulators via stimulation or tapering of immune responses from associated signal transduction events. Binding of HLTs to gangliosides can increase proliferation of T-cells, increase cytokine release, augment mucosal/systemic antibody responses, and increase the effectiveness of antigen presenting cells. Subunit components also independently stimulate certain immune responses. Mutant forms of HLTs have potent immunomodulatory effects without the toxicity associated with holotoxins.Expert opinion: HLTs have been the subject of abundant research exploring their use as vaccine adjuvants, in the treatment of autoimmune conditions, in cancer therapy, and for weight loss, proving that these molecules are promising tools in the field of immunotherapy.

Heat-Labile Toxin from Enterotoxigenic Escherichia coli Causes Systemic Impairment in Zebrafish Model

Heat-labile toxin I (LT-I), produced by strains of enterotoxigenic Escherichia coli (ETEC), causes profuse watery diarrhea in humans. Different in vitro and in vivo models have already elucidated the mechanism of action of this toxin; however, their use does not always allow for more specific studies on how the LT-I toxin acts in systemic tracts and intestinal cell lines. In the present work, zebrafish (Danio rerio) and human intestinal cells (Caco-2) were used as models to study the toxin LT-I. Caco-2 cells were used, in the 62nd passage, at different cell concentrations. LT-I was conjugated to FITC to visualize its transport in cells, as well as microinjected into the caudal vein of zebrafish larvae, in order to investigate its effects on survival, systemic traffic, and morphological formation. The internalization of LT-I was visualized in 3 × 104 Caco-2 cells, being associated with the cell membrane and nucleus. The systemic traffic of LT-I in zebrafish larvae showed its presence in the cardiac cavity, yolk, and regions of the intestine, as demonstrated by cardiac edema (100%), the absence of a swimming bladder (100%), and yolk edema (80%), in addition to growth limitation in the larvae, compared to the control group. There was a reduction in heart rate during the assessment of larval survival kinetics, demonstrating the cardiotoxic effect of LT-I. Thus, in this study, we provide essential new depictions of the features of LT-I.

Prospects for mucosal vaccine: shutting the door on SARS-CoV-2

The sudden emergence of a highly transmissible and pathogenic coronavirus SARS-CoV-2 in December 2019 from China and its rapid global spread has posed an international health emergency. The rapid development of an effective vaccine is imperative to control the spread of SARS-CoV-2. A number of concurrent efforts to find an effective therapeutic agent or vaccine for COVID-19 (coronavirus disease 2019) are being undertaken globally. Oral and nasal mucosal surfaces serve as the primary portal of entry for pathogens like coronaviruses in the human body. As evidenced by studies on similar coronaviruses (SARS-CoV and MERS-CoV), mucosal vaccination can provide a safe and effective means for the induction of long-lasting systemic and mucosal immunity to confer protection against SARS-CoV-2. This article summarizes the approaches to an effective mucosal vaccine formulation which can be a rewarding approach to combat the unprecedented threat posed by this emerging global pandemic.

Type IIb Heat Labile Enterotoxin B Subunit as a Mucosal Adjuvant to Enhance Protective Immunity against H5N1 Avian Influenza Viruses

Human infections with highly pathogenic avian influenza H5N1 viruses persist as a major global health concern. Vaccination remains the primary protective strategy against H5N1 and other novel avian influenza virus infections. We investigated the use of E. coli type IIb heat labile enterotoxin B subunit (LTIIb-B5) as a mucosal adjuvant for intranasal immunizations with recombinant HA proteins against H5N1 avian influenza viruses. Use of LTIIb-B5 adjuvant elicited more potent IgG, IgA, and neutralizing antibody titers in both sera and bronchoalveolar lavage fluids, thus increasing protection against lethal virus challenges. LTIIb-B5 mucosal adjuvanticity was found to trigger stronger Th17 cellular response in spleen lymphocytes and cervical lymph nodes. Studies of anti-IL-17A monoclonal antibody depletion and IL-17A knockout mice also suggest the contribution from Th17 cellular response to anti-H5N1 protective immunity. Our results indicate a link between improved protection against H5N1 live virus challenges and increased Th17 response due to the use of LTIIb-B5 mucosal adjuvant with HA subunit proteins.

Highly Pathogenic Avian Influenza H5 Hemagglutinin Fused with the A Subunit of Type IIb Escherichia coli Heat Labile Enterotoxin Elicited Protective Immunity and Neutralization by Intranasal Immunization in Mouse and Chicken Models

Highly pathogenic avian influenza viruses are classified by the World Organization for Animal Health (OIE) as causes of devastating avian diseases. This study aimed to develop type IIb Escherichiacoli heat-labile enterotoxin (LTIIb) as novel mucosal adjuvants for mucosal vaccine development. The fusion protein of H5 and LTIIb-A subunit was expressed and purified for mouse and chicken intranasal immunizations. Intranasal immunization with the H5-LTIIb-A fusion protein in mice elicited potent neutralizing antibodies in sera and bronchoalveolar lavage fluids, induced stronger Th1 and Th17 cellular responses in spleen and cervical lymph nodes, and improved protection against H5N1 influenza virus challenge. More interestingly, intranasal immunization with the H5-LTIIb-A fusion protein in chickens elicited high titers of IgY, IgA, hemagglutinin inhibition (HAI), and neutralizing antibodies in their antisera. This study employed the novel adjuvants of LTIIb for the development of a new generation of mucosal vaccines against highly pathogenic avian influenza viruses.

Adjuvant Strategies for Lactic Acid Bacterial Mucosal Vaccines

Lactic acid bacteria (LAB) are Gram-positive, acid-tolerant bacteria that have long been used in food fermentation and are generally recognized as safe (GRAS). LAB are a part of a normal microbiome and act as probiotics, improving the gastrointestinal microbiome and health when consumed. An increasing body of research has shown the importance of the microbiome on both mucosal immune heath and immune response to pathogens and oral vaccines. Currently, there are few approved mucosal vaccines, and most are attenuated viruses or bacteria, which necessitates cold chain, carries the risk of reversion to virulence, and can have limited efficacy in individuals with poor mucosal health. On account of these limitations, new types of mucosal vaccine vectors are necessary. There has been increasing interest and success in developing recombinant LAB as next generation mucosal vaccine vectors due to their natural acid and bile resistance, stability at room temperature, endogenous activation of innate and adaptive immune responses, and the development of molecular techniques that allow for manipulation of their genomes. To enhance the immunogenicity of these LAB vaccines, numerous adjuvant strategies have been successfully employed. Here, we review these adjuvant strategies and their mechanisms of action which include: Toll-like receptor ligands, secretion of bacterial toxins, secretion of cytokines, direct delivery to antigen presenting cells, and enterocyte targeting. The ability to increase the immune response to LAB vaccines gives them the potential to be powerful mucosal vaccine vectors against mucosal pathogens.

Recognition of the microbiota by Nod2 contributes to the oral adjuvant activity of cholera toxin through the induction of interleukin-1β

The role of symbiotic bacteria in the development of antigen-specific immunity remains poorly understood. Previous studies showed that sensing of symbiotic bacteria by nucleotide-binding oligomerization domain-containing protein 2 (Nod2) regulates antibody responses in response to nasal immunization with antigen and cholera toxin (CT). In this study, we examined the role of the microbiota in the adjuvant activity of CT induced after oral immunization with antigen. Germ-free (GF) mice showed impaired production of antibody responses and T-cell-specific cytokines after oral immunization when compared with that observed in conventionally raised mice. Similar to GF mice, Nod2-deficient mice showed reduced humoral responses upon oral immunization with antigen and CT. Treatment with CT enhanced the production of interleukin-1β (IL-1β), but not tumor necrosis factor-α or IL-12p40, induced by stimulation of dendritic cells with muramyl dipeptide, the Nod2 ligand. Mechanistically, the enhanced production of IL-1β induced by muramyl dipeptide and CT stimulation required Nod2 and was mediated by both increased synthesis of pro-IL-1β and caspase-1 activation. Furthermore, antigen-specific antibody and cytokine responses induced by CT were impaired in orally immunized IL-1β-deficient mice. Collectively, our results indicate that Nod2 stimulation by symbiotic bacteria contributes to optimal CT-mediated antigen-specific oral vaccination through the induction of IL-1β production.

Protease-Activated Receptor 2 Agonist as Adjuvant: Augmenting Development of Protective Memory CD8 T Cell Responses Induced by Influenza Virosomes

Protease-activated receptor 2 (PAR-2) is expressed in various tissues, including lung, and when activated, promotes inflammation, differentiation, and migration of dendritic cells. We found that combining influenza virosomes containing hemagglutinin and neuraminidase with a PAR-2 agonist peptide (PAR-2AP) in an intranasal prime boost approach increased survival of mice challenged weeks later with lethal influenza virus over that by virosome or PAR-2AP prime boost alone. No weight loss occurred from influenza challenge after virosome-plus-PAR-2AP prime boost compared with either virosomes or PAR-2AP alone. Thus, virosomes plus PAR-2AP prevented morbidity as well as mortality. Through adoptive transfer, CD8⁺ lung T cells but not CD4⁺ T cells from virosomes plus PAR-2AP-primed mice protected from lethal influenza virus challenge and enhanced survival with less weight loss and faster recovery. Virosome-plus-PAR-2AP prime boost resulted in greater percentages of T effector memory phenotype cells (Tem) in lung, and higher frequencies of CD8 Tem and T central memory cells displayed effector functions in response to virus challenge in vivo. Virosome-plus-PAR-2AP prime boost also resulted in greater percentages of Ag-specific CD8⁺ T cells, both Tem and T central memory cells, in lungs of animals subsequently challenged with live influenza virus. Our findings indicate that PAR-2AP, a short peptide, may be a new and useful mucosal adjuvant.

Humoral immune and adjuvant responses of mucosally-administered Tinospora cordifolia immunomodulatory protein in BALB/c mice

Background

In traditional medicine, guduchi (Tinospora cordifolia) is considered as an adaptogen with immunomodulatory prowess. A 25 kDa protein from guduchi stem has been characterized as an immunomodulatory protein (ImP).

Objectives

The aim of this study was to evaluate the intrinsic immunogenicity of guduchi ImP and adjuvant activity using ovalbumin (OVA) as antigen in BALB/c mice.

Materials and Methods

Mice were given guduchi ImP (30 and 60 μg) by intranasal administration to respective groups (n = 6) on days 1, 14 and thereafter weekly till day 42. Immunogenic response was monitored by serum IgG/IgA levels (days 14, 35 and 50). The adjuvant activity was measured by serum anti-OVA IgG/IgA responses to administration of 30 μg OVA with guduchi ImP. The effect of guduchi ImP on the spleen status was examined by splenic weight (day 50).

Results

Guduchi ImP administration displayed a significant increase in anti-guduchi ImP IgG (5–7 fold) and anti-guduchi ImP IgA (3–4 fold) on day 50 vs. control. Guduchi ImP showed a significant increase in anti-OVA IgG (6–7 fold) and anti-OVA IgA (4–5 fold) on day 50 vs. control. The splenic index of guduchi ImP group increased significantly in both the immune and adjuvant response groups; however, the splenic index in the adjuvant response group was markedly higher.

Conclusion

The results indicate that guduchi ImP is a strong immunogen by itself and enhances the immunogenicity of mucosally-administered antigen in BALB/c mice. Based on the results of this animal study, it appears that guduchi ImP shows a potential for future studies in humans.

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Evaluated the immune responses of guduchi immunomodulatory protein (ImP) in BALB/c mice.

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Guduchi ImP (30/60 μg) given intranasally on days 1, 14; thereafter, weekly till day 42.

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Increase in anti-guduchi ImP IgG (5–7 fold) and anti-guduchi ImP IgA (3–4 fold) on day 50.

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Increase in ovalbumin-specific IgG (6–7 fold) and IgA (4–5 fold) on day 50 vs. control.

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Splenic index of guduchi ImP group increased in the immune/adjuvant response groups.

Exploring dengue proteome to design an effective epitope-based vaccine against dengue virus

Dengue, a mosquito-borne disease, is caused by four known dengue serotypes. This infection causes a range of symptoms from a mild fever to a sever homorganic fever and death. It is a serious public health problem in subtropical and tropical countries. There is no specific vaccine currently available for clinical use and study on this issue is ongoing. In this study, bioinformatics approaches were used to predict antigenic, immunogenic, non-allergenic, and conserved B and T-cell epitopes as promising targets to design an effective peptide-based vaccine against dengue virus. Molecular docking analysis indicated the deep binding of the identified epitopes in the binding groove of the most popular human MHC I allele (human leukocyte antigens [HLA] A*0201). The final vaccine construct was created by conjugating the B and T-cell identified epitopes using proper linkers and adding an appropriate adjuvant at the N-terminal. The characteristics of the new subunit vaccine demonstrated that the epitope-based vaccine was antigenic, non-toxic, stable, and soluble. Other physicochemical properties of the new designed construct including isoelectric point value, aliphatic index, and grand average of hydropathicity were biologically considerable. Molecular docking of the engineered vaccine with Toll-like receptor 2 (TLR2) model revealed the hydrophobic interaction between the adjuvant and the ligand binding regions in the hydrophobic channel of TLR2. The study results indicated the high potential capability of the new multi-epitope vaccine to induce cellular and humoral immune responses against the dengue virus. Further experimental tests are required to investigate the immune protection capacity of the new vaccine construct in animal models. Communicated by Ramaswamy H. Sarma.

The Biology of Gangliosides

Gangliosides comprise a varied family of glycosphingolipid structures bearing one or more sialic acid residues. They are found in all mammalian tissues but are most abundant in the brain, where they represent the quantitatively major class of sialoglycans. As prominent molecular determinants on cell surfaces, they function as molecular-recognition partners for diverse glycan-binding proteins ranging from bacterial toxins to endogenous cell-cell adhesion molecules. Gangliosides also regulate the activity of plasma membrane proteins, including protein tyrosine kinases, by lateral association in the same membranes in which they reside. Their roles in molecular recognition and membrane protein regulation implicate gangliosides in human physiology and pathology, including infectious diseases, diabetes, cancer, and neurodegeneration. The varied structures and biosynthetic pathways of gangliosides are presented here, along with representative examples of their biological functions in health and disease.

ADP-ribosylating enterotoxins as vaccine adjuvants

Most infections are caused by pathogens that access the body at mucosal sites. Hence, development of mucosal vaccines to prevent local infection or invasion of pathogens appears highly warranted, especially since only mucosal immunization will stimulate strong local IgA responses and tissue resident memory CD4 and CD8 T cells. The most significant obstacle to developing such vaccines is the lack of approved adjuvants that can effectively and safely enhance relevant mucosal and systemic immune responses. The most potent mucosal adjuvants known today are the adenosine diphosphate (ADP)-ribosylating bacterial enterotoxins cholera toxin (CT) and Escherichia coli heat-labile toxins (LTs). Unfortunately, these molecules are also very toxic, which precludes their clinical use. However, much effort has been devoted to developing derivatives of these enterotoxins with low or no toxicity and retained adjuvant activity. Although it is fair to say that we know more about how these toxins affect the immune system than ever before, we still lack a detailed understanding of how and why these toxins are effective adjuvants. In the present review, we provide a state-of-the-art overview of the mechanism of action of the holotoxins and the strategies used for improving the toxin-based adjuvants.

The chromosomal nature of LT-II enterotoxins solved: a lambdoid prophage encodes both LT-II and one of two novel pertussis-toxin-like toxin family members in type II enterotoxigenic Escherichia coli

Heat-labile enterotoxins (LT) of enterotoxigenic Escherichia coli (ETEC) are structurally and functionally related to cholera toxin (CT). LT-I toxins are plasmid-encoded and flanked by IS elements, while LT-II toxins of type II ETEC are chromosomally encoded with flanking genes that appear phage related. Here, I determined the complete genomic sequence of the locus for the LT-IIa type strain SA53, and show that the LT-IIa genes are encoded by a 51 239 bp lambdoid prophage integrated at the rac locus, the site of a defective prophage in E. coli K12 strains. Of 50 LT-IIa and LT-IIc, 46 prophages also encode one member of two novel two-gene ADP-ribosyltransferase toxin families that are both related to pertussis toxin, which I named eplBA or ealAB, respectively. The eplBA and ealAB genes are syntenic with the Shiga toxin loci in their lambdoid prophages of the enteric pathogen enterohemorrhagic E. coli. These novel AB₅ toxins show pertussis-toxin-like activity on tissue culture cells, and like pertussis toxin bind to sialic acid containing glycoprotein ligands. Type II ETEC are the first mucosal pathogens known to simultaneously produce two ADP-ribosylating toxins predicted to act on and modulate activity of both stimulatory and inhibitory alpha subunits of host cell heterotrimeric G-proteins.

Two novel pertussis-toxin-like toxins are also present in the genome of the prophage that also encodes the LT-II enterotoxin genes in type II enterotoxigenic Escherichi coli.

Comparative Safety of Vaccine Adjuvants: A Summary of Current Evidence and Future Needs

Use of highly pure antigens to improve vaccine safety has led to reduced vaccine immunogenicity and efficacy. This has led to the need to use adjuvants to improve vaccine immunogenicity. The ideal adjuvant should maximize vaccine immunogenicity without compromising tolerability or safety. Unfortunately, adjuvant research has lagged behind other vaccine areas such as antigen discovery, with the consequence that only a very limited number of adjuvants based on aluminium salts, monophosphoryl lipid A and oil emulsions are currently approved for human use. Recent strategic initiatives to support adjuvant development by the National Institutes of Health should translate into greater adjuvant choices in the future. Mechanistic studies have been valuable for better understanding of adjuvant action, but mechanisms of adjuvant toxicity are less well understood. The inflammatory or danger-signal model of adjuvant action implies that increased vaccine reactogenicity is the inevitable price for improved immunogenicity. Hence, adjuvant reactogenicity may be avoidable only if it is possible to separate inflammation from adjuvant action. The biggest remaining challenge in the adjuvant field is to decipher the potential relationship between adjuvants and rare vaccine adverse reactions, such as narcolepsy, macrophagic myofasciitis or Alzheimer's disease. While existing adjuvants based on aluminium salts have a strong safety record, there are ongoing needs for new adjuvants and more intensive research into adjuvants and their effects.

Role of virulence factors on host inflammatory response induced by diarrheagenic Escherichia coli pathotypes

ABSTRACT 

Pathogens are able to breach the intestinal barrier, and different bacterial species can display different abilities to colonize hosts and induce inflammation. Inflammatory response studies induced by enteropathogens as Escherichia coli are interesting since it has acquired diverse genetic mobile elements, leading to different E. coli pathotypes. Diarrheagenic E. coli secrete toxins, effectors and virulence factors that exploit the host cell functions to facilitate the bacterial colonization. Many bacterial proteins are delivered to the host cell for subverting the inflammatory response. Hereby, we have highlighted the specific processes used by E. coli pathotypes, by that subvert the inflammatory pathways. These mechanisms include an arrangement of pro- and anti-inflammatory responses to favor the appropriate environmental niche for the bacterial survival and growth.

Intradermal administration of the Type II heat-labile enterotoxins LT-IIb and LT-IIc of enterotoxigenic Escherichia coli enhances humoral and CD8+ T cell immunity to a co-administered antigen

Vaccinations are extremely effective at combating infectious diseases. Many conserved antigen (Ag) targets, however, are poorly immunogenic. Protein subunit vaccines frequently elicit only humoral immune responses and fail to confer protection against serious intracellular pathogens. These barriers to vaccine development are often overcome by the use of appropriate adjuvants. Heat-labile enterotoxins (HLT) produced by enterotoxigenic strains of Escherichia coli are potent adjuvants when administered by mucosal or systemic routes. The efficacy of the type II HLT, however, has not been well-defined when administered by the intradermal (ID) route. Using a murine ID immunization model, the adjuvant properties of LT-IIb and LT-IIc, two type II HLTs, were compared with those of LT-I, a prototypical type I HLT. While all three HLT adjuvants enhanced Ag-specific humoral responses to similar levels, LT-IIb and LT-IIc, in contrast to LT-I, induced a more vigorous Ag-specific CD8⁺ T cell response and proffered faster clearance of Listeria monocytogenes in a challenge model. Additionally, LT-IIb and LT-IIc induced distinct differences in the profiles of the Ag-specific CD8⁺ T cell responses. While LT-IIc stimulated a robust and rapid primary CD8⁺ T cell response, LT-IIb exhibited slower CD8⁺ T cell expansion and contraction kinetics with the formation of higher percentages of effector memory cells. In comparison to LT-I and LT-IIc, LT-IIb evoked better long-term protection after immunization. Furthermore, LT-IIb and LT-IIc enhanced the total number of dendritic cells (DC) in the draining lymph node (DLN) and expression of costimulatory molecules CD80, CD86, and CD40 on DCs. In contrast to LT-I, LT-IIb and LT-IIc induced less edema, cellular infiltrates, and general inflammation at the site of ID injection. Thus, LT-IIb and LT-IIc are attractive comprehensive ID adjuvants with unique characteristic that enhance humoral and cellular immunity to a co-administered protein Ag.

Intranasal DNA Vaccine for Protection against Respiratory Infectious Diseases: The Delivery Perspectives

Intranasal delivery of DNA vaccines has become a popular research area recently. It offers some distinguished advantages over parenteral and other routes of vaccine administration. Nasal mucosa as site of vaccine administration can stimulate respiratory mucosal immunity by interacting with the nasopharyngeal-associated lymphoid tissues (NALT). Different kinds of DNA vaccines are investigated to provide protection against respiratory infectious diseases including tuberculosis, coronavirus, influenza and respiratory syncytial virus (RSV) etc. DNA vaccines have several attractive development potential, such as producing cross-protection towards different virus subtypes, enabling the possibility of mass manufacture in a relatively short time and a better safety profile. The biggest obstacle to DNA vaccines is low immunogenicity. One of the approaches to enhance the efficacy of DNA vaccine is to improve DNA delivery efficiency. This review provides insight on the development of intranasal DNA vaccine for respiratory infections, with special attention paid to the strategies to improve the delivery of DNA vaccines using non-viral delivery agents.

The role of Staphylococcal enterotoxin A in achieving oral tolerance to myelin basic protein in adult mice

BACKGROUND

Oral tolerance is the biological process explaining the non-responsiveness of gut lymphoid tissue to intestinal content. Our study tested a new approach for the enhancement of oral tolerance to a multiple sclerosis-triggering auto-antigen-myelin basic protein, by its oral administration with the Staphylococcal enterotoxin A.

METHODS

Immune tolerance thus stimulated was assessed in adult BALB/c mice, by measuring different cytokines from the supernatant of mesenteric lymph nodes cells (IFN-γ, IL-4, IL-10, IL-17, and TGF-β), and in a SJL/E mouse model of experimental autoimmune encephalomyelitis, by evaluating the development of regulatory T cells in mesenteric lymph nodes and the clinical outcome of the intervention.

RESULTS

We obtained a significant rise in the levels of IL-10 and TGF-β compared with control and a significant decrease of IFN-γ, IL-4 (p < 0.05). Regulatory T cells were increased compared with control (p < 0.05). These results were attributable both to myelin basic protein and to Staphylococcal enterotoxin A. The clinical outcome of experimental autoimmune encephalomyelitis was influenced only by the administration of myelin basic protein.

CONCLUSION

In our experiment, Staphylococcal enterotoxin A enhanced the immune tolerance to myelin basic protein in the gut mucosa, but had no impact on the clinical evolution of experimental autoimmune encephalomyelitis.

Recent advances in understanding enteric pathogenic Escherichia coli

Although Escherichia coli can be an innocuous resident of the gastrointestinal tract, it also has the pathogenic capacity to cause significant diarrheal and extraintestinal diseases. Pathogenic variants of E. coli (pathovars or pathotypes) cause much morbidity and mortality worldwide. Consequently, pathogenic E. coli is widely studied in humans, animals, food, and the environment. While there are many common features that these pathotypes employ to colonize the intestinal mucosa and cause disease, the course, onset, and complications vary significantly. Outbreaks are common in developed and developing countries, and they sometimes have fatal consequences. Many of these pathotypes are a major public health concern as they have low infectious doses and are transmitted through ubiquitous mediums, including food and water. The seriousness of pathogenic E. coli is exemplified by dedicated national and international surveillance programs that monitor and track outbreaks; unfortunately, this surveillance is often lacking in developing countries. While not all pathotypes carry the same public health profile, they all carry an enormous potential to cause disease and continue to present challenges to human health. This comprehensive review highlights recent advances in our understanding of the intestinal pathotypes of E. coli.

Nanoemulsion nasal adjuvant W₈₀5EC induces dendritic cell engulfment of antigen-primed epithelial cells

Nanoemulsions are adjuvants that enhance antigen penetration in the nasal mucosa, increase cellular uptake of antigens by both epithelial dendritic cells, and promote migration of antigen-loaded dendritic cells to regional lymph nodes within a day of vaccine administration. The objective of this study was to determine whether the W(80)5EC nanoemulsion adjuvant enhances immune response not only by direct uptake of antigen by dendritic cells, but also indirectly, by phagocytosis of antigen-primed, apoptotic, epithelial cells. Consistent with this, we show that exposure of both epithelial cells (TC-1s) and dendritic cells (JAWS II or bone marrow derived dendritic cells (BMDCs)) to nanoemulsion exhibited augmented antigen uptake in cell culture. TC-1 cells subsequently underwent G(2)/M cell cycle arrest and apoptosis, and when co-cultured with JAWS II or BMDCs were rapidly engulfed by the dendritic cells, which responded by up-regulating dendritic cell maturation marker CD86. Altogether these results suggest that the effectiveness of nanoemulsions as adjuvants stems, at least in part, from the engulfment of antigen-loaded epithelial cells, leading to enhanced antigen processing and a strong and balanced mucosal and systemic immune response.

Type II heat-labile enterotoxins: structure, function, and immunomodulatory properties

The heat-labile enterotoxins (HLTs) of Escherichia coli and Vibrio cholerae are classified into two major types on the basis of genetic, biochemical, and immunological properties. Type I and Type II HLT have been intensively studied for their exceptionally strong adjuvant activities. Despite general structural similarities, these molecules, in intact or derivative (non-toxic) forms, display notable differences in their mode of immunomodulatory action. The molecular basis of these differences has remained largely uncharacterized until recently. This review focuses on the Type II HLTs and their immunomodulatory properties which depend largely on interactions with unique gangliosides and Toll-like receptors that are not utilized by the Type I HLTs.

Type II heat-labile enterotoxins from 50 diverse Escherichia coli isolates belong almost exclusively to the LT-IIc family and may be prophage encoded

Some enterotoxigenic Escherichia coli (ETEC) produce a type II heat-labile enterotoxin (LT-II) that activates adenylate cyclase in susceptible cells but is not neutralized by antisera against cholera toxin or type I heat-labile enterotoxin (LT-I). LT-I variants encoded by plasmids in ETEC from humans and pigs have amino acid sequences that are ≥ 95% identical. In contrast, LT-II toxins are chromosomally encoded and are much more diverse. Early studies characterized LT-IIa and LT-IIb variants, but a novel LT-IIc was reported recently. Here we characterized the LT-II encoding loci from 48 additional ETEC isolates. Two encoded LT-IIa, none encoded LT-IIb, and 46 encoded highly related variants of LT-IIc. Phylogenetic analysis indicated that the predicted LT-IIc toxins encoded by these loci could be assigned to 6 subgroups. The loci corresponding to individual toxins within each subgroup had DNA sequences that were more than 99% identical. The LT-IIc subgroups appear to have arisen by multiple recombinational events between progenitor loci encoding LT-IIc1- and LT-IIc3-like variants. All loci from representative isolates encoding the LT-IIa, LT-IIb, and each subgroup of LT-IIc enterotoxins are preceded by highly-related genes that are between 80 and 93% identical to predicted phage lysozyme genes. DNA sequences immediately following the B genes differ considerably between toxin subgroups, but all are most closely related to genomic sequences found in predicted prophages. Together these data suggest that the LT-II loci are inserted into lambdoid type prophages that may or may not be infectious. These findings raise the possibility that production of LT-II enterotoxins by ETEC may be determined by phage conversion and may be activated by induction of prophage, in a manner similar to control of production of Shiga-like toxins by converting phages in isolates of enterohemmorhagic E. coli.

Escherichia coli and the mucosal immune system

The meeting was held in the beautiful city of Ghent, Belgium, to bring together basic scientists and clinicians working on Escherichia coli and the mucosal immune system; in particular focusing on cellular interactions, immune modulation and vaccination strategies in humans and animals. The aim was to exchange knowledge on the pathogenicity of different types of E. coli and recent advances in the area of mucosal immunity. The meeting was timely given the recent outbreak in northern Germany of an emergent Shiga toxigenic E. coli strain that was associated with the deaths of over 45 people and caused hemolytic uremic syndrome in nearly 800 individuals according to the European Centre for Disease Prevention and Control.

Use of inactivated Escherichia coli enterotoxins to enhance respiratory mucosal adjuvanticity during vaccination in swine

In order to augment responses to respiratory vaccines in swine, various adjuvants were intranasally coadministered with a foot-and-mouth disease virus (FMDV) antigen to pigs. Detoxified Escherichia coli enterotoxins LTK63 and LTR72 enhanced antigen-specific mucosal and systemic immunity, demonstrating their efficacy as adjuvants for nonreplicating antigens upon intranasal immunization in swine.

Distinctive immunomodulatory and inflammatory properties of the Escherichia coli type II heat-labile enterotoxin LT-IIa and its B pentamer following intradermal administration

The type I and type II heat-labile enterotoxins (LT-I and LT-II) are strong mucosal adjuvants when they are coadministered with soluble antigens. Nonetheless, data on the parenteral adjuvant activities of LT-II are still limited. Particularly, no previous study has evaluated the adjuvant effects and induced inflammatory reactions of LT-II holotoxins or their B pentameric subunits after delivery via the intradermal (i.d.) route to mice. In the present report, the adjuvant and local skin inflammatory effects of LT-IIa and its B subunit pentamer (LT-IIaB(5)) were determined. When coadministered with ovalbumin (OVA), LT-IIa and, to a lesser extent, LT-IIaB(5) exhibited serum IgG adjuvant effects. In addition, LT-IIa but not LT-IIaB(5) induced T cell-specific anti-OVA responses, particularly in respect to induction of antigen-specific cytotoxic CD8(+) T cell responses. LT-IIa and LT-IIaB(5) induced differential tissue permeability and local inflammatory reactions after i.d. injection. Of particular interest was the reduced or complete lack of local reactions, such as edema and tissue induration, in mice i.d. inoculated with LT-IIa and LT-IIaB(5,) respectively, compared with mice immunized with LT-I. In conclusion, the present results show that LT-IIa and, to a lesser extent, LT-IIaB(5) exert adjuvant effects when they are delivered via the i.d. route. In addition, the low inflammatory effects of LT-IIa and LT-IIaB(5) in comparison to those of LT-I support the usefulness of LT-IIa and LT-IIaB(5) as parenterally delivered vaccine adjuvants.

Characterization of antigen-presenting cells induced by intragastric immunization with recombinant chimeric immunogens constructed from Streptococcus mutans AgI/II and type I or type II heat-labile enterotoxins

Intragastric (i.g.) immunization with recombinant chimeric proteins constructed from the saliva-binding region (SBR) of Streptococcus mutans surface antigen AgI/II and the A2/B subunits of enterobacterial heat-labile enterotoxins has been successfully used to induce salivary and circulating antibodies against S. mutans that have protective potential against dental caries. To investigate the mode of action of these vaccine constructs, mice were immunized i.g. with chimeric proteins constructed from SBR and cholera toxin (CT) or the type II enterotoxins of Escherichia coli, LT-IIa and LT-IIb. Antigen-presenting cells (APC) in Peyer's patches (PP) and mesenteric lymph nodes (MLN) were characterized by flow cytometry. Compared with immunization with SBR alone, chimeric proteins SBR-LTIIaA2/B and SBR-LTIIbA2/B increased the number of B cells and macrophages in PP and diminished B cell numbers in MLN, whereas SBR-CTA2/B diminished the numbers of B cells and macrophages in PP and MLN. Immunization with all three chimeric proteins led to upregulation of MHC class II molecules and co-stimulatory receptors CD40, CD80, and CD86 especially on dendritic cells in PP and also on APC in MLN. The results provide a molecular basis for the enhanced immune responses induced by chimeric proteins compared with uncoupled antigen, and for differential responses to chimeric proteins based on CT or type II enterotoxins.

Genome sequencing and comparative transcriptomics of the model entomopathogenic fungi Metarhizium anisopliae and M. acridum

Metarhizium spp. are being used as environmentally friendly alternatives to chemical insecticides, as model systems for studying insect-fungus interactions, and as a resource of genes for biotechnology. We present a comparative analysis of the genome sequences of the broad-spectrum insect pathogen Metarhizium anisopliae and the acridid-specific M. acridum. Whole-genome analyses indicate that the genome structures of these two species are highly syntenic and suggest that the genus Metarhizium evolved from plant endophytes or pathogens. Both M. anisopliae and M. acridum have a strikingly larger proportion of genes encoding secreted proteins than other fungi, while ∼30% of these have no functionally characterized homologs, suggesting hitherto unsuspected interactions between fungal pathogens and insects. The analysis of transposase genes provided evidence of repeat-induced point mutations occurring in M. acridum but not in M. anisopliae. With the help of pathogen-host interaction gene database, ∼16% of Metarhizium genes were identified that are similar to experimentally verified genes involved in pathogenicity in other fungi, particularly plant pathogens. However, relative to M. acridum, M. anisopliae has evolved with many expanded gene families of proteases, chitinases, cytochrome P450s, polyketide synthases, and nonribosomal peptide synthetases for cuticle-degradation, detoxification, and toxin biosynthesis that may facilitate its ability to adapt to heterogenous environments. Transcriptional analysis of both fungi during early infection processes provided further insights into the genes and pathways involved in infectivity and specificity. Of particular note, M. acridum transcribed distinct G-protein coupled receptors on cuticles from locusts (the natural hosts) and cockroaches, whereas M. anisopliae transcribed the same receptor on both hosts. This study will facilitate the identification of virulence genes and the development of improved biocontrol strains with customized properties.

Aside from playing a crucial role in natural ecosystems, entomopathogenic fungi are being developed as environmentally friendly alternatives for the control of insect pests. We conducted the first genomic study of two of the best characterized entomopathogens, Metarhizium anisopliae and M. acridum. M. anisopliae is a ubiquitous pathogen of >200 insect species and a plant growth promoting colonizer of rhizospheres. M. acridum is a specific pathogen of locusts. Important findings of this study included: 1) Both M. anisopliae and M. acridum have a very large number of genes encoding secreted proteins, and many of these play roles in fungus-insect interactions. 2) M. anisopliae has more genes than M. acridum, which may be associated with adaptation to multiple insect hosts. 3) Unlike M. acridum, the M. anisopliae genome contains many more transposase genes and shows no evidence of repeat-induced point mutations. The lack of repeat-induced mutations may have allowed the lineage-specific gene duplications that have contributed to its adaptability. 4) High-throughput transcriptomics identified the strategies by which these fungi overcome their insect hosts and achieve specificity. These genome sequences will provide the basis for a comprehensive understanding of fungal–plant–insect interactions and will contribute to our understanding of fungal evolution and ecology.

LT-IIc, a new member of the type II heat-labile enterotoxin family, exhibits potent immunomodulatory properties that are different from those induced by LT-IIa or LT-IIb

A plethora of human pathogens invade and/or colonize mucosal surfaces. Elaboration of strong, protective immune responses against those pathogens by mucosal vaccination, however, is hampered by endogenous regulatory systems in the mucosae that dampen responses to foreign antigens (Ags). To overcome those natural barriers, mucosal adjuvants must be employed. Using a mouse mucosal immunization model and AgI/II, a weak immunogen from Streptococcus mutans, LT-IIc, a new member of the type II subgroup of the heat-labile enterotoxin family, was shown to have potent mucosal adjuvant properties. In comparison to mice intranasally immunized only with AgI/II, co-administration of AgI/II with LT-IIc enhanced production of Ag-specific IgA antibodies in the saliva and vaginal fluids and Ag-specific IgA and IgG in the serum. Secretion of IL-2, IL-6, IL-17, IFN-γ, and TNF-α was enhanced in cultures of AgI/II-stimulated splenic cells isolated from mice that had received LT-IIc as a mucosal adjuvant. In contrast, secretion of IL-10 was suppressed in those cells. This pattern of cytokine secretion suggested that LT-IIc stimulates both Th1 and Th2 immune responses. In contrast to LT-IIa and LT-IIb, the original members of the type II subgroup that also are mucosal adjuvants, LT-IIc dramatically enhanced secretion of IL-1α and IL-1β in peritoneal macrophages that had been co-cultured with LPS. Furthermore, the B pentameric subunit of LT-IIc augmented uptake of Ag by bone marrow-derived dendritic cells to levels that exceeded those attained by use of LPS or by the B pentamers of LT-IIa or LT-IIb. These data confirmed that LT-IIc is a strong mucosal adjuvant with immunomodulatory properties that are distinguishable from those of LT-IIa and LT-IIb and which has immunomodulatory properties that may be exploitable in vaccine development.

Different types of cell death induced by enterotoxins

The infection of bacterial organisms generally causes cell death to facilitate microbial invasion and immune escape, both of which are involved in the pathogenesis of infectious diseases. In addition to the intercellular infectious processes, pathogen-produced/secreted enterotoxins (mostly exotoxins) are the major weapons that kill host cells and cause diseases by inducing different types of cell death, particularly apoptosis and necrosis. Blocking these enterotoxins with synthetic drugs and vaccines is important for treating patients with infectious diseases. Studies of enterotoxin-induced apoptotic and necrotic mechanisms have helped us to create efficient strategies to use against these well-characterized cytopathic toxins. In this article, we review the induction of the different types of cell death from various bacterial enterotoxins, such as staphylococcal enterotoxin B, staphylococcal alpha-toxin, Panton-Valentine leukocidin, alpha-hemolysin of Escherichia coli, Shiga toxins, cytotoxic necrotizing factor 1, heat-labile enterotoxins, and the cholera toxin, Vibrio cholerae. In addition, necrosis caused by pore-forming toxins, apoptotic signaling through cross-talk pathways involving mitochondrial damage, endoplasmic reticulum stress, and lysosomal injury is discussed.