Immune response to a Murray Valley encephalitis virus epitope expressed in the flagellin of an attenuated strain of Salmonella

Flagellin as a vaccine adjuvant

INTRODUCTION

Bacterial flagellin, as a pathogen-associated molecular pattern (PAMP), can activate both innate and adaptive immunity. Its unique structural characteristics endow an effective and flexible adjuvant activity, which allow the design of different types of vaccine strategies to prevent various diseases. This review will discuss recent progress in the mechanism of action of flagellin and its prospects for use as a vaccine adjuvant.

AREAS COVERED

Herein we summarize various types of information related to flagellin adjuvants from PubMed, including structures, signaling pathways, natural immunity, and extensive applications in vaccines, and it discusses the immunogenicity, safety, and efficacy of flagellin-adjuvanted vaccines in clinical trials.

EXPERT COMMENTARY

It is widely accepted that as an adjuvant, flagellin can induce an enhanced antigen-specific immune response. Flagellin adjuvants will allow more effective flagellin-based vaccines to enter clinical trials. Furthermore, vaccine formulations containing PAMPs are crucial to exert the maximum potential of vaccine antigens. Therefore, combinations of flagellin-adjuvanted vaccines with other adjuvants that act in a synergistic manner, particularly TLR ligands, represent a promising method for tailoring targeted vaccines to meet specific requirements.

The flagellin hypervariable region is a potential flagella display domain in probiotic Escherichia coli strain Nissle 1917

The most studied probiotic, Escherichia coli strain Nissle 1917 (EcN) possesses flagella of serotype H1. To explore the potential to use EcN flagellin in flagella display applications, we investigated the effect of deleting amino acids in the hypervariable region of flagellin on EcNc (EcN cured of its two cryptic plasmids pMUT1 and pMUT2). Two EcNc flagellin isogenic mutants with deletions of amino acid residual from 277 to 286 and from 287 to 296 in the hypervariable domain were constructed. Both mutants were flagellated, adherent to IPEC-J2 cells, and colonized BALB/c mice. These hypervariable regions may have future utility in the display of heterologous epitopes.

The use of live attenuated bacteria as a delivery system for heterologous antigens

Live attenuated mutants of several pathogenic bacteria have been exploited as potential vaccine vectors for heterologous antigen delivery by the mucosal route. Such live vectors offer the advantage of potential delivery in a single oral, intranasal or inhalational dose, stimulating both systemic and mucosal immune responses. Over the years, a range of strategies have been developed to allow controlled and stable delivery of antigens and improved immunogenicity where required. Most of these approaches have been evaluated in Salmonella vaccine vectors and, as a result, several live attenuated recombinant Salmonella vaccines are now in human clinical trials. In this review, these strategies and their use in the development of a delivery system for the Yersinia pestis V antigen are described.

Salmonella: immune responses and vaccines

Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Vaccination is an effective tool for the prevention of Salmonella infections. Host resistance to Salmonella relies initially on the production of inflammatory cytokines leading to the infiltration of activated inflammatory cells in the tissues. Thereafter T- and B-cell dependent specific immunity develops allowing the clearance of Salmonella microorganisms from the tissues and the establishment of long-lasting acquired immunity to re-infection. The increased resistance that develops after primary infection/ vaccination requires T-cells cytokines such as IFNgamma TNFalpha and IL12 in addition to opsonising antibody. However for reasons that are not fully understood seroconversion and/or the presence of detectable T-cell memory do not always correlate with the development of acquired resistance to infection.Whole-cell killed vaccines and subunit vaccines are used in the prevention of Salmonella infection in animals and in humans with variable results. A number of early live Salmonella vaccines derived empirically by chemical or u.v. mutagenesis proved to be immunogenic and protective and are still in use despite the need for repeated parenteral administration. Recent progress in the knowledge of the genetics of Salmonella virulence and modern recombinant DNA technology offers the possibility to introduce multiple defined attenuating and irreversible mutations into the bacterial genome. This has recently allowed the development of Salmonella strains devoid of significant side effects but still capable of inducing solid immunity after single oral administration. Live attenuated Salmonella vaccines have been used for the expression of heterologous antigens/proteins that can be successfully delivered to the immune system. Furthermore Salmonella can transfer plasmids encoding foreign antigens under the control of eukaryotic promoters (DNA vaccines) to antigen-presenting cells resulting in targeted delivery of DNA vaccines to these cells. Despite the great recent advances in the development of Salmonella vaccines a large proportion of the work has been conducted in laboratory rodents and more research in other animal species is required.

Peptide display on bacterial flagella: principles and applications

Expression of foreign peptides as fusions to bacterial cell surface proteins has gained increasing attention in basic, as well as applied research during the last decade. A wide range of heterologous peptides have been expressed, and the spectrum of available carrier proteins is also wide. The choice of carrier protein is frequently ruled by the application of the fusion protein constructed. This review is focused on flagella display, which is based on genetic fusion of foreign peptides into a surface-exposed, dispensable region of flagellin, the flagellar major subunit present in thousands of copies per filament. Expression of these constructs in flagellin-deficient host strains results in hybrid flagella carrying the heterologous peptides in thousands of intimately-associated copies. The first and still most frequent application of flagella display is the construction of novel recombinant vaccines. Flagella display has also been used in peptide display as an alternative to the phage-display technique. One application involves fusion into a disulfide loop of Escherichia coli thioredoxin that has been inserted into flagellin, this system facilitates expression of random peptides in a conformationally constrained manner readily accessible on the flagellar surface. The random peptide library has been applied in antibody epitope mapping and is suitable for biopanning procedures in the study of ligand-receptor interactions. Many bacterial adhesins are of complex nature and thereby difficult to analyse by conventional methods. Direct flagella display has proven to be applicable also in bacterial adhesion technology since large fragments, up to 302 amino acid residues in length, of bacterial adhesins can be functionally expressed as fusions to flagellin. Hybrid flagella are easily purified and can easily be analysed for binding to various targets, such as immobilized proteins, tissue sections, as well as cell cultures.

Immune mediated and inherited defences against flaviviruses

BACKGROUND

Flavivirus infection elicits an abundant immune response in the host which is directed against a number of the viral proteins. Resistance to flavivirus-induced disease can also be controlled via a non-immune mechanism involving the product of a naturally occurring murine gene, Flv.

OBJECTIVES

To review studies that have reported the mapping of epitopes on flavivirus proteins that elicit T- or B-cell immune responses in mice or humans and to discuss a possible mechanism for flavivirus-specific genetic resistance.

STUDY DESIGN

Purified viral proteins and synthetic peptides were used to map B-cell epitopes. Purified proteins, vaccinia-expressed viral protein fragments and synthetic peptides were used to map T-cell epitopes. Congenic-resistant, C3H/RV and congenic susceptible, C3H/He mice and cell cultures were used to study the mechanism of genetic resistance to flavivirus infection.

RESULTS

T- and B-cell epitopes have been mapped to the E, NS1 and NS3 proteins of several flaviviruses. Immune responses to the C, PreM, NS2a, NS4a, and NS5 proteins have also been documented. Data suggest that the Flv gene product acts intracellularly to suppress the synthesis of viral genomic RNA.

CONCLUSIONS

Although flavivirus infection elicits an abundant immune response, this response is not always rapid enough to protect the host from developing encephalitis. During secondary infections both the humoral and cellular flavivirus-specific responses can confer protection. Dengue haemorrhagic fever (DHF) and dengue shock syndrome (DSS) appear to be caused by an overly vigorous immune response. In genetically resistant animals reduced production of virus results in a slower spread of the infection, which in turn allows time for the immune response to develop and to clear the infection before disease symptoms appear.

The immune response to a B-cell epitope delivered by Salmonella is enhanced by prior immunological experience

Attenuated, heterologous strains of Salmonella have shown potential as live, recombinant vaccines against foreign pathogens. Studies in animal models have demonstrated that immunization with these heterologous vaccines is an effective way to induce both cellular and humoral immune responses against Salmonella and the foreign antigen. We studied the consequence of priming mice with Salmonella dublin 3-6 months before intraperitoneal administration with the same strain carrying a model B-cell epitope. Mice primed with the carrier strain demonstrated enhanced serum Ig titres against the foreign antigen. This immune enhancement was observed up to approximately 6 months after priming. These findings suggest that previous immunological experience with Salmonella does not limit the immune response to a foreign antigen carried by the same organism. In fact, prior exposure to Salmonella appears to enhance the response to the foreign antigen.