Construction of an invertible DNA segment for improved antigen expression by a hybrid Salmonella vaccine strain

Construction of highly attenuated Salmonella enterica serovar Typhimurium live vectors for delivering heterologous antigens by chromosomal integration

Attenuated live Salmonella enterica serovar Typhimurium is a versatile organism for the generation of live recombinant vaccines for mucosal immunization and various approaches were devised for the stable and efficient expressions of heterologous antigens by attenuated S. enterica strains. Phage lamda Red recombinase has recently been devised for gene replacements in S. enterica after introduction of PCR products as a one-step deletion approach and FLP-mediated recombination allows the subsequent removal of antibiotic resistance markers. As an extension of this method, we have developed an approach that allows the sequential integration of multiple recombinant expression cassettes for heterologous antigens into the chromosome of S. enterica. We observed the stable expression of model antigens without selective pressure. In addition, the method allows the simultaneous generation of double-attenuating mutations by gene deletions. This approach allows the rapid and efficient construction of recombinant Salmonella strains as vaccine carriers.

Rapid method for the construction of Salmonella enterica Serovar Typhimurium vaccine carrier strains

Salmonella enterica serovar Typhimurium is a versatile organism for the generation of live recombinant vaccines for mucosal immunization. Various strategies have been devised for the stable and efficient expression of heterologous antigens by attenuated S. enterica strains, but these methods often require complex manipulations. Use of phage lambda Red recombinase has recently been devised for gene replacements in Escherichia coli and S. enterica after introduction of PCR products. Based on this method, we have developed an approach that allows the integration of recombinant expression cassettes for heterologous antigens in a single step. The recombinant construct is integrated into the chromosome and is devoid of any selective marker such as antibiotic resistance. We observed the stable expression of model antigens without selective pressure. In addition, the method allows the simultaneous generation of attenuating mutations by gene deletions. The novel "knock-in" approach allows the rapid and efficient construction of recombinant Salmonella strains as vaccine carriers.

Salmonella vaccines for use in humans: present and future perspectives

In recent years there has been significant progress in the development of attenuated Salmonella enterica serovar Typhi strains as candidate typhoid fever vaccines. In clinical trials these vaccines have been shown to be well tolerated and immunogenic. For example, the attenuated S. enterica var. Typhi strains CVD 908-htrA (aroC aroD htrA), Ty800 (phoP phoQ) and chi4073 (cya crp cdt) are all promising candidate typhoid vaccines. In addition, clinical trials have demonstrated that S. enterica var. Typhi vaccines expressing heterologous antigens, such as the tetanus toxin fragment C, can induce immunity to the expressed antigens in human volunteers. In many cases, the problems associated with expression of antigens in Salmonella have been successfully addressed and the future of Salmonella vaccine development is very promising.

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.

Mice are protected from Helicobacter pylori infection by nasal immunization with attenuated Salmonella typhimurium phoPc expressing urease A and B subunits

Live Salmonella typhimurium phoPc bacteria were tested as mucosal vaccine vectors to deliver Helicobacter pylori antigens. The genes encoding the A and B subunits of H. pylori urease were introduced into S. typhimurium phoPc and expressed under the control of a constitutive tac promoter (tac-ureAB) or a two-phase T7 expression system (cT7-ureAB). Both recombinant Salmonella strains expressed the two urease subunits in vitro and were used to nasally immunize BALB/c mice. The plasmid carrying cT7-ureAB was stably inherited by bacteria growing or persisting in the spleen, lungs, mesenteric or cervical lymph nodes, and Peyer's patches of immunized mice, while the plasmid carrying tac-ureAB was rapidly lost. Spleen and Peyer's patch CD4+ lymphocytes from mice immunized with S. typhimurium phopc cT7-ureAB proliferated in vitro in response to urease, whereas cells from mice given S. typhimurium phoPc alone did not. Splenic CD4+ cells from mice immunized with phoPc cT7-ureAB secreted gamma interferon and interleukin 10, while Peyer's patch CD4+ cells did not secrete either cytokine. Specific H. pylori anti-urease immunoglobulin G1 (IgG1) and IgG2A antibodies were detected following immunization, confirming that both Th1- and Th2-type immune responses were generated by the live vaccine. Sixty percent of the mice (9 of 15) immunized with S. typhimurium phoPc cT7-ureAB were found to be resistant to infection by H. pylori, while all mice immunized with phoPc tac-ureAB (15 of 15) or phoPc (15 of 15) were infected. Our data demonstrate that H. pylori urease delivered nasally by using a vaccine strain of S. typhimurium can trigger Th1- and Th2-type responses and induce protective immunity against Helicobacter infection.

Induction of feline immunodeficiency virus specific antibodies in cats with an attenuated Salmonella strain expressing the Gag protein

Salmonella typhimurium aroA strains (SL3261), expressing high levels of the Gag protein of feline immunodeficiency virus (FIV) fused with maltose binding protein (SL3261-MFG), were constructed using an invertible promoter system that allows the stable expression of heterologous antigens at levels toxic for bacteria. A SL3261 strain expressing the B subunit of cholera toxin by a similar system (SL3261-CtxB) served as a control in FIV-immunization experiments. Cats immunized once orally or intraperitoneally with SL3261-MFG or SL3261-CtxB all developed serum antibodies to SL3261 lipopolysaccharide and against maltose binding protein or the B subunit of cholera toxin, respectively. Two intraperitoneal immunizations with SL3261-MFG also resulted in the development of Gag specific serum antibodies. Two oral immunizations with SL3261-MFG primed for a Gag specific response, which was demonstrated upon FIV challenge. All challenged cats became infected and no significant differences in viral loads were found between SL3261-MFG and SL3261-CtxB immunized cats.

Mixed population approach for vaccination with live recombinant Salmonella strains

Attenuated strains of enteropathogenic species, such as Salmonella, represent useful carries for the delivery of heterologous recombinant antigens to the immune system. A frequently encountered obstacle, however, is the negative influence of high-level antigen production on the stability of carrier strains and the maintenance of their specific properties concerning tissue colonization and viability during infection. To solve this problem we have established an expression system based on genetic variation. This generates two sub-populations of a recombinant vaccine strain, i.e., one consisting of viable cells which maintain all characteristics of the native carrier strain and generate a second population of cells producing antigen(s) of interest at a very high level. This novel expression system offers unique applications and advantages over common live recombinant vaccine approaches.

Construction and evaluation of an expression vector allowing the stable expression of foreign antigens in a Salmonella typhimurium vaccine strain

Salmonella strains have great potential as live carriers of heterologous antigens to induce immunity against a variety of infectious diseases. However, the amount of heterologous antigen required to induce an adequate immune response may be toxic for the bacterium and result in cell death, overattenuation or loss of expression of the heterologous antigen. To solve this problem an expression vector was developed with a strong promoter located on a DNA fragment which is inverted at random. Antigen is only expressed in one particular orientation of the promoter. Thus a bacterial population harbouring the plasmid will consist of a subpopulation which does not produce heterologous antigen, and is therefore not affected in growth, persistence and dissemination within the host. Further, this non-producing population will continuously segregate antigen-producing bacteria. To evaluate the system, CtxB was used as a model antigen. Analysis of the plasmid DNA isolated from Salmonella revealed a selection against the promoter orientation that directs transcription of the ctxB gene. In spite of this, the vector was stably maintained in vivo and induced CtxB-specific IgA and IgG in mice. These results indicate that this kind of expression vector may offer a solution to the problem of unstable expression of foreign antigens in live bacterial vaccine strains.

Live attenuated Salmonella vaccines and their potential as oral combined vaccines carrying heterologous antigens

Live attenuated salmonellae are protective, and are candidate vaccines against invasive salmonella infections in man and animals. Different attenuating mutations have been described, and more than one can be incorporated in a vaccine for added safety. Combined salmonella vaccines express target carbohydrate and protein antigens or epitopes from viruses, bacteria and eukaryotic parasites, either within or on the surface of the cell, as capsules, fimbriae, or in the flagellin. Humoral, secretory and cellular responses to the recombinant antigens has been demonstrated. Experimental protection against diseases including streptococcal infection, tetanus, influenza and malaria has been obtained.