Protective immunity evoked by oral administration of attenuated aroA Salmonella typhimurium expressing cloned streptococcal M protein

An Overview of Vaccine Adjuvants: Current Evidence and Future Perspectives

Vaccinations are one of the most important preventive tools against infectious diseases. Over time, many different types of vaccines have been developed concerning the antigen component. Adjuvants are essential elements that increase the efficacy of vaccination practises through many different actions, especially acting as carriers, depots, and stimulators of immune responses. For many years, few adjuvants have been included in vaccines, with aluminium salts being the most commonly used adjuvant. However, recent research has focused its attention on many different new compounds with effective adjuvant properties and improved safety. Modern technologies such as nanotechnologies and molecular biology have forcefully entered the production processes of both antigen and adjuvant components, thereby improving vaccine efficacy. Microparticles, emulsions, and immune stimulators are currently in the spotlight for their huge potential in vaccine production. Although studies have reported some potential side effects of vaccine adjuvants such as the recently recognised ASIA syndrome, the huge worth of vaccines remains unquestionable. Indeed, the recent COVID-19 pandemic has highlighted the importance of vaccines, especially in regard to managing future potential pandemics. In this field, research into adjuvants could play a leading role in the production of increasingly effective vaccines.

Streptococcus pyogenes genes that promote pharyngitis in primates

Streptococcus pyogenes (group A streptococcus; GAS) causes 600 million cases of pharyngitis annually worldwide. There is no licensed human GAS vaccine despite a century of research. Although the human oropharynx is the primary site of GAS infection, the pathogenic genes and molecular processes used to colonize, cause disease, and persist in the upper respiratory tract are poorly understood. Using dense transposon mutant libraries made with serotype M1 and M28 GAS strains and transposon-directed insertion sequencing, we performed genome-wide screens in the nonhuman primate (NHP) oropharynx. We identified many potentially novel GAS fitness genes, including a common set of 115 genes that contribute to fitness in both genetically distinct GAS strains during experimental NHP pharyngitis. Targeted deletion of 4 identified fitness genes/operons confirmed that our newly identified targets are critical for GAS virulence during experimental pharyngitis. Our screens discovered many surface-exposed or secreted proteins - substrates for vaccine research - that potentially contribute to GAS pharyngitis, including lipoprotein HitA. Pooled human immune globulin reacted with purified HitA, suggesting that humans produce antibodies against this lipoprotein. Our findings provide new information about GAS fitness in the upper respiratory tract that may assist in translational research, including developing novel vaccines.

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.

Commensal Streptococcus mitis is a unique vector for oral mucosal vaccination

The development of vaccine approaches that induce mucosal and systemic immune responses is critical for the effective prevention of several infections. Here, we report on the use of the abundant human oral commensal bacterium Streptococcus mitis as a delivery vehicle for mucosal immunization. Using homologous recombination we generated a stable rS. mitis expressing a Mycobacterium tuberculosis protein (Ag85b). Oral administration of rS. mitis in gnotobiotic piglets resulted in efficient oral colonization and production of oral and systemic anti-Ag85b specific IgA and IgG antibodies. These results support that the commensal S. mitis is potentially a useful vector for mucosal vaccination.

Live-attenuatedSalmonellaas a prototype vaccine vector for passenger immunogens in humans: are we there yet?

It has been nearly 20 years since the first Phase I clinical trial of a live-attenuated bacterial vaccine was created by recombinant DNA methods, opening the door to the use of these organisms as mucosal delivery vehicles for passenger antigens. Over this time, a number of animal studies have indicated the feasibility of this approach. These include studies showing that bacteria can deliver antigens expressed by the bacterium itself and that bacteria can deliver DNA vaccines to be expressed in target eukaryotic cells. Concomitant studies have identified a number of attenuating mutations that render the bacterial vectors both safe and immunogenic in humans. Both avenues of research indicate the significant promise of this approach to mucosal vaccine development; however, this promise remains largely unrealized at the level of human clinical trials. This review sketches the history of this problem and points toward possible solutions using Salmonella vaccine vectors as the prototypes.

Engineering of cytomegalovirus genomes for recombinant live herpesvirus vaccines

The advances of sequence knowledge and genetic engineering hold a great promise for a rational approach to vaccine development. Herpesviruses are important pathogens of all vertebrates. They cause acute and chronic infections and persist in their hosts for life. In man there are eight herpesviruses known and most of them can be linked to diseases. To date only one licensed vaccine against a human herpesvirus exists and there is no proven successful concept on rational design for herpesvirus vaccines available. Here, we use new reverse genetic systems, based on the 230-kb mouse cytomegalovirus genome to explore new methods of vaccine delivery and of virus attenuation. With regard to virus delivery, we show that the bacterial transfer of the infectious DNA in vivo is theoretically possible but not yet a practical option. With regard to a rational approach of virus attenuation, we consider a selective deletion of viral genes that modulate the immune response of the host.

Asparaginase display of polypeptides in the periplasm of Escherichia coli: Potential rapid pepscan technique for antigen epitope mapping

Asparaginase of Escherichia coli, a tetramer of identical subunits, was tested as a vector to display linear peptides on the surface of each enzyme subunit. A recombinant gene encoding a chimeric protein composed of asparaginase, a tetanus toxin peptide (TTP) spacer (831-854 fragment), and the foreign cholesteryl ester transfer protein C-terminal fragment (CETPC) was expressed and targeted to the periplasm of E. coli. The purified chimeric enzyme exhibited approximately 83% activity of the native enzyme, allowing the rapid screening of recombinant clones. In contrast, an asparaginase-CETPC fusion protein without the TTP spacer produced only about 23% activity of the native enzyme. Rats immunized with bacterial cells containing the chimeric enzymes induced CETP-specific immunoresponse. In contrast, rats inoculated with the cells expressing asparaginase only did not generate specific anti-CETP antibodies. Our study showed that asparaginase of E. coli was an effective carrier for displaying foreign peptides or epitopes. Moreover, the use of the TTP spacer appeared to play a critical role in maintaining the catalytic activity of the chimeric enzymes by redirecting the foreign CETPC peptide to the surface of the enzyme. The chimeric enzyme constructs fusing asparaginase with foreign peptides via a TTP spacer could be utilized as a rapid pepscan technique for antigen epitope mapping. The fusion protein of asparaginase-TTP-CETPC could also be useful for the development of a vaccine against atherosclerosis.

Vaccination of mice with bacteria carrying a cloned herpesvirus genome reconstituted in vivo

Bacterial delivery systems are gaining increasing interest as potential vaccination vectors to deliver either proteins or nucleic acids for gene expression in the recipient. Bacterial delivery systems for gene expression in vivo usually contain small multicopy plasmids. We have shown before that bacteria containing a herpesvirus bacterial artificial chromosome (BAC) can reconstitute the virus replication cycle after cocultivation with fibroblasts in vitro. In this study we addressed the question of whether bacteria containing a single plasmid with a complete viral genome can also reconstitute the viral replication process in vivo. We used a natural mouse pathogen, the murine cytomegalovirus (MCMV), whose genome has previously been cloned as a BAC in Escherichia coli. In this study, we tested a new application for BAC-cloned herpesvirus genomes. We show that the MCMV BAC can be stably maintained in certain strains of Salmonella enterica serovar Typhimurium as well and that both serovar Typhimurium and E. coli harboring the single-copy MCMV BAC can reconstitute a virus infection upon injection into mice. By this procedure, a productive virus infection is regenerated only in immunocompromised mice. Virus reconstitution in vivo causes elevated titers of specific anti-MCMV antibodies, protection against lethal MCMV challenge, and strong expression of additional genes introduced into the viral genome. Thus, the reconstitution of infectious virus from live attenuated bacteria presents a novel concept for multivalent virus vaccines launched from bacterial vectors.

Potential of lipid core peptide technology as a novel self-adjuvanting vaccine delivery system for multiple different synthetic peptide immunogens

This study demonstrates the effectiveness of a novel self-adjuvanting vaccine delivery system for multiple different synthetic peptide immunogens by use of lipid core peptide (LCP) technology. An LCP formulation incorporating two different protective epitopes of the surface antiphagocytic M protein of group A streptococci (GAS)--the causative agents of rheumatic fever and subsequent rheumatic heart disease--was tested in a murine parenteral immunization and GAS challenge model. Mice were immunized with the LCP-GAS formulation, which contains an M protein amino-terminal type-specific peptide sequence (8830) in combination with a conserved non-host-cross-reactive carboxy-terminal C-region peptide sequence (J8) of the M protein. Our data demonstrated immunogenicity of the LCP-8830-J8 formulation in B10.BR mice when coadministered in complete Freund's adjuvant and in the absence of a conventional adjuvant. In both cases, immunization led to induction of high-titer GAS peptide-specific serum immunoglobulin G antibody responses and induction of highly opsonic antibodies that did not cross-react with human heart tissue proteins. Moreover, mice were completely protected from GAS infection when immunized with LCP-8830-J8 in the presence or absence of a conventional adjuvant. Mice were not protected, however, following immunization with an LCP formulation containing a control peptide from a Schistosoma sp. These data support the potential of LCP technology in the development of novel self-adjuvanting multi-antigen component vaccines and point to the potential application of this system in the development of human vaccines against infectious diseases.

Protection of mice from group A streptococcal infection by intranasal immunisation with a peptide vaccine that contains a conserved M protein B cell epitope and lacks a T cell autoepitope

Infection with group A streptococci (GAS) can lead to rheumatic fever (RF) and rheumatic heart disease (RHD) which are a major health concern particularly in indigenous populations worldwide, and especially in Australian Aboriginals. A primary route of GAS infection is via the upper respiratory tract, and therefore, a major goal of research is the development of a mucosal-based GAS vaccine. The majority of the research to date has focused on the GAS M protein since immunity to GAS is mediated by M protein type-specific opsonic antibodies. There are two major impediments to the development of a vaccine-the variability in M proteins and the potential for the induction of an autoimmune response. To develop a safe and broad-based vaccine, we have therefore focused on the GAS M protein conserved C-region, and have identified peptides, J8 and the closely related J8 peptide (J14), which may be important in protective immunity to GAS infection. Using a mucosal animal model system, our data have shown a high degree of throat GAS colonisation in B10.BR mice 24h following intranasal immunisation with the mucosal adjuvant, cholera toxin B subunit (CTB), and/or diptheria toxoid (dT) carrier, or PBS alone, and challenge with the M1 GAS strain. However, GAS colonisation of the throat was significantly reduced following intranasal immunisation of mice with the vaccine candidate J8 conjugated to dT or J14-dT when administered with CTB. Moreover, J8-dT/CTB and J14-dT/CTB-immunised mice had a significantly higher survival when compared to CTB and PBS-immunised control mice. These data indicate that immunity to GAS infection can be evoked by intranasal immunisation with a GAS M protein C-region peptide vaccine that contains a protective B cell epitope and lacks a T cell autoepitope.

Parenteral and mucosal delivery of a novel multi-epitope M protein-based group A streptococcal vaccine construct: investigation of immunogenicity in mice

Primary vaccine strategies against group A streptococci (GAS) have focused on the M protein--the target of opsonic antibodies important for protective immunity. We have previously reported protection of mice against GAS infection following parenteral delivery of a multi-epitope vaccine construct, referred to as a heteropolymer. This current report has assessed mucosal (intranasal (i.n.) and oral) delivery of the heteropolymer in mice with regard to the induction and specificity of mucosal and systemic antibody responses, and compared this to parenteral delivery. GAS-specific IgA responses were detected in saliva and gut upon i.n. and oral delivery of the heteropolymer co-administered with cholera toxin B subunit, respectively. High titre serum IgG responses were elicited to the heteropolymer following all routes of delivery when administered with adjuvant. Moreover, as with parenteral delivery, serum IgG antibodies were detected to the individual heteropolymer peptides following i.n. but not oral delivery. These data support the potential of the i.n. route in the mucosal delivery of a GAS vaccine.

Construction and characterization of a live, attenuated aroA deletion mutant of Pseudomonas aeruginosa as a candidate intranasal vaccine

Antibodies to the lipopolysaccharide O antigen of Pseudomonas aeruginosa mediate high-level immunity, but protective epitopes have proven to be poorly immunogenic, while nonprotective or minimally protective O-antigen epitopes often elicit the best immune responses. With the goal of developing a broadly protective P. aeruginosa vaccine, we used a gene replacement system based on the Flp recombinase to construct an unmarked aroA deletion mutant of the P. aeruginosa serogroup O2/O5 strain PAO1. The resultant aroA deletion mutant of PAO1 is designated PAO1 Delta aroA. The aroA deletion was confirmed by both PCR and failure of the mutant to grow on minimal media lacking aromatic amino acids. When evaluated for safety and immunogenicity in mice, PAO1 Delta aroA could be applied either intranasally or intraperitoneally at doses up to 5 x 10(9) CFU per mouse without adverse effects. No dissemination of PAO1 Delta aroA to blood, liver, or spleen was detected after intranasal application, and histological evidence of pneumonia was minimal. Intranasal immunization of mice and rabbits elicited high titers of immunoglobulin G to whole bacterial cells and to heat-stable bacterial antigens of all seven prototypic P. aeruginosa serogroup O2/O5 strains. The mouse antisera mediated potent phagocytic killing of most of the prototypic serogroup O2/O5 strains, while the rabbit antisera mediated phagocytic killing of several serogroup-heterologous strains in addition to killing all O2/O5 strains. This live, attenuated P. aeruginosa strain PAO1 Delta aroA appears to be safe for potential use as an intranasal vaccine and elicits high titers of opsonic antibodies against multiple strains of the P. aeruginosa O2/O5 serogroup.

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.

Biotechnological applications of phage and cell display

In recent years, the use of surface-display vectors for displaying polypeptides on the surface of bacteriophage and bacteria, combined with in vitro selection technologies, has transformed the way in which we generate and manipulate ligands, such as enzymes, antibodies and peptides. Phage display is based on expressing recombinant proteins or peptides fused to a phage coat protein. Bacterial display is based on expressing recombinant proteins fused to sorting signals that direct their incorporation on the cell surface. In both systems, the genetic information encoding for the displayed molecule is physically linked to its product via the displaying particle. Using these two complementary technologies, we are now able to design repertoires of ligands from scratch and use the power of affinity selection to select those ligands having the desired (biological) properties from a large excess of irrelevant ones. With phage display, tailor-made proteins (fused peptides, antibodies, enzymes, DNA-binding proteins) may be synthesized and selected to acquire the desired catalytic properties or affinity of binding and specificity for in vitro and in vivo diagnosis, for immunotherapy of human disease or for biocatalysis. Bacterial surface display has found a range of applications in the expression of various antigenic determinants, heterologous enzymes, single-chain antibodies, and combinatorial peptide libraries. This review explains the basis of phage and bacterial surface display and discusses the contributions made by these two leading technologies to biotechnological applications. This review focuses mainly on three areas where phage and cell display have had the greatest impact, namely, antibody engineering, enzyme technology and vaccine development.

Pathogenesis of group A streptococcal infections

Group A streptococci are model extracellular gram-positive pathogens responsible for pharyngitis, impetigo, rheumatic fever, and acute glomerulonephritis. A resurgence of invasive streptococcal diseases and rheumatic fever has appeared in outbreaks over the past 10 years, with a predominant M1 serotype as well as others identified with the outbreaks. emm (M protein) gene sequencing has changed serotyping, and new virulence genes and new virulence regulatory networks have been defined. The emm gene superfamily has expanded to include antiphagocytic molecules and immunoglobulin-binding proteins with common structural features. At least nine superantigens have been characterized, all of which may contribute to toxic streptococcal syndrome. An emerging theme is the dichotomy between skin and throat strains in their epidemiology and genetic makeup. Eleven adhesins have been reported, and surface plasmin-binding proteins have been defined. The strong resistance of the group A streptococcus to phagocytosis is related to factor H and fibrinogen binding by M protein and to disarming complement component C5a by the C5a peptidase. Molecular mimicry appears to play a role in autoimmune mechanisms involved in rheumatic fever, while nephritis strain-associated proteins may lead to immune-mediated acute glomerulonephritis. Vaccine strategies have focused on recombinant M protein and C5a peptidase vaccines, and mucosal vaccine delivery systems are under investigation.

Extending the CD4(+) T-cell epitope specificity of the Th1 immune response to an antigen using a Salmonella enterica serovar typhimurium delivery vehicle

We analyzed the CD4 T-cell immunodominance of the response to a model antigen (Ag), MalE, when delivered by an attenuated strain of Salmonella enterica serovar Typhimurium (SL3261*pMalE). Compared to purified MalE Ag administered with adjuvant, the mapping of the peptide-specific proliferative responses showed qualitative differences when we used the Salmonella vehicle. We observed the disappearance of one out of eight MalE peptides' T-cell reactivity upon SL3261*pMalE immunization, but this phenomenon was probably due to a low level of T-cell priming, since it could be overcome by further immunization. The most striking effect of SL3261*pMalE administration was the activation and stimulation of new MalE peptide-specific T-cell responses that were silent after administration of purified Ag with adjuvant. Ag presentation assays performed with MalE-specific T-cell hybridomas showed that infection of Ag-presenting cells by this intracellular attenuated bacterium did not affect the processing and presentation of the different MalE peptides by major histocompatibility complex (MHC) class II molecules and therefore did not account for immunodominance modulation. Thus, immunodominance of the T-cell response to microorganisms is governed not only by the frequency of the available T-cell repertoire or the processing steps in Ag-presenting cells that lead to MHC presentation but also by other parameters probably related to the infectious process and to the bacterial products. Our results indicate that, upon infection by a microorganism, the specificity of the T-cell response induced against its Ags can be much more effective than with purified Ags and that it cannot completely be mimicked by purified Ags administered with adjuvant.

Viral recombinant vaccines to the E6 and E7 antigens of HPV-16

Most cancerous lesions of the uterine cervix are linked to persistent infections with human papillomaviruses (HPV), most notably HPV-16 or -18. Vaccine-induced immune responses to the HPV early antigens E6 and E7, which contribute to cell transformation and are thus expressed in these cervical cancers, could potentially eradicate malignant cells. We generated recombinant vaccines based on E1-deleted adenovirus human strain 5 or on vaccinia virus strain Copenhagen expressing either the E6 or E7 oncoproteins of HPV-16. The different vaccines were compared in two experimental mouse tumor models employing Balb/c or C57Bl/6 mice. Data presented here demonstrate that depending on the model either CD4(+) or CD8(+) T cells provide protection to tumor cell challenge, resulting in striking differences in the efficacy of the four vaccines under investigation.

Immunogenicity of the extracellular copper/zinc superoxide dismutase of the filarial parasite Acanthocheilonema viteae delivered by a two-phase vaccine strain of Salmonella typhimurium

The recombinant extracellular copper/zinc superoxide dismutase of the filarial parasite Acanthocheilonema viteae (AVSOD2) was cloned in an expression vector under control of the bacteriophage T7 promoter and the resulting plasmid pLAT7 was introduced in tha aroA attenuated Salmonella typhimurium vaccine strain SL3261:pYZ84. This vaccine strain carries a chromosomally integrated two phase expression system containing inducible T7 RNA polymerase. The recombinant AVSOD2 was efficiently expressed, constituting up to 5% of the total bacterial protein. Furthermore, the plasmid vector containing the AVSOD2 cDNA was shown to be stable over a long period of time in the vaccine strain without antibiotic selection in vitro and in vivo. Jirds which were immunised orally with the recombinant vaccine strain expressing the A. viteae EC-SOD produced a strong humoral immune response.

Modulation of autoimmune disease in the MRL-lpr/lpr mouse by IL-2 and TGF-beta1 gene therapy using attenuated Salmonella typhimurium as gene carrier

We have investigated the effects of interleukin-2 (IL-2) and transforming growth factor-beta (TGF-beta) gene therapy on the progress of autoimmune disease in MRL-lpr/lpr mice, a murine model of systemic lupus erythematosus (SLE). These mice have uncontrolled proliferation of T cells, an impaired response to T cell mitogen and produce autoantibodies against nuclear antigens, including DNA. Immune complexes formed by these autoantibodies are believed to cause glomerulonephritis and vasculitis in lupus mice and human SLE. Since there is an imbalance of cytokine production in both SLE patients and lupus mice, we examined the effects of cytokine gene therapy on the progression of autoimmune disease in MRL-lpr/lpr mice. The mice were treated orally with a non-pathogenic strain of Salmonella typhimurium bearing the aroA-aroD- mutations and carrying the murine genes encoding IL-2 and TGF-beta. The bacteria synthesise and slowly release the cytokines in vivo. Our results show that, contrary to expectation, TGF-beta gene therapy produced no improvement in pathology and generally had opposite effects to those of IL-2. IL-2 gene therapy restored the defective T cell proliferative response to mitogen and suppressed the autoantibody response, glomerulonephritis and growth of lymphoid tumours.

Vaccine strategies to prevent rheumatic fever

Group A streptococci (GAS) are responsible for numerous human illnesses, ranging from pharyngitis to severe invasive infections, such as necrotizing fascitis and toxic shock syndrome to the postinfectious sequelae, acute rheumatic fever (ARF), and glomerulonephritis. To date, to develop a vaccine, studies have focused on the M protein. However, designing a vaccine to prevent GAS infection based on this molecule has been hampered by the vast number of M protein serotypes and the possibility that it may induce potentially harmful autoimmune reactions. In this article, the authors discuss recent approaches to overcoming the problems of an M protein-based vaccine. In addition, recent studies identifying the protective properties of other streptococcal antigens and their potential as vaccine candidates are discussed.

Genetic control of immune response to recombinant antigens carried by an attenuated Salmonella typhimurium vaccine strain: Nramp1 influences T-helper subset responses and protection against leishmanial challenge

Attenuated strains of Salmonella typhimurium have been widely used as vehicles for delivery and expression of vaccine antigens in murine models of infectious disease. In mice, early bacterial replication following infection with S. typhimurium is controlled by the gene (Nramp1, formerly Ity/Lsh/Bcg) encoding the natural-resistance-associated macrophage protein (Nramp1). Nramp1 regulates macrophage activation and has multiple pleiotropic effects, including regulation of tumor necrosis factor alpha, interleukin 1beta (IL-1beta), and major histocompatibility complex class II molecules, all of which influence antigen processing and presentation. Nramp1 also has a direct effect on antigen processing, possibly by regulating the activity of proteases in the late endosomal compartment. Hence, there are multiple ways (regulation of bacterial load or recombinant antigen dose, class II molecule expression, costimulatory or adjuvant activity, and antigen processing) that Nramp1 might influence responses to recombinant salmonella vaccines. To test the hypothesis that Nramp1 influences responses to vaccination, congenic mouse strains have been used to analyze immune responses to recombinant antigens (tetanus toxoid antigen and leishmanial gp63) carried by live attenuated S. typhimurium aroA aroD mutants. Results show that congenic mice carrying the wild-type (S. typhimurium resistance) Nramp1 allele mount a predominantly T-helper-1 (IL-2 and gamma interferon) response to vaccination and show enhanced resolution of lesions following challenge infection with Leishmania major. In contrast, mice carrying mutant (S. typhimurium susceptibility) Nramp1 mount a T-helper-2 (immunoglobulin E and IL-4) response and show exacerbated lesion growth upon challenge.

Expression and immunogenicity of V3 loop epitopes of HIV-1, isolates SC and WMJ2, inserted in Salmonella flagellin

Synthetic oligonucleotides corresponding to specific V3 loop portions of two HIV-1 isolates, SC and WMJ2, were expressed in the flagella of a Salmonella live-vaccine strain. Expression of the inserted epitopes in flagellin and their exposure at the surface of flagellar filaments were shown by immunoblotting and immunogold labeling with anti-flagellin (Salmonella d) and anti-HIV-1(IIIB) V3 loop peptide sera. Live recombinant Salmonella strains expressing either one of the two V3 loop inserts were administered intraperitoneally to BALB/c mice. All these animals developed antibodies specific for the heterologous glycoprotein 120 (gp120) of HIV-1 MN strain, as detected by enzyme-linked immunosorbent assays (ELISA), two of the sera had neutralizing activity against the heterologous HIV-1 MN strain. Moreover, oral administration of the live Salmonella recombinant strains to mice evoked specific IgA directed against gp120.

Bacterial surface display: trends and progress

Heterologous surface display on Gram-negative bacterial was first described a decade ago and is now an active research area. More recently, strategies for surface display on Gram-positive bacterial have also been devised and these carry some inherent advantages. Bacterial surface display has found a range of applications in the expression of various antigenic determinants, heterologous enzymes, single-chain antibodies, polyhistidyl tags and even entire peptide libraries. This article explains the basis of bacterial surface display and discusses current uses and possible future trends of this emerging technology.

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.

Immunization of Macaca mulatta with aroA attenuated Salmonella typhimurium expressing the SIVp27 antigen

Attenuated bacteria expressing foreign antigens stimulate both systemic and mucosal immune responses to the recombinant protein. We studied the infection of rhesus macaques with an attenuated Salmonella typhimurium expressing the simian immunodeficiency virus p27 capsid protein. Juvenile rhesus macaques were inoculated by intragastric intubation with doses ranging from 3 to 9 x 10(9) viable aroA attenuated S. typhimurium. The bacterial infection was self-limiting with no overt clinical signs. Salmonella were shed in the feces of macaques for approximately five days. Salmonella were isolated from fecal material to examine the in vivo stability of both the attenuating mutation and the integrated SIVp27 expression cassette. All Salmonella isolates retained both the attenuating mutation and the recombinant expression construct. In vitro analysis showed that a minimum of 7.2 microg of p27 was delivered by a single oral dose with attenuated, recombinant S. typhimurium.

Control by H-2 genes of the Th1 response induced against a foreign antigen expressed by attenuated Salmonella typhimurium

Attenuated salmonellae represent an attractive vehicle for the delivery of heterologous protective antigens to the immune system. Here, we have investigated the influence of the genetic background of the host which regulates the growth and elimination of Salmonella cells on the cellular response induced against a foreign antigen delivered by an aroA Salmonella strain. We have tested CD4+ T-cell responses (cell proliferation and cytokine production) in various mouse strains following immunization with Salmonella typhimurium SL3261 expressing a high level of the recombinant Escherichia coli MalE protein. We were able to detect a CD4+ T-cell response against the recombinant MalE protein only in a restricted number of mouse strains, whereas all mice produced good levels of anti-MalE immunoglobulin G antibodies. The Ity gene did not play a major role in these differences in T-cell responses, since both Ity-resistant and -susceptible strains of mice were found to be unresponsive to MalE delivered by recombinant salmonellae. In contrast, when B10 congenic mice were used, a correlation was established between MalE-specific T-cell unresponsiveness and H-2 genes. The discrepancies described in this paper in the ability of various strains of mice to develop an efficient Th1 response against a recombinant antigen displayed by a live Salmonella vaccine underscore the difficulties that can be encountered in the vaccination of human populations by such a strategy.

Effect of Haemophilus influenzae type b conjugate vaccine in combination with peroral immunization with Escherichia coli on experimental otitis media

The protective ability of a conjugated Haemophilus influenzae type b vaccine, ACT-HIB, used singly or in combination with orally administered Escherichia coli, was investigated in a rat model for acute otitis media. The humoral response to ACT-HIB was also analyzed. The study demonstrated that ACT-HIB vaccination resulted in a prompt antibody response, and that ACT-HIB was efficient in preventing middle ear infections caused by Haemophilus influenzae type b. The efficiency increased if the vaccine was combined with Escherichia coli. The results suggest that Escherichia coli could possibly be useful in the future as a vaccine vehicle, and since Haemophilus influenzae acute mastoiditis seems to be almost exclusively due to serotype b, the incidence of this infection may be reduced with the conjugated Haemophilus influenzae type b vaccines.

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.

Targeting vaccinia virus-expressed secretory beta subunit of human chorionic gonadotropin to the cell surface induces antibodies

We carried out experiments designed to study the effect of a protein's localization on its immunogenicity. A novel cell-surface protein was generated from a small, glycosylated secretory protein. The DNA sequence encoding the entire precursor of the human chorionic gonadotropin beta (beta hCG) subunit was fused in the correct reading frame to the DNA sequence encoding the transmembrane and cytoplasmic domains of vesicular stomatitis virus glycoprotein. This chimeric gene was introduced into the vaccinia virus genome to generate a recombinant virus. The recombinant virus, when used to infect animal cells, expressed a 135-amino-acid beta hCG subunit anchored in cellular membranes by the 48 carboxy-terminal amino acids of vesicular stomatitis virus glycoprotein. The immunogenicity of this recombinant virus with respect to its ability to generate anti-hCG antibodies was compared with that of a second recombinant vaccinia virus expressing the native secretory form of beta hCG. All animals immunized with the vaccinia virus expressing beta hCG on the cell surface elicited high titers of anti-hCG antibodies. Even after a single immunization with the recombinant vaccinia virus, the anti-hCG antibody titers persisted for a long period of time (more than 6 months). None of the animals immunized with vaccinia virus expressing the native secretory form of beta hCG showed any hCG-specific antibody response.

Surface display compared to periplasmic expression of a malarial antigen in Salmonella typhimurium and its implications for immunogenicity

Two different expression systems were investigated for the production of an 80 amino acid polypeptide, M3, from the C-terminus of the Plasmodium falciparum blood stage antigen Pf155/RESA in an attenuated Salmonella typhimurium vaccine strain. Upon expression, the malarial polypeptide was targeted either to the periplasm as a soluble fusion protein containing two IgG-binding domains (ZZ) from the staphylococcal protein A or, to the bacterial surface as an insert within a chimeric outer membrane protein A (OmpA) derived from Escherichia coli and Shigella dysenteriae. Both the ZZM3 and the OmpAM3 proteins were stably expressed in the periplasm or on the surface of Salmonella, respectively. The ZZ expression system yielded 10-100 times more malarial immunogen than did the OmpA system. Live recombinant Salmonella expressing ZZM3 or OmpAM3 were used to immunize mice intraperitoneally. Both the ZZM3 and OmpAM3 genes persisted for up to three weeks in bacteria isolated from different lymphoid organs. Bacteria expressing ZZM3 induced antibodies to M3, ZZ and to the Pf155/RESA antigen whereas, bacteria producing OmpAM3 induced similar levels of antibodies reactive with M3 but not with Pf155/RESA. Both recombinants induced a memory response of antibodies reactive with both M3 and Pf155/RESA. The high levels of M3 produced by the ZZ expression system make it suitable for the expression of heterologous antigens in Salmonella. Nevertheless, in spite of the quantitative difference in M3 expression, the ZZ and OmpA constructs elicited comparable immune responses to M3.

Construction and immunogenicity of Salmonella typhimurium vaccine vectors that express HIV-1 gp120

Since the human immunodeficiency virus (HIV-1) is transmitted either parenterally or sexually, both mucosal and systemic immune responses may be required to provide protective immunity. Attenuated Salmonella vectors expressing heterologous antigen can stimulate responses in both compartments. To evaluate the utility of Salmonella vectors as an HIV-1 vector vaccine, a gene expression cassette encoding recombinant HIV-1 gp120 (rgp120) was integrated into the hisOGD locus of Salmonella typhimurium aroA strain, SL3261 (SL3261::120). To test if increased antigen expression potentiates immunogenicity, strains were constructed that express rgp120 from a multicopy asd-stabilized plasmid (SL7207 pYA:120). Immunoblot analysis demonstrated that SL7207 pYA:120 expressed approximately 50-fold more rgp120 than SL3261::120. Oral immunization of BALB/c mice with these strains did not stimulate an env-specific CTL response or a significant rise in antigp120 antibody titer as compared to controls. However, splenic T cells from SL7207 pYA::120 immunized mice proliferated upon restimulation with gp120 in vitro while splenocytes from SL3261::120 immunized mice did not, gp120 restimulated splenic T cells from SL7207 pYA:120 immune mice also produced IFN-gamma but no IL-5. Two conclusions can be drawn from these results. First, high level expression of rgp120 in Salmonella vectors is necessary to stimulate a gp120-specific immune response in mice. Second, Salmonella::rgp120 stimulates a gp120-specific Th1 response in mice. This is the first report to describe the construction of a Salmonella::rgp120 vector vaccine that is immunogenic in mice.

An assessment of mucosal immunisation in protection against Streptococcus equi ('Strangles') infections in horses

The ability of mucosally administered antigen to provide protection against Streptococcus equi ('Strangles') infections in horses was examined. First, an enzyme linked immunosorbent assay (ELISA) was developed to detect the immune status of horses to S. equi. This assay was used to select Strangles-naive horses for the study and also to monitor their response to immunisation. Potential vaccine candidates were: (a) orally administered paraformaldehyde killed S. equi; (b) intraperitoneally (IP) administered paraformaldehyde killed S. equi in a non-inflammatory adjuvant; (c) orally administered live avirulent S. equi; (d) orally administered microencapsulated streptococcal M protein. The latter three preparations were first assessed in a rat model, using rate of lung bacterial clearance following intratracheal inoculation of live virulent bacteria as an indication of efficacy. Candidates (a) and (b) were then assessed in an equine model. IP immunisation of horses was shown to effectively induce production of specific antibody in mucosal and systemic sites. Four weeks after initial immunisation, horses were challenged intranasally with live virulent S. equi. Both groups of immunised horses demonstrated partial protection following vaccination. Of the IP immunised horses, only two out of four developed clinical signs of Strangles following live challenge. The orally immunised horses all developed submandibular abscesses containing S. equi. However, none of the immunised horses became as ill as the control horses in terms of fever, anorexia, loss of condition and general malaise.

Current perspectives in salmonellosis

Salmonellosis remains an important human and animal problem worldwide and, despite extensive research effort, many of the details of its pathogenesis are not known. While there have been recent advances in some aspects of pathogenesis, other areas are not understood. The host adaptation shown by several serotypes and the recent dramatic changes in the predominance of particular serotypes are examples. Molecular techniques using in vitro model systems have identified several genes involved in adhesion and invasion, though their function and even their relevance to disease remain poorly defined. Similarly, several potential toxins have been identified and the genes cloned, although their significance is far from clear. Some of the essential genes on the large virulence plasmids have been defined, and these are known to be necessary for the establishment of systemic infection. Two of these genes are regulatory, but the function of the other genes is unknown. A general theme has been the identification of gene systems involved in regulation of virulence. New vaccines, based on 'rational attenuation' are being designed, and these have also been used to carry heterologous antigens; such vaccines are currently undergoing trials. The improved understanding of the pathogenesis of salmonellosis may also provide a model of wide applicability to a more general understanding of bacterial pathogenesis. New techniques, including the polymerase chain reaction, are being applied to diagnose salmonellosis.

Influence of preimmunization with tetanus toxoid on immune responses to tetanus toxin fragment C-guest antigen fusions in a Salmonella vaccine carrier

We have previously described a new system for the delivery of recombinant antigens in live Salmonella vaccines as genetic fusions to the C terminus of fragment C of tetanus toxin (TetC) driven by the anaerobically inducible nirB promoter. It has been reported that preimmunization with tetanus toxoid (TT) can suppress the antibody response to peptides chemically coupled to TT (epitope-specific suppression) in both animals and humans, which could interfere with efficacy of the Salmonella-TetC delivery system. We report that preimmunization of BALB/c mice with TT in alum did not suppress the response to either of two protective antigens of Schistosoma mansoni, the full-length S. mansoni P28 glutathione S-transferase (P28) and a construct consisting of eight tandem copies of the protective peptide comprising amino acids 115 to 131 of P28. The guest antigens were expressed in the aroA Salmonella typhimurium SL3261 vaccine strain as fusions to TetC. Preimmunization with TT 10 weeks before administration of the recombinant salmonellae did not alter the antibody response to the full-length P28, whereas the response to the peptide comprising amino acids 115 to 131 was increased by preimmunization with TT, with the increase seen mainly in the immunoglobulin G1 isotype. The antitetanus response was increased by preimmunization with TT in all groups receiving salmonellae expressing TetC. The results could be important when one is considering the use of the Salmonella-TetC delivery system in populations preimmunized with TT.

Oral vaccination with an attenuated Salmonella typhimurium strain expressing Borrelia burgdorferi OspA prevents murine Lyme borreliosis

Borrelia burgdorferi is the causative agent of Lyme disease. In the mouse model, protection is correlated with the development of antibodies to a major outer surface protein, OspA. In this study, we expressed OspA in an attenuated strain of Salmonella typhimurium and tested the efficacy of the transformed strain in protecting against disease. We show that mice inoculated by gavage developed high titers of anti-OspA antibodies and were protected against an intradermal challenge with the spirochete.

Expression and immunogenicity of an 18-residue epitope of HIV1 gp41 inserted in the flagellar protein of a Salmonella live vaccine

A synthetic oligonucleotide specifying residues 735-752 of the product of the env gene of HIV1 IIIB was inserted by blunt-end ligation at restriction sites in the hypervariable, antigenically determinant region IV of two flagellin genes. Its integration, in frame and correct orientation, into gene fliC(d) in plasmid pLS408 allowed production of functional flagella when the plasmid was placed in a flagellin-negative aroA live-vaccine Salmonella dublin strain, SL5928. Bacteria thus made motile were immobilized and agglutinated by anti-(735-752 peptide) serum; expression was also shown by immunoelectron-microscopy and by Western blot of whole-cell lysates. Enzyme immunoassay (EIA) of sera of mice given three doses by intraperitoneal injection of the live-vaccine strain producing chimeric flagellin, or of concentrated flagella from it, showed production of antibody with affinity for the peptide, and in some sera, also for r-gp160. Pooled serum from mice given five i.p. doses of the live vaccine strain expressing the gp41 epitope at the surface of its flagellar filaments had higher titres of anti-peptide and anti-r-gp160 antibody and weak neutralizing activity on the IIIB isolate (90% neutralization at 1/100). The sera of nine mice given two doses of the live vaccine by the oral route had either no or only very low titres of antibody to flagellar antigen d; they were therefore not tested for anti-peptide activity.

Intracellular processing and presentation of T cell epitopes, expressed by recombinant Escherichia coli and Salmonella typhimurium, to human T cells

Vaccines based on recombinant attenuated bacteria represent a potentially safe and effective immunization strategy. A carrier system was developed to analyze in vitro whether foreign T cell epitopes, inserted in the outer membrane protein PhoE of Escherichia coli and expressed by recombinant bacteria, are efficiently processed and presented via human leukocyte antigen (HLA) class I and II molecules by bacterial infected human macrophages. A well-defined HLA-B27-restricted cytotoxic T cell (CTL) epitope and an HLA-DR53 restricted T helper (Th) epitope of the fusion protein of measles virus were genetically inserted in a surface-exposed region of PhoE, and the chimeric proteins were expressed in E. coli and Salmonella typhimurium. Macrophages infected with both recombinant bacteria presented the Th epitope to the specific CD4+ T cell clone, but failed to present the CTL epitope to the specific CD8+ T cell clone. Presentation of the Th epitope by the infected macrophages was inhibited by cytochalasin D, indicating that phagocytic processing of intact bacteria within infected macrophages was essential for antigen presentation via HLA class II. Presentation of the Th epitope to the CD4+ T cell clone by infected macrophages was blocked by brefeldin A and cycloheximide, indicating the requirement of nascent HLA class II molecules for presentation. The efficiency of macrophages to process and present the inserted Th epitope was similar for both the recombinant E. coli and S. typhimurium strains.

Expression of TGF-beta in attenuated Salmonella typhimurium: oral administration leads to the reduction of inflammation, IL-2 and IFN-gamma, but enhancement of IL-10, in carrageenin-induced oedema in mice

Mice injected with carrageenin in the footpad developed local inflammation which peaked at 48 hr. This was significantly reduced in mice inoculated orally with an attenuated Salmonella construct expressing transforming growth factor-beta (TGF-beta). Administration of the Salmonella construct alone had no effect on inflammation. High levels of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) were secreted by draining lymph node cells from mice injected with carrageenin following stimulation in vitro. Prior inoculation with Salmonella enhanced the production of IL-2 and IFN-gamma from the draining lymph node cells. Administration of the Salmonella-TGF-beta construct significantly inhibited the production of these cytokines. In contrast, IL-10 only was secreted from draining lymph node cells of animals inoculated with the Salmonella-TGF-beta construct. Thus, oral administration of TGF-beta can significantly inhibit local inflammation and alter the cytokine secretion pattern of cells from lymph nodes draining the site of inflammation.

Analysis of immunization route-related variation in the immune response to heat-killed Salmonella typhimurium in mice

In examinations of the factors regulating the quality and quantity of the immune response to Salmonella typhimurium, we have shown previously that live and heat-killed preparations of S. typhimurium can induce gamma interferon-dominant and interleukin-4-dominant immune responses, respectively, upon intraperitoneal (i.p.) immunization of BALB/c mice. Using this system to investigate the role of the route of immunization in the immune response, we show in the present study that i.p. immunization with heat-killed S. typhimurium generates a quantitatively better immune response than does intradermal (i.d.) immunization. The quantitative differences observed between the i.p. and i.d. routes are apparent in the amount of S. typhimurium-specific antibodies produced, the extent of responses in T-cell proliferation assays, and the quantities of lymphokines generated. However, the ratios of immunoglobulin (Ig) isotypes [IgG1/IgG2a] are comparable and the relative dominance of interleukin-4 over gamma interferon is seen in both i.p.- and i.d.-immunized mice, suggesting that the predominant T-cell effector pathways triggered are not qualitatively dependent on the route of immunization. An examination of the antigenic profile recognised by the B-cell and T-cell responses in i.p.- versus i.d.-immunized mice shows that while the Western immunoblot patterns recognized by serum antibodies from the two groups of mice were not significantly different, T cells from i.p.-immunized mice recognized a broader spectrum of antigens in an immunoblot assay than did those from i.d.-immunized mice. These data suggest that there may be a significant difference in the antigen-processing ability of peritoneal and dermal antigen-presenting cells for complex antigenic formulations such as bacterial vaccines.

Bacillus anthracis protective antigen, expressed in Salmonella typhimurium SL 3261, affords protection against anthrax spore challenge

The protective antigen (PA) gene from Bacillus anthracis has been expressed in Salmonella typhimurium SL 3261 (aroA). Expression was achieved by cloning the gene after the plac promoter in a high copy number plasmid. The recombinant PA was exported into the periplasm. This construct was unstable in vivo and also reduced the colonization ability of the host S. typhimurium. Mouse-passaging of the recombinant Salmonella resulted in a strain with enhanced colonization ability and increased stability of the plasmid in vivo. This effect appeared to be due to a reduction in copy number of the PA-encoding plasmid. Mice were vaccinated with recombinant S. typhimurium and adjuvanted PA and challenged with virulent B. anthracis. Only mice vaccinated with adjuvanted PA or orally with the mouse-passaged recombinant showed partial protection. The degree of protection observed after oral vaccination with the recombinant S. typhimurium was similar to the degree of protection afforded by adjuvanted PA and suggested that the use of S. typhimurium to deliver PA is an effective approach for inducing protection against B. anthracis. The results presented also suggest that the degree of protection demonstrated in the mouse may not fully indicate the potential of the recombinant Salmonella as an effective vaccine in other species.

Genetic control of antibody responses induced against an antigen delivered by recombinant attenuated Salmonella typhimurium

Recombinant derivatives of nonpathogenic bacteria such as attenuated Salmonella typhi have the potential to be used for delivery of heterologous antigens to the immune system. Genetic factors may modulate the immune responses to these live attenuated organisms and could therefore modify the immunogenicity of future human vaccines. In the present study, we compared the antibody responses of Ity or H-2 congenic strains of mice to a foreign antigen expressed by the murine attenuated aroA S. typhimurium strain. Our results demonstrate that the Ity gene may modulate the antibody responses to the foreign antigen but that the major genetic influence is exerted by H-2 genes, which control the capacity of mice to respond to the antigen expressed by recombinant attenuated Salmonella cells. This genetic control is related to differences in responsiveness of different strains of mice to low doses of antigen. Increasing the amount of foreign antigen expressed by recombinant Salmonella cells overcame the genetic restriction of these responses. These findings are potentially of great importance for the design of live vaccines for humans and show that care must be taken to optimize the amount of foreign antigen delivered to the immune system.

Parameters that influence the efficiency of processing antigenic epitopes expressed in Salmonella typhimurium

We investigated parameters that affect the efficiency with which antigenic epitopes from Salmonella typhimurium are processed for presentation to T lymphocytes. As a model system, the hen egg white lysozyme 52-61 [HEL(52-61)] epitope, which binds the murine major histocompatibility complex class II (MHC-II) molecule I-Ak, was expressed in soluble fusion proteins in S. typhimurium. Murine peritoneal macrophages mediated phagocytic processing of viable S. typhimurium expressing fusion proteins of the HEL epitope for presentation via I-Ak regardless of the bacterial compartment in which the epitope was contained (i.e., surface exposed, facing the periplasmic space, or in the cytoplasm). Minor differences in processing efficiency observed with different epitope compartmentalizations could be overcome by altering the relative expression level, indicating that epitope abundance is an important factor for efficient processing of epitopes from S. typhimurium. This processing pathway required phagocytosis of bacteria followed by passage through an acidic compartment, suggesting a pathway involving phagolysosomal degradation of the bacteria to liberate epitopes that bind MHC-II. HEL(52-61) was processed more efficiently from heat-killed S. typhimurium than from viable bacteria, and in addition, the HEL epitope was processed more efficiently from a rough lipopolysaccharide (LPS) strain than from its isogenic smooth LPS counterpart, most likely because of enhanced phagocytosis of the rough LPS strain. These data suggest that the efficiency of epitope processing from S. typhimurium for presentation via MHC-II is affected by bacterial viability, epitope abundance, and LPS phenotype, factors which may be important to consider in development of recombinant S. typhimurium vaccine strains.

Protection of immunosuppressed mice against translocation of Pseudomonas aeruginosa from the gut by oral immunization with recombinant Pseudomonas aeruginosa outer membrane protein I expressing Salmonella dublin

AroA Salmonella dublin was used as recipient for a plasmid coding for the outer membrane protein I (OprI) of Pseudomonas aeruginosa. Oral immunization of Balb/c mice with recombinant S. dublin induced serum IgG and IgA antibodies against P. aeruginosa. In spleen and Peyer's patches anti-P. aeruginosa IgG- and IgA-secreting cells could be measured by the ELISPOT technique. In an oral challenge of immunosuppressed mice with P. aeruginosa the orally immunized animals had a 58-fold higher LD50 than control animals.

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.