Expression, extracellular secretion, and immunogenicity of the Plasmodium falciparum sporozoite surface protein 2 in Salmonella vaccine strains

Biotechnological applications of type 1 secretion systems

Bacteria have evolved a diverse range of secretion systems to export different substrates across their cell envelope. Although secretion of proteins into the extracellular space could offer advantages for recombinant protein production, the low secretion titers of the secretion systems for some heterologous proteins remain a clear drawback of their utility at commercial scales. Therefore, a potential use of most of secretion systems as production platforms at large scales are still limited. To overcome this limitation, remarkable efforts have been made toward improving the secretion efficiency of different bacterial secretion systems in recent years. Here, we review the progress with respect to biotechnological applications of type I secretion system (T1SS) of Gram-negative bacteria. We will also focus on the applicability of T1SS for the secretion of heterologous proteins as well as vaccine development. Last but not least, we explore the employed engineering strategies that have enhanced the secretion efficiencies of T1SS. Attention is also paid to directed evolution approaches that may offer a more versatile approach to optimize secretion efficiency of T1SS.

Prospects for Malaria Vaccines: Pre-Erythrocytic Stages, Blood Stages, and Transmission-Blocking Stages

Malaria is a disease of public health importance in many parts of the world. Currently, there is no effective way to eradicate malaria, so developing safe, efficient, and cost-effective vaccines against this disease remains an important goal. Current research on malaria vaccines is focused on developing vaccines against pre-erythrocytic stage parasites and blood-stage parasites or on developing a transmission-blocking vaccine. Here, we briefly describe the progress made towards a vaccine against Plasmodium falciparum, the most pathogenic of the malaria parasite species to infect humans.

Protective immunity against enteral stages of Trichinella spiralis elicited in mice by live attenuated Salmonella vaccine that secretes a 30-mer parasite epitope fused to the molecular adjuvant C3d-P28

The development of a veterinary vaccine against T. spiralis infection is an alternative strategy to control trichinellosis. In an effort to develop an efficient vaccine, BALB/c mice were immunized with attenuated Salmonella enterica serovar Typhimurium SL3261 that expresses a 30-mer peptide (Ag30) derived from the gp43 of T. spiralis muscle larvae fused to three copies of the molecular adjuvant P28 (Ag30-P283) and it was either displayed on the surface or secreted by recombinant Salmonella strains. Salmonella strain secreting Ag30-P283, reduced the adult worm burden 92.8% following challenge with T. spiralis muscle larvae compared to 42% achieved by recombinant Salmonella displaying Ag30-P283 on the surface. The protection induced by secreted Ag30-P283 was associated with a mixed Th1/Th2 with predominance of Th2 phenotype, which was characterized by the production of IgG1, intestinal IgA antibodies and IL-5 secretion. This finding could provide an efficient platform technology for the design of novel vaccination strategies.

Oral vaccine delivery: can it protect against non-mucosal pathogens?

Vaccination is an efficient and cost-effective form of preventing infectious diseases. However, most currently available vaccines are delivered by injection, which makes mass immunization more costly and less safe, particularly in resource-poor developing countries. Oral vaccines have several attractive features compared with parenteral vaccines, but studies on their use have been limited almost exclusively to protection against mucosally transmitted pathogens. Their potential for controlling non-mucosally transmitted diseases has not yet been appreciated in general. In this article, we provide evidence that oral immunization is a feasible alternative for preventing infections transmitted through non-mucosal routes, including infections such as malaria, Japanese encephalitis and hepatitis B. Although there are still hurdles to overcome before such approaches can be deployed widely, recent progress in the oral vaccination field and the availability of a range of delivery systems offers hope for the development of a larger number of oral vaccines.

Cancer immunotherapy based on recombinant Salmonella enterica serovar Typhimurium aroA strains secreting prostate-specific antigen and cholera toxin subunit B

Prostate cancer is the most common malignant tumor in men and is normally associated with increased serum levels of prostate-specific antigen (PSA). Therefore, PSA is one potential target for a prostate cancer vaccine. In this study we analyzed the functionality of new bacterial PSA vaccines, expressed and secreted via the hemolysin (HlyA) secretion system of Escherichia coli, the prototype of Type I secretion systems (T1SS) using an attenuated Salmonella enterica serovar Typhimurium aroA strain as carrier. The data demonstrate that a bacterial live vaccine encompassing T1SS in combination with cholera toxin subunit B can be successfully used for delivery of PSA to induce cytotoxic CD8+ T-cell responses resulting in an efficient prevention of tumor growth in mice.

Enhanced immunity to Plasmodium falciparum circumsporozoite protein (PfCSP) by using Salmonella enterica serovar Typhi expressing PfCSP and a PfCSP-encoding DNA vaccine in a heterologous prime-boost strategy

Two Salmonella enterica serovar Typhi strains that express and export a truncated version of Plasmodium falciparum circumsporozoite surface protein (tCSP) fused to Salmonella serovar Typhi cytolysin A (ClyA) were constructed as a first step in the development of a preerythrocytic malaria vaccine. Synthetic codon-optimized genes (t-csp1 and t-csp2), containing immunodominant B- and T-cell epitopes present in native P. falciparum circumsporozoite surface protein (PfCSP), were fused in frame to the carboxyl terminus of the ClyA gene (clyA::t-csp) in genetically stabilized expression plasmids. Expression and export of ClyA-tCSP1 and ClyA-tCSP2 by Salmonella serovar Typhi vaccine strain CVD 908-htrA were demonstrated by immunoblotting of whole-cell lysates and culture supernatants. The immunogenicity of these constructs was evaluated using a "heterologous prime-boost" approach consisting of mucosal priming with Salmonella serovar Typhi expressing ClyA-tCSP1 and ClyA-tCSP2, followed by parenteral boosting with PfCSP DNA vaccines pVR2510 and pVR2571. Mice primed intranasally on days 0 and 28 with CVD 908-htrA(pSEC10tcsp2) and boosted intradermally on day 56 with PfCSP DNA vaccine pVR2571 induced high titers of serum NANP immunoglobulin G (IgG) (predominantly IgG2a); no serological responses to DNA vaccination were observed in the absence of Salmonella serovar Typhi-PfCSP priming. Mice primed with Salmonella serovar Typhi expressing tCSP2 and boosted with PfCSP DNA also developed high frequencies of gamma interferon-secreting cells, which surpassed those produced by PfCSP DNA in the absence of priming. A prime-boost regimen consisting of mucosal delivery of PfCSP exported from a Salmonella-based live-vector vaccine followed by a parenteral PfCSP DNA boosting is a promising strategy for the development of a live-vector-based malaria vaccine.

Immunization of volunteers with Salmonella enterica serovar Typhi strain Ty21a elicits the oligoclonal expansion of CD8+ T cells with predominant Vbeta repertoires

CD8(+) T cells are likely to play an important role in host defense against Salmonella enterica serovar Typhi by several effector mechanisms, including lysis of infected cells (cytotoxicity) and gamma interferon (IFN-gamma) secretion. In an effort to better understand these responses, we studied the T-cell receptor (TCR) repertoire of serovar Typhi-specific CD8(+) T cells in humans. To this end, we determined the TCR beta chain (Vbeta) usage of CD8(+) T cells from three volunteers orally immunized with Ty21a typhoid vaccine by flow cytometry using a panel of monoclonal antibodies. Although TCR Vbeta usage varied among volunteers, we identified oligoclonal Vbeta subset expansions in individual volunteers (Vbeta 2, 5.1, 8, 17, and 22 in volunteer 1; Vbeta 1, 2, 5.1, 14, 17, and 22 in volunteer 2; and Vbeta 3, 8, 14, and 16 in volunteer 3). These subsets were antigen specific, as shown by cytotoxicity and IFN-gamma secretion assays on Vbeta sorted cells and on T-cell clones derived from these volunteers. Moreover, eight-color flow cytometric analysis showed that these clones exhibited a T effector memory phenotype (i.e., CCR7(-) CD27(-) CD45RO(+) CD62L(-)) and coexpressed gut homing molecules (e.g., high levels of integrin alpha4beta7, intermediate levels of CCR9, and low levels of CD103). In conclusion, our results show that long-term T-cell responses to serovar Typhi in Ty21a vaccinees are oligoclonal, involving multiple TCR Vbeta families. Moreover, these serovar Typhi-specific CD8(+) T cells bearing defined Vbeta specificities are phenotypically and functionally consistent with T effector memory cells with preferential gut homing potential.

Orally delivered malaria vaccines: not too hard to swallow

Vaccines offer efficient and cost-effective protection against a wide range of infectious diseases. Unfortunately, no effective vaccine is yet available against malaria, and this infection remains one of the most important causes of human morbidity and mortality in the developing world. Over the past two decades a number of candidate proteins for inclusion in a subunit vaccine have been identified. Malariologists believe that an effective malaria vaccine will need to include multiple proteins that induce protective immune responses against different stages of the Plasmodium life cycle. The construction of such multivalent vaccines is beset by considerable logistical difficulties, not least of which is how to deliver them to a population living in endemic areas. Compared with other routes of vaccine administration, oral delivery has several advantages that make it an attractive strategy for vaccine development. This review summarises the progress towards an oral vaccine delivery system for malaria and discusses the feasibility of this approach.

Use of the α-hemolysin secretion system of Escherichia coli for antigen delivery in the Salmonella typhi Ty21a vaccine strain

This study examined the suitability of the hemolysin secretion system of Escherichia coli for expression and delivery of alpha-hemolysin (HlyA) by the S. typhi Ty21a strain, the only live oral Salmonella vaccine strain licensed for human use, under in vitro and in vivo conditions. For this purpose, two plasmid vectors encoding either the whole alpha-hemolysin of E. coli (pANN202-812/pMOhly2) or the hemolysin secretion signal (pMOhly1) were transferred into S. typhi Ty21a. S. typhi Ty21a carrying pANN202-812/pMOhly2 revealed efficient secretion of hemolysin in vitro. After formulation according to a process suitable for commercial production of Salmonella-based live bacterial vaccines, plasmids were shown to be stable in Ty21a and hemolysin secretion was demonstrated even after storage of the strains under real-time and stress conditions. After intranasal immunization of mice with S. typhi Ty21a/pANN202-812 plasmids are stable in vivo, and immunization induced a profound immune response against the heterologous HlyA antigen. Therefore, the combination of the hemolysin secretion system and S. typhi Ty21a could form the basis for a new generation of live bacterial vaccines.

Identification of a human HLA-E-restricted CD8+ T cell subset in volunteers immunized with Salmonella enterica serovar Typhi strain Ty21a typhoid vaccine

Our previous studies in volunteers immunized with Salmonella enterica serovar Typhi (S. Typhi) have suggested an important role for CD8+ T cells in host defense. In this study we describe a novel subset of nonclassical human HLA-E-restricted S. Typhi-specific CD8+ T cells derived from PBMC of Ty21a typhoid vaccinees. CD3+CD8+CD4-CD56- T cells effectively killed S. Typhi-infected targets regardless of whether they share classical HLA class I molecules with them, by a FAS-independent, granule-dependent mechanism, as evidenced by induction of granzyme B release and the blocking effects of concanamycin and strontium ions. The expression of HLA-E Ags, but not CD1-a, -b, or -c, on the membrane of S. Typhi-infected targets rendered them susceptible to lysis. Moreover, anti-HLA-E Abs partially blocked these responses. We also demonstrated that presentation of S. Typhi Ags via HLA-E could stimulate IFN-gamma production. Increases in the net frequency of IFN-gamma spot-forming cells were observed in the presence of targets coated with peptides that contain S. Typhi GroEL HLA-E binding motifs. These results demonstrate that HLA-E binds nonamer peptides derived from bacterial proteins and trigger CD8+-mediated lysis and IFN-gamma production when exposed to infected targets, raising the possibility that this novel effector mechanism might contribute to host defense against intracellular bacterial infections.

Recombinant Salmonella enterica serovar Typhi in a prime-boost strategy

This study investigated the utility of attenuated Salmonella enterica serovar Typhi strain CVD 908-htrA (908 h) in a heterologous prime-boost strategy. Mice primed intranasally (i.n.) with 908 h expressing fragment C (Frag C) of tetanus toxin and boosted intramuscularly (i.m.) with tetanus toxoid (TT) mounted enhanced and accelerated serum IgG anti-Frag C responses in comparison to unprimed, vector-primed and homologously-primed and boosted mice. Serum antitoxin responses were also determined; mice that were vaccinated following a heterologous prime-boost regimen exhibited the highest levels of Frag C-specific toxin neutralizing antibodies 1 week after boosting. Mice primed and boosted i.m. with TT developed a significantly greater proportion of serum IgG1 antibodies and weaker IFN-gamma levels in contrast to those primed intranasally (i.n.) with rS. Typhi that were homologously or heterologously boosted. These encouraging pre-clinical data provide a rational basis for undertaking a pilot clinical trial to evaluate this strategy. An ability to stimulate enhanced, accelerated responses to parenteral vaccination following mucosal priming may be advantageous in the immunoprophylaxis of many infectious diseases, including those of biodefense importance.

Channel-tunnels: outer membrane components of type I secretion systems and multidrug efflux pumps of Gram-negative bacteria

For translocation across the cell envelope of Gram-negative bacteria, substances have to overcome two permeability barriers, the inner and outer membrane. Channel-tunnels are outer membrane proteins, which are central to two distinct export systems: the type I secretion system exporting proteins such as toxins or proteases, and efflux pumps discharging antibiotics, dyes, or heavy metals and thus mediating drug resistance. Protein secretion is driven by an inner membrane ATP-binding cassette (ABC) transporter while drug efflux occurs via an inner membrane proton antiporter. Both inner membrane transporters are associated with a periplasmic accessory protein that recruits an outer membrane channel-tunnel to form a functional export complex. Prototypes of these export systems are the hemolysin secretion system and the AcrAB/TolC drug efflux pump of Escherichia coli, which both employ TolC as an outer membrane component. Its remarkable conduit-like structure, protruding 100 A into the periplasmic space, reveals how both systems are capable of transporting substrates across both membranes directly from the cytosol into the external environment. Proteins of the channel-tunnel family are widespread within Gram-negative bacteria. Their involvement in drug resistance and in secretion of pathogenic factors makes them an interesting system for further studies. Understanding the mechanism of the different export apparatus could help to develop new drugs, which block the efflux pumps or the secretion system.

Haemolysin A and listeriolysin--two vaccine delivery tools for the induction of cell-mediated immunity

Haemolysin A of Escherichia coli and listeriolysin of Listeria monocytogenes represent important bacterial virulence factors. While such cytolysins are usually the reason for morbidity and even mortality, vaccine researchers have turned haemolysin A and listeriolysin into tools for vaccine delivery. Both cytolysins have found widespread application in vaccine research and are highly suitable for the elicitation of cell-mediated immunity. In this paper, we will review vaccine delivery mediated by the haemolysin A secretion system and listeriolysin and will highlight their use in vaccination approaches against protozoan parasites.

Animal models paving the way for clinical trials of attenuated Salmonella enterica serovar Typhi live oral vaccines and live vectors

Attenuated Salmonella enterica serovar Typhi (S. Typhi) strains can serve as safe and effective oral vaccines to prevent typhoid fever and as live vectors to deliver foreign antigens to the immune system, either by the bacteria expressing antigens through prokaryotic expression plasmids or by delivering foreign genes carried on eukaryotic expression systems (DNA vaccines). The practical utility of such live vector vaccines relies on achieving a proper balance between minimizing the vaccine's reactogenicity and maximizing its immunogenicity. To advance to clinical trials, vaccine candidates need to be pre-clinically evaluated in relevant animal models that attempt to predict what their safety and immunogenicity profile will be when administered to humans. Since S. Typhi is a human-restricted pathogen, a major obstacle that has impeded the progress of vaccine development has been the shortcomings of the animal models available to assess vaccine candidates. In this review, we summarize the usefulness of animal models in the assessment of the degree of attenuation and immunogenicity of novel attenuated S. Typhi strains as vaccine candidates for the prevention of typhoid fever and as live vectors in humans.

Delivery of protein antigens and DNA by attenuated intracellular bacteria

On the basis of attenuated intracellular bacteria, we have developed two delivery systems for either heterologous proteins or DNA vaccine vectors. The first system utilizes attenuated strains of Gram-negative bacteria which are engineered to secrete heterologous antigens via the alpha-hemolysin secretion system (type I) of Escherichia coli. The second system is based on attenuated suicide strains of Listeria monocytogenes, which are used for the direct delivery of eukaryotic antigen expression vectors into professional antigen-presenting cells (APC) like macrophages and dendritic cells in vitro and can be also used in animal models.

The E. coli alpha-hemolysin secretion system and its use in vaccine development

Many Gram-negative bacteria use a type I secretion system to translocate proteins, including pore-forming toxins, proteases, lipases and S-layer proteins, across both the inner and outer membranes into the extracellular surroundings. The Escherichia coli alpha-hemolysin (HlyA) secretion system is the prototypical and best characterized type I secretion system. The structure and function of the components of the HlyA secretion apparatus, HlyB, HlyD and TolC, have been studied in great detail. The functional characteristics of this secretion system enable it to be used in a variety of different applications, including the presentation of heterologous antigens in live-attenuated bacterial vaccines. Such vaccines can be an effective delivery system for heterologous antigens, and the use of a type I secretion system allows the antigens to be actively exported from the cytoplasm of the bacterial carrier rather than only becoming accessible to the host immune system after bacterial disintegration.