Efficacy of intracellular activated promoters for generation of Salmonella-based vaccines

Engineered bacteria launch and control an oncolytic virus

The ability of bacteria and viruses to selectively replicate in tumors has led to synthetic engineering of new microbial therapies. Here we design a cooperative strategy whereby S. typhimurium bacteria transcribe and deliver the Senecavirus A RNA genome inside host cells, launching a potent oncolytic viral infection. Then, we engineer the virus to require a bacterially delivered protease in order to achieve virion maturation, demonstrating bacterial control over the virus. This work extends bacterially delivered therapeutics to viral genomes, and the governing of a viral population through engineered microbial interactions.

One-Sentence Summary

Bacteria are engineered to act as a synthetic "capsid" delivering Senecavirus A genome and controlling its spread.

Changes in the gut microbiota of NOD mice in response to an oral Salmonella-based vaccine against type 1 diabetes

We developed an oral Salmonella-based vaccine that prevents and reverses diabetes in non-obese diabetic (NOD) mice. Related to this, the gastrointestinal tract harbors a complex dynamic population of microorganisms, the gut microbiome, that influences host homeostasis and metabolism. Changes in the gut microbiome are associated with insulin dysfunction and type 1 diabetes (T1D). Oral administration of diabetic autoantigens as a vaccine can restore immune balance. However, it was not known if a Salmonella-based vaccine would impact the gut microbiome. We administered a Salmonella-based vaccine to prediabetic NOD mice. Changes in the gut microbiota and associated metabolome were assessed using next-generation sequencing and gas chromatography-mass spectrometry (GC-MS). The Salmonella-based vaccine did not cause significant changes in the gut microbiota composition immediately after vaccination although at 30 days post-vaccination changes were seen. Additionally, no changes were noted in the fecal mycobiome between vaccine- and control/vehicle-treated mice. Significant changes in metabolic pathways related to inflammation and proliferation were found after vaccine administration. The results from this study suggest that an oral Salmonella-based vaccine alters the gut microbiome and metabolome towards a more tolerant composition. These results support the use of orally administered Salmonella-based vaccines that induced tolerance after administration.

A SARS-CoV-2 oral vaccine development strategy based on the attenuated Salmonella type III secretion system

Aims

This study aimed to provide a safe, stable and efficient SARS‐CoV‐2 oral vaccine development strategy based on the type III secretion system of attenuated Salmonella and a reference for the development of a SARS‐CoV‐2 vaccine.

Methods and Results

The attenuated Salmonella mutant ΔhtrA‐VNP was used as a vector to secrete the antigen SARS‐CoV‐2 based on the type III secretion system (T3SS). The Salmonella pathogenicity island 2 (SPI‐2)‐encoded T3SS promoter (sifB) was screened to express heterologous antigens (RBD, NTD, S2), and the SPI‐2‐encoded secretion system (sseJ) was employed to secrete this molecule (psifB‐sseJ‐antigen, abbreviated BJ‐antigen). Both immunoblotting and fluorescence microscopy revealed effective expression and secretion of the antigen into the cytosol of macrophages in vitro. The mixture of the three strains (BJ‐RBD/NTD/S2, named AisVax) elicited a marked increase in the induction of IgA or IgG S‐protein Abs after oral gavage, intraperitoneal and subcutaneous administration. Flow cytometric analysis proved that AisVax caused T‐cell activation, as shown by a significant increase in CD44 and CD69 expression. Significant production of IgA or IgG N‐protein Abs was also detected by using psifB‐sseJ‐N(FL), indicating the universality of this strategy.

Conclusions

Delivery of multiple SARS‐CoV‐2 antigens using the type III secretion system of attenuated Salmonella ΔhtrA‐VNP is a potential COVID‐19 vaccine strategy.

Significance and Impact of the Study

The attenuated Salmonella strain ΔhtrA‐VNP showed excellent performance as a vaccine vector. The Salmonella SPI‐2‐encoded T3SS showed highly efficient delivery of SARS‐COV‐2 antigens. Anti‐loss elements integrated into the plasmid stabilized the phenotype of the vaccine strain. Mixed administration of antigen‐expressing strains improved antibody induction.

Oral Salmonella msbB Mutant as a Carrier for a Salmonella-Based Vaccine for Prevention and Reversal of Type 1 Diabetes

A therapy that includes an oral vaccine for type 1 diabetes (T1D) using live attenuated Salmonella MvP728 (ΔhtrA/ΔpurD), cytokines (IL10 and TGFβ) and preproinsulin (PPI) antigen in combination with a sub-therapeutic dose of anti-CD3 mAb was developed by our team. The vaccine combination therapy reduced insulitis and prevented and reversed diabetes in non-obese diabetic (NOD) mice. Here, we show the effectiveness of an alternative Salmonella mutant (ΔmsbB) as a carrier strain, which is anticipated to have lower risks of an inflammatory response and septicemia as a result of modification in the lipopolysaccharide (LPS) via detoxification of lipid A. This mutant strain proved to have highly reduced pathogenic side effects. Salmonella strain ΔmsbB expressed autoantigens and in combination with cytokines and anti-CD3 mAb, successfully prevented and reversed T1D to levels comparable to the previously used carrier strain ΔhtrA/ΔpurD. Additionally, the Salmonella msbB mutant resulted in higher rates of host cell infection. These results further demonstrate the potential of an oral Salmonella-based combined therapy in the treatment of early T1D.

Tracking of an Oral Salmonella-Based Vaccine for Type 1 Diabetes in Non-obese Diabetic Mice

Type 1 diabetes (T1D) arises secondary to immune-driven destruction of pancreatic β-cells and manifests as insulin-deficient hyperglycemia. We showed that oral vaccination with live attenuated Salmonella, which simultaneously delivers autoantigens and a TGFβ expression vector to immune cells in the gut mucosa, provides protection against the progression of T1D in non-obese diabetic (NOD) mice. In this study we employed the Sleeping Beauty (SB) transposon system that is composed of a transposase and transposon encoding the td-Tomato to express red fluorescent protein (RFP) to permanently mark the cells that take up the Salmonella vaccine. After animal vaccination, the transposon labeled-dendritic cells (DCs) with red fluorescence appeared throughout the secondary lymphoid tissues. Furthermore, Sleeping Beauty containing tgfβ1 gene (SB-tgfβ1) co-expressed TGFβ and RFP. The labeled DCs were detected predominantly in Peyer's patches (PP) and mesenteric lymph nodes (MLN) and expressed CD103 surface marker. CD103+ DCs induced tolerogenic effects and gut homing. TGFβ significantly increased programmed death-ligand-1 (PDL-1 or CD274) expression in the DCs in the MLN and PP of treated mice. Also, TGFβ increased cytotoxic T-lymphocyte-associated protein-4 (CTLA-4) levels in CD4+ cells in MLN and PP. Interestingly, DCs increased in all lymphatic organs of mice vaccinated with oral live Salmonella-based vaccine expressing preproinsulin (PPI), in combination with TGFβ, IL10, and subtherapeutic-doses of anti-CD3 mAb compared with vehicle-treated mice. These DCs are mostly tolerogenic in MLN and PP. Furthermore the DCs obtained from vaccine-treated but not vehicle-treated mice suppressed in vitro T cell proliferation. These data suggest that the MLN and the PP are a central hub for the beneficial anti-diabetic effects of an oral Salmonella-based vaccine prevention of diabetes in rodents.

Reversal of New Onset Type 1 Diabetes by Oral Salmonella-Based Combination Therapy and Mediated by Regulatory T-Cells in NOD Mice

Autoimmune diseases such as type 1 diabetes (T1D) involve the loss of regulatory mechanisms resulting in increased tissue-specific cytotoxicity. The result is destruction of pancreatic insulin-producing β-cells and loss of glucose homeostasis. We are developing a novel oral vaccine using live attenuated Salmonella to deliver TGFβ, IL10, and the diabetic autoantigen preproinsulin combined with low-doses of anti-CD3 mAb. Here we show that oral administration of Salmonella-based anti-CD3 mAb combined therapy reverses new-onset T1D in non-obese diabetic (NOD) mice. The therapeutic effect of the combined therapy was associated with induction of immune suppressive CD4⁺CD25⁺Foxp3⁺ Treg and CD4⁺CD49b⁺LAG3⁺ Tr1 cells. In adoptive transfer experiments, adding or depleting Treg or Tr1 cells indicated that both are important for preventing diabetes in combined therapy-treated mice, but that Tr1 cells may have a more central role. Furthermore, induced Tr1 cells were found to be antigen-specific responding to peptide stimulation by secreting tolerance inducing IL10. These preclinical data demonstrate a role for Treg and Tr1 cells in combined therapy-mediated induction of tolerance in NOD mice. These results also demonstrate the potential of oral Salmonella-based combined therapy in the treatment of early T1D.

Factors affecting Salmonella-based combination immunotherapy for prevention of type 1 diabetes in non-obese diabetic mice

We previously reported the development of an oral vaccine for diabetes based on live attenuated Salmonella-expressing preproinsulin (PPI) as the autoantigen. When combined with host cell-expressed TGFβ, the vaccine prevented the onset of diabetes in non-obese diabetic (NOD) mice. Herein, we investigated factors that could affect vaccine efficacy including vaccination number, optimization of the autoantigen codon sequence, Salmonella SPI2-TTSS promoter/effector combinations, concurrent short-course low-dose anti-CD3. We also evaluated autoantigen GAD65 and cytokine IL10 treatment upon vaccine efficacy. T-cells we employed to elucidate the mechanism of the vaccine action. Our results showed that GAD65+TGFβ or PPI+TGFβ+IL10 prevented the onset of diabetes in the NOD mice and maintained glucose tolerance. However, increasing the number of vaccine doses, codon-optimization of the autoantigen(s) or use of other Salmonella promoter/effector combinations had no in vivo effect. Interestingly, two doses of vaccine (PPI+TGFβ+IL10) combined with a sub-therapeutic dose of anti-CD3 prevented diabetes and decreased hyperglycemia in mice. The combined therapy also increased splenic Tregs and local Tregs in pancreatic lymph nodes (PLN) and increased regulatory (IL10 and IL2) but reduced inflammatory (IFNγ and TNFα) cytokines. Together, these results indicate that the combination of low vaccine dose number, less vaccine autoantigen expression and short-course low-dose anti-CD3 can increase regulatory mechanisms and suppress autoimmunity.

Usage of Murine T-cell Hybridoma Cells as Responder Cells Reveals Interference of Helicobacter Pylori with Human Dendritic Cell-mediated Antigen Presentation

Direct effects of Helicobacter pylori (H. pylori) on human CD4⁺ T-cells hamper disentangling a possible bacterial-mediated interference with major histocompatibility complex class II (MHC-II)-dependent antigen presentation to these cells. To overcome this limitation, we employed a previously described assay, which enables assessing human antigen-processing cell function by using murine T-cell hybridoma cells restricted by human leukocyte antigen (HLA) alleles. HLA-DR1⁺ monocyte-derived dendritic cells were exposed to H. pylori and pulsed with the antigen 85B from Mycobacterium tuberculosis (M. tuberculosis). Interleukin-2 (IL-2) secretion by AG85B_aa97-112-specific hybridoma cells was then evaluated as an integral reporter of cognate antigen presentation. This methodology enabled revealing of interference of H. pylori with the antigen-presenting capacity of human dendritic cells.

RNA-seq Brings New Insights to the Intra-Macrophage Transcriptome of Salmonella Typhimurium

Salmonella enterica serovar Typhimurium is arguably the world’s best-understood bacterial pathogen. However, crucial details about the genetic programs used by the bacterium to survive and replicate in macrophages have remained obscure because of the challenge of studying gene expression of intracellular pathogens during infection. Here, we report the use of deep sequencing (RNA-seq) to reveal the transcriptional architecture and gene activity of Salmonella during infection of murine macrophages, providing new insights into the strategies used by the pathogen to survive in a bactericidal immune cell. We characterized 3583 transcriptional start sites that are active within macrophages, and highlight 11 of these as candidates for the delivery of heterologous antigens from Salmonella vaccine strains. A majority (88%) of the 280 S. Typhimurium sRNAs were expressed inside macrophages, and SPI13 and SPI2 were the most highly expressed pathogenicity islands. We identified 31 S. Typhimurium genes that were strongly up-regulated inside macrophages but expressed at very low levels during in vitro growth. The SalComMac online resource allows the visualisation of every transcript expressed during bacterial replication within mammalian cells. This primary transcriptome of intra-macrophage S.-Typhimurium describes the transcriptional start sites and the transcripts responsible for virulence traits, and catalogues the sRNAs that may play a role in the regulation of gene expression during infection.

The burden of Salmonellosis remains unacceptably high throughout the world and control measures have had limited success. Because Salmonella bacteria can be transmitted from the wider environment to animals and humans, the bacteria encounter diverse environments that include food, water, plant surfaces and the extracellular and intracellular phases of infection of eukaryotic hosts. An intricate transcriptional network has evolved to respond to a variety of environmental signals and control the “right time/ right place” expression of virulence genes. To understand how transcription is rewired during intracellular infection, we determined the primary transcriptome of Salmonella enterica serovar Typhimurium within murine macrophages. We report the coding genes, sRNAs and transcriptional start sites that are expressed within macrophages at 8 hours after infection, and use these to infer gene function. We identified gene promoters that are specifically expressed within macrophages and could drive the intracellular delivery of antigens by S. Typhimurium vaccine strains. These data contribute to our understanding of the mechanisms used by Salmonella to regulate virulence gene expression whilst replicating inside mammalian cells.

Antigen Delivery System II

This chapter reviews papers mostly written since 2005 that report results using live attenuated bacterial vectors to deliver after administration through mucosal surfaces, protective antigens, and DNA vaccines, encoding protective antigens to induce immune responses and/or protective immunity to pathogens that colonize on or invade through mucosal surfaces. Papers that report use of such vaccine vector systems for parenteral vaccination or to deal with nonmucosal pathogens or do not address induction of mucosal antibody and/or cellular immune responses are not reviewed.

A bivalent typhoid live vector vaccine expressing both chromosome- and plasmid-encoded Yersinia pestis antigens fully protects against murine lethal pulmonary plague infection

Live attenuated bacteria hold great promise as multivalent mucosal vaccines against a variety of pathogens. A major challenge of this approach has been the successful delivery of sufficient amounts of vaccine antigens to adequately prime the immune system without overattenuating the live vaccine. Here we used a live attenuated Salmonella enterica serovar Typhi strain to create a bivalent mucosal plague vaccine that produces both the protective F1 capsular antigen of Yersinia pestis and the LcrV protein required for secretion of virulence effector proteins. To reduce the metabolic burden associated with the coexpression of F1 and LcrV within the live vector, we balanced expression of both antigens by combining plasmid-based expression of F1 with chromosomal expression of LcrV from three independent loci. The immunogenicity and protective efficacy of this novel vaccine were assessed in mice by using a heterologous prime-boost immunization strategy and compared to those of a conventional strain in which F1 and LcrV were expressed from a single low-copy-number plasmid. The serum antibody responses to lipopolysaccharide (LPS) induced by the optimized bivalent vaccine were indistinguishable from those elicited by the parent strain, suggesting an adequate immunogenic capacity maintained through preservation of bacterial fitness; in contrast, LPS titers were 10-fold lower in mice immunized with the conventional vaccine strain. Importantly, mice receiving the optimized bivalent vaccine were fully protected against lethal pulmonary challenge. These results demonstrate the feasibility of distributing foreign antigen expression across both chromosomal and plasmid locations within a single vaccine organism for induction of protective immunity.

Effective cancer vaccine platform based on attenuated salmonella and a type III secretion system

Vaccines explored for cancer therapy have been based generally on injectable vector systems used to control foreign infectious pathogens, to which the immune system evolved to respond naturally. However, these vectors may not be effective at presenting tumor-associated antigens (TAA) to the immune system in a manner that is sufficient to engender antitumor responses. We addressed this issue with a novel orally administered Salmonella-based vector that exploits a type III secretion system to deliver selected TAA in the cytosol of professional antigen-presenting cells in situ. A systematic comparison of candidate genes from the Salmonella Pathogenicity Island 2 (SPI2) locus was conducted in the vaccine design, using model antigens and a codon-optimized form of the human TAA survivin (coSVN), an oncoprotein that is overexpressed in most human cancers. In a screen of 20 SPI2 promoter:effector combinations, a PsifB::sseJ combination exhibited maximal potency for antigen translocation into the APC cytosol, presentation to CD8 T cells, and murine immunogenicity. In the CT26 mouse model of colon carcinoma, therapeutic vaccination with a lead PsifB::sseJ-coSVN construct (p8032) produced CXCR3-dependent infiltration of tumors by CD8 T cells, reversed the CD8:Treg ratio at the tumor site, and triggered potent antitumor activity. Vaccine immunogenicity and antitumor potency were enhanced by coadministration of the natural killer T-cell ligand 7DW8-5, which heightened the production of IL12 and IFNγ. Furthermore, combined treatment with p8032 and 7DW8-5 resulted in complete tumor regression in A20 lymphoma-bearing mice, where protective memory was demonstrated. Taken together, our results demonstrate how antigen delivery using an oral Salmonella vector can provide an effective platform for the development of cancer vaccines.

Improved delivery of the OVA-CD4 peptide to T helper cells by polymeric surface display on Salmonella

Background

Autotransporter proteins represent a treasure trove for molecular engineers who modify Gram-negative bacteria for the export or secretion of foreign proteins across two membrane barriers. A particularly promising direction is the development of autotransporters as antigen display or secretion systems. Immunologists have been using ovalbumin as a reporter antigen for years and have developed sophisticated tools to detect specific T cells that respond to ovalbumin. Although ovalbumin-expressing bacteria are being used to trace T cell responses to colonizing or invading pathogens, current constructs for ovalbumin presentation have not been optimized.

Results

The activation of T helper cells in response to ovalbumin was improved by displaying the OVA-CD4 reporter epitope as a multimer on the surface of Salmonella and fused to the autotransporter MisL. Expression was optimized by including tandem in vivo promoters and two post-segregational killing systems for plasmid stabilization.

Conclusions

The use of an autotransporter protein to present relevant epitope repeats on the surface of bacteria, combined with additional techniques favoring stable and efficient in vivo transcription, optimizes antigen presentation to T cells. The technique of multimeric epitope surface display should also benefit the development of new Salmonella or other enterobacterial vaccines.

Avian influenza: virology, diagnosis and surveillance

Avian influenza virus (AIV) is the causative agent of a zoonotic disease that affects populations worldwide with often devastating economic and health consequences. Most AIV subtypes cause little or no disease in waterfowl, but outbreaks in poultry can be associated with high mortality. Although transmission of AIV to humans occurs rarely and is strain dependent, the virus has the ability to mutate or reassort into a form that triggers a life-threatening infection. The constant emergence of new influenza strains makes it particularly challenging to predict the behavior, spread, virulence or potential for human-to-human transmission. Because it is difficult to anticipate which viral strain or what location will initiate the next pandemic, it is difficult to prepare for that event. However, rigorous implementation of biosecurity, vaccination and education programs can minimize the threat of AIV. Global surveillance programs help record and identify newly evolving and potentially pandemic strains harbored by the reservoir host.

Identification of salmonella pathogenicity island-2 type III secretion system effectors involved in intramacrophage replication of S. enterica serovar typhimurium: implications for rational vaccine design

Salmonella enterica serovars cause severe diseases in humans, such as gastroenteritis and typhoid fever. The development of systemic disease is dependent on a type III secretion system (T3SS) encoded by Salmonella pathogenicity island-2 (SPI-2). Translocation of effector proteins across the Salmonella-containing vacuole, via the SPI-2 T3SS, enables bacterial replication within host cells, including macrophages. Here, we investigated the contribution of these effectors to intramacrophage replication of Salmonella enterica serovar Typhimurium using Fluorescence Dilution, a dual-fluorescence tool which allows direct measurement of bacterial replication. Of 32 strains, each carrying single mutations in genes encoding effectors, 10 (lacking sifA, sseJ, sopD2, sseG, sseF, srfH, sseL, spvD, cigR, or steD) were attenuated in replication in mouse bone marrow-derived macrophages. The replication profiles of strains combining deletions in effector genes were also investigated: a strain lacking the genes sseG, sopD2, and srfH showed an increased replication defect compared to single-mutation strains and was very similar to SPI-2 T3SS-deficient bacteria with respect to its replication defect. This strain was substantially attenuated in virulence in vivo and yet retained intracellular vacuole integrity and a functional SPI-2 T3SS. Moreover, this strain was capable of SPI-2 T3SS-mediated delivery of a model antigen for major histocompatibility complex (MHC) class I-dependent T-cell activation. This work establishes a basis for the use of a poly-effector mutant strain as an attenuated vaccine carrier for delivery of heterologous antigens directly into the cytoplasm of host cells.

Live attenuated strains of Salmonella enterica serotype Typhi have generated much interest in the search for improved vaccines against typhoid fever and as vaccine vectors for the delivery of heterologous antigens. A promising vaccine candidate is the ΔaroC ΔssaV S. Typhi strain, which owes its attenuation mainly to lack of a type III secretion system (SPI-2 T3SS). The SPI-2 T3SS is important for bacterial proliferation inside macrophages, but not all of the effectors involved in this process have been identified. Here, we show that 10 effectors of the related strain S. Typhimurium contribute to intracellular replication in macrophages. Moreover, we establish that a poly-effector mutant strain of S. Typhimurium can have a severe replication defect and maintain a functional SPI-2 T3SS, which can be exploited for delivery of heterologous antigens.

Novel methods for expression of foreign antigens in live vector vaccines

Bacterial live vector vaccines represent a vaccine development strategy that offers exceptional flexibility. In this approach, genes encoding protective antigens of unrelated bacterial, viral or parasitic pathogens are expressed in an attenuated bacterial vaccine strain that delivers these foreign antigens to the immune system, thereby eliciting relevant immune responses. Rather than expressing these antigens using low copy expression plasmids, here we pursue expression of foreign proteins from the live vector chromosome. Our strategy is designed to compensate for the inherent disadvantage of loss of gene dosage (vs. plasmid-based expression) by integrating antigen-encoding gene cassettes into multiple chromosomal sites already inactivated in an attenuated Salmonella enterica serovar Typhi vaccine candidate. We tested expression of a cassette encoding the green fluorescent protein (GFPuv) integrated separately into native guaBA, htrA or clyA chromosomal loci. Using single integrations, we show that expression levels of GFPuv are significantly affected by the site of integration, regardless of the inclusion of additional strong promoters within the incoming cassette. Using cassettes integrated into both guaBA and htrA, we observe cumulative synthesis levels from two integration sites superior to single integrations. Most importantly, we observe that GFPuv expression increases in a growth phase-dependent manner, suggesting that foreign antigen synthesis may be “tuned” to the physiology of the live vaccine. We expect this novel platform expression technology to prove invaluable in the development of a wide variety of multivalent live vector vaccines, capable of expressing multiple antigens from both chromosomal and plasmid-based expression systems within a single strain.

Evaluation of Salmonella enterica type III secretion system effector proteins as carriers for heterologous vaccine antigens

Live attenuated strains of Salmonella enterica have a high potential as carriers of recombinant vaccines. The type III secretion system (T3SS)-dependent translocation of S. enterica can be deployed for delivery of heterologous antigens to antigen-presenting cells. Here we investigated the efficacy of various effector proteins of the Salmonella pathogenicity island (SPI2)-encoded T3SS for the translocation of model antigens and elicitation of immune responses. The SPI2 T3SS effector proteins SifA, SteC, SseL, SseJ, and SseF share an endosomal membrane-associated subcellular localization after translocation. We observed that all effector proteins could be used to translocate fusion proteins with the model antigens ovalbumin and listeriolysin into the cytosol of host cells. Under in vitro conditions, fusion proteins with SseJ and SteC stimulated T-cell responses that were superior to those triggered by fusion proteins with SseF. However, in mice vaccinated with Salmonella carrier strains, only fusion proteins based on SseJ or SifA elicited potent T-cell responses. These data demonstrate that the selection of an optimal SPI2 effector protein for T3SS-mediated translocation is a critical parameter for the rational design of effective Salmonella-based recombinant vaccines.

Salmonella enterica as a vaccine carrier

Salmonella enterica is an invasive, facultative intracellular gastrointestinal pathogen causing human diseases such as gastroenteritis and typhoid fever. Virulence-attenuated strains of this pathogen have interesting capacities for the generation of live vaccines. Attenuated live typhoidal and nontyphoidal Salmonella strains can be used for vaccination against Salmonella infections and to target tumor tissue. Such strains may also serve as live carriers for the development of vaccination strategies against other bacterial, viral or parasitic pathogens. Various strategies have been developed to deploy regulatory circuits and protein secretion systems for efficient expression and delivery of foreign antigens by Salmonella carrier strains. One prominent example is the use of type III secretion systems to translocate recombinant antigens into antigen presenting cells. In this review, we will describe the recent developments in strategies that utilize live attenuated Salmonella as vaccine carriers for prophylactic vaccination against infectious diseases and therapeutic vaccination against tumors. Considerations for generating safe, attenuated carrier strains, designing stable expression systems and the use of adjuvants for live carrier strategies are discussed.

Oral delivery of the Sj23LHD-GST antigen by Salmonella typhimurium type III secretion system protects against Schistosoma japonicum infection in mice

BACKGROUND

Schistosomiasis japonica is a zoonotic parasitic disease and oral vaccine delivery system would be benefit for prevention of this disease. Although attenuated salmonella has been used as an antigen expression vector for oral vaccine development, the membrane-bound vacuoles in which bacteria reside hinders the presentation of expressed heterologous antigens to the major histocompatibility complex (MHC) molecules. The present work used an attenuated Salmonella typhimurium strain VNP20009 to secretory expression of Sj23LHDGST bivalent antigen from Schistosoma japonicum and tested the protective efficacy against S. japonicum infection in orally immunized mice.

METHODOLOGY/PRINCIPAL FINDINGS

Promoters (nirB or pagC) were used to express the antigen (Sj23LHDGST) and the Salmonella type III or α-hemolysin secretion system was employed to secrete it. The immunoblotting analysis and fluorescent microscopy revealed that the antigen was effectively expressed and delivered to the cytosol of macrophages in vitro. Among recombinant vaccine strains, an engineered VNP20009 which expressed the antigen by nirB promoter and secreted it through type III secretion system (nirB-sopE(1-104)-Sj23LHD-GST) efficiently protected against S. japonicum infection in a mouse model. This strain elicited a predominantly IgG(2a) antibody response and a markedly increase in the production of IL-12 and IFN-γ. The flow cytometric analysis demonstrated that this strain caused T cell activation as evidenced by significantly increased expression of CD44 and CD69.

CONCLUSION/SIGNIFICANCE

Oral delivery of antigen by nirB-driven Salmonella typhimurium type III secretion system is a novel, safe, inexpensive, efficient and convenient approach for schistosome vaccine development.

Comparison of a regulated delayed antigen synthesis system with in vivo-inducible promoters for antigen delivery by live attenuated Salmonella vaccines

Induction of strong immune responses against a vectored antigen in hosts immunized with live attenuated Salmonella vaccines is related in part to the amount of antigen delivered and the overall fitness of the Salmonella vector in relation to its ability to stimulate the host immune system. Constitutive high-level antigen synthesis causes a metabolic burden to the vaccine vector strain that can reduce the vaccine strain's ability to interact with host lymphoid tissues, resulting in a compromised immune response. A solution to this problem is the use of systems that regulate antigen gene expression, permitting high levels of antigen synthesis only after the vaccine strain has reached its target tissues. In vivo-inducible promoters (IVIPs) are often used to accomplish this. We recently developed an alternative strategy, a regulated delayed antigen synthesis (RDAS) system, in which the LacI-repressible P(trc) promoter controls antigen gene expression by adding arabinose. In this paper, we compared the RDAS system with two commonly used IVIPs, P(ssaG) and P(pagC). Three nearly identical plasmids, differing only in the promoter used to direct transcription of the pneumococcal pspA gene, P(trc), P(ssaG), or P(pagC), were constructed and introduced into isogenic Salmonella vaccine strains with or without arabinose-inducible LacI synthesis. Mice immunized with the RDAS strain developed slightly higher titers of mucosal and serum anti-PspA antibodies than P(pagC)-immunized mice, while titers in mice immunized with the P(ssaG) strain were 100-fold lower. Both the RDAS and P(pagC) strains conferred similar levels of protection against Streptococcus pneumoniae challenge, significantly greater than those for the P(ssaG) strain or controls. Thus, RDAS provides another choice for inclusion in the live vaccine design to increase immunogenicity.