The Asd(+)-DadB(+) dual-plasmid system offers a novel means to deliver multiple protective antigens by a recombinant attenuated Salmonella vaccine

The JMU-SalVac-System: A Novel, Versatile Approach to Oral Live Vaccine Development

Salmonella enterica Serovar Typhi Ty21a (Ty21a) is the only licensed oral vaccine against typhoid fever. Due to its excellent safety profile, it has been used as a promising vector strain for the expression of heterologous antigens for mucosal immunization. As the efficacy of any bacterial live vector vaccine correlates with its ability to express and present sufficient antigen, the genes for antigen expression are traditionally located on plasmids with antibiotic resistance genes for stabilization. However, for use in humans, antibiotic selection of plasmids is not applicable, leading to segregational loss of the antigen-producing plasmid. Therefore, we developed an oral Ty21a-based vaccine platform technology, the JMU-SalVac-system (Julius-Maximilians-Universität Würzburg) in which the antigen delivery plasmids (pSalVac-plasmid-series) are stabilized by a ΔtyrS/tyrS+-based balanced-lethal system (BLS). The system is made up of the chromosomal knockout of the essential tyrosyl-tRNA-synthetase gene (tyrS) and the in trans complementation of tyrS on the pSalVac-plasmid. Further novel functional features of the pSalVac-plasmids are the presence of two different expression cassettes for the expression of protein antigens. In this study, we present the construction of vaccine strains with BLS plasmids for antigen expression. The expression of cytosolic and secreted mRFP and cholera toxin subunit B (CTB) proteins as model antigens is used to demonstrate the versatility of the approach. As proof of concept, we show the induction of previously described in vivo inducible promoters cloned into pSalVac-plasmids during infection of primary macrophages and demonstrate the expression of model vaccine antigens in these relevant human target cells. Therefore, antigen delivery strains developed with the JMU-SalVac technology are promising, safe and stable vaccine strains to be used against mucosal infections in humans.

Vaccines to Control Salmonella in Poultry

This review is focused on describing and analyzing means by which Salmonella enterica serotype strains have been genetically modified with the purpose of developing safe, efficacious vaccines to present Salmonella-induced disease in poultry and to prevent Salmonella colonization of poultry to reduce transmission through the food chain in and on eggs and poultry meat. Emphasis is on use of recently developed means to generate defined deletion mutations to eliminate genetic sequences conferring antimicrobial resistance or residual elements that might lead to genetic instability. Problems associated with prior means to develop vaccines are discussed with presentation of various means by which these problems have been lessened, if not eliminated. Practical considerations are also discussed in hope of facilitating means to move lab-proven successful vaccination procedures and vaccine candidates to the marketplace to benefit the poultry industry.

Multiple immunodominant O-epitopes co-expression in live attenuated Salmonella serovars induce cross-protective immune responses against S. Paratyphi A, S. Typhimurium and S. Enteritidis

Salmonella enterica subsp. enterica (S. enterica) is a significant public health concern and is estimated to cause more than 300,000 deaths annually. Nowadays, the vaccines available for human Salmonellosis prevention are all targeting just one serovar, i.e., S. Typhi, leaving a huge potential risk of Salmonella disease epidemiology change. In this study, we explored the strategy of multiple immunodominant O-epitopes co-expression in S. enterica serovars and evaluated their immunogenicity to induce cross-immune responses and cross-protections against S. Paratyphi A, S. Typhimurium and S. Enteritidis. We found that nucleotide sugar precursors CDP-Abe and CDP-Par (or CDP-Tyv) could be utilized by S. enterica serovars simultaneously, exhibiting O2&O4 (or O4&O9) double immunodominant O-serotypes without obvious growth defects. More importantly, a triple immunodominant O2&O4&O9 O-serotypes could be achieved in S. Typhimurium by improving the substrate pool of CDP-Par, glycosyltransferase WbaV and flippase Wzx via a dual-plasmid overexpressing system. Through immunization in a murine model, we found that double or triple O-serotypes live attenuated vaccine candidates could induce significantly higher heterologous serovar-specific antibodies than their wild-type parent strain. Meanwhile, the bacterial agglutination, serum bactericidal assays and protection efficacy experiments had all shown that these elicited serum antibodies are cross-reactive and cross-protective. Our work highlights the potential of developing a new type of live attenuated Salmonella vaccines against S. Paratyphi A, S. Typhimurium and S. Enteritidis simultaneously.

Mucosal Delivery of a Self-destructing Salmonella-Based Vaccine Inducing Immunity Against Eimeria

A programmed self-destructive Salmonella vaccine delivery system was developed to facilitate efficient colonization in host tissues that allows release of the bacterial cell contents after lysis to stimulate mucosal, systemic, and cellular immunities against a diversity of pathogens. Adoption and modification of these technological improvements could form part of an integrated strategy for cost-effective control and prevention of infectious diseases, including those caused by parasitic pathogens. Avian coccidiosis is a common poultry disease caused by Eimeria. Coccidiosis has been controlled by medicating feed with anticoccidial drugs or administering vaccines containing low doses of virulent or attenuated Eimeria oocysts. Problems of drug resistance and nonuniform administration of these Eimeria resulting in variable immunity are prompting efforts to develop recombinant Eimeria vaccines. In this study, we designed, constructed, and evaluated a self-destructing recombinant attenuated Salmonella vaccine (RASV) lysis strain synthesizing the Eimeria tenella SO7 antigen. We showed that the RASV lysis strain χ11791(pYA5293) with a ΔsifA mutation enabling escape from the Salmonella-containing vesicle (or endosome) successfully colonized chicken lymphoid tissues and induced strong mucosal and cell-mediated immunities, which are critically important for protection against Eimeria challenge. The results from animal clinical trials show that this vaccine strain significantly increased food conversion efficiency and protection against weight gain depression after challenge with 105E. tenella oocysts with concomitant decreased oocyst output. More importantly, the programmed regulated lysis feature designed into this RASV strain promotes bacterial self-clearance from the host, lessening persistence of vaccine strains in vivo and survival if excreted, which is a critically important advantage in a vaccine for livestock animals. Our approach should provide a safe, cost-effective, and efficacious vaccine to control coccidiosis upon addition of additional protective Eimeria antigens. These improved RASVs can also be modified for use to control other parasitic diseases infecting other animal species.

Uncoupling of invasive bacterial mucosal immunogenicity from pathogenicity

There is the notion that infection with a virulent intestinal pathogen induces generally stronger mucosal adaptive immunity than the exposure to an avirulent strain. Whether the associated mucosal inflammation is important or redundant for effective induction of immunity is, however, still unclear. Here we use a model of auxotrophic Salmonella infection in germ-free mice to show that live bacterial virulence factor-driven immunogenicity can be uncoupled from inflammatory pathogenicity. Although live auxotrophic Salmonella no longer causes inflammation, its mucosal virulence factors remain the main drivers of protective mucosal immunity; virulence factor-deficient, like killed, bacteria show reduced efficacy. Assessing the involvement of innate pathogen sensing mechanisms, we show MYD88/TRIF, Caspase-1/Caspase-11 inflammasome, and NOD1/NOD2 nodosome signaling to be individually redundant. In colonized animals we show that microbiota metabolite cross-feeding may recover intestinal luminal colonization but not pathogenicity. Consequent immunoglobulin A immunity and microbial niche competition synergistically protect against Salmonella wild-type infection.

Attenuated Salmonella Typhimurium expressing Salmonella Paratyphoid A O-antigen induces protective immune responses against two Salmonella strains

Salmonella enterica serovar Paratyphi A is the main causative agent of paratyphoid fever in many Asian countries. As paratyphoid is spread by the fecal-oral route, the most effective means of controlling S. Paratyphi A infection is through the availability of clean water supplies and working sanitation services. Because sanitation facilities improve slowly in these poor areas and antibiotic resistance is severe, the development of a safe and effective vaccine remains a priority for controlling the spread of paratyphoid disease. In this study, we investigated the strategy of heterologous O-antigenic O2 serotype (S. Paratyphi A characterized) conversion in S. Typhimurium to prevent paratyphoid infections. A series of S. Typhimurium mutants were constructed with replacement of abe, wzxB1 and wbaVB1 genes with respective prt-tyvA1, wzxA1 and wbaVA1, and the results showed that only three genes including prt, wbaVA1 and wzxA1 from S. Paratyphi A presence enable S. Typhimurium to sufficiently express O2 antigen polysaccharide. We also constructed a series of live attenuated S. Typhimurium vaccine candidates expressing heterologous O2 O-antigens, and a mouse model was used to evaluate the immunogenicity of live vaccines. ELISA data showed that vaccine candidates could induce a comparatively high level of S. Paratyphi A and/or S. Typhimurium LPS-specific IgG and IgA responses in murine model, and IgG2a levels were consistently higher than IgG1 levels. Moreover, the functional properties of serum antibodies were evaluated using in vitro C3 complement deposition and opsonophagocytic assays. Our work highlights the potential for developing S. Typhimurium live vaccines against S. Paratyphi A.

Live bacterial vaccine vector and delivery strategies of heterologous antigen: A review

Live bacteria, including attenuated bacteria and probiotics, can be engineered to deliver target antigen to excite the host immune system. The preponderance of these live bacterial vaccine vectors is that they can stimulate durable humoral and cellular immunity. Moreover, delivery strategies of heterologous antigen in live bacterial promote the applications of new vaccine development. Genetic technologies are evolving, which potentiate the developing of heterologous antigen delivery systems, including bacterial surface display system, bacterial secretion system and balanced lethal vector system. Although the live bacterial vaccine vector is a powerful adjuvant, certain disadvantages, such as safety risk, must also be taken into account. In this review, we compare the development of representative live bacterial vectors, and summarize the main characterizations of the various delivery strategies of heterologous antigen in live vector vaccines.

Bacterial type III secretion system as a protein delivery tool for a broad range of biomedical applications

A protein delivery tool based on bacterial type III secretion system (T3SS) has been broadly applied in biomedical researches. In this review, we summarize various applications of the T3SS-mediate protein delivery which enables translocation of proteins directly into mammalian cells without protein purification. Some of the remarkable advancements include delivery of antigens for therapeutic vaccines, nucleases for genome editing, transcription factors for cellular reprogramming and stem cells differentiation, and signaling molecules for post-translational proteomics studies. With continued improvement of the T3SS-mediated protein delivery tools, even wider application of the technology is anticipated.

A bivalent vaccine derived from attenuated Salmonella expressing O-antigen polysaccharide provides protection against avian pathogenic Escherichia coli O1 and O2 infection

Avian pathogenic Escherichia coli (APEC), a leading cause of avian airsacculitis and colibacillosis, is responsible for significant economic loss in the poultry industry. APEC serogroups O1, O2, and O78 are predominantly associated with disease. Lipopolysaccharide (LPS) O-antigen has been shown to be a potent antigen for inducing specific protective immune responses. Therefore, we sought to develop a multivalent polysaccharide vaccine to prevent most APEC infections. We previously reported the stable expression of plasmid pSS27 encoding the APEC O1 O-antigen gene cluster (10.8 kb) in attenuated Salmonella enterica serovar Typhimurium S740 provided excellent protection against APEC O1 challenge. In this study, the plasmid pSS28 harboring the APEC O2 O-antigen polysaccharide gene cluster (15.5 kb) was constructed. Biosynthesis of pSS28-encoded APEC O2 O-antigen in Salmonella vaccine strain S740 was validated by Western blot. The recombinant Salmonella vaccine strain S740 (pSS28) elicited homologous protection against virulent wild-type APEC O2 challenge in a chicken model. Furthermore, through equal-volume mixing the two monovalent vaccine strains S740 (pSS27) and S740 (pSS28), a bivalent vaccine candidate against both APEC O1 and O2 was developed. Immunization of chickens with the bivalent vaccine elicited production of serum IgG and mucosal sIgA antibodies against the LPS of both APEC O1 and O2. Moreover, antibodies induced by the bivalent vaccine promoted opsonization, provoked complement-mediated bactericidal activity, and elicited protection against lethal challenge with both virulent APEC O1 and O2 strains. These results demonstrate that the bivalent vaccine comprised of S740 (pSS27) and S740 (pSS28) is a promising vaccine candidate against APEC O1 and O2 infection.

Salmonella Vaccines: Conduits for Protective Antigens

Vaccines afford a better and more cost-effective approach to combatting infectious diseases than continued reliance on antibiotics or antiviral or antiparasite drugs in the current era of increasing incidences of diseases caused by drug-resistant pathogens. Recombinant attenuated Salmonella vaccines (RASVs) have been significantly improved to exhibit the same or better attributes than wild-type parental strains to colonize internal lymphoid tissues and persist there to serve as factories to continuously synthesize and deliver rAgs. Encoded by codon-optimized pathogen genes, Ags are selected to induce protective immunity to infection by that pathogen. After immunization through a mucosal surface, the RASV attributes maximize their abilities to elicit mucosal and systemic Ab responses and cell-mediated immune responses. This article summarizes many of the numerous innovative technologies and discoveries that have resulted in RASV platforms that will enable development of safe efficacious RASVs to protect animals and humans against a diversity of infectious disease agents.

Two Novel Salmonella Bivalent Vaccines Confer Dual Protection against Two Salmonella Serovars in Mice

Non-typhoidal Salmonella includes thousands of serovars that are leading causes of foodborne diarrheal illness worldwide. In this study, we constructed three bivalent vaccines for preventing both Salmonella Typhimurium and Salmonella Newport infections by using the aspartate semialdehyde dehydrogenase (Asd)-based balanced-lethal vector-host system. The constructed Asd⁺ plasmid pCZ11 carrying a subset of the Salmonella Newport O-antigen gene cluster including the wzx-wbaR-wbaL-wbaQ-wzy-wbaW-wbaZ genes was introduced into three Salmonella Typhimurium mutants: SLT19 (Δasd) with a smooth LPS phenotype, SLT20 (Δasd ΔrfbN) with a rough LPS phenotype, and SLT22 (Δasd ΔrfbN ΔpagL::T araC P_BADrfbN) with a smooth LPS phenotype when grown with arabinose. Immunoblotting demonstrated that SLT19 harboring pCZ11 [termed SLT19 (pCZ11)] co-expressed the homologous and heterologous O-antigens; SLT20 (pCZ11) exclusively expressed the heterologous O-antigen; and when arabinose was available, SLT22 (pCZ11) expressed both types of O-antigens, while in the absence of arabinose, SLT22 (pCZ11) expressed only the heterologous O-antigen. Exclusive expression of the heterologous O-antigen in Salmonella Typhimurium decreased the swimming ability of the bacterium and its susceptibility to polymyxin B. Next, the crp gene was deleted from the three recombinant strains for attenuation purposes, generating the three bivalent vaccine strains SLT25 (pCZ11), SLT26 (pCZ11), and SLT27 (pCZ11), respectively. Groups of BALB/c mice (12 mice/group) were orally immunized with 10⁹ CFU of each vaccine strain twice at an interval of 4 weeks. Compared with a mock immunization, immunization with all three vaccine strains induced significant serum IgG responses against both Salmonella Typhimurium and Salmonella Newport LPS. The bacterial loads in the mouse tissues were significantly lower in the three vaccine-strain-immunized groups than in the mock group after either Salmonella Typhimurium or Salmonella Newport lethal challenge. All of the mice in the three vaccine-immunized groups survived the lethal Salmonella Typhimurium challenge. In contrast, SLT26 (pCZ11) and SLT27 (pCZ11) conferred full protection against lethal Salmonella Newport challenge, but SLT25 (pCZ11) provided only 50% heterologous protection. Thus, we developed two novel Salmonella bivalent vaccines, SLT26 (pCZ11) and SLT27 (pCZ11), suggesting that the delivery of a heterologous O-antigen in attenuated Salmonella strains is a prospective approach for developing Salmonella vaccines with broad serovar coverage.

aroA-Deficient Salmonella enterica Serovar Typhimurium Is More Than a Metabolically Attenuated Mutant

Recombinant attenuated Salmonella enterica serovar Typhimurium strains are believed to act as powerful live vaccine carriers that are able to elicit protection against various pathogens. Auxotrophic mutations, such as a deletion of aroA, are commonly introduced into such bacteria for attenuation without incapacitating immunostimulation. In this study, we describe the surprising finding that deletion of aroA dramatically increased the virulence of attenuated Salmonella in mouse models. Mutant bacteria lacking aroA elicited increased levels of the proinflammatory cytokine tumor necrosis factor alpha (TNF-α) after systemic application. A detailed genetic and phenotypic characterization in combination with transcriptomic and metabolic profiling demonstrated that ΔaroA mutants display pleiotropic alterations in cellular physiology and lipid and amino acid metabolism, as well as increased sensitivity to penicillin, complement, and phagocytic uptake. In concert with other immunomodulating mutations, deletion of aroA affected flagellin phase variation and gene expression of the virulence-associated genes arnT and ansB. Finally, ΔaroA strains displayed significantly improved tumor therapeutic activity. These results highlight the importance of a functional shikimate pathway to control homeostatic bacterial physiology. They further highlight the great potential of ΔaroA-attenuated Salmonella for the development of vaccines and cancer therapies with important implications for host-pathogen interactions and translational medicine.

Recombinant attenuated bacterial vector systems based on genetically engineered Salmonella have been developed as highly potent vaccines. Due to the pathogenic properties of Salmonella, efficient attenuation is required for clinical applications. Since the hallmark study by Hoiseth and Stocker in 1981 (S. K. Hoiseth and B. A. D. Stocker, Nature 291:238–239, 1981, http://dx.doi.org/10.1038/291238a0), the auxotrophic ΔaroA mutation has been generally considered safe and universally used to attenuate bacterial strains. Here, we are presenting the remarkable finding that a deletion of aroA leads to pronounced alterations of gene expression, metabolism, and cellular physiology, which resulted in increased immunogenicity, virulence, and adjuvant potential of Salmonella. These results suggest that the enhanced immunogenicity of aroA-deficient Salmonella strains might be advantageous for optimizing bacterial vaccine carriers and immunotherapy. Accordingly, we demonstrate a superior performance of ΔaroA Salmonella in bacterium-mediated tumor therapy. In addition, the present study highlights the importance of a functional shikimate pathway to sustain bacterial physiology and metabolism.

Generation of influenza virus from avian cells infected by Salmonella carrying the viral genome

Domestic poultry serve as intermediates for transmission of influenza A virus from the wild aquatic bird reservoir to humans, resulting in influenza outbreaks in poultry and potential epidemics/pandemics among human beings. To combat emerging avian influenza virus, an inexpensive, heat-stable, and orally administered influenza vaccine would be useful to vaccinate large commercial poultry flocks and even migratory birds. Our hypothesized vaccine is a recombinant attenuated bacterial strain able to mediate production of attenuated influenza virus in vivo to induce protective immunity against influenza. Here we report the feasibility and technical limitations toward such an ideal vaccine based on our exploratory study. Five 8-unit plasmids carrying a chloramphenicol resistance gene or free of an antibiotic resistance marker were constructed. Influenza virus was successfully generated in avian cells transfected by each of the plasmids. The Salmonella carrier was engineered to allow stable maintenance and conditional release of the 8-unit plasmid into the avian cells for recovery of influenza virus. Influenza A virus up to 10⁷ 50% tissue culture infective doses (TCID50)/ml were recovered from 11 out of 26 co-cultures of chicken embryonic fibroblasts (CEF) and Madin-Darby canine kidney (MDCK) cells upon infection by the recombinant Salmonella carrying the 8-unit plasmid. Our data prove that a bacterial carrier can mediate generation of influenza virus by delivering its DNA cargoes into permissive host cells. Although we have made progress in developing this Salmonella influenza virus vaccine delivery system, further improvements are necessary to achieve efficient virus production, especially in vivo.

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.

Multicopy integration of mini-Tn7 transposons into selected chromosomal sites of a Salmonella vaccine strain

Chromosomal integration of expression modules for transgenes is an important aspect for the development of novel Salmonella vectors. Mini-Tn7 transposons have been used for the insertion of one such module into the chromosomal site attTn7, present only once in most Gram-negative bacteria. However, integration of multiple mini-Tn7 copies might be suitable for expression of appropriate amounts of antigen or combination of different modules. Here we demonstrate that integration of a 9.6 kb mini-Tn7 harbouring the luciferase luxCDABE (lux) occurs at the natural attTn7 site and simultaneously other locations of the Salmonella chromosome, which were engineered using λ-Red recombinase to contain one or two additional artificial attTn7 sites (a-attTn7). Multicopy integration even at closely spaced attTn7 sites was unexpected in light of the previously reported distance-dependent Tn7 target immunity. Integration of multiple copies of a mini-Tn7 containing a gfp cassette resulted in increasing green fluorescence of bacteria. Stable consecutive integration of two mini-Tn7 encoding lacZ and lux was achieved by initial transposition of lacZ-mini-Tn7, subsequent chromosomal insertion of a-attTn7 and a second round of transposition with lux-mini-Tn7. Mini-Tn7 thus constitutes a versatile method for multicopy integration of expression cassettes into the chromosome of Salmonella and possibly other bacteria.

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.

Development of Streptococcus pneumoniae Vaccines Using Live Vectors

Streptococcus pneumoniae still causes severe morbidity and mortality worldwide, especially in young children and the elderly. Much effort has been dedicated to developing protein-based universal vaccines to conquer the current shortcomings of capsular vaccines and capsular conjugate vaccines, such as serotype replacement, limited coverage and high costs. A recombinant live vector vaccine delivering protective antigens is a promising way to achieve this goal. In this review, we discuss the researches using live recombinant vaccines, mainly live attenuated Salmonella and lactic acid bacteria, to deliver pneumococcal antigens. We also discuss both the limitations and the future of these vaccines.

Evaluation of the immunogenicity of Campylobacter jejuni CjaA protein delivered by Salmonella enterica sv. Typhimurium strain with regulated delayed attenuation in chickens

Campylobacter spp. are regarded as the most common bacterial cause of gastroenteritis worldwide, and consumption of chicken meat contaminated by Campylobacter is considered to be one of the most frequent sources of human infection in developed countries. Here we evaluated the immunogenicity and protective efficacy of Salmonella Typhimurium χ9718 producing the Campylobacter jejuni CjaA protein as a chicken anti-Campylobacter vaccine. In this study chickens were orally immunized with a new generation S. Typhimurium strain χ9718 with regulated delayed attenuation in vivo and displaying delayed antigen expression. The immunization with the S. Typhimurium χ9718 strain producing C. jejuni CjaA antigen induced strong immune responses against CjaA in both serum IgY and intestinal IgA, however, it did not result in the significant reduction of intestinal colonization by Campylobacter strain. The low level of protection might arise due to a lack of T cell response. Our results demonstrated that a Salmonella strain with regulated delayed attenuation and displaying regulated delayed antigen expression might be an efficient vector to induce immune response against Campylobacter. It seems that an efficient anti-Campylobacter subunit vaccine should be multicomponent. Since S. Typhimurium χ9718 contains two compatible balanced-lethal plasmids, it can provide the opportunity of cloning several Campylobacter genes encoding immunodominant proteins. It may also be used as a delivery vector of eukaryotic genes encoding immunostimulatory molecules to enhance or modulate functioning of chicken immune system.

Serotype-independent pneumococcal vaccines

Streptococcus pneumoniae remains an important cause of disease with high mortality and morbidity, especially in children and in the elderly. The widespread use of the polysaccharide conjugate vaccines in some countries has led to a significant decrease in invasive disease caused by vaccine serotypes, but an increase in disease caused by non-vaccine serotypes has impacted on the overall efficacy of these vaccines on pneumococcal disease. The obvious solution to overcome such shortcomings would be the development of new formulations that provide serotype-independent immunity. This review focuses on the most promising approaches, including protein antigens, whole cell pneumococcal vaccines, and recombinant bacteria expressing pneumococcal antigens. The protective capacity of these vaccine candidates against the different stages of pneumococcal infection, including colonization, mucosal disease, and invasive disease in animal models is reviewed. Some of the human trials that have already been performed or that are currently ongoing are presented. Finally, the feasibility and the possible shortcomings of these candidates in relation to an ideal vaccine against pneumococcal infections are discussed.

Oral administration of attenuated Salmonella typhimurium containing a DNA vaccine against rabbit haemorrhagic disease

The use of attenuated Salmonella typhimurium as a bactofection vehicle for the oral delivery of a DNA vaccine against rabbit haemorrhagic disease virus (RHDV) was investigated. The DNA vaccine plasmid pcDNA3.1-VP60, which encodes the viral capsid protein VP60, was transformed into the attenuated S. typhimurium strain SL7207. The resulting recombinant bacteria, named as SL/pcDNA3.1-VP60, were orally used to immunise rabbits. The successful delivery of the DNA plasmid was confirmed by the detected VP60 transcription in the rabbit intestines through the reverse transcription polymerase chain reaction. In addition, the RHDV-specific humoral and cell-mediated immune response that was induced by SL/pcDNA3.1-VP60 was detected by the enzyme-linked immunosorbent assay as well as the assays for T lymphocyte proliferation and cytokines secretion. The significant protection of immunised rabbits against the RHDV strain XA/China/2010 at 42 d post-immunisation was demonstrated. This study is the first report about the efficient usage of attenuated Salmonella as a live vector for the oral delivery of a DNA vaccine against RHDV.

New technologies in developing recombinant attenuated Salmonella vaccine vectors

Recombinant attenuated Salmonella vaccine (RASV) vectors producing recombinant gene-encoded protective antigens should have special traits. These features ensure that the vaccines survive stresses encountered in the gastrointestinal tract following oral vaccination to colonize lymphoid tissues without causing disease symptoms and to result in induction of long-lasting protective immune responses. We recently described ways to achieve these goals by using regulated delayed in vivo attenuation and regulated delayed in vivo antigen synthesis, enabling RASVs to efficiently colonize effector lymphoid tissues and to serve as factories to synthesize protective antigens that induce higher protective immune responses. We also developed some additional new strategies to increase vaccine safety and efficiency. Modification of lipid A can reduce the inflammatory responses without compromising the vaccine efficiency. Outer membrane vesicles (OMVs) from Salmonella-containing heterologous protective antigens can be used to increase vaccine efficiency. A dual-plasmid system, possessing Asd+ and DadB+ selection markers, each specifying a different protective antigen, can be used to develop multivalent live vaccines. These new technologies have been adopted to develop a novel, low-cost RASV synthesizing multiple protective pneumococcal protein antigens that could be safe for newborns/infants and induce protective immunity to diverse Streptococcus pneumoniae serotypes after oral immunization.