Swine immunity to an attenuated Salmonella typhimurium mutant containing a recombinant plasmid which codes for production of a 31-kilodalton protein of Brucella abortus

Vaccines as alternatives to antibiotics for food producing animals. Part 2: new approaches and potential solutions

Vaccines and other alternative products are central to the future success of animal agriculture because they can help minimize the need for antibiotics by preventing and controlling infectious diseases in animal populations. To assess scientific advancements related to alternatives to antibiotics and provide actionable strategies to support their development, the United States Department of Agriculture, with support from the World Organisation for Animal Health, organized the second International Symposium on Alternatives to Antibiotics. It focused on six key areas: vaccines; microbial-derived products; non-nutritive phytochemicals; immune-related products; chemicals, enzymes, and innovative drugs; and regulatory pathways to enable the development and licensure of alternatives to antibiotics. This article, the second part in a two-part series, highlights new approaches and potential solutions for the development of vaccines as alternatives to antibiotics in food producing animals; opportunities, challenges and needs for the development of such vaccines are discussed in the first part of this series. As discussed in part 1 of this manuscript, many current vaccines fall short of ideal vaccines in one or more respects. Promising breakthroughs to overcome these limitations include new biotechnology techniques, new oral vaccine approaches, novel adjuvants, new delivery strategies based on bacterial spores, and live recombinant vectors; they also include new vaccination strategies in-ovo, and strategies that simultaneously protect against multiple pathogens. However, translating this research into commercial vaccines that effectively reduce the need for antibiotics will require close collaboration among stakeholders, for instance through public–private partnerships. Targeted research and development investments and concerted efforts by all affected are needed to realize the potential of vaccines to improve animal health, safeguard agricultural productivity, and reduce antibiotic consumption and resulting resistance risks.

TLR2 and TLR4 signaling pathways are required for recombinant Brucella abortus BCSP31-induced cytokine production, functional upregulation of mouse macrophages, and the Th1 immune response in vivo and in vitro

Brucella abortus is a zoonotic Gram-negative pathogen that causes brucelosis in ruminants and humans. Toll-like receptors (TLRs) recognize Brucella abortus and initiate antigen-presenting cell activities that affect both innate and adaptive immunity. In this study, we focused on recombinant Brucella cell-surface protein 31 (rBCSP31) to determine its effects on mouse macrophages. Our results demonstrated that rBCSP31 induced TNF-α, IL-6 and IL-12p40 production, which depended on the activation of mitogen-activated protein kinases (MAPKs) by stimulating the rapid phosphorylation of p38 and JNK and the activation of transcription factor NF-κB in macrophages. In addition, continuous exposure (>24 h) of RAW264.7 cells to rBCSP31 significantly enhanced IFN-γ-induced expression of MHC-II and the ability to present rBCSP31 peptide to CD4(+) T cells. Furthermore, we found that rBCSP31 could interact with both TLR2 and TLR4. The rBCSP31-induced cytokine production by macrophages from TLR2(-/-) and TLR4(-/-) mice was lower than that from C57BL/6 macrophages, and the activation of NF-κB and MAPKs was attenuated in macrophages from TLR2(-/-) and TLR4(-/-) mice. In addition, CD4(+) T cells from C57BL/6 mice immunized with rBCSP31 produced higher levels of IFN-γ and IL-2 compared with CD4(+) T cells from TLR2(-/-) and TLR4(-/-) mice. Macrophages from immunized C57BL/6 mice produced higher levels of IL-12p40 than those from TLR2(-/-) and TLR4(-/-) mice. Furthermore, immunization with rBCSP31 provided better protection in C57BL/6 mice than in TLR2(-/-) and TLR4(-/-) mice after B. abortus 2308 challenge. These results indicate that rBCSP31 is a TLR2 and TLR4 agonist that induces cytokine production, upregulates macrophage function and induces the Th1 immune response.

Progress in Brucella vaccine development

Brucella spp. are zoonotic, facultative intracellular pathogens, which cause animal and human disease. Animal disease results in abortion of fetuses; in humans, it manifests flu-like symptoms with an undulant fever, with osteoarthritis as a common complication of infection. Antibiotic regimens for human brucellosis patients may last several months and are not always completely effective. While there are no vaccines for humans, several licensed live Brucella vaccines are available for use in livestock. The performance of these animal vaccines is dependent upon the host species, dose, and route of immunization. Newly engineered live vaccines, lacking well-defined virulence factors, retain low residual virulence, are highly protective, and may someday replace currently used animal vaccines. These also have possible human applications. Moreover, due to their enhanced safety and efficacy in animal models, subunit vaccines for brucellosis show great promise for their application in livestock and humans. This review summarizes the progress of brucellosis vaccine development and presents an overview of candidate vaccines.

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.

Oral immunization of swine with attenuated Salmonella typhimurium aroA SL3261 expressing a recombinant antigen of Mycoplasma hyopneumoniae (NrdF) primes the immune system for a NrdF specific secretory IgA response in the lungs

Salmonella typhimurium SL3261 (aroA mutant) expressing a recombinant Mycoplasma hyopneumoniae antigen was used to orally immunize swine against porcine enzootic pneumonia. This construct, designated S. typhimurium aro A SL3261 (pKF1), expressed a recombinant protein containing the carboxy-terminal 11 kDa of a 42 kDa M. hyopneumoniae NrdF ribonucleotide reductase R2 subunit protein. Here we demonstrate that this antigen is present in all seven geographically diverse strains of M. hyopneumoniae tested, and is recognized by the swine immune system after experimental infection with the virulent M. hyopneumoniae Beaufort strain. The immune response of swine orally immunized twice with S. typhimurium SL3261 (pKF1) on day 0 and day 14 was evaluated. Oral immunization with S. typhimurium SL3261 (pKF1) primed the immune system to elicit a significant (P<0.05) secretory IgA response against the 15 kDa NrdF antigen in the respiratory tract of swine, post-challenge, compared to control groups. Blood lymphocytes from swine immunized with S. typhimurium SL3261 (pKF1) proliferated significantly (P<0.05) following stimulation with M. hyopneumoniae whole-cell extracts compared to control groups 14 days post-vaccination. Following challenge with virulent M. hyopneumoniae, swine immunized with S. typhimurium SL3261 (pKF1) showed higher average daily weight gains and reduced lung pathology compared to control groups.

Expression and immunogenicity of an Echinococcus granulosus fatty acid-binding protein in live attenuated Salmonella vaccine strains

Fatty acid-binding proteins (FABPs) are candidate molecules for vaccines against several parasitic platyhelminths. A FABP from the cestode Echinococcus granulosus (EgDf1) was expressed in Salmonella vaccine strains as a C-terminal fusion to fragment C of tetanus toxin (TetC) by using expression vector pTECH. The fusion protein was equally expressed in several attenuated vaccine strains derived from bacteria with different genetic backgrounds and different attenuating mutations. Single-dose immunization experiments with the aroA Salmonella typhimurium strain SL3261 carrying the pTECH-EgDf1 construct were conducted with mice, using both the intravenous and the oral routes. Surprisingly, the antibody response to EgDf1 and the antigen-specific cytokine production in spleen cells were stronger in mice immunized orally. Furthermore, immune mouse sera strongly reacted with fixed sections of the worm's larval stage. Analysis of the isotype distribution of the specific anti-EgDf1 antibodies showed similar production of immunoglobulin G1 (IgG1) and IgG2a together with specific IgA antibodies. In addition, stimulation of spleen cells from mice immunized with the different constructs with either Salmonella lysate, TetC, or EgDf1 showed that, together with Th1-related cytokines (gamma interferon and interleukin 2 [IL-2]), significant levels of a Th2 cytokine (IL-5) were produced specifically, indicating a Th2 component to the response to the Salmonella carrier and to the recombinant antigens. Salmonellae expressing the TetC-rEgDfl fusion are currently under evaluation as potential vaccines against E. granulosus.

Immunisation of mice using Salmonella typhimurium expressing human papillomavirus type 16 E7 epitopes inserted into hepatitis B virus core antigen

Live vaccines based on BRD509, an attenuated S. typhimurium (aroA, aroD) strain, were constructed that directed the expression of hepatitis B core antigen particles (HBcAg) (BRD969) or HBcAg harbouring human papillomavirus type 16 E7 protein sequences (BRD974), under the control of the in vivo inducible nirB promoter. These strains were used to orally or intravenously immunise different inbred mouse strains and humoral, secretory and cellular anti-E7 and anti-HBcAg responses were monitored. Both BRD969 and BRD974 induced anti-HBcAg humoral IgG responses following oral or intravenous immunisation of B10 mice, although responses were higher in BRD969 immunised animals. IgG subclass analysis revealed a predominantly IgG2a response in these animals. BRD974, but not BRD969, induced anti-E7 humoral IgG responses. Anti-HBcAg (BRD969 and BRD974) and anti-E7 (BRD974) IgA responses were detected in the intestines of orally immunised mice. Anti-Salmonella but not anti-HBcAg or anti-E7 T helper cell responses were detected in mice immunised with BRD509, BRD969 and BRD974. Thus Salmonella vaccine strains can be used to efficiently deliver HBcAg and E7 epitopes to the mucosal and systemic immune systems.

Characterization of transmissible gastroenteritis coronavirus S protein expression products in avirulent S. typhimurium delta cya delta crp: persistence, stability and immune response in swine

The spike protein from transmissible gastroenteritis virus (TGEV) was expressed in attenuated S. typhimurium delta cya delta crp delta asd chi 3987. Three partially overlapping fragments of TGEV S gene, encoding the amino-terminal, intermediate, and carboxy-terminal end of the protein, as well as the full length gene were inserted into the asd+ plasmid pYA292 to generate recombinant plasmids pYATS-1, pYATS-2, pYATS-3, and pYATS-4, respectively, which were transformed into S. typhimurium chi 3987. Recombinant S. typhimurium chi 3987 (pYATS-1) and chi 3987 (pYATS-4) expressing constitutively a 53 kDa amino-terminal fragment of the S protein and the full length protein (144 kDa), respectively, showed high stability. After 50 generations in vitro 60% and 20% of the bacteria transformed with pYATS-1 and pYATS-4, respectively, expressed the S-protein antigen. Since S. typhimurium chi 3987 (pYATS-1) showed a better level of expression and stability in vitro, this recombinant strain was selected as a potential bivalent vector to induce both immunity to Salmonella and TGEV in swine. In order to study colonization of swine tissues by S. typhimurium delta cya delta crp, a gene conferring resistance to rifampicin was cloned into the chromosome of S. typhimurium chi 3987, generating chi 4509 strain. Both S. typhimurium chi 4509 (pYA292) and chi 4509 (pYATS-1) colonized the ileum of orally inoculated swine with clearance of bacteria between days 10-20 post-infection. The expression of the amino-terminal fragment of the S protein diminished the ability of S. typhimurium chi 4509 (pYATS-1) to colonize deep tissues. The recombinant strain S. typhimurium chi 3987 (pYATS-1) induced TGEV specific antibodies in both serum and saliva of orally inoculated swine.

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.

Oral immunization with recombinant Salmonella typhimurium expressing surface protein antigen A of Streptococcus sobrinus: dose response and induction of protective humoral responses in rats

An attenuated, recombinant Salmonella typhimurium mutant, chi 4072(pYA2905), expressing the surface protein antigen A (SpaA) of Streptococcus sobrinus was investigated for its effectiveness in inducing protective immune responses against S. sobrinus-induced dental caries in an experimental caries model. Fischer rats were orally immunized with either 10(8) or 10(9) CFU of S. typhimurium chi 4072(pYA2905). Persistence of salmonellae in Peyer's patches and spleens and the induction of immune responses were determined. Maximum numbers of salmonellae were recovered from Peyer's patches of rats within the first week of immunization, with higher numbers recovered from rats given 10(9) CFU than from those given 10(8) CFU. Serum anti-Salmonella and anti-SpaA responses increased more rapidly in rats given 10(9) CFU than in rats given 10(8) CFU. The salivary antibody response to SpaA increased with time, but the response varied in the two groups. In a separate study, rats were orally immunized with the recombinant Salmonella mutant and then challenged with cariogenic S. sobrinus 6715. The levels of serum and salivary antibody and caries activity were assessed at the termination of the experiment. Higher levels of salivary immunoglobulin A antibody to SpaA and Salmonella carrier were detected in rats given 10(9) CFU than in those given 10(8) CFU, and these responses were higher than those in nonimmunized controls. Mandibular molars from immunized rats had lower numbers of recoverable streptococci and less extensive carious lesions than those from nonimmunized, control rats. These data indicate that oral immunization with an attenuated recombinant S. typhimurium expressing SpaA of S. sobrinus induces the production of antigen-specific mucosal antibody and confers protection against dental caries.

Oral immunization with recombinant Salmonella typhimurium expressing surface protein antigen A of Streptococcus sobrinus: persistence and induction of humoral responses in rats

Recombinant Salmonella typhimurium has been used as an oral vaccine for various microbial pathogens. Here we report immune responses in Fischer rats orally immunized with a recombinant S. typhimurium strain encoding surface protein antigen A (SpaA) of Streptococcus sobrinus. The attenuated S. typhimurium chi 4072 delta cya delta crp delta asd mutant used in this study contains the Asd+ plasmid pYA2905 expressing a fragment of the SpaA protein. Salmonella cells were cleared from spleens by 7 days and from Peyer's patches by 14 days in rats receiving a single oral immunization of 10(9) CFU of chi 4072. In animals receiving multiple (i.e., days 0 and 7 or days 0, 7, and 21) immunizations, Salmonella cells were cleared from the Peyer's patches by 25 days following the initial immunization. Antigen-specific systemic and mucosal antibody responses were greater in rats receiving multiple immunizations than in those receiving a single immunization. Serum anti-Salmonella activity was potentiated following boosting on day 21. Mucosal immunoglobulin A antibody responses were also greater in rats receiving multiple immunizations than in rats receiving a single immunization. Anti-Salmonella and anti-Streptococcus immunoglobulin A activity persisted longer in rats boosted on day 21 than in rats immunized on days 0 and 7. These data indicate that oral immunization of rats with the recombinant S. typhimurium chi 4072(pYA2905) vaccine induces systemic as well as mucosal antibody responses specific to the Salmonella cells and to the cloned SpaA protein. This is the first report of the use of an attenuated mutant of the murine pathogen S. typhimurium as an oral vaccine in rats.

Recent advances in bovine vaccine technology

A description of new commercial and experimental vaccines for viral and bacterial diseases of cattle can be broadly divided into those used for both beef and dairy cows and those used predominantly in dairy cattle. For both types of cattle, newer and experimental vaccines are directed against several of the important viral (e.g., bovine herpesvirus 1, bovine viral diarrhea virus, bovine respiratory syncytial virus, parainfluenza type 3, and foot-and-mouth disease virus) and bacterial pathogens (e.g., Pasteurella spp., Haemophilus somnus). The viral vaccines include gene-deleted, modified live, subunit, and peptide antigens. Newer bacterial vaccines, particularly those for Pasteurella spp., are composed of either modified-live vaccines or bacterins supplemented with toxoid or surface antigens. Haemophilus somnus vaccine research has concentrated mainly on defining unique surface antigens. Novel dairy cow vaccines would include the lipopolysaccharide-core (J5) antigen approach, which has been used for successful immunization against coliform mastitis. Core antigen vaccines also have reduced calf mortality from Gram-negative pathogens. Staphylococcal mastitis vaccines that contain capsular antigens, toxoids, or the staphylococcal fibronectin receptor are of active research interest. Vaccines against mastitis induced by Streptococcus agalactiae and Streptococcus uberis also are areas of intensive research. Delivery of multiple subunit antigens with optimal immune response induction has led to the investigation of attenuated heterologous viral and bacterial expression vectors such as bovine herpesvirus 1, vaccinia, and Salmonella spp. This discussion also demonstrates that molecular biology is being used to advance bovine vaccine technology.

Immune responses to Streptococcus sobrinus surface protein antigen A expressed by recombinant Salmonella typhimurium

In this study, we used a vaccine strain of Salmonella typhimurium to express antigenic determinants of the SpaA antigen of Streptococcus sobrinus, which is involved in the caries-forming process. We cloned either a single repeat (pYA2901) or three tandem repeats (pYA2905) of the 0.48-kb fragment of the spaA gene, which codes for an important component of the SpaA protein, plus a 1.2-kb minor antigenic determinant and measured the resulting immune responses to SpaA in orally immunized BALB/c mice. The single or triple repeat of the spaA gene fragment was inserted into the Asd+ vector pYA292 and was transformed into the S. typhimurium delta cya delta crp vaccine strain chi 4072 containing delta asd in the chromosome. Female BALB/c mice were then orally immunized with two doses of the S. typhimurium containing either of the two SpaA constructs, and the immune responses to the expressed SpaA protein were assessed. Significant serum immunoglobulin G (IgG) anti-SpaA titers were detected in mice immunized with chi 4072(pYA2905) but not chi 4072(pYA2901). Salivary anti-SpaA IgA titers were minimal and were only detected in mice immunized with S. typhimurium expressing the SpaA encoded by pYA2905. Intestinal anti-SpaA IgA titers, however, were detected in both groups of mice, particularly in mice immunized with chi 4072(pYA2905). An oral booster 26 weeks after the initial series of immunizations resulted in increased serum IgG titers in both chi 4072(pYA2901)- and chi 4072(pYA2905)-immunized animals, particularly in the chi 4072(pYA2905)-immunized animals. No anamnestic IgA response was detected in the saliva following the booster immunization.

Oral immunization of mice and swine with an attenuated Salmonella choleraesuis [delta cya-12 delta(crp-cdt)19] mutant containing a recombinant plasmid

Salmonella choleraesuis chi 3781, an attenuated [delta cya-12 delta(crp-cdt)19] mutant, was electroporated with the plasmid pBA31-R7, which codes for the expression of a 31-kDa protein from Brucella abortus (BCSP31). Recombinant S. choleraesuis chi 3781 stably maintained the pBA31-R7 plasmid and continued to express the cloned protein following recovery of the organism from orally inoculated animals. Unlike previous studies using S. typhimurium chi 4064(pBA31-R7), S. choleraesuis chi 3781(pBA31-R7) was able to colonize both the gut mucosa and deep tissues of both BALB/cByJ mice and crossbred swine. Orally inoculated mice developed serum antibodies to both the cloned 31-kDa protein (rBCSP31) and to S. choleraesuis chi 3781 endotoxin. These mice also developed a local intestinal antibody response to Salmonella endotoxin but not to rBCSP31. Similarly, mice inoculated with recombinant S. choleraesuis chi 3781 did not develop a delayed-type hypersensitivity (DTH) footpad response following injection with rBCSP31; however, these mice did respond to S. choleraesuis chi 3781 soluble antigen. Conversely, orally inoculated swine did not develop significant serum or intestinal antibody responses to cloned protein or Salmonella endotoxin, but DTH responses to both cloned protein and S. choleraesuis chi 3781 soluble antigen were strongly positive. The cell-mediated nature of these DTH responses was confirmed by histological examination. Results suggest that S. choleraesuis chi 3781 may be a suitable choice for further studies of vaccine efficacy in swine, especially for diseases which require cell-mediated immunity for resolution.

Characterization and protective properties of attenuated mutants of Salmonella choleraesuis

We have constructed crp::Tn10 and cya::Tn10 Salmonella choleraesuis mutants and their fusaric acid-resistant derivatives with deletions (delta) of the Tn10 and adjacent DNA sequences and found them to be avirulent and able to induce protection against a wild-type challenge in 8-week-old BALB/c mice. Mice survived infection with the crp and cya mutants at doses of more than 7 x 10(3) times the oral (p.o.) 50% lethal dose (LD50) and more than 8 x 10(2) times the intraperitoneal LD50 of the wild-type S. choleraesuis parent. Mice vaccinated with attenuated strains were protected against challenge with more than 1.6 x 10(4) times the p.o. LD50 and more than 80 times the intraperitoneal LD50 of the wild-type virulent S. choleraesuis parent. One deletion mutation isolated in the crp region extends to an adjacent gene(s) that was shown to be associated with avirulence. This gene or operon has been designated cdt (colonization of deep tissues). A delta (crp-cdt)19 strain, when complemented with the wild-type crp gene and promoter on a pBR-derived plasmid, had p.o. LD50 values 10(3) times higher than those for the wild type. A delta cya delta (crp-cdt)19 double mutant was less virulent than and afforded more complete protection against a challenge with the wild-type strain than a delta crp-11 delta cya double mutant or the individual cya, crp, or crp+/cdt mutants. The deletion derivatives exhibited reduced invasion of CHO cells in vitro, and the numbers of the mutants recovered from mouse tissues were less than that of the parent strain. These studies suggest that one or more of the genes involved in cell attachment to and/or invasion of S. choleraesuis may be under catabolite repression. In addition, we describe a new deletion of a gene(s) located in the crp region between cysG and argD that is associated with virulence in S. choleraesuis.