Bacterial virulence, proinflammatory cytokines and host immunity: how to choose the appropriate Salmonella vaccine strain?

Salmonella Typhimurium Lacking YjeK as a Candidate Live Attenuated Vaccine Against Invasive Salmonella Infection

Non-typhoidal Salmonella (NTS) causes gastrointestinal infection, which is commonly self-limiting in healthy humans but may lead to invasive infection at extraintestinal sites, leading to bacteremia and focal systemic infections in the immunocompromised. However, a prophylactic vaccine against invasive NTS has not yet been developed. In this work, we explored the potential of a ΔyjeK mutant strain as a live attenuated vaccine against invasive NTS infection. YjeK in combination with YjeA is required for the post-translational modification of elongation factor P (EF-P), which is critical for bacterial protein synthesis. Therefore, malfunction of YjeK and YjeA-mediated EF-P activation might extensively influence protein expression during Salmonella infection. Salmonella lacking YjeK showed substantial alterations in bacterial motility, antibiotics resistance, and virulence. Interestingly, deletion of the yjeK gene increased the expression levels of Salmonella pathogenicity island (SPI)-1 genes but decreased the transcription levels of SPI-2 genes, thereby influencing bacterial invasion and survival abilities in contact with host cells. In a mouse model, the ΔyjeK mutant strain alleviated the levels of splenomegaly and bacterial burdens in the spleen and liver in comparison with the wild-type strain. However, mice immunized with the ΔyjeK mutant displayed increased Th1- and Th2-mediated immune responses at 28 days post-infection, promoting cytokines and antibodies production. Notably, the Th2-associated antibody response was highly induced by administration of the ΔyjeK mutant strain. Consequently, vaccination with the ΔyjeK mutant strain protected 100% of the mice against challenge with lethal invasive Salmonella and significantly relieved bacterial burdens in the organs. Collectively, these results suggest that the ΔyjeK mutant strain can be exploited as a promising live attenuated NTS vaccine.

Evaluation of Bifidobacteria and Lactobacillus Probiotics as Alternative Therapy for Salmonella typhimurium Infection in Broiler Chickens

Simple Summary

Salmonella is an important foodborne pathogen that represents a very critical threat to poultry industry worldwide. This study concerns an important aspect of human food and health problem by treating a common zoonotic bacterial disease in poultry industry. Owing to the increased resistance to antibiotics among Salmonellaenterica serotypes, we aimed to explore the beneficial effects of different probiotics strains as alternative sources of protection against infection in broiler chickens. Three probiotic strains Lactobacillus (Lacticaseibacillus) casei ATTC334, Bifidobacterium breve JCM1192 and Bifidobacterium infantis BL2416) improved body weight gain and prevented the deleterious effects and mortality induced by Salmonella infection in chicks through different mechanisms, including competitive exclusion and the promotion of cytokines’ release.

Abstract

Chicken Salmonella enterica serovars are enteric bacteria associated with massive public health risks and economic losses. There is a widespread antimicrobial resistance among S.enterica serotypes, and innovative solutions to antibiotic resistance are needed. We aimed to use probiotics to reduce antibiotic resistance and identify the major probiotic players that modify the early interactions between S.enterica and host cells. One-day-old cobb broiler chicks were challenged with S. typhimurium after oral inoculation with different probiotic strains for 3 days. The adherence of different probiotic strains to Caco-2 intestinal epithelial cells was studied in vitro. Lactobacillus (Lacticaseibacillus) casei ATTC334 and Bifidobacterium breve JCM1192 strains attached to Caco-2 cells stronger than B. infantis BL2416. L. casei ATTC334 and B. breve JCM1192 reduced S. typhimurium recovery from the cecal tonsils by competitive exclusion mechanism. Although B. infantis BL2416 bound poorly to Caco-2 epithelial cells, it reduced S. typhimurium recovery and increased IFN-γ and TNF-α production. L. casei ATTC334, B. breve JCM1192 and B. infantis BL2416 improved body weight gain and the food conversion rate in S. typhimurium-infected broilers. B. longum Ncc2785 neither attached to epithelial cells nor induced IFN-γ and TNF-α release and consequently did not prevent S. typhimurium colonization in broiler chickens. In conclusion, probiotics prevented the intestinal colonization of S. typhimurium in infected chickens by competitive exclusion or cytokine production mechanisms.

Adenosine receptor Adora2b antagonism attenuates Brucella abortus 544 infection in professional phagocyte RAW 264.7 cells and BALB/c mice

Brucella as a stealthy intracellular pathogen avoids activation of innate immune response. Here we investigated the contribution of an adenosine receptor, Adora2b, during Brucella infection in professional phagocyte RAW 264.7 cells and in a murine model. Adora2b-deficient cells showed attenuated Brucella internalization and intracellular survival with enhanced release of IL-6, TNF-α, IL-12 and MCP-1. In addition, blockade of Adora2b using MRS 1754 treatment in mice resulted in increased total weight of the spleens but suppressed bacterial burden in these organs accompanied by elevated levels of IL-6, IFN-γ, TNF-α, IL-12 and MCP-1, while reduced IL-10. Overall, we proposed that the Adora2b participates in the successful phagocytic pathway and intracellular survival of Brucella in RAW 264.7 cells, and could be a potential therapeutic target for the treatment of acute brucellosis in animals.

Live-Attenuated Bacterial Vectors for Delivery of Mucosal Vaccines, DNA Vaccines, and Cancer Immunotherapy

Vaccines save millions of lives each year from various life-threatening infectious diseases, and there are more than 20 vaccines currently licensed for human use worldwide. Moreover, in recent decades immunotherapy has become the mainstream therapy, which highlights the tremendous potential of immune response mediators, including vaccines for prevention and treatment of various forms of cancer. However, despite the tremendous advances in microbiology and immunology, there are several vaccine preventable diseases which still lack effective vaccines. Classically, weakened forms (attenuated) of pathogenic microbes were used as vaccines. Although the attenuated microbes induce effective immune response, a significant risk of reversion to pathogenic forms remains. While in the twenty-first century, with the advent of genetic engineering, microbes can be tailored with desired properties.

In this review, I have focused on the use of genetically modified bacteria for the delivery of vaccine antigens. More specifically, the live-attenuated bacteria, derived from pathogenic bacteria, possess many features that make them highly suitable vectors for the delivery of vaccine antigens. Bacteria can theoretically express any heterologous gene or can deliver mammalian expression vectors harboring vaccine antigens (DNA vaccines). These properties of live-attenuated microbes are being harnessed to make vaccines against several infectious and noninfectious diseases. In this regard, I have described the desired features of live-attenuated bacterial vectors and the mechanisms of immune responses manifested by live-attenuated bacterial vectors. Interestingly anaerobic bacteria are naturally attracted to tumors, which make them suitable vehicles to deliver tumor-associated antigens thus I have discussed important studies investigating the role of bacterial vectors in immunotherapy. Finally, I have provided important discussion on novel approaches for improvement and tailoring of live-attenuated bacterial vectors for the generation of desired immune responses.

HU-Lacking Mutants of Salmonella enterica Enteritidis Are Highly Attenuated and Can Induce Protection in Murine Model of Infection

Salmonella enterica infection is a major public health concern worldwide, particularly when associated with other medical conditions. The serovars Typhimurium and Enteritidis are frequently associated with an invasive illness that primarily affects immunocompromised adults and children with HIV, malaria, or malnutrition. These serovars can also cause infections in a variety of animal hosts, and they are the most common isolates in poultry materials. Here, we described S. Enteritidis mutants, where hupA and hupB genes were deleted, and evaluated their potential use as live-attenuated vaccine candidates. In vitro, the mutants behaved like S. Typhimurium described previously, but there were some particularities in macrophage invasion and survival experiments. The virulence and immunogenicity of the mutant lacking both hupA and hupB (PT4ΔhupAB) were evaluated in a BALB/c mice model. This mutant was highly attenuated and could, therefore, be administrated at doses higher than 10⁹ CFU/treatment, which was sufficient to protect all treated mice challenged with the wild-type parental strain with a single dose. Additionally, the PT4ΔhupAB strain induced production of specific IgG and IgA antibodies against Salmonella and TH1-related cytokines (IFN-γ and TNF-α), indicating that this strain can induce systemic and mucosal protection in the murine model. Additional studies are needed to better understand the mechanisms that lead to attenuation of the double-mutant PT4ΔhupAB and to elucidate the immune response induced by immunization using this strain. However, our data allow us to state that hupAB mutants could be potential candidates to be explore as live-attenuated vaccines.

Different innate immunity and clearance of Salmonella Pullorum in macrophages from White Leghorn and Tibetan Chickens

Salmonella enterica serovar Gallinarum biovar Pullorum ( S. Pullorum) is responsible for the systemic salmonellosis in different breeds of chickens. Macrophages, as host cells, play a key role in the innate immune response following infection with S. Pullorum. In this study, we first generated macrophages from two breeds of chicken (White Leghorn (WL) and Tibetan Chickens (TC)) peripheral blood monocytes in vitro. Then, we showed that the production of interleukin-1β (IL-1β), macrophage inflammatory protein-1β (MIP-1β) and interleukin-10 (IL-10) in lipopolysaccharide (LPS)-treated macrophages was significantly higher compared with the unstimulated cells in TC. LPS triggered only more expression of IL-10 in WL macrophages. Furthermore, macrophages from TC eliminated intracellular bacteria more efficiently than those from WL after S. Pullorum infection at a multiplicity of infection (MOI) 1. In addition, the variation between individuals and sex had the crucial effect on the immune response to LPS and S. Pullorum invasion.

phoP, SPI1, SPI2 and aroA mutants of Salmonella Enteritidis induce a different immune response in chickens

Poultry is the most frequent reservoir of non-typhoid Salmonella enterica for humans. Understanding the interactions between chickens and S. enterica is therefore important for vaccine design and subsequent decrease in the incidence of human salmonellosis. In this study we therefore characterized the interactions between chickens and phoP, aroA, SPI1 and SPI2 mutants of S. Enteritidis. First we tested the response of HD11 chicken macrophage-like cell line to S. Enteritidis infection monitoring the transcription of 36 genes related to immune response. All the mutants and the wild type strain induced inflammatory signaling in the HD11 cell line though the response to SPI1 mutant infection was different from the rest of the mutants. When newly hatched chickens were inoculated, the phoP as well as the SPI1 mutant did not induce an expression of any of the tested genes in the cecum. Despite this, such chickens were protected against challenge with wild-type S. Enteritidis. On the other hand, inoculation of chickens with the aroA or SPI2 mutant induced expression of 27 and 18 genes, respectively, including genes encoding immunoglobulins. Challenge of chickens inoculated with these two mutants resulted in repeated induction of 11 and 13 tested genes, respectively, including the genes encoding immunoglobulins. In conclusion, SPI1 and phoP mutants induced protective immunity without inducing an inflammatory response and antibody production. Inoculation of chickens with the SPI2 and aroA mutants also led to protective immunity but was associated with inflammation and antibody production. The differences in interaction between the mutants and chicken host can be used for a more detailed understanding of the chicken immune system.

Safety and immunogenicity of an attenuated Salmonella enterica serovar Paratyphi A vaccine candidate

Enteric fever caused by Salmonella enterica serovar Paratyphi A has progressively increased in recent years and became a global health issue. Currently licensed typhoid vaccines do not confer adequate cross-immunoprotection against S. Paratyphi A infection. Therefore, vaccines specifically against enteric fever caused by S. Paratyphi A are urgently needed. In the present study, an attenuated vaccine strain was constructed from S. Paratyphi A CMCC50093 by the deletions of aroC and yncD. The obtained strain SPADD01 showed reduced survival within THP-1 cells and less bacterial burden in spleens and livers of infected mice compared with the wild-type strain. The 50% lethal doses of SPADD01 and the wild-type strain were assessed using a murine infection model. The virulence of SPADD01 is approximately 40,000-fold less than that of the wild-type strain. In addition, SPADD01 showed an excellent immunogenicity in mouse model. Single intranasal inoculation elicited striking humoral and mucosal immune responses in mice and yielded effective protection against lethal challenge of the wild-type strain. A high level of cross-reactive humoral immune response against LPS of Salmonella enterica serovar Typhi was also detected in immunized mice. However, SPADD01 vaccination only conferred a low level of cross-protection against S. Typhi. Our data suggest that SPADD01 is a promising vaccine candidate against S. Paratyphi A infection and deserves further evaluation in clinical trial. To date, no study has demonstrated a good cross-protection between serovars of S. Typhi and S. Paratyphi A, suggesting that the dominant protective antigens of both serovars are likely different and need to be defined in future study.

Salmonella Typhimurium TTSS-2 deficient mig-14 mutant shows attenuation in immunocompromised mice and offers protection against wild-type Salmonella Typhimurium infection

Background

Development of Salmonella enterica serovar Typhimurium (S. Typhimurium) live attenuated vaccine carrier strain to prevent enteric infections has been a subject of intensive study. Several mutants of S. Typhimurium have been proposed as an effective live attenuated vaccine strain. Unfortunately, many such mutant strains failed to successfully complete the clinical trials as they were suboptimal in delivering effective safety and immunogenicity. However, it remained unclear, whether the existing live attenuated S. Typhimurium strains can further be attenuated with improved safety and immune efficacy or not.

Results

We deleted a specific non-SPI (Salmonella Pathogenicity Island) encoded virulence factor mig-14 (an antimicrobial peptide resistant protein) in ssaV deficient S. Typhimurium strain. The ssaV is an important SPI-II gene involved in Salmonella replication in macrophages and its mutant strain is considered as a potential live attenuated strain. However, fatal systemic infection was previously reported in immunocompromised mice like Nos2^−/− and Il-10^−/− when infected with ssaV deficient S. Typhimurium. Here we reported that attenuation of S. Typhimurium ssaV mutant in immunocompromised mice can further be improved by introducing additional deletion of gene mig-14. The ssaV, mig-14 double mutant was as efficient as ssaV mutant, with respect to host colonization and eliciting Salmonella-specific mucosal sIgA and serum IgG response in wild-type C57BL/6 mice. Interestingly, this double mutant did not show any systemic infection in immunocompromised mice.

Conclusions

This study suggests that ssaV, mig-14 double mutant strain can be effectively used as a potential vaccine candidate even in immunocompromised mice. Such attenuated vaccine strain could possibly used for expression of heterologous antigens and thus for development of a polyvalent vaccine strain.

Boots for Achilles: progesterone's reduction of cholesterol is a second-order adaptation

Progesterone and cholesterol are both vital to pregnancy. Among other functions, progesterone downregulates inflammatory responses, allowing for maternal immune tolerance of the fetal allograft. Cholesterol a key component of cell membranes, is important in intracellular transport, cell signaling, nerve conduction, and metabolism Despite the importance of each substance in pregnancy, one exercises an antagonistic effect on the other, as periods of peak progesterone correspond with reductions in cholesterol availability, a consequence of progesterone's negative effects on cholesterol biosynthesis. This arrangement is understandable in light of the threat posed by pathogens early in pregnancy. Progesterone-induced immunomodulation entails increased vulnerability to infection, an acute problem in the first trimester, when fetal development is highly susceptible to insult. Many pathogens rely on cholesterol for cell entry, egress, and replication. Progesterone's antagonistic effects on cholesterol thus partially compensate for the costs entailed by progesterone-induced immunomodulation. Among pathogens to which the host's vulnerability is increased by progesterone's effects, approximately 90% utilize cholesterol, and this is notably true of pathogens that pose a risk during pregnancy. In addition to having a number of possible clinical applications, our approach highlights the potential importance of second-order adaptations, themselves a consequence of the lack of teleology in evolutionary processes.

Immunological characterization of a gidA mutant strain of Salmonella for potential use in a live-attenuated vaccine

Background

Salmonella is often associated with gastrointestinal disease outbreaks in humans throughout the world due to the consumption of contaminated food. Our previous studies have shown that deletion of glucose-inhibited division gene (gidA) significantly attenuated Salmonella enterica serovar Typhimurium (STM) virulence in both in vitro and in vivo models of infection. Most importantly, immunization with the gidA mutant protected mice from a lethal dose challenge of wild-type STM. In this study, we further characterize the gidA mutant STM strain for potential use in a live-attenuated vaccine.

Results

The protective efficacy of immunization with the gidA mutant was evaluated by challenging immunized mice with a lethal dose of wild-type STM. Sera levels of IgG2a and IgG1, passive transfer of sera and cells, and cytokine profiling were performed to study the induction of humoral and cellular immune responses induced by immunization with the gidA mutant strain. Additionally, a lymphocyte proliferation assay was performed to gauge the splenocyte survival in response to treatment with STM cell lysate. Mice immunized with the gidA mutant strain were fully protected from a lethal dose challenge of wild-type STM. Naïve mice receiving either cells or sera from immunized mice were partially protected from a lethal dose challenge of wild-type STM. The lymphocyte proliferation assay displayed a significant response of splenocytes from immunized mice when compared to splenocytes from non-immunized control mice. Furthermore, the immunized mice displayed significantly higher levels of IgG1 and IgG2a with a marked increase in IgG1. Additionally, immunization with the gidA mutant strain evoked higher levels of IL-2, IFN-γ, and IL-10 cytokines in splenocytes induced with STM cell lysate.

Conclusions

Together, the results demonstrate that immunization with the gidA mutant strain protects mice by inducing humoral and cellular immune responses with the humoral immune response potentially being the main mechanism of protection.

Role of inflammasomes in salmonella infection

Pattern recognition receptors (PRRs) play a crucial role in both the detection of pathogens and the activation of the innate immune system. Nod-like receptors (NLR) family members are cytosolic PRRs that sense bacterial products or endogenous danger signals. Recent evidence suggests that NLRs contribute to the detection of Salmonella through the activation of inflammasomes, molecular platforms that promotes the maturation of the proinflammatory cytokines IL-1β and IL-18. During enteric Salmonella infection the activation of caspase-1 and the production of IL-1β and IL-18 result in a protective host response. In macrophages, the activation of caspase-1 induced by Salmonella is mainly mediated by the NLR family member NLRC4 that senses cytosolic flagellin. Recent data suggest that an effective innate immune response against Salmonella requires the engagement of multiple inflammasomes in both hematopoietic and non-hematopoietic cell lineages. Further understanding of the innate immune response mediated by inflammasomes should provide new insights into the mechanisms of host defense and the pathogenesis of inflammatory diseases.

ppGpp conjures bacterial virulence

Like for all microbes, the goal of every pathogen is to survive and replicate. However, to overcome the formidable defenses of their hosts, pathogens are also endowed with traits commonly associated with virulence, such as surface attachment, cell or tissue invasion, and transmission. Numerous pathogens couple their specific virulence pathways with more general adaptations, like stress resistance, by integrating dedicated regulators with global signaling networks. In particular, many of nature's most dreaded bacteria rely on nucleotide alarmones to cue metabolic disturbances and coordinate survival and virulence programs. Here we discuss how components of the stringent response contribute to the virulence of a wide variety of pathogenic bacteria.

Impaired germinal center responses and suppression of local IgG production during intracellular bacterial infection

Germinal centers (GCs) are specialized microenvironments in secondary lymphoid organs that facilitate the development of high-affinity, isotype-switched Abs, and immunological memory; consequently, many infections require GC-derived IgG for pathogen clearance. Although Ehrlichia muris infection elicits a robust expansion of splenic, IgM-secreting plasmablasts, we detected only very low frequencies of isotype-switched IgG-secreting cells in mouse spleens, until at least 3 wk postinfection. Instead, Ag-specific IgG was produced in lymph nodes, where it required CD4 T cell help. Consistent with these findings, organized GCs and phenotypically defined splenic GC B cells were found in lymph nodes, but not spleens. Ehrlichial infection also inhibited spleen IgG responses against a coadministered T cell-dependent Ag, hapten 4-hydroxy-3-nitrophenyl acetyl (NP)-conjugated chicken gamma globulin in alum. NP-specific B cells failed to undergo expansion and differentiation into GC B cells in the spleen, Ab titers were reduced, and splenic IgG production was inhibited nearly 10-fold when the Ag was administered during infection. Our data provide a mechanism whereby an intracellular bacterial infection can compromise local immunity to coinfecting pathogens or antigenic challenge.

Salmonella enterica serovar Typhimurium-induced placental inflammation and not bacterial burden correlates with pathology and fatal maternal disease

Food-borne infections caused by Salmonella enterica species are increasing globally, and pregnancy poses a high risk. Pregnant mice rapidly succumb to S. enterica serovar Typhimurium infection. To determine the mechanisms involved, we addressed the role of inflammation and bacterial burden in causing placental and systemic disease. In vitro, choriocarcinoma cells were a highly conducive niche for intracellular S. Typhimurium proliferation. While infection of mice with S. Typhimurium wild-type (WT) and mutant (Delta aroA and Delta invA) strains led to profound pathogen proliferation and massive burden within placental cells, only the virulent WT S. Typhimurium infection evoked total fetal loss and adverse host outcome. This correlated with substantial placental expression of granulocyte colony-stimulating factor (G-CSF), interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-alpha) and increased serum inflammatory cytokines/chemokines, such as G-CSF, IL-6, CCL1, and KC, evoked by WT S. Typhimurium infection. In contrast, infection with high doses of S. Typhimurium Delta aroA, despite causing massive placental infection, resulted in reduced inflammatory cellular and cytokine response. While S. Typhimurium WT bacteria were dispersed in large numbers across all regions of the placenta, including the deeper labyrinth trophoblast, S. Typhimurium Delta aroA bacteria localized primarily to the decidua. This correlated with the widespread placental necrosis accompanied by neutrophil infiltration evoked by the S. Typhimurium WT bacteria. Thus, the ability of Salmonella to localize to deeper layers of the placenta and the nature of inflammation triggered by the pathogen, rather than bacterial burden, profoundly influenced placental integrity and host survival.

Comparative analysis of Salmonella enterica serovar Enteritidis mutants with a vaccine potential

If any new live Salmonella vaccine is introduced in the future, it is quite probable that detailed characterisation of its attenuation will be required. In this study we therefore compared 34 isogenic mutants of S. Enteritidis in aroA, aroD, galE, ssrA, sseA, phoP, rpoS, ompR, htrA, clpP, lon, rfaL, rfaG, rfaC, hfq, sodCI, hilA, sipA, avrA, sopB, sopA, sopE, sifA, shdA, fliC, fur, relA, spoT, rel-spoT, misL, rmbA, STM4258, STM4259 and spvBC genes for their resistance to stresses likely to be expected in the host and for their virulence and immunogenicity in Balb/C mice. We found that the cold and bile resistances essentially did not correlate with the resistances to other stress factors. Resistance to acid pH, heat, polymyxin and serum correlated with each other and also with the attenuation. When the residual virulence and immunogenicity were both considered, mutants in htrA, ompR, aroA, aroD and lon performed the best in mice. Furthermore, when a detailed comparison of polymyxin and serum sensitive mutants was performed, the serum sensitive mutants were more immunogenic.

"Gently rough": the vaccine potential of a Salmonella enterica regulatory lipopolysaccharide mutant

BACKGROUND

An alternative to multivalent vaccines could be to construct strains capable of conferring broad protection through shared antigens. Down-regulation of immunodominant major antigens has been proposed to enhance the immunogenicity of conserved antigens.

METHODS

The protection provided by an aroA as well as structural and regulatory lipopolysaccharide (LPS) mutants of Salmonella enterica serovar Typhimurium against homologous and heterologous challenges was assessed in the murine model of typhoid. The reactivity and cross-reactivity of the immune sera raised was tested by enzyme-linked immunospot assay and immunoblots. Conserved outer membrane proteins were identified by mass spectrometry.

RESULTS

Unlike any structural LPS mutants, the regulatory mutant lacking RfaH was finely balanced between safety and immunogenicity, and its vaccine potential was comparable to that of the well-characterized DeltaaroA mutant. Loss of the transcriptional antiterminator RfaH resulted in a heterogeneous length of LPS chains, designated here as the "gently rough" phenotype. Our study also provides evidence that the rough phenotype enhances the immunogenicity of minor antigens, which may improve cross-protection against heterologous bacteria. A panel of conserved antigens shared by members of the Enterobacteriaceae family was identified as abundant porins and lipoprotein antigens.

CONCLUSIONS

Fine-tuned down-regulation of immunodominant epitopes can create live vaccine strains that are not only desirably attenuated but that also exhibit an improved cross-protective potential.

Evaluation of new generation Salmonella enterica serovar Typhimurium vaccines with regulated delayed attenuation to induce immune responses against PspA

Increasing the immunogenicity to delivered antigens by recombinant attenuated Salmonella vaccines (RASV) has been the subject of intensive study. With this goal in mind, we have designed and constructed a new generation of RASV that exhibit regulated delayed attenuation. These vaccine strains are phenotypically wild type at the time of immunization and become attenuated after colonization of host tissues. The vaccine strains are grown under conditions that allow expression of genes required for optimal invasion and colonization of host tissues. Once established in the host, these virulence genes are turned off, fully attenuating the vaccine strain. In this study, we compared 2 of our newly developed regulated delayed attenuation Salmonella enterica serovar Typhimurium strains chi9088 and chi9558 with the Deltacya Deltacrp Deltaasd strain chi8133, for their abilities to express and present a secreted form of the alpha-helical region of pneumococcal surface protein A (PspA) to the mouse immune system. All 3 strains induced high levels of serum antibodies specific for PspA as well as to Salmonella antigens in orally immunized mice. However, both RASVs expressing delayed attenuation elicited significantly greater anti-PspA immune responses, including serum IgG and T cell secretion of IL-4 and IFN-gamma, than other groups. Also, vaccination with delayed attenuation strains resulted in a greater degree of protection against Streptococcus pneumoniae challenge than in mice vaccinated with chi8133 (71-86% vs. 21% survival, P </= 0.006). Together, the results demonstrate that the regulated attenuation strategy results in highly immunogenic antigen delivery vectors for oral vaccination.

Pathogen proliferation governs the magnitude but compromises the function of CD8 T cells

CD8+ T cell memory is critical for protection against many intracellular pathogens. However, it is not clear how pathogen virulence influences the development and function of CD8+ T cells. Salmonella typhimurium (ST) is an intracellular bacterium that causes rapid fatality in susceptible mice and chronic infection in resistant strains. We have constructed recombinant mutants of ST, expressing the same immunodominant Ag OVA, but defective in various key virulence genes. We show that the magnitude of CD8+ T cell response correlates directly to the intracellular proliferation of ST. Wild-type ST displayed efficient intracellular proliferation and induced increased numbers of OVA-specific CD8+ T cells upon infection in mice. In contrast, mutants with defective Salmonella pathogenicity island II genes displayed poor intracellular proliferation and induced reduced numbers of OVA-specific CD8+ T cells. However, when functionality of the CD8+ T cell response was measured, mutants of ST induced a more functional response compared with the wild-type ST. Infection with wild-type ST, in contrast to mutants defective in pathogenicity island II genes, induced the generation of mainly effector-memory CD8+ T cells that expressed little IL-2, failed to mediate efficient cytotoxicity, and proliferated poorly in response to Ag challenge in vivo. Taken together, these results indicate that pathogens that proliferate rapidly and chronically in vivo may evoke functionally inferior memory CD8+ T cells which may promote the survival of the pathogen.

Attenuated Salmonella enterica serovar Typhimurium lacking the ZnuABC transporter confers immune-based protection against challenge infections in mice

Salmonella enterica has long been recognised as an important zoonotic pathogen of economic significance, both in animals and humans. We have recently shown that inactivation of the ZnuABC high affinity zinc transporter significantly affects the pathogenicity of S. enterica, likely due to zinc shortage in the eukaryotic tissues. Here, we demonstrate that a S. enterica serovar Typhimurium znuABC deleted strain is able to induce a short lasting infection in mice. On the same time, it primes a cell-mediated immune response, which confers a solid and durable immune-based protection against challenge infections with virulent strains of S. Typhimurium. These findings suggest the possibility to explore the use of S. enterica ZnuABC deleted mutants for the production on novel vaccines.

Avirulant Salmonella typhimurium strains prevent food allergy in mice

Oral tolerance to foods can be regulated by microorganisms in the gut lumen. We hypothesized that pretreatment with avirulent Salmonella typhimurium strains could prevent food allergy in mice. Mice were administered S. typhimurium PhoPc (STPhoPc) or S. typhimurium AroA prior to oral sensitization to beta-lactoglobulin in the presence of cholera toxin. An oral antigen challenge after sensitization assessed antigen-induced anaphylaxis. Antigen-specific antibody titres were measured by enzyme-linked immunosorbent assay in the serum and enzyme-linked immunospot (ELISPOT) in the spleen, and cytokine-secreting cells were measured by ELISPOT in the Peyer's patches, lamina propria and epithelium cells. We showed first that S. typhimurium could up-regulate interleukin (IL)-12 and IL-10 secretion by gut T cells. Mice pretreated with STPhoPc had decreased anaphylaxis upon challenge, along with decreased immumoglobulin G1 (IgG1) and IgE antibody titres. Mice having received S. typhimurium AroA had partly decreased anaphylaxis as well as decreased serum IgG1 antibody titres in the serum, and increased serum IgA antibody titres. Antibody titres could be correlated with increased numbers of spleen and Peyer's patches antibody-producing cells. STPhoPc-treated mice showed significantly decreased anaphylaxis when compared with the control mice, while S. typhimurium AroA-pretreated mice had a similar immune response together with increased secretory IgA titres. Our experiments have proved a potential immunomodulatory protective effect by two avirulent S. typhimurium strains.

Cytokine response of porcine cell lines to Salmonella enterica serovar typhimurium and its hilA and ssrA mutants

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a facultative intracellular bacterium which can infect and colonize pigs. After contact with enterocytes and macrophages, S. Typhimurium induces production of cytokines thus triggering the innate immune response. In this study we evaluated the cytokine response of two porcine cell lines, IPI-2I and 3D4/31, of epithelial or macrophage origins, respectively, to the wild-type S. Typhimurium and its hilA and ssrA mutants. We observed that the 3D4/31 cell line essentially did not respond to S. Typhimurium infection when a medium with foetal calf serum was used. However when the 3D4 cell line was incubated overnight in the presence of porcine serum, it efficiently responded to the wild-type strain and the ssrA mutant but not to the noninvasive hilA mutant as measured by mRNA quantification of TNF-alpha, IL-8 and GM-CSF by the real-time RT-PCR. In IPI-2I, all the cytokines were also induced by the wild-type S. Typhimurium and the ssrA mutant although the induction of TNF-alpha was lower than that induced by the wild-type strain. The hilA mutant was unable to induce any of the cytokines tested. The ssrA mutant can therefore be considered as more suitable for further vaccine development as the stimulation of innate immune response is important for animal protection against a challenge with virulent strains.

An oralSalmonellavaccine promotes the down-regulation of cell surface Toll-like receptor 4 (TLR4) and TLR2 expression in mice

A single oral immunization with the Lon-protease-deficient Salmonella enterica serovar Typhimurium (strain CS2022) induced protective immunity in mice against a subcutaneous challenge with virulent Listeria monocytogenes as well as virulent Salmonella serovar Typhimurium. The populations of cell surface Toll-like receptor 4 (TLR4) and TLR2 on peritoneal macrophages decreased at week 6 after immunization. This population decrease was not reversed after a challenge with either Salmonella or Listeria. These results suggest that oral immunization with CS2022 induced immune tolerance correlated with the down-regulation of cell surface TLR expression. This down-regulation may in part account for the development of cross-protection against a Listeria challenge by immunization with CS2022.

Use of a genetically defined double mutant strain of Bordetella bronchiseptica lacking adenylate cyclase and type III secretion as a live vaccine

While most vaccines consisting of killed bacteria induce high serum antibody titers, they do not always confer protection as effective as that induced by infection, particularly against mucosal pathogens. Bordetella bronchiseptica is a gram-negative respiratory pathogen that is endemic in many nonhuman mammalian populations and causes substantial disease in a variety of animals. At least 14 different live attenuated vaccines against this pathogen are available for use in a variety of livestock and companion animals. However, there are few published data on the makeup or efficacy of these vaccines. Here we report the use of a genetically engineered double mutant of B. bronchiseptica, which lacks adenylate cyclase and type III secretion, as a vaccine candidate. This strain is safe at high doses, even for highly immunocompromised animals, and induces immune responses that are protective against highly divergent B. bronchiseptica strains, preventing colonization in the lower respiratory tract and decreasing the bacterial burden in the upper respiratory tract. This novel B. bronchiseptica vaccine candidate induces strong local immunity while eliminating damage caused by the two predominant cytotoxic mechanisms.

Salmonella infections: immune and non-immune protection with vaccines

Salmonella enterica in poultry remains a major political issue. S. enterica serovar Enteritidis, particularly, remains a world-wide problem. Control in poultry by immunity, whether acquired or innate, is a possible means of containing the problem. Widespread usage of antibiotics has led to the emergence of multiple antibiotic-resistant bacteria. This problem has indicated an increasing requirement for effective vaccines to control this important zoonotic infection. An attempt is made in the present review to explain the relatively poor success in immunizing food animals against these non-host-specific Salmonella serotypes that usually produce food-poisoning, compared with the success obtained with the small number of serotypes that more typically produce systemic "typhoid-like" diseases. New examinations of old problems such as the carrier state and vertical transmission, observed with S. Pullorum, is generating new information of relevance to immunity. Newer methods of attenuation are being developed. Live vaccines, if administered orally, demonstrate non-specific and rapid protection against infection that is of biological and practical interest. However, from the point of view of consumer safety, there is a school of thought that considers inactivated or sub-unit vaccines to be the safest. The benefits of developing effective killed or sub-unit vaccines over the use of live vaccines are enormous. Recently, there have been significant advances in the development of adjuvants (e.g. microspheres) that are capable of potent immuno-stimulation, targeting different arms of the immune system. The exploitation of such technology in conjunction with the ongoing developments in identifying key Salmonella virulence determinants should form the next generation of Salmonella sub-unit vaccines for the control of this important group of pathogens. There are additional areas of concern associated with the use of live vaccines, particularly if these are generated by genetic manipulation.

Pronounced susceptibility to infection by Salmonella enterica serovar Typhimurium in mice chronically exposed to lead correlates with a shift to Th2-type immune responses

Persistent exposure to inorganic lead (Pb) is known to adversely affect the immune system. In the present study, we assessed the effect of chronic Pb exposure on susceptibility to infection by the facultative intracellular pathogen Salmonella enterica serovar Typhimurium. Mice were exposed to 10 mM Pb-acetate in drinking water for approximately 16 weeks, resulting in a significant level of Pb in the blood (106.2+/-8.9 microg/dl). Pb exposure rendered mice susceptible to Salmonella infection, manifested by increased bacterial burden in target organs and heightened mortality. Flow cytometric analysis of the splenic cellular composition in normal and Pb-exposed mice revealed no gross alteration in the ratios of B and T lymphocytes or myeloid cells. Similarly, the capacity of B and T cells to upregulate the expression of activation antigens in response to mitogenic or inflammatory stimuli was not hindered by Pb exposure. Analysis of the ability of ex vivo-cultured splenocytes to secrete cytokines demonstrated a marked reduction in IFN-gamma and IL-12p40 production associated with Pb exposure. In contrast, secretion of IL-4 by splenocytes of Pb-treated mice was 3- to 3.6-fold higher than in normal mice. The increased capacity to produce IL-4 correlated with a shift in the in vivo anti-Salmonella antibody response from the protective IgG2a isotype to the Th2-induced IgG1 isotype. We conclude that chronic exposure to high levels of Pb results in a state of immunodeficiency which is not due to an overt cytotoxic or immunosuppressive mechanism, but rather is largely caused by a shift in immune responsiveness to Th2-type reactions.

Delayed expansion and contraction of CD8+ T cell response during infection with virulent Salmonella typhimurium

Ag presentation to CD8(+) T cells often commences immediately after infection, which facilitates their rapid expansion and control of infection. Subsequently, the primed cells undergo rapid contraction. We report that this paradigm is not followed during infection with virulent Salmonella enterica, serovar Typhimurium (ST), an intracellular bacterium that replicates within phagosomes of infected cells. Although susceptible mice die rapidly (approximately 7 days), resistant mice (129 x 1SvJ) harbor a chronic infection lasting approximately 60-90 days. Using rOVA-expressing ST (ST-OVA), we show that T cell priming is considerably delayed in the resistant mice. CD8(+) T cells that are induced during ST-OVA infection undergo delayed expansion, which peaks around day 21, and is followed by protracted contraction. Initially, ST-OVA induces a small population of cycling central phenotype (CD62L(high)IL-7Ralpha(high)CD44(high)) CD8(+) T cells. However, by day 14-21, majority of the primed CD8(+) T cells display an effector phenotype (CD62L(low)IL-7Ralpha(low)CD44(high)). Subsequently, a progressive increase in the numbers of effector memory phenotype cells (CD62L(low)IL-7Ralpha(high)CD44(high)) occurs. This differentiation program remained unchanged after accelerated removal of the pathogen with antibiotics, as majority of the primed cells displayed an effector memory phenotype even at 6 mo postinfection. Despite the chronic infection, CD8(+) T cells induced by ST-OVA were functional as they exhibited killing ability and cytokine production. Importantly, even memory CD8(+) T cells failed to undergo rapid expansion in response to ST-OVA infection, suggesting a delay in T cell priming during infection with virulent ST-OVA. Thus, phagosomal lifestyle may allow escape from host CD8(+) T cell recognition, conferring a survival advantage to the pathogen.

Attenuation and persistence of and ability to induce protective immunity to a Staphylococcus aureus aroA mutant in mice

Staphylococcus aureus is the most important etiological agent of bovine mastitis, a disease that causes significant economic losses to the dairy industry. Several vaccines to prevent the disease have been tested, with limited success. The aim of this study was to obtain a suitable attenuated aro mutant of S. aureus by transposon mutagenesis and to demonstrate its efficacy as a live vaccine to induce protective immunity in a murine model of intramammary infection. To do this, we transformed S. aureus RN6390 with plasmid pTV1ts carrying Tn917. After screening of 3,493 erythromycin-resistant colonies, one mutant incapable of growing on plates lacking phenylalanine, tryptophan, and tyrosine was isolated and characterized. Molecular characterization of the mutant showed that the affected gene was aroA and that the insertion occurred 756 bp downstream of the aroA start codon. Complementation of the aroA mutant with a plasmid carrying aroA recovered the wild-type phenotype. The mutant exhibited a 50% lethal dose (1 x 10(6) CFU/mouse) higher than that of the parental strain (4.3 x 10(4) CFU/mouse). The aroA mutant showed decreased ability to persist in the lungs, spleens, and mammary glands of mice. Intramammary immunization with the aroA mutant stimulated both Th1 and Th2 responses in the mammary gland, as ascertained by reverse transcription-PCR, and induced significant protection from challenge with either the parental wild-type or a heterologous strain isolated from a cow with mastitis.

Evaluation of the Lon-deficient Salmonella strain as an oral vaccine candidate

We evaluated the efficacy of CS2022 (the Lon protease-deficient mutant strain of Salmonella enterica serovar Typhimurium) as a candidate live oral vaccine strain against subsequent oral challenge with a virulent strain administered to BALB/c and C57BL/6 mice. CS2022 persistently resided in the spleen, mesenteric lymph nodes, Peyer's patches, and cecum of both strains of mice after a single oral inoculation with 1 x 10(8) colony-forming units. Finally, CS2022 almost disappeared from each tissue sample by week 12 in BALB/c mice, whereas CS2022 still resided in each tissue type at week 12 after inoculation of C57BL/6 mice. A significant increase in the serovar Typhimurium lipopolysaccharide-specific secretory immunoglobulin A (s-IgA), as measured for one of the mucosal immune responses, was detected in bile and intestinal samples of both strains of immunized mice at week 4 after immunization. In addition, the expression of gamma interferon mRNA in the spleens of both strains of immunized mice, especially those of C57BL/6 mice, was significantly increased at week 4 after immunization and was boosted during the following 5 days after the challenge was administered to the mice. Furthermore, peritoneal macrophages isolated from immunized mice at week 4 after immunization exhibited an increase in intracellular killing activity against both virulent and avirulent Salmonella. The present results suggested that salmonellae-specific s-IgA on the mucosal surfaces induced by immunization with CS2022 generally prevented mice from succumbing to an oral challenge with a virulent strain. Simultaneously, CS2022 promoted the protective immunity associated with macrophages in both strains of mice.

Rational design of Salmonella-based vaccination strategies

A permanently growing body of information is becoming available about the quality of protective immune responses induced by mucosal immunization. Attenuated live bacterial vaccines can be administered orally and induce long-lasting protective immunity in humans without causing major side effects. An attenuated Salmonella enterica serovar Typhi strain is registered as live oral vaccine against typhoid fever and has been in use for more than two decades. Recombinant attenuated Salmonella strains are also an attractive means of delivering heterologous antigens to the immune system, thereby, stimulating strong mucosal and systemic immune responses and consequently provide an efficient platform technology to design novel vaccination strategies. This includes the choice of heterologous protective antigens and their expression under the control of appropriate promoters within the carrier strain. The availability of well-characterized attenuated mutants of Salmonella concomitantly supports fine tuning of immune response triggered against heterologous antigens. Exploring different mucosal sites as a potential route of immunization has to be taken into account as an additional important way to modulate immune responses according to clinical requirements. This article focuses on the rational design of strategies to modulate appropriate immunological effector functions on the basis of selection of (i) attenuating mutations of the Salmonella strains, (ii) specific expression systems for the heterologous antigens, and (iii) route of mucosal administration.

CD154 Is Essential for Protective Immunity in ExperimentalSalmonellaInfection: Evidence for a Dual Role in Innate and Adaptive Immune Responses

CD40-CD154 interactions are of central importance in the induction of humoral and cellular immune responses. In the present study, CD154-deficient (CD154-/-) mice were used to assess the role of CD40-CD154 interactions in regulating the immune response to a systemic Salmonella infection. Compared with C57BL/6 (CD154+/+) controls, CD154-/- mice were hypersusceptible to infection by an attenuated strain of Salmonella enterica serovar Typhimurium (S. typhimurium), as evidenced by decreased survival rate and mean time to death, which correlated with increased bacterial burden and persistence in target organs. CD154-/- mice exhibited a defect both in the production of IL-12, IFN-gamma, and NO during the acute phase of the disease and in the generation of Salmonella-specific Ab responses and Ig isotype switching. Furthermore, when CD154-/- animals were administered a sublethal dose of attenuated S. typhimurium and subsequently challenged with a virulent homologous strain, all mice succumbed to an overwhelming infection. Similar treatment of CD154+/+ mice consistently resulted in > or =90% protection. The lack of protective immunity in CD154-/- mice correlated with a decreased T cell recall response to Salmonella Ags. Significant protection against virulent challenge was conferred to presensitized CD154-/- mice by transfer of serum or T cells from immunized CD154+/+ mice. For best protection, however, a combination of immune serum and T cells was required. We conclude that intercellular communications via the CD40-CD154 pathway play a critical role in the induction of type 1 cytokine responses, memory T cell generation, Ab formation, and protection against primary as well as secondary Salmonella infections.

Immune response induced by Salmonella typhimurium defective in ppGpp synthesis

Systemic infection by Salmonella typhimurium requires coordinated expression of virulence genes found primarily in Salmonella Pathogenecity Islands (SPIs). We have previously reported that the intracellular signal that induces these virulence genes is a stringent signal molecule, ppGpp [Song et al. J Biol Chem 2003;279:34183]. In this study, we found that relA and spoT double mutant Salmonella (DeltappGpp strain), which is defective in ppGpp synthesis, was virtually avirulent in BALB/c mice. Subsequently, the live vaccine potential of the avirulent DeltappGpp Salmonella strain was determined. A single immunization with live DeltappGpp Salmonella efficiently protected mice from challenge with wild-type Salmonella at a dose 10(6)-fold above the LD50 30 days after immunization. Various assays revealed that immunization of mice with the DeltappGpp strain elicited both systemic and mucosal antibody responses, in addition to cell-mediated immunity.

Tumor Vaccination by Salmonella typhimurium After Transformation with a Eukaryotic Expression Vector in Mice

Transformed attenuated Salmonella typhimurium (ST) have been suggested as an efficient means of tumor vaccination. However, ST themselves might be immunosuppressive, and the question has arisen as to whether this impedes vaccination efficacy even if ST are transformed with a eukaryotic expression vector such that "tumor antigen" will be transcribed by the host. The question was evaluated using a mutant SL7207, where the yej operon, which interferes with MHC I-mediated presentation, had been inactivated (SL7207DeltayejE). Mice were vaccinated with SL7207 or SL7207DeltayejE transformed with a eukaryotic expression vector carrying the lacZ or the gp100 gene and later received lacZ-transfected RENCA or YC8 or gp100-expressing B16F1 tumor cells. In vaccinated mice, tumor growth started with a delay and some animals remained tumor-free; however, the tumor growth rate remained unaltered. No significant difference was seen between SL7207DeltayejE versus SL7207 vaccinated mice. The latter finding contrasted with ex vivo analyses where vaccination with SL7207DeltayejE, compared with SL7207, induced a significantly stronger response, including nonadaptive defense mechanisms. The failure to detect a superior vaccination efficacy of SL7207DeltayejE in vivo could be attributed to a stronger effect of the yej operon on MHC-mediated antigen presentation when driven by a prokaryotic promoter. Also, additional Salmonella genes apparently interfere with maintenance of a sustained immune response. Thus, the immunosuppressive yej operon affects innate and adaptive immunity. However, when ST are carriers for eukaryotic-expressed tumor antigens, yej does not severely hamper induction of an immune response.

Protection of gnotobiotic pigs against Salmonella enterica serotype Typhimurium by rough mutant of the same serotype is accompanied by the change of local and systemic cytokine response

We have demonstrated that severe systemic disease caused by virulent LT2 strain Salmonella enterica serotype Typhimurium in gnotobiotic piglets can be alleviated by oral inoculation with an avirulent rough (R) mutant of the same serotype 24 h before challenge with the virulent strain. Protected piglets had no signs of enteritis. The concentrations of TNF-alpha, IL-1beta, IL-8 and IL-10 were measured by ELISA in ileal washings and plasma of uninfected and infected pigs. The cytokines were not detected in plasma of germ-free piglets, and low concentrations of IL-1beta and IL-8 were found in their ileal washings. The pre-inoculation of the rough mutant induced an increase in IL-8 and decrease in IL-1beta and IL-10 in plasma. The virulent LT2 strain induced very high TNF-alpha concentrations in the ileum which were reduced in the pigs pre-inoculated with the R mutant.

IntracellularSalmonellaInhibit Antigen Presentation by Dendritic Cells

Dendritic cells (DC) are important APCs linking innate and adaptive immunity. During analysis of the intracellular activities of Salmonella enterica in DC, we observed that viable bacteria suppress Ag-dependent T cell proliferation. This effect was dependent on the induction of inducible NO synthase by DC and on the function of virulence genes in Salmonella pathogenicity island 2 (SPI2). Intracellular activities of Salmonella did not affect the viability, Ag uptake, or maturation of DC, but resulted in reduced presentation of antigenic peptides by MHC class II molecules. Increased resistance to reinfection was observed after vaccination of mice with SPI2-deficient Salmonella compared with mice vaccinated with SPI2-proficient Salmonella, and this correlated with an increased amount of CD4(+) as well as CD8(+) T cells. Our study is the first example of interference of an intracellular bacterial pathogen with Ag presentation by DC. The subversion of DC functions is a novel strategy deployed by this pathogen to escape immune defense, colonize host organs, and persist in the infected host.

Salmonella typhimurium strains carrying independent mutations display similar virulence phenotypes yet are controlled by distinct host defense mechanisms

The outcome of Salmonella infection in the mammalian host favors whoever succeeds best in disturbing the equilibrium between coordinate expression of bacterial (virulence) genes and host defense mechanisms. Intracellular persistence in host cells is critical for pathogenesis and disease, because Salmonella typhimurium strains defective in this property are avirulent. We examined whether similar host defense mechanisms are required for growth control of two S. typhimurium mutant strains. Salmonella pathogenicity island 2 (SPI2) and virulence plasmid-cured Salmonella mutants display similar virulence phenotypes in immunocompetent mice, yet their gene loci participate in independent virulence strategies. We determined the role of TNF-alpha and IFN-gamma as well as different T cell populations in infection with these Salmonella strains. After systemic infection, IFN-gamma was essential for growth restriction of plasmid-cured S. typhimurium, while SPI2 mutant infections were controlled in the absence of IFN-gamma. TNFRp55-deficiency restored systemic virulence to both Salmonella mutants. After oral inoculation, control of plasmid-cured bacteria substantially relied on both IFN-gamma and TNF-alpha signaling while control of SPI2 mutants did not. However, for both mutants, ultimate clearance of bacteria from infected mice depended on alphabeta T cells.

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

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

A single dose of recombinant Salmonella typhimurium induces specific humoral immune responses against heterologous Eimeria tenella antigens in chicken

Salmonella typhimurium vaccine strains were used as antigen delivery system for oral immunisation of chickens against two antigens of the coccidian parasite Eimeria tenella. The cDNAs of the known E. tenella proteins, SO7 and TA4, were isolated from total RNA and subcloned into the expression vectors pQE30 and pTECH2. Subcutaneous immunisation of chickens with Escherichia coli-expressed SO7 and TA4 revealed that both proteins were immunogenic. Both cDNAs were subcloned into plasmids of the pTECH2 vector system, which allows them to be expressed as fusion proteins with the highly immunogenic fragment C of the tetanus toxin under control of the anaerobically inducible nirB promoter. Plasmids were introduced into the S. typhimurium vaccine strains SL3261, C5aroD and C5htrA. SDS-PAGE and Western blot analysis revealed expression of both fusion proteins in all strains under anaerobic culture conditions. Three-week-old white leghorn chickens were orally immunised with 10(9) CFU per animal. The stability of the recombinant bacteria was revealed by recovery of viable Salmonella containing the respective plasmids from the liver of the immunised chickens at day 3 after inoculation. Specific serum IgG antibodies against the SO7-or TA4-antigens were detectable by ELISA 2 weeks after oral immunisation and remained for at least 6 weeks, while specific IgA antibodies were restricted to the bile of the birds. All chickens produced serum IgG and IgA to S. typhimurium lipopolysaccharides. Our data show that a single oral inoculation with recombinant S. typhimurium SL3261, C5aroD and C5htrA can induce specific antibody responses to heterologous Eimeria antigens in chickens, suggesting that recombinant Salmonella are a suitable delivery system for vaccines against Eimeria infections.

Introduction: microbiology and immunology: lessons learned from Salmonella

Salmonella enterica, a Gram-negative bacterium, causes significant morbidity and mortality worldwide, and is an excellent model to study bacterial pathogenesis and cellular immune responses. With the development of powerful new technologies, there has been a fusion of research on immunology, molecular biology and cellular microbiology of S. enterica infections. This multidisciplinary research will enhance our understanding of the basic mechanisms of bacterial infections and immunity; it also provides new approaches towards therapeutic and control measures.