Cutting edge: role of B lymphocytes in protective immunity against Salmonella typhimurium infection

B cell activating factor (BAFF) from neutrophils and dendritic cells is required for protective B cell responses against Salmonella typhimurium infection

Mice lacking B cells are more susceptible to S. typhimurium infection. How B cells contribute to protective immunity against Salmonella and what signals drive their activation are still unclear. Neutrophils (Nphs), monocytes (MOs), and dendritic cells (DCs) are involved in early immune responses to control the initial replication of S. typhimurium. These cells can produce B cell activating factor (BAFF) required for mature B cell survival and may help regulate B cell responses during Salmonella infection. Using BAFF reporter mice (BAFF-RFP+/-), we discovered that an i.p. infection with a virulent strain of S. typhimurium increased BAFF expression in splenic conventional DCs (cDC) and inflammatory Ly6Chi MOs/DCs four days post-infection. S. typhimurium infection induced the release of BAFF from Nphs, a decrease of BAFF-RFP expression and expansion of BAFF-RFP+ Nphs in the spleen and peritoneal cavity. After S. typhimurium infection, serum BAFF levels and immature and mature B cell subsets and plasma cells increased substantially. Conditional knockout (cKO) mice lacking BAFF in either Nphs or cDCs compared to control Bafffl/fl mice had reduced up-regulation of systemic BAFF levels and reduced expansion of mature and germinal center B cell subsets after infection. Importantly, the cKO mice lacking BAFF from either Nphs or cDCs had impaired induction of Salmonella-specific IgM Abs, and were more susceptible to S. typhimurium infection. Thus, Nphs and cDCs are major cellular sources of BAFF driving B cell responses, required for mounting optimal protective immunity against lethal Salmonella infection.

B Cells Control Mucosal-Associated Invariant T Cell Responses to Salmonella enterica Serovar Typhi Infection Through the CD85j HLA-G Receptor

Mucosal-associated invariant T (MAIT) cells are an innate-like population of T cells that display a TCR Vα7.2+ CD161+ phenotype and are restricted by the nonclassical MHC-related molecule 1 (MR1). Although B cells control MAIT cell development and function, little is known about the mechanisms underlying their interaction(s). Here, we report, for the first time, that during Salmonella enterica serovar Typhi (S. Typhi) infection, HLA-G expression on B cells downregulates IFN-γ production by MAIT cells. In contrast, blocking HLA-G expression on S. Typhi-infected B cells increases IFN-γ production by MAIT cells. After interacting with MAIT cells, kinetic studies show that B cells upregulate HLA-G expression and downregulate the inhibitory HLA-G receptor CD85j on MAIT cells resulting in their loss. These results provide a new role for HLA-G as a negative feedback loop by which B cells control MAIT cell responses to antigens.

T-Cell Cytokine Response in Salmonella Typhimurium-Vaccinated versus Infected Pigs

Vaccination with the live attenuated vaccine Salmoporc is an effective measure to control Salmonella Typhimurium (STM) in affected swine populations. However, the cellular immune response evoked by the Salmoporc vaccine including differences in vaccinated pigs versus non-vaccinated pigs upon STM infection have not been characterized yet. To investigate this, tissue-derived porcine lymphocytes from different treatment groups (vaccination-only, vaccination and infection, infection-only, untreated controls) were stimulated in vitro with heat-inactivated STM and abundances of IFN-γ, TNF-α and/or IL-17A-producing T-cell subsets were compared across organs and treatment groups. Overall, our results show the induction of a strong CD4⁺ T-cell response after STM infection, both locally and systemically. Low-level induction of STM-specific cytokine-producing CD4⁺ T cells, notably for the IFN-γ/TNF-α co-producing phenotype, was detected after vaccination-only. Numerous significant contrasts in cytokine-producing T-cell phenotypes were observed after infection in vaccinated and infected versus infected-only animals. These results suggest that vaccine-induced STM-specific cytokine-producing CD4⁺ T cells contribute to local immunity in the gut and may limit the spread of STM to lymph nodes and systemic organs. Hence, our study provides insights into the underlying immune mechanisms that account for the efficacy of the Salmoporc vaccine.

Antibodies and Protection in Systemic Salmonella Infections: Do We Still Have More Questions than Answers?

Salmonella causes grave systemic infections in humans and other animals and provides a paradigm for other diseases in which the bacteria have both intracellular and extracellular lifestyles. New generations of vaccines rely on the essential contribution of the antibody responses for their protection. The quality, antigen specificity, and functions associated with antibody responses to this pathogen have been elusive for a long time. Recent approaches that combine studies in humans and genetically manipulated experimental models and that exploit awareness of the location and within-host life cycle of the pathogen are shedding light on how humoral immunity to Salmonella operates. However, this area of research remains full of controversy and discrepancies. The overall scenario indicates that antibodies are essential for resistance against systemic Salmonella infections and can express the highest protective function when operating in conjunction with cell-mediated immunity. Antigen specificity, isotype profile, Fc-gamma receptor usage, and complement activation are all intertwined factors that still arcanely influence antibody-mediated protection to Salmonella.

Salmonella Virulence and Immune Escape

Salmonella genus represents the most common foodborne pathogens causing morbidity, mortality, and burden of disease in all regions of the world. The introduction of antimicrobial agents and Salmonella-specific phages has been considered as an effective intervention strategy to reduce Salmonella contamination. However, data from the United States, European countries, and low- and middle-income countries indicate that Salmonella cases are still a commonly encountered cause of bacterial foodborne diseases globally. The control programs have not been successful and even led to the emergence of some multidrug-resistant Salmonella strains. It is known that the host immune system is able to effectively prevent microbial invasion and eliminate microorganisms. However, Salmonella has evolved mechanisms of resisting host physical barriers and inhibiting subsequent activation of immune response through their virulence factors. There has been a high interest in understanding how Salmonella interacts with the host. Therefore, in the present review, we characterize the functions of Salmonella virulence genes and particularly focus on the mechanisms of immune escape in light of evidence from the emerging mainstream literature.

Bacteria That Cause Enteric Diseases Stimulate Distinct Humoral Immune Responses

Bacterial enteric pathogens individually and collectively represent a serious global health burden. Humoral immune responses following natural or experimentally-induced infections are broadly appreciated to contribute to pathogen clearance and prevention of disease recurrence. Herein, we have compared observations on humoral immune mechanisms following infection with Citrobacter rodentium, the model for enteropathogenic Escherichia coli, Vibrio cholerae, Shigella species, Salmonella enterica species, and Clostridioides difficile. A comparison of what is known about the humoral immune responses to these pathogens reveals considerable variance in specific features of humoral immunity including establishment of high affinity, IgG class-switched memory B cell and long-lived plasma cell compartments. This article suggests that such variance could be contributory to persistent and recurrent disease.

Diet-induced obese mice exhibit altered immune responses to early Salmonella Typhimurium oral infection

Obesity is a chronic disease associated with different metabolic diseases as well as alterations in immune cell function. It is characterized by a chronic systemic low grade inflammation. There are several studies demonstrating the influence of obesity on the impaired immune response to infection. However, it is not completely clear whether the obese environment influences the development or maintenance of the immune response against infections. The aim of this study was to determine how obesity induced by a high-fat diet affects the immune response to an early oral Salmonella infection. Four groups of mice were kept in separate cages. Two of these designated as controls, fed with a normal diet; whereas other two groups were fed with a high fat diet for 10 weeks. Some mice were used for Salmonella oral infection. After 7 days of oral infection with S. Thypimurium the proportions of spleen cell subsets expressing activation markers in normal diet and HFD obese mice were stained with monoclonal antibodies and analyzed by flow cytometry. Also, mRNA levels of different cytokines were quantified by RT-PCR. It was found that obesity affects the function of the immune system against an early oral Salmonella infection, decreasing NK cells, altering the expression of activation molecules as well as cytokines mRNA levels. Interestingly, the expression some activation molecules on T lymphocytes was reestablished after Salmonella infection, but not the CD25 expression. Immune alterations could lead to immunosuppression or increased susceptibility to infections in HFD obese mice.

Enhanced survival following oral and systemic Salmonella enterica serovar Typhimurium infection in polymeric immunoglobulin receptor knockout mice

Background

Polymeric immunoglobulin receptor (pIgR) transport of secretory immunoglobulin A (SIgA) to mucosal surfaces is thought to promote gut integrity and immunity to Salmonella enterica serovar Typhimurium (S. Typhimurium), an invasive pathogen in mice. To elucidate potential mechanisms, we assessed intestinal barrier function and both oral and systemic S. Typhimurium virulence in pIgR knockout (KO) and wildtype (WT) mice.

Methods

In uninfected animals, we harvested jejunal segments for Ussing chamber analyses of transepithelial resistance (TER); mesenteric lymph nodes (mLN) for bacterial culture; and serum and stool for IgA. Separately, we infected mice either orally or intravenously (IV) with S. Typhimurium to compare colonization, tissue dynamics, and inflammation between KOs and WTs.

Results

Uninfected KOs displayed decreased TER and dramatically increased serum IgA and decreased fecal IgA vs. WT; however, KO mLNs yielded fewer bacterial counts. Remarkably, WTs challenged orally with S. Typhimurium exhibited increased splenomegaly, tissue colonization, and pro-inflammatory cytokines vs. pIgR KOs, which showed increased survival following either oral or IV infection.

Conclusions

Absence of pIgR compromises gut integrity but does not exacerbate bacterial translocation nor S. Typhimurium infection. These findings raise the possibility that immune adaptation to increased gut permeability and elevated serum IgA in the setting of SIgA deficiency provides compensatory protection against invasive gut pathogens.

Salmonella infection: Interplay between the bacteria and host immune system

Salmonella infection causes morbidity and mortality throughout the world with the host immune response varying depending on whether the infection is acute and limited, or systemic and chronic. Additionally, Salmonella bacteria have evolved multiple mechanisms to avoid or subvert immunity to its own benefit and often the anatomical location of infection plays a role in both the immune response and bacterial fate. Here, we provide an overview of the interplay between the immune system and Salmonella, while discussing how different host and bacterial factors influence the outcome of infection.

Memory B Cells of Mice and Humans

We comprehensively review memory B cells (MBCs), covering the definition of MBCs and their identities and subsets, how MBCs are generated, where they are localized, how they are maintained, and how they are reactivated. Whereas naive B cells adopt multiple fates upon stimulation, MBCs are more restricted in their responses. Evolving work reveals that the MBC compartment in mice and humans consists of distinct subpopulations with differing effector functions. We discuss the various approaches to define subsets and subset-specific roles. A major theme is the need to both deliver faster effector function upon reexposure and readapt to antigenically variant pathogens while avoiding burnout, which would be the result if all MBCs generated only terminal effector function. We discuss cell-intrinsic differences in gene expression and signaling that underlie differences in function between MBCs and naive B cells and among MBC subsets and how this leads to memory responses.

Oral Challenge with Wild-Type Salmonella Typhi Induces Distinct Changes in B Cell Subsets in Individuals Who Develop Typhoid Disease

A novel human oral challenge model with wild-type Salmonella Typhi (S. Typhi) was recently established by the Oxford Vaccine Group. In this model, 104 CFU of Salmonella resulted in 65% of participants developing typhoid fever (referred here as typhoid diagnosis -TD-) 6-9 days post-challenge. TD was diagnosed in participants meeting clinical (oral temperature ≥38°C for ≥12h) and/or microbiological (S. Typhi bacteremia) endpoints. Changes in B cell subpopulations following S. Typhi challenge remain undefined. To address this issue, a subset of volunteers (6 TD and 4 who did not develop TD -NoTD-) was evaluated. Notable changes included reduction in the frequency of B cells (cells/ml) of TD volunteers during disease days and increase in plasmablasts (PB) during the recovery phase (>day 14). Additionally, a portion of PB of TD volunteers showed a significant increase in activation (CD40, CD21) and gut homing (integrin α4β7) molecules. Furthermore, all BM subsets of TD volunteers showed changes induced by S. Typhi infections such as a decrease in CD21 in switched memory (Sm) CD27+ and Sm CD27- cells as well as upregulation of CD40 in unswitched memory (Um) and Naïve cells. Furthermore, changes in the signaling profile of some BM subsets were identified after S. Typhi-LPS stimulation around time of disease. Notably, naïve cells of TD (compared to NoTD) volunteers showed a higher percentage of cells phosphorylating Akt suggesting enhanced survival of these cells. Interestingly, most these changes were temporally associated with disease onset. This is the first study to describe differences in B cell subsets directly related to clinical outcome following oral challenge with wild-type S. Typhi in humans.

Specific Monoclonal Antibody Overcomes the Salmonella enterica Serovar Typhimurium's Adaptive Mechanisms of Intramacrophage Survival and Replication

Salmonella-specific antibodies play an important role in host immunity; however, the mechanisms of Salmonella clearance by pathogen-specific antibodies remain to be completely elucidated since previous studies on antibody-mediated protection have yielded inconsistent results. These inconsistencies are at least partially attributable to the use of polyclonal antibodies against Salmonella antigens. Here, we developed a new monoclonal antibody (mAb)-449 and identified its related immunogen that protected BALB/c mice from infection with Salmonella enterica serovar Typhimurium. In addition, these data indicate that the mAb-449 immunogen is likely a major protective antigen. Using in vitro infection studies, we also analyzed the mechanism by which mAb-449 conferred host protection. Notably, macrophages infected with mAb-449-treated S. Typhimurium showed enhanced pathogen uptake compared to counterparts infected with control IgG-treated bacteria. Moreover, these macrophages produced elevated levels of pro-inflammatory cytokine TNFα and nitric oxide, indicating that mAb-449 enhanced macrophage activation. Finally, the number of intracellular bacteria in mAb-449-activated macrophages decreased considerably, while the opposite was found in IgG-treated controls. Based on these findings, we suggest that, although S. Typhimurium has the potential to survive and replicate within macrophages, host production of a specific antibody can effectively mediate macrophage activation for clearance of intracellular bacteria.

Vaccination Method Affects Immune Response and Bacterial Growth but Not Protection in the Salmonella Typhimurium Animal Model of Typhoid

Understanding immune responses elicited by vaccines, together with immune responses required for protection, is fundamental to designing effective vaccines and immunisation programs. This study examines the effects of the route of administration of a live attenuated vaccine on its interactions with, and stimulation of, the murine immune system as well as its ability to increase survival and provide protection from colonisation by a virulent challenge strain. We assess the effect of administration method using the murine model for typhoid, where animals are infected with S. Typhimurium. Mice were vaccinated either intravenously or orally with the same live attenuated S. Typhimurium strain and data were collected on vaccine strain growth, shedding and stimulation of antibodies and cytokines. Following vaccination, mice were challenged with a virulent strain of S. Typhimurium and the protection conferred by the different vaccination routes was measured in terms of challenge suppression and animal survival. The main difference in immune stimulation found in this study was the development of a secretory IgA response in orally-vaccinated mice, which was absent in IV vaccinated mice. While both strains showed similar protection in terms of challenge suppression in systemic organs (spleen and liver) as well as survival, they differed in terms of challenge suppression of virulent pathogens in gut-associated organs. This difference in gut colonisation presents important questions around the ability of vaccines to prevent shedding and transmission. These findings demonstrate that while protection conferred by two vaccines can appear to be the same, the mechanisms controlling the protection can differ and have important implications for infection dynamics within a population.

Adaptive Immune Responses during Salmonella Infection

The interaction betweenSalmonella and its host is complex and dynamic: the host mounts an immune defense against the pathogen, which in turn acts to reduce, evade, or exploit these responses to successfully colonize the host. Although the exact mechanisms mediating protective immunity are poorly understood, it is known that T cells are a critical component of immunity to Salmonella infection, and a robust T-cell response is required for both clearance of primary infection and resistance to subsequent challenge. B-cell functions, including but not limited to antibody production, are also required for generation of protective immunity. Additionally, interactions among host cells are essential. For example, antigen-presenting cells (including B cells) express cytokines that participate in CD4+ T cell activation and differentiation. Differentiated CD4+ T cells secrete cytokines that have both autocrine and paracrine functions, including recruitment and activation of phagocytes, and stimulation of B cell isotype class switching and affinity maturation. Multiple bacterium-directed mechanisms, including altered antigen expression and bioavailability and interference with antigen-presenting cell activation and function, combine to modify Salmonella's "pathogenic signature" in order to minimize its susceptibility to host immune surveillance. Therefore, a more complete understanding of adaptive immune responses may provide insights into pathogenic bacterial functions. Continued identification of adaptive immune targets will guide rational vaccine development, provide insights into host functions required to resist Salmonella infection, and correspondingly provide valuable reagents for defining the critical pathogenic capabilities of Salmonella that contribute to their success in causing acute and chronic infections.

Salmonella induces PD-L1 expression in B cells

Salmonella persists for a long time in B cells; however, the mechanism(s) through which infected B cells avoid effector CD8 T cell responses has not been characterized. In this study, we show that Salmonella infects and survives within all B1 and B2 cell subpopulations. B cells are infected with a Salmonella typhimurium strain expressing an ovalbumin (OVA) peptide (SIINFEKL) to evaluate whether B cells process and present Salmonella antigens in the context of MHC-I molecules. Our data showed that OVA peptides are presented by MHC class I K(b)-restricted molecules and the presented antigen is generated through proteasomal degradation and vacuolar processing. In addition, Salmonella-infected B cells express co-stimulatory molecules such as CD40, CD80, and CD86 as well as inhibitory molecules such as PD-L1. Thus, the cross-presentation of Salmonella antigens and the expression of activation molecules suggest that infected B cells are able to prime and activate specific CD8(+) T cells. However, the Salmonella infection-stimulated expression of PD-L1 suggests that the PD-1/PD-L1 pathway may be involved in turning off the cytotoxic effector response during Salmonella persistent infection, thereby allowing B cells to become a reservoir for the bacteria.

Protective host immune responses to Salmonella infection

Salmonella enterica serovars Typhi and Paratyphi are the causative agents of human typhoid fever. Current typhoid vaccines are ineffective and are not widely used in endemic areas. Greater understanding of host-pathogen interactions during Salmonella infection should facilitate the development of improved vaccines to combat typhoid and nontyphoidal Salmonellosis. This review will focus on our current understanding of Salmonella pathogenesis and the major host immune components that participate in immunity to Salmonella infection. In addition, recent findings regarding host immune mechanisms in response to Salmonella infection will be also discussed, providing a new perspective on the utility of improved tools to study the immune response to Salmonella infections.

Mucosal immunization of BALB/c mice with DNA vaccines encoding the SEN1002 and SEN1395 open reading frames of Salmonella enterica serovar Enteritidis induces protective immunity

Salmonella Enteritidis is the main cause of foodborne salmonellosis worldwide. The limited effectiveness of current interventions against this pathogen has been the main incentive to develop new methods for the efficient control of this infection. To investigate the use of DNA vaccines against S. Enteritidis in humans, immune responses stimulated by two plasmids containing the genes designated SEN1002, located in the pathogenicity island SPI-19 and encoding a Hcp protein involved in transport mechanisms, and SEN1395, located in the genomic island ΦSE14 and encoding a protein of a new superfamily of lysozymes, were evaluated. Humoral and cellular responses following intranasal immunization of two groups of BALB/c mice with the plasmids pV1002 and pV1395 plus adjuvant were evaluated and it was observed that the IgG2a/IgG1 ratios were sixfold higher than control groups. Both plasmids stimulated specific secretory IgA production. Increased proliferation of lymphocytes and IFN-γ production were detected in both experimental groups. DNA-vaccinated mice developed protective immunity against a virulent strain of S. Enteritidis, with nearly 2 logs of protection level compared to the negative control values in the spleen. Therefore, DNA vaccines are efficient at stimulating cellular and humoral immune responses at systemic and mucosal levels.

Infection with Salmonella enterica Serovar Typhimurium Leads to Increased Proportions of F4/80+ Red Pulp Macrophages and Decreased Proportions of B and T Lymphocytes in the Spleen

Infection of mice with Salmonella enterica serovar Typhimurium (Salmonella) causes systemic inflammatory disease and enlargement of the spleen (splenomegaly). Splenomegaly has been attributed to a general increase in the numbers of phagocytes, lymphocytes, as well as to the expansion of immature CD71+Ter119+ reticulocytes. The spleen is important for recycling senescent red blood cells (RBCs) and for the capture and eradication of blood-borne pathogens. Conservation of splenic tissue architecture, comprised of the white pulp (WP), marginal zone (MZ), and red pulp (RP) is essential for initiation of adaptive immune responses to captured pathogens. Using flow cytometry and four color immunofluorescence microscopy (IFM), we show that Salmonella-induced splenomegaly is characterized by drastic alterations of the splenic tissue architecture and cell population proportions, as well as in situ cell distributions. A major cause of splenomegaly appears to be the significant increase in immature RBC precursors and F4/80+ macrophages that are important for recycling of heme-associated iron. In contrast, the proportions of B220+, CD4+ and CD8+ lymphocytes, as well as MZ MOMA+ macrophages decrease significantly as infection progresses. Spleen tissue sections show visible tears and significantly altered tissue architecture with F4/80+ macrophages and RBCs expanding beyond the RP and taking over most of the spleen tissue. Additionally, F4/80+ macrophages actively phagocytose not only RBCs, but also lymphocytes, indicating that they may contribute to declining lymphocyte proportions during Salmonella infection. Understanding how these alterations of spleen microarchitecture impact the generation of adaptive immune responses to Salmonella has implications for understanding Salmonella pathogenesis and for the design of more effective Salmonella-based vaccines.

Suppressive functions of B cells in infectious diseases

B lymphocytes are often essential to successfully control invading pathogens and play a primary role in the protection afforded by successful vaccines through the production of specific antibodies. However, recent studies have highlighted the complex roles of B cells in infectious diseases, showing unexpectedly that some activated B cells limited host defense towards pathogens. This B-cell function involves production of regulatory cytokines including IL-10 and IL-35 and is reminiscent of the regulatory functions of B cells initially defined in autoimmune diseases. It is now known that various types of microbes including bacteria, helminths and viruses can induce IL-10-expressing B cells with inhibitory functions, indicating that this response is a general component of anti-microbial immunity. Interestingly, IL-10-producing B cells induced in the course of some microbial infections can inhibit concurrent immune responses directed towards unrelated antigens in a bystander manner and as a consequence ameliorate the course of autoimmune or allergic diseases. This could explain how some micro-organisms might provide protection from these pathologies, as formulated in the 'hygiene hypothesis'. In this review, we discuss the regulatory functions of B cells in bacterial, parasitic and viral infections, taking into account the phenotype of the B cells implicated, the signals controlling their induction and the cell types targeted by their suppressive activities.

Haloarchaeal gas vesicle nanoparticles displaying Salmonella antigens as a novel approach to vaccine development

A safe, effective, and inexpensive vaccine against typhoid and other Salmonella diseases is urgently needed. In order to address this need, we are developing a novel vaccine platform employing buoyant, self-adjuvanting gas vesicle nanoparticles (GVNPs) from the halophilic archaeon Halobacterium sp. NRC-1, bioengineered to display highly conserved Salmonella enterica antigens. As the initial antigen for testing, we selected SopB, a secreted inosine phosphate effector protein injected by pathogenic S. enterica bacteria during infection into the host cells. Two highly conserved sopB gene segments near the 3'-region, named sopB4 and sopB5, were each fused to the gvpC gene, and resulting SopB-GVNPs were purified by centrifugally accelerated flotation. Display of SopB4 and SopB5 antigenic epitopes on GVNPs was established by Western blotting analysis using antisera raised against short synthetic peptides of SopB. Immunostimulatory activities of the SopB4 and B5 nanoparticles were tested by intraperitoneal administration of SopB-GVNPs to BALB/c mice which had been immunized with S. enterica serovar Typhimurium 14028 ΔpmrG-HM-D (DV-STM-07), a live attenuated vaccine strain. Proinflammatory cytokines IFN-γ, IL-2, and IL-9 were significantly induced in mice boosted with SopB5-GVNPs, consistent with a robust Th1 response. After challenge with virulent S. enterica serovar Typhimurium 14028, bacterial burden was found to be diminished in spleen of mice boosted with SopB4-GVNPs and absent or significantly diminished in liver, mesenteric lymph node, and spleen of mice boosted with SopB5-GVNPs, indicating that the C-terminal portions of SopB displayed on GVNPs elicit a protective response to Salmonella infection in mice. SopB antigen-GVNPs were also found to be stable at elevated temperatures for extended periods without refrigeration. The results show that bioengineered GVNPs are likely to represent a valuable platform for antigen delivery and development of improved vaccines against Salmonella and other diseases.

rIL-22 as an adjuvant enhances the immunogenicity of rGroEL in mice and its protective efficacy against S. Typhi and S. Typhimurium

Salmonella infection, ranging from mild, self-limiting diarrhea to severe gastrointestinal, septicemic disease and enteric fever, is a global health problem both in humans and animals. Rapid development of microbial drug resistance has led to a need for efficacious and affordable vaccines against Salmonella. Microbial heat shock proteins (HSPs), including HSP60 and HSP70, are the dominant antigens that promote the host immune response. Co-administration of these antigens with cytokines, such as IL-22, which plays an important role in antimicrobial defense, can enhance the immune response and protection against pathogens. Therefore, the aim of the present study was to determine the immunogenicity of rGroEL (Hsp60) of S. Typhi, alone or administered in combination with murine rIL-22, and its protective efficacy against lethal infection with Salmonella, in mice. There was appreciable stimulation of the humoral and cell-mediated immune responses in mice immunized with rGroEL alone. However, co-administration of rGroEL with rIL-22 further boosted the antibody titers (IgG, IgG1 and IgG2a), T-cell proliferative responses and the secretion of both Th1 and Th2 cytokines. Additionally, rGroEL alone accorded 65%-70% protection against lethal challenge with S. Typhi and S. Typhimurium, which increased to 90% when co-administered with rIL-22.

Natural and vaccine-mediated immunity to Salmonella Typhimurium is impaired by the helminth Nippostrongylus brasiliensis

Background

The impact of exposure to multiple pathogens concurrently or consecutively on immune function is unclear. Here, immune responses induced by combinations of the bacterium Salmonella Typhimurium (STm) and the helminth Nippostrongylus brasiliensis (Nb), which causes a murine hookworm infection and an experimental porin protein vaccine against STm, were examined.

Methodology/Principal Findings

Mice infected with both STm and Nb induced similar numbers of Th1 and Th2 lymphocytes compared with singly infected mice, as determined by flow cytometry, although lower levels of secreted Th2, but not Th1 cytokines were detected by ELISA after re-stimulation of splenocytes. Furthermore, the density of FoxP3+ T cells in the T zone of co-infected mice was lower compared to mice that only received Nb, but was greater than those that received STm. This reflected the intermediate levels of IL-10 detected from splenocytes. Co-infection compromised clearance of both pathogens, with worms still detectable in mice weeks after they were cleared in the control group. Despite altered control of bacterial and helminth colonization in co-infected mice, robust extrafollicular Th1 and Th2-reflecting immunoglobulin-switching profiles were detected, with IgG2a, IgG1 and IgE plasma cells all detected in parallel. Whilst extrafollicular antibody responses were maintained in the first weeks after co-infection, the GC response was less than that in mice infected with Nb only. Nb infection resulted in some abrogation of the longer-term development of anti-STm IgG responses. This suggested that prior Nb infection may modulate the induction of protective antibody responses to vaccination. To assess this we immunized mice with porins, which confer protection in an antibody-dependent manner, before challenging with STm. Mice that had resolved a Nb infection prior to immunization induced less anti-porin IgG and had compromised protection against infection.

Conclusion

These findings demonstrate that co-infection can radically alter the development of protective immunity during natural infection and in response to immunization.

Vaccination studies in animal models have focused on understanding responses in young, previously naïve mice. In reality, humans are vaccinated or respond to infection in the context of a life-time of accumulated exposure to multiple, systemic infections and other vaccines, some of which are themselves attenuated live organisms. This is even more pronounced in areas that are endemic for infectious diseases. We wished to examine the impact infectious history can have on the immune response against infection and the efficacy of vaccination. To do this, we used two classes of pathogens that model clinically important invasive infections. One pathogen is the bacterium, Salmonella Typhimurium against which we have also developed an experimental porin vaccine, and the second is an invasive helminth, Nippostrongylus brasiliensis, that models aspects of hookworm infections. Our studies indicate that exposure to a second, unrelated pathogen can reduce the efficiency of immunity generated during natural infection and immunity generated after vaccination. These results are important as they help to identify potential strategies for improving immune-mediated control of infection and the success of vaccination in infection-endemic regions.

Salmonella modulates B cell biology to evade CD8(+) T cell-mediated immune responses

Although B cells and antibodies are the central effectors of humoral immunity, B cells can also produce and secrete cytokines and present antigen to helper T cells. The uptake of antigen is mainly mediated by endocytosis; thus, antigens are often presented by MHC-II molecules. However, it is unclear if B cells can present these same antigens via MHC-I molecules. Recently, Salmonella bacteria were found to infect B cells, allowing possible antigen cross-processing that could generate bacterial peptides for antigen presentation via MHC-I molecules. Here, we will discuss available knowledge regarding Salmonella antigen presentation by infected B cell MHC-I molecules and subsequent inhibitory effects on CD8(+) T cells for bacterial evasion of cell-mediated immunity.

Complex adaptive immunity to enteric fevers in humans: lessons learned and the path forward

Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, and S. Paratyphi A and B, causative agents of paratyphoid fever, are major public health threats throughout the world. Although two licensed typhoid vaccines are currently available, they are only moderately protective and immunogenic necessitating the development of novel vaccines. A major obstacle in the development of improved typhoid, as well as paratyphoid vaccines is the lack of known immunological correlates of protection in humans. Considerable progress has been made in recent years in understanding the complex adaptive host responses against S. Typhi. Although the induction of S. Typhi-specific antibodies (including their functional properties) and memory B cells, as well as their cross-reactivity with S. Paratyphi A and S. Paratyphi B has been shown, the role of humoral immunity in protection remains undefined. Cell mediated immunity (CMI) is likely to play a dominant role in protection against enteric fever pathogens. Detailed measurements of CMI performed in volunteers immunized with attenuated strains of S. Typhi have shown, among others, the induction of lymphoproliferation, multifunctional type 1 cytokine production, and CD8(+) cytotoxic T-cell responses. In addition to systemic responses, the local microenvironment of the gut is likely to be of paramount importance in protection from these infections. In this review, we will critically assess current knowledge regarding the role of CMI and humoral immunity following natural S. Typhi and S. Paratyphi infections, experimental challenge, and immunization in humans. We will also address recent advances regarding cross-talk between the host's gut microbiota and immunization with attenuated S. Typhi, mechanisms of systemic immune responses, and the homing potential of S. Typhi-specific B- and T-cells to the gut and other tissues.

Cellular requirements for systemic control of Salmonella enterica serovar Typhimurium infections in mice

The rational design of vaccines requires an understanding of the contributions of individual immune cell subsets to immunity. With this understanding, targeted vaccine delivery approaches and adjuvants can be developed to maximize vaccine efficiency and to minimize side effects (S. H. E. Kaufmann et al., Immunity 33:555-577, 2010; T. Ben-Yedidia and R. Arnon, Hum. Vaccines 1:95-101, 2005). We have addressed the contributions of different immune cell subsets and their ability to contribute to the control and clearance of the facultative intracellular pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium) in a murine model. Using a systematic and reproducible model of experimental attenuated S. Typhimurium infection, we show that distinct lymphocyte deficiencies lead to one of four different infection outcomes: clearance, chronic infection, early death, or late death. Our study demonstrates a high level of functional redundancy in the ability of different lymphocyte subsets to provide interferon gamma (IFN-γ), a critical cytokine in Salmonella immunity. Whereas early control of the infection was entirely dependent on IFN-γ but not on any particular lymphocyte subset, clearance of the infection critically required CD4(+) T cells but appeared to be independent of IFN-γ. These data reinforce the idea of a bimodal immune response against Salmonella: an early T cell-independent but IFN-γ-dependent phase and a late T cell-dependent phase that may be IFN-γ independent.

LcrV delivered via type III secretion system of live attenuated Yersinia pseudotuberculosis enhances immunogenicity against pneumonic plague

Here, we constructed a Yersinia pseudotuberculosis mutant strain with arabinose-dependent regulated and delayed shutoff of crp expression (araC P(BAD) crp) and replacement of the msbB gene with the Escherichia coli msbB gene to attenuate it. Then, we inserted the asd mutation into this construction to form χ10057 [Δasd-206 ΔmsbB868::P(msbB) msbB(EC) ΔP(crp21)::TT araC P(BAD) crp] for use with a balanced-lethal Asd-positive (Asd(+)) plasmid to facilitate antigen synthesis. A hybrid protein composed of YopE (amino acids [aa]1 to 138) fused with full-length LcrV (YopE(Nt138)-LcrV) was synthesized in χ10057 harboring an Asd(+) plasmid (pYA5199, yopE(Nt138)-lcrV) and could be secreted through a type III secretion system (T3SS) in vitro and in vivo. Animal studies indicated that mice orally immunized with χ10057(pYA5199) developed titers of IgG response to whole-cell lysates of Y. pestis (YpL) and subunit LcrV similar to those seen with χ10057(pYA3332) (χ10057 plus an empty plasmid). However, only immunization of mice with χ10057(pYA5199) resulted in a significant secretory IgA response to LcrV. χ10057(pYA5199) induced a higher level of protection (80% survival) against intranasal (i.n.) challenge with ~240 median lethal doses (LD50) (2.4 × 10(4) CFU) of Y. pestis KIM6+(pCD1Ap) than χ10057(pYA3332) (40% survival). Splenocytes from mice vaccinated with χ10057(pYA5199) produced significant levels of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-17 (IL-17) after restimulation with LcrV and YpL antigens. Our results suggest that it is possible to use an attenuated Y. pseudotuberculosis strain delivering the LcrV antigen via the T3SS as a potential vaccine candidate against pneumonic plague.

Sequential acquisition of T cells and antibodies to nontyphoidal Salmonella in Malawian children

BACKGROUND

Salmonella Typhimurium (STm) remain a prominent cause of bacteremia in sub-Saharan Africa. Complement-fixing antibodies to STm develop by 2 years of age. We hypothesized that STm-specific CD4⁺ T cells develop alongside this process.

METHODS

Eighty healthy Malawian children aged 0-60 months were recruited. STm-specific CD4⁺ T cells producing interferon γ, tumor necrosis factor α, and interleukin 2 were quantified using intracellular cytokine staining. Antibodies to STm were measured by serum bactericidal activity (SBA) assay, and anti-STm immunoglobulin G antibodies by enzyme-linked immunosorbent assay.

RESULTS

Between 2006 and 2011, STm bacteremias were detected in 449 children <5 years old. STm-specific CD4⁺ T cells were acquired in infancy, peaked at 14 months, and then declined. STm-specific SBA was detectable in newborns, declined in the first 8 months, and then increased to a peak at age 35 months. Acquisition of SBA correlated with acquisition of anti-STm-lipopolysaccharide (LPS) immunoglobulin G (r = 0.329 [95% confidence interval, .552-.062]; P = .01) but not anti-STm-outer membrane protein or anti-STm-flagellar protein (FliC).

CONCLUSIONS

Acquisition of STm-specific CD4⁺ T cells in early childhood is consistent with early exposure to STm or cross-reactive protein antigens priming this T-cell development. STm-specific CD4⁺ T cells seem insufficient to protect against invasive nontyphoidal Salmonella disease, but sequential acquisition of SBA to STm LPS is associated with a decline in its incidence.

Antibody response toSalmonella: its induction and role in protection against avian enteric salmonellosis

Human enteritis resulting from the consumption of poultry products contaminated with serovars of Salmonella enterica remains a major public-health concern. Reducing food contamination by preventing or controlling infection in the chicken during rearing is an attractive solution. An accurate understanding of the mechanisms of immunity to Salmonella infection in the chicken will help to focus the development of vaccines for birds and prevent contaminated products from entering the human food chain. Infection is primarily restricted to the intestinal lumen when chickens are infected with S. enterica serovars Typhimurium or Enteritidis, where they persist for many weeks. High titers of Salmonella-specific antibodies are observed following infection and demonstrate a high degree of cross-reactivity against other serovars. However, depletion of B cells and, therefore, removal of the capacity for antibody production in the chicken does not exacerbate the infection following either primary or secondary challenge.

Characterization of CD4+ subpopulations and CD25+ cells in ileal lymphatic tissue of weaned piglets infected with Salmonella Typhimurium with or without Enterococus faecium feeding

The aim of the present study was to test the effect of Enterococcus faecium NCIMB 10415 (E. faecium) on CD4+ T helper immune cell subpopulations and CD25+ cells in ileal lymphatic tissue after challenge with Salmonella (S.) Typhimurium DT 104. German Landrace piglets treated with E. faecium (n=16) as a feed additive and untreated controls (n=16) were challenged with S. Typhimurium 10 days after weaning. The expression of lineage specific T helper cell subtype master transcription factors on mRNA level was measured in the whole tissue of the gut associated lymphoid tissues (ileocecal mesenteric lymph node, ileum with Peyer's patches and papilla ilealis) and in magnetically sorted T helper cells from blood and ileocecal mesenteric lymph nodes at two and 28 days post infection. CD25 protein expression of T helper cells was studied by flow cytometry in ileal Peyer's patches, lymph nodes and blood. Distribution and morphology of CD25+ cells was demonstrated in situ by immunohistochemistry in paraffin embedded specimens of the ileum and the ileocecal mesenteric lymph nodes. The data provide evidence for a higher T helper 2 cell driven immune response in the control group compared to the E. faecium treated group (P<0.05) in CD4+ magnetically sorted lymphocytes from the ileocecal mesenteric lymph nodes at two and 28 days post infection. We did not observe differences for CD25+ cells in immunohistochemistry and flow cytometry between E. faecium fed pigs and the control group, but provided a detailed description of the occurrence and morphology of these cells in the gut associate lymphoid tissues of piglets. In conclusion we suggest that (i) prolonged feeding with E. faecium can result in changes of the T helper cell response leading to a stronger infection with S. Typhimurium and (ii) that it is important to examine purified immune cells to be able to detect effects on T helper cell subpopulations.

Dietary supplementation with white button mushrooms augments the protective immune response to Salmonella vaccine in mice

We previously showed that dietary white button mushrooms (WBMs) enhanced natural killer cell activity and that in vitro WBM supplementation promotes maturation and function of dendritic cells (DCs). The current study investigated whether WBM consumption would enhance pathogen-specific immune response using a Salmonella vaccination and infection animal model. C57BL/6 mice were fed diets containing 0%, 2%, or 5% WBM for 4 wk before oral vaccination with live attenuated Salmonella typhimurium SL1479. Four weeks after immunization, mice were orally infected with virulent Salmonella typhimurium SL1344. Immunization increased animal survival and, among immunized mice, the 2% WBM group had a higher survival rate than the other groups. Next, we fed mice 2% WBMs to determine the immunological mechanism underlying the WBM-potentiated protective effect. We found that WBM supplementation increased Salmonella-specific blood immunoglobulin (Ig) G and fecal IgA concentrations. WBM-fed mice also had a higher IgG2a and unchanged IgG1 production, leading to an elevated IgG2a:IgG1 ratio and indicating an enhanced T helper 1 response. Consistent with these results, WBM-fed mice had higher interferon-γ, tumor necrosis factor (TNF)-α, and interleukin (IL)-17A production and unchanged IL-4 production in their splenocytes after polyclonal (anti-CD3/CD28) or antigen-specific stimulation. Furthermore, WBM-fed mice had more DCs in the spleen, and these DCs expressed higher levels of activation markers CD40 and major histocompatibility complex-II. These mice also produced more IL-12 and TNF-α postimmunization. Together, these results suggest that WBMs may improve Salmonella vaccine efficacy through an enhanced adaptive immune response.

Helicobacter and salmonella persistent infection strategies

Some host-adapted bacterial pathogens are capable of causing persistent infections in humans. For example, Helicobacter pylori inhabits the human gastric mucosa and persistence can be lifelong. Salmonella enterica serovar Typhi causes systemic infections that involve colonization of the reticuloendothelial system and some individuals become lifelong carriers. In this review, I compare and contrast the different lifestyles of Helicobacter and Salmonella within the host and the strategies they have evolved to persist in mammalian hosts. Persistently infected carriers serve as the reservoirs for these pathogens, and the carrier state is an essential feature that is required for survival of the bacteria within a restricted host population. Therefore, investigating the chronic carrier state should provide insight into bacterial survival strategies, as well as new therapeutic approaches for treatments.

miR-155: an ancient regulator of the immune system

MicroRNAs (miRNAs) are a newly recognized class of regulatory genes which repress the expression of protein-coding genes. Numerous studies have uncovered a complex role for miRNAs regulating many aspects of a variety of cellular processes including cell growth, differentiation, and lineage commitment. In the immune system, miR-155 is unique in its ability to shape the transcriptome of activated myeloid and lymphoid cells controlling diverse biological functions ranging from inflammation to immunological memory. Not surprisingly, a tight control of miR-155 expression is required to avoid malignant transformation, as evidenced by miR-155 overexpression in many cancers of B-cell origin. In this review, we discuss the potential of miR-155 as a molecular target for therapeutic intervention and discuss the function of miR-155 in the context of protective immunity. We first look back into the emergence of miR-155 in evolution, which is coincidental with the emergence of the ancestors of the antigen receptors. We then summarize what we have learned about the role of miR-155 in the regulation of lymphoid subsets at the cellular and molecular level in the context of recent progress in this field.

Recombinant Salmonella enterica serovar Typhimurium as a vaccine vector for HIV-1 Gag

The HIV/AIDS epidemic remains a global health problem, especially in Sub-Saharan Africa. An effective HIV-1 vaccine is therefore badly required to mitigate this ever-expanding problem. Since HIV-1 infects its host through the mucosal surface, a vaccine for the virus needs to trigger mucosal as well as systemic immune responses. Oral, attenuated recombinant Salmonella vaccines offer this potential of delivering HIV-1 antigens to both the mucosal and systemic compartments of the immune system. So far, a number of pre-clinical studies have been performed, in which HIV-1 Gag, a highly conserved viral antigen possessing both T- and B-cell epitopes, was successfully delivered by recombinant Salmonella vaccines and, in most cases, induced HIV-specific immune responses. In this review, the potential use of Salmonella enterica serovar Typhimurium as a live vaccine vector for HIV-1 Gag is explored.

Immunization with Escherichia coli outer membrane vesicles protects bacteria-induced lethality via Th1 and Th17 cell responses

Outer membrane vesicles (OMVs), secreted from Gram-negative bacteria, are spherical nanometer-sized proteolipids enriched with outer membrane proteins. OMVs, also known as extracellular vesicles, have gained interests for use as nonliving complex vaccines and have been examined for immune-stimulating effects. However, the detailed mechanism on how OMVs elicit the vaccination effect has not been studied extensively. In this study, we investigated the immunological mechanism governing the protective immune response of OMV vaccines. Immunization with Escherichia coli-derived OMVs prevented bacteria-induced lethality and OMV-induced systemic inflammatory response syndrome. As verified by adoptive transfer and gene-knockout studies, the protective effect of OMV immunization was found to be primarily by the stimulation of T cell immunity rather than B cell immunity, especially by the OMV-Ag-specific production of IFN-γ and IL-17 from T cells. By testing the bacteria-killing ability of macrophages, we also demonstrated that IFN-γ and IL-17 production is the main factor promoting bacterial clearances. Our findings reveal that E. coli-derived OMV immunization effectively protects bacteria-induced lethality and OMV-induced systemic inflammatory response syndrome primarily via Th1 and Th17 cell responses. This study therefore provides a new perspective on the immunological detail regarding OMV vaccination.

Contribution of Thy1+ NK cells to protective IFN-γ production during Salmonella typhimurium infections

IFN-γ is critical for immunity against infections with intracellular pathogens, such as Salmonella enterica. However, which of the many cell types capable of producing IFN-γ controls Salmonella infections remains unclear. Using a mouse model of systemic Salmonella infection, we observed that only a lack of all lymphocytes or CD90 (Thy1)(+) cells, but not the absence of T cells, Retinoic acid-related orphan receptor (ROR)-γt-dependent lymphocytes, (NK)1.1(+) cells, natural killer T (NKT), and/or B cells alone, replicated the highly susceptible phenotype of IFN-γ-deficient mice to Salmonella infection. A combination of antibody depletions and adoptive transfer experiments revealed that early protective IFN-γ was provided by Thy1-expressing natural killer (NK) cells and that these cells improved antibacterial immunity through the provision of IFN-γ. Further analysis of NK cells producing IFN-γ in response to Salmonella indicated that less mature NK cells were more efficient at mediating antibacterial effector function than terminally differentiated NK cells. Inspired by recent reports of Thy1(+) NK cells contributing to immune memory, we analyzed their role in secondary protection against otherwise lethal WT Salmonella infections. Notably, we observed that a newly generated Salmonella vaccine strain not only conferred superior protection compared with conventional regimens but that this enhanced efficiency of recall immunity was afforded by incorporating CD4(-)CD8(-)Thy1(+) cells into the secondary response. Taken together, these findings demonstrate that Thy1-expressing NK cells play an important role in antibacterial immunity.

Emerging trends in the formation and function of tuberculosis granulomas

The granuloma is an elaborated aggregate of immune cells found in non-infectious as well as infectious diseases. It is a hallmark of tuberculosis (TB). Predominantly thought as a host-driven strategy to constrain the bacilli and prevent dissemination, recent discoveries indicate the granuloma can also be modulated into an efficient tool to promote microbial pathogenesis. The aim of future studies will certainly focus on better characterization of the mechanisms driving the modulation of the granuloma functions. Here, we provide unique perspectives from both the innate and adaptive immune system in the formation and the role of the TB granuloma. As macrophages (Mϕs) comprise the bulk of granulomas, we highlight the emerging concept of Mϕ polarization and its potential impact in the microbicide response, and other activities, that may ultimately shape the fate of granulomas. Alternatively, we shed light on the ability of B-cells to influence inflammatory status within the granuloma.

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.

Cutting edge: B cells are essential for protective immunity against Salmonella independent of antibody secretion

Typhoid fever and nontyphoidal bacteremia caused by Salmonella remain critical human health problems. B cells are required for protective immunity to Salmonella, but the mechanism of protection remains unclear. In this study, we immunized wild-type, B cell-deficient, Ab-deficient, and class-switched Ab-deficient mice with attenuated Salmonella and examined protection against secondary infection. As expected, wild-type mice were protected and B cell-deficient mice succumbed to secondary infection. Interestingly, mice with B cells but lacking secreted Ab or class-switched Ab had little deficiency in resistance to Salmonella infection. The susceptibility of B cell-deficient mice correlated with marked reductions in CD4 T cell IFN-γ production after secondary infection. Taken together, these data suggest that the primary role of B cells in acquired immunity to Salmonella is via the development of protective T cell immunity.

Humoral and cellular immune response generated by different vaccine programs before and after Salmonella Enteritidis challenge in chickens

The poultry industry has a high demand for Salmonella vaccines in order to generate safer Salmonella-free food for consumers around the world. Vaccination against S. Enteritidis (SE) is vastly undertaken in many countries, although the criteria for the use of live vaccine (LV) or killed vaccine (KV) should also depend on the immune mechanisms triggered by each. In this study, a commercial bacterin (KV) and an attenuated SG mutant (LV) were used in four different vaccine programs (LV; LV+LV; KV; LV+KV). At 1 day before (dbi) and 1, 6 and 9 days after SE challenge (dpi), humoral (IgM, IgG and secretory IgA) and cellular (CD8(+) T cells) immune responses were evaluated along with the production of IL-10, IL-12 and IFN-γ. Although after challenge, all birds from each group had an influx of CD8(+) T cells, birds which received KV had lower levels of these cells in organs and significantly higher levels of immunoglobulins. The expression of the cytokines was up-regulated in all groups post-vaccination, although, after challenge, cytokine expression decreased in the vaccinated groups, and increased in the unvaccinated group A. IL-10 levels were significantly higher at 1 day post-infection in the group that received KV, which may be involved in the weak cellular immune response observed within this group. In caecal tonsils, IFN-γ expression at 1 dbi was higher in birds which received two vaccine doses, and after challenge, the population of CD8(+) T cells constantly increased in birds that were only vaccinated with the LV. This study demonstrated that the development of a mature immune response by CD8(+) T cells, provided by the use of the LV, had better efficacy in comparison to the high antibody levels in the serum stimulated by the KV. However, high secretory IgA levels in the intestinal lumen associated with influx CD8(+) T cells may be indicative of protection as noticed in group E (LV+KV).

Polyfunctional CD4+ T cell responses to immunodominant epitopes correlate with disease activity of virulent Salmonella

Salmonella enterica serovars are intracellular bacteria capable of causing typhoid fever and gastroenteritis of significant morbidity and mortality worldwide. Current prophylactic and therapeutic treatment is hampered by the emergence of multidrug-resistant (MDR) strains of Salmonella, and vaccines provide only temporal and partial protection in vaccinees. To develop more effective Salmonella vaccines, it is important to understand the development of protective adaptive immunity to virulent Salmonella. Here we report the identification of novel CD4(+) T cell peptide epitopes, which are conserved among Salmonella serovars. Immunization of Salmonella-infected mice with these peptide epitopes reduces the burden of Salmonella disease. Furthermore, we show that distinct polyfunctional (interferon-γ(+), tumor necrosis factor(+), and interleukin-2(+)) Salmonella-specific CD4(+) T cell responses develop with respect to magnitude and kinetics. Moreover, we found that CD4(+) T cell responses against immunodominant epitopes are predictive for active Salmonella disease. Collectively, these data could contribute to improved diagnosis of Salmonella-related diseases and rational design of Salmonella vaccines.

Increased susceptibility to Salmonella infection in signal regulatory protein α-deficient mice

Recent studies have shed light on the connection between elevated erythropoetin production/spleen erythropoiesis and increased susceptibility to Salmonella infection. In this article, we provide another mouse model, the SIRPα-deficient (Sirpα⁻/⁻) mouse, that manifests increased erythropoiesis as well as heightened susceptibility to Salmonella infection. Sirpα⁻/⁻ mice succumbed to systemic infection with attenuated Salmonella, possessing significantly higher bacterial loads in both the spleen and the liver. Moreover, Salmonella-specific Ab production and Ag-specific CD4 T cells were reduced in Sirpα⁻/⁻ mice compared with wild-type controls. To further characterize the potential mechanism underlying SIRPα-dependent Ag-specific CD4 T cell priming, we demonstrate that lack of SIRPα expression on dendritic cells results in less efficient Ag processing and presentation in vitro. Collectively, these findings demonstrate an indispensable role of SIRPα for protective immunity to Salmonella infection.

Identification of a common immune signature in murine and human systemic Salmonellosis

Despite the importance of Salmonella infections in human and animal health, the target antigens of Salmonella-specific immunity remain poorly defined. We have previously shown evidence for antibody-mediating protection against invasive Salmonellosis in mice and African children. To generate an overview of antibody targeting in systemic Salmonellosis, a Salmonella proteomic array containing over 2,700 proteins was constructed and probed with immune sera from Salmonella-infected mice and humans. Analysis of multiple inbred mouse strains identified 117 antigens recognized by systemic antibody responses in murine Salmonellosis. Importantly, many of these antigens were independently identified as target antigens using sera from Malawian children with Salmonella bacteremia, validating the study of the murine model. Furthermore, vaccination with SseB, the most prominent antigenic target in Malawian children, provided mice with significant protection against Salmonella infection. Together, these data uncover an overlapping immune signature of disseminated Salmonellosis in mice and humans and provide a foundation for the generation of a protective subunit vaccine.

Deletion of the aceE gene (encoding a component of pyruvate dehydrogenase) attenuates Salmonella enterica serovar Enteritidis

Salmonella enterica serovar Enteritidis (S. Enteritidis) is a major food-borne pathogen. From a transposon insertion mutant library created previously using S. Enteritidis 10/02, one of the mutants was identified to have a 50% lethal dose (LD(50) ) at least 100 times that of the parental strain in young chicks, with an attenuation in a poorly studied gene encoding a component of pyruvate dehydrogenase, namely the aceE gene. Evaluation of the in vitro virulence characteristics of the ΔaceE∷kan mutant revealed that it was less able to invade epithelial cells, less resistant to reactive oxygen intermediate, less able to survive within a chicken macrophage cell line and had a retarded growth rate compared with the parental strain. Young chicks vaccinated with 2 × 10(9) CFU of the ΔaceE∷kan mutant were protected from the subsequent challenge of the parental strain, with the mutant colonized in the liver and spleen in a shorter time than the group infected with the parental strain. In addition, compared with the parental strain, the ΔaceE∷kan mutant did not cause persistent eggshell contamination of vaccinated hens.

CD31 is required on CD4+ T cells to promote T cell survival during Salmonella infection

Hematopoietic cells constitutively express CD31/PECAM1, a signaling adhesion receptor associated with controlling responses to inflammatory stimuli. Although expressed on CD4(+) T cells, its function on these cells is unclear. To address this, we have used a model of systemic Salmonella infection that induces high levels of T cell activation and depends on CD4(+) T cells for resolution. Infection of CD31-deficient (CD31KO) mice demonstrates that these mice fail to control infection effectively. During infection, CD31KO mice have diminished numbers of total CD4(+) T cells and IFN-γ-secreting Th1 cells. This is despite a higher proportion of CD31KO CD4(+) T cells exhibiting an activated phenotype and an undiminished capacity to prime normally and polarize to Th1. Reduced numbers of T cells reflected the increased propensity of naive and activated CD31KO T cells to undergo apoptosis postinfection compared with wild-type T cells. Using adoptive transfer experiments, we show that loss of CD31 on CD4(+) T cells alone is sufficient to account for the defective CD31KO T cell accumulation. These data are consistent with CD31 helping to control T cell activation, because in its absence, T cells have a greater propensity to become activated, resulting in increased susceptibility to become apoptotic. The impact of CD31 loss on T cell homeostasis becomes most pronounced during severe, inflammatory, and immunological stresses such as those caused by systemic Salmonella infection. This identifies a novel role for CD31 in regulating CD4 T cell homeostasis.

Development of protective immunity to Salmonella, a mucosal pathogen with a systemic agenda

Salmonella infections can cause a range of intestinal and systemic diseases in human and animal hosts. Although some Salmonella serovars initiate a localized intestinal inflammatory response, others use the intestine as a portal of entry to initiate a systemic infection. Considerable progress has been made in understanding bacterial invasion and dissemination strategies, as well as the nature of the Salmonella-specific immune response to oral infection. Innate and adaptive immunity are rapidly initiated after oral infection, but these effector responses can also be hindered by bacterial evasion strategies. Furthermore, although Salmonella resides within intramacrophage phagosomes, recent studies have highlighted a surprising collaboration of CD4 Th1, Th17, and B-cell responses in mediating resistance to Salmonella infection.

Cellular aspects of immunity to intracellular Salmonella enterica

Salmonella enterica is a frequent gastrointestinal pathogen with ability to cause diseases ranging from local gastrointestinal inflammation and diarrhea to life-threatening typhoid fever. Salmonella is an invasive, facultative intracellular pathogen that infects various cell types of the host and can survive and proliferate in different populations of immune cells. During pathogenesis, Salmonella is confronted with various lines of immune defense. To successfully colonize host organisms, the pathogen deploys a set of sophisticated mechanisms of immune evasion and direct manipulation of immune cell functions. In addition to resistance against innate immune mechanisms, including the ability to avoid killing by macrophages and dendritic cells (DCs), Salmonella interferes with antigen presentation by DCs and the formation of an efficient adaptive immune response. In this review, we describe the current understanding of Salmonella virulence factors during intracellular life and focus on the recent advances in the understanding of interference of intracellular Salmonella with cellular functions of immune cells.