Cellular and cytokine responses to Salmonella enterica serotype Typhi proteins in patients with typhoid fever in Bangladesh

Genomic and phenotypic comparison of two variants of multidrug-resistant Salmonella enterica serovar Heidelberg isolated during the 2015-2017 multi-state outbreak in cattle

Salmonella enterica subspecies enterica serovar Heidelberg (Salmonella Heidelberg) has caused several multistate foodborne outbreaks in the United States, largely associated with the consumption of poultry. However, a 2015-2017 multidrug-resistant (MDR) Salmonella Heidelberg outbreak was linked to contact with dairy beef calves. Traceback investigations revealed calves infected with outbreak strains of Salmonella Heidelberg exhibited symptoms of disease frequently followed by death from septicemia. To investigate virulence characteristics of Salmonella Heidelberg as a pathogen in bovine, two variants with distinct pulse-field gel electrophoresis (PFGE) patterns that differed in morbidity and mortality during the multistate outbreak were genotypically and phenotypically characterized and compared. Strain SX 245 with PFGE pattern JF6X01.0523 was identified as a dominant and highly pathogenic variant causing high morbidity and mortality in affected calves, whereas strain SX 244 with PFGE pattern JF6X01.0590 was classified as a low pathogenic variant causing less morbidity and mortality. Comparison of whole-genome sequences determined that SX 245 lacked ~200 genes present in SX 244, including genes associated with the IncI1 plasmid and phages; SX 244 lacked eight genes present in SX 245 including a second YdiV Anti-FlhC(2)FlhD(4) factor, a lysin motif domain containing protein, and a pentapeptide repeat protein. RNA-sequencing revealed fimbriae-related, flagella-related, and chemotaxis genes had increased expression in SX 245 compared to SX 244. Furthermore, SX 245 displayed higher invasion of human and bovine epithelial cells than SX 244. These data suggest that the presence and up-regulation of genes involved in type 1 fimbriae production, flagellar regulation and biogenesis, and chemotaxis may play a role in the increased pathogenicity and host range expansion of the Salmonella Heidelberg isolates involved in the bovine-related outbreak.

Reduced immunogenicity of a live Salmonella enterica serovar Typhimurium vaccine in aged mice

Introduction

Non-typhoidal Salmonella (NTS) is responsible for a high burden of foodborne infections and deaths worldwide. In the United States, NTS infections are the leading cause of hospitalizations and deaths due to foodborne illnesses, and older adults (≥65 years) are disproportionately affected by Salmonella infections. Due to this public health concern, we have developed a live attenuated vaccine, CVD 1926 (I77 ΔguaBA ΔclpP ΔpipA ΔhtrA), against Salmonella enterica serovar Typhimurium, a common serovar of NTS. Little is known about the effect of age on oral vaccine responses, and due to the decline in immune function with age, it is critical to evaluate vaccine candidates in older age groups during early product development.

Methods

In this study, adult (six-to-eight-week-old) and aged (18-month-old) C57BL/6 mice received two doses of CVD 1926 (109 CFU/dose) or PBS perorally, and animals were evaluated for antibody and cell-mediated immune responses. A separate set of mice were immunized and then pre-treated with streptomycin and challenged orally with 108 CFU of wild-type S. Typhimurium SL1344 at 4 weeks postimmunization.

Results

Compared to PBS-immunized mice, adult mice immunized with CVD 1926 had significantly lower S. Typhimurium counts in the spleen, liver, and small intestine upon challenge. In contrast, there were no differences in bacterial loads in the tissues of vaccinated versus PBS aged mice. Aged mice exhibited reduced Salmonella-specific antibody titers in the serum and feces following immunization with CVD 1926 compared to adult mice. In terms of T cell responses (T-CMI), immunized adult mice showed an increase in the frequency of IFN-γ- and IL-2-producing splenic CD4 T cells, IFN-γ- and TNF-α-producing Peyer's Patch (PP)-derived CD4 T cells, and IFN-γ- and TNF-α-producing splenic CD8 T cells compared to adult mice administered PBS. In contrast, in aged mice, T-CMI responses were similar in vaccinated versus PBS mice. CVD 1926 elicited significantly more PP-derived multifunctional T cells in adult compared to aged mice.

Conclusion

These data suggest that our candidate live attenuated S. Typhimurium vaccine, CVD 1926, may not be sufficiently protective or immunogenic in older humans and that mucosal responses to live-attenuated vaccines decrease with increasing age.

Genetic Susceptibility to Enteric Fever in Experimentally Challenged Human Volunteers

Infections with Salmonella enterica serovars Typhi and Paratyphi A cause an estimated 14 million cases of enteric fever annually. Here, the controlled nature of challenge studies is exploited to identify genetic variants associated with enteric fever susceptibility. Human challenge participants were genotyped by Illumina OmniExpress-24 BeadChip array (n = 176) and/or transcriptionally profiled by RNA sequencing (n = 174). While the study was underpowered to detect any single nucleotide polymorphisms (SNPs) significant at the whole-genome level, two SNPs within CAPN14 and MIATNB were identified with P < 10-5 for association with development of symptoms or bacteremia following oral S. Typhi or S. Paratyphi A challenge. Imputation of classical human leukocyte antigen (HLA) types from genomic and transcriptomic data identified HLA-B*27:05, previously associated with nontyphoidal Salmonella-induced reactive arthritis, as the HLA type most strongly associated with enteric fever susceptibility (P = 0.011). Gene sets relating to the unfolded protein response/heat shock and endoplasmic reticulum-associated protein degradation were overrepresented in HLA-B*27:05+ participants following challenge. Furthermore, intracellular replication of S. Typhi is higher in C1R cells transfected with HLA-B*27:05 (P = 0.02). These data suggest that activation of the unfolded protein response by HLA-B*27:05 misfolding may create an intracellular environment conducive to S. Typhi replication, increasing susceptibility to enteric fever.

Revisiting Persistent Salmonella Infection and the Carrier State: What Do We Know?

One characteristic of the few Salmonella enterica serovars that produce typhoid-like infections is that disease-free persistent infection can occur for months or years in a small number of individuals post-convalescence. The bacteria continue to be shed intermittently which is a key component of the epidemiology of these infections. Persistent chronic infection occurs despite high levels of circulating specific IgG. We have reviewed the information on the basis for persistence in S. Typhi, S. Dublin, S. Gallinarum, S. Pullorum, S. Abortusovis and also S. Typhimurium in mice as a model of persistence. Persistence appears to occur in macrophages in the spleen and liver with shedding either from the gall bladder and gut or the reproductive tract. The involvement of host genetic background in defining persistence is clear from studies with the mouse but less so with human and poultry infections. There is increasing evidence that the organisms (i) modulate the host response away from the typical Th1-type response normally associated with immune clearance of an acute infection to Th2-type or an anti-inflammatory response, and that (ii) the bacteria modulate transformation of macrophage from M1 to M2 type. The bacterial factors involved in this are not yet fully understood. There are early indications that it might be possible to remodulate the response back towards a Th1 response by using cytokine therapy.

IL-9 contributes to the host immune response against Helicobacter pylori and helps limit infection in a Mouse Model

BACKGROUND

As an important mediator in lots of diseases, interleukin-9 (IL-9) can be a protector or pro-inflammatory cytokine depending on the complicated inflammatory milieu. Helicobacter pylori (H. pylori) induced a series of immunology cells and cytokines change, and however, the role of IL-9 in H. pylori infection remains unknown.

MATERIALS AND METHODS

Wild-type and IL-9 deficient mice were infected with H. pylori by means of intragastric administration. The colonization of H. pylori bacteria was measured by detecting specific 16s rDNA, and the intensity of inflammation was observed by H&E stain. The expression level of inflammation cytokines was determined by ELISA and quantitative real-time PCR.

RESULTS

IL-9 was increased due to the attack of H. pylori, besides deletion of Il9 aggravated the bacterial colonization and inflammation intensity. In addition, treatment of rmIL-9 reduced colonized H. pylori and inflammation level, indicated that IL-9 was a protector for the host against this bacterium. Followed by the H. pylori infection, interferon (IFN)-γ and interleukin (IL)-17A were up-regulated as expected, and nevertheless, the expression of IL-17A shared a positive relationship with IL-9 while IFN-γ negative associated with IL-9. Moreover, we also proved that Treg cells were not involved in the protective effect of IL-9, and meanwhile, CD4⁺ CD25^- T cells secreted more IFN-γ and less IL-17A in vitro due to the deletion of Il9.

CONCLUSIONS

IL-9 plays a protective role against H. pylori and the protection associated with cytokines change including IFN-γ and IL-17A.

Human Genetic Variation Influences Enteric Fever Progression

In the 21st century, enteric fever is still causing a significant number of mortalities, especially in high-risk regions of the world. Genetic studies involving the genome and transcriptome have revealed a broad set of candidate genetic polymorphisms associated with susceptibility to and the severity of enteric fever. This review attempted to explain and discuss the past and the most recent findings on human genetic variants affecting the progression of Salmonella typhoidal species infection, particularly toll-like receptor (TLR) 4, TLR5, interleukin (IL-) 4, natural resistance-associated macrophage protein 1 (NRAMP1), VAC14, PARK2/PACRG, cystic fibrosis transmembrane conductance regulator (CFTR), major-histocompatibility-complex (MHC) class II and class III. These polymorphisms on disease susceptibility or progression in patients could be related to multiple mechanisms in eliminating both intracellular and extracellular Salmonella typhoidal species. Here, we also highlighted the limitations in the studies reported, which led to inconclusive results in association studies. Nevertheless, the knowledge obtained through this review may shed some light on the development of risk prediction tools, novel therapies as well as strategies towards developing a personalised typhoid vaccine.

Investigating chemical changes during Snake fruit and Black tea Kombucha fermentation and the associated immunomodulatory activity in Salmonella typhi-infected mice

This study uncovered the chemical changes during kombucha's fermentation process and revealed the associated immunomodulatory activity in Salmonella typhi-infected mice. The snake fruit juice and black tea extract were processed into kombucha (a beverage known for its health benefits) by fermentation with SCOBY culture at room temperature for  14 days. Snake fruit kombucha showed high changes in fermentation parameters (total acidity, pH, and total sugar), as well as bioactive compounds and antioxidant activity. Salmonella typhi demonstrated a reduction in the population of CD8+TNFα+ and CD4+IFNγ+ of infected experimental animals. Both snake fruit kombucha and black tea kombucha have the potential to be utilized as an immunomodulator to circumvent unstable conditions of the immune system caused by Salmonella typhi. Black tea kombucha and snake fruit kombucha cancan raise the production of CD8+TNFα+ and CD4+IFNγ+ in mice infected with Salmonella typhi. In the group of normal mice, black tea and snake fruit kombucha were able to lower down the production of CD8+TNFα+, which is a potent mechanism to modulate the immune system. Further research is required to highlight the mechanism and role of black tea kombucha and snake fruit kombucha in the immune response that modulates and treats infection by Salmonella typhi.

Immune Modulation and the Development of Fowl Typhoid: A Model of Human Disease?

Salmonella enterica serovar Gallinarum (S. Gallinarum) is the cause of typhoid in chickens but the immune factors that may facilitate the development of typhoid have not been fully elucidated. We show that, in contrast to non-typhoid S. Enteritidis infection, S. Gallinarum significantly reduced nitrite ion production and expression of mRNA for heterophil granulocyte chemoattractants CXCLi2 and IL-6 in chicken monocyte-derived macrophages (chMDMs) (p < 0.05) at 6 h post-infection (pi). S. Gallinarum also reduced IFN-γ and IL-17 expression by CD4⁺ lymphocytes cultured with infected chMDMs for 5 days but did not induce a Th2 phenotype or anergy. In vivo, S. Gallinarum also induced significantly lower expression of CXCLi1, CXCLi2, IL-1β, IL-6 and iNOS mRNA in the caecal tonsil by day 2 pi (p < 0.05–0.01) and consistently lower levels of IFN-γ, IL-18, IL-12, and IL-17. In the spleen, S. Gallinarum induced significantly lower levels of iNOS and IFN-γ (p < 0.01 and 0.05 respectively) and consistently lower levels of IL-18 and IL-12 but significantly greater (p < 0.01) expression of anti-inflammatory IL-10 at day 4 and 5 pi when compared to S. Enteritidis. This immune phenotype was associated with transit from the intestinal tissues to the liver by S. Gallinarum, not observed following S. Enteritidis infection. In conclusion, we report an immune mechanism that may facilitate typhoid disease in S. Gallinarum-infected chickens. However, down-regulation of inflammatory mediators, upregulation of IL-10, and associated liver colonisation are also characteristic of human typhoid, suggesting that this may also be a useful model of typhoid in humans.

Human Salmonella Typhi exposure generates differential multifunctional cross-reactive T-cell memory responses against Salmonella Paratyphi and invasive nontyphoidal Salmonella

Objective

There are no vaccines for most of the major invasive Salmonella strains causing severe infection in humans. We evaluated the specificity of adaptive T memory cell responses generated after Salmonella Typhi exposure in humans against other major invasive Salmonella strains sharing capacity for dissemination.

Methods

T memory cells from eleven volunteers who underwent controlled oral challenge with wtS. Typhi were characterised by flow cytometry for cross‐reactive cellular cytokine/chemokine effector responses or evidence of degranulation upon stimulation with autologous B‐lymphoblastoid cells infected with either S. Typhi, Salmonella Paratyphi A (PA), S. Paratyphi B (PB) or an invasive nontyphoidal Salmonella strain of the S. Typhimurium serovar (iNTSTy).

Results

Blood T‐cell effector memory (T_EM) responses after exposure to S. Typhi in humans evolve late, peaking weeks after infection in most volunteers. Induced multifunctional CD4⁺ Th1 and CD8⁺ T_EM cells elicited after S. Typhi challenge were cross‐reactive with PA, PB and iNTSTy. The magnitude of multifunctional CD4⁺ T_EM cell responses to S. Typhi correlated with induction of cross‐reactive multifunctional CD8⁺ T_EM cells against PA, PB and iNTSTy. Highly multifunctional subsets and T central memory and T effector memory cells that re‐express CD45 (T_EMRA) demonstrated less heterologous T‐cell cross‐reactivity, and multifunctional Th17 elicited after S. Typhi challenge was not cross‐reactive against other invasive Salmonella.

Conclusion

Gaps in cross‐reactive immune effector functions in human T‐cell memory compartments were highly dependent on invasive Salmonella strain, underscoring the importance of strain‐dependent vaccination in the design of T‐cell‐based vaccines for invasive Salmonella.

Diversity of Salmonella Typhi-responsive CD4 and CD8 T cells before and after Ty21a typhoid vaccination in children and adults

Typhoid fever is a life-threatening disease caused by the human-restricted pathogen Salmonella enterica serovar Typhi (S. Typhi). The oral live attenuated Ty21a typhoid vaccine protects against this severe disease by eliciting robust, multifunctional cell-mediated immunity (CMI), shown to be associated with protection in wild-type S. Typhi challenge studies. Ty21a induces S. Typhi-responsive CD8+ and CD4+ T cells but little is known about the response to this vaccine in children. To address this important gap in knowledge, we have used mass cytometry to analyze pediatric and adult pre- and post-Ty21a vaccination CMI in an autologous S. Typhi antigen presentation model. Here, using conventional supervised analytical tools, we show adult T cells are more multifunctional at baseline than those obtained from children. Moreover, pediatric and adult T cells respond similarly to Ty21a vaccination, but adult responders remain more multifunctional. The use of the unsupervised dimensionality reduction tool tSNE (t-distributed Stochastic Neighbor Embedding) allowed us to confirm these findings, as well as to identify increases and decreases in well-defined specific CD4+ and CD8+ T-cell populations that were not possible to uncover using the conventional gating strategies. These findings evidenced age-associated maturation of multifunctional S. Typhi-responsive T-cell populations, including those which we have previously shown to be associated with protection from, and/or delayed onset of, typhoid disease. These findings are likely to play an important role in improving pediatric vaccination strategies against S. Typhi and other enteric pathogens.

Cell mediated immune responses elicited in volunteers following immunization with candidate live oral Salmonella enterica serovar Paratyphi A attenuated vaccine strain CVD 1902

The incidence of Salmonella enterica serovar Paratyphi A (PA) infection is on the rise and no licensed vaccines are available. We evaluated cell mediated immune (CMI) responses elicited in volunteers following immunization with a single dose (10⁹ or 10¹⁰ cfu) of a novel attenuated live oral PA-vaccine strain (CVD 1902). Results showed increases in PA-lipopolysaccharide-specific IgG- and/or IgA B-memory cells and production of IFN-γ, TNF-α, IL-10, IL-23 and RANTES following stimulation with PA-antigens by peripheral blood mononuclear cells obtained 28 days post immunization. Flow cytometry assays revealed that vaccine elicited PA-specific CD8+ and/or CD4+ T effector/memory cells were predominantly multifunctional concomitantly expressing CD107a and/or producing IFN-γ, TNF-α and/or IL-2. Similar proportions of these MF cells expressed, or not, the gut homing marker integrin α4β7. The results suggest that immunization with CVD 1902 elicits CMI responses against PA supporting its further evaluation as a potential vaccine candidate against paratyphoid A fever.

Genome divergence and increased virulence of outbreak associated Salmonella enterica subspecies enterica serovar Heidelberg

Salmonella enterica serotype Heidelberg is primarily a poultry adapted serotype of Salmonella that can also colonize other hosts and cause human disease. In this study, we compared the genomes of outbreak associated non-outbreak causing Salmonella ser. Heidelberg strains from diverse hosts and geographical regions. Human outbreak associated strains in this study were from a 2015 multistate outbreak of Salmonella ser. Heidelberg involving 15 states in the United States which originated from bull calves. Our clinicopathologic examination revealed that cases involving Salmonella ser. Heidelberg strains were predominantly young, less than weeks-old, dairy calves. Pre-existing or concurrent disease was found in the majority of the calves. Detection of Salmonella ser. Heidelberg correlated with markedly increased death losses clinically comparable to those seen in herds infected with S. Dublin, a known serious pathogen of cattle. Whole genome based single nucleotide polymorphism based analysis revealed that these calf isolates formed a distinct cluster along with outbreak associated human isolates. The defining feature of the outbreak associated strains, when compared to older isolates of S. Heidelberg, is that all isolates in this cluster contained Saf fimbrial genes which are generally absent in S. Heidelberg. The acquisition of several single nucleotide polymorphisms and the gain of Saf fimbrial genes may have contributed to the increased disease severity of these Salmonella ser. Heidelberg strains.

Model of Persistent Salmonella Infection: Salmonella enterica Serovar Pullorum Modulates the Immune Response of the Chicken from a Th17-Type Response towards a Th2-Type Response

Salmonella enterica infection affects a wide range of animals and humans, and a small number of serovars cause typhoid-like infections, one characteristic of which is persistent infection in convalescents. Avian-specific S. enterica serovar Pullorum produces systemic disease in young chickens, which is followed by a carrier state in convalescent birds, leading to infection of the ovary at sexual maturity and vertical transmission. However, the immunological basis of persistent infection remains unclear. S. enterica serovar Enteritidis is taxonomically closely related but does not show this characteristic. Differences in the immune responses between S Pullorum and S Enteritidis were compared by using Salmonella-infected chicken monocyte-derived macrophages (chMDMs) and CD4⁺ T lymphocytes that had been cocultured with infected chMDMs or chicken splenocytes in vitro and also in 2-day-old chickens in vivo In comparison with S Enteritidis, S Pullorum-infected chMDMs showed reduced mRNA expression levels of interleukin-12α (IL-12α) and IL-18 and stimulated the proliferation of Th2 lymphocytes, with reduced expression of gamma interferon (IFN-γ) and IL-17 and increased expression levels of IL-4 and IL-13 There was little evidence of clonal anergy or immune suppression induced by S Pullorum in vitro. S Pullorum also increased the levels of expression of IL-4 and decreased the levels of IFN-γ in the spleen and cecal tonsil of infected birds. This suggests that S Pullorum is able to modulate host immunity from a dominant IFN-γ-producing Th17 response toward a Th2 response, which may promote persistent infection in chickens. S Pullorum in chickens is presented as a good model of the typhoid group to study persistent infection.

Clonal analysis of Salmonella-specific effector T cells reveals serovar-specific and cross-reactive T cell responses

To tackle the complexity of cross-reactive and pathogen-specific T cell responses against related Salmonella serovars, we used mass cytometry, unbiased single-cell cloning, live fluorescence barcoding, and T cell-receptor sequencing to reconstruct the Salmonella-specific repertoire of circulating effector CD4⁺ T cells, isolated from volunteers challenged with Salmonella enterica serovar Typhi (S. Typhi) or Salmonella Paratyphi A (S. Paratyphi). We describe the expansion of cross-reactive responses against distantly related Salmonella serovars and of clonotypes recognizing immunodominant antigens uniquely expressed by S. Typhi or S. Paratyphi A. In addition, single-amino acid variations in two immunodominant proteins, CdtB and PhoN, lead to the accumulation of T cells that do not cross-react against the different serovars, thus demonstrating how minor sequence variations in a complex microorganism shape the pathogen-specific T cell repertoire. Our results identify immune-dominant, serovar-specific, and cross-reactive T cell antigens, which should aid in the design of T cell-vaccination strategies against Salmonella.

Analysis of Spleen-Induced Fimbria Production in Recombinant Attenuated Salmonella enterica Serovar Typhimurium Vaccine Strains

Salmonella enterica serovar Typhimurium genome encodes 13 fimbrial operons. Most of the fimbriae encoded by these operons are not produced under laboratory conditions but are likely to be synthesized in vivo. We used an in vivo expression technology (IVET) strategy to identify four fimbrial operons, agf, saf, sti, and stc that are expressed in the spleen. When any three of these operons were deleted, the strain retained wild-type virulence. However, when all four operons were deleted, the resulting strain was completely attenuated, indicating that these four fimbriae play functionally redundant roles critical for virulence. In mice, oral doses of as low as 1 × 10⁵ CFU of the strain with four fimbrial operons deleted provided 100% protection against challenge with 1 × 10⁹ CFU of wild-type S. Typhimurium. We also examined the possible effect of these fimbriae on the ability of a Salmonella vaccine strain to deliver a guest antigen. We modified one of our established attenuated vaccine strains, χ9088, to delete three fimbrial operons while the fourth operon was constitutively expressed. Each derivative was modified to express the Streptococcus pneumoniae antigen PspA. Strains that constitutively expressed saf or stc elicited a strong Th1 response with significantly greater levels of anti-PspA serum IgG and greater protective efficacy than strains carrying saf or stc deletions. The isogenic strain in which all four operons were deleted generated the lowest anti-PspA levels and did not protect against challenge with virulent S. pneumoniae. Our results indicate that these fimbriae play important roles, as yet not understood, in Salmonella virulence and immunogenicity.

Salmonella enterica is the leading cause of bacterial food-borne infection in the United States. S. Typhimurium is capable of producing up to 13 distinct surface structures called fimbriae that presumably mediate its adherence to surfaces. The roles of most of these fimbriae in disease are unknown. Identifying fimbriae produced during infection will provide important insights into how these bacterial structures contribute to disease and potentially induce protective immunity to Salmonella infection. We identified four fimbriae that are produced during infection. Deletion of all four of these fimbriae results in a significant reduction in virulence. We explored ways in which the expression of these fimbriae may be exploited for use in recombinant Salmonella vaccine strains and found that production of Saf and Stc fimbriae are important for generating a strong immune response against a vectored antigen. This work provides new insight into the role of fimbriae in disease and their potential for improving the efficacy of Salmonella-based vaccines.

Importance of Salmonella Typhi-Responsive CD8+ T Cell Immunity in a Human Typhoid Fever Challenge Model

Typhoid fever, caused by the human-restricted organism Salmonella enterica serovar Typhi (S. Typhi), constitutes a major global health problem. The development of improved attenuated vaccines is pressing, but delayed by the lack of appropriate preclinical models. Herein, we report that high levels of S. Typhi-responsive CD8+ T cells at baseline significantly correlate with an increased risk of disease in humans challenged with a high dose (~10⁴ CFU) wild-type S. Typhi. Typhoid fever development was associated with higher multifunctional S. Typhi-responsive CD8+ T effector memory cells at baseline. Early decreases of these cells in circulation following challenge were observed in both S. Typhi-responsive integrin α₄β₇− and integrin α₄β₇+ CD8+ T effector memory (T_EM) cells, suggesting their potential to home to both mucosal and extra-intestinal sites. Participants with higher baseline levels of S. Typhi-responsive CD8+ T memory cells had a higher risk of acquiring disease, but among those who acquired disease, those with a higher baseline responses took longer to develop disease. In contrast, protection against disease was associated with low or absent S. Typhi-responsive T cells at baseline and no changes in circulation following challenge. These data highlight the importance of pre-existing S. Typhi-responsive immunity in predicting clinical outcome following infection with wild-type S. Typhi and provide novel insights into the complex mechanisms involved in protective immunity to natural infection in a stringent human model with a high challenge dose. They also contribute important information on the immunological responses to be assessed in the appraisal and selection of new generation typhoid vaccines.

Live Attenuated Human Salmonella Vaccine Candidates: Tracking the Pathogen in Natural Infection and Stimulation of Host Immunity

Salmonellosis, caused by members of the genus Salmonella, is responsible for considerable global morbidity and mortality in both animals and humans. In this review, we will discuss the pathogenesis of Salmonella enterica serovar Typhi and Salmonella enterica serovar Typhimurium, focusing on human Salmonella infections. We will trace the path of Salmonella through the body, including host entry sites, tissues and organs affected, and mechanisms involved in both pathogenesis and stimulation of host immunity. Careful consideration of the natural progression of disease provides an important context in which attenuated live oral vaccines can be rationally designed and developed. With this in mind, we will describe a series of attenuated live oral vaccines that have been successfully tested in clinical trials and demonstrated to be both safe and highly immunogenic. The attenuation strategies summarized in this review offer important insights into further development of attenuated vaccines against other Salmonella for which live oral candidates are currently unavailable.

Cross-reactive multifunctional CD4+ T cell responses against Salmonella enterica serovars Typhi, Paratyphi A and Paratyphi B in humans following immunization with live oral typhoid vaccine Ty21a

The live oral typhoid vaccine Ty21a elicits predominantly CD8+, as well as CD4+ T cells mediated immune responses. Clinical field studies showed that Ty21a is moderately effective against S. Typhi and S. Paratyphi B, but not S. Paratyphi A infections. In this study we describe the in depth characterization of S. Typhi, S. Paratyphi A and S. Paratyphi B cross-reactive CD4+ T cell responses elicited following immunization with Ty21a. PBMC samples were collected from 16 healthy volunteers before and 42/84days after Ty21a immunization and stimulated ex-vivo with Salmonella-infected targets. Multiparametric flow cytometry was used to detect the vaccine elicited Salmonella-specific responses in T effector/memory (T_EM) and CD45RA+ T effector/memory (T_EMRA) CD4+ cell subsets, by measuring CD4+ multifunctional (MF) cells that concomitantly produced IFN-γ, TNF-α, IL-2, MIP-1β, IL-17A and/or expressed CD107a. Post-vaccination increases in S. Typhi-specific MF cells were observed in CD4+ T_EM and T_EMRA subsets which predominantly produced IFN-γ and/or TNF-α, while IL-2 was produced by a smaller cell subset. A small proportion of those MF cells also produced MIP-1β, IL-17A and expressed CD107a (a marker associated with cytotoxicity). Approximately one third of these specific MF cells have the potential to migrate to the gut mucosa, as evidenced by co-expression of the gut-homing molecule integrin α₄β₇. In contrast to our previous observations with CD8+ T cells, MF CD4+ T cell responses to the different Salmonella serovars evaluated were similar in magnitude and characteristics. We conclude that although induction of cross-reactive CD4+ MF effector T cells suggest a possible role in Salmonella-immunity, these responses are unlikely to provide an immunological basis for the observed efficacy of Ty21a against S. Typhi and S. Paratyphi B, but not to S. Paratyphi A.

Cryptosporidium Priming Is More Effective than Vaccine for Protection against Cryptosporidiosis in a Murine Protein Malnutrition Model

Cryptosporidium is a major cause of severe diarrhea, especially in malnourished children. Using a murine model of C. parvum oocyst challenge that recapitulates clinical features of severe cryptosporidiosis during malnutrition, we interrogated the effect of protein malnutrition (PM) on primary and secondary responses to C. parvum challenge, and tested the differential ability of mucosal priming strategies to overcome the PM-induced susceptibility. We determined that while PM fundamentally alters systemic and mucosal primary immune responses to Cryptosporidium, priming with C. parvum (10⁶ oocysts) provides robust protective immunity against re-challenge despite ongoing PM. C. parvum priming restores mucosal Th1-type effectors (CD3⁺CD8⁺CD103⁺ T-cells) and cytokines (IFNγ, and IL12p40) that otherwise decrease with ongoing PM. Vaccination strategies with Cryptosporidium antigens expressed in the S. Typhi vector 908htr, however, do not enhance Th1-type responses to C. parvum challenge during PM, even though vaccination strongly boosts immunity in challenged fully nourished hosts. Remote non-specific exposures to the attenuated S. Typhi vector alone or the TLR9 agonist CpG ODN-1668 can partially attenuate C. parvum severity during PM, but neither as effectively as viable C. parvum priming. We conclude that although PM interferes with basal and vaccine-boosted immune responses to C. parvum, sustained reductions in disease severity are possible through mucosal activators of host defenses, and specifically C. parvum priming can elicit impressively robust Th1-type protective immunity despite ongoing protein malnutrition. These findings add insight into potential correlates of Cryptosporidium immunity and future vaccine strategies in malnourished children.

Cryptosporidium attributable morbidities in malnourished children are increasingly recognized. Exactly how malnutrition interferes with host mucosal immunity to diarrheal pathogens and mucosal vaccine responses remains unclear. Dissecting these interactions in an experimental model of cryptosporidiosis can uncover new insights into novel therapeutic approaches against a pathogen for which effective therapies and vaccines are currently unavailable. We demonstrate that although malnutrition diminishes baseline (primary) Th1-type mucosal immunity these deficits can be partially overcome via non-specific mucosal strategies (S. Typhi and CpG) and completely restored after a sub-clinical (low-dose) exposure to viable C. parvum. These results add insight into preventive strategies to help alleviate Cryptosporidium-specific diarrhea in children in low-resource settings and abrogate prolonged post-infection sequelae.

Salmonella Typhi-specific multifunctional CD8+ T cells play a dominant role in protection from typhoid fever in humans

BACKGROUND

Typhoid fever, caused by the human-restricted organism Salmonella Typhi (S. Typhi), is a major public health problem worldwide. Development of novel vaccines remains imperative, but is hampered by an incomplete understanding of the immune responses that correlate with protection.

METHODS

Recently, a controlled human infection model was re-established in which volunteers received ~10(3) cfu wild-type S. Typhi (Quailes strain) orally. Twenty-one volunteers were evaluated for their cell-mediated immune (CMI) responses. Ex vivo PBMC isolated before and up to 1 year after challenge were exposed to three S. Typhi-infected targets, i.e., autologous B lymphoblastoid cell-lines (B-LCL), autologous blasts and HLA-E restricted AEH B-LCL cells. CMI responses were evaluated using 14-color multiparametric flow cytometry to detect simultaneously five intracellular cytokines/chemokines (i.e., IL-17A, IL-2, IFN-g, TNF-a and MIP-1b) and a marker of degranulation/cytotoxic activity (CD107a).

RESULTS

Herein we provide the first evidence that S. Typhi-specific CD8+ responses correlate with clinical outcome in humans challenged with wild-type S. Typhi. Higher multifunctional S. Typhi-specific CD8+ baseline responses were associated with protection against typhoid and delayed disease onset. Moreover, following challenge, development of typhoid fever was accompanied by decreases in circulating S. Typhi-specific CD8+ T effector/memory (TEM) with gut homing potential, suggesting migration to the site(s) of infection. In contrast, protection against disease was associated with low or no changes in circulating S. Typhi-specific TEM.

CONCLUSIONS

These studies provide novel insights into the protective immune responses against typhoid disease that will aid in selection and development of new vaccine candidates.

Immunization with Ty21a live oral typhoid vaccine elicits crossreactive multifunctional CD8+ T-cell responses against Salmonella enterica serovar Typhi, S. Paratyphi A, and S. Paratyphi B in humans

Previously we have extensively characterized Salmonella enterica serovar Typhi (S. Typhi)-specific cell-mediated immune (CMI) responses in volunteers orally immunized with the licensed Ty21a typhoid vaccine. In this study we measured Salmonella-specific multifunctional (MF) CD8+ T-cell responses to further investigate whether Ty21a elicits crossreactive CMI against S. Paratyphi A and S. Paratyphi B that also cause enteric fever. Ty21a-elicited crossreactive CMI responses against all three Salmonella serotypes were predominantly observed in CD8+ T effector/memory (T(EM)) and, to a lesser extent, in CD8+CD45RA+ T(EM) (T(EMRA)) subsets. These CD8+ T-cell responses were largely mediated by MF cells coproducing interferon-γ and macrophage inflammatory protein-1β and expressing CD107a with or without tumor necrosis factor-α. Significant proportions of Salmonella-specific MF cells expressed the gut-homing molecule integrin α4β7. In most subjects, similar MF responses were observed to S. Typhi and S. Paratyphi B, but not to S. Paratyphi A. These results suggest that Ty21a elicits MF CMI responses against Salmonella that could be critical in clearing the infection. Moreover, because S. Paratyphi A is a major public concern and Ty21a was shown in field studies not to afford cross-protection to S. Paratyphi A, these results will be important in developing a S. Typhi/S. Paratyphi A bivalent vaccine against enteric fevers.

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.