Generation of specific effector and memory T cells with gut- and secondary lymphoid tissue- homing potential by oral attenuated CVD 909 typhoid vaccine in humans

Prevention and Control of Human Salmonella enterica Infections: An Implication in Food Safety

Salmonella is a foodborne zoonotic pathogen causing diarrhoeal disease to humans after consuming contaminated water, animal, and plant products. The bacterium is the third leading cause of human death among diarrhoeal diseases worldwide. Therefore, human salmonellosis is of public health concern demanding integrated interventions against the causative agent, Salmonella enterica. The prevention of salmonellosis in humans is intricate due to several factors, including an immune-stable individual infected with S. enterica continuing to shed live bacteria without showing any clinical signs. Similarly, the asymptomatic Salmonella animals are the source of salmonellosis in humans after consuming contaminated food products. Furthermore, the contaminated products of plant and animal origin are a menace in food industries due to Salmonella biofilms, which enhance colonization, persistence, and survival of bacteria on equipment. The contaminated food products resulting from bacteria on equipment offset the economic competition of food industries and partner institutions in international business. The most worldwide prevalent broad-range Salmonella serovars affecting humans are Salmonella Typhimurium and Salmonella Enteritidis, and poultry products, among others, are the primary source of infection. The broader range of Salmonella serovars creates concern over multiple strategies for preventing and controlling Salmonella contamination in foods to enhance food safety for humans. Among the strategies for preventing and controlling Salmonella spread in animal and plant products include biosecurity measures, isolation and quarantine, epidemiological surveillance, farming systems, herbs and spices, and vaccination. Other measures are the application of phages, probiotics, prebiotics, and nanoparticles reduced and capped with antimicrobial agents. Therefore, Salmonella-free products, such as beef, pork, poultry meat, eggs, milk, and plant foods, such as vegetables and fruits, will prevent humans from Salmonella infection. This review explains Salmonella infection in humans caused by consuming contaminated foods and the interventions against Salmonella contamination in foods to enhance food safety and quality for humans.

Controlled human infectious models, a path forward in uncovering immunological correlates of protection: Lessons from enteric fevers studies

Enteric infectious diseases account for more than a billion disease episodes yearly worldwide resulting in approximately 2 million deaths, with children under 5 years old and the elderly being disproportionally affected. Enteric pathogens comprise viruses, parasites, and bacteria; the latter including pathogens such as Salmonella [typhoidal (TS) and non-typhoidal (nTS)], cholera, Shigella and multiple pathotypes of Escherichia coli (E. coli). In addition, multi-drug resistant and extensively drug-resistant (XDR) strains (e.g., S. Typhi H58 strain) of enteric bacteria are emerging; thus, renewed efforts to tackle enteric diseases are required. Many of these entero-pathogens could be controlled by oral or parenteral vaccines; however, development of new, effective vaccines has been hampered by lack of known immunological correlates of protection (CoP) and limited knowledge of the factors contributing to protective responses. To fully comprehend the human response to enteric infections, an invaluable tool that has recently re-emerged is the use of controlled human infection models (CHIMs) in which participants are challenged with virulent wild-type (wt) organisms. CHIMs have the potential to uncover immune mechanisms and identify CoP to enteric pathogens, as well as to evaluate the efficacy of therapeutics and vaccines in humans. CHIMs have been used to provide invaluable insights in the pathogenesis, host-pathogen interaction and evaluation of vaccines. Recently, several Oxford typhoid CHIM studies have been performed to assess the role of multiple cell types (B cells, CD8+ T, Tregs, MAIT, Monocytes and DC) during S. Typhi infection. One of the key messages that emerged from these studies is that baseline antigen-specific responses are important in that they can correlate with clinical outcomes. Additionally, volunteers who develop typhoid disease (TD) exhibit higher levels and more activated cell types (e.g., DC and monocytes) which are nevertheless defective in discrete signaling pathways. Future critical aspects of this research will involve the study of immune responses to enteric infections at the site of entry, i.e., the intestinal mucosa. This review will describe our current knowledge of immunity to enteric fevers caused byS. Typhi and S. Paratyphi A, with emphasis on the contributions of CHIMs to uncover the complex immunological responses to these organisms and provide insights into the determinants of protective immunity.

CD4+ T cell immunity to Salmonella is transient in the circulation

While Salmonella enterica is seen as an archetypal facultative intracellular bacterial pathogen where protection is mediated by CD4⁺ T cells, identifying circulating protective cells has proved very difficult, inhibiting steps to identify key antigen specificities. Exploiting a mouse model of vaccination, we show that the spleens of C57BL/6 mice vaccinated with live-attenuated Salmonella serovar Typhimurium (S. Typhimurium) strains carried a pool of IFN-γ⁺ CD4⁺ T cells that could adoptively transfer protection, but only transiently. Circulating Salmonella-reactive CD4⁺ T cells expressed the liver-homing chemokine receptor CXCR6, accumulated over time in the liver and assumed phenotypic characteristics associated with tissue-associated T cells. Liver memory CD4⁺ T cells showed TCR selection bias and their accumulation in the liver could be inhibited by blocking CXCL16. These data showed that the circulation of CD4⁺ T cells mediating immunity to Salmonella is limited to a brief window after which Salmonella-specific CD4⁺ T cells migrate to peripheral tissues. Our observations highlight the importance of triggering tissue-specific immunity against systemic infections.

Helper T cells are essential for controlling infections by bacterial pathogens, such as Salmonella enterica var Typhimurium (S. Typhimurium). While it is well-established that this role is related to their provision of IFN-γ, when and where helper T cells elicit their protective function in vivo remains unresolved. We identified a protective helper T cell population in the circulation of mice early after inoculation with growth-attenuated S. Typhimurium strains; this population waned overtime. We observed that circulating helper T cell immunity can adoptively protect naïve recipient mice against lethal S. Typhimurium infection when harvested from a short time-window. In comparing helper T cell responses between spleen and liver in Salmonella-infected mice, we have observed a previously uncharacterized trafficking of helper T cells to the liver followed by the residence of S. Typhimurium-specific T cell memory in the organ. Taken together these findings identify that protective immunity to Salmonella infections is transient in the circulation and the liver as a preferential site of helper T memory cells.

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.

B and T Cell Immunity in Tissues and Across the Ages

B and T cells are key components of the adaptive immune system and coordinate multiple facets of immunity including responses to infection, vaccines, allergens, and the environment. In humans, B- and T-cell immunity has been determined using primarily peripheral blood specimens. Conversely, human tissues have scarcely been studied but they host multiple adaptive immune cells capable of mounting immune responses to pathogens and participate in tissue homeostasis. Mucosal tissues, such as the intestines and respiratory track, are constantly bombarded by foreign antigens and contain tissue-resident memory T (T_RM) cells that exhibit superior protective capacity to pathogens. Also, tissue-resident memory B (B_RM) cells have been identified in mice but whether humans have a similar population remains to be confirmed. Moreover, the immune system evolves throughout the lifespan of humans and undergoes multiple changes in its immunobiology. Recent studies have shown that age-related changes in tissues are not necessarily reflected in peripheral blood specimens, highlighting the importance of tissue localization and subset delineation as essential determinants of functional B and T cells at different life stages. This review describes our current knowledge of the main B- and T-cell subsets in peripheral blood and tissues across age groups.

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.

Effect of the live oral attenuated typhoid vaccine, Ty21a, on systemic and terminal ileum mucosal CD4+ T memory responses in humans

Our current understanding of CD4+ T-cell-mediated immunity (CMI) elicited by the oral live attenuated typhoid vaccine Ty21a is primarily derived from studies using peripheral blood. Very limited data are available in humans regarding mucosal immunity (especially CD4+ T) at the site of infection (e.g. terminal ileum; TI). Here using multiparametric flow cytometry, we examined the effect of Ty21a immunization on TI-lamina propria mononuclear cells (LPMC) and peripheral blood CD4+ T memory (TM) subsets in volunteers undergoing routine colonoscopy. Interestingly, we observed significant increases in the frequencies of LPMC CD4+ T cells following Ty21a immunization, restricted to the T effector/memory (TEM)-CD45RA+ (TEMRA) subset. Importantly, Ty21a immunization elicited Salmonella Typhi-responsive LPMC CD4+ T cells in all major TM subsets [interferon (IFN)γ and interleukin (IL)-17A in TEM; IFNγ and macrophage inflammatory protein (MIP)1β in T central/memory (TCM); and IL-2 in TEMRA]. Subsequently, we analyzed LPMC S. Typhi-responsive CD4+ T cells in depth for multifunctional (MF) effectors. We found that LPMC CD4+ TEM responses were mostly MF, except for those cells exhibiting the characteristics associated with IL-17A responses. Finally, we compared mucosal to systemic responses and observed that LPMC CD4+S. Typhi-specific responses were unique and distinct from their systemic counterparts. This study provides the first demonstration of S. Typhi-specific CD4+ TM responses in the human TI mucosa and provides valuable information about the generation of mucosal immune responses following oral Ty21a immunization.

Differential functional patterns of memory CD4+ and CD8+ T-cells from volunteers immunized with Ty21a typhoid vaccine observed using a recombinant Escherichia coli system expressing S. Typhi proteins

It is widely accepted that CD4⁺ and CD8⁺ T-cells play a significant role in protection against Salmonella enterica serovar Typhi (S. Typhi), the causative agent of the typhoid fever. However, the antigen specificity of these T-cells remains largely unknown. Previously, we demonstrated the feasibility of using a recombinant Escherichia coli (E. coli) expression system to uncover the antigen specificity of CD4⁺ and CD8⁺ T cells. Here, we expanded these studies to include the evaluation of 12 additional S. Typhi proteins: 4 outer membrane proteins (OmpH, OmpL, OmpR, OmpX), 3 Vi-polysaccharide biosynthesis proteins (TviA, TviB, TviE), 3 cold shock proteins (CspA, CspB, CspC), and 2 conserved hypothetical proteins (Chp 1 and Chp2), all selected based on the bioinformatic analyses of the content of putative T-cell epitopes. CD4⁺ and CD8⁺ T cells from 15 adult volunteers, obtained before and 42 days after immunization with oral live attenuated Ty21a vaccine, were assessed for their functionality (i.e., production of cytokines and cytotoxic expression markers in response to stimulation with selected antigens) as measured by flow cytometry. Although volunteers differed on their T-cell antigen specificity, we observed T-cell immune responses against all S. Typhi proteins evaluated. These responses included 9 proteins, OmpH, OmpR, TviA, TviE, CspA, CspB, CspC, Chp 1 and Chp 2, which have not been previously reported to elicit T-cell responses. Interestingly, we also observed that, regardless of the protein, the functional patterns of the memory T-cells were different between CD4⁺ and CD8⁺ T cells. In sum, these studies demonstrated the feasibility of using bioinformatic analysis and the E. coli expressing system described here to uncover novel immunogenic T-cell proteins that could serve as potential targets for the production of protein-based vaccines.

Association between S. Typhi-specific memory CD4+ and CD8+ T responses in the terminal ileum mucosa and in peripheral blood elicited by the live oral typhoid vaccine Ty21a in humans

CD4+ and CD8+ T subsets are essential components of the adaptive immune system which act in concert at the site of infections to effectively protect against pathogens. Very limited data is available in humans regarding the relationship between CD4+ and CD8+ S. Typhi responsive cells in the terminal ileum mucosa (TI) and peripheral blood following Ty21a oral typhoid immunization. Here, we compared TI lamina propria mononuclear cells (LPMC) and peripheral blood CD4+ and CD8+ T memory (T_M) subsets responses and their relationship by Spearman’s correlation following Ty21a immunization in volunteers undergoing routine colonoscopy. We observed that Ty21a immunization (i) influences the homing and accumulation of both CD4+ and CD8+ T cells in the TI, particularly integrin α4β7+ CCR9+ CD8+ T cells, (ii) elicits significantly higher frequencies of LPMC S. Typhi-responsive CD8+ T multifunctional (CD107a, IFNγ, IL-17A and/or MIP1β) cells than their CD4+ T counterparts, and (iii) results in the correlation of LPMC CD4+ Teffector/memory (T_EM) S. Typhi responses (CD107a, IFNγ, TNFα, IL-17A and/or MIP1β) to their LPMC CD8+ T_EM counterparts. Moreover, we demonstrated that these positive correlations between CD4+ and CD8+ T_EM occur primarily in TI LPMC but not in PBMC, suggesting important differences in responses between the mucosal and systemic compartments following oral Ty21a immunization. This study provides the first demonstration of the correlation of S. Typhi-specific CD4+ and CD8+ T_M responses in the human terminal ileum mucosa and provides valuable information regarding the generation of mucosal and systemic immune responses following oral Ty21a-immunization which might impact future vaccine design and development.

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.

Longevity of duodenal and peripheral T-cell and humoral responses to live-attenuated Salmonella Typhi strain Ty21a

BACKGROUND

We have previously demonstrated that polyfunctional Ty21a-responsive CD4+ and CD8+ T cells are generated at the duodenal mucosa 18 days following vaccination with live-attenuated S. Typhi (Ty21a). The longevity of cellular responses has been assessed in peripheral blood, but persistence of duodenal responses is unknown.

METHODS

We vaccinated eight healthy adults with Ty21a. Peripheral blood and duodenal samples were acquired after a median of 1.5 years (ranging from 1.1 to 3.7 years) following vaccination. Cellular responses were assessed in peripheral blood and at the duodenal mucosa by flow cytometry. Levels of IgG and IgA were also assessed in peripheral blood by enzyme-linked immunosorbent assay.

RESULTS

No T-cell responses were observed at the duodenal mucosa, but CD4+ T-cell responses to Ty21a and FliC were observed in peripheral blood. Peripheral anti-lipopolysaccharide IgG and IgA responses were also observed. Early immunoglobulin responses were not associated with the persistence of long-term cellular immune responses.

CONCLUSIONS

Early T-cell responses which we have previously observed at the duodenal mucosa 18 days following oral vaccination with Ty21a could not be detected at a median of 1.5 years. Peripheral responses were observed at this time. Immunoglobulin responses observed shortly after vaccination were not associated with cellular immune responses at 1.5 years, suggesting that the persistence of cellular immunity is not associated with the strength of the initial humoral response to vaccination.

Use of a novel antigen expressing system to study the Salmonella enterica serovar Typhi protein recognition by T cells

Salmonella enterica serovar Typhi (S. Typhi), the causative agent of the typhoid fever, is a pathogen of great public health importance. Typhoid vaccines have the potential to be cost-effective measures towards combating this disease, yet the antigens triggering host protective immune responses are largely unknown. Given the key role of cellular-mediated immunity in S. Typhi protection, it is crucial to identify S. Typhi proteins involved in T-cell responses. Here, cells from individuals immunized with Ty21a typhoid vaccine were collected before and after immunization and used as effectors. We also used an innovative antigen expressing system based on the infection of B-cells with recombinant Escherichia coli (E. coli) expressing one of four S. Typhi gene products (i.e., SifA, OmpC, FliC, GroEL) as targets. Using flow cytometry, we found that the pattern of response to specific S. Typhi proteins was variable. Some individuals responded to all four proteins while others responded to only one or two proteins. We next evaluated whether T-cells responding to recombinant E. coli also possess the ability to respond to purified proteins. We observed that CD4⁺ cell responses, but not CD8⁺ cell responses, to recombinant E. coli were significantly associated with the responses to purified proteins. Thus, our results demonstrate the feasibility of using an E. coli expressing system to uncover the antigen specificity of T-cells and highlight its applicability to vaccine studies. These results also emphasize the importance of selecting the stimuli appropriately when evaluating CD4⁺ and CD8⁺ cell responses.

Salmonella enterica serovar Typhi (S. Typhi) is the causative agent of the life-threatening typhoid fever that affects 11.9–20.6 million individuals annually in low-income and middle-income countries. The T-cells, CD4⁺ and CD8⁺ T cells, play a significant role in protection against S. Typhi infection. Yet, the antigens triggering host protective immune responses recognized by these cells are largely unknown. To address this shortcoming, in this study we used an E. coli expression system methodology for identifying immunogenic proteins of S. Typhi. We found that although the pattern of response to individual S. Typhi proteins was variable among the typhoid vaccinees, the E. coli expressing system uncovered the antigen specificity of T-cells, and highlight its applicability to vaccine studies.

Waning Immunity and Microbial Vaccines-Workshop of the National Institute of Allergy and Infectious Diseases

Since the middle of the 20th century, vaccines have made a significant public health impact by controlling infectious diseases globally. Although long-term protection has been achieved with some vaccines, immunity wanes over time with others, resulting in outbreaks or epidemics of infectious diseases. Long-term protection against infectious agents that have a complex life cycle and antigenic variation remains a key challenge. Novel strategies to characterize the short- and long-term immune responses to vaccines and to induce immune responses that mimic natural infection have recently emerged. New technologies and approaches in vaccinology, such as adjuvants, delivery systems, and antigen formulations, have the potential to elicit more durable protection and fewer adverse reactions; together with in vitro systems, these technologies have the capacity to model and accelerate vaccine development. The National Institute of Allergy and Infectious Diseases (NIAID) held a workshop on 19 September 2016 that focused on waning immunity to selected vaccines (for Bordetella pertussis, Salmonella enterica serovar Typhi, Neisseria meningitidis, influenza, mumps, and malaria), with an emphasis on identifying knowledge gaps, future research needs, and how this information can inform development of more effective vaccines for infectious diseases.

Vaccination against Salmonella Infection: the Mucosal Way

Salmonella enterica subspecies enterica includes several serovars infecting both humans and other animals and leading to typhoid fever or gastroenteritis. The high prevalence of associated morbidity and mortality, together with an increased emergence of multidrug-resistant strains, is a current global health issue that has prompted the development of vaccination strategies that confer protection against most serovars. Currently available systemic vaccine approaches have major limitations, including a reduced effectiveness in young children and a lack of cross-protection among different strains. Having studied host-pathogen interactions, microbiologists and immunologists argue in favor of topical gastrointestinal administration for improvement in vaccine efficacy. Here, recent advances in this field are summarized, including mechanisms of bacterial uptake at the intestinal epithelium, the assessment of protective host immunity, and improved animal models that closely mimic infection in humans. The pros and cons of existing vaccines are presented, along with recent progress made with novel formulations. Finally, new candidate antigens and their relevance in the refined design of anti-Salmonella vaccines are discussed, along with antigen vectorization strategies such as nanoparticles or secretory immunoglobulins, with a focus on potentiating mucosal vaccine efficacy.

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.

Impact of CD4+ T Cell Responses on Clinical Outcome following Oral Administration of Wild-Type Enterotoxigenic Escherichia coli in Humans

Enterotoxigenic Escherichia coli (ETEC) is a non-invasive enteric pathogen of considerable public health importance, being one of the most common attributable causes of diarrheal illness in infants and young children in developing countries and the most common cause of traveler’s diarrhea. To enhance study-to-study consistency of our experimental challenge model of ETEC in volunteers, and to allow concomitant multi-site trials to evaluate anti-ETEC immunoprophylactic products, hundreds of vials, each containing a standardized inoculum of virulent wild-type (wt) ETEC strain H10407 (serotype O78:H11 expressing colonization factor antigen I and heat-labile and heat-stable enterotoxins), were prepared under current Good Manufacturing Practices (cGMP) and frozen. Following thawing, the contents of each vial can be used (diluted as necessary) to prepare consistent challenge inoculum, even at different study sites. A preliminary human experimental challenge study using this cGMP inoculum was conducted on a research isolation ward and the clinical and cell-mediated immune responses evaluated. Of the 6 healthy adult volunteers challenged 83% (5/6) developed diarrhea and 50% developed moderate-to-severe diarrhea (MSD). Moderate and severe diarrhea were defined as passage of ≥ 1 liter or ≥ 3 liters of diarrheal stool respectively. We compared the CD4+ T cell responses of volunteers who developed MSD against those who did not and identified significant differences in ETEC-specific cytokine production and gut homing potential. We furthermore demonstrated that increased expression of the gut-homing molecule integrin α4β7 by peripheral T follicular helper cells (pT_fh) correlated with decreased stool volume and increased ETEC-specific IgA B memory cell (B_M) development. Collectively, despite small numbers of volunteers, our results indicate a potential role for CD4+ T cells, in particular pT_fh, in modulating disease outcome following exposure to wt ETEC in a volunteer experimental challenge model.

Enterotoxigenic Escherichia coli (ETEC) is an important cause of diarrheal illness in infants and young children in the developing world, as well as in individuals traveling to endemic areas. Due to the lack of suitable animal models for human ETEC infection, we performed a human challenge study in which volunteers ingested wild-type ETEC in a controlled clinical setting. In addition to closely monitoring their clinical status, we studied their ETEC-specific T cell responses prior to and after challenge and studied the presence of associations between CD4+ T cell responses and clinical outcome. We observed differences in the immunological responses of individuals who developed moderate to severe diarrhea following challenge compared to those who did not. These results indicate that T cells may be an important component of the immune response against ETEC.

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.

Refined live attenuated Salmonella enterica serovar Typhimurium and Enteritidis vaccines mediate homologous and heterologous serogroup protection in mice

Invasive nontyphoidal Salmonella (NTS) infections constitute a major health problem among infants and toddlers in sub-Saharan Africa; these infections also occur in infants and the elderly in developed countries. We genetically engineered a Salmonella enterica serovar Typhimurium strain of multilocus sequence type 313, the predominant genotype circulating in sub-Saharan Africa. We evaluated the capacities of S. Typhimurium and Salmonella enterica serovar Enteritidis ΔguaBA ΔclpX live oral vaccines to protect mice against a highly lethal challenge dose of the homologous serovar and determined protection against other group B and D serovars circulating in sub-Saharan Africa. The vaccines S. Typhimurium CVD 1931 and S. Enteritidis CVD 1944 were immunogenic and protected BALB/c mice against 10,000 50% lethal doses (LD50) of S. Typhimurium or S. Enteritidis, respectively. S. Typhimurium CVD 1931 protected mice against the group B serovar Salmonella enterica serovar Stanleyville (91% vaccine efficacy), and S. Enteritidis CVD 1944 protected mice against the group D serovar Salmonella enterica serovar Dublin (85% vaccine efficacy). High rates of survival were observed when mice were infected 12 weeks postimmunization, indicating that the vaccines elicited long-lived protective immunity. Whereas CVD 1931 did not protect against S. Enteritidis R11, CVD 1944 did mediate protection against S. Typhimurium D65 (81% efficacy). These findings suggest that a bivalent (S. Typhimurium and S. Enteritidis) vaccine would provide broad protection against the majority of invasive NTS infections in sub-Saharan Africa.

Activation of Salmonella Typhi-specific regulatory T cells in typhoid disease in a wild-type S. Typhi challenge model

Salmonella Typhi (S. Typhi), the causative agent of typhoid fever, causes significant morbidity and mortality worldwide. Currently available vaccines are moderately efficacious, and identification of immunological responses associated with protection or disease will facilitate the development of improved vaccines. We investigated S. Typhi-specific modulation of activation and homing potential of circulating regulatory T cells (Treg) by flow and mass cytometry using specimens obtained from a human challenge study. Peripheral blood mononuclear cells were obtained from volunteers pre- and at multiple time-points post-challenge with wild-type S. Typhi. We identified differing patterns of S. Typhi-specific modulation of the homing potential of circulating Treg between volunteers diagnosed with typhoid (TD) and those who were not (No TD). TD volunteers demonstrated up-regulation of the gut homing molecule integrin α4ß7 pre-challenge, followed by a significant down-regulation post-challenge consistent with Treg homing to the gut. Additionally, S. Typhi-specific Treg from TD volunteers exhibited up-regulation of activation molecules post-challenge (e.g., HLA-DR, LFA-1). We further demonstrate that depletion of Treg results in increased S. Typhi-specific cytokine production by CD8+ TEM in vitro. These results suggest that the tissue distribution of activated Treg, their characteristics and activation status may play a pivotal role in typhoid fever, possibly through suppression of S. Typhi-specific effector T cell responses. These studies provide important novel insights into the regulation of immune responses that are likely to be critical in protection against typhoid and other enteric infectious diseases.

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.

Characterization and functional properties of gastric tissue-resident memory T cells from children, adults, and the elderly

T cells are the main orchestrators of protective immunity in the stomach; however, limited information on the presence and function of the gastric T subsets is available mainly due to the difficulty in recovering high numbers of viable cells from human gastric biopsies. To overcome this shortcoming we optimized a cell isolation method that yielded high numbers of viable lamina propria mononuclear cells (LPMC) from gastric biopsies. Classic memory T subsets were identified in gastric LPMC and compared to peripheral blood mononuclear cells (PBMC) obtained from children, adults, and the elderly using an optimized 14 color flow cytometry panel. A dominant effector memory T (T_EM) phenotype was observed in gastric LPMC CD4⁺ and CD8⁺ T cells in all age groups. We then evaluated whether these cells represented a population of gastric tissue-resident memory T (T_RM) cells by assessing expression of CD103 and CD69. The vast majority of gastric LPMC CD8⁺ T cells either co-expressed CD103/CD69 (>70%) or expressed CD103 alone (~20%). Gastric LPMC CD4⁺ T cells also either co-expressed CD103/CD69 (>35%) or expressed at least one of these markers. Thus, gastric LPMC CD8⁺ and CD4⁺ T cells had the characteristics of T_RM cells. Gastric CD8⁺ and CD4⁺ T_RM cells produced multiple cytokines (IFN-γ, IL-2, TNF-α, IL-17A, MIP-1β) and up-regulated CD107a upon stimulation. However, marked differences were observed in their cytokine and multi-cytokine profiles when compared to their PBMC T_EM counterparts. Furthermore, gastric CD8⁺ T_RM and CD4⁺ T_RM cells demonstrated differences in the frequency, susceptibility to activation, and cytokine/multi-cytokine production profiles among the age groups. Most notably, children’s gastric T_RM cells responded differently to stimuli than gastric T_RM cells from adults or the elderly. In conclusion, we demonstrate the presence of gastric T_RM, which exhibit diverse functional characteristics in children, adults, and the elderly.

Vaccines against invasive Salmonella disease: current status and future directions

Though primarily enteric pathogens, Salmonellae are responsible for a considerable yet under-appreciated global burden of invasive disease. In South and South-East Asia, this manifests as enteric fever caused by serovars Typhi and Paratyphi A. In sub-Saharan Africa, a similar disease burden results from invasive nontyphoidal Salmonellae, principally serovars Typhimurium and Enteritidis. The existing Ty21a live-attenuated and Vi capsular polysaccharide vaccines target S. Typhi and are not effective in young children where the burden of invasive Salmonella disease is highest. After years of lack of investment in new Salmonella vaccines, recent times have seen increased interest in the area led by emerging-market manufacturers, global health vaccine institutes and academic partners. New glycoconjugate vaccines against S. Typhi are becoming available with similar vaccines against other invasive serovars in development. With other new vaccines under investigation, including live-attenuated, protein-based and GMMA vaccines, now is an exciting time for the Salmonella vaccine field.

Live oral Salmonella enterica serovar Typhi vaccines Ty21a and CVD 909 induce opsonophagocytic functional antibodies in humans that cross-react with S. Paratyphi A and S. Paratyphi B

Live oral Salmonella enterica serovar Typhi vaccine Ty21a induces specific antibodies that cross-react against Salmonella enterica serovar Paratyphi A and Salmonella enterica serovar Paratyphi B, although their functional role in clearance remains unknown. We utilized an in vitro assay with THP-1 macrophages to compare the phagocytosis and survival of Salmonella opsonized with heat-inactivated human sera obtained before and after vaccination with Ty21a or a live oral S. Typhi vaccine, CVD 909. Opsonization with postvaccination sera predominantly increased the phagocytosis of S. Typhi relative to the corresponding prevaccination sera, and increases were also observed with S. Paratyphi A and S. Paratyphi B, albeit of lower magnitudes. Relative to prevaccination sera, opsonization with the postvaccination sera reduced the survival inside macrophages of S. Typhi but not of S. Paratyphi A or S. Paratyphi B. Higher anti-S. Typhi O antigen (lipopolysaccharide [LPS]) IgG, but not IgA, antibody titers correlated significantly with postvaccination increases in opsonophagocytosis. No differences were observed between immunization with four doses of Ty21a or one dose of CVD 909. Ty21a and CVD 909 induced cross-reactive functional antibodies, predominantly against S. Typhi. IgG anti-LPS antibodies may be important in phagocytic clearance of these organisms. Therefore, measurement of functional antibodies might be important in assessing the immunogenicity of a new generation of typhoid and paratyphoid A vaccines. (The CVD 909 study has been registered at ClinicalTrials.gov under registration no. NCT00326443.).

B cells modulate mucosal associated invariant T cell immune responses

A common finding when measuring T cell immunity to enteric bacterial vaccines in humans is the presence of background responses among individuals before immunization. Yet the nature of these background responses remains largely unknown. Recent findings show the presence in uninfected individuals of mucosal associated invariant T (MAIT) cells that mount broad spectrum immune responses against a variety of microorganisms including Mycobacterium tuberculosis and enteric bacteria such as Escherichia coli and Salmonella. Therefore, we investigated whether MAIT immune responses to intestinal bacteria might account for the background responses observed before immunization. Here we measured MAIT immune responses to commensal and enteric pathogenic bacteria in healthy individuals with no history of oral immunization with enteric bacteria. We found that MAIT cells were activated by B cells infected with various bacteria strains (commensals and pathogens from the Enterobacteriaceae family), but not by uninfected cells. These responses were restricted by the non-classical MHC-related molecule 1 (MR1) and involved the endocytic pathway. The quality of these responses (i.e., cytokine profile) was dependent on bacterial load but not on the level expression of MR1 or bacterial antigen on B cell surface, suggesting that a threshold level of MR1 expression is required to trigger MAIT activation. These results provide important insights into the role of B cells as a source of antigen-presenting cells to MAIT cells and the gut immune surveillance of commensal microbiota.

Impact of oral typhoid vaccination on the human gut microbiota and correlations with s. Typhi-specific immunological responses

The resident microbial consortia of the human gastrointestinal tract play an integral role in modulating immune responses both locally and systemically. However, detailed information regarding the effector immune responses after vaccine administration in relation to the gastrointestinal microbiota is absent. In this study, the licensed oral live-attenuated typhoid vaccine Ty21a was administered in a clinical study to investigate whether oral immunization resulted in alterations of the microbiota and to identify whether a given microbiota composition, or subsets of the community, are associated with defined S. Typhi-specific immunological responses. The fecal microbiota composition and temporal dynamics were characterized using bacterial 16S rRNA pyrosequencing from individuals who were either immunized with the Ty21a typhoid vaccine (n = 13) or served as unvaccinated controls (n = 4). The analysis revealed considerable inter- and intra-individual variability, yet no discernible perturbations of the bacterial assemblage related to vaccine administration were observed. S. Typhi-specific cell mediated immune (CMI) responses were evaluated by measurement of intracellular cytokine production using multiparametric flow cytometry, and humoral responses were evaluated by measurement of serum anti-LPS IgA and IgG titers. Volunteers were categorized according to the kinetics and magnitude of their responses. While differences in microbial composition, diversity, or temporal stability were not observed among individuals able to mount a positive humoral response, individuals displaying multiphasic CMI responses harbored more diverse, complex communities. In line with this preliminary observation, over two hundred operational taxonomic units (OTUs) were found to differentiate multiphasic and late CMI responders, the vast majority of which classified within the order Clostridiales. These results provide an unprecedented view into the dramatic temporal heterogeneity of both the gut microbiota and host immune responses.

In vitro Intestinal Mucosal Epithelial Responses to Wild-Type Salmonella Typhi and Attenuated Typhoid Vaccines

Typhoid fever, caused by S. Typhi, is responsible for approximately 200,000 deaths per year worldwide. Little information is available regarding epithelium-bacterial interactions in S. Typhi infection. We have evaluated in vitro the effects of wild-type S. Typhi, the licensed Ty21a typhoid vaccine and the leading strains CVD 908-htrA and CVD 909 vaccine candidates on intestinal barrier function and immune response. Caco2 monolayers infected with wild-type S. Typhi exhibited alterations in the organization of tight junctions, increased paracellular permeability, and a rapid decrease in Trans-Epithelial Electrical Resistance as early as 4 h post-exposure. S. Typhi triggered the secretion of interleukin (IL)-8 and IL-6. Caco2 cells infected with the attenuated strains exhibited a milder pro-inflammatory response with minimal disruption of the barrier integrity. We conclude that wild-type S. Typhi causes marked transient alterations of the intestinal mucosa that are more pronounced than those observed with Ty21a or new generation attenuated typhoid vaccine candidates.

Vaccines for typhoid fever and other salmonelloses

PURPOSE OF REVIEW

This review summarizes the recent advances in vaccination against Salmonella enterica serovar Typhi and highlights the data supporting the development of next generation vaccines to address paratyphoid fever and invasive nontyphoidal Salmonella (iNTS) disease.

RECENT FINDINGS

There has been increasing awareness of the disease burden caused by S. Typhi particularly in Africa and greater recognition of S. Paratyphi A's contribution to enteric fever episodes throughout Asia. Groups have been working to improve the existing typhoid vaccines and provide comprehensive data on the feasibility of their implementation in endemic settings. These data have resulted in modifications to the recommendations for typhoid vaccination in traveller markets and endemic settings, and has also led to the development of S. Paratyphi A vaccine components that can be combined with existing typhoid vaccines to generate bivalent formulations against enteric fever. The epidemiology of iNTS serovars as cause of appreciable morbidity and mortality in Africa, and the need for vaccines, has also become more widely appreciated.

SUMMARY

Current typhoid vaccines, although moderately effective for short periods of time, cannot be used in all age groups and only target one of the clinically relevant Salmonella serovars. Greater effort must be placed on the development and implementation of improved vaccines for the disease burden resulting from Typhi, Paratyphi A or iNTS infections.

Heterogeneity of multifunctional IL-17A producing S. Typhi-specific CD8+ T cells in volunteers following Ty21a typhoid immunization

Salmonella enterica serovar Typhi (S. Typhi), the causative agent of typhoid fever, continues to cause significant morbidity and mortality world-wide. CD8+ T cells are an important component of the cell mediated immune (CMI) response against S. Typhi. Recently, interleukin (IL)-17A has been shown to contribute to mucosal immunity and protection against intracellular pathogens. To investigate multifunctional IL-17A responses against S. Typhi antigens in T memory subsets, we developed multiparametric flow cytometry methods to detect up to 6 cytokines/chemokines (IL-10, IL-17A, IL-2, interferon-γ (IFN-γ), tumor necrosis factor-α (TNF-α) and macrophage inflammatory protein-1β (MIP-1β)) simultaneously. Five volunteers were immunized with a 4 dose regimen of live-attenuated S. Typhi vaccine (Ty21a), peripheral blood mononuclear cells (PBMC) were isolated before and at 11 time points after immunization, and CMI responses were evaluated. Of the 5 immunized volunteers studied, 3 produced detectable CD8+ T cell responses following stimulation with S. Typhi-infected autologous B lymphoblastoid cell lines (B-LCL). Additionally, 2 volunteers had detectable levels of intracellular cytokines in response to stimulation with S. Typhi-infected HLA-E restricted cells. Although the kinetics of the responses differed among volunteers, all of the responses were bi- or tri-phasic and included multifunctional CD8+ T cells. Virtually all of the IL-17A detected was derived from multifunctional CD8+ T cells. The presence of these multifunctional IL-17A+ CD8+ T cells was confirmed using an unsupervised analysis program, flow cytometry clustering without K (FLOCK). This is the first report of IL-17A production in response to S. Typhi in humans, indicating the presence of a Tc17 response which may be important in protection. The presence of IL-17A in multifunctional cells co-producing Tc1 cytokines (IL-2, IFN-γ and TNF-α) may also indicate that the distinction between Tc17 and Tc1 responses in humans is not as clearly delineated as suggested by in vitro experiments and animal models.

Live oral typhoid vaccine Ty21a induces cross-reactive humoral immune responses against Salmonella enterica serovar Paratyphi A and S. Paratyphi B in humans

Enteric fever caused by Salmonella enterica serovar Paratyphi A infection has emerged as an important public health problem. Recognizing that in randomized controlled field trials oral immunization with attenuated S. enterica serovar Typhi live vaccine Ty21a conferred significant cross-protection against S. Paratyphi B but not S. Paratyphi A disease, we undertook a clinical study to ascertain whether humoral immune responses could explain the field trial results. Ty21a immunization of adult residents of Maryland elicited predominantly IgA antibody-secreting cells (ASC) that recognize S. Typhi lipopolysaccharide (LPS). Cross-reactivity to S. Paratyphi A LPS was significantly lower than that to S. Paratyphi B LPS. ASC producing IgG and IgA that bind LPS from each of these Salmonella serovars expressed CD27 and integrin α4β7 (gut homing), with a significant proportion coexpressing CD62L (secondary lymphoid tissue homing). No significant differences were observed in serum antibody against LPS of the different serovars. Levels of IgA B memory (B(M)) cells to S. Typhi LPS were significantly higher than those against S. Paratyphi A or B LPS, with no differences observed between S. Paratyphi A and B. The response of IgA B(M) to outer membrane proteins (OMP) from S. Typhi was significantly stronger than that to OMP of S. Paratyphi A but similar to that to OMP of S. Paratyphi B. The percentages of IgG or IgA B(M) responders to LPS or OMP from these Salmonella strains were similar. Whereas cross-reactive humoral immune responses to S. Paratyphi A or B antigens are demonstrable following Ty21a immunization, they cannot explain the efficacy data gleaned from controlled field trials.

Immunology of gut mucosal vaccines

Understanding the mechanisms underlying the induction of immunity in the gastrointestinal mucosa following oral immunization and the cross-talk between mucosal and systemic immunity should expedite the development of vaccines to diminish the global burden caused by enteric pathogens. Identifying an immunological correlate of protection in the course of field trials of efficacy, animal models (when available), or human challenge studies is also invaluable. In industrialized country populations, live attenuated vaccines (e.g. polio, typhoid, and rotavirus) mimic natural infection and generate robust protective immune responses. In contrast, a major challenge is to understand and overcome the barriers responsible for the diminished immunogenicity and efficacy of the same enteric vaccines in underprivileged populations in developing countries. Success in developing vaccines against some enteric pathogens has heretofore been elusive (e.g. Shigella). Different types of oral vaccines can selectively or inclusively elicit mucosal secretory immunoglobulin A and serum immunoglobulin G antibodies and a variety of cell-mediated immune responses. Areas of research that require acceleration include interaction between the gut innate immune system and the stimulation of adaptive immunity, development of safe yet effective mucosal adjuvants, better understanding of homing to the mucosa of immunologically relevant cells, and elicitation of mucosal immunologic memory. This review dissects the immune responses elicited in humans by enteric vaccines.

Oral priming with Salmonella Typhi vaccine strain CVD 909 followed by parenteral boost with the S. Typhi Vi capsular polysaccharide vaccine induces CD27+IgD-S. Typhi-specific IgA and IgG B memory cells in humans

Attenuated live oral typhoid vaccine candidate CVD 909 constitutively expresses Salmonella Typhi capsular polysaccharide antigen (Vi). A randomized, double-blind, heterologous prime-boost clinical study was conducted to determine whether immunity to licensed parenteral Vi vaccine could be enhanced by priming with CVD 909. Priming with CVD 909 elicited higher and persistent, albeit not significant, anti-Vi IgG and IgA following immunization with Vi, than placebo-primed recipients. Vi-specific IgA B memory (B(M)) cells were significantly increased in CVD 909-primed subjects. S. Typhi-specific LPS and flagella IgA B(M) cells were observed in subjects immunized with CVD 909 or with the licensed Vi-negative oral typhoid vaccine Ty21a. CVD 909-induced B(M) cells exhibited a classical B(M) phenotype (i.e., CD3(-)CD19(+)IgD(-)CD27(+)). This is the first demonstration of classical B(M) cells specific for bacterial polysaccharide or protein antigens following typhoid immunization. The persistent IgA B(M) responses demonstrate the capacity of oral typhoid vaccines to prime mucosally relevant immune memory.

Ex Vivo kinetics of early and long-term multifunctional human leukocyte antigen E-specific CD8+ cells in volunteers immunized with the Ty21a typhoid vaccine

T cells are likely to play an important role in the host defense against Salmonella enterica serovar Typhi, the causative agent of typhoid fever. We have shown that HLA-E can function as a restriction element for S. Typhi-specific CD8(+) T cells. Because of the potential importance of HLA-E-restricted CD8(+) responses in resistance to Salmonella infection, we characterized these responses and investigated their kinetics of appearance and persistence in volunteers immunized orally with the licensed attenuated Ty21a strain typhoid vaccine. Cells were obtained from volunteers before and at days 2, 4, 7, 10, 14, 28, 42, 56, 120, 180, 360, and 720 after immunization. An ex vivo multicolor staining panel including antibodies to CD107a and -b, interleukin-2, gamma interferon (IFN-gamma), and tumor necrosis factor alpha (TNF-alpha) was used to functionally assess memory T-cell subsets by flow cytometry. Increases in cytokine-secreting CD8(+) cells were observed in the T effector/memory (T(EM)) and CD45RA(+) T(EM) (T(EMRA)) subsets as early as 4 days after immunization and persisted, particularly in the T(EMRA) subset, up to 2 years after immunization. The majority of HLA-E-restricted CD8(+) cells 28 to 56 days after immunization coexpressed CD107, IFN-gamma, and TNF-alpha, showing characteristic features of multifunctional T cells. In summary, the multifunctionality and longevity of the HLA-E-restricted CD8 responses observed in this study highlight their significance in adaptive immunity to S. Typhi. Finally, this is the first demonstration, in either animals or humans, of the presence of long-term multifunctional HLA-E-restricted CD8(+) cells after immunization.

Characterization of T-cell responses in macaques immunized with a single dose of HIV DNA vaccine

The optimization of immune responses (IR) induced by HIV DNA vaccines in humans is one of the great challenges in the development of an effective vaccine against AIDS. Ideally, this vaccine should be delivered in a single dose to immunize humans. We recently demonstrated that the immunization of mice with a single dose of a DNA vaccine derived from pathogenic SHIV(KU2) (Delta4SHIV(KU2)) induced long-lasting, potent, and polyfunctional HIV-specific CD8(+) T-cell responses (G. Arrode, R. Hegde, A. Mani, Y. Jin, Y. Chebloune, and O. Narayan, J. Immunol. 178:2318-2327, 2007). In the present work, we expanded the characterization of the IR induced by this DNA immunization protocol to rhesus macaques. Animals immunized with a single high dose of Delta4SHIV(KU2) DNA vaccine were monitored longitudinally for vaccine-induced IR using multiparametric flow cytometry-based assays. Interestingly, all five immunized macaques developed broad and polyfunctional HIV-specific T-cell IR that persisted for months, with an unusual reemergence in the blood following an initial decline but in the absence of antibody responses. The majority of vaccine-specific CD4(+) and CD8(+) T cells lacked gamma interferon production but showed high antigen-specific proliferation capacities. Proliferative CD8(+) T cells expressed the lytic molecule granzyme B. No integrated viral vector could be detected in mononuclear cells from immunized animals, and this high dose of DNA did not induce any detectable autoimmune responses against DNA. Taken together, our comprehensive analysis demonstrated for the first time the capacity of a single high dose of HIV DNA vaccine alone to induce long-lasting and polyfunctional T-cell responses in the nonhuman primate model, bringing new insights for the design of future HIV vaccines.

Generation of heterogeneous memory T cells by live attenuated tularemia vaccine in humans

There is very limited evidence concerning the phenotype, function, and homing characteristics of memory T (T(M)) cells elicited by vaccination against intracellular bacteria in humans. Here we studied T(M) subsets elicited by exposure to Francisella tularensis in humans as a model of immunity to intracellular bacteria. To this end, T(M) cells were evaluated in two groups: (1) subjects immunized with live attenuated tularemia vaccine by skin scarification and (2) tularemia naturally infected subjects. In both groups the immune responses were mediated by CD4(+) and CD8(+) effector T(M) cells, mostly CD45RA(-)CD62L(-) and CD45RA(+)CD62L(-). Based on the expression of CD27, integrins alpha(4)/beta(7,) and alpha(4)/beta(1), it is likely that some of these T(M) cells have lytic potential and the ability to enter both mucosal and non-mucosal sites. Thus, regardless of whether by immunization or natural exposure, tularemia antigens elicited a broad spectrum of specific T(M) subsets with diverse homing characteristics.

Priming of Salmonella enterica serovar typhi-specific CD8(+) T cells by suicide dendritic cell cross-presentation in humans

BACKGROUND

The emergence of antibiotic-resistant strains of Salmonella enterica serovar Typhi (S. Typhi), the etiologic agent of typhoid fever, has aggravated an already important public health problem and added new urgency to the development of more effective typhoid vaccines. To this end it is critical to better understand the induction of immunity to S. Typhi. CD8(+) T cells are likely to play an important role in host defense against S. Typhi by several effector mechanisms, including killing of infected cells and IFN-gamma secretion. However, how S. Typhi regulates the development of specific CD8(+) responses in humans remains unclear. Recent studies in mice have shown that dendritic cells (DC) can either directly (upon uptake and processing of Salmonella) or indirectly (by bystander mechanisms) elicit Salmonella-specific CD8(+) T cells.

METHODOLOGY/PRINCIPAL FINDINGS

We report here that upon infection with live S. Typhi, human DC produced high levels of pro-inflammatory cytokines IL-6, IL-8 and TNF-alpha, but low levels of IL-12 p70 and IFN-gamma. In contrast, DC co-cultured with S. Typhi-infected cells, through suicide cross-presentation, uptake S. Typhi-infected human cells and release high levels of IFN-gamma and IL-12p70, leading to the subsequent presentation of bacterial antigens and triggering the induction of memory T cells, mostly CD3(+)CD8(+)CD45RA(-)CD62L(-) effector/memory T cells.

CONCLUSIONS/SIGNIFICANCE

This study is the first to demonstrate the effect of S. Typhi on human DC maturation and on their ability to prime CD8(+) cells and highlights the significance of these phenomena in eliciting adaptive immunity to S. Typhi.

Salmonella enterica serovar Typhi live vector vaccines finally come of age

Attenuated Salmonella Typhi vaccine strains hold great promise as live vectors for presentation of foreign antigens from unrelated bacterial, viral and parasitic pathogens to the immune system. Although this approach has proved quite successful in experimental animal models for eliciting antigen-specific mucosal, humoral and cellular responses, results have been disappointing for clinical trials carried out thus far. We hypothesize that the paucity of human responses to foreign antigens delivered by live vectors suggests that the strains and genetic approaches used to date have resulted in overattenuated vaccine strains with severely reduced immunogenicity. However, remarkable advances have now been made in the genetics of foreign antigen expression, understanding mechanisms of live vector immunity and refining immunization strategies. The time has now come for development of multivalent live vectors in which stable antigen expression and export is balanced with metabolic fitness to create highly immunogenic vaccines.