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

From Eberthella typhi to Salmonella Typhi: The Fascinating Journey of the Virulence and Pathogenicity of Salmonella Typhi

Salmonella Typhi (S. Typhi), the invasive typhoidal serovar of Salmonella enterica that causes typhoid fever in humans, is a severe threat to global health. It is one of the major causes of high morbidity and mortality in developing countries. According to recent WHO estimates, approximately 11-21 million typhoid fever illnesses occur annually worldwide, accounting for 0.12-0.16 million deaths. Salmonella infection can spread to healthy individuals by the consumption of contaminated food and water. Typhoid fever in humans sometimes is accompanied by several other critical extraintestinal complications related to the central nervous system, cardiovascular system, pulmonary system, and hepatobiliary system. Salmonella Pathogenicity Island-1 and Salmonella Pathogenicity Island-2 are the two genomic segments containing genes encoding virulent factors that regulate its invasion and systemic pathogenesis. This Review aims to shed light on a comparative analysis of the virulence and pathogenesis of the typhoidal and nontyphoidal serovars of S. enterica.

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.

Salmonella Biofilm Formation, Chronic Infection, and Immunity Within the Intestine and Hepatobiliary Tract

Within the species of Salmonella enterica, there is significant diversity represented among the numerous subspecies and serovars. Collectively, these account for microbes with variable host ranges, from common plant and animal colonizers to extremely pathogenic and human-specific serovars. Despite these differences, many Salmonella species find commonality in the ability to form biofilms and the ability to cause acute, latent, or chronic disease. The exact outcome of infection depends on many factors such as the growth state of Salmonella, the environmental conditions encountered at the time of infection, as well as the infected host and immune response elicited. Here, we review the numerous biofilm lifestyles of Salmonella (on biotic and abiotic surfaces) and how the production of extracellular polymeric substances not only enhances long-term persistence outside the host but also is an essential function in chronic human infections. Furthermore, careful consideration is made for the events during initial infection that allow for gut transcytosis which, in conjunction with host immune functions, often determine the progression of disease. Both typhoidal and non-typhoidal salmonellae can cause chronic and/or secondary infections, thus the adaptive immune responses to both types of bacteria are discussed with particular attention to the differences between Salmonella Typhi, Salmonella Typhimurium, and invasive non-typhoidal Salmonella that can result in differential immune responses. Finally, while strides have been made in our understanding of immunity to Salmonella in the lymphoid organs, fewer definitive studies exist for intestinal and hepatobiliary immunity. By examining our current knowledge and what remains to be determined, we provide insight into new directions in the field of Salmonella immunity, particularly as it relates to chronic infection.

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.

Intravesical Ty21a Vaccine Promotes Dendritic Cells and T Cell-Mediated Tumor Regression in the MB49 Bladder Cancer Model

Preclinical data show that intravesical instillation of Ty21a/Vivotif, a commercial vaccine against typhoid fever, is an effective alternative option to standard Bacillus Calmette-Guérin (BCG) immunotherapy for non-muscle-invasive bladder cancer (NMIBC). Here, we characterized the inflammatory effects of Ty21a on the bladder and investigated the immune mechanisms underlying tumor regression toward the use of this bacterial vaccine in NMIBC patients. MB49 bladder tumor-bearing mice had significantly improved survival after intravesical instillations of Ty21a doses of 10⁶ to 10⁸ colony-forming units. By IHC and morphology, both BCG and Ty21a instillations were associated with bladder inflammation, which was decreased with the use of low, but effective doses of Ty21a. Flow-cytometry analysis showed a significant infiltration of T cells, natural killer (NK) cells, and myeloid cells, compared with controls, after a single dose of Ty21a, whereas this was only observed after multiple doses of BCG. The induced myeloid cells were predominantly neutrophils and Ly6C⁺CD103⁺ dendritic cells (DC), the latter being significantly more numerous after instillation of Ty21a than BCG. Ex vivo infection of human leukocytes with Ty21a, but not BCG, similarly significantly increased DC frequency. CD4⁺ and CD8⁺ T cells, but not NK cells nor neutrophils, were required for effective bladder tumor regression upon Ty21a treatment. Thus, the generation of antitumor adaptive immunity was identified as a key process underlying Ty21a-mediated treatment efficacy. Altogether, these results demonstrate mechanisms behind intravesical Ty21a therapy and suggest its potential as a safe and effective treatment for NMIBC patients.

Is a Human CD8 T-Cell Vaccine Possible, and if So, What Would It Take? CD8 T-Cell-Mediated Protective Immunity and Vaccination against Enteric Bacteria

Although induction of CD8⁺ responses is widely accepted as critical in clearing viral infections and necessary for effective vaccines against viruses, much less is known regarding the role of these cells in bacterial and other infections, particularly those that enter the host via the gastrointestinal tract. In this commentary, I discuss the likelihood that CD8⁺ responses are also important in protection from intestinal Gram-negative bacteria, as well as the many factors that should be taken into consideration during the development of vaccines, based on eliciting long-term protection predominantly mediated by CD8⁺ responses against these organisms.

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.

Challenge of Humans with Wild-type Salmonella enterica Serovar Typhi Elicits Changes in the Activation and Homing Characteristics of Mucosal-Associated Invariant T Cells

Gastrointestinal infections by Salmonella enterica serovar Typhi (S. Typhi) are rare in industrialized countries. However, they remain a major public health problem in the developing world with an estimated 26.9 million new cases annually and significant mortality when untreated. Recently, we provided the first direct evidence that CD8⁺ MAIT cells are activated and have the potential to kill cells exposed to S. Typhi, and that these responses are dependent on bacterial load. However, MAIT cell kinetics and function during bacterial infections in humans remain largely unknown. In this study, we characterize the human CD8⁺ MAIT cell immune response to S. Typhi infection in subjects participating in a challenge clinical trial who received a low- or high dose of wild-type S. Typhi. We define the kinetics of CD8⁺ MAIT cells as well as their levels of activation, proliferation, exhaustion/apoptosis, and homing potential. Regardless of the dose, in volunteers resistant to infection (NoTD), the levels of CD8⁺ MAIT cells after S. Typhi challenge fluctuated around their baseline values (day 0). In contrast, volunteers susceptible to the development of typhoid disease (TD) exhibited a sharp decline in circulating MAIT cells during the development of typhoid fever. Interestingly, MAIT cells from low-dose TD volunteers had higher levels of CD38 coexpressing CCR9, CCR6, and Ki67 during the development of typhoid fever than high-dose TD volunteers. No substantial perturbations on the levels of these markers were observed in NoTD volunteers irrespective of the dose. In sum, we describe, for the first time, that exposure to an enteric bacterium, in this case S. Typhi, results in changes in MAIT cell activation, proliferation, and homing characteristics, suggesting that MAIT cells are an important component of the human host response to bacterial infection.

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.

Homologous prime-boost immunization with live attenuated Salmonella enterica serovar Senftenberg and its preventive efficacy against experimental challenge with various strains of S. Senftenberg

Background

The heterogeneity observed regarding persistence, and subsequent fecal shedding pattern of the Salmonella Senftenberg (S. Senftenberg) serovar in chicken’s calls for development of the optimized immunization strategy which can provide protection against various S. Senftenberg isolated. Optimization of an immunization strategy with a live attenuated S. Senftenberg (Δlon and ΔcpxR) vaccine candidate (JOL1587) was undertaken in this study to evaluate the ability of a homologous prime-boost immunization strategy (using JOL1587) to confer protection against four different S. Senftenberg isolates in chickens.

Results

After oral immunization with JOL1587, the humoral, mucosal and cell-mediated immune responses were significantly higher in double immunized chickens than in single immunized and control group chickens. A significant increase in the multifunctional cytokine IL-6 and in helper and cytotoxic T cell populations after a booster immunization also indicated the advantage of double over single immunization. The four different S. Senftenberg field isolates were characterized by their persistence levels in chickens, and were subsequently used for challenge experiments to evaluate the differences in protective efficacy conferred by single and double immunization. Chickens from the doubleimmunized group exhibited significant reduction in the shedding of all four wild-type S. Senftenberg challenge strains below the detection limit in the fecal samples. Single immunized chickens showed a decrease in fecal shedding, but failed to exhibit complete protection against all the challenge strains.

Conclusion

Although single immunization with JOL1587 showed a reduction in the fecal shedding of challenge strains, only the homologous prime-boost immunization strategy provided an adequate immune response for increased protection against all four challenge strains of S. Senftenberg from the feces of chickens.

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.

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.

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.

Selective infection of antigen-specific B lymphocytes by Salmonella mediates bacterial survival and systemic spreading of infection

BACKGROUND

The bacterial pathogen Salmonella causes worldwide disease. A major route of intestinal entry involves M cells, providing access to B cell-rich Peyer's Patches. Primary human B cells phagocytose Salmonella typhimurium upon recognition by the specific surface Ig receptor (BCR). As it is unclear how Salmonella disseminates systemically, we studied whether Salmonella can use B cells as a transport device for spreading.

METHODOLOGY/PRINCIPAL FINDINGS

Human primary B cells or Ramos cell line were incubated with GFP-expressing Salmonella. Intracellular survival and escape was studied in vitro by live cell imaging, flow cytometry and flow imaging. HEL-specific B cells were transferred into C57BL/6 mice and HEL-expressing Salmonella spreading in vivo was analyzed investigating mesenteric lymph nodes, spleen and blood. After phagocytosis by B cells, Salmonella survives intracellularly in a non-replicative state which is actively maintained by the B cell. Salmonella is later excreted followed by reproductive infection of other cell types. Salmonella-specific B cells thus act both as a survival niche and a reservoir for reinfection. Adoptive transfer of antigen-specific B cells before oral infection of mice showed that these B cells mediate in vivo systemic spreading of Salmonella to spleen and blood.

CONCLUSIONS/SIGNIFICANCE

This is a first example of a pathogenic bacterium that abuses the antigen-specific cells of the adaptive immune system for systemic spreading for dissemination of infection.

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.

New insights in cellular immune response in colostrum-deprived pigs after immunization with subunit and commercial vaccines against Glässer's disease

Four groups of colostrum-deprived pigs were immunized with Porcilis Glässer® (PG) or with subunit vaccines developed by us (rTbpA, NPAPT(M) or NPAPT(Cp)) against Glässer's disease, and they were challenged with 3×10(8)CFU of Haemophilus parasuis. A strong reduction in CD3(+)γδTCR(+) cells was seen in non-immunized control and scarcely protected (rTbpA) groups, suggesting that these cells could represent a target of H. parasuis infection. A significant increase in CD172α(+)CD163(+) cells was detected in all groups but PG, while a reduction in SLAIIDR(+) molecules expression was observed after challenge in control animals. Significant increases in CD3ε(+)CD8α(+)CD8β(+) and B cells were detected respectively in control and NPAPT groups, and in scarcely (rTbpA) and well-protected (NPAPT(M) and NPAPT(Cp)) groups. Finally, a greater response in CD4(+)CD8α(-) cells was observed in NPAPT(Cp) compared to NPAPT(M) and PG groups. These results state the potential of NPAPT antigen for developing effective vaccines against Glässer's disease.

Single dose novel Salmonella vaccine enhances resistance against visceralizing L. major and L. donovani infection in susceptible BALB/c mice

Visceral leishmaniasis is a major neglected tropical disease, with an estimated 500,000 new cases and more than 50,000 deaths attributable to this disease every year. Drug therapy is available but costly and resistance against several drug classes has evolved. Despite all efforts, no commercial, let alone affordable, vaccine is available to date. Thus, the development of cost effective, needle-independent vaccines is a high priority. Here, we have continued efforts to develop live vaccine carriers based on recombinant Salmonella. We used an in silico approach to select novel Leishmania parasite antigens from proteomic data sets, with selection criteria based on protein abundance, conservation across Leishmania species and low homology to host species. Five chosen antigens were differentially expressed on the surface or in the cytosol of Salmonella typhimurium SL3261. A two-step procedure was developed to select optimal Salmonella vaccine strains for each antigen, based on bacterial fitness and antigen expression levels. We show that vaccine strains of Salmonella expressing the novel Leishmania antigens LinJ08.1190 and LinJ23.0410 significantly reduced visceralisation of L. major and enhanced systemic resistance against L. donovani in susceptible BALB/c mice. The results show that Salmonella are valid vaccine carriers for inducing resistance against visceral leishmaniasis but that their use may not be suitable for all antigens.

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.

Immunity to salmonellosis

Salmonella enterica is a genetically broad species harboring isolates that display considerable antigenic heterogeneity and significant differences in virulence potential. Salmonella generally exhibit an invasive potential and they can survive for extended periods within cells of the immune system. They cause acute or chronic infections that can be local (e.g. gastroenteritis) or systemic (e.g. typhoid). In vivo Salmonella infections are complex with multiple arms of the immune system being engaged. Both humoral and cellular responses can be detected and characterized, but full protective immunity is not always induced, even following natural infection. The murine model has proven to be a fertile ground for exploring immune mechanisms and observations in the mouse have often, although not always, correlated with those in other infectable species, including humans. Host genetic studies have identified a number of mammalian genes that are central to controlling infection, operating both in innate and acquired immune pathways. Vaccines, both oral and parenteral, are available or under development, and these have been used with some success to explore immunity in both model systems and clinically in humans.

Interferon-γ and proliferation responses to Salmonella enterica Serotype Typhi proteins in patients with S. Typhi Bacteremia in Dhaka, Bangladesh

Background

Salmonella enterica serotype Typhi is a human-restricted intracellular pathogen and the cause of typhoid fever. Cellular immune responses are required to control and clear Salmonella infection. Despite this, there are limited data on cellular immune responses in humans infected with wild type S. Typhi.

Methodology/Principal Findings

For this work, we used an automated approach to purify a subset of S. Typhi proteins identified in previous antibody-based immuno-affinity screens and antigens known to be expressed in vivo, including StaF-putative fimbrial protein-STY0202, StbB-fimbrial chaperone-STY0372, CsgF-involved in curli production-STY1177, CsgD- putative regulatory protein-STY1179, OppA-periplasmic oligopeptide binding protein precursor-STY1304, PagC-outer membrane invasion protein-STY1878, and conserved hypothetical protein-STY2195; we also generated and analyzed a crude membrane preparation of S. Typhi (MP). In comparison to samples collected from uninfected Bangladeshi and North American participants, we detected significant interferon-γ responses in PBMCs stimulated with MP, StaF, StbB, CsgF, CsgD, OppA, STY2195, and PagC in patients bacteremic with S. Typhi in Bangladesh. The majority of interferon-γ expressing T cells were CD4 cells, although CD8 responses also occurred. We also assessed cellular proliferation responses in bacteremic patients, and confirmed increased responses in infected individuals to MP, StaF, STY2195, and PagC in convalescent compared to acute phase samples and compared to controls. StaF is a fimbrial protein homologous to E. coli YadK, and contains a Pfam motif thought to be involved in cellular adhesion. PagC is expressed in vivo under the control of the virulence-associated PhoP-regulon required for intra-macrophage survival of Salmonella. STY2195 is a conserved hypothetical protein of unknown function.

Conclusion/Significance

This is the first analysis of cellular immune responses to purified S. Typhi antigens in patients with typhoid fever. These results indicate that patients generate significant CD4 and CD8 interferon-γ responses to specific S. Typhi antigens during typhoid fever, and that these responses are elevated at the time of clinical presentation. These observations suggest that an interferon-γ based detection system could be used to diagnose individuals with typhoid fever during the acute stage of illness.

Salmonella enterica serotype Typhi infection is a significant global public health problem and the cause of typhoid fever. Salmonella are intracellular pathogens, and cellular immune responses are required to control and clear Salmonella infections. Despite this, there are limited data on cellular immune responses during wild type S. Typhi infection in humans. Here we report the assessment of cellular immune responses in humans with S. Typhi bacteremia through a screening approach that permitted us to evaluate interferon-γ and proliferation responses to a number of S. Typhi antigens. We detected significant interferon-γ CD4 and CD8 responses, as well as proliferative responses, to a number of recombinantly purified S. Typhi proteins as well as membrane preparation in infected patients. Antigen-specific interferon-γ responses were present at the time of clinical presentation in patients and absent in healthy controls. These observations could assist in the development of interferon-γ-based diagnostic assays for typhoid fever.

Antigen-specific B cells reactivate an effective cytotoxic T cell response against phagocytosed Salmonella through cross-presentation

BACKGROUND

The eradication of facultative intracellular bacterial pathogens, like Salmonella typhi, requires the concerted action of both the humoral immune response and the cytotoxic CD8(+) T cell response. Dendritic cells (DCs) are considered to orchestrate the cytotoxic CD8(+) T cell response via cross-presentation of bacterial antigens onto MHC class I molecules. Cross-presentation of Salmonella by DCs however, is accompanied by the induction of apoptosis in the DCs. Besides antibody production, B cells are required to clear Salmonella infection for other unknown reasons.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that Salmonella-specific B cells that phagocytose Salmonella upon BCR-ligation reactivate human memory CD8(+) T cells via cross-presentation yielding a Salmonella-specific cytotoxic T cell response. The reactivation of CD8(+) T cells is dependent on CD4(+) T cell help. Unlike the DCs, B cell-mediated cross-presentation of Salmonella does not coincide with apoptosis.

CONCLUSIONS/SIGNIFICANCE

B cells form a new player in the activation of the cytotoxic effector arm of the immune response and the generation of effective adaptive immunity in Salmonella infection.

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.