Capacity of recombinant gamma interferon to activate macrophages for Salmonella-killing activity

The protected physiological state of intracellular Salmonella enterica persisters reduces host cell-imposed stress

During infectious diseases, small subpopulations of bacterial pathogens enter a non-replicating (NR) state tolerant to antibiotics. After phagocytosis, intracellular Salmonella enterica serovar Typhimurium (STM) forms persisters able to subvert immune defenses of the host. Physiological state and sensing properties of persisters are difficult to analyze, thus poorly understood. Here we deploy fluorescent protein reporters to detect intracellular NR persister cells, and to monitor their stress response on single cell level. We determined metabolic properties of NR STM during infection and demonstrate that NR STM persisters sense their environment and respond to stressors. Since persisters showed a lower stress response compared to replicating (R) STM, which was not consequence of lower metabolic capacity, the persistent state of STM serves as protective niche. Up to 95% of NR STM were metabolically active at beginning of infection, very similar to metabolic capacity of R STM. Sensing and reacting to stress with constant metabolic activity supports STM to create a more permissive environment for recurrent infections. Stress sensing and response of persister may be targeted by new antimicrobial approaches.

Schulte et al. show that non-replicating Salmonella enterica serovar Typhimurium persisters, which are tolerant to antibiotics, sense their environment and respond to stressors. This study suggests that stress sensing and response of persisters may be targeted as an antimicrobial strategy.

Competitive Exclusion of Intra-Genus Salmonella in Neonatal Broilers

Salmonellosis is a zoonotic infection caused by Salmonella enterica serotypes contracted from contaminated products. We hypothesized that competitive exclusion between Salmonella serotypes in neonatal broilers would reduce colonization and affect the host immune response. Day of hatch broilers were randomly allocated to one of six treatment groups: (1) control, which received saline, (2) Salmonella Kentucky (SK) only on day 1 (D1), (3) Salmonella Typhimurium (ST) or Salmonella Enteritidis (SE) only on D1, (4) SK on D1 then ST or SE on day 2 (D2), (5) ST or SE on D1 then SK on D2, and (6) SK and ST or SE concurrently on D1. Salmonella gut colonization and incidence were measured from cecal contents. Livers and spleens were combined and macerated to determine systemic translocation. Relative mRNA levels of interleukin-1β (IL-1β), IL-6, IL-10, IL-18, and gamma interferon (IFN-γ) were measured in cecal tonsils and liver to investigate local and systemic immune responses. When a serotype was administered first, it was able to significantly reduce colonization of the following serotype. Significant changes were found in mRNA expression of cytokines. These results suggest competitive exclusion by Salmonella enterica serotypes affect local and systemic immune responses.

Human Genetic Variation Influences Enteric Fever Progression

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

Salmonella Persistence and Host Immunity Are Dictated by the Anatomical Microenvironment

The intracellular bacterial pathogen Salmonella is able to evade the immune system and persist within the host. In some cases, these persistent infections are asymptomatic for long periods and represent a significant public health hazard because the hosts are potential chronic carriers, yet the mechanisms that control persistence are incompletely understood. Using a mouse model of chronic typhoid fever combined with major histocompatibility complex (MHC) class II tetramers to interrogate endogenous, Salmonella-specific CD4⁺ helper T cells, we show that certain host microenvironments may favorably contribute to a pathogen's ability to persist in vivo We demonstrate that the environment in the hepatobiliary system may contribute to the persistence of Salmonella enterica subsp. enterica serovar Typhimurium through liver-resident immunoregulatory CD4⁺ helper T cells, alternatively activated macrophages, and impaired bactericidal activity. This contrasts with lymphoid organs, such as the spleen and mesenteric lymph nodes, where these same cells appear to have a greater capacity for bacterial killing, which may contribute to control of bacteria in these organs. We also found that, following an extended period of infection of more than 2 years, the liver appeared to be the only site that harbored Salmonella bacteria. This work establishes a potential role for nonlymphoid organ immunity in regulating chronic bacterial infections and provides further evidence for the hepatobiliary system as the site of chronic Salmonella infection.

Humoral Immunity vs. Salmonella

In primary infection with Salmonella, it has been reported—without consideration of Salmonella's functions—that humoral immunity plays no role in the clearance of bacteria. In fact, Salmonella targets and suppresses several aspects of humoral immunity, including B cell lymphopoiesis, B cell activation, and IgG production. In particular, the suppression of IgG-secreting plasma cell maintenance allows the persistence of Salmonella in tissues. Therefore, the critical role(s) of humoral immunity in the response to Salmonella infection, especially at the late phase, should be re-investigated. The suppression of IgG plasma cell memory strongly hinders vaccine development against non-typhoidal Salmonella (NTS) because Salmonella can also reduce humoral immune memory against other bacteria and viruses, obtained from previous vaccination or infection. We propose a new vaccine against Salmonella that would not impair humoral immunity, and which could also be used as a treatment for antibody-dependent autoimmune diseases to deplete pathogenic long-lived plasma cells, by utilizing the Salmonella's own suppression mechanism of humoral immunity.

Pathogen-dependent role of turbot (Scophthalmus maximus) interferon-gamma

Interferon-gamma has been typically described as a pro-inflammatory cytokine playing an important role in the resolution of both viral and bacterial diseases. Nevertheless, some anti-inflammatory functions are also attributed to this molecule. In this work we have characterized for the first time the turbot (Scophthalmus maximus) interferon-gamma gene (ifng) and its expression pattern under basal conditions, after type I IFNs administration, and viral and bacterial infection. The intramuscular injection of an expression plasmid encoding turbot Ifng (pMCV1.4-ifng) was not able to affect the transcription of numerous immune genes directly related to the activity of IFN-gamma, with the exception of macrophage-colony stimulating factor (csf1). It was also unable to reduce the mortality caused by a Viral Hemorrhagic Septicemia Virus (VHSV) or Aeromonas salmonicida challenge. Interestingly, at 24 h post-infection, turbot previously inoculated with pMCV1.4-ifng and infected with VHSV showed an increase in the expression of pro-inflammatory cytokines and type I IFNs compared to those fish not receiving expression plasmid, indicating a synergic effect of Ifng and VHSV. On the other hand, some macrophage markers, such as the macrophage receptor with collagenous structure (marco), were down-regulated by Ifng during the viral infection. Ifng had the opposite effect in those turbot infected with the bacteria, showing a reduction in the transcription of pro-inflammatory and type I IFNs genes, and an increase in the expression of genes related to the activity of macrophages.

Interferon-γ-Induced Nitric Oxide Synthase-2 Contributes to Blood/Brain Barrier Dysfunction and Acute Mortality in Experimental Streptococcus pneumoniae Meningitis

The proinflammatory cytokine interferon-gamma (IFNγ) recently was shown to play a crucial role in experimental pneumococcal meningitis (PM) pathogenesis, and we aimed in this study to investigate IFNγ-driven nitric oxide synthase-2 (NOS2)-mediated pathogenesis of murine PM. We demonstrate that costimulation of toll-like receptors and IFNγ receptors was synergistic for NOS2 expression in cultured murine microglia. Using an experimental PM model, wild-type mice treated with anti-IFNγ antibody, as well as IFNγ and NOS2 gene knockout (GKO) mice, were inoculated intracerebroventricularly with 10(3) colony-forming units of Streptococcus pneumoniae (WU2 strain). Mice were monitored daily during a 200-h disease course to assess survival rate and blood-brain barrier (BBB) permeability measured at 48 h. IFNγ deficiency was protective in PM, with an approximate 3-fold increase in survival rates in both antibody-treated and IFNγ GKO mice compared to controls (P < 0.01). At 48 h postinoculation, brain NOS2 mRNA expression was significantly increased in an IFNγ-dependent manner. Mortality was significantly delayed in NOS2 GKO mice compared to controls (P < 0.01), and BBB dysfunction was reduced by 54% in IFNγ GKO mice and abolished in NOS2 GKO. These data suggest that IFNγ-dependent expression of NOS2 in the brain contributes to BBB breakdown and early mortality in murine PM.

Genetic susceptibility to invasive Salmonella disease

Invasive Salmonella disease, in the form of enteric fever and invasive non-typhoidal Salmonella (iNTS) disease, causes substantial morbidity and mortality in children and adults in the developing world. The study of genetic variations in humans and mice that influence susceptibility of the host to Salmonella infection provides important insights into immunity to Salmonella. In this Review, we discuss data that have helped to elucidate the host genetic determinants of human enteric fever and iNTS disease, alongside data from the mouse model of Salmonella infection. Considered together, these studies provide a detailed picture of the immunobiology of human invasive Salmonella disease.

Natural Killer Cells Mediate Protection against Yersinia pseudotuberculosis in the Mesenteric Lymph Nodes

Natural killer cells play a crucial role in the initial defense against bacterial pathogens. The crosstalk between host cells infected with intracellular pathogens and NK cells has been studied intensively, but not much attention has been given to characterize the role of NK cells in the response to extracellular bacterial pathogens such as yersiniae. In this study we used antibody-mediated NK cell depletion to address the importance of this immune cell type in controlling a Y. pseudotuberculosis infection. Analysis of the bacterial counts was used to follow the infection and flow cytometry was performed to characterize the composition and dynamic of immune cells. Depletion of NK cells led to higher bacterial loads within the mesenteric lymph nodes. We further show that in particular CD11b⁺ CD27⁺ NK cells which express higher levels of the activation marker CD69 increase within the mesenteric lymph nodes during a Y. pseudotuberculosis infection. Moreover, in response to the activation NK cells secrete higher levels of IFNy, which in turn triggers the production of the proinflammatory cytokine TNFα. These results suggest, that NK cells aid in the clearance of Y. pseudotuberculosis infections mainly by triggering the expression of proinflammatory cytokines manipulating the host immune response.

Human B cells promote T-cell plasticity to optimize antibody response by inducing coexpression of T(H)1/T(FH) signatures

BACKGROUND

B cells mediate humoral immunity against pathogens but also direct CD4(+) T-cell responses. Recent plasticity studies in mice have challenged the concept of strict fate commitment during CD4(+) T-cell differentiation into distinct subsets.

OBJECTIVE

We sought to elucidate the contribution of human antigen-primed B cells in CD4(+) T-cell responses that support humoral immunity.

METHODS

CD4(+) T-cell differentiation by primary human B cells was investigated in in vitro cocultures by using tetanus toxoid and Salmonella species as antigen models. T-cell differentiation was assessed by using intracellular cytokines and subset-specific transcription factors and markers. IgM and IgG formation was analyzed by means of ELISA.

RESULTS

Human B cells, but not dendritic cells, induce prominent and stable coexpression of TH1 and follicular helper T (TFH) cell characteristics during priming and on antigen recall. TH1/TFH cells coexpress the TH1 and TFH effector cytokines IFN-γ and IL-21 and the TFH marker CXCR5, demonstrating that the coexpressed TH1 and TFH subset-specifying transcription factors T-box transcription factor (T-bet) and B cell lymphoma 6 are both functionally active. B cell-derived IL-6 and IL-12 controlled respective expression of IL-21 and IFN-γ, with IL-21 being key for humoral immunity.

CONCLUSION

Human B cells exploit CD4(+) T-cell plasticity to create flexibility in the effector T-cell response. Induction of a T-cell subset coexpressing IL-21 and IFN-γ might combine IL-21-mediated T-cell aid for antibody production while maintaining TH1 cytokine expression to support other cellular immune defenses.

IFNγ expression by an attenuated strain of Salmonella enterica serovar Typhimurium improves vaccine efficacy in susceptible TLR4-defective C3H/HeJ mice

C3H/HeJ mice carry a mutated allele of TLR4 gene (TLR4 ( d )) and thus are hyporesponsive to the lethal effects of lipopolysaccharide (LPS). Characteristically, however, the mice are also hypersusceptible to infections, particularly by Gram-negative bacteria such as Salmonella enterica serovar Typhimurium (S. typhimurium) and are known to be difficult to vaccinate against virulent exposure. This is observed despite the expression of wild-type allele of Nramp1 gene, another important determinant of Salmonella susceptibility. In contrast, C3H/HeN mice (TLR4 ( n ) Nramp1 ( n )) express a functional TLR4 protein and are resistant to infection, even by virulent strains of S. typhimurium. In the present study, we describe the immune system-enhancing properties of an attenuated strain of S. typhimurium engineered to express murine IFN-γ. This strain (designated GIDIFN) was able to modulate immune responses following systemic inoculation by upregulating the production of inflammatory mediators (IL-6 and IL-12) and anti-bacterial effector molecules (nitric oxide; NO). Consequently, this led to a more effective control of bacterial proliferation in systemic target organs in both C3H/HeJ and C3H/HeN mice. Although evidence for the enhancement in immune responses could be observed as early as few hours post-inoculation, sustained improvements required 2-3 days to manifest. Vaccination of C3H/HeJ mice with GIDIFN strain, even at low doses, conferred a significantly higher degree of protection against challenge with virulent Salmonella in susceptible C3H/HeJ mice. Our data demonstrate that IFNγ-expressing Salmonella are immunogenic and confer excellent protection against virulent challenge in susceptible C3H/HeJ mice; in addition they may be used as an effective mucosal delivery vectors against virulent infection and for boosting immune responses in immunodeficient hosts.

NKRP1A+ γδ and αβ T cells are preferentially induced in patients with Salmonella infection

NKRP1A(+) γδ and αβ T cells play an important role at the early phase of Salmonella infection in mice. Meanwhile, association between NKRP1A(+) T cells and human Salmonella infection has not been reported. The objective of this study was to investigate the role of the peripheral NKRP1A(+) T cells in immune response to Salmonella infection. Expression of NKRP1A in peripheral γδ and αβ T cells and production of interferon (IFN) γ and interleukin (IL)-4 in NKRP1A(+) γδ and αβ T cells were analyzed in 28 patients with acute phase Salmonella infection, 23 patients with acute bacterial enterocolitis other than Salmonella infection (disease controls) and 44 normal controls by flow cytometry. The proportion of γδ T cells expressing NKRP1A and that of IFNγ-producing cells in NKRP1A(+) γδ cells were significantly higher in Salmonella group than those in other two groups. Compared with normal controls, the proportion of αβ T cells expressing NKRP1A and that of IL-4-producing cells in NKRP1A(+) αβ cells were significantly higher in Salmonella group. These data suggested that NKRP1A(+) T cells might play an important role in the early defense mechanism against Salmonella infection.

Stromal IFN-γR-signaling modulates goblet cell function during Salmonella Typhimurium infection

Enteropathogenic bacteria are a frequent cause of diarrhea worldwide. The mucosal defenses against infection are not completely understood. We have used the streptomycin mouse model for Salmonella Typhimurium diarrhea to analyze the role of interferon gamma receptor (IFN-γR)-signaling in mucosal defense. IFN-γ is known to contribute to acute S. Typhimurium diarrhea. We have compared the acute mucosal inflammation in IFN-γR(-/-) mice and wild type animals. IFN-γR(-/-) mice harbored increased pathogen loads in the mucosal epithelium and the lamina propria. Surprisingly, the epithelium of the IFN-γR(-/-) mice did not show the dramatic "loss" of mucus-filled goblet cell vacuoles, a hallmark of the wild type mucosal infection. Using bone marrow chimeric mice we established that IFN-γR-signaling in stromal cells (e.g. goblet cells, enterocytes) controlled mucus excretion/vacuole loss by goblet cells. In contrast, IFN-γR-signaling in bone marrow-derived cells (e.g. macrophages, DCs, PMNs) was required for restricting pathogen growth in the gut tissue. Thus IFN-γR-signaling influences different mucosal responses to infection, including not only pathogen restriction in the lamina propria, but, as shown here, also goblet cell function.

Nitric oxide-mediated intracellular growth restriction of pathogenic Rhodococcus equi can be prevented by iron

Rhodococcus equi is an intracellular pathogen which causes pneumonia in young horses and in immunocompromised humans. R. equi arrests phagosome maturation in macrophages at a prephagolysosome stage and grows inside a privileged compartment. Here, we show that, in murine macrophages activated with gamma interferon and lipopolysaccharide, R. equi does not multiply but stays viable for at least 24 h. Whereas infection control of other intracellular pathogens by activated macrophages is executed by enhanced phagosome acidification or phagolysosome formation, by autophagy or by the interferon-inducible GTPase Irgm1, none of these mechanisms seems to control R. equi infection. Growth control by macrophage activation is fully mimicked by treatment of resting macrophages with nitric oxide donors, and inhibition of bacterial multiplication by either activation or nitric oxide donors is annihilated by cotreatment of infected macrophages with ferrous sulfate. Transcriptional analysis of the R. equi iron-regulated gene iupT demonstrates that intracellular R. equi encounters iron stress in activated, but not in resting, macrophages and that this stress is relieved by extracellular addition of ferrous sulfate. Our results suggest that nitric oxide is central to the restriction of bacterial access to iron in activated macrophages.

Early interferon-γ production in human lymphocyte subsets in response to nontyphoidal Salmonella demonstrates inherent capacity in innate cells

Background

Nontyphoidal Salmonellae frequently cause life-threatening bacteremia in sub-Saharan Africa. Young children and HIV-infected adults are particularly susceptible. High case-fatality rates and increasing antibiotic resistance require new approaches to the management of this disease. Impaired cellular immunity caused by defects in the T helper 1 pathway lead to intracellular disease with Salmonella that can be countered by IFNγ administration. This report identifies the lymphocyte subsets that produce IFNγ early in Salmonella infection.

Methodology

Intracellular cytokine staining was used to identify IFNγ production in blood lymphocyte subsets of ten healthy adults with antibodies to Salmonella (as evidence of immunity to Salmonella), in response to stimulation with live and heat-killed preparations of the D23580 invasive African isolate of Salmonella Typhimurium. The absolute number of IFNγ-producing cells in innate, innate-like and adaptive lymphocyte subpopulations was determined.

Principal Findings

Early IFNγ production was found in the innate/innate-like lymphocyte subsets: γδ-T cells, NK cells and NK-like T cells. Significantly higher percentages of such cells produced IFNγ compared to adaptive αβ-T cells (Student's t test, P<0.001 and ≤0.02 for each innate subset compared, respectively, with CD4⁺- and CD8⁺-T cells). The absolute numbers of IFNγ-producing cells showed similar differences. The proportion of IFNγ-producing γδ-T cells, but not other lymphocytes, was significantly higher when stimulated with live compared with heat-killed bacteria (P<0.0001).

Conclusion/Significance

Our findings indicate an inherent capacity of innate/innate-like lymphocyte subsets to produce IFNγ early in the response to Salmonella infection. This may serve to control intracellular infection and reduce the threat of extracellular spread of disease with bacteremia which becomes life-threatening in the absence of protective antibody. These innate cells may also help mitigate against the effect on IFNγ production of depletion of Salmonella-specific CD4⁺-T lymphocytes in HIV infection.

Metallomic analysis of macrophages infected with Histoplasma capsulatum reveals a fundamental role for zinc in host defenses

The fungal pathogen Histoplasma capsulatum evades the innate and adaptive immune responses and thrives within resting macrophages. Cytokines that induce antimicrobial activity, such as granulocyte macrophage colony-stimulating factor (GM-CSF), inhibit H. capsulatum growth in macrophages. Conversely, interleukin 4 inhibits the killing of intracellular pathogens. Using inductively coupled plasma mass spectrometry, we examined alterations in the metal homeostasis of murine H. capsulatum-infected macrophages that were exposed to activating cytokines. Decreases in the levels of iron (Fe(2+) and Fe(3+)) and zinc (Zn(2+)) were observed in infected, GM-CSF-treated macrophages compared with those in infected controls. Interleukin 4 reversed the antifungal activity of GM-CSF-activated macrophages and was associated with increased intracellular Zn(2+) levels. Chelation of Zn(2+) inhibited yeast replication in both the absence of macrophages and the presence of macrophages. Treatment of cells with GM-CSF altered the host Zn(2+) binding species profile. These results establish that Zn(2+) deprivation may be a host defense mechanism utilized by macrophages.

Dynamics of intracellular bacterial replication at the single cell level

Several important pathogens cause disease by surviving and replicating within host cells. Bacterial proliferation is the product of both replication and killing undergone by the population. However, these processes are difficult to distinguish, and are usually assessed together by determination of net bacterial load. In addition, measurement of net load does not reveal heterogeneity within pathogen populations. This is particularly important in persistent infections in which slow or nongrowing bacteria are thought to have a major impact. Here we report the development of a reporter system based on fluorescence dilution that enables direct quantification of the replication dynamics of Salmonella enterica serovar Typhimurium (S. Typhimurium) in murine macrophages at both the population and single-cell level. We used this technique to demonstrate that a major S. Typhimurium virulence determinant, the Salmonella pathogenicity island 2 type III secretion system, is required for bacterial replication but does not have a major influence on resistance to killing. Furthermore, we found that, upon entry into macrophages, many bacteria do not replicate, but appear to enter a dormant-like state. These could represent an important reservoir of persistent bacteria. The approach could be extended to other pathogens to study the contribution of virulence and host resistance factors to replication and killing, and to identify and characterize nonreplicating bacteria associated with chronic or latent infections.

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Production of IL-12 and IFN-γ secretion are important components of the protective host response against the intracellular bacterial pathogen, Salmonella typhimurium. While infection with Salmonella does elicit this T helper type 1 response, its magnitude does not appear to be sufficient to prevent infection or limit pathogenesis. Therefore we have investigated factors which might limit a T helper type 1 response following infection. Previously we found that infection of antigen presenting cells with Salmonella dramatically increases cyclooxygenase-2 (COX-2) activity, resulting in high levels of prostaglandin E₂ (PGE₂). Since PGE₂ production can have profound effects on initiation of T helper type 1 responses, we questioned whether this mediator might limit antigen-specific T cell activation. Here we show that blockage of COX-2 activity with the selective inhibitor celecoxib leads to enhancement of the T helper type 1 components stimulated by Salmonella infection. In vitro studies demonstrate the induction of IL-12 and IFN-γ upon Salmonella exposure, which are further increased following COX-2 inhibition. Taken together these in vitro studies suggest that COX-2 activity can limit a salmonella-initiated T helper type 1 response.

Interferon-gamma limits the availability of iron for intramacrophage Salmonella typhimurium

In stimulating effector functions of mononuclear phagocytes, IFN-gamma is of pivotal importance in host defense against intramacrophage pathogens including salmonellae. As the activity of IFN-gamma is modulated by iron and since a sufficient availability of iron is essential for the growth of pathogens, we investigated the regulatory effects of IFN-gamma on iron homeostasis and immune function in murine macrophages infected with Salmonella typhimurium. In Salmonella-infected phagocytes, IFN-gamma caused a significant reduction of iron uptake via transferrin receptor 1 and resulted in an increased iron efflux caused by an enhanced expression of the iron exporter ferroportin 1. Moreover, the expression of haem oxygenase 1 and of the siderophore-capturing antimicrobial peptide lipocalin 2 was markedly elevated following bacterial invasion, with IFN-gamma exerting a super-inducing effect. This observed regulatory impact of IFN-gamma reduced the intracellular iron pools within infected phagocytes, thus restricting the acquisition of iron by engulfed Salmonella typhimurium while concomitantly promoting NO and TNF-alpha production. Our data suggest that the modulation of crucial pathways of macrophage iron metabolism in response to IFN-gamma concordantly aims at withdrawing iron from intracellular Salmonella and at strengthening macrophage immune response functions. These regulations are thus consistent with the principles of nutritional immunity.

Hemophagocytic macrophages harbor Salmonella enterica during persistent infection

Salmonella enterica subspecies can establish persistent, systemic infections in mammals, including human typhoid fever. Persistent S. enterica disease is characterized by an initial acute infection that develops into an asymptomatic chronic infection. During both the acute and persistent stages, the bacteria generally reside within professional phagocytes, usually macrophages. It is unclear how salmonellae can survive within macrophages, cells that evolved, in part, to destroy pathogens. Evidence is presented that during the establishment of persistent murine infection, macrophages that contain S. enterica serotype Typhimurium are hemophagocytic. Hemophagocytic macrophages are characterized by the ingestion of non-apoptotic cells of the hematopoietic lineage and are a clinical marker of typhoid fever as well as certain other infectious and genetic diseases. Cell culture assays were developed to evaluate bacterial survival in hemophagocytic macrophages. S. Typhimurium preferentially replicated in macrophages that pre-phagocytosed viable cells, but the bacteria were killed in macrophages that pre-phagocytosed beads or dead cells. These data suggest that during persistent infection hemophagocytic macrophages may provide S. Typhimurium with a survival niche.

Analysis of cells targeted by Salmonella type III secretion in vivo

The type III secretion systems (TTSS) encoded in Salmonella pathogenicity island-1 and -2 (SPI-1 and -2) are virulence factors required for specific phases of Salmonella infection in animal hosts. However, the host cell types targeted by the TTSS have not been determined. To investigate this, we have constructed translational fusions between the ß-lactamase reporter and a broad array of TTSS effectors secreted via SPI-1, SPI-2, or both. Secretion of the fusion protein to a host cell was determined by cleavage of a specific fluorescent substrate. In cultured cells, secretion of all six effectors could be observed. However, two to four days following i.p. infection of mice, only effectors secreted by SPI-2 were detected in spleen cells. The cells targeted were identified via staining with nine different cell surface markers followed by FACS analysis as well as by conventional cytological methods. The targeted cells include B and T lymphocytes, neutrophils, monocytes, and dendritic cells, but not mature macrophages. To further investigate replication in these various cell types, Salmonella derivatives were constructed that express a red fluorescent protein. Bacteria could be seen in each of the cell types above; however, most viable bacteria were present in neutrophils. We find that Salmonella is capable of targeting most phagocytic and non-phagocytic cells in the spleen but has a surprisingly high preference for neutrophils. These findings suggest that Salmonella specifically target splenic neutrophils presumably to attenuate their microbicidal functions, thereby promoting intracellular survival and replication in the mouse.

Bacteria of the Salmonella genus are important human pathogens and a leading cause of food-borne illness. Salmonella species' ability to cause disease relies on the activities of two sophisticated molecular syringes that allow the bacteria to pump proteins into cells that they infect. The activities of these syringes have been studied extensively in cells grown under laboratory conditions and shown to be essential for the infectious process in animal models. However, the specific cells within infected organs that are targeted by these syringes have not been identified. In this work we describe the specific spleen cells targeted by Salmonella in the mouse. We find that Salmonella is capable of targeting most cell types using their molecular syringes. Quite surprisingly, we find that Salmonella mostly targets neutrophils, a cell type not thought to be associated with live Salmonella in host tissues. These findings challenge our current views of Salmonella infection and may lead to new insight for treating the disease.

Primary macrophages from HIV-infected adults show dysregulated cytokine responses to Salmonella, but normal internalization and killing

BACKGROUND

Adults with advanced HIV are susceptible to invasive and recrudescent infections with nontyphoidal salmonellae.

OBJECTIVES

To examine whether persistence and recurrence of salmonella infection results from HIV-related defects in macrophage internalization and intracellular killing or from ineffective type 1 cytokine responses. Such defects could be a direct consequence of macrophage HIV infection or secondary to reduced enhancement of macrophage effector functions by interferon-gamma (IFNgamma) as CD4 cell count falls.

DESIGN

Ex-vivo scientific case-control study.

METHODS

Primary ex-vivo human alveolar macrophages (huAM) from HIV-negative and HIV-positive subjects were challenged with Salmonella typhimurium under unprimed and IFNgamma-primed conditions to study internalization and intracellular killing of bacteria and cytokine responses of huAM.

RESULTS

Priming of huAM with IFNgamma reduced bacterial internalization but enhanced microbicidal activity against intracellular salmonellae. HuAM from HIV-positive subjects showed unimpaired internalization and intracellular killing of salmonellae, with and without IFNgamma priming. Opsonic and mannose receptor (CD206)-mediated entry was not required for optimal internalization. HuAM from HIV-positive subjects, however, exhibited increased secretion of tumour necrosis factor alpha (TNFalpha), interleukin (IL)-10 and IL-12 in response to S. typhimurium challenge, regardless of IFNgamma priming. This cytokine dysregulation showed a trend to a curvilinear relationship with peripheral CD4 cell count, with marked decline at values < 250 cell/mul.

CONCLUSIONS

Dysregulation of proinflammatory cytokine release, including IL-12, by macrophages during salmonella infection may underlie the susceptibility to severe salmonellosis in patients with AIDS. This defect was not reversed by IFNgamma and may represent a proinflammatory effect of HIV infection upon the macrophage or the alveolar milieu.

Salmonella induces prominent gene expression in the rat colon

Background

Salmonella enteritidis is suggested to translocate in the small intestine. In vivo it induces gene expression changes in the ileal mucosa and Peyer's patches. Stimulation of Salmonella translocation by dietary prebiotics fermented in colon suggests involvement of the colon as well. However, effects of Salmonella on colonic gene expression in vivo are largely unknown. We aimed to characterize time dependent Salmonella-induced changes of colonic mucosal gene expression in rats using whole genome microarrays. For this, rats were orally infected with Salmonella enteritidis to mimic a foodborne infection and colonic gene expression was determined at days 1, 3 and 6 post-infection (n = 8 rats per time-point). As fructo-oligosaccharides (FOS) affect colonic physiology, we analyzed colonic mucosal gene expression of FOS-fed versus cellulose-fed rats infected with Salmonella in a separate experiment. Colonic mucosal samples were isolated at day 2 post-infection.

Results

Salmonella affected transport (e.g. Chloride channel calcium activated 6, H⁺/K⁺ transporting Atp-ase), antimicrobial defense (e.g. Lipopolysaccharide binding protein, Defensin 5 and phospholipase A2), inflammation (e.g. calprotectin), oxidative stress related genes (e.g. Dual oxidase 2 and Glutathione peroxidase 2) and Proteolysis (e.g. Ubiquitin D and Proteosome subunit beta type 9). Furthermore, Salmonella translocation increased serum IFNγ and many interferon-related genes in colonic mucosa. The gene most strongly induced by Salmonella infection was Pancreatitis Associated Protein (Pap), showing >100-fold induction at day 6 after oral infection. Results were confirmed by Q-PCR in individual rats. Stimulation of Salmonella translocation by dietary FOS was accompanied by enhancement of the Salmonella-induced mucosal processes, not by induction of other processes.

Conclusion

We conclude that the colon is a target tissue for Salmonella, considering the abundant changes in mucosal gene expression.

Abortion due to Salmonella enterica serovar Abortusovis (S. Abortusovis) in ewes is associated to a lack of production of IFN-γ and can be prevented by immunization with inactivated S. Abortusovis vaccine

Salmonellosis due to Salmonella enterica serovar Abortusovis (S. Abortusovis) is mainly characterized by abortion in sheep. Little is known about the immune response, which develops in the host as a result of infection. We evaluated the immune response of pregnant ewes vaccinated and successively exposed to full virulent S. Abortusovis. We found that vaccine constituted by inactivated S. Abortusovis induced both humoral and cellular-mediated immune response and that it provided protection against a challenge infection due to a fully virulent S. Abortusovis. Furthermore, we found an association between the lack of capability to produce IFN-gamma and abortion. This evidence suggests that protection against abortion can be associated to an IFN-gamma mediated mechanism. Our findings represent an interesting insight to better understand the interplay between host and S. Abortusovis and the effector mechanisms underpinning immune-based protection.

Proteinase-activated receptors induce nonoxidative, antimicrobial peptides and increased antimicrobial activity in human mononuclear phagocytes

As thrombin and SFLLRNPNDKYEPF (SFLLRN-14), a synthetic ligand, mainly of the proteinase-activated receptor-1 (PAR-1), induce in monocytes the synthesis and secretion of chemokines, the PAR pathway can be viewed as a mononuclear phagocyte-activating principle. Classically, antimicrobial activity of mononuclear phagocytes is the measure for activation. Here, we investigated whether thrombin or SFLLRN-14 increases the antimicrobial activity of human monocytes and compared these effects to those of IFN-gamma. Furthermore, we measured the effects of these agents on the secretion of reactive oxygen intermediates and the antimicrobial activity of acid peptide extracts from monocytes. Human monocytes were exposed to maximally active concentrations of thrombin, SFLLRN-14, and IFN-gamma. Human monocytes treated with thrombin or SFLLRN-14 and then challenged with Salmonella enterica serovar typhimurium, including its attenuated mutant phoP, or Listeria monocytogenes killed, within 3 h, significantly more bacteria than control cells, an effect comparable with or surpassing the effect of IFN-gamma. This finding establishes the proteinase-PAR pathway as a potent, alternate activation pathway of mononuclear phagocytes. Thrombin and SFLLRN-14 had no significant effects on the amount of H(2)O(2) secreted by monocytes. This was in contrast to IFN-gamma, which as expected, increased the secretion of H(2)O(2) by approximately fourfold. Thrombin and SFLLRN-14, but not IFN-gamma, however, significantly increased the antimicrobial activity of acid peptide extracts of monocytes in a radial diffusion assay. Taken together, these findings suggest that IFN-gamma and thrombin differentially regulate oxidative and nonoxidative killing systems of human monocytes.

Role for inducible costimulator in control of Salmonella enterica serovar Typhimurium infection in mice

Inducible costimulator (ICOS) is expressed on activated T cells and plays a key role in sustaining and enhancing the effector function of CD4 T cells. Given the function of this molecule in sustaining T-cell responses, we reasoned that ICOS might play an important role in a prolonged infection model, such as Salmonella infection of mice. To test this hypothesis, wild-type (WT) and ICOS-deficient (ICOS-/-) mice were infected systemically with a Salmonella enterica serovar Typhimurium strain expressing the chicken ovalbumin gene (Salmonella-OVA). ICOS-/- mice exhibited greater splenomegaly than WT mice and showed delayed bacterial clearance. The acquired immune response in this model was slow to develop. Maximal T-cell responses to Salmonella-OVA were detected at 3 weeks postinfection in both WT and ICOS-/- mice. CD4 T-cell-dependent gamma interferon production and a class switch to immunoglobulin G2a were severely reduced in ICOS-/- mice. ICOS-/- mice also exhibited a substantial defect in antigen-specific CD8 T-cell responses. In vitro, the effect of anti-ICOS on CD8 T-cell division was greater when CD8 T cells rather than CD4 T cells expressed ICOS, suggesting that the in vivo effects of ICOS on CD8 T cells could be direct. Taken together, these studies show that ICOS plays a critical role in control of Salmonella infection in mice, with effects on antibody, Th1, and CD8 T-cell responses.

Murine osteoblasts respond to LPS and IFN-gamma similarly to macrophages

Osteoblasts are bone-forming mesenchymal cells, while macrophages are cells of hematopoietic origin responsible for innate immunity. Lipopolysaccharide (LPS) can induce tolerance in macrophages, whereas interferon (IFN)-gamma can activate macrophages to produce cytokines, exert bactericidal effects, and present antigens. In this study, we examined such macrophagic phenotypes regulated by LPS and IFN-gamma in murine osteoblasts. In both primary calvarial osteoblasts and osteoblastic MC3T3-E1 cells, LPS pretreatment resulted in reduced production of IL-6 in response to a subsequent LPS stimulation or to Salmonella infection, indicating the existence of LPS-induced tolerance in osteoblasts. Furthermore, IFN-gamma treatment of MC3T3-E1 cells resulted in both enhanced IL-6 production in response to LPS and upregulation of major histocompatibility complex class II (MHC II). Following infection, Salmonella-containing vacuoles (SCVs) were formed in MC3T3-E1 cells, and IFN-gamma pretreatment enhanced bactericidal effects on intracellular Salmonella. Taken together, these observations indicate that osteoblasts can exhibit a subset of phenotypes reminiscent of macrophages in the course of bacterial infection.

Repression of SPI2 transcription by nitric oxide-producing, IFNgamma-activated macrophages promotes maturation of Salmonella phagosomes

By remodeling the phagosomal membrane, the type III secretion system encoded within the Salmonella pathogenicity island-2 (SPI2) helps Salmonella thrive within professional phagocytes. We report here that nitric oxide (NO) generated by IFNγ-activated macrophages abrogates the intracellular survival advantage associated with a functional SPI2 type III secretion system. NO congeners inhibit overall expression of SPI2 effectors encoded both inside and outside the SPI2 gene cluster, reflecting a reduced transcript level of the sensor kinase SsrA that governs overall SPI2 transcription. Down-regulation of SPI2 expression in IFNγ-treated macrophages does not seem to be the result of global NO cytotoxicity, because transcription of the housekeeping rpoD sigma factor remains unchanged, whereas the expression of the hmpA-encoded, NO-metabolizing flavohemoprotein is stimulated. Because of the reduced SPI2 expression, Salmonella-containing vacuoles interact more efficiently with compartments of the late endosomal/lysosomal system in NO-producing, IFNγ-treated macrophages. These findings demonstrate that inhibition of intracellular SPI2 transcription by NO promotes the interaction of Salmonella phagosomes with the degradative compartments required for enhanced antimicrobial activity. Transcriptional repression of a type III secretion system that blocks phagolysosome biogenesis represents a novel mechanism by which NO mediates resistance of IFNγ-activated phagocytes to an intracellular pathogen.

Gamma interferon enhances internalization and early nonoxidative killing of Salmonella enterica serovar Typhimurium by human macrophages and modifies cytokine responses

Gamma interferon (IFN-gamma) is a critical cytokine in host defense against salmonella infections, but its role in phagocytic killing of intracellular Salmonella spp. has been investigated mainly in animal rather than human cells. We measured the effect of recombinant IFN-gamma (rIFN-gamma) priming on bacterial internalization, intracellular killing, oxidative burst, and cytokine release during phagocytosis of Salmonella enterica serovar Typhimurium by human monocyte-derived macrophages (MDM). Eleven-day-old MDM, primed for 72 h with rIFN-gamma (100 ng/ml) exhibited an increased proportion of cells with associated bacteria (31% versus 26%, P = 0.036) and a 67% increase in internalized bacteria per cell compared to unprimed cells (P = 0.025). Retrieval of viable bacteria following internalization was reduced 3.6-fold in 72-h primed versus unprimed MDM (interquartile range, 3.1 to 6.4) at 0.5 h due to enhanced early intracellular killing, and this difference was maintained up to 24 h. In contrast, cells primed for only 24 h exhibited no increase in early killing. MDM were competent to produce an early oxidative burst when stimulated with phorbol myristate acetate, which was fully abrogated by the respiratory burst inhibitor diphenyleneiodonium chloride (DPI), but infection of MDM with S. enterica serovar Typhimurium did not cause an increase in the early respiratory burst under unprimed or primed conditions, and DPI had no effect on the early killing of bacteria by primed or unprimed MDM. During 24 h following infection, rIFN-gamma-primed MDM released more interleukin-12 (IL-12) and less IL-10 relative to unprimed cells. We conclude that 72-h priming with rIFN-gamma increases the efficiency of internalization and nonoxidative early intracellular killing of S. enterica serovar Typhimurium by human macrophages and modifies subsequent cytokine release.

Salmonella carrier state in chicken: comparison of expression of immune response genes between susceptible and resistant animals

Asymptomatic Salmonella enterica serovar Enteritidis carrier state in poultry has serious consequences on food safety and public health due to the risks of food poisoning following consumption of contaminated products. An understanding the mechanisms of persistence of Salmonella in the digestive tract of chicken can be achieved by a better knowledge of the defects in the control of infection in susceptible versus resistant animals. The gene expression of innate immune response factors including anti-microbial molecules, inflammatory and anti-infectious cytokines was studied in the caecal lymphoid tissue associated with the carrier state. Expression levels of these genes were assessed by real-time PCR and were compared in two inbred lines of chickens differing in resistance to the carrier state following oral inoculation of S. enterica serovar Enteritidis at 1 week of age. No correlation was observed between resistance/susceptibility to caecal carrier state and level of interleukin (IL)-1beta, IL-8, IL-18, inducible NO synthase (iNOS) and natural resistance associated macrophage protein 1 (NRAMP1). A high baseline level of defensin gene expression was recorded in young animals from the susceptible line. In contrast, a significantly low expression of interferon-gamma (IFN-gamma) gene was observed in these susceptible infected animals in comparison to resistant ones and healthy counterparts. IFN-gamma expression level represents a valuable indication of immunodeficiency associated with persistence of Salmonella in the chicken digestive tract, and IFN-gamma thus represents a factor to consider in the development of prophylactic measures for the reduction of Salmonella carrier state.

Cytokines in Salmonellosis

The recruitment and activation of phagocytic cells in infected tissues and the induction of T-cell- and B-cell-dependent acquired immunity are crucial for the control and resolution of Salmonella infections. These complex processes require the interaction of bacteria with a multitude of cell surface receptors and the controlled production of soluble mediators. The mechanisms of cytokine induction in response to Salmonella and the role of cytokine networks in Salmonella infections are the main foci of this review. Pathogen-associated molecular pattern receptors play an important role in recognition of bacteria by the host. Effective immunity against the bacterium therefore relies on the ability of the host to recruit phagocytes in the tissues and to enhance the antibacterial functions of these inflammatory cells. TNF-a, IFN-?, IL12, IL15, and IL18 are needed for the full expression of innate host resistance to Salmonella. The genes for mammalian cytokines can be cloned into suitable vectors and expressed in Salmonella as functional proteins. The in vivo production of cytokines by Salmonella carriers can have therapeutic applications and can modulate immune functions in the host. The possibility to modulate antigen-specific immune responses by expressing cytokines in Salmonella is illustrated by the increase in Salmonella-specific IgA responses induced by administration of IL-5-expressing bacteria. The same cytokines that are responsible for endotoxic shock are elevated in the late stages of lethal Salmonella infections, indicating that the toxicity of Salmonella lipopolysaccharide (LPS) may actually be contributing to the death of the host.

Cyclooxygenase-2-mediated prostaglandin E2 production in mesenteric lymph nodes and in cultured macrophages and dendritic cells after infection with Salmonella

Although numerous studies have demonstrated the ability of intestinal epithelial cells to produce PGs after infection with wild-type strains of Salmonella, few studies have focused on Salmonella-induced prostanoids in mucosal lymphoid tissues. This is surprising in view of the profound effects PGs can have on the host response. To begin to address PG production at mucosal sites, mice were orally inoculated with Salmonella, and at varying times postinfection cyclooxygenase-2 (COX-2) mRNA expression and PGE(2) synthesis were investigated. COX-2 mRNA expression was highly inducible in the mesenteric lymph nodes, whereas COX-1 mRNA levels were constitutive. PGE(2) production also increased significantly in the mesenteric lymph nodes following exposure to viable Salmonella, but not after exposure to killed bacteria. This increased PGE(2) response could be blocked by treatment of mice with the selective COX-2 inhibitor, celecoxib. Treatment of mice with celecoxib during salmonellosis resulted in increased viable bacteria in the mesenteric lymph nodes by day 3 postinfection. However, celecoxib treatment prolonged the survival of lethally infected animals. In vitro studies demonstrated Salmonella-induced up-regulation of COX-2 mRNA expression and PGE(2) secretion by both macrophages and dendritic cells, which could also be blocked in the presence of celecoxib. Interestingly, exposure of these cultured APCs to viable Salmonella was a much greater stimulus for induction of PGE(2) synthesis than exposure to Salmonella-derived LPS. The present study demonstrates induction of PGE(2) synthesis in mesenteric lymph nodes, macrophages, and dendritic cells after infection with wild-type salmonella.

Intracellular adhesion molecule 1 plays a key role in acquired immunity to salmonellosis

This study investigated the role of intracellular adhesion molecule 1 (ICAM-1) during Salmonella enterica serovar Typhimurium infection of mice. We show that ICAM-1 is expressed in and around granulomas on day 4 of infection in wild-type mice. However, when naive ICAM-1(-/-) mice were challenged with a sublethal dose of serovar Typhimurium, there were no detectable differences in systemic bacterial burden over the first 9 days of infection compared to wild-type control mice. When mice were immunized with the S. enterica serovar Typhimurium vaccine strain SL2361 and then challenged with the virulent S. enterica serovar Typhimurium strain C5, 100% of the ICAM-1(-/-) mice succumbed to infection, compared to 30% of wild-type mice. T-cell responses, as measured by activation via interleukin-2 production, as well as antibody responses were comparable in the ICAM-1(-/-) and wild-type mice. Following challenge, counts in organs were significantly higher in the ICAM-1(-/-) mice, and histological examination of organs showed pathological differences. Strain SL3261-immunized wild-type mice had cellular infiltrate and normal granuloma formation in the liver and spleen on days 5 and 10 after challenge with strain C5. ICAM-1(-/-) mice had a similar infiltrate on day 5, whereas on day 10 the infiltrate was more widespread and there were fewer macrophages associated with the granulomas. High circulating levels of tumor necrosis factor alpha and gamma interferon, as well as a high burden of strain C5 in the blood, accompanied the differences in histopathology. In this study we show that ICAM-1 plays a critical role during rechallenge of immunized mice with virulent S. enterica serovar Typhimurium.

Induction of antimicrobial pathways during early-phase immune response to Salmonella spp. in murine macrophages: gamma interferon (IFN-gamma) and upregulation of IFN-gamma receptor alpha expression are required for NADPH phagocytic oxidase gp91-stimulated oxidative burst and control of virulent Salmonella spp

The effect of gamma interferon (IFN-gamma) on elevation of reactive oxygen species and the viability of virulent wild-type and avirulent mutants of Salmonella enterica serovar Typhimurium and S. enterica serovar Infantis was studied in a murine macrophage cell line (J774.2 cells). S. enterica serovar Typhimurium 14028 phoP and a rough lipopolysaccharide mutant of S. enterica serovar Infantis 1326/28 (phi(r)) (avirulent mutants) induced NADPH phagocytic oxidase gp91 (gp91(phox)) activity and a significant (P < 0.05) elevation of reactive oxygen species within 12 h without coculture with IFN-gamma. This coincided with reduced survival of S. enterica serovar Typhimurium14028 phoP or stasis of S. enterica serovar Infantis phi(r). Fluorometric studies indicated that expression of IFN-gamma on infected J774.2 cells was not significantly (P > 0.05) elevated. However, studies with the virulent S. enterica serovar Typhimurium strains showed that a comparable level of control of bacterial numbers could only be achieved by coculture with IFN-gamma. This coincided with significant upregulation of IFN-gamma receptor alpha expression on the surface of J774.2 cells and was completely abolished by N-acetyl-L-cysteine captopril (an inhibitor of reactive oxygen species). Delay in reactive oxygen species induction due to a requirement for IFN-gamma and upregulation of IFN-gamma receptor alpha in macrophages infected with virulent salmonellae may result in greater dissemination of virulent salmonellae in host tissue.

Salmonella-induced macrophage death: the role of caspase-1 in death and inflammation

Salmonella typhimurium invades host macrophages and can induce either an almost immediate cell death or establish an intracellular niche within the phagocytic vacuole. Rapid cell death depends on the Salmonella pathogenicity island SPI1 and the host protein caspase-1, a member of the pro-apoptotic caspase family of proteases. Caspase-1-dependent cell death leads to the activation of the potent pro-inflammatory cytokines interleukin (IL)-1beta and IL-18 to produce bioactive cytokines. Animal studies indicate that the activation of these cytokines is necessary for efficient colonization of the mouse gastrointestinal tract. Salmonella that reside in the phagocytic vacuole do not cause this early cell death and can trigger a macrophage death at a much later time point. This late-phase cell death is dependent on SPI2-encoded genes and ompR.

Cytokines in host defense against Salmonella

Cytokines are key communication molecules between host cells in the defense against the enteric pathogen, Salmonella. Infection with Salmonella induces expression of multiple chemokines and proinflammatory cytokines in cultured intestinal epithelial cells and macrophages. In animal models, protective roles have been shown for IL-1alpha, TNFalpha, IFN-gamma, IL-12, IL-18 and IL-15, whereas IL-4 and IL-10 inhibit host defenses against Salmonella.

Cytokine gene expression in lymph node and spleen of sheep in response to Salmonella infection by two serotypes displaying different host specificity

In order to investigate the determinism of the host specificity and to better understand the host resistance mechanisms, infections of sheep were performed with either S. abortusovis, serotype specific for ovine species, or with S. dublin, serotype adapted to cattle and accidentally transmissible to human. Following a subcutaneous challenge, S. dublin disseminated more rapidly towards lymphoid tissues than S. abortusovis. However, S. abortusovis tended to persist in spleen more efficiently than S. dublin. Using a quantitative RT-PCR method, the expression level of ovine cytokines genes was measured in the draining lymph node and in the spleen, in the course of infection. Inflammatory cytokine response was characterised by an early and strong increase of IL-1beta and TNFalpha mRNA in both lymphoid organs following S. dublin infection, while S. abortusovis challenge only induced IL-1beta mRNA increase in the spleen at day 3 post-inoculation. Likewise, S. dublin infection provoked a marked increase of IL-12 mRNA and a slight up-regulation of IFNgamma gene transcription in the local lymphoid site, in contrast to S. abortusovis infection. Elsewhere, both serotypes induced a strong and early IL-10 mRNA production and had no effect on IL-4 gene expression. Finally, taken together, these data suggest that the intensity of inflammatory and anti-infectious cytokine responses, but not the type 2 cytokine response, is serotype-dependent. They also suggest that the host-specific serotype, by limiting the host cytokine-mediated defence, could favour its persistence within lymphoid organs.

Disruption of the genes for ClpXP protease in Salmonella enterica serovar Typhimurium results in persistent infection in mice, and development of persistence requires endogenous gamma interferon and tumor necrosis factor alpha

The enteric pathogen Salmonella enterica serovar Typhimurium, similar to other facultative intracellular pathogens, has been shown to respond to the hostile conditions inside macrophages of the host organism by producing a set of stress proteins that are also induced by various environmental stresses. The stress-induced ClpXP protease is a member of the ATP-dependent proteases, which are known to be responsible for more than 90% of all proteolysis in Escherichia coli. To investigate the contribution of the ClpXP protease to the virulence of serovar Typhimurium we initially cloned the clpP and clpX operon from the pathogenic strain serovar Typhimurium chi3306 and then created insertional mutations in the clpP and/or clpX gene. The Delta clpP and Delta clpX mutants were used to inoculate BALB/c mice by either the intraperitoneal or the oral route and found to be limited in their ability to colonize organs of the lymphatic system and to cause systemic disease in the host. A variety of experiments were performed to determine the possible reasons for the loss of virulence. An oxygen-dependent killing assay using hydrogen peroxide and paraquat (a superoxide anion generator) and a serum killing assay using murine serum demonstrated that all of the serovar Typhimurium Delta clpP and Delta clpX mutants were as resistant to these killing mechanisms as the wild-type strain. On the other hand, the macrophage survival assay revealed that all these mutants were more sensitive to the intracellular environment than the wild-type strain and were unable to grow or survive within peritoneal macrophages of BALB/c mice. In addition, it was revealed that the serovar Typhimurium ClpXP-depleted mutant was not completely cleared but found to persist at low levels within spleens and livers of mice. Interferon gamma-deficient mice and tumor necrosis factor alpha-deficient mice failed to survive the attenuated serovar Typhimurium infections, suggesting that both endogenous cytokines are essential for regulation of persistent infection with serovar Typhimurium.

Salmonella: immune responses and vaccines

Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Vaccination is an effective tool for the prevention of Salmonella infections. Host resistance to Salmonella relies initially on the production of inflammatory cytokines leading to the infiltration of activated inflammatory cells in the tissues. Thereafter T- and B-cell dependent specific immunity develops allowing the clearance of Salmonella microorganisms from the tissues and the establishment of long-lasting acquired immunity to re-infection. The increased resistance that develops after primary infection/ vaccination requires T-cells cytokines such as IFNgamma TNFalpha and IL12 in addition to opsonising antibody. However for reasons that are not fully understood seroconversion and/or the presence of detectable T-cell memory do not always correlate with the development of acquired resistance to infection.Whole-cell killed vaccines and subunit vaccines are used in the prevention of Salmonella infection in animals and in humans with variable results. A number of early live Salmonella vaccines derived empirically by chemical or u.v. mutagenesis proved to be immunogenic and protective and are still in use despite the need for repeated parenteral administration. Recent progress in the knowledge of the genetics of Salmonella virulence and modern recombinant DNA technology offers the possibility to introduce multiple defined attenuating and irreversible mutations into the bacterial genome. This has recently allowed the development of Salmonella strains devoid of significant side effects but still capable of inducing solid immunity after single oral administration. Live attenuated Salmonella vaccines have been used for the expression of heterologous antigens/proteins that can be successfully delivered to the immune system. Furthermore Salmonella can transfer plasmids encoding foreign antigens under the control of eukaryotic promoters (DNA vaccines) to antigen-presenting cells resulting in targeted delivery of DNA vaccines to these cells. Despite the great recent advances in the development of Salmonella vaccines a large proportion of the work has been conducted in laboratory rodents and more research in other animal species is required.

Avian T helper one/two immune response balance can be shifted toward inflammation by antigen delivery to scavenger receptors

Whether immune responses are dominated by inflammation or antibody production is often key to surviving infections. Therefore, differential control of these immune pathways determined by CD4 T cells is of fundamental interest for vaccine design. Little is known about how inflammatory [T helper cell (Th) type 1 (Th1)] versus antibody-inducing (Th2) choices are controlled in domestic fowl. To address this, MHC-matched chickens were immunized to test whether antibody-dominated Th2 or inflammatory Th1 responses could be preferentially activated, and our findings subsequently extended to outbred broiler breeders. Strategies used were known to shift the response in mice from Th2 to Th1 by delivering the injected antigen preferentially to macrophages. The model antigen, BSA, was maleylated to allow binding to scavenger receptors (SR) present on mammalian macrophages. Maleyl-BSA bound well in receptor-specific fashion to a chicken macrophage cell line. Compared with native BSA, immunization with SR-binding, maleyl-BSA modulated the immune response toward the Th1 pathway, as evident by increases in the magnitude of in vivo inflammatory reactions and declines in antibody-making responses. Initiation of a maleyl-BSA Th1 pathway is further supported by the enhanced ability of splenocytes to express mRNA for interferon-gamma in response to antigens. Together, these data establish the presence and functional relevance of SR in domestic fowl as well as provide a system for investigating the mechanisms controlling Th1/Th2 pathways in chickens. Moreover, the ability to direct immune responses toward either pathway by antigen maleylation will contribute significantly to the development of better vaccines for poultry diseases.

Salmonella infection does not increase expression and activity of the high affinity IL-12 receptor

Expression of high affinity IL-12 receptors is required for IL-12-mediated IFN-gamma production. Activation of this pathway has been shown to be critical in generating optimal cell-mediated immunity. Therefore, increased IL-12 receptor expression might be expected in the host response after infection by an intracellular bacterial pathogen. In the present study, we have made the surprising discovery that infection with Salmonella results in an early reduction of high affinity IL-12 receptor expression and activation. After oral inoculation with Salmonella, the level of mRNA expression encoding IL-12 receptor beta2 (IL-12Rbeta2) subunit was diminished 12 h postinfection in the mesenteric lymph nodes and subsequently in the spleen. Furthermore, decreased IL-12Rbeta2 mRNA expression was observed in CD4+ T lymphocytes isolated from the mesenteric lymph nodes and spleens of infected mice. Attenuated IL-12Rbeta2 mRNA expression correlated with reduced receptor signaling, as demonstrated by reduced IL-12-induced STAT4 phosphorylation in enriched T lymphocytes isolated from the mesenteric lymph nodes and spleens of Salmonella-infected mice. These in vivo results were substantiated with an in vitro model system. In this model system, T lymphocytes cocultured with Salmonella-infected macrophages expressed less IL-12Rbeta2 mRNA. The cocultured T cells were also less responsive to IL-12 as assessed by reduced phosphorylation of STAT4 and limited IFN-gamma secretion. Together, these studies suggest that Salmonella can limit an optimal host immune response by reducing the expression and activity of high affinity IL-12 receptors.

Antimicrobial actions of the NADPH phagocyte oxidase and inducible nitric oxide synthase in experimental salmonellosis. II. Effects on microbial proliferation and host survival in vivo

The roles of the NADPH phagocyte oxidase (phox) and inducible nitric oxide synthase (iNOS) in host resistance to virulent Salmonella typhimurium were investigated in gp91phox(-/)-, iNOS(-/)-, and congenic wild-type mice. Although both gp91phox(-/)- and iNOS(-/)- mice demonstrated increased susceptibility to infection with S. typhimurium compared with wild-type mice, the kinetics of bacterial replication were dramatically different in the gp91phox(-/)- and iNOS(-/)- mouse strains. Greater bacterial numbers were present in the spleens and livers of gp91phox(-/)- mice compared with C57BL/6 controls as early as day 1 of infection, and all of the gp91phox(-/)- mice succumbed to infection within 5 d. In contrast, an increased bacterial burden was detected within reticuloendothelial organs of iNOS(-/)- mice only beyond the first week of infection. Influx of inflammatory CD11b(+) cells, granuloma formation, and serum interferon gamma levels were unimpaired in iNOS(-/)- mice, but the iNOS-deficient granulomas were unable to limit bacterial replication. The NADPH phagocye oxidase and iNOS are both required for host resistance to wild-type Salmonella, but appear to operate principally at different stages of infection.

Salmonella exploits caspase-1 to colonize Peyer's patches in a murine typhoid model

Salmonella typhimurium invades host macrophages and induces apoptosis and the release of mature proinflammatory cytokines. SipB, a protein translocated by Salmonella into the cytoplasm of macrophages, is required for activation of Caspase-1 (Casp-1, an interleukin [IL]-1beta-converting enzyme), which is a member of a family of cysteine proteases that induce apoptosis in mammalian cells. Casp-1 is unique among caspases because it also directly cleaves the proinflammatory cytokines IL-1beta and IL-18 to produce bioactive cytokines. We show here that mice lacking Casp-1 (casp-1(-/)- mice) had an oral S. typhimurium 50% lethal dose (LD(50)) that was 1,000-fold higher than that of wild-type mice. Salmonella breached the M cell barrier of casp-1(-/)- mice efficiently; however, there was a decrease in the number of apoptotic cells, intracellular bacteria, and the recruitment of polymorphonuclear lymphocytes in the Peyer's patches (PP) as compared with wild-type mice. Furthermore, Salmonella did not disseminate systemically in the majority of casp-1(-/)- mice, as demonstrated by significantly less colonization in the PP, mesenteric lymph nodes, and spleens of casp-1(-/)- mice after an oral dose of S. typhimurium that was 100-fold higher than the LD(50). The increased resistance in casp-1(-/)- animals appears specific for Salmonella infection since these mice were susceptible to colonization by another enteric pathogen, Yersinia pseudotuberculosis, which normally invades the PP. These results show that Casp-1, which is both proapoptotic and proinflammatory, is essential for S. typhimurium to efficiently colonize the cecum and PP and subsequently cause systemic typhoid-like disease in mice.

Identification of genes required for chronic persistence of Brucella abortus in mice

The genetic basis for chronic persistence of Brucella abortus in lymphoid organs of mice, cows, and humans is currently unknown. We identified B. abortus genes involved in chronic infection, by assessing the ability of 178 signature-tagged mutants to establish and maintain persistent infection in mice. Each mutant was screened for its ability to colonize the spleens of mice at 2 and 8 weeks after inoculation. Comparison of the results from both time points identified two groups of mutants attenuated for chronic infection in mice. The first group was not recovered at either 2 or 8 weeks postinfection and was therefore defective in establishing infection. Mutants in this group carried transposon insertions in genes involved in lipopolysaccharide biosynthesis (wbkA), in aromatic amino acid biosynthesis, and in type IV secretion (virB1 and virB10). The second group, which was recovered at wild-type levels 2 weeks postinfection but not 8 weeks postinfection was able to establish infection but was unable to maintain chronic infection. One mutant in this group carried a transposon insertion in a gene with homology to gcvB of Mycobacterium tuberculosis, encoding glycine dehydrogenase, an enzyme whose activity is increased during the state of nonreplicating persistence. These results suggest that some mechanisms for long-term persistence may be shared among chronic intracellular pathogens. Furthermore, identification of two groups of genes, those required for initiating infection and those required only for long-term persistence, suggests that B. abortus uses distinct sets of virulence determinants to establish and maintain chronic infection in mice.