Serum TNF alpha in mouse typhoid and enhancement of a Salmonella infection by anti-TNF alpha antibodies

Salmonella Bloodstream Infections

Salmonella is a major foodborne pathogen of both animals and humans. This bacterium is responsible for considerable morbidity and mortality world-wide. Different serovars of this genus cause diseases ranging from self-limiting gastroenteritis to a potentially fatal systemic disease known as enteric fever. Gastrointestinal infections with Salmonella are usually self-limiting and rarely require medical intervention. Bloodstream infections, on the other hand, are often fatal even with hospitalization. This review describes the routes and underlying mechanisms of the extraintestinal dissemination of Salmonella and the chronic infections that sometimes result. It includes information on the pathogenicity islands and individual virulence factors involved in systemic dissemination as well as a discussion of the host factors that mediate susceptibility. Also, the major outbreaks of invasive Salmonella disease in the tropics are described.

Salmonella infection induces the reorganization of follicular dendritic cell networks concomitant with the failure to generate germinal centers

Germinal centers (GCs) are sites where plasma and memory B cells form to generate high-affinity, Ig class-switched antibodies. Specialized stromal cells called follicular dendritic cells (FDCs) are essential for GC formation. During systemic Salmonella Typhimurium (STm) infection GCs are absent, whereas extensive extrafollicular and switched antibody responses are maintained. The mechanisms that underpin the absence of GC formation are incompletely understood. Here, we demonstrate that STm induces a reversible disruption of niches within the splenic microenvironment, including the T and B cell compartments and the marginal zone. Alongside these effects after infection, mature FDC networks are strikingly absent, whereas immature FDC precursors, including marginal sinus pre-FDCs (MadCAM-1+) and perivascular pre-FDCs (PDGFRβ+) are enriched. As normal FDC networks re-establish, extensive GCs become detectable throughout the spleen. Therefore, the reorganization of FDC networks and the loss of GC responses are key, parallel features of systemic STm infections.

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

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

Within-host spatiotemporal dynamic of systemic salmonellosis: Ways to track infection, reaction to vaccination and antimicrobial treatment

During the last two decades our understanding of the complex in vivo host-pathogen interactions has increased due to technical improvements and new research tools. The rapid advancement of molecular biology, flow cytometry and microscopy techniques, combined with mathematical modelling, have empowered in-depth studies of systemic bacterial infections across scales from single molecules, to cells, to organs and systems to reach the whole organism level. By tracking subpopulations of bacteria in vivo using molecular or fluorescent tags, it has been possible to reconstruct the spread of infection within and between organs, allowing unprecedented quantification of the effects of antimicrobial treatment and vaccination. This review illustrates recent advances in the study of heterogeneous traits of the infection process and illustrate approaches to investigate the reciprocal interactions between antimicrobial treatments, bacterial growth/death as well as inter- and intra-organ spread. We also discuss how vaccines impact the in vivo behaviour of bacteria and how these findings can guide vaccine design and rational antimicrobial treatment.

Invasive Nontyphoidal Salmonella Disease in Africa

Nontyphoidal salmonellae (NTS) are a major cause of invasive (iNTS) disease in sub-Saharan Africa, manifesting as bacteremia and meningitis. Available epidemiological data indicate that iNTS disease is endemic in much of the region. Antimicrobial resistance is common and case fatality rates are high. There are well-characterized clinical associations with iNTS disease, including young age, HIV infection, malaria, malnutrition, anemia, and sickle cell disease. However, the clinical presentation of iNTS disease is often with fever alone, so clinical diagnosis is impossible without blood culture confirmation. No vaccine is currently available, making this a priority area for global health research. Over the past ten years, it has emerged that iNTS disease in Africa is caused by distinct pathovars of Salmonella Typhimurium, belonging to sequence type ST313, and Salmonella Enteritidis. These are characterized by genome degradation and appear to be adapting to an invasive lifestyle. Investigation of rare patients with primary immunodeficiencies has suggested a key role for interferon gamma-mediated immunity in host defense against NTS. This concept has been supported by recent population-based host genetic studies in African children. In contrast, immunoepidemiological studies from Africa indicate an important role for antibody for protective immunity, supporting the development of antibody-inducing vaccines against iNTS disease. With candidate O-antigen-based vaccines due to enter clinical trials in the near future, research efforts should focus on understanding the relative contributions of antibody and cell-mediated immunity to protection against iNTS disease in humans.

The phorbol 12-myristate-13-acetate differentiation protocol is critical to the interaction of THP-1 macrophages with Salmonella Typhimurium

THP-1 cells differentiated with phorbol 12-myristate 13-acetate (PMA) are widely used as a model for function and biology of human macrophages. However, the conditions used for differentiation, particularly the concentration of PMA and the duration of treatment, vary widely. Here we compare several differentiation conditions and compare the ability of THP-1 macrophages to interact with the facultative intracellular pathogen Salmonella enterica serovar Typhimurium. The results show that THP-1 macrophages differentiated in high concentrations of PMA rapidly died following infection whereas those differentiated in low concentrations of PMA survived and were able to control the intracellular bacteria similar to primary human macrophages.

Monocyte activation and cytokine production in Malawian children presenting with P. falciparum malaria

Malaria in malaria‐naïve adults is associated with an inflammatory response characterized by expression of specific activation markers on innate immune cells. Here, we investigate activation and adhesion marker expression, and cytokine production in monocytes from children presenting with cerebral malaria (CM, n = 36), severe malarial anaemia (SMA, n = 42) or uncomplicated malaria (UM, n = 66), and healthy aparasitemic children (n = 52) in Blantyre, Malawi. In all malaria groups, but particularly in the two severe malaria groups, monocyte expression of CD11b, CD11c, CD18, HLA‐DR and CD86, and percentages of TNF‐α‐ and IL‐6‐producing monocytes were lower than in healthy controls, while expression of CD11a, TLR2 and TLR4 was lower in children with severe malaria compared with controls. These levels mostly normalized during convalescence, but percentages of cytokine‐producing monocytes remained suppressed in children with SMA. In all malaria groups, especially the SMA group, a greater proportion of monocytes were loaded with haemozoin than among controls. In a P. falciparum hyperendemic area, monocytes in children with acute symptomatic malaria have reduced expression of adhesion molecules and activation markers and reduced inflammatory cytokine production. This immune suppression could be due to accumulation of haemozoin and/or previous exposure to P. falciparum.

Effect of in vivo neutralization of tumor necrosis alpha on the efficacy of antibiotic treatment in systemic Salmonella enterica infections

Immunity can co-operate with antibiotics, but can also antagonize drug efficacy by segregating the bacteria to areas of the body that are less accessible to antimicrobials, and by selecting for subpopulations with low division rates that are often difficult to eradicate. We studied the effect of an anti-inflammatory/immunosuppressive anti-TNFα treatment, which accelerates bacterial growth in the tissues and inhibits or reverses the formation of granulomas, on the efficacy of ampicillin and ciprofloxacin during a systemic Salmonella enterica infection of the mouse. The anti-TNFα treatment neither precluded nor enhanced the efficacy of antibiotic treatment. However, the anti-TNFα treatment rendered the animals susceptible to the rapid relapse of the infection seen after cessation of the antibiotic treatment. Reactivation of an established infection, due to late administration of anti-TNFα antibodies, could be successfully controlled by antibiotics, but full clearance of the bacterial load from the tissues was not achieved. We conclude that the lack of TNFα does not preclude the efficacy of antibiotic treatment and must be monitored with care due to post-treatment relapses. Combinations of anti-cytokine compounds and antibiotic molecules may not be the best way to treat persistent infections with intracellular bacteria like Salmonella.

Immunology, epidemiology and mathematical modelling towards a better understanding of invasive non-typhoidal Salmonella disease and rational vaccination approaches

INTRODUCTION

Invasive non-typhoidal Salmonella (iNTS) infections cause a high burden of lethal sepsis in young children and HIV patients, often associated with malaria, anaemia, malnutrition and sickle-cell disease. Vaccines against iNTS are urgently needed but none are licensed yet. Areas covered: This review illustrates how immunology, epidemiology and within-host pathogen behaviour affect invasive Salmonella infections and highlights how this knowledge can assist the improvement and choice of vaccines. Expert Commentary: Control of iNTS disease requires approaches that reduce transmission and improve diagnosis and treatment. These are often difficult to implement due to the fragile ecology and economies in endemic countries. Vaccines will be key tools in the fight against iNTS disease. To optimise vaccine design, we need to better define protective antigens and mechanisms of resistance to disease in susceptible populations even in those individuals where innate immunity may be impaired by widespread comorbidities.

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.

Dynamics of spread of Salmonella enterica in the systemic compartment

Traditional microbiological and immunological tools, combined with modern imaging, and molecular and mathematical approaches, have revealed the dispersive nature of Salmonella infections. Bacterial escape from infected cells, spread in the tissues and attempts to restrain this process by the host give rise to fascinating scenarios that underpin the pathogenesis of salmonelloses.

Bacterial persistence and toxin-antitoxin loci

Bacterial persistence is caused by the presence of rare, slowly growing bacteria among populations of rapidly growing cells. The slowly growing bacteria are tolerant of antibiotics and other environmental insults, whereas their isogenic, rapidly growing siblings are sensitive. Recent research has shown that persistence of the model organism Escherichia coli depends on toxin-antitoxin (TA) loci. Deletion of type II TA loci reduces the level of persistence significantly. Lon protease but no other known ATP-dependent proteases is required for persistence. Polyphosphate and (p)ppGpp also are required for persistence. These observations led to the proposal of a simple and testable model that explains the persistence of E. coli. It is now important to challenge this model and to test whether the persistence of pathogenic bacteria also depends on TA loci.

Mouse models to assess the efficacy of non-typhoidal Salmonella vaccines: revisiting the role of host innate susceptibility and routes of challenge

Non-typhoidal Salmonella enterica (NTS) serovars Typhimurium and Enteritidis are important causes of bacterial gastroenteritis in the USA and worldwide. In sub-Saharan Africa these two serovars are emerging as agents associated with lethal invasive disease (e.g., bacteremia, meningitis). The development of NTS vaccines, based on mucosally administered live attenuated strains and parenteral non-living antigens, could diminish the NTS disease burden globally. Mouse models of S. Typhimurium and S. Enteritidis invasive disease can accelerate the development of NTS vaccines. Live attenuated NTS vaccines elicit both cellular and humoral immunity in mice and their efficacy is well established. In contrast, non-living vaccines that primarily elicit humoral immunity have demonstrated variable efficacy. An analysis of the reported studies with non-living vaccines against S. Typhimurium and S. Enteritidis reveals that efficacy is influenced by two important independent variables: (1) the innate susceptibility to NTS infection that differs dramatically between commonly used mouse strains and (2) the virulence of the NTS strain used for challenge. Protection by non-living vaccines has generally been seen only in host-pathogen interactions where a sub-lethal infection results, such as challenging resistant mice with either highly virulent or weakly virulent strains or susceptible mice with weakly virulent strains. The immunologic basis of this discrepancy and the implications for human NTS vaccine development are reviewed herein.

Role for neutrophils in host immune responses and genetic factors that modulate resistance to Salmonella enterica serovar typhimurium in the inbred mouse strain SPRET/Ei

Infection with Salmonella enterica serovar Typhimurium is a complex disease in which the host-bacterium interactions are strongly influenced by genetic factors of the host. We demonstrate that SPRET/Ei, an inbred mouse strain derived from Mus spretus, is resistant to S. Typhimurium infections. The kinetics of bacterial proliferation, as well as histological examinations of tissue sections, suggest that SPRET/Ei mice can control bacterial multiplication and spreading despite significant attenuation of the cytokine response. The resistance of SPRET/Ei mice to S. Typhimurium infection is associated with increased leukocyte counts in the circulation and enhanced neutrophil influx into the peritoneum during the course of infection. A critical role of neutrophils was confirmed by neutrophil depletion: neutropenic SPRET/Ei mice were sensitive to infection with S. Typhimurium and showed much higher bacterial loads. To identify genes that modulate the natural resistance of SPRET/Ei mice to S. Typhimurium infection, we performed a genome-wide study using an interspecific backcross between C3H/HeN and SPRET/Ei mice. The results of this analysis demonstrate that at least two loci, located on chromosomes 6 and 11, affect survival following lethal infection with S. Typhimurium. These two loci contain several interesting candidate genes which may have important implications for the search for genetic factors controlling Salmonella infections in humans and for our understanding of complex host-pathogen interactions in general.

Type 2 diabetes mellitus is associated with impaired cytokine response and adhesion molecule expression in human endotoxemia

PURPOSE

Type 2 diabetes is associated with an increased risk of acquiring infectious diseases and developing sepsis. This may partly be due to immune dysfunction. We investigated the in vivo innate immune response of type 2 diabetic persons to an intravenous injection of E. coli lipopolysaccharide (LPS).

METHODS

After ethics approval, informed consent and a thorough physical examination, 19 type 2 diabetic patients and 23 healthy controls were included. LPS was given as an intravenous bolus injection of 0.3 ng/kg. Physiological variables, white blood cell count, and plasma concentrations of tumour necrosis factor (TNF), interleukin-6 (IL-6), interleukin-1 receptor antagonist (IL-1ra), and the adhesion molecules E-selectin, vascular adhesion molecule (VCAM)-1, and intracellular adhesion molecule (ICAM)-1 were measured hourly for 8 h.

RESULTS

LPS injection induced a systemic inflammatory response with increases in neutrophils, temperature, heart rate and plasma concentrations of cytokines and adhesion molecules in healthy and type 2 diabetic volunteers. Type 2 diabetes was associated with less pronounced LPS-induced increases in TNF, IL-1ra, VCAM-1 and ICAM-1. There was a trend towards an attenuated upregulation of E-selectin in diabetics, even though the plasma concentration tended to be generally higher compared to healthy controls.

CONCLUSIONS

Patients with type 2 diabetes exhibit an attenuated increase in plasma levels of TNF and IL-1ra, as well as an attenuated upregulation of VCAM-1 and ICAM-1 to LPS in vivo. This finding may provide a mechanistic explanation for the adverse outcome seen during infectious diseases in diabetic patients.

Streptococcus pneumoniae autolysis prevents phagocytosis and production of phagocyte-activating cytokines

Streptococcus pneumoniae is a major pathogen in humans. The pathogenicity of this organism is related to its many virulence factors, the most important of which is the thick pneumococcal capsule that minimizes phagocytosis. Another virulence-associated trait is the tendency of this bacterium to undergo autolysis in stationary phase through activation of the cell wall-bound amidase LytA, which breaks down peptidoglycan. The exact function of autolysis in pneumococcal pathogenesis is, however, unclear. Here, we show the selective and specific inefficiency of wild-type S. pneumoniae for inducing production of phagocyte-activating cytokines in human peripheral blood mononuclear cells (PBMC). Indeed, clinical pneumococcal strains induced production of 30-fold less tumor necrosis factor (TNF), 15-fold less gamma interferon (IFN-gamma), and only negligible amounts of interleukin-12 (IL-12) compared with other closely related Streptococcus species, whereas the levels of induction of IL-6, IL-8, and IL-10 production were similar. If pneumococcal LytA was inactivated by mutation or by culture in a medium containing excess choline, the pneumococci induced production of significantly more TNF, IFN-gamma, and IL-12 in PBMC, whereas the production of IL-6, IL-8, and IL-10 was unaffected. Further, adding autolyzed pneumococci to intact bacteria inhibited production of TNF, IFN-gamma, and IL-12 in a dose-dependent manner but did not inhibit production of IL-6, IL-8, and IL-10 in response to the intact bacteria. Fragments from autolyzed bacteria inhibited phagocytosis of intact bacteria and reduced the in vitro elimination of pneumococci from human blood. Our results suggest that fragments generated by autolysis of bacteria with reduced viability interfere with phagocyte-mediated elimination of live pneumococci.

Dexamethasone modulates Salmonella enterica serovar Typhimurium infection in vivo independently of the glucocorticoid-inducible protein annexin-A1

Salmonella enterica serovar Typhimurium (S. Typhimurium) infection causes an inflammatory response through activation of Toll-like receptor 4 by lipopolysaccharide. Dexamethasone, a glucocorticoid analogue, suppresses inflammatory responses by many mechanisms including inhibition of the lipopolysaccharide-induced production of proinflammatory mediators. There is little information on the effect of glucocorticoids on murine salmonellosis. In this study, we treated susceptible BALB/c mice by subcutaneous implantation of slow-release dexamethasone pellets before infection with S. Typhimurium. Dexamethasone promotes bacterial growth early in infection and induces a dose-dependent increase in bacterial growth within mouse livers and spleens. The bacterial load in organs from infected placebo-treated mice was lower than that in dexamethasone-treated mice. Glucocorticoids inhibit lipopolysaccharide-induced inflammation partially through the steroid-inducible protein annexin-A1 (ANXA1). Infection of wild-type and ANXA1 knock-out mice with S. Typhimurium led to similar organ bacterial loads. ANXA1 also did not affect the bacterial load in organs from infected dexamethasone-treated mice. This suggests that glucocorticoids, independently of ANXA1, accelerate S. Typhimurium growth in vivo in susceptible BALB/c mice.

Virulent Salmonella enterica infections can be exacerbated by concomitant infection of the host with a live attenuated S. enterica vaccine via Toll-like receptor 4-dependent interleukin-10 production with the involvement of both TRIF and MyD88

During systemic disease in mice, Salmonella enterica grows intracellularly within discrete foci of infection in the spleen and liver. In concomitant infections, foci containing different S. enterica strains are spatially separated. We have investigated whether functional interactions between bacterial populations within the same host can occur despite the known spatial separation of the foci and independence of growth of salmonellae residing in different foci. In this study we have demonstrated that bacterial numbers of virulent S. enterica serovar Typhimurium C5 strain in mouse tissues can be increased by the presence of the attenuated aroA S. Typhimurium SL3261 vaccine strain in the same tissue. Disease exacerbation does not require simultaneous coinjection of the attenuated bacteria. SL3261 can be administered up to 48 hr after or 24 hr before the administration of C5 and still determine higher tissue numbers of the virulent bacteria. This indicates that intravenous administration of a S. enterica vaccine strain could potentially exacerbate an established infection with wild-type bacteria. These data also suggest that the severity of an infection with a virulent S. enterica strain can be increased by the prior administration of a live attenuated vaccine strain if infection occurs within 48 hr of vaccination. Exacerbation of the growth of C5 requires Toll-like receptor 4-dependent interleukin-10 production with the involvement of both Toll/interleukin-1 receptor-domain-containing adaptor inducing interferon-beta and myeloid differentiation factor 88.

Fcgamma receptors are crucial for the expression of acquired resistance to virulent Salmonella enterica serovar Typhimurium in vivo but are not required for the induction of humoral or T-cell-mediated immunity

Antibodies play an important role in immunity to Salmonella enterica. Here we evaluated the requirement for Fcgamma receptors in host resistance to S. enterica using an in vivo model of systemic infection. We show that mice lacking FcgammaRI, II and III can control and clear a primary infection with S. enterica micro-organisms of low virulence, but are impaired in the expression of vaccine-induced acquired immunity to oral challenge with virulent bacteria. We also show that, in vivo, FcgammaRI, II, III(-/-) mice were able to mount efficient T-helper 1 type T-cell responses and antibody responses specific for S. enterica. The work indicates that targeting S. enterica to FcgammaR is needed for the expression of vaccine-induced acquired immunity, but is not essential for the engenderment of T- and B-cell immunity to the bacterium in vivo.

Enhanced resistance to Salmonella enterica serovar Typhimurium infection in mice after coumarin treatment

Coumarin and its derivatives are naturally occurring substances with multiple biological activities. Here we demonstrate that prophylactic peroral administration of coumarin or 7-hydroxycoumarin (7-OHC) enhances resistance to subsequent lethal Salmonella enterica Serovar Typhimurium infection in mice. 7-OHC decreased bacterial load in liver and spleen, and enhanced phagocytosis and bacterial killing by macrophages when applied in vitro and in vivo. 7-OHC treatment induced significant NO release in peritoneal macrophage cultures. The observed protective effect correlated with the induction of Th1-associated cytokines, such as IL-12, IFN-gamma, and TNF-alpha. These data demonstrate a clear immunomodulatory potential of coumarins which might have important therapeutic implications to enhance resistance to infection.

Apolipoprotein CI stimulates the response to lipopolysaccharide and reduces mortality in Gram‐negative sepsis

Gram-negative sepsis is a major death cause in intensive care units. Accumulating evidence indicates the protective role of plasma lipoproteins such as high-density lipoprotein (HDL) in sepsis. It has recently been shown that septic HDL is almost depleted from apolipoprotein CI (apoCI), suggesting that apoCI may be a protective factor in sepsis. Sequence analysis revealed that apoCI possesses a highly conserved consensus KVKEKLK binding motif for lipopolysaccharide (LPS), an outer-membrane component of gram-negative bacteria. Through avid binding to LPS involving this motif, apoCI improved the presentation of LPS to macrophages in vitro and in mice, thereby stimulating the inflammatory response to LPS. Moreover, apoCI dose-dependently increased the early inflammatory response to Klebsiella pneumoniae-induced pneumonia, reduced the number of circulating bacteria, and protected mice against fatal sepsis. Our data support the hypothesis that apoCI is a physiological protector against infection by enhancing the early inflammatory response to LPS and suggest that timely increase of apoCI levels could be used to efficiently prevent and treat early sepsis.

NEUTRALIZATION OF TUMOR NECROSIS FACTOR IN PRECLINICAL MODELS OF SEPSIS

Tumor necrosis factor (TNFalpha), a cardinal early mediator of the innate host inflammatory response, has been an attractive target for therapeutic intervention in human sepsis. However, pooled data from 12 completed randomized controlled trials show only a very modest impact on mortality in a highly heterogeneous population of patients. To gain insight into the preclinical in vivo biology of TNFalpha that might aid in better identifying appropriate patient populations for therapeutic intervention, we undertook a systematic review of published reports of preclinical studies assessing the consequences of neutralization of TNFalpha in models of acute infection or inflammation. We identified 143 reports incorporating 484 unique experimental comparisons in seven different animal species. The effects of neutralization of TNFalpha in these were quite variable. Neutralization of TNFalpha was beneficial in endotoxemia, or after systemic challenge with gram-negative organisms, Staphylococcus aureus, or Group B streptococci. On the other hand, neutralization was detrimental in infections caused by Streptococcus pneumoniae, Candida spp., or intracellular pathogens such as Listeria and Mycobacterium tuberculosis, and in models of pneumonia. Treatment was more efficacious when delivered before infectious challenge, and the therapeutic signal increased as the baseline mortality in the placebo group increased. Evidence of neutralization of TNFalpha bioactivity, and of attenuation of inflammation, was typically accompanied by evidence of impairment of antimicrobial defenses. Multiple specific and nonspecific therapeutic strategies were identified. We conclude that the beneficial effects of TNF in systemic inflammation occur at the cost of impaired antimicrobial defenses, and that a better understanding of the consequences of neutralization of TNFalpha in vivo could aid in better defining optimal patient populations for therapeutic intervention.

Sublethal infection of C57BL/6 mice with Salmonella enterica Serovar Typhimurium leads to an increase in levels of Toll-like receptor 1 (TLR1), TLR2, and TLR9 mRNA as well as a decrease in levels of TLR6 mRNA in infected organs

Sublethal infection of C57BL/6 mice with Salmonella enterica serovar Typhimurium M525P initiates a strong inflammatory response. We measured organ expression of mRNA for Toll-like receptors and their associated signaling molecules during S. enterica serovar Typhimurium infection. During infection, the Toll-lie receptor 1 (TLR1), TLR2, and TLR9 mRNA levels increased, while TLR6 mRNA expression decreased.

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.

Pathogenesis of malaria and clinically similar conditions

There is now wide acceptance of the concept that the similarity between many acute infectious diseases, be they viral, bacterial, or parasitic in origin, is caused by the overproduction of inflammatory cytokines initiated when the organism interacts with the innate immune system. This is also true of certain noninfectious states, such as the tissue injury syndromes. This review discusses the historical origins of these ideas, which began with tumor necrosis factor (TNF) and spread from their origins in malaria research to other fields. As well the more established proinflammatory mediators, such as TNF, interleukin-1, and lymphotoxin, the roles of nitric oxide and carbon monoxide, which are chiefly inhibitory, are discussed. The established and potential roles of two more recently recognized contributors, overactivity of the enzyme poly(ADP-ribose) polymerase 1 (PARP-1) and the escape of high-mobility-group box 1 (HMGB1) protein from its normal location into the circulation, are also put in context. The pathogenesis of the disease caused by falciparum malaria is then considered in the light of what has been learned about the roles of these mediators in these other diseases, as well as in malaria itself.

Characterization and development of T-Cell immune responses in B-cell-deficient (Igh-6(-/-)) mice with Salmonella enterica serovar Typhimurium infection

Infection of mice with Salmonella enterica serovar Typhimurium induces strong Th1 T-cell responses that are central to the control of the infection. In the present study, we examined the role of B cells in the development of Th1 T-cell responses to Salmonella by using gene-targeted B-cell-deficient mice (Igh-6(-/-) mice). The development of Th1 T-cell responses in Igh-6(-/-) mice was impaired in the early stage of a primary infection. This impairment persisted throughout the course of the disease. The ability of T cells to produce the Th1 cytokine gamma interferon and the frequency at which they did so were lower in Igh-6(-/-) mice than in control mice. We also observed a transient switch toward Th2 cytokine production in Igh-6(-/-) mice. Thus, B cells are important for the induction of protective Th1 T-cell responses in the early phase of a Salmonella infection. Activated B cells express high levels of major histocompatibility complex and costimulatory molecules and are nearly as effective as dendritic cells in their antigen-presenting cell (APC) activity. However, their importance as APCs in infection and their role in initiating and/or maintaining T-cell responses are unknown. Here, we show that B cells upregulate costimulatory molecules upon in vitro stimulation with S. enterica serovar Typhimurium and that they can present Salmonella antigens to Salmonella-specific CD4(+) T cells. Our results show that B cells are important for the development of T-cell responses in the early stage of a Salmonella infection and that this property may be due to their ability to present antigens to T cells.

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.

Toll-like receptor expression in C3H/HeN and C3H/HeJ mice during Salmonella enterica serovar Typhimurium infection

Here, we have investigated the mRNA expression of Toll-like receptor 2 (TLR-2), TLR-4, and MD-2 in spleens and livers of C3H/HeN mice (carrying wild-type TLR-4) and C3H/HeJ mice (carrying mutated TLR-4) in response to Salmonella infection. During Salmonella infections, TLR-4 is activated, leading to increased TLR-2 and decreased TLR-4 expression.

Anti-cytokine therapeutics and infections

In view of the increasing use of anti-cytokine-based therapies to treat autoimmune diseases, the role of specific cytokines in host defense against infection has become a highly relevant area of investigation. There are over 300,000 patients worldwide being treated with agents that specifically block the biological activities of interleukin-1 (IL-1) or tumor necrosis factor (TNF) for reducing the severity of autoimmune diseases such as rheumatoid arthritis, Crohn's disease or psoriasis. Those patients receiving anti-TNF-alpha or IL-1 blocking therapies are treated on a chronic basis. Studies suggest that other chronic inflammatory diseases will benefit from anti-cytokine therapies. However, there is a growing body of clinical evidence that neutralization of TNF-alpha is associated with an increased risk of opportunistic infections, including mycobacterial diseases. Blockade of IL-1 activity with the IL-1 receptor antagonist (IL-1Ra) appears, at present, to be relatively safe. However, because of physician under reporting (some estimates of reporting being less than 5% of these infections), the true incidence of infections, both serious and non-serious, will remain unknown. Does the increase in infections associated with anti-cytokine-based therapies come as a surprise? Of the two components of host defense, the innate and the acquired responses, which are affected by anti-cytokine therapies? From a wealth of rodent studies using live infection models, the following conclusions can be drawn: (1) neutralization or gene deletion for TNF-alpha is frequently associated with reduction of host defense in models of live Gram-positive or Gram-negative infections as well as infection by intracellular microbes such as Salmonella and Listeria; (2) absence of the IL-1 receptor can also result in decreased resistance to Listeria or Gram-positive bacteria and (3) TNF-alpha and IFN-gamma are required for defense against infection caused by Mycobacterium tuberculosis.

Stimulation of Toll-like receptor 4 by lipopolysaccharide during cellular invasion by live Salmonella typhimurium is a critical but not exclusive event leading to macrophage responses

Invasion of macrophages by salmonellae induces cellular responses, with the bacterial inducers likely to include a number of pathogen-associated molecular patterns. LPS is one of the prime candidates, but its precise role in the process, especially when presented as a component of live infecting bacteria, is unclear. We thus investigated this question using the lipid A antagonist E5531, the macrophage-like cell line RAW 264.7, and primary macrophage cultures from C3H/HeJ and Toll-like receptor 4(-/-) (TLR-4(-/-)) mice. We show that LPS presented on live salmonellae provides an essential signal, via functional TLR-4, for macrophages to produce NO and TNF-alpha. Furthermore, the mitogen-activated protein kinase c-Jun N-terminal kinase and p38 are activated, and the transcription factor NF-kappa B is translocated to the nucleus when RAW 264.7 cells are presented with purified LPS or live salmonellae. Purified LPS stimulates rapid, transitory mitogen-activated protein kinase activation that is inhibited by E5531, whereas bacterial invasion stimulates delayed, prolonged activation, unaffected by E5531. Both purified LPS and bacterial invasion caused translocation of NF-kappa B, but whereas E5531 always inhibited activation by purified LPS, activation by bacterial invasion was only inhibited at later time points. In conclusion, we show for the first time that production of NO and TNF-alpha is critically dependent on activation of TLR-4 by LPS during invasion of macrophages by salmonellae, but that different patterns of activation of intracellular signaling pathways are induced by purified LPS vs live salmonellae.

Risk and the efficacy of antiinflammatory agents: retrospective and confirmatory studies of sepsis

We investigated whether a relationship between risk of death and treatment effect could explain the disparate results between the preclinical and clinical sepsis trials of antiinflammatory agents over the last decade. A metaregression analysis of cited preclinical studies showed that the treatment effects of these agents were highly dependent on risk of death (p = 0.0001) and that animals were studied at significantly higher control mortality rates than humans (median [25th-75th quartile], 88% [79-96%] versus 39% [32-42%], p = 0.0001). An analysis of the clinical trials showed that antiinflammatory agents were also significantly more efficacious in septic patients with higher risk of death (p = 0.002) and were harmful in those with low risk. To test this relationship prospectively, we studied antiinflammatory agents in models employing differing doses of bacterial challenge to produce the full range of risk of death. We found that the efficacy of these agents, although very beneficial at high control mortality rates, was much reduced (p = 0.0001) and similar to those in human trials at moderate control mortality rates (i.e., 30 to 40%). The efficacy of antiinflammatory agents during sepsis is dependent on the risk of death, an observation that explains the apparent contradiction between preclinical and clinical trial results. Accounting for this relationship may be necessary for the safe and effective development of antiinflammatory therapies for sepsis.

The innate immune response differs in primary and secondary Salmonella infection

This study examines innate immunity to oral Salmonella during primary infection and after secondary challenge of immune mice. Splenic NK and NKT cells plummeted early after primary infection, while neutrophils and macrophages (Mphi) increased 10- and 3-fold, respectively. In contrast, immune animals had only a modest reduction in NK cells, no loss of NKT cells, and a slight increase in phagocytes following secondary challenge. During primary infection, the dominant sources of IFN-gamma were, unexpectedly, neutrophils and Mphi, the former having intracellular stores of IFN-gamma that were released during infection. IFN-gamma-producing phagocytes greatly outnumbered IFN-gamma-producing NK cells, NKT cells, and T cells during the primary response. TNF-alpha production was also dominated by neutrophils and Mphi, which vastly outnumbered NKT cells producing this cytokine. Neither T cells nor NK cells produced TNF-alpha early during primary infection. The TNF-alpha response was reduced in a secondary response, but remained dominated by neutrophils and Mphi. Moreover, no significant IFN-gamma production by Mphi was associated with the secondary response. Indeed, only NK1.1(+) cells and T cells produced IFN-gamma in these mice. These studies provide a coherent view of innate immunity to oral Salmonella infection, reveal novel sources of IFN-gamma, and demonstrate that immune status influences the nature of the innate response.

Fimbriated Salmonella enterica serovar typhimurium abates initial inflammatory responses by macrophages

Oral immunization of mice with a Salmonella vaccine expressing colonization factor antigen I (CFA/I) from enterotoxigenic Escherichia coli results in the rapid onset of interleukin-4 (IL-4) and IL-5 production, which explains the observed elevations in mucosal immunoglobulin A (IgA) and serum IgG1 antibodies. In contrast, oral immunization with the Salmonella vector does not result in the production of Th2-type cytokines. To begin to assess why such differences exist between the two strains, it should be noted that in vitro infection of RAW 264.7 macrophages resulted in the absence of nitric oxide (NO) production in cells infected with the Salmonella-CFA/I vaccine. This observation suggests differential proinflammatory cytokine production by these isogenic Salmonella strains. Upon measurement of proinflammatory cytokines, minimal to no tumor necrosis factor alpha (TNF-alpha), IL-1alpha, IL-1beta, or IL-6 was produced by Salmonella-CFA/I-infected RAW 264.7 or peritoneal macrophages, but production was greatly induced in Salmonella vector-infected macrophages. Only minute levels of IL-12 p70 were induced by Salmonella vector-infected macrophages, and none was induced by Salmonella-CFA/I-infected macrophages. The absence of IL-12 was not due to overt increases in production of either IL-12 p40 or IL-10. CFU measurements taken at 8 h postinfection showed no differences in colonization in RAW 264.7 cells infected with either Salmonella construct, but there were differences in peritoneal macrophages. However, after 24 h, the Salmonella vector strain colonized to a greater extent in RAW 264.7 cells than in peritoneal macrophages. Infection of RAW 264.7 cells or peritoneal macrophages with either Salmonella construct showed no difference in macrophage viabilities. This evidence shows that the expression of CFA/I fimbriae alters how macrophages recognize or process salmonellae and prevents the rapid onset of proinflammatory cytokines which is typical during Salmonella infections.

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.

Bacterial virulence, proinflammatory cytokines and host immunity: how to choose the appropriate Salmonella vaccine strain?

Salmonella infection in its mammalian host can be dissected into two main components. The co-ordinate expression of bacterial virulence genes which are designed to evade, subvert or circumvent the host response on the one hand, and the host defence mechanisms which are designed to restrict bacterial survival and replication on the other hand. The outcome of infection is determined by the one which succeeds in disturbing this equilibrium more efficiently. This delicate balance between Salmonella virulence and host immunity/inflammation has important implications for vaccine development or therapeutic intervention. Novel Salmonella vaccine candidates and live carriers for heterologous antigens are attenuated strains with defined genetic modifications of metabolic or virulence functions. Although genetic defects of different gene loci can lead to similar degrees of attenuation, effects on the course of infection may vary, thereby altering the quality of the elicited immune response. Studies with gene-deficient animals indicate that Salmonella typhimurium strains with mutations in aroA, phoP/phoQ or ssrA/ssrB invoke different immune responses and that a differential repertoire of pro-inflammatory cytokines is required for clearance. Consequently, Salmonella mutants defective in distinct virulence functions offer the potential to specifically modulate the immune response for defined medical applications.

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.

Production of chemokines in vivo in response to microbial stimulation

Members of the chemokine gene superfamily are known to play a central role in leukocyte extravasation; however, their involvement in acute inflammation in response to micro-organisms has not yet been well studied. We have therefore investigated the role of murine macrophage-inflammatory protein (muMIP) 1alpha and muMIP-2 in the inflammatory response mounted against the bacteria Salmonella enteritidis and the Sacchromyces cerevisiae cell wall component, zymosan. Leukocyte extravasation was monitored in murine s.c. air pouches. Both agonists induced accumulation of leukocytes in a dose- and time-dependent manner, with the response peaking after 4 h and declining thereafter. The inflammatory exudate comprised mainly neutrophils; however, an increase in eosinophil accumulation was also observed in response to zymosan. The production of both muMIP-1alpha and muMIP-2 increased with time in response to both the agonists, although production was more sustained in response to the bacteria. Prior treatment of mice with neutralizing Abs against muMIP-1alpha or muMIP-2, either alone or in combination, failed to attenuate the accumulation of leukocytes in response to the agonists. In contrast, the anti-muMIP-2 Abs significantly inhibited leukocyte recruitment in response to S. enteritidis in complement-deficient mice. Taken together, these data show that while muMIP-1alpha and muMIP-2 are produced in response to phagocytosis of micro-organisms in s.c. tissue, under these circumstances components of the complement pathway appear to play a dominant role in the recruitment of neutrophils.

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.

Defective localization of the NADPH phagocyte oxidase to Salmonella-containing phagosomes in tumor necrosis factor p55 receptor-deficient macrophages

Tumor necrosis factor receptor (TNFR) p55-knockout (KO) mice are susceptible profoundly to Salmonella infection. One day after peritoneal inoculation, TNFR-KO mice harbor 1,000-fold more bacteria in liver and spleen than wild-type mice despite the formation of well organized granulomas. Macrophages from TNFR-KO mice produce abundant quantities of reactive oxygen and nitrogen species in response to Salmonella but nevertheless exhibit poor bactericidal activity. Treatment with IFN-gamma enhances killing by wild-type macrophages but does not restore the killing defect of TNFR-KO cells. Bactericidal activity of macrophages can be abrogated by a deletion in the gene encoding TNFalpha but not by saturating concentrations of TNF-soluble receptor, suggesting that intracellular TNFalpha can regulate killing of Salmonella by macrophages. Peritoneal macrophages from TNFR-KO mice fail to localize NADPH oxidase-containing vesicles to Salmonella-containing vacuoles. A TNFR-KO mutation substantially restores virulence to an attenuated mutant bacterial strain lacking the type III secretory system encoded by Salmonella pathogenicity island 2 (SPI2), suggesting that TNFalpha and SPI2 have opposing actions on a common pathway of vesicular trafficking. TNFalpha-TNFRp55 signaling plays a critical role in the immediate innate immune response to an intracellular pathogen by optimizing the delivery of toxic reactive oxygen species to the phagosome.

Early responses to Salmonella typhimurium infection in mice occur at focal lesions in infected organs

Salmonella typhimurium causes an invasive disease in mice that has similarities to human typhoid, with key roles for cytokines and possibly also inducible nitric oxide synthase (iNOS), in mediating host responses to infection. In this paper we demonstrate that iNOS mRNA, protein and enzyme activity is induced within spleens and livers of infected mice as early as 5 h post-infection. Immunohistochemistry and in situ hybridization indicated that iNOS expression occurs predominantly in macrophages in localized, discrete foci in the infected organs. iNOS activity in spleen and liver was not detectable in uninfected control mice. The presence of mRNA encoding pro-inflammatory cytokines (TNFalpha, IL-1beta and IFNgamma) in infected organs was measured using RT-PCR, all three being present from 2 h post-infection onwards, but not before. These data show that there is a very early host response to S. typhimurium infection in mice, limited to foci within the infected organs.

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.

Igh-6(-/-) (B-cell-deficient) mice fail to mount solid acquired resistance to oral challenge with virulent Salmonella enterica serovar typhimurium and show impaired Th1 T-cell responses to Salmonella antigens

In the present study we evaluated the role of B cells in acquired immunity to Salmonella infection by using gene-targeted B-cell-deficient innately susceptible mice on a C57BL/6 background (Igh-6(-/-)). Igh-6(-/-) mice immunized with a live, attenuated aroA Salmonella enterica serovar Typhimurium vaccine strain showed impaired long-term acquired resistance against the virulent serovar Typhimurium strain C5. Igh-6(-/-) mice were able to control a primary infection and to clear the inoculum from the reticuloendothelial system. However, Igh-6(-/-) mice, unlike Igh-6(+/+) C57BL/6 controls, did not survive an oral challenge with strain C5 at 4 months after vaccination. Transfer of immune serum did not restore resistance in Igh-6(-/-) mice. Total splenocytes and purified CD4(+) T cells obtained from Igh-6(-/-) mice 4 months after vaccination showed reduced ability to release Th1-type cytokines (interleukin 2 and gamma interferon) upon in vitro restimulation with serovar Typhimurium soluble cell extracts compared to cells obtained from Igh-6(+/+) C57BL/6 control mice. Therefore, the impaired resistance to oral challenge with virulent serovar Typhimurium observed in B-cell-deficient mice, which cannot be restored by passive transfer of Salmonella-immune serum, may be in part due to a reduced serovar Typhimurium-specific T-cell response following primary immunization.

Host cytokine response and resistance to Salmonella infection

Knowledge of the host response, of the resistance process, and of the mediators committed against Salmonella infection is essential to progress towards better means of prophylaxis and eradication. In this context, the present contribution attempts to interconnect, with the pivotal role of the macrophage, the early resistance process under the control of the Nramp1 gene and the cytokine response for resolving infection. IL-12 produced by macrophages is an inducer of IFN-gamma production, which in turn activates the macrophage antibacterial activity and synergizes its effects with TNF-alpha. All three of these cytokines are powerful actors in the first line of anti-Salmonella defence. It can be pointed out that susceptible and resistant individuals do not seem to see the cytokine environment the same way, the former being unresponsive to IL-1 or GM-CSF treatment and deficient in IFN-gamma production. These discrepancies may rely on cell signalling events that could be defective in macrophages of the susceptible phenotype.

Interleukin 18 contributes to host resistance and gamma interferon production in mice infected with virulent Salmonella typhimurium

Spleen and peritoneal macrophages obtained from innately resistant A/J mice released low levels of interleukin 18 (IL-18) upon infection with Salmonella typhimurium C5 RP4. Incubating the cells with recombinant gamma interferon (rIFN-gamma) enhanced IL-18 production. A/J mice treated in vivo with anti-IL-18 antibodies showed impaired resistance to infection, with increased bacterial loads in the liver and spleen. Administration of rIL-18 could protect A/J mice from challenge with a lethal dose of virulent salmonellae, with a dramatic reduction in bacterial numbers in the tissues. rIL-18 administration did not ameliorate the disease in IFN-gamma-R-/- mice. IL-18 proved to be required for IFN-gamma production by mouse splenocytes from conventional, scid, and rag-1(-/-) mice; in vivo IL-18 neutralization caused a decrease in circulating IFN-gamma levels. Thus, IL-18 is a key factor in early host resistance to Salmonella and probably acts via IFN-gamma.

Interleukin-12 is required for control of the growth of attenuated aromatic-compound-dependent salmonellae in BALB/c mice: role of gamma interferon and macrophage activation

The attenuated S. typhimurium SL3261 (aroA) strain causes mild infections in BALB/c mice. We were able to exacerbate the disease by administering anti-interleukin-12 (IL-12) antibodies, resulting in bacterial counts in the spleens and livers of anti-IL-12-treated mice that were 10- to 100-fold higher than the ones normally observed in premortem mice; yet the animals showed only mild signs of illness. Nevertheless, they eventually died of a slow, progressive disease. Mice infected with salmonellae become hypersusceptible to endotoxin. We found that IL-12 neutralization prevented the death of infected mice following subcutaneous injection of lipopolysaccharide. Granulomatous lesions developed in the spleens and livers of control animals, as opposed to a widespread infiltration of mononuclear cells seen in the organs of anti-IL-12-treated mice. In the latter (heavily infected), salmonellae were seen within mononuclear cells, indicating an impairment of the bactericidal or bacteriostatic ability of the phagocytes in the absence of biologically active IL-12. Gamma interferon (IFN-gamma) levels were reduced in the sera and tissue homogenates from anti-IL-12-treated mice compared to those in control animals. Furthermore, fluorescence-activated cell sorter analysis on spleen cells showed that IL-12 neutralization impaired the upregulation of I-Ad/I-Ed antigens on macrophages from infected mice. Inducible nitric oxide synthase and IFN-gamma mRNA production was down-regulated in anti-IL-12-treated mice, which also showed an increased production of IL-10 mRNA and a decrease in nitric oxide synthase activity in the tissues. Administration of recombinant IFN-gamma to anti-IL-12-treated mice was able to restore host resistance, granuloma formation, and expression of major histocompatibility complex class II antigens in F4/80(+) and CD11b+ spleen cells.

Susceptibility to Salmonella typhimurium infection and effectiveness of vaccination in mice deficient in the tumor necrosis factor alpha p55 receptor

Mice defective in the ability to produce the tumor necrosis factor alpha p55 receptor (TNFalphap55R) were orally challenged with a number of Salmonella typhimurium HWSH derivatives that differ in virulence. In comparison to TNFalphap55R+/+ mice, TNFalphap55R-/- mice succumbed earlier to challenge with wild-type S. typhimurium HWSH and S. typhimurium HWSH purE. In contrast, TNFalphap55R-/- mice were able to control an S. typhimurium HWSH aroA challenge, although greater numbers of Salmonella organisms were present in the tissues for a longer time period than was observed with TNFalphap55R+/+ mice. Vaccination of normal and TNFalphap55R knockout animals with S. typhimurium HWSH aroA showed that TNFalphap55R-/- mice, unlike TNFalphap55R+/+ mice, were not protected against a virulent S. typhimurium HWSH challenge. Splenocytes from TNFalphap55R-/- mice exhibited a reduced ability to proliferate in the presence of S. typhimurium antigen compared to TNFalphap55R+/+ mice. Thus, TNFalphap55R is essential for controlling Salmonella growth in tissues and for recall of immunity in murine salmonellosis.

Anti-inflammatory cytokine profile and mortality in febrile patients

BACKGROUND

An anti-inflammatory cytokine profile on whole-blood stimulation in vitro is associated with fatal outcome of meningococcal disease. We investigated whether an anti-inflammatory cytokine profile in the circulation is associated with adverse outcome in other infectious diseases.

METHODS

We enrolled 464 consecutive patients (272 men, 192 women) who presented to hospital with fever (> or = 38.2 degrees C). On admission we measured plasma interleukin 10 (IL-10) and tumour necrosis factor alpha (TNF alpha), and collected clinical and microbiological data on the febrile illness, then followed up all patients for clinical outcome.

FINDINGS

In at least 399 of the 464 patients fever was caused by infection. 33 patients died after a median hospital stay of 11 days (interquartile range 3-20). Concentrations of IL-10 were significantly higher in non-survivors (median 169 pg/mL [IQR 83-530]) than in survivors (median 88 pg/mL [42-235], p=0.042). When dichotomised around the median, the mortality risk was two times higher in patients who had high concentrations of IL-10 than in those with low concentrations (relative risk 2.39 [95% CI 1.07-5.33]), in patients with low and high concentrations of TNF alpha. In the 406 patients without haemodynamic deterioration in the first 24 h, IL-10 was higher and TNF alpha lower in patients who died than in those who survived. The ratio of IL-10 to TNF alpha was higher in non-survivors (median 6.9 [3.0-21.0]) than in survivors (median 3.9 [2.0-7.0], p=0.040). This ratio was highest in patients who died without underlying disease (median 21.5 [5.0-25.0]). Age, sex, and duration of fever before admission did not explain the differences in IL-10 and TNF alpha.

INTERPRETATION

An anti-inflammatory cytokine profile of a high ratio of IL-10 to TNF alpha is associated with fatal outcome in febrile patients with community-acquired infection. Our findings caution against a widespread use of proinflammatory cytokine inhibition in patients with sepsis.