Host immune response to Salmonella enterica serovar Typhimurium infection in mice derived from wild strains

Early transcriptional responses to human enteric fever challenge

Enteric fever, caused by oral infection with typhoidal Salmonella serovars, presents as a non-specific febrile illness preceded by an incubation period of 5 days or more. The enteric fever human challenge model provides a unique opportunity to investigate the innate immune response during this incubation period, and how this response is altered by vaccination with the Vi polysaccharide or conjugate vaccine. We find that on the same day as ingestion of typhoidal Salmonella, there is already evidence of an immune response, with 199 genes upregulated in the peripheral blood transcriptome 12 hours post-challenge (false discovery rate <0.05). Gene sets relating to neutrophils, monocytes, and innate immunity were over-represented (false discovery rate <0.05). Estimating cell proportions from gene expression data suggested a possible increase in activated monocytes 12 hours post-challenge (P = 0.036, paired Wilcoxon signed-rank test). Furthermore, plasma TNF-α rose following exposure (P = 0.011, paired Wilcoxon signed-rank test). There were no significant differences in gene expression (false discovery rate <0.05) in the 12 hours response between those who did and did not subsequently develop clinical or blood culture confirmed enteric fever or between vaccination groups. Together, these results demonstrate early perturbation of the peripheral blood transcriptome after enteric fever challenge and provide initial insight into early mechanisms of protection.

Comparative analysis of the caecal tonsil transcriptome in two chicken lines experimentally infected with Salmonella Enteritidis

Managing Salmonella enterica Enteritidis (SE) carriage in chicken is necessary to ensure human food safety and enhance the economic, social and environmental sustainability of chicken breeding. Salmonella can contaminate poultry products, causing human foodborne disease and economic losses for farmers. Both genetic selection for a decreased carriage and gut microbiota modulation strategies could reduce Salmonella propagation in farms. Two-hundred and twenty animals from the White Leghorn inbred lines N and 61 were raised together on floor, infected by SE at 7 days of age, transferred into isolators to prevent oro-fecal recontamination and euthanized at 12 days post-infection. Caecal content DNA was used to measure individual Salmonella counts (ISC) by droplet digital PCR. A RNA sequencing approach was used to measure gene expression levels in caecal tonsils after infection of 48 chicks with low or high ISC. The analysis between lines identified 7516 differentially expressed genes (DEGs) corresponding to 62 enriched Gene Ontology (GO) Biological Processes (BP) terms. A comparison between low and high carriers allowed us to identify 97 DEGs and 23 enriched GO BP terms within line 61, and 1034 DEGs and 288 enriched GO BP terms within line N. Among these genes, we identified several candidate genes based on their putative functions, including FUT2 or MUC4, which could be involved in the control of SE infection, maybe through interactions with commensal bacteria. Altogether, we were able to identify several genes and pathways associated with differences in SE carriage level. These results are discussed in relation to individual caecal microbiota compositions, obtained for the same animals in a previous study, which may interact with host gene expression levels for the control of the caecal SE load.

Polymorphism detection of promoter region of IFN-γ and IL-2 genes and their association with productive traits in Mazandaran native breeder fowls

To identify polymorphism in interferon gamma (IFN-γ) and interleukin-2 (IL-2) genes, blood samples were collected from 380 breeder hens of the Mazandaran native fowls breeding station. DNA extraction was performed through a modified saltingout method and fragments of 670 and 659 bp from the promoter regions of IFN-γ and IL-2 genes were amplified by using specific primers, respectively. Following genotyping in the IFN-γ gene using the Tsp509I restriction enzyme, two alleles of A and G with the frequencies of 0.55 and 0.45 and three genotypes of AA, AG and GG were observed with the frequencies of 0.32, 0.46 and 0.22, respectively. For the IL-2 gene, two alleles of A and G were also detected using the MnlI restriction enzyme with the frequencies of 0.58 and 0.42 and three genotypes of AA, AG and GG with the frequencies of 0.33, 0.50 and 0.17, respectively. Statistical analysis revealed significant associations between IL-2 gene single-nucleotide polymorphism and productive traits including the average egg weight (EW) at 345-375 days of age, egg number (EN) at 345-375 days of age and body weight (BW) at 8weeks of age traits (P<0.05). Further, in a mean comparison analysis, there were also significant differences between different genotypes of the IL-2 gene in average EWat 28 and 30weeks of age, in which AG genotypes showed higher performance. Additionally, for the IFN-γ gene, a significant difference was found between the genotypes in average EW at 28 weeks of age trait. Therefore, it can be concluded that the above-mentioned polymorphisms could be considered as the pivotal geneticmakers to improveMazandaran native fowl breeding programmes to achieve the optimum performance in productive traits more efficiently.

Diet-induced obese mice exhibit altered immune responses to early Salmonella Typhimurium oral infection

Obesity is a chronic disease associated with different metabolic diseases as well as alterations in immune cell function. It is characterized by a chronic systemic low grade inflammation. There are several studies demonstrating the influence of obesity on the impaired immune response to infection. However, it is not completely clear whether the obese environment influences the development or maintenance of the immune response against infections. The aim of this study was to determine how obesity induced by a high-fat diet affects the immune response to an early oral Salmonella infection. Four groups of mice were kept in separate cages. Two of these designated as controls, fed with a normal diet; whereas other two groups were fed with a high fat diet for 10 weeks. Some mice were used for Salmonella oral infection. After 7 days of oral infection with S. Thypimurium the proportions of spleen cell subsets expressing activation markers in normal diet and HFD obese mice were stained with monoclonal antibodies and analyzed by flow cytometry. Also, mRNA levels of different cytokines were quantified by RT-PCR. It was found that obesity affects the function of the immune system against an early oral Salmonella infection, decreasing NK cells, altering the expression of activation molecules as well as cytokines mRNA levels. Interestingly, the expression some activation molecules on T lymphocytes was reestablished after Salmonella infection, but not the CD25 expression. Immune alterations could lead to immunosuppression or increased susceptibility to infections in HFD obese mice.

Wild-derived mice: from genetic diversity to variation in immune responses

Classical inbred mouse strains have historically been instrumental in mapping immunological traits. However, most of the classical strains originate from a relatively limited number of founder animals, largely within the Mus musculus domesticus subspecies. Therefore, their genetic diversity is ultimately limited. For this reason, it is not feasible to use these mice for exhaustive interrogation of immune signaling pathways. In order to investigate networks through forward genetic analysis, larger genetic diversity is required than is introduced under laboratory conditions. Recently, inbred strains from other mouse subspecies were established such as Mus musculus castaneus and Mus musculus musculus, which diverged from a shared common ancestor with Mus musculus domesticus more than one million years ago. A direct genomic comparison clearly demonstrates the evolutionary divergence that has occurred between wild-derived mice and the classical inbred strains. When compared to classical inbred strains, wild-derived mice exhibit polymorphisms every 100-200 base pairs. Studying the molecular basis of these traits provides us with insight into how the immune system can evolve regulatory features to accommodate environment-specific constraints. Because most wild-derived strains are able to breed with classical inbred mice, they represent a rich source of evolutionarily significant diversity for forward genetic studies. These organisms are an emerging, though still largely unexplored, model for the identification and study of novel immunological genes.

Terminal Deoxynucleotidyl Transferase Is Not Required for Antibody Response to Polysaccharide Vaccines against Streptococcus pneumoniae and Salmonella enterica Serovar Typhi

B cell antigen receptor (BCR) diversity increases by several orders of magnitude due to the action of terminal deoxynucleotidyl transferase (TdT) during V(D)J recombination. Unlike adults, infants have limited BCR diversity, in part due to reduced expression of TdT. Since human infants and young mice respond poorly to polysaccharide vaccines, such as the pneumococcal polysaccharide vaccine Pneumovax23 and Vi polysaccharide (ViPS) of Salmonella enterica serovar Typhi, we tested the contribution of TdT-mediated BCR diversity in response to these vaccines. We found that TdT^+/- and TdT^-/- mice generated comparable antibody responses to Pneumovax23 and survived Streptococcus pneumoniae challenge. Moreover, passive immunization of B cell-deficient mice with serum from Pneumovax23-immunized TdT^+/- or TdT^-/- mice conferred protection. TdT^+/- and TdT^-/- mice generated comparable levels of anti-ViPS antibodies and antibody-dependent, complement-mediated bactericidal activity against S Typhi in vitro To test the protective immunity conferred by ViPS immunization in vivo, TdT^+/- and TdT^-/- mice were challenged with a chimeric Salmonella enterica serovar Typhimurium strain expressing ViPS, since mice are nonpermissive hosts for S Typhi infection. Compared to their unimmunized counterparts, immunized TdT^+/- and TdT^-/- mice challenged with ViPS-expressing S Typhimurium exhibited a significant reduction in the bacterial burden and liver pathology. These data suggest that the impaired antibody response to the Pneumovax23 and ViPS vaccines in the young is not due to limited TdT-mediated BCR diversification.

An Unmutated IgM Response to the Vi Polysaccharide of Salmonella Typhi Contributes to Protective Immunity in a Murine Model of Typhoid

T cell-dependent B cell responses typically develop in germinal centers. Abs generated during such responses are isotype switched and have a high affinity to the Ag because of somatic hypermutation of Ab genes. B cell responses to purified polysaccharides are T cell independent and do not result in the formation of bona fide germinal centers, and the dominant Ab isotype produced during such responses is IgM with very few or no somatic mutations. Activation-induced cytidine deaminase (AID) is required for both somatic hypermutation and Ig isotype switching in humans and mice. To test the extent to which unmutated polysaccharide-specific IgM confers protective immunity, we immunized wildtype and AID^-/- mice with either heat-killed Salmonella enterica serovar Typhi (S. Typhi) or purified Vi polysaccharide (ViPS). We found that wildtype and AID^-/- mice immunized with heat-killed S. Typhi generated similar anti-ViPS IgM responses. As expected, wildtype, but not AID^-/- mice generated ViPS-specific IgG. However, the differences in the Ab-dependent killing of S. Typhi mediated by the classical pathway of complement activation were not statistically significant. In ViPS-immunized wildtype and AID^-/- mice, the ViPS-specific IgM levels and S. Typhi bactericidal Ab titers at 7 but not at 28 d postimmunization were also comparable. To test the protective immunity conferred by these immunizations, mice were challenged with a chimeric S. Typhimurium strain expressing ViPS. Compared with their naive counterparts, immunized wildtype and AID^-/- mice exhibited significantly reduced bacterial burden regardless of the route of infection. These data indicate that an unmutated IgM response to ViPS contributes to protective immunity to S. Typhi.

Identification of new loci involved in the host susceptibility to Salmonella Typhimurium in collaborative cross mice

Background

Salmonella is a Gram-negative bacterium causing a wide range of clinical syndromes ranging from typhoid fever to diarrheic disease. Non-typhoidal Salmonella (NTS) serovars infect humans and animals, causing important health burden in the world. Susceptibility to salmonellosis varies between individuals under the control of host genes, as demonstrated by the identification of over 20 genetic loci in various mouse crosses. We have investigated the host response to S. Typhimurium infection in 35 Collaborative Cross (CC) strains, a genetic population which involves wild-derived strains that had not been previously assessed.

Results

One hundred and forty-eight mice from 35 CC strains were challenged intravenously with 1000 colony-forming units (CFUs) of S. Typhimurium. Bacterial load was measured in spleen and liver at day 4 post-infection. CC strains differed significantly (P < 0.0001) in spleen and liver bacterial loads, while sex and age had no effect. Two significant quantitative trait loci (QTLs) on chromosomes 8 and 10 and one suggestive QTL on chromosome 1 were found for spleen bacterial load, while two suggestive QTLs on chromosomes 6 and 17 were found for liver bacterial load. These QTLs are caused by distinct allelic patterns, principally involving alleles originating from the wild-derived founders. Using sequence variations between the eight CC founder strains combined with database mining for expression in target organs and known immune phenotypes, we were able to refine the QTLs intervals and establish a list of the most promising candidate genes. Furthermore, we identified one strain, CC042/GeniUnc (CC042), as highly susceptible to S. Typhimurium infection.

Conclusions

By exploring a broader genetic variation, the Collaborative Cross population has revealed novel loci of resistance to Salmonella Typhimurium. It also led to the identification of CC042 as an extremely susceptible strain.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-4667-0) contains supplementary material, which is available to authorized users.

Differential Levels of Cecal Colonization by Salmonella Enteritidis in Chickens Triggers Distinct Immune Kinome Profiles

Salmonella enterica serovar Enteritidis are facultative intracellular bacteria that cause disease in numerous species. Salmonella-related infections originating from poultry and/or poultry products are a major cause of human foodborne illness with S. Enteritidis the leading cause worldwide. Despite the importance of Salmonella to human health and chickens being a reservoir, little is known of the response to infection within the chicken gastrointestinal tract. Using chicken-specific kinome immune peptide arrays we compared a detailed kinomic analysis of the chicken jejunal immune response in a single line of birds with high and low Salmonella loads. Four-day-old chicks were challenged with S. Enteritidis (10⁵ cfu) and cecal content and a section of jejunum collected at three times: early [4-7 days post-infection (dpi)], middle (10-17 dpi), and late (24-37 dpi). Salmonella colonization was enumerated and birds with the highest (n = 4) and lowest (n = 4) loads at each time were selected for kinomic analyses. Key biological processes associated with lower loads of Salmonella clustered around immune responses, including cell surface receptor signaling pathway, positive regulation of cellular processes, defense response, innate immune response, regulation of immune response, immune system process, and regulation of signaling. Further evaluation showed specific pathways including chemokine, Jak-Stat, mitogen activated protein kinase, and T cell receptor signaling pathways were also associated with increased resistance. Collectively, these findings demonstrate that it is possible to identify key mechanisms and pathways that are associated with increased resistance against S. Enteritidis cecal colonization in chickens. Therefore, providing a foundation for future studies to identify specific proteins within these pathways that are associated with resistance, which could provide breeders additional biomarkers to identify birds naturally more resistant to this important foodborne pathogen.

Studies on the therapeutic effect of propolis along with standard antibacterial drug in Salmonella enterica serovar Typhimurium infected BALB/c mice

BACKGROUND

Antibiotic resistance is an emerging public health problem. Centers for Disease Control and Prevention (CDC) has described antibiotic resistance as one of the world's most pressing health problems in 21^st century. WHO rated antibiotic resistance as "one of the three greatest threats to human health". One important strategy employed to overcome this resistance is the use of combination of drugs. Many plants, natural extracts have been shown to exhibit synergistic response with standard drugs against microorganisms. The present study focused on the antibacterial potential of propolis in combination with the standard antibiotic Cefixime against the typhoid causing bacteria i.e. Salmonella.

METHODS

Ethanolic extract of propolis was taken for the present work. For the experiment BALB/c mice were taken as animal model and divided into ten groups. Along with normal and infected control groups, four different combinations of cefixime and propolis were used. Biochemical, hematological and histopathological indices were studied by following the standard protocols.

RESULTS

In BALB/c mice, Salmonella causes severe biochemical, hematological and histopathological alterations by 5^th day of infection. Ethanolic extract of propolis at a dose of 300 mg/kg body weight of mice when used alone to treat Salmonella infection in mice gave significant results by 30^th day of treatment. Similarly, when cefixime (4 mg/kg body weight of mice) was used to treat infection in mice, significant results as compared to infected control were observed after 5^th day. But when propolis and cefixime were used together in different concentrations in combination therapy, evident results were observed after 5 days of treatment. The levels of various liver and kidney function enzymes, blood indices and the histopathology of liver, spleen and kidney were restored to near normal after 5 days of treatment and at much lower doses as compared to the effective dose when used alone.

CONCLUSION

The study confirmed that significant results were observed in three combinations of cefixime and propolis as compared to infected controls. Propolis acted synergistically with cefixime and enhanced the efficacy of antibiotic and reduced its effective dose in combined therapy.

Genetic dissection of the ity3 locus identifies a role for ncf2 co-expression modules and suggests selp as a candidate gene underlying the ity3.2 locus

Typhoid fever and salmonellosis, which are caused by Salmonella typhi and typhimurium, respectively, are responsible for significant morbidity and mortality in both developed and developing countries. We model typhoid fever using mice infected with Salmonella typhimurium, which results in a systemic disease, whereby the outcome of infection is variable in different inbred strains of mice. This model recapitulates several clinical aspects of the human disease and allows the study of the host response to Salmonella typhimurium infection in vivo. Previous work in our laboratory has identified three loci (Ity, Ity2, and Ity3) in the wild-derived MOLF/Ei mice influencing survival after infection with Salmonella typhimurium. Fine mapping of the Ity3 locus indicated that two sub-loci contribute collectively to the susceptibility of B6.MOLF-Ity/Ity3 congenic mice to Salmonella infection. In the current paper, we provided further evidence supporting a role for Ncf2 (neutrophil cytosolic factor 2 a subunit of NADPH oxidase) as the gene underlying the Ity3.1 sub-locus. Gene expression profiling indicated that the Ity3.1 sub-locus defined a global gene expression signature with networks articulated around Ncf2. Furthermore, based on differential expression and complementation analysis using Selp (selectin-P) knock-out mice, Selp was identified as a strong candidate gene for the Ity3.2 sub-locus.

Selection for pro-inflammatory mediators yields chickens with increased resistance against Salmonella enterica serovar Enteritidis

Salmonella is a leading cause of foodborne illness and can be transmitted through consumption of contaminated poultry; therefore, increasing a flock's natural resistance to Salmonella could improve food safety. Previously, we characterized the heterophil-mediated innate immune response of 2 parental broiler lines and F1 reciprocal crosses and showed that increased heterophil function and expression of pro-inflammatory mediators corresponds with increased resistance against diverse pathogens. A preliminary selection trial showed that individual sires had varying inherent levels of pro-inflammatory mediators and selection based on a high or low phenotype was passed onto progeny. Based on these results, we hypothesized selection of broilers for higher levels of the pro-inflammatory mediators IL-6, CXCLi2, and CCLi2 would produce progeny with increased resistance against Salmonella Enteritidis. Peripheral blood leukocytes were isolated from 75 commercial broiler sires, screened, and 10 naturally high and low expressing sires were selected and mated to randomly selected dams to produce the first generation of "high" and "low" progeny. The mRNA expression of CXCLi2 and CCLi2 were significantly (P ≤ 0.02) higher in the high progeny and were more resistant to liver and spleen organ invasion by Salmonella Enteritidis compared with low progeny. Production of the second generation yielded progeny that had differences (P ≤ 0.03) in all 3 mediators and further improved resistance against Salmonella Enteritidis. Feed conversion ratio and percent breast meat yield were calculated and were equal, whereas the high birds weighed slightly, but significantly, less than the low birds. These data clearly demonstrate that selection based on a higher phenotype of key pro-inflammatory mediators is a novel means to produce broilers that are naturally more resistant to Salmonella, one of the most important foodborne pathogens affecting the poultry industry.

Ecto-5'-nucleotidase (CD73) regulates host inflammatory responses and exacerbates murine salmonellosis

Food-borne Salmonella spp., are a major cause of hospitalization and death. Adenosine, an important immune regulator of inflammation, limits tissue damage during infection. CD39 (nucleoside triphosphate dephosphorylase) combined with ecto-5′-nucleotidase (CD73) metabolizes ATP to adenosine. We studied the expressions of CD39 and CD73 in tissues, and T helper cells in mice after Salmonella infection and evaluated the role of CD73 in regulating immune responses and bacterial clearance in wild-type and CD73-deficient (CD73^−/−) mice. Both CD39 and CD73 transcript levels declined in the infected wild-type mice. Compared to wild-type mice, tissues from infected CD73^−/− mice had significantly higher expression of pro-inflammatory cytokines and reduced anti-inflammatory responses. CD73^−/− mice were more resistant to infection and had a greater inflammatory responses and a significantly lower bacterial load in the liver compared to wild-type mice. Thus, CD73 expression attenuates inflammation during murine Salmonellosis and impairs immunity, leading to increased bacterial colonization and prolonged infection.

Recombinant Salmonella enterica serovar Typhimurium as a vaccine vector for HIV-1 Gag

The HIV/AIDS epidemic remains a global health problem, especially in Sub-Saharan Africa. An effective HIV-1 vaccine is therefore badly required to mitigate this ever-expanding problem. Since HIV-1 infects its host through the mucosal surface, a vaccine for the virus needs to trigger mucosal as well as systemic immune responses. Oral, attenuated recombinant Salmonella vaccines offer this potential of delivering HIV-1 antigens to both the mucosal and systemic compartments of the immune system. So far, a number of pre-clinical studies have been performed, in which HIV-1 Gag, a highly conserved viral antigen possessing both T- and B-cell epitopes, was successfully delivered by recombinant Salmonella vaccines and, in most cases, induced HIV-specific immune responses. In this review, the potential use of Salmonella enterica serovar Typhimurium as a live vaccine vector for HIV-1 Gag is explored.

Dietary rice bran promotes resistance to Salmonella enterica serovar Typhimurium colonization in mice

Background

Dietary rice bran consists of many bioactive components with disease fighting properties; including the capacity to modulate the gut microbiota. Studies point to the important roles of the gut microbiota and the mucosal epithelium in the establishment of protection against enteric pathogens, such as Salmonella. The ability of rice bran to reduce the susceptibility of mice to a Salmonella infection has not been previously investigated. Therefore, we hypothesized that the incorporation of rice bran into the diet would inhibit the colonization of Salmonella in mice through the induction of protective mucosal responses.

Results

Mice were fed diets containing 0%, 10% and 20% rice bran for one week prior to being orally infected with Salmonella enterica serovar Typhimurium. We found that mice consuming the 10 and 20% rice bran diets exhibited a reduction in Salmonella fecal shedding for up to nine days post-infection as compared to control diet fed animals (p < 0.05). In addition, we observed decreased concentrations of the pro-inflammatory cytokines, TNF-alpha, IFN-gamma, and IL-12 (p < 0.05) as well as increased colonization of native Lactobacillus spp. in rice bran fed mice (p < 0.05). Furthermore, in vitro experiments revealed the ability of rice bran extracts to reduce Salmonella entry into mouse small intestinal epithelial cells.

Conclusions

Increasing rice bran consumption represents a novel dietary means for reducing susceptibility to enteric infection with Salmonella and potentially via induction of native Lactobacillus spp.

Refinement of the genetics of the host response to Salmonella infection in MOLF/Ei: regulation of type 1 IFN and TRP3 pathways by Ity2

Typhoid fever, which is caused by Salmonella typhi and paratyphi, is a severe systemic disease that remains a major public health issue in several areas of the world. We can model the human disease using mice infected with a related bacterium, Salmonella typhimurium. This model recapitulates several clinical aspects of the human disease and allows for the study of the host response to Salmonella typhimurium infection in vivo. Previous work in our laboratory has identified three Immunity to typhimurium loci (Ity, Ity2 and Ity3) in the wild-derived MOLF/Ei mice, influencing survival after infection with Salmonella typhimurium. The MOLF/Ei alleles at Ity and Ity2 are protective, while the MOLF/Ei allele at Ity3 confers susceptibility. In this paper, we have generated a novel cross combination between the highly susceptible strain, MOLF/Ei, and the resistant strain, 129S6, to better define the genetic architecture of susceptibility to infection in MOLF/Ei. Using this cross, we have replicated the locus on chr 11 (Ity2) and identified a novel locus on chr 13 (Ity13). Using microarrays and transcriptional profiling, we examined the response of uninfected and infected Ity2 congenic mice. These analyses demonstrate a role for both type-1-interferon (IFN) and TRP53 signaling in the pathogenesis of Salmonella infection.

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.

Proteins from latex of Calotropis procera prevent septic shock due to lethal infection by Salmonella enterica serovar Typhimurium

AIM OF THE STUDY

The latex of Calotropis procera has been used in traditional medicine to treat different inflammatory diseases. The anti-inflammatory activity of latex proteins (LP) has been well documented using different inflammatory models. In this work the anti-inflammatory protein fraction was evaluated in a true inflammatory process by inducing a lethal experimental infection in the murine model caused by Salmonella enterica Subsp. enterica serovar Typhimurium.

MATERIALS AND METHODS

Experimental Swiss mice were given 0.2 ml of LP (30 or 60 mg/kg) by the intraperitoneal route 24 h before or after lethal challenge (0.2 ml) containing 10(6) CFU/ml of Salmonella Typhimurium using the same route of administration.

RESULTS

All the control animals succumbed to infection within 6 days. When given before bacterial inoculums LP prevented the death of mice, which remained in observation until day 28. Even, LP-treated animals exhibited only discrete signs of infection which disappeared latter. LP fraction was also protective when given orally or by subcutaneous route. Histopathological examination revealed that necrosis and inflammatory infiltrates were similar in both the experimental and control groups on days 1 and 5 after infection. LP activity did not clear Salmonella Typhimurium, which was still present in the spleen at approximately 10(4) cells/g of organ 28 days after challenge. However, no bacteria were detected in the liver at this stage. LP did not inhibit bacterial growth in culture medium at all. In the early stages of infection bacteria population was similar in organs and in the peritoneal fluid but drastically reduced in blood. Titration of TNF-alpha in serum revealed no differences between experimental and control groups on days 1 and 5 days after infection while IL-12 was only discretely diminished in serum of experimental animals on day 5. Moreover, cultured macrophages treated with LP and stimulated by LPS released significantly less IL-1beta.

CONCLUSIONS

LP-treated mice did not succumb to septic shock when submitted to a lethal infection. LP did not exhibit in vitro bactericidal activity. It is thought that protection of LP-treated mice against Salmonella Typhimurium possibly involves down-regulation of pro-inflammatory cytokines (other than TNF-alpha). LP inhibited IL-1beta release in cultured macrophages and discretely reduced IL-12 in serum of animals given LP. Results reported here support the folk use of latex to treat skin infections by topic application.

Selection of broilers with improved innate immune responsiveness to reduce on-farm infection by foodborne pathogens

Economic pressure on the modern poultry industry has directed the selection process towards fast-growing broilers that have a reduced feed conversion ratio. Selection based heavily on growth characteristics could adversely affect immune competence leaving chickens more susceptible to disease. Since the innate immune response directs the acquired immune response, efforts to select poultry with an efficient innate immune response would be beneficial. Our laboratories have been evaluating the innate immune system of two parental broiler lines to assess their capacity to protect against multiple infections. We have shown increased in vitro heterophil function corresponds with increased in vivo resistance to Gram-positive and Gram-negative bacterial infections. Additionally, there are increased mRNA expression levels of pro-inflammatory cytokines/chemokines in heterophils isolated from resistant lines compared to susceptible lines. Collectively, all data indicate there are measurable differences in innate responsiveness under genetic control. Recently, a small-scale selection trial was begun. We identified sires within a broiler population with higher and/or lower-than-average pro-inflammatory cytokine/chemokine mRNA expression levels and subsequently utilized small numbers of high-expressing and low-expressing sires to produce progeny with increased or decreased, respectively, pro-inflammatory cytokine/chemokine profiles. This novel approach should allow us to improve breeding stock by improving the overall immunological responsiveness. This will produce a line of chickens with an effective pro-inflammatory innate immune response that should improve resistance against diverse pathogens, improve responses to vaccines, and increase livability. Ongoing work from this project is providing fundamental information for the development of poultry lines that will be inherently resistant to colonization by pathogenic and food-poisoning microorganisms. Utilization of pathogen-resistant birds by the poultry production industry would significantly enhance the microbiological safety of poultry products reaching the consumer.

A mutation in Irak2c identifies IRAK-2 as a central component of the TLR regulatory network of wild-derived mice

In a phenotypic screen of the wild-derived mouse strain MOLF/Ei, we describe an earlier and more potent toll-like receptor (TLR)–mediated induction of IL-6 transcription compared with the classical inbred strain C57BL/6J. The phenotype correlated with increased activity of the IκB kinase axis as well as p38, but not extracellular signal-regulated kinase or c-Jun N-terminal kinase, mitogen-activated protein kinase (MAPK) phosphorylation. The trait was mapped to the Why1 locus, which contains Irak2, a gene previously implicated as sustaining the late phase of TLR responses. In the MOLF/Ei TLR signaling network, IRAK-2 promotes early nuclear factor κB (NF-κB) activity and is essential for the activation of p38 MAPK. We identify a deletion in the MOLF/Ei promoter of the inhibitory Irak2c gene, leading to an increased ratio of pro- to antiinflammatory IRAK-2 isoforms. These findings demonstrate that IRAK-2 is an essential component of the early TLR response in MOLF/Ei mice and show a distinct pathway of p38 and NF-κB activation in this model organism. In addition, they demonstrate that studies in evolutionarily divergent model organisms are essential to complete dissection of signal transduction pathways.

Presence of wild-type and attenuated Salmonella enterica strains in brain tissues following inoculation of mice by different routes

Salmonella enterica serovar Typhi and Typhimurium vaccine candidates elicit significant immune responses in mice by intranasal (i.n.) immunization. Because of the proximity of the cribriform plate of the ethmoid bone, we were concerned that Salmonella bacteria delivered i.n. might access the brain. Accordingly, wild-type and attenuated (by single and double mutations) strains of S. enterica serovars Typhimurium and Typhi were recovered at low numbers initially from the olfactory lobe and then from the brain for 3 to 4 days after i.n. immunization. This was independent of invA gene function. Although the presence of bacteria in blood 1 to 3 h after i.n. inoculation was sometimes observed, this was infrequent compared to the frequency of bacteria detected in brain tissues. In confirmation of recent observations by Wickham et al. (M. E. Wickham, N. F. Brown, J. Provias, B. B. Finlay, and B. K. Coombes, BMC Infect. Dis. 7:65, 2007) that oral inoculation with wild-type S. enterica serovar Typhimurium strains lead to bacteria in blood with subsequent colonization of brain tissues with neurological symptoms of disease, we found similar results by using the i.n. and intraperitoneal (i.p.) routes of inoculation for wild-type but not for attenuated strains of S. enterica serovar Typhimurium. In contrast, a highly modified attenuated S. enterica serovar Typhimurium strain was not present in brain tissues when administered at higher doses by the oral, i.n., and i.p. routes than the wild-type strain even though the presence of bacteria in blood was detectable 1 to 3 h after inoculation by each of the three routes. Our results indicate that i.n. and possibly even oral delivery of live Salmonella vaccines may be unsafe although it is possible to reduce this risk by appropriate genetic modifications.

Forward genetic analysis of Toll-like receptor responses in wild-derived mice reveals a novel antiinflammatory role for IRAK1BP1

Although inflammatory cytokines produced by activation of Toll-like receptors (TLRs) are essential for early host defense against infection, they also mediate a vast array of pathologies, including autoimmune disease, hypersensitivity reactions, and sepsis. Thus, numerous regulatory mechanisms exist in parallel with proinflammatory pathways to prevent excessive release of these potent effector molecules. We report elucidation of a novel regulatory function for interleukin receptor-associated kinase (IRAK)-1 binding protein 1 (IRAK1BP1, also known as SIMPL) through quantitative trait locus mapping of the TLR response in wild-derived mouse strains. This gene emerged as a negative regulator of TLR2-mediated interleukin (IL)-6 production in MOLF/Ei mice, which expressed IRAK1BP1 mRNA in an allele-specific manner when crossed with the C57BL/6J strain. Human peripheral blood mononuclear cells and primary macrophages from two other wild-derived mouse strains also induced IRAK1BP1 mRNA by 4 hours after stimulation with agonists of various TLRs. Examination of its effects on IL-6 and other cytokines demonstrated that IRAK1BP1 regulates transcription of a specific subset of TLR-responsive genes, producing an overall antiinflammatory profile. Our results reveal that IRAK1BP1 is a critical factor in preventing dangerous overproduction of proinflammatory cytokines by the innate immune system and in influencing the specificity of TLR responses. Furthermore, these results show that the genetic diversity of wild-derived mouse strains makes them a valuable model of important human gene functions that have been lost in some laboratory-inbred strains.

Genetic analysis of resistance to infections in mice: A/J meets C57BL/6J

Susceptibility to infectious diseases has long been known to have a genetic component in human populations. This genetic effect is often complex and difficult to study as it is further modified by environmental factors including the disease-causing pathogen itself. The laboratory mouse has proved a useful alternative to implement a genetic approach to study host defenses against infections. Our laboratory has used genetic analysis and positional cloning to characterize single and multi-gene effects regulating inter-strain differences in the susceptibility of A/J and C57BL/6J mice to infection with several bacterial and parasitic pathogens. This has led to the identification of several proteins including Nrampl (Slc11a1), Birc1e, Icsbp, C5a, and others that play critical roles in the antimicrobial defenses of macrophages against intracellular pathogens. The use of AcB/BcA recombinant congenic strains has further facilitated the characterization of single gene effects in complex traits such as susceptibility to malaria. The genetic identification of erythrocyte pyruvate kinase (Pklr) and myeloid pantetheinase enzymes (Vnn1/3) as key regulators of blood-stage parasitemia has suggested that cellular redox potential may be a key biochemical determinant of Plasmodium parasite replication. Expanding these types of studies to additional inbred strains and to emerging stocks of mutagenized mice will undoubtedly continue to unravel the molecular basis of host defense against infections.

Interspecific recombinant congenic strains between C57BL/6 and mice of the Mus spretus species: a powerful tool to dissect genetic control of complex traits

Complex traits are under the genetic control of multiple genes, often with weak effects and strong epistatic interactions. We developed two new collections of mouse strains to improve genetic dissection of complex traits. They are derived from several backcrosses of the Mus spretus SEG/Pas or STF/Pas strains on the C57BL/6J background. Each of the 55 interspecific recombinant congenic strains (IRCSs) carries up to eight SEG/Pas chromosomal segments with an average size of 11.7 Mb, totalizing 1.37% of the genome. The complete series covers 39.7% of the SEG/Pas genome. As a complementary resource, six partial or complete interspecific consomic strains were developed and increased genome coverage to 45.6%. To evaluate the usefulness of these strains for QTL mapping, 16 IRCSs were compared with C57BL/6J for seven hematological parameters. Strain 66H, which carries three SEG/Pas chromosomal segments, had lower red blood cell volume and higher platelet count than C57BL/6J. Each chromosomal segment was isolated in a congenic strain to evaluate individual effects. Congenic strains were combined to assess epistasis. Our data show that both traits were controlled by several genes with complex epistatic interactions. IRCSs are therefore useful to unravel QTL with small effects and gene-by-gene interactions.

Icsbp1/IRF-8 is required for innate and adaptive immune responses against intracellular pathogens

The chronic myeloid leukemia syndrome of the BXH-2 mouse strain (Mus musculus) is caused by a recessive mutation (R294C) in the transcriptional regulator Icsbp1/IRF-8. In trans activation assays using an IL-12p40 gene reporter construct introduced in RAW 264.7 mouse macrophages, we show that the Icsbp1(C294) isoform behaves as a partial loss-of-function. The Icsbp1(C294) hypomorph allele appears to have a threshold effect on IL-12 production, with pleiotropic consequences on resistance to different types of infections in vivo. Despite the presence of a resistance Nramp1(G169) allele, BXH-2 mice (Icsbp1(C294)) show impaired control of Mycobacterium bovis (bacille Calmette-Guérin) multiplication both early and late during infection, with uncontrolled replication linked to inability to form granulomas in infected liver and spleen. Studies in informative (BXH-2 x BALB/cJ)F(2) mice show that homozygosity for Icsbp1(C294) causes susceptibility to Salmonella enterica serovar Typhimurium to a level comparable to that seen for mice lacking functional Nramp1 or TLR4. Finally, impaired Icsbp1(C294) function is associated with the following: 1) increased replication of the Plasmodium chabaudi AS malarial parasite during the first burst of blood parasitemia, and 2) recurring waves of high blood parasitemia late during infection. These results show that Icsbp1 is required for orchestrating early innate responses and also long-term immune protection against unrelated intracellular pathogens.

Salmonella enterica serovar Typhimurium strain lacking pmrG-HM-D provides excellent protection against salmonellosis in murine typhoid model

The superiority of live attenuated vaccines in systemic salmonellosis has been proven over killed and subunit vaccines, because of its ability to induce protective cell mediated immunity by CD8+ T cells. A live attenuated Salmonella enterica serovar Typhimurium vaccine has been developed by systematic site directed deletion of the pmrG-HM-D chromosomal genomic loci. This gene confers involved in antimicrobial peptide resistance and is involved in LPS modification, both of which are the major immune evasive mechanisms in Salmonella. The efficacy of the newly developed strain in inducing protection against mortality after challenge with the virulent wild type Salmonella typhimurium 12023 was evaluated in mice model of typhoid fever. Animals were immunized and then boosted on days 7 and 14. Following challenge with virulent S. typhimurium 12023, organ burden and mortality of vaccinated mice were less compared to non-immunized controls. The vaccine strain also induced elevated CD8+ T cells in the vaccinated mice. This multiple mutant vaccine candidate appears to be safe for use in pregnant mice and provides a model for the development of live vaccine candidates against naturally occurring salmonellosis and typhoid fever.

Genetic analysis of the innate immune responses in wild-derived inbred strains of mice

The vertebrate immune system has evolved to recognize nucleic acids of bacterial and viral origin. Microbial DNA, as well as synthetic oligonucleotides based on these motifs, activates innate immune pathways mediated by the family of Toll-like receptors (TLR) initiating a cascade of signals in immune cells necessary for responses to pathogens. However, not all of the proteins that participate in TLR-mediated responses have been identified. In studies described herein, we observed significant variation in innate immune responses among selected wild-derived strains of mice. Specifically, we show that mice of MOLF/Ei, Czech/Ei, and MSM/Ms strains are hypo-responsive to polyinosinic-polycytidylic acid (poly(I:C)) because of a mutation in Tlr3. In addition, we discovered a hypo-response to cytosine guanine dinucleotide in MOLF/Ei mice and established that it is not linked to Tlr9, but to another locus. Further inquiry revealed that this hypo-response is transmitted as a monogenic dominant trait that can be mapped and cloned through positional cloning methods. These results suggest the existence of a novel molecule that can alter pro-inflammatory signals or activate additional signal transduction pathways. In addition, they support the wild-derived mouse strain as a forward genetic tool for the identification of novel immunological phenotypes.

Tlr5 is not primarily associated with susceptibility to Salmonella Typhimurium infection in MOLF/Ei mice

The extreme susceptibility to infection with Salmonella Typhimurium of wild-derived MOLF/Ei mice has been linked to one genomic region on Chromosome 1 (Ity3). A member of the Toll-like receptors family, Tlr5, located on distal Chromosome 1, was previously shown to be a candidate gene for Ity3 based on expression studies and sequencing analysis. The candidacy of Tlr5 as a Salmonella-susceptibility gene was evaluated functionally by comparing Tlr5 C57BL/6J and MOLF/Ei alleles in vitro and in vivo. In vitro studies showed that the MOLF/Ei Tlr5 allele is more transcriptionally active when the gene is removed from its natural genomic environment. This observation was supported by in vivo studies in B6.MOLF-Ity3 congenic mice that showed that mice homozygous for the MOLF/Ei allele at Ity3, including Tlr5, had an increased response to flagellin as measured by IL-6 and CXCL-1 secretion in the serum compared with parental MOLF/Ei mice. Despite the fact that both MOLF/Ei and B6.MOLF-Ity/Ity3 mice are more susceptible to Salmonella Typhimurium infection than B6.MOLF-Ity mice, they exhibit a different phenotype with respect to Tlr5 expression and Tlr5 signaling, supporting the prediction that Tlr5 is not primarily involved in the disease phenotype underlying the Ity3 locus in MOLF/Ei mice.

Sequencing, expression, and functional analyses support the candidacy of Ncf2 in susceptibility to Salmonella typhimurium infection in wild-derived mice

A recessive Salmonella Typhimurium susceptibility locus (immunity to Typhimurium (Ity3) was reported previously on distal mouse chromosome 1 using a cross between C57BL/6J and wild-derived MOLF/Ei mice. This quantitative trait locus is located in a genomic region spanning 84 Mb, rich in candidate genes for which a role in host resistance to Salmonella infection is either known or can be envisioned. In this study, we report the evaluation of neutrophil cytosolic factor 2 (Ncf2) as a candidate Salmonella susceptibility gene for Ity3. Ncf2 encodes p67phox, a subunit of the multiprotein enzyme complex NADPH oxidase, known to be responsible for the generation of superoxides. Congenic mice carrying the Ity3 region from MOLF/Ei, B6.MOLF-Ity/Ity3 were more susceptible to infection compared with control mice heterozygous at Ity3, B6.MOLF-Ity/Ity3(MOLF/B6), confirming the existence of a recessive Salmonella susceptibility locus on distal chromosome 1. Spleen Ncf2 expression levels were lower in infected congenic mice homozygous for the MOLF/Ei allele at Ity3 compared with mice heterozygous at Ity3. C57BL/6J and MOLF/Ei Ncf2 sequence comparisons revealed one nonconservative amino acid change (R394Q) in the functional and highly conserved Phox and Bem1 domain of the protein. Functional analysis revealed that the MOLF/Ei allele had reduced PMA- and Salmonella-induced superoxide induction as compared with their wild-type counterparts ex vivo. The R394Q substitution seems to occur on an amino acid involved in electrostatic interactions with p40phox, crucial in its activation. Moreover, a human mutation in the corresponding R395W, resulting in chronic granulatomous disease, is known to lead to reduced superoxide levels. These results support the candidacy of Ncf2 as the gene underlying Ity3.

Inhibition of IFN-gamma-stimulated proinflammatory cytokines by vasoactive intestinal peptide (VIP) correlates with increased survival of Salmonella enterica serovar typhimurium phoP in murine macrophages

Vasoactive intestinal peptide (VIP)is a novel Th2 cytokine that has been shown previously to rescue rats and mice from the lethal effect of bacterial lipopolysaccharide (LPS). We report that VIP inhibited production of the proinflammatory cytokines, tumor necrosis factor-alpha(TNF-alpha)and interleukin-1beta (IL-1beta), at the mRNA level and that the inhibitory effect of VIP was maintained when macrophages were cocultured with an immunostimulatory concentration of interferon-gamma (IFN-gamma)(100 U/ml). The concentration of VIP that had optimal inhibitory effect was (1010) M. Furthermore, VIP prevented macrophage killing of a phoP mutant of Salmonella enterica serovar typhimurium, which is usually attenuated for virulence as a result of its inability to survive inside macrophages. However, although the effect of VIP on inducible nitric oxide synthase (iNOS) was less clear, N-monoethyl arginine (NEMA)(an iNOS inhibitor)did not rescue S. typhimurium from IFN- gamma-induced death, in accordance with previous reports that suggest that iNOS is not an important Salmonella killing pathway in macrophages within the first 24 h. VIP is a potent inhibitor of inflammatory pathways that lead to significant pathologic conditions. However, it increases survival of the normally avirulent phoP mutant and is able to inhibit IFN-gamma-stimulated killing of wild-type S. typhimurium in murine macrophages. Thus, VIP inhibits the proinflammatory type 1 response, thus favoring Salmonella survival.

Identification and functional characterization of chicken toll-like receptor 5 reveals a fundamental role in the biology of infection with Salmonella enterica serovar typhimurium

Toll-like receptors (TLRs) are a major component of the pattern recognition receptor repertoire that detect invading microorganisms and direct the vertebrate immune system to eliminate infection. In chickens, the differential biology of Salmonella serovars (systemic versus gut-restricted localization) correlates with the presence or absence of flagella, a known TLR5 agonist. Chicken TLR5 (chTLR5) exhibits conserved sequence and structural similarity with mammalian TLR5 and is expressed in tissues and cell populations of immunological and stromal origin. Exposure of chTLR5+ cells to flagellin induced elevated levels of chicken interleukin-1beta (chIL-1beta) but little upregulation of chIL-6 mRNA. Consistent with the flagellin-TLR5 hypothesis, an aflagellar Salmonella enterica serovar Typhimurium fliM mutant exhibited an enhanced ability to establish systemic infection. During the early stages of infection, the fliM mutant induced less IL-1beta mRNA and polymorphonuclear cell infiltration of the gut. Collectively, the data represent the identification and functional characterization of a nonmammalian TLR5 and indicate a role in restricting the entry of flagellated Salmonella into systemic sites of the chicken.

Murein lipoprotein is a critical outer membrane component involved in Salmonella enterica serovar typhimurium systemic infection

Lipopolysaccharide (LPS) and Braun (murein) lipoprotein (Lpp) are major components of the outer membrane of gram-negative enteric bacteria that function as potent stimulators of inflammatory and immune responses. In a previous paper, we provided evidence that two functional copies of the lipoprotein gene (lppA and lppB) located on the chromosome of Salmonella enterica serovar Typhimurium contributed to bacterial virulence. In this study, we characterized lppA and lppB single-knockout (SKO) mutants and compared them with an lpp double-knockout (DKO) mutant using in vitro and in vivo models. Compared to the lpp DKO mutant, which was nonmotile, the motility of the lpp SKO mutants was significantly increased (73 to 77%), although the level of motility did not reach the level of wild-type (WT) S. enterica serovar Typhimurium. Likewise, the cytotoxicity was also significantly increased when T84 human intestinal epithelial cells and RAW264.7 murine macrophages were infected with the lpp SKO mutants compared to the cytotoxicity when cells were infected with the lpp DKO mutant. The level of interleukin-8 (IL-8) in polarized T84 cells infected with the lppB SKO mutant was significantly higher (two- to threefold higher), reaching the level in cells infected with WT S. enterica serovar Typhimurium, than the level in host cells infected with the lppA SKO mutant. The lpp DKO mutant induced minimal levels of IL-8. Similarly, sera from mice infected with the lppB SKO mutant contained 4.5- to 10-fold-higher levels of tumor necrosis factor-alpha and IL-6; the levels of these cytokines were 1.7- to 3.0-fold greater in the lppA SKO mutant-infected mice than in animals challenged with the lpp DKO mutant. The increased cytokine levels observed with the lppB SKO mutant in mice correlated with greater tissue damage in the livers and spleens of these mice than in the organs of animals infected with the lppA SKO and lpp DKO mutants. Moreover, the lppB SKO mutant-infected mice had increased susceptibility to death. Since the lpp DKO mutant retained intact LPS, we constructed an S. enterica serovar Typhimurium triple-knockout (TKO) mutant in which the lppA and lppB genes were deleted from an existing msbB mutant (msbB encodes an enzyme required for the acylation of lipid A). Compared to the lpp DKO and msbB SKO mutants, the lpp-msbB TKO mutant was unable to induce cytotoxicity and to produce cytokines and chemokines in vitro and in vivo. These studies provided the first evidence of the relative contributions of Lpp and lipid A acylation to Salmonella pathogenesis.

Phenotypic expression of pyruvate kinase deficiency and protection against malaria in a mouse model

The recombinant congenic mouse strains AcB55 and AcB61 are extremely resistant to malaria (Plasmodium chabaudi AS) despite the presence of susceptibility alleles at the known Char1/Char2 resistance loci. Resistance in AcB55 and AcB61 is controlled by a locus on chromosome 3 (Char4) shown to be allelic with or tightly linked to a loss-of-function mutation in pyruvate kinase (Pklr). AcB55 and AcB61 show important splenomegaly prior to infection caused by the expansion of the red pulp, and display histological signs of extramedullary erythropoiesis in the liver. Examination of splenic cell populations by flow cytometry demonstrates elevated numbers of TER119-positive erythroid precursor cells (>30% of total spleen cells), while RNA expression studies show elevated expression of erythrocyte-specific transcripts such as globin, transferrin receptor, and Nramp2/Slc11a2 in the spleen of both strains. Hematological profiling in both strains is consistent with the presence of anemia as evidenced by low total erythrocyte counts, decreased hemoglobin, as well as abnormally high numbers of circulating reticulocytes (15-20%). These results strongly suggest that the mutant Pklr allele (Pklr(269A)) of AcB55/61 strains causes hemolytic anemia compensated by constitutive erythropoiesis, which in turn protects the mice against P. chabaudi infection. The possible molecular basis of the Pklr protective effect is discussed and is under current investigation in these two strains.

Genetic control of susceptibility to bacterial infections in mouse models

Historically, the laboratory mouse (Mus musculus) has been the experimental model of choice to study pathophysiology of infection with bacterial pathogens, including natural and acquired host defence mechanisms. Inbred mouse strains differ significantly in their degree of susceptibility to infection with various human pathogens such as Mycobacterium, Salmonella, Legionella and many others. Segregation analyses and linkage studies have indicated that some of these differences are under simple genetic control whereas others behave as complex traits. Major advances in genome technologies have greatly facilitated positional cloning of single gene effects. Thus, a number of genes playing a key role in initial susceptibility, progression and outcome of infection have been uncovered and the functional characterization of the encoded proteins has provided new insight into the molecular basis of antimicrobial defences of polymorphonuclear leukocytes, macrophages, as well as T and B lymphocytes. The multigenic control of susceptibility to infection with certain human pathogens is beginning to be characterized by quantitative trait locus mapping in genome wide scans. This review summarizes recent progress on the mapping, cloning and characterization of genes and proteins that affect susceptibility to infection with major intracellular bacterial pathogens.

Genetic regulation of host responses to Salmonella infection in mice

Salmonella spp are Gram-negative bacteria capable of infecting a wide range of host species, including humans, domesticated and wild mammals, reptiles, birds and insects. The outcome of an encounter between Salmonella and its host is dependent upon multiple factors including the host genetic background. To facilitate the study of the genetic factors involved in resistance to this pathogen, mouse models of Salmonella infection have been developed and studied for years, allowing identification of several genes and pathways that may influence the disease outcome. In this review, we will cover some of the genes involved in mouse resistance to Salmonella that were identified through the study of congenic mouse strains, cloning of spontaneous mouse mutations, use of site-directed mutagenesis or quantitative trait loci analysis. In parallel, the relevant information pertaining to genes involved in resistance to Salmonella in humans will be discussed.