Mapping of genetic modulators of natural resistance to infection with Salmonella typhimurium in wild-derived mice

The Collaborative Cross: A Systems Genetics Resource for Studying Host-Pathogen Interactions

Host genetic variation has a major impact on infectious disease susceptibility. The study of pathogen resistance genes, largely aided by mouse models, has significantly advanced our understanding of infectious disease pathogenesis. The Collaborative Cross (CC), a newly developed multi-parental mouse genetic reference population, serves as a tractable model system to study how pathogens interact with genetically diverse populations. In this review, we summarize progress utilizing the CC as a platform to develop improved models of pathogen-induced disease and to map polymorphic host response loci associated with variation in susceptibility to pathogens.

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.

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.

The genetic basis for susceptibility to Rift Valley fever disease in MBT/Pas mice

The large variation in individual response to infection with Rift Valley fever virus (RVFV) suggests that host genetic determinants play a role in determining virus-induced disease outcomes. These genetic factors are still unknown. The systemic inoculation of mice with RVFV reproduces major pathological features of severe human disease, notably the hepatitis and encephalitis. A genome scan performed on 546 (BALB/c × MBT) F2 progeny identified three quantitative trait loci (QTLs), denoted Rvfs-1 to Rvfs-3, that were associated with disease susceptibility in MBT/Pas mice. Non-parametric interval-mapping revealed one significant and two suggestive linkages with survival time on chromosomes 2 (Rvfs-1), 5 (Rvfs-3) and 11 (Rvfs-2) with respective logarithm of odds (LOD) scores of 4.58, 2.95 and 2.99. The two-part model, combining survival time and survival/death, identified one significant linkage to Rvfs-2 and one suggestive linkage to Rvfs-1 with respective LOD scores of 5.12 and 4.55. Under a multiple model, with additive effects and sex as a covariate, the three QTLs explained 8.3% of the phenotypic variance. Sex had the strongest influence on susceptibility. The contribution of Rvfs-1, Rvfs-2 and Rvfs-3 to survival time of RVFV-infected mice was further confirmed in congenic mice.

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.

Fine-mapping and phenotypic analysis of the Ity3 Salmonella susceptibility locus identify a complex genetic structure

Experimental animal models of Salmonella infections have been widely used to identify genes important in the host immune response to infection. Using an F2 cross between the classical inbred strain C57BL/6J and the wild derived strain MOLF/Ei, we have previously identified Ity3 (Immunity to Typhimurium locus 3) as a locus contributing to the early susceptibility of MOLF/Ei mice to infection with Salmonella Typhimurium. We have also established a congenic strain (B6.MOLF-Ity/Ity3) with the MOLF/Ei Ity3 donor segment on a C57BL/6J background. The current study was designed to fine map and characterize functionally the Ity3 locus. We generated 12 recombinant sub-congenic strains that were characterized for susceptibility to infection, bacterial load in target organs, cytokine profile and anti-microbial mechanisms. These analyses showed that the impact of the Ity3 locus on survival and bacterial burden was stronger in male mice compared to female mice. Fine mapping of Ity3 indicated that two subloci contribute collectively to the susceptibility of B6.MOLF-Ity/Ity3 congenic mice to Salmonella infection. The Ity3.1 sublocus controls NADPH oxidase activity and is characterized by decreased ROS production, reduced inflammatory cytokine response and increased bacterial burden, thereby supporting a role for Ncf2 (neutrophil cytosolic factor 2 a subunit of NADPH oxidase) as the gene underlying this sublocus. The Ity3.2 sub-locus is characterized by a hyperresponsive inflammatory cytokine phenotype after exposure to Salmonella. Overall, this research provides support to the combined action of hormonal influences and complex genetic factors within the Ity3 locus in the innate immune response to Salmonella infection in wild-derived MOLF/Ei mice.

Innate immunity of finfish: primordial conservation and function of viral RNA sensors in teleosts

During the past decade, huge progress has been made in research into teleost PAMPs (pathogen-associated molecule patterns) recognition receptors (PRRs). Numerous fish PRR genes have been identified, and the primordial functions of PRRs involved in the innate immune response to viral infection (especially those responsible for sensing viral RNA) have been increasingly clarified in teleosts. Particular progress has been made in our understanding of Toll-like receptors (TLRs) and retinoic acid inducible gene I (RIG-I)-like receptors (RLRs). However, there are important evolutionary differences between teleosts and mammals; for instance, seven TLR repertoires (TLR5S, -14, -19, -20, -21, -22 and -23) are present in teleosts but not in mammals, indicating that some TLRs likely possess different functions. Thus, comparison of PRRs in teleosts and mammals may help us understand the immune responses triggered by host-pathogen interactions in teleosts. In this article, the evolutionary conservations and divergences in the PRR mechanisms of teleosts and mammals are examined, with a focus on their molecular features and the recognition of viral RNA by fish TLRs and RLRs. In addition, the mechanism of type I interferon gene expression in teleosts, which is enhanced after the recognition of viral RNA by fish TLRs and RLRs, is also introduced.

Susceptibility to progressive Cryptococcus neoformans pulmonary infection is regulated by loci on mouse chromosomes 1 and 9

Genetic factors that regulate the pathogenesis of pneumonia caused by the fungus Cryptococcus neoformans are poorly understood. Through a phenotypic strain survey we observed that inbred C3H/HeN mice develop a significantly greater lung fungal burden than mice of the resistant CBA/J strain 4 weeks following intratracheal infection with C. neoformans ATCC 24067. The aim of the present study was to characterize the inflammatory response of C3H/HeN mice following C. neoformans pulmonary infection and to identify genetic loci that regulate host defense. Following cryptococcal infection, C3H/HeN mice demonstrated a Th2 immune response with heightened airway and tissue eosinophilia, goblet cell metaplasia, and significantly higher lung interleukin-5 (IL-5) and IL-13 protein expression relative to CBA/J mice. Conversely, CBA/J mice exhibited greater airway and tissue neutrophilia that was associated with significantly higher pulmonary expression of gamma interferon, CXCL10, and IL-17 proteins than C3H/HeN mice. Using the fungal burden at 4 weeks postinfection as a phenotype, genome-wide quantitative trait locus (QTL) analysis among 435 segregating (C3H/HeN × CBA/J)F2 (C3HCBAF2) hybrids identified two significant QTLs on chromosomes 1 (Cnes4) and 9 (Cnes5) that control susceptibility to cryptococcal pneumonia in an additive manner. Susceptible C3H/HeN mice carry a resistance allele at Cnes4 and a susceptibility allele at Cnes5. These studies reveal additional genetic complexity of the host response to C. neoformans that is associated with divergent patterns of pulmonary inflammation.

Allelic variation on murine chromosome 11 modifies host inflammatory responses and resistance to Bacillus anthracis

Anthrax is a potentially fatal disease resulting from infection with Bacillus anthracis. The outcome of infection is influenced by pathogen-encoded virulence factors such as lethal toxin (LT), as well as by genetic variation within the host. To identify host genes controlling susceptibility to anthrax, a library of congenic mice consisting of strains with homozygous chromosomal segments from the LT-responsive CAST/Ei strain introgressed on a LT-resistant C57BL/6 (B6) background was screened for response to LT. Three congenic strains containing CAST/Ei regions of chromosome 11 were identified that displayed a rapid inflammatory response to LT similar to, but more severe than that driven by a LT-responsive allele of the inflammasome constituent NRLP1B. Importantly, increased response to LT in congenic mice correlated with greater resistance to infection by the Sterne strain of B. anthracis. The genomic region controlling the inflammatory response to LT was mapped to 66.36–74.67 Mb on chromosome 11, a region that encodes the LT-responsive CAST/Ei allele of Nlrp1b. However, known downstream effects of NLRP1B activation, including macrophage pyroptosis, cytokine release, and leukocyte infiltration could not fully explain the response to LT or the resistance to B. anthracis Sterne in congenic mice. Further, the exacerbated response in congenic mice is inherited in a recessive manner while the Nlrp1b-mediated response to LT is dominant. Finally, congenic mice displayed increased responsiveness in a model of sepsis compared with B6 mice. In total, these data suggest that allelic variation of one or more chromosome 11 genes in addition to Nlrp1b controls the severity of host response to multiple inflammatory stimuli and contributes to resistance to B. anthracis Sterne. Expression quantitative trait locus analysis revealed 25 genes within this region as high priority candidates for contributing to the host response to LT.

We show that genetic variation within an 8.3 Mb region on mouse chromosome 11 controls host response to anthrax lethal toxin (LT) and resistance to infection by the Sterne strain of Bacillus anthracis. Specifically, congenic C57BL/6 mice in which this region of chromosome 11 is derived from a genetically divergent CAST/Ei strain presented with a rapid and strong innate immune response to LT and displayed increased survival following infection with Sterne spores. CAST/Ei chromosome 11 encodes a dominant LT-responsive allele of Nlrp1b that may partially account for the severe response to LT. However, the strength of this response was attenuated in mice with only one copy of chromosome 11 derived from CAST/Ei indicating the existence of a recessive modifier of the inflammatory response to LT. In addition, congenic mice displayed a pronounced immune response using an experimental model of sepsis, indicating that one or more genes within the chromosome 11 region control host response to multiple inflammatory stimuli. Analyzing the influence of allelic variation on gene expression identified 25 genes as candidates for controlling these responses. In summary, we report a genetic model to study inflammatory responses beneficial to the host during anthrax.

Linkage mapping of toll-like receptors (TLRs) in Japanese flounder, Paralichthys olivaceus

Toll-like receptors (TLRs) are responsible for the recognition of specific pathogen-associated molecular patterns and consequently activate signal pathways leading to inflammatory and interferon responses. The region surrounding several TLRs was previously found to be associated with resistance to specific disease. Hence, we determined the location of 11 TLRs in Japanese flounder (Paralichthys olivaceus) using polymorphic microsatellite markers. TLR1 and TLR3 were located on linkage group (LG) 21 and 7, respectively. Membrane TLR5 and soluble TLR5 were mapped to LG22. TLR7 and TLR8 were mapped to LG3. TLR9 was found on LG1 and TLR14 and TLR21 were located on the same linkage group, LG10. TLR22 was found on LG8. Interestingly, TLR2 was mapped with the previously reported Poli9-8TUF microsatellite marker which is tightly associated with lymphocystis virus disease resistance. Therefore, TLR2 is a candidate gene for resistance to lymphocystis disease. These results imply that the location of a TLR associated with a particular disease may be valuable for the research on the relationship between host immune response and disease resistance.

A locus on chromosome 9 is associated with differential response of 129S1/SvImJ and FVB/NJ strains of mice to systemic LPS

Although polymorphisms in TLR receptors and downstream signaling molecules affect the innate immune response, these variants account for only a portion of the ability of the host to respond to microorganisms. To identify novel genes that regulate the host response to systemic lipopolysaccharide (LPS), we created an F2 intercross between susceptible (FVB/NJ) and resistant (129S1/SvImJ) strains, challenged F2 progeny with LPS via intraperitoneal injection, and phenotyped 605 animals for survival and another 500 mice for serum concentrations of IL-1β and IL-6. Genome-wide scans were performed on pools of susceptible and resistant mice for survival, IL-1β, and IL-6. This approach identified a locus on the telomeric end of the q arm of chromosome 9 (0-40 Mb) that was associated with the differences in morbidity and serum concentrations of IL-1β and IL-6 following systemic LPS in FVB/NJ and 129S1/SvImJ strains of mice. Fine mapping narrowed the locus to 3.7 Mb containing 11 known genes, among which are three inflammatory caspases. We studied expression of genes within the locus by quantitative RT-PCR and showed that Casp1 and Casp12 levels are unaffected by LPS in both strains, whereas Casp4 is highly induced by LPS in FVB/NJ but not in 129S1/SvImJ mice. In conclusion, our mapping results indicate that a 3.7-Mb region on chromosome 9 contains a gene that regulates differential response to LPS in 129S1/SvImJ and FVB/NJ strains of mice. Differences in the induction of Casp4 expression by LPS in the two strains suggest that Casp4 is the most likely candidate gene in this region.

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.

Mapping behavioural evolution onto brain evolution: the strategic roles of conserved organization in individuals and species

The pattern of individual variation in brain component structure in pigs, minks and laboratory mice is very similar to variation across species in the same components, at a reduced scale. This conserved pattern of allometric scaling resembles robotic architectures designed to be robust to changes in computing power and task demands, and may reflect the mechanism by which both growing and evolving brains defend basic sensory, motor and homeostatic functions at multiple scales. Conserved scaling rules also have implications for species-specific sensory and social communication systems, motor competencies and cognitive abilities. The role of relative changes in neuron number in the central nervous system in producing species-specific behaviour is thus highly constrained, while changes in the sensory and motor periphery, and in motivational and attentional systems increase in probability as the principal loci producing important changes in functional neuroanatomy between species. By their nature, these loci require renewed attention to development and life history in the initial organization and production of species-specific behavioural abilities.

Gene expression profiles of RAW264.7 macrophages stimulated with preparations of LPS differing in isolation and purity

Lipopolysaccharide is a major component of the cell wall of Gram-negative bacteria and a potent stimulator of innate immune response via TLR4. Studies on the LPS action both in vivo and in vitro have used different preparations of LPS, including ultra-pure LPS (LIST) and a less pure but less expensive form (Sigma) isolated from Escherichia coli serotype O111:B4. The difference between the effects of these compounds has not been well studied although this information is important in understanding TLR stimulation. In this study, we compared response of RAW264.7 macrophage cells treated LIST or Sigma LPS for 6 h and 24 h. Gene expression data were analyzed to identify specific genes and pathways that are in common and unique to the two LPS preparations. Seven hundred fifty-five genes were differentially expressed at 6 h in response to Sigma LPS and 973 were differentially expressed following LIST LPS treatment, with 503 in common. At 24 h, Sigma LPS induced or repressed 901 genes while 1646 genes were differentially regulated by LIST LPS treatment; 701 genes were shared by two forms of LPS. Although considerably more genes were differentially expressed in response to LIST LPS, similar molecular pathways and transcriptional networks were activated by the two LPS preparations. We also treated bone marrow-derived macrophages (BMMs) from three strains of mice with different concentrations of LIST and Sigma LPS and showed that BMMs produced more IL-6 and TNF-α in response to LIST LPS at low LPS concentrations but, at higher LPS concentrations, more cytokines were produced in response to stimulation by Sigma LPS. Together, these findings suggest that, despite activation of similar molecular pathways by LIST and Sigma LPS preparations, residual protein impurities in the Sigma LPS preparation may nevertheless influence the transcriptional profile attributed to TLR4 stimulation.

Fine mapping of the chicken salmonellosis resistance locus (SAL1)

Salmonella enterica serovar Typhimurium is a Gram-negative bacterium that has a significant impact on both human and animal health. It is one of the most common food-borne pathogens responsible for a self-limiting gastroenteritis in humans and a similar disease in pigs, cattle and chickens. In contrast, intravenous challenge with S. Typhimurium provides a valuable model for systemic infection, often causing a typhoid-like infection, with bacterial replication resulting in the destruction of the spleen and liver of infected animals. Resistance to systemic salmonellosis in chickens is partly genetically determined, with bacterial numbers at systemic sites in resistant lines being up to 1000-fold fewer than in susceptible lines. Identification of genes contributing to disease resistance will enable genetic selection of resistant lines that will reduce Salmonella levels in poultry flocks. We previously identified a novel resistance locus on Chromosome 5, designated SAL1. Through the availability of high-density SNP panels in the chicken, combined with advanced back-crossing of the resistant and susceptible lines, we sought to refine the SAL1 locus and identify potential positional candidate genes. Using a 6(th) generation backcross mapping population, we have confirmed and refined the SAL1 locus as lying between 54.0 and 54.8 Mb on the long arm of Chromosome 5 (F = 8.72, P = 0.00475). This region spans 14 genes, including two very striking functional candidates; CD27-binding protein (Siva) and the RAC-alpha serine/threonine protein kinase homolog, AKT1 (protein kinase B, PKB).

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.

Genomic Approaches to the Host Response to Pathogens

The need to better understand host–pathogen interactions has risen with the expansion in genomics and related technologies. This chapter focuses on two aspects of the host response to pathogens where major advances are being made using genomic approaches. The availability of complete genomic sequences of an expanding number of pathogens, the human and mouse genome sequences, and the advent of genome-wide genotyping and gene expression profiling has opened up new avenues of investigation in the field. The genotype of the pathogen plays a major role in the response of the host to infection with more virulent pathogenic strains possessing the capability to interfere with the host immune response. In addition, different individuals in a population can have very different responses to a genetically identical pathogen. Part of the differential response is governed by the underlying genetic differences between individuals. The advent of genome-wide genotyping using single nucleotide polymorphisms or microsatellite markers is leading to major advances in molecular epidemiology. The future impact of genomic approaches on the development of diagnostics and therapeutics is discussed for infectious diseases. This includes defining the basis of genetic susceptibility to infection and system-wide molecular response to a pathogen.

Identification of novel genes that mediate innate immunity using inbred mice

Innate immunity is the first line of defense against microbial infections. Although polymorphisms in toll-like receptors (TLRs) and downstream signaling molecules (CD14, TLR2, TLR4, TLR5, and IRAK4) affect the innate immune response, these variants account for only a portion of the ability of the host to respond to bacteria, fungi, and viruses. To identify other genes involved in the innate immune response, we challenged 16 inbred murine strains with lipopolysaccharide (LPS) systemically and measured serum concentrations of pro-inflammatory cytokines IL-1beta, IL-6, and TNFalpha, and the chemokine KC 6 hr post-treatment. Loci that segregate with strain phenotypes were identified by whole genome association (WGA) mapping of cytokine concentrations. Published gene expression profiles and quantitative trait loci (QTL) were then utilized to prioritize loci and genes that potentially regulate the host response to LPS. Sixteen loci were selected for further investigation by combining WGA analysis with previously published QTL for murine response to LPS or gram negative bacteria. Thirty-eight genes within these loci were then selected for further investigation on the basis of the significance of the identified locus, transcriptional response to LPS, and biological plausibility. RNA interference-mediated inhibition of 4 of 38 candidate genes was shown to block the production of IL-6 in J774A.1 macrophages. In summary, our analysis identified 4 genes that have not previously been implicated in innate immunity, namely, 1110058L19Rik, 4933415F23Rik, Fbxo9, and Ipo7. These genes could represent potential sepsis biomarkers or therapeutic targets that should be further investigated in human populations.

Comparative genomics of Toll-like receptor signalling in five species

BACKGROUND

Over the last decade, several studies have identified quantitative trait loci (QTL) affecting variation of immune related traits in mammals. Recent studies in humans and mice suggest that part of this variation may be caused by polymorphisms in genes involved in Toll-like receptor (TLR) signalling. In this project, we used a comparative approach to investigate the importance of TLR-related genes in comparison with other immunologically relevant genes for resistance traits in five species by associating their genomic location with previously published immune-related QTL regions.

RESULTS

We report the genomic localisation of TLR1-10 and ten associated signalling molecules in sheep and pig using in-silico and/or radiation hybrid (RH) mapping techniques and compare their positions with their annotated homologues in the human, cattle and mouse whole genome sequences. We also report medium-density RH maps for porcine chromosomes 8 and 13. A comparative analysis of the positions of previously published relevant QTLs allowed the identification of homologous regions that are associated with similar health traits in several species and which contain TLR related and other immunologically relevant genes. Additional evidence was gathered by examining relevant gene expression and association studies.

CONCLUSION

This comparative genomic approach identified eight genes as potentially causative genes for variations of health related traits. These include susceptibility to clinical mastitis in dairy cattle, general disease resistance in sheep, cattle, humans and mice, and tolerance to protozoan infection in cattle and mice. Four TLR-related genes (TLR1, 6, MyD88, IRF3) appear to be the most likely candidate genes underlying QTL regions which control the resistance to the same or similar pathogens in several species. Further studies are required to investigate the potential role of polymorphisms within these genes.

Quantitative trait loci associated with susceptibility to bladder and kidney infections induced by Escherichia coli in female C3H/HeJ mice

BACKGROUND

The C3H/HeJ mouse strain develops severe bladder and kidney infections after receiving intravesical inoculation with uropathogenic Escherichia coli. This susceptibility is genetically determined, but the specific genes involved have not been completely defined. The objective of the present study was to use quantitative trait locus (QTL) mapping to identify chromosomal sites associated with susceptibility to infection in C3H/HeJ mice.

METHODS

Female mice from a backcross of C3H/HeJ and (BALB/cxC3H/HeJ)F1 mice were inoculated with E. coli, and the number of E. coli colony-forming units present in the bladder and kidneys was quantified 10 days later. Genomic DNA was scanned using microsatellite markers to localize chromosomal segments derived from parental strains. Statistical analyses associated infection phenotypes with chromosomal sites.

RESULTS

A highly significant QTL for susceptibility to bladder infection was identified on chromosome 4, and C3H/HeJ alleles at this locus interacted with BALB/c alleles on chromosome 19 to increase the severity of infection. A significant QTL on chromosome 6 was associated with severe kidney infections.

CONCLUSIONS

Increased susceptibility to E. coli bladder and kidney infections in female C3H/HeJ mice is associated with specific chromosomal sites located near genes contributing to host resistance to infection. The results demonstrate the multigenic nature of susceptibility to urinary tract infections.

Use of high-throughput mass spectrometry to elucidate host-pathogen interactions in Salmonella

Capabilities in mass spectrometry are evolving rapidly, with recent improvements in sensitivity, data analysis and, most important from the standpoint of this review, much higher throughput, allowing analysis of many samples in a single day. This short review describes how these improvements in mass spectrometry can be used to dissect host-pathogen interactions using Salmonella as a model system. This approach has enabled direct identification of the majority of annotated Salmonella proteins, quantitation of expression changes under various in vitro growth conditions and new insights into virulence and expression of Salmonella proteins within host cells. One of the most significant findings is that a relatively high percentage of all the annotated genes (>20%) in Salmonella are regulated post-transcriptionally. In addition, new and unexpected interactions have been identified for several Salmonella virulence regulators that involve protein-protein interactions, suggesting additional functions of these regulators in coordinating virulence expression. Overall high-throughput mass spectrometry provides a new view of host-pathogen interactions, emphasizing the protein products and defining how protein interactions determine the outcome of infection.

Genetic resistance to infection influences a male's sexual attractiveness and modulation of testosterone

Females may be attracted to males genetically resistant to infectious diseases, and one potential mechanism for this mating bias is that such males may be better able to maintain high testosterone. To test these two hypotheses, we collected scent-marks from male house mice (Mus domesticus) genetically resistant and susceptible to Salmonella due to a single locus (Nramp 1, also known as Slc11a1). We tested whether females are more attracted to the scent-marks of resistant males, and whether such males are better able to maintain testosterone concentrations during an experimental Salmonella infection. We found that females preferred the scent-marks of genetically resistant males compared to susceptible ones; but they showed no preferences 5d after males were infected. As predicted, genetically resistant males maintained their testosterone concentrations during the experimental infection, whereas susceptible males showed a significant decline 14 d after inoculation. These differences in the males' ability to modulate testosterone, however, do not explain females' attraction to resistant males. Thus, our results indicate that females sometimes prefer males genetically resistant to infection, and they provide the first evidence that males modulate their testosterone depending upon their genetic resistance to infection; however, we found no evidence to link these two findings.

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.

Molecular genetic analysis of two loci (Ity2 and Ity3) involved in the host response to infection with Salmonella typhimurium using congenic mice and expression profiling

Numerous genes have been identified to date that contribute to the host response to systemic Salmonella Typhimurium infection in mice. We have previously identified two loci, Ity2 and Ity3, that control survival to Salmonella infection in the wild-derived inbred MOLF/Ei mouse using a (C57BL/6J x MOLF/Ei)F(2)cross. We validated the existence of these two loci by creating congenic mice carrying each quantitative trait locus (QTL) in isolation. Subcongenic mice generated for each locus allowed us to define the critical intervals underlying Ity2 and Ity3. Furthermore, expression profiling was carried out with the aim of identifying differentially expressed genes within the critical intervals as potential candidate genes. Genomewide expression arrays were used to interrogate expression differences in the Ity2 congenics, leading to the identification of a new candidate gene (Havcr2, hepatitis A virus cellular receptor 2). Interval-specific oligonucleotide arrays were created for Ity3, identifying one potential candidate gene (Chi3l1, chitinase 3-like 1) to be pursued further. The combination of the use of congenics in QTL confirmation and fine mapping and in the identification of candidate genes by expression profiling has been successful and represents a step toward quantitative gene(s) identification.

Genetics of susceptibility to leishmaniasis in mice: four novel loci and functional heterogeneity of gene effects

Symptoms of human leishmaniasis range from subclinical to extensive systemic disease with splenomegaly, hepatomegaly, skin lesions, anemia and hyperglobulinemia, but the basis of this variation is unknown. Association of progression of the disease with Th2 lymphocyte response was reported in mice but not in humans. As most genetic studies in Leishmania major (L. major)-infected mice were restricted to skin lesions, we analyzed the symptomatology of leishmaniasis in mice by monitoring skin lesions, hepatomegaly, splenomegaly and seven immunological parameters. We detected and mapped 17 Leishmania major response (Lmr) gene loci that control the symptoms of infection. Surprisingly, the individual Lmr loci control 13 different combinations of pathological and immunological symptoms. Seven loci control both pathological and immunological parameters, 10 influence immunological parameters only. Moreover, the genetics of clinical symptoms is also very heterogeneous: loci Lmr13 and Lmr4 determine skin lesions only, Lmr5 and Lmr10 skin lesions and splenomegaly, Lmr14 and Lmr3 splenomegaly and hepatomegaly, Lmr3 (weakly) skin lesions, and Lmr15 hepatomegaly only. Only two immunological parameters, IgE and interferon-gamma serum levels, correlate partly with clinical manifestations. These findings extend the paradigm for the genetics of host response to infection to include numerous genes, each controlling a different set of organ-specific and systemic effects.

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.

Complex genetic control of host susceptibility to coxsackievirus B3-induced myocarditis

The pathogenesis of viral myocarditis is a multifactorial process involving host genetics, viral genetics and the environment in which they interact. We have used a model of infection with coxsackievirus B3 (CVB3) to characterize the contribution of host genetics to viral myocarditis in mice of different genetic backgrounds but with a common H2 haplotype: A/J and B10.A-H2(a). Here we have used Evans blue dye as a quantitative biomarker for susceptibility to CVB3-induced myocarditis in addition to histopathological semiquantitative measures. We have found evidence of linkage between susceptibility to viral myocarditis and three loci. A locus on chromosome 1 centered on D1Mit200 was linked to sarcolemmal disruption in males (P=0.00005), a second locus on chromosome 4 centered on D4Mit81 was also linked to sarcolemmal disruption in males (P=0.0022). A third locus on distal chromosome 3 centered on D3Mit19 was linked to myocardial infiltration, with a logarithm of odds (LOD) score of 4.7 (P=0.0045), as well as sarcolemmal disruption in females (P=0.0015). These results provide strong evidence for the presence of loci contributing to the susceptibility of mice to viral myocarditis.

Host genetics of mycobacterial diseases in mice and men: forward genetic studies of BCG-osis and tuberculosis

In humans, genetic factors have long been suspected to contribute to the onset and outcome of tuberculosis. Such effects are difficult to identify owing to their complex inheritance, and to the confounding impact of environmental factors, notably pathogen-associated virulence determinants. Recently, forward genetic approaches in mouse models and in human populations have been used to elucidate a molecular basis for predisposition to mycobacterial diseases. The genetic dissection of host predisposition to infection with Mycobacterium bovis BCG and M. tuberculosis will help to define the key molecules involved in host antituberculous immunity and should provide new insights into this important infectious disease.

Complexity in the host response to Salmonella Typhimurium infection in AcB and BcA recombinant congenic strains

The host response to Salmonella infection is controlled by its genetic makeup. Using the mouse model of typhoid fever, several genes were found to influence the outcome of Salmonella infection, including Nramp1 (Slc11a1). In order to improve our knowledge of genetic determinants of the mouse response to acute Salmonella Typhimurium infection, we performed a systematic screening of a set of A/J and C57BL/6J recombinant congenic strains (RCS) for their resistance to infection. While we knew that the parental strains differ in their susceptibility to Salmonella because C57BL/6J mice carry a non-functional allele at Nramp1, we hypothesized that other genes would influence the response to Salmonella and segregate in the RCS. We identified several RCS that showed a non-expected phenotype given their known Nramp1 genotype proving that the response to Salmonella in A/J and C57BL/6J mice is complex. Based on these findings, we selected two RCS for generation of fully informative F2 crosses, (AcB61 x 129S6) and (AcB64 x DBA/2J). Genetic analyses performed on these crosses identified five novel Salmonella susceptibility QTL mapping to chromosomes 3 (Ity4), 2 (Ity5), 14 (Ity6), 7 (Ity7) and 15 (Ity8). These results illustrate the genetic complexity associated with the mouse response to Salmonella Typhimurium.

Genetic determinants of acute inflammation regulate Salmonella infection and modulate Slc11a1 gene (formerly Nramp1) effects in selected mouse lines

Two lines of mice selected to produce maximal (AIRmax) or minimal (AIRmin) acute inflammatory reactions (AIR) differ in their susceptibility to infection by Salmonella enterica serotype Typhimurium (S. Typhimurium). The LD(50) for AIRmax mice is 1000 times higher than that observed for AIRmin mice, and higher frequencies of Slc11a1 alleles (known to confer either resistance (R) or high susceptibility (S) to S. Typhimurium) were consistently found in AIRmax and AIRmin mouse lines, respectively. In order to evaluate the effect of the quantitative trait loci (QTL) segregated in AIRmax and AIRmin mice on Slc11a1 dependent susceptibility to S. Typhimurium, the R and S alleles were fixed in homozygosity in AIRmax and AIRmin backgrounds by genotype assisted breedings. These new lines were named AIRmax(RR), AIRmax(SS), AIRmin(RR), and AIRmin(SS). Acute inflammation of Slc11a1(RR) animals was more severe in comparison to their Slc11a1(SS) counterparts, implicating Slc11a1 (or other linked genes) in AIR regulation. The LD(50) of S. Typhimurium was 800-times higher for AIRmax(SS) than for AIRmin(SS), demonstrating that AIR QTL can act as modifiers of the Slc11a1(SS) susceptibility gene. Four microsatellite markers for S. Typhimurium susceptibility QTL described in other mouse lines showed specific allele fixation in AIRmax or AIRmin mice, suggesting that these chromosomal regions also segregate with inflammatory 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.

Influence of Slc11a1 on the outcome of Salmonella enterica serovar Enteritidis infection in mice is associated with Th polarization

Genetic analyses identified Ses1 as a significant quantitative trait locus influencing the carrier state of 129S6 mice following a sublethal challenge with Salmonella enterica serovar Enteritidis. Previous studies have determined that Slc11a1 was an excellent candidate gene for Ses1. Kinetics of infection in 129S6 mice and Slc11a1-deficient (129S6-Slc11a1(tm1Mcg)) mice demonstrated that the wild-type allele of Slc11a1 contributed to the S. enterica serovar Enteritidis carrier state as early as 7 days postinfection. Gene expression profiling demonstrated that 129S6 mice had a significant up-regulation of proinflammatory genes associated with macrophage activation at day 10 postinfection, followed by a gradual increase in immunoglobulin transcripts, whereas 129S6-Slc11a1(tm1Mcg) mice had higher levels of immunoglobulins earlier in the infection. Quantitative reverse transcription-PCR revealed an increase in Th1 cytokine (Ifng and Il12) and Th1-specific transcription factor Tbx21 expression during infection in both the 129S6 and 129S6-Slc11a1(tm1Mcg) strains. However, the expression of Gata3, a transcription factor involved in Th2 polarization, Cd28, and Il4 was markedly increased in Slc11a1-deficient mice during infection, suggesting a predominant Th2 phenotype in 129S6-Slc11a1(tm1Mcg) animals following S. enterica serovar Enteritidis infection. A strong immunoglobulin G2a response, reflecting Th1 activity, was observed only in 129S6 mice. All together, these results are consistent with an impact of Slc11a1 on Th cell differentiation during chronic S. enterica serovar Enteritidis infection. The presence of a Th2 bias in Slc11a1-deficient mice is associated with improved bacterial clearance.

Genetic susceptibility to infectious disease: lessons from mouse models of leishmaniasis

Susceptibility to infectious disease is influenced by multiple host genes, most of which are low penetrance QTLs that are difficult to map in humans. Leishmaniasis is a well-studied infectious disease with a variety of symptoms and well-defined immunological features. Mouse models of this disease have revealed more than 20 QTLs as being susceptibility genes, studies of which have made important contributions to our understanding of the host response to infection. The functional effects of individual QTLs differ widely, indicating a networked regulation of these effects. Several of these QTLs probably also influence susceptibility to other infections, indicating that their characterization will contribute to our understanding of susceptibility to infectious disease in general.

Mapping of interactions and mouse congenic strains identified novel epistatic QTLs controlling the persistence of Salmonella Enteritidis in mice

The host response to infection in humans is multifactorial and involves the complex interaction between two genomes (the host and the pathogen) and the environment. Using an experimental mouse model of chronic infection, we have previously identified the individual effect of three significant and one suggestive quantitative trait loci (QTLs) (Ses1, Ses2, Ses3 and Ses1.1) on Salmonella Enteritidis persistence in target organs of 129S6/SvEvTac mice. Congenic strain construction was performed by transferring each of these QTLs from C57BL/6J onto the 129S6/SvEvTac background, and phenotypic analysis confirmed that Ses1 and Ses1.1 contribute to bacterial clearance. Additional QTLs regulating Salmonella carriage in 129S6/SvEvTac mice were identified using a two-locus epistasis QTL linkage mapping approach conducted separately in females and males. The epistatic model for females included the individual effect of Ses3 and two significant interactions (Ses1-D7Mit267 and Ses1-DXMit48) accounting for 47% of the total phenotypic variance. The model for males included the individual effect of Ses1.1, three interactions (Ses1-D9Mit218, D2Mit197-D4Mit2 and D3Mit256-D13Mit36) and explained 47% of the phenotypic variance. Our results suggest that the oligogenic nature of Salmonella persistence and epistasis are important constituents of the genetic architecture of the host response to chronic Salmonella infection.

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.

Quantitative trait loci in Chromosomes 3, 8, and 9 regulate antibody production against Salmonella flagellar antigensin the mouse

Two mouse lines were produced by bidirectional selection according to the high (HIII) or low (LIII) antibody responsiveness against Salmonella flagellar antigens (Selection III). In the present work we conducted a genomewide scan to map the quantitative trait loci (QTL) involved in the antibody response regulation in these selected mice. HIII and LIII genomes were screened with microsatellite markers and those found polymorphic between the lines (146) were used for linkage analysis in F2 (HIII x LIII) intercross. Simple interval mapping analysis was performed using Mapmanager QTX software. Three highly significant QTL linked to antibody production against Salmonella flagellar antigens have been demonstrated in Chromosomes 3, 8, and 9. HIII and LIII lines differ in the resistance to several diseases, therefore, the relevance of these QTL with the genetic factors involved in infections, autoimmunity, and neoplastic disease progression is discussed.

Cystathionine beta-lyase is important for virulence of Salmonella enterica serovar Typhimurium

The biosynthesis of methionine in bacteria requires the mobilization of sulfur from Cys by the formation and degradation of cystathionine. Cystathionine beta-lyase, encoded by metC in bacteria and STR3 in Schizosaccharomyces pombe, catalyzes the breakdown of cystathionine to homocysteine, the penultimate step in methionine biosynthesis. This enzyme has been suggested to be the target for pyridinamine antimicrobial agents. We have demonstrated, by using purified enzymes from bacteria and yeast, that cystathionine beta-lyase is not the likely target of these agents. Nonetheless, an insertional inactivation of metC in Salmonella enterica serovar Typhimurium resulted in the attenuation of virulence in a mouse model of systemic infection. This result confirms a previous chemical validation of the Met biosynthetic pathway as a target for the development of antibacterial agents and demonstrates that cystathionine beta-lyase is important for bacterial virulence.

Molecular Koch's postulates applied to bacterial pathogenicity--a personal recollection 15 years later

Koch's postulates were derived from Robert Koch's work on infectious diseases, such as anthrax and tuberculosis, which still engage us to this day. These guidelines were an attempt to establish a standard for identifying the specific causation of an infectious disease and to convince sceptics that microorganisms could cause disease. They were also established to encourage an increasing number of novice microbiologists to use more rigorous criteria before claiming a causal relationship between a microorganism and a disease.

Genetic control of the innate immune response

BACKGROUND

Susceptibility to infectious diseases is directed, in part, by the interaction between the invading pathogen and host macrophages. This study examines the influence of genetic background on host-pathogen interactions, by assessing the transcriptional responses of macrophages from five inbred mouse strains to lipopolysaccharide (LPS), a major determinant of responses to gram-negative microorganisms.

RESULTS

The mouse strains examined varied greatly in the number, amplitude and rate of induction of genes expressed in response to LPS. The response was attenuated in the C3H/HeJlpsd strain, which has a mutation in the LPS receptor Toll-like receptor 4 (TLR4). Variation between mouse strains allowed clustering into early (C57Bl/6J and DBA/2J) and delayed (BALB/c and C3H/ARC) transcriptional phenotypes. There was no clear correlation between gene induction patterns and variation at the Bcg locus (Slc11A1) or propensity to bias Th1 versus Th2 T cell activation responses.

CONCLUSION

Macrophages from each strain responded to LPS with unique gene expression profiles. The variation apparent between genetic backgrounds provides insights into the breadth of possible inflammatory responses, and paradoxically, this divergence was used to identify a common transcriptional program that responds to TLR4 signalling, irrespective of genetic background. Our data indicates that many additional genetic loci control the nature and the extent of transcriptional responses promoted by a single pathogen-associated molecular pattern (PAMP), such as LPS.

Overexpression of Toll-like receptor 4 amplifies the host response to lipopolysaccharide and provides a survival advantage in transgenic mice

Toll-like receptors are transmembrane proteins that are involved in the innate immune recognition of microbial constituents. Among them, Toll-like receptor 4 (Tlr4) is a crucial signal transducer for LPS, the major component of Gram-negative bacteria outer cell membrane. The contribution of Tlr4 to the host response to LPS and to infection with virulent Salmonella typhimurium was studied in four transgenic (Tg) strains including three overexpressing Tlr4. There was a good correlation between the level of Tlr4 mRNA expression and the sensitivity to LPS both in vitro and in vivo: Tg mice possessing the highest number of Tlr4 copies respond the most to LPS. Overexpression of Tlr4 by itself appears to have a survival advantage in Tg mice early during infection: animals possessing more than two copies of the gene survived longer and in a greater percentage to Salmonella infection. The beneficial effect of Tlr4 overexpression is greatly enhanced when the mice present a wild-type allele at natural resistance-associated macrophage protein 1, another critical innate immune gene involved in resistance to infection with Salmonella. Tlr4 and natural resistance-associated macrophage protein 1 exhibit functional epistatic interaction to improve the capacity of the host to control bacterial replication. However, this early improvement in disease resistance is not conducted later during infection, because mice overexpressing Tlr4 developed an excessive inflammatory response detrimental to the host.

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.

Identification of microsatellites in expressed muscle genes: assessment of a desmin (CT) dinucleotide repeat as a marker for meat quality

We investigated the presence of dinucleotide repeats (microsatellites) in a porcine skeletal muscle cDNA library. In total, microsatellites were found in nearly 4% of all muscle cDNA clones, of which CT- and TG-repeats were the most prevalent. We found a near full-length desmin cDNA clone with a CT-repeat sited downstream from its stop codon. From 44 commercially reared pigs, six allelic length polymorphisms were identified at this microsatellite locus (128, 131, 134, 135, 136, and 138 bp), whose desmin genotypes were associated with pH change after slaughter (P=0.031) and colour variation of meat (P<0.001 for hue, P<0.005 for lightness, and P=0.014 for b colour). Two of the genotypes (131/131 bp and 131/135 bp) were consistent with paler meat. This finding indicates the usefulness of the cDNA library approach to generate markers for marker-assisted selection and suggests that the desmin microsatellite could be a possible selection marker.

Allelic variation in TLR4 is linked to susceptibility to Salmonella enterica serovar Typhimurium infection in chickens

Toll-like receptor 4 (TLR4) is part of a group of evolutionarily conserved pattern recognition receptors involved in the activation of the immune system in response to various pathogens and in the innate defense against infection. We describe here the cloning and characterization of the avian orthologue of mammalian TLR4. Chicken TLR4 encodes a 843-amino-acid protein that contains a leucine-rich repeat extracellular domain, a short transmembrane domain typical of type I transmembrane proteins, and a Toll-interleukin-1R signaling domain characteristic of all TLR proteins. The chicken TLR4 protein shows 46% identity (64% similarity) to human TLR4 and 41% similarity to other TLR family members. Northern blot analysis reveals that TLR4 is expressed at approximately the same level in all tissues tested, including brain, thymus, kidney, intestine, muscle, liver, lung, bursa of Fabricius, heart, and spleen. The probe detected only one transcript of ca. 4.4 kb in length for all tissues except muscle where the size of TLR4 mRNA was ca. 9.6 kb. We have mapped TLR4 to microchromosome E41W17 in a region harboring the gene for tenascin C and known to be well conserved between the chicken and mammalian genomes. This region of the chicken genome was shown previously to harbor a Salmonella susceptibility locus. By using linkage analysis, TLR4 was shown to be linked to resistance to infection with Salmonella enterica serovar Typhimurium in chickens (likelihood ratio test of 10.2, P = 0.00138), suggesting a role of TLR4 in the host response of chickens to Salmonella infection.

Co-localization of quantitative trait loci regulating resistance to Salmonella typhimurium infection and specific antibody production phenotypes

Salmonella enterica serotype typhimurium is a facultative intracellular bacteria that induces systemic infection in mice. Resistance to this pathogen is under polygenic control in which Nramp1 is the major gene involved. Lines of mice obtained by selective breeding for high (HIII) or low (LIII) antibody response to flagellar antigens of salmonellae showed significant susceptibility differences, although both the lines display Nramp1(R) alleles. The HIII line was extremely susceptible to infection, while the LIII line was resistant. In order to examine the cellular and genetic mechanisms involved in this distinct pattern of resistance, HIII and LIII mice were analyzed for IFNgamma and IL4 production and screened for quantitative trait loci involved in S. typhimurium infection, using several polymorphic microsatellites. In the present work, HIII mice showed an IFNgamma downregulation in the early phase of infection when compared with LIII animals. No interline differences in IL4 production were verified. The loci screening was performed on immunized F2 intercrosses obtained from HIII and LIII mice. Three antibody-controlling chromosomal regions were coincident, and another was mapped near one of the four loci known to affect susceptibility to S. typhimurium. These results indicate a major role of IFNgamma in our model, and suggest the co-localization of quantitative trait loci modulating both infection and antibody production phenotypes.

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.

Candidate genes for resistance to Salmonella enteritidis colonization in chickens as detected in a novel genetic cross

Salmonellosis is a zoonotic disease that is problematic for both animal production and food safety. A novel genetic cross, named the Iowa Salmonella response resource population (ISRRP), was established to elucidate the genetic control of resistance to Salmonella enteritidis (SE) colonization in young chicks, to characterize unique resistance alleles, and to estimate gene interaction effects. Outbred broiler sires were mated with dams of diverse, highly inbred, light-bodied lines to produce an F(1) generation that was informative for all heterozygous alleles of the sires. Mating F(1) sires back to dams of the corresponding inbred line produced a backcross generation. To mimic the natural route of exposure and thus afford the opportunity to investigate mucosal immunity, pathogenic SE were inoculated into the esophagus of day-old chicks. After 1 week, the SE colonizing the cecal lumen and the spleen were enumerated. Candidate genes were selected for analysis based upon one of the two criteria. Functional candidates were genes with reported activity related to the tested traits. Positional candidates were genes mapped near microsatellites that were linked, in other phases of this project, with antibody levels to SE vaccine. Broiler sire alleles of the MHC class I, NRAMP1, PSAP, and IAP1 genes showed association with SE colonization in the F(1) generation of this novel disease resistance resource population.

Identification of quantitative trait loci controlling acute virulence in Toxoplasma gondii

Strains of Toxoplasma gondii can be grouped into three predominant clonal lineages with members of the type I group being uniformly lethal in mice. To elucidate the basis of this extreme virulence, a genetic cross was performed between a highly virulent type I strain (GT-1) and a less-virulent type III strain (CTG), and the phenotypes of resulting progeny were analyzed by genetic linkage mapping. Analysis of independent recombinant progeny identified several quantitative trait loci that contributed to acute virulence. A major quantitative trait locus located on chromosome VII accounted for approximately 50% of the virulence phenotype, whereas a minor locus on chromosome IV, linked to the ROP1 gene, accounted for approximately 10%. These loci are conserved in other type I strains, indicating that acute virulence is controlled by discrete genes common to the type I lineage.

Identification of genetic loci controlling bacterial clearance in experimental Salmonella enteritidis infection: an unexpected role of Nramp1 (Slc11a1) in the persistence of infection in mice

The Gram-negative bacteria, Salmonella, cause a broad spectrum of clinical diseases in both animals and humans ranging from asymptomatic carriage to life-threatening sepsis. We have developed a model to study the contribution of genetic factors to the susceptibility of 129sv and C57BL/6J inbred mice to Salmonella enteritidis during the late phase of infection. C57BL/6J mice were able to eliminate completely sublethal inoculums of S. enteritidis from their reticuloendothelial system, whereas 129sv mice could not even after 60 days post inoculation. A genome scan performed on 302 (C57BL/6J x 129sv) F2 progeny identified three dominant loci (designated Ses1 to Ses3) that are associated with disease susceptibility in 129sv mice. Two highly significant linkages were identified on chromosomes 1 (Ses1) and 7 (Ses2) with respective LOD scores of 9.9 (P = 1.4 x 10(-11)) at D1Mcg5 and 4.0 (P = 1.9 x 10(-5)) at D7Mit62. One highly suggestive QTL was located on chromosomes15 (Ses3) with a LOD score 3.4 (P = 1.2 x 10(-4)). The estimated effects of Ses1, Ses2 and Ses3 on the bacterial clearance were greater in females. Using a model of three loci, with interaction between Ses1 and Ses2 and sex as a covariate, the three QTLs explained 32% of the phenotypic variance. The candidacy of Nramp1 as the gene for Ses1 was evaluated using mice carrying a null allele at Nramp1 (129sv-Nramp1(tm1Mcg)). These mice have a significantly lower spleen bacterial load compared to the wild-type 129sv mice, strongly suggesting the involvement of Nramp1 in controlling S. enteritidis clearance during the late phase of infection.