The major goal of the research program in my laboratory is to identify and characterize genes involved in the host immune response to pathogenic Salmonella using mouse models of the disease. We are using different models of infection (typhoid-like, chronic carriage and typhlocolitis) and genetic approaches (positional cloning of Mendelian and complex phenotypes and ENU chemical mutagenesis) to study the complex mechanisms underlying the host response to Salmonella infection. Salmonella infections in humans cause diseases (typhoid fever, salmonellosis and invasive non-typhoidal salmonellosis) that are an increasingly important public health issue both in developed and developing countries. We have identified four major Salmonella susceptibility genes (Nramp1, Tlr4, Pklr and Usp18) and defined the genetic architecture of the host response to Salmonella infection in models of acute and chronic infections.
Salmonella are Gram-negative bacteria causing two important syndromes in humans, enteric fever (typhoid), caused by Salmonella Typhi and Salmonella Paratyphi, and a range of clinical diseases, including a self-limiting gastroenteritis (salmonellosis) caused by a variety of non-typhoidal Salmonella serovars (NTS). Food and waterborne diseases caused by Salmonella are important public health issue, both in developed and developing countries. Enteric fever represents a major problem in many developing countries with 28 million cases reported annually worldwide and 200,000 associated deaths. In Canada and USA, salmonellosis is one of the most common and widely distributed foodborne diseases and is considered emerging because the incidence and severity of cases have increased recently.
Mouse models of Salmonella infection:
The use of mouse models has helped significantly to improve our understanding of the pathogenesis of Salmonella infection and has allowed the identification of several key genes and loci influencing the outcome of Salmonella infection. There are several mouse models of Salmonella infection. The most widely used model is one of systemic disease caused by Salmonella Typhimurium resembling typhoid fever for which the outcome of infection relies on the activation of both the innate and adaptive immune responses of the host. Different strains of mice show different degree of susceptibility to infection with Salmonella Typhimurium and have been used to study the impact of the innate immune response on the infection. Chronic models of infection have been developed to study Salmonella pathophysiology later during the course of infection. In these models, the mice do not succumb to the infection and carry the bacteria for prolonged period of time in the reticuloendothelial system and mesenteric lymph nodes. The chronic models have been developed using Salmonella Typhimurium of attenuated virulence in susceptible mice, sublethal infection with Salmonella Typhimurium in resistant mice or sublethal infection with Salmonella Enteritidis. Finally, a mouse model for Salmonella Typhimurium-induced typhlocolitis has been developed more recently and uses mice pretreated with a single dose of streptomycin. The antibiotic disrupts the normal intestinal flora and Salmonella Typhimurium efficiently colonizes the large intestine and triggers a severe acute diffuse inflammation of the ceacum and colon. These mice eventually develop a systemic disease in parallel to the typhlocolitis.
Genetic basis of susceptibility to Salmonella infection:
Several key studies, by us and others led to the discovery of major Salmonella susceptibility genes in mice (1).
The typhoid model has been used successfully to study the involvement of known genes (NADPH oxidase, NO, IFN-g, etc.) in the early host response to infection with Salmonella.
On the other hand, the study of the natural variation of the host response to infection in spontaneous mouse mutants have identified novel innate immune genes and pathways having Mendelian contribution to disease susceptibility including Nramp1G169D in C57BL/6J and BALB/cJ, Tlr4P712H in C3H/HeJ, or Usp18L361P in Ity9ENU mice (2-5).
Nramp1 is an excellent example of innate immune gene identified in mouse mutants (Bcg) that has been evaluated as candidate genes in related diseases in humans and animals.
Nramp1 has been shown in multiple studies to be of prime importance in the outcome of infection with intracellular pathogens including Salmonella Typhimurium in mice. A role for NRAMP1 in host defence against other intracellular pathogens (M. tuberculosis and M. leprae) was demonstrated in humans (6) and against salmonellosis and chronic Salmonella carriage in mouse and chicken (7-9).
Additional work in our laboratory has shown that the phenotypic diversity of the host response to Salmonella Typhimurium observed among different inbred mouse strains was also due to the presence of specific mutations that are inherited as complex patterns.
These studies identified 7 loci affecting the host response to the typhoid model of infection (Ity2 to Ity8) (10-11) and 10 loci (Ses1 to Ses10) linked to bacterial persistence (12-13).
Strong candidate genes have been identified for some of these loci: Nramp1 for Ses1 (14); Ncf2 for Ity3 (15) and Pklr for Ity4 (16).
1. Vidal, SM, Malo D, Marquis J-F and Gros P. 2008. Forward genetic dissection of immunity to infection in the mouse. Ann Review Immunol 26: 81-132.
2. Vidal SM, Malo D, Vogan K, Skamene E, Gros P. 1993. Natural resistance to infection with intracellular parasites: isolation of a candidate gene for Bcg. Cell 73: 469-85.
3. Vidal S., Tremblay M., Govoni G., Gauthier S., *Sebastiani G., Malo D., Skamene E., Olivier M., Jotly S., Gros P. The Ity/Lsh/Bcg locus: natural resistance to infection with intracellular parasites is abrogated by disruption of the Nramp1 gene. J. Exp. Med.182, 655-666. 1995.
4. Qureshi S.T., Larivière L., Leveque G., Clermont S., Moore K., Gros P., Malo D. Endotoxin-tolerant mice have mutations in Toll-like receptor 4. J. Exp. Med. 189:615-625; 189:1519-1520. 1999.
5. Richer E., Prendergast C., Boivin G., Zhang D.E., Qureshi S.T., Vidal S.M., and Malo D. 2009. Novel ENU-induced mutation in Usp18 increases susceptibility to Salmonella Typhimurium: important role of IFNa/b signaling.
6. Malik, S., L. Abel, H. Tooker, A. Poon, L. Simkin, M. Girard, G.J. Adams, J.R. Starke, K.C. Smith, E.A. Graviss, J.M. Musser, and E. Schurr. 2005. Alleles of the NRAMP1 gene are risk factors for pediatric tuberculosis disease. Proc Natl Acad Sci U S A 102:12183-12188.
7. Beaumont, C., J. Protais, F. Pitel, G. Leveque, D. Malo, F. Lantier, F. Plisson-Petit, P. Colin, M. Protais, P. Le Roy, J.M. Elsen, D. Milan, I. Lantier, A. Neau, G. Salvat, and A. Vignal. 2003. Effect of two candidate genes on the Salmonella carrier state in fowl. Poult Sci 82:721-726.
8. Hu, J., N. Bumstead, P. Barrow, G. Sebastiani, L. Olien, K. Morgan, and D. Malo. 1997. Resistance to salmonellosis in the chicken is linked to NRAMP1 and TNC. Genome Res 7:693-704.
9. Leveque, G., V. Forgetta, S. Morroll, A.L. Smith, N. Bumstead, P. Barrow, J.C. Loredo-Osti, K. Morgan, and D. Malo. 2003. Allelic variation in TLR4 is linked to susceptibility to Salmonella enterica serovar Typhimurium infection in chickens. Infect Immun 71:1116-1124.
10. Sebastiani, G., L. Olien, S. Gauthier, E. Skamene, K. Morgan, P. Gros, and D. Malo. 1998. Mapping of genetic modulators of natural resistance to infection with Salmonella typhimurium in wild-derived mice. Genomics 47:180-186.
10. Sancho-Shimizu, V., R. Khan, S. Mostowy, L. Lariviere, R. Wilkinson, N. Riendeau, M. Behr, and D. Malo. 2007. 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. Genetics 177:1125-1139.
11. Roy M-F, Riendeau N, Loredo-Osti JC, Malo D. 2006. Complexity in the host response to Salmonella Typhimurium infection in AcB and BcA recombinant congenic strains. Genes Immun 7:655-666.
12. Caron J, Loredo-Osti JC, Laroche L, Skamene E. Morgan K, Malo D. 2002. Identification of genetic loci controlling bacterial clearance in experimental Salmonella enteritidis infection: an unexpected role of Nramp1 in the persistence of infection in mice Genes Immun. 3:196-204.
13. Caron, J., J.C. Loredo-Osti, K. Morgan, and D. Malo. 2005. Mapping of interactions and mouse congenic strains identified novel epistatic QTLs controlling the persistence of Salmonella Enteritidis in mice. Genes Immun 6:500-508.
14. Caron, J., Larivière, L., Tam, M.F., Stevenson, M.M., McKerly, C., Gros, P., Malo, D. 2006. The influence of Slc11a1 on the outcome of Salmonella Enteritidis infection in mice is associated with TH polarization. Infect Immun 74:2787-2802.
15. Sancho-Shimizu, V., and D. Malo. 2006. Sequencing, expression, and functional analyses support the candidacy of Ncf2 in susceptibility to Salmonella typhimurium infection in wild-derived mice. J Immunol 176:6954-6961.
16. Roy, M.F., N. Riendeau, C. Bédard, P. Hélie, G. Min-Oo, K. Turcotte, P. Gros, F. Canonne-Hergaux, and D. Malo. 2007. Pyruvate kinase deficiency confers susceptibility to Salmonella typhimurium infection in mice. J Exp Med 204:2949-2961.