Kartierung und Identifikation von Genen in der Maus, deren verschiedenen Allele die Resistenz gegen eine akute systematische Chlamydia Infektion beeinflussen
Final Report Abstract
Chlamydia trachomatis is an obligate intracellular bacterial pathogen that is the cause of a wide spectrum of human diseases. The host response to infections with Chlamydia plays an important role in the pathogenesis of the disease, since pathological outcomes of C. trachomatis infection are intimately linked to the vigor and quality of the host immune response. To better understand the interplay between this prevalent human pathogen and its host, we aimed to identify host factors involved in resistance to Chlamydia infections. To accomplish this goal, we adapted a murine model of systemic infection for use in genetic analysis. In this model, C. trachomatis colonization and replication is measured in the spleens of mice shortly after intravenous delivery of C. trachomatis. We were able to show that C57BL/6J (B6) and C3H/HeJ (C3H) inbred mice are differentially susceptible to systemic infection in vivo and took advantage of this natural variation to map quantitative trait loci (QTL) on Chromosomes 2, 3, and 11 that segregate with the bacterial load in F2 cross progeny during the acute phase of C. trachomatis infection . We have named the Chromosome 2, 3 and 11 loci, respectively: Ctrq-1, Ctrq-2, and Ctrq-3 for C. trachomatis resistance QTL. We used a fine structure mapping approach to localize the effect of the Ctrq-3 locus on chromosome 11 to a 1.4 megabase interval of genomic DNA that contains IrgblO and IrgmS/Igtp, two members of a large family of immunity related GTPases (IRGs). The IRG protein family has been widely implicated in resistance to protozoan and intracellular bacterial pathogens in mouse and recently also in humans. Expression analysis in resistant (B6) and susceptible (C3H) mouse embryonic fibroblasts (MEFs) treated with IFNy demonstrated that expression of IrgblO (but not Irgm3/Igtp) differs between resistant and susceptible MEFs, with relatively resistant MEFs expressing more IrgblO than relatively susceptible MEFs. However, eliminating or decreasing expression of either IrgblO or Irgm3/Igtp resulted in increased susceptibility to C. trachomatis infection in vitro. This led us to conclude that while a diminished IrgblO expression in C3H mice is likely to be responsible for the effect of Ctrq-3 on immunity to C, trachomatis, both genes play a role in resistance to C, trachomatis. At least 23 IRG genes are found in the mouse genome. In contrast, humans possess only two IRG genes: IRGC, which is exclusively expressed in the male gonad similar to its mouse ortholog and IRGM, which is a truncated, non-inducible ortholog of the mouse Irgm subclass. Surprisingly, human IRGM in spite of its truncation and lack of IFN mediated induction has been implicated in cell-autonomous resistance to Mycobacteria and shown to play a role in the induction of autophagy. However, the relative subtlety of the observed phenotype of IRGM depleted human cells compared to the dramatic phenotypes of the IRG knockout mice suggests that IRG mediated immunity is of far less importance in humans than it is in mice. It has previously been reported that the mouse-adapted pathogen Chlamydia muridarum (in contrast to the closely related species C. trachomatis) can evade IFNy activated cell-autonomous restriction of bacterial growth in mouse cells but not in human cells. I therefore tested the hypothesis that C. muridarum can specifically evade IRG mediated host resistance. I demonstrated that ectopic expression of IrgblO in the absence of a general IFNy response is sufficient to reduce bacterial yield of C. trachomatis but not C. muridarum. The differential effect of Irgb10 expression on Chlamydia growth correlated with the distinct subcellular localization of Irgb10 in cells infected with C. trachomatis as compared to those infected with C. muridarum. Specifically, I was able to show that Irgbl10 protein decorates an inclusion formed by C. trachomatis, whereas Irgb10 is absent from C. muridarum inclusions. Therefore, I have suggested a model in which Irgb10 is required to localize to the inclusion in order to limit bacterial replication, and have proposed that C. muridarum has evolved a mechanism to restrict access of IRG proteins to its inclusion.
Publications
- (2006) Genetic analysis of susceptibility to Chlamydia trachomatis in mouse. Genes Immun. 7: 122-129
Bernstein-Hanley, I., Balsara, Z.R., Ulmer, W., Coers, J., Starnbach, M.N. and Dietrich, W.F.
- (2006) The p47 GTPases Igtp and Irgb10 map to the Chlamydia trachomatis susceptibility locus Ctrq-3 and mediate cellular resistance in mice. Proc Natl Acad Sci USA. 103: 14092-14097
Bernstein-Hanley, I., Coers, J., Balsara, Z.R., Taylor, G.A., Starnbach, M.N. and Dietrich, W.F.
- (2007) Restriction of Legionella pneumophila growth in macrophages requires the concerted action of cytokine and Naip5/Ipaf signalling pathways. Cell Microbiol. 9: 2344-2357
Coers, J., Vance, R.E., Fontana, M.F. and Dietrich, W.F.
