Multigenic control of disease severity after virulent Mycobacterium tuberculosis infection in mice

Abstract

Following challenge with virulent Mycobacterium tuberculosis, mice of the I/St inbred strain exhibit shorter survival time, more rapid body weight loss, higher mycobacterial loads in organs, and more severe lung histopathology than mice of the A/Sn strain. We previously performed a genome-wide scan for quantitative trait loci (QTLs) that control the severity of M. tuberculosis-triggered disease in [(A/Sn x I/St) F1 x I/St] backcross-1 (BC1) mice and described several QTLs that are significantly or suggestively linked to body weight loss. In the present study we expanded our analysis by including the survival time phenotype and by genotyping 406 (A/Sn x I/St) F2 mice for the previously identified chromosomal regions of interest. The previously identified 12-cM-wide QTL on distal mouse chromosome 3 was designated tbs1 (tuberculosis severity 1); the location of the QTL on proximal chromosome 9 was narrowed to a 9-cM interval, and this QTL was designated tbs2. Allelic variants of the tbs2 locus appeared to be involved in control of both body weight loss and survival time. Also, the data strongly suggested that a QTL located in the vicinity of the H-2 complex on chromosome 17 is involved in control of tuberculosis in mice of both genders, whereas the tbs1 locus seemed to have an effect on postinfection body weight loss in female mice. Interestingly, these loci appeared to interact with each other, which suggests that there might be a basic genetic network for the control of intracellular parasites. Overall, linkage data reported here for F2 mice are in agreement with, and add to, our previous findings concerning the control of M. tuberculosis-triggered disease in the BC1 segregation.

FIG. 1.

QTL linkage analysis of TB severity following infection of mice with M. tuberculosis H37Rv: LOD score plots for gender-adjusted postinfection relative body weight loss and survival time from analyses of BC1 (A and D) and F2 (B, C, and E) animals. The LOD score peaks between D9Mit23 and D9M142 in panel A were not designated since they were not supported by a marker.

FIG. 2.

Segregation of mortality in (A/Sn × I/St) F2 mice following M. tuberculosis H37Rv challenge. Animals that died early after challenge formed a rather homogeneous group (indicated by bars). Defining these animals as susceptible and the rest of the animals as resistant gave the following segregation results: 95 susceptible and 81 resistant female mice and 104 susceptible and 126 resistant male mice.

FIG. 3.

Effect of the chromosome X locus on the distribution of TB severity according to genotype at the chromosome 17 locus (A) and the tbs2 locus (B). The means and standard errors for gender-adjusted postinfection relative body weight loss in (A/Sn × I/St) F2 mice are shown. The numbers above the symbols are the numbers of mice.

FIG. 4.

Interaction among the tbs1, chromosome 17, and tbs2 loci in control of TB severity in (A/Sn × I/St) F2 mice. The TB severity measurements used to determine the LOD scores and graphs were gender adjusted postinfection relative to body weight loss, and the TB severity measurements used for the MANOVA were adjusted postinfection relative to body weight loss and survival time. The means and standard errors for mice grouped on the basis of their genotypes at the three loci are shown. The numbers above the symbols are the numbers of mice. (A) LOD score for D17Mit175, including mice that were tbs1^a/a. MANOVA was performed for the gender-chromosome 17 locus-tbs1 interaction. (B) LOD score for D3Mit215, including mice that were tbs2^i/a. MANOVA was performed for the gender-tbs2-tbs1 interaction.