Susceptibility to tuberculosis: a locus on mouse chromosome 19 (Trl-4) regulates Mycobacterium tuberculosis replication in the lungs

Abstract

The mouse DBA/2 (D2) strain is very susceptible to infection with virulent Mycobacterium tuberculosis, whereas C57BL/6 (B6) is much more resistant. Infection of D2 and B6 mice with M. tuberculosis H37Rv by the respiratory route is biphasic: during the first 3 weeks, there is rapid bacterial growth in the lung of both strains, whereas beyond this point replication stops in B6 but continues in D2, causing rapidly fatal pulmonary disease. To identify the genes regulating growth of M. tuberculosis in the lungs of these two strains, 98 informative (B6 x D2) F2 mice were infected by the respiratory route with M. tuberculosis H37Rv (2 x 102 colony-forming units), and the extent of bacterial replication in the lungs at 90 days was used as a quantitative measure of susceptibility in a whole-genome scan. Quantitative trait locus mapping identified a major locus on chromosome 19 (Tuberculosis resistance locus-4, Trl-4; logarithm of odds 5.6), which regulated pulmonary replication of M. tuberculosis and accounted for 25% of the phenotypic variance. B6 alleles at Trl-4 were inherited in an incompletely dominant fashion and associated with reduced bacterial replication. An additional effect of a locus (Trl-3), previously shown to affect survival to i.v. infection with M. tuberculosis, was also noted. F2 mice homozygous for B6 alleles at both Trl-3 and Trl-4 were as resistant as B6 parents, whereas mice homozygous for D2 alleles were as susceptible as D2 parents. These results suggest a strong genetic interaction between Trl-3 and Trl-4 in regulating pulmonary replication of M. tuberculosis.

Fig. 1.

Replication of M. tuberculosis in the lungs of B6, D2, and (B6 × D2) F₂ mice. Resistant B6 and susceptible D2 control mice were infected via the aerosol route with 2 × 10² M. tuberculosis H37Rv. (A) The number of M. tuberculosis present in the lungs was determined (log₁₀ cfu) at preset time points, where each data point corresponds to the mean cfu counts for groups of five mice. (B) Lung cfus (log₁₀) of B6 and D2 controls and of individual (B6 × D2) F₂ mice 90 days after aerosol infection with 2 × 10² M. tuberculosis H37Rv. Horizontal bars represent means of cfus for each group.

Fig. 2.

Linkage analysis by whole-genome scan of susceptibility to infection with M. tuberculosis. lod score traces along Chr. 19 (A) and 7 (B), for which highly significant (lod >5.0) and suggestive (lod >3) QTL controlling bacterial replication of M. tuberculosis H37Rv after aerosol infection with 2 × 10² cfus in (B6 × D2) F₂ mice was detected. For comparison, the lod score plot for Chr. 7 from an independent genome scan for a significant QTL (Trl-3) that controls survival of (B6 × D2) F₂ mice after i.v. infection with 1 × 10⁵ cfus of M. tuberculosis H37Rv is shown [C reproduced with permission from ref. (copyright 2000, Nature Publishing Group)]. The map positions of microsatellite markers used are indicated, and chromosomal lengths are shown to scale.

Fig. 3.

Effect of haplotype combination at individual QTLs on M. tuberculosis replication in the lungs. The effect of parental allele combinations at Trl-4 (B, D19Mit54), Trl-3 (C, D7Mit228), as well as Trl-3 and Trl-4 in combination (D and E) on bacterial replication (log₁₀ cfu at day 90 postinfection) is shown. The D2 and B6 parental alleles are identified as d and b, respectively, and the number of animals (n) in each group is shown beneath the figure. Each data point represents a single mouse, and horizontal bars indicate mean cfus in each group. The log₁₀ cfus in the parental B6 and D2 groups are shown in A for comparison.