Host Genetics in Granuloma Formation: Human-Like Lung Pathology in Mice with Reciprocal Genetic Susceptibility to M. tuberculosis and M. avium

Development of lung granulomata is a hallmark of infections caused by virulent mycobacteria, reflecting both protective host response that restricts infection spreading and inflammatory pathology. The role of host genetics in granuloma formation is not well defined. Earlier we have shown that mice of the I/St strain are extremely susceptible to Mycobacterium tuberculosis but resistant to M. avium infection, whereas B6 mice show a reversed pattern of susceptibility. Here, by directly comparing: (i) characteristics of susceptibility to two infections in vivo; (ii) architecture of lung granulomata assessed by immune staining; and (iii) expression of genes encoding regulatory factors of neutrophil influx in the lung tissue, we demonstrate that genetic susceptibility of the host largely determines the pattern of lung pathology. Necrotizing granuloma surrounded by hypoxic zones, as well as a massive neutrophil influx, develop in the lungs of M. avium-infected B6 mice and in the lungs of M. tuberculosis-infected I/St mice, but not in the lungs of corresponding genetically resistant counterparts. The mirror-type lung tissue responses to two virulent mycobacteria indicate that the level of genetic susceptibility of the host to a given mycobacterial species largely determines characteristics of pathology, and directly demonstrate the importance of host genetics in pathogenesis

The lung tissue surrounding necrotizing granuloma centers in mice susceptible to <i>M. tuberculosis</i> and <i>M. avium</i> is markedly hypoxic.

<p>I/St mice 6 wk after <i>M. tuberculosis</i> challenge (A) and B6 mice 16 wk after <i>M. avium</i> challenge (B) were injected with 60 mg/kg body weight of Hypoxyprobe™-1 and sacrificed 3 h later. Lung cryosections were obtained and developed for indirect peroxidase staining to detect hypoxia gradients (×200).</p

The picture of leukocyte infiltration of the lung tissue of I/St (left) and B6 (right) mice infected with 2×10³ CFU of <i>M. avium</i> via aerosol route 16 weeks earlier.

<p>See legend to <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0010515#pone-0010515-g002" target="_blank">Fig. 2</a>.</p

The picture of leukocyte infiltration of the lung tissue of I/St (left) and B6 (right) mice infected with 2×10³ CFU of <i>M. avium</i> via aerosol route 8 weeks earlier.

<p>Peroxidase immune staining with hematoxylin counter-staining (×150). Cell populations are indicated on the left side. See text for the description.</p

<i>M. avium</i> infection does not render active hypoxia in the lungs of resistant mice, but induces numerous necrotizing granuloma surrounded by hypoxic zones in the lungs of susceptible mice.

<p>(A)  =  I/St; (B)  =  B6. Hypoxiprobe peroxidase staining (×25).</p

B6 mice are more resistant to <i>M. tuberculosis</i> infection compared to I/St mice.

<p>Their survival time (A, <i>P</i><0.001, Gohen's criterion for survival curves) is longer and lung CFU counts (B, <i>P</i><0.01-0.001 at different time points, ANOVA) are lower. Lung macrophages of I/St, but not of B6, mice inhibit multiplication of <i>M. avium</i> after in vitro infection within a high range of MOI (C). The rate of mycobacterial growth was measured by [³H]-uracil uptake at 72 h after establishing co-cultures. 1 µCi/well [³H]-uracil was added for the last 18 h of incubation. The wells containing mycobacteria alone at numbers corresponding to each MOI served as controls. Results obtained in one of three similar experiments are expressed as mean CPMs ± SD for triplicate cultures; interstrain differences are statistically significant (<i>P</i><0.01, Mann-Whitney's U-test).</p