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

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

<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