Lung bacillary load and histopathological characteristics during Mtb CDC1551 infection of rabbits. A: Kinetics of Mtb growth in the infected lungs of untreated and triamcinolone (immune suppression) treated rabbits. Complete clearance of cultivable bacilli is noted at 20 weeks p.i.; 4 weeks of triamcinolone treatment (arrow) resumed bacillary growth in the infected rabbit lungs. B: Kinetics of bacillary growth and clearance in the lungs of rabbits infected with a low or high dose of Mtb CDC1551. No bacterial CFUs are obtained in the rabbit lungs after 12 or 24 weeks in the low- and high-dose infections, respectively. Values are given as mean ± SD, obtained from 91 rabbits in four separate experiments, with four to five animals per time point (A and B). C–H: Histopathological characteristics of rabbit lungs infected with Mtb CDC1551. C and D: Lungs of infected, untreated rabbits at 8 weeks p.i., showing well-organized granulomas containing aggregates of mononuclear cells (arrows) surrounded by a lymphocytic cuff. E and F: Mtb CDC1551-infected rabbit lungs at 24 weeks p.i., without triamcinolone treatment. In both E and F, almost normal lung parenchyma and alveoli with moderately elevated cellularity can be seen. G and H: Histological characteristics of Mtb-infected rabbit lungs, treated with triamcinolone for 4 weeks, initiated at 20 weeks p.i. Large suppurative infiltration with many macrophages and polymorphonuclear leukocytes (arrows) that contain red granules in their cytoplasm are seen. Scale bars: 100 μm (C, E, and G); 50 μm (D, F, and H). All sections were stained with H&E and photographed. Original magnification: ×10 (C, E, and G); ×40 (D, F, and H).