Pathogenesis and Animal Models of Post-Primary (Bronchogenic) Tuberculosis, A Review

Abstract

Primary and post-primary tuberculosis (TB) are different diseases caused by the same organism. Primary TB produces systemic immunity. Post-primary TB produces cavities to support massive proliferation of organisms for transmission of infection to new hosts from a person with sufficient immunity to prevent systemic infection. Post-primary, also known as bronchogenic, TB begins in humans as asymptomatic bronchial spread of obstructive lobular pneumonia, not as expanding granulomas. Most lesions regress spontaneously. However, some undergo caseation necrosis that is coughed out through the necrotic bronchi to form cavities. Caseous pneumonia that is not expelled through the bronchi is retained to become the focus of fibrocaseous disease. No animal reproduces this entire process. However, it appears that many mammals utilize similar mechanisms, but fail to coordinate them as do humans. Understanding this makes it possible to use human tuberculous lung sections to guide manipulation of animals to produce models of particular human lesions. For example, slowly progressive and reactivation TB in mice resemble developing human bronchogenic TB. Similarly, bronchogenic TB and cavities resembling those in humans can be induced by bronchial infection of sensitized rabbits. Granulomas in guinea pigs have characteristics of both primary and post primary TB. Mice can be induced to produce a spectrum of human like caseating granulomas. There is evidence that primates can develop bronchogenic TB. We are optimistic that such models developed by coordinated study of human and animal tissues can be used with modern technologies to finally address long-standing questions about host/parasite relationships in TB, and support development of targeted therapeutics and vaccines.

Keywords: animal model; bronchogenic; human; mouse; pathogenesis; pathology; post-primary; primate; rabbit; tuberculosis.

Figure 1

Tuberculosis as a three-act play: A new paradigm for the pathogenesis of pulmonary tuberculosis. The key new component is that post-primary TB begins as a prolonged bronchogenic spread of asymptomatic subclinical infection in alveolar macrophages. Most lesions regress spontaneously. After 1–2 years, a few undergo sudden caseation necrosis to produce cavities and post-primary fibrocaseous disease [22]. Granulomas of post-primary TB are different from those of primary TB in that they contain ghosts of alveolar walls, that are the remnants of caseous pneumonia.

Figure 2

Bronchial obstruction and intracellular accumulation of MTB antigens in developing post-primary TB. The tree-in-bud sign on high resolution CT scan (A) is characteristic of developing post-primary TB. It is formed by obstructed bronchioles (tree) and alveoli filled with foamy macrophages (buds). Bronchial obstruction (arrow in B) is found in 100% of cases of TB (H&E, 40 magnification). Immunohistochemistry (IHC) stain with rabbit polyclonal antibody against whole killed MTB (C,D) shows MTB antigens to be intracellular in alveolar macrophages in viable lung tissue. MTB are seldom found in such lesions by either IHC or AFB staining suggesting that very few MTB secrete antigens that are stored for a prolonged period in these cells prior to induction of caseous pneumonia. (C 700×, D 100× magnification).

Figure 3

Multiple stages of post-primary TB in a single rabbit lung. Sensitized rabbits infected with MTB via bronchoscopy develop multiple lesions that resemble various stages of the human disease. The earliest stage (A) is collection of alveolar macrophages in focal areas of alveoli. Next, (B) the alveolar walls become thickened with lymphocytes and the macrophages become variably foamy. Bronchial obstruction (B insert) is associated with these lesions. Larger masses of the pneumonia undergo caseation necrosis and soften to produce cavities (C). The caseation necrosis may expand in some areas (C arrow). A higher magnification of this area (D) shows developing caseous pneumonia adjacent to viable alveoli containing macrophages. (all H&E A 400×, B 200×, C 40× and D 400×).

Figure 4

Caseating Granulomas in Humans and Mice (35). C57Bl/6 mice produce classic pulmonary caseating granulomas following injection of a high dose of MTB in oil into the lungs of a sensitized animals (LEFT). Human caseating granuloma for comparison (MIDDLE). Mice injected i.v. with MTB one day after i.p injection of TDM in oil produce caseating granulomas with a fibrous capsule in the peritoneal cavity. (RIGHT, Trichrome connective tissue stain). This lesion contained many MTB after most had been eliminated from other parts of the body. Rupture of the capsule of this type of lesion was associated with reactivation TB in the lungs (Photos at 20× Mag).

Figure 5

Comparison of early post-primary TB in humans with slowly progressive TB in mice [32]. Both lesions are obstructive lobular pneumonias characterized by foamy alveolar macrophages with very few AFB (A,B) (H&E stain 100×) and bronchial obstruction (C,D) (32) (H&E stain A, B, C 100×, D 200×).

Figure 6

Reactivation Tuberculosis in the Mouse at 200 days. The Cornell model is produced by inducing latent TB in mice by treatment with antibiotics. The disease then reactivates months later as typical bronchogenic TB. It is a tuberculous lipid pneumonia quite different from the granulomas of primary murine TB. Like human post primary TB, it typically begins as sub pleural wedge shaped pneumonic lesions. Bronchial obstructions are frequently found at the apex of the wedge (arrow and insert). Acid fast bacilli are present in low numbers in alveolar macrophages. No granulomas are present in these animals. The dark purple areas are primarily interstitial lymphocytes while the pale areas they surround are alveoli filled with foamy macrophages. (H&E 100×).

Figure 7

MTB antigen in a Macaque lung at 4 months after infection. MTB antigen (brown IHC stain) is present alveolar macrophages just outside of a granuloma. Earlier time points showed MTB antigen only in granulomas. This pattern of MTB antigen in alveolar macrophages outside of granulomas is typical of the earliest stage of post-primary TB in humans and mice. (IHC stain for MTB at 400× left and 40× right).