Different types of pulmonary granuloma necrosis in immunocompetent vs. TNFRp55-gene-deficient mice aerogenically infected with highly virulent Mycobacterium avium

The immunopathogenesis of mycobacterial infections frequently involves the formation of caseating granulomas which cause tissue destruction and, in the case of tuberculosis (TB), may lead to cavity formation. Both intravenous and aerosol models of Mycobacterium tuberculosis infection in mice do not reflect the pulmonary lesions characteristic of TB patients. Using both low- dose (10² colony-forming units, cfu) and high-dose (10⁵ cfu) aerosol infection with a highly virulent strain of Mycobacterium avium (TMC724) in C57BL/6 mice, it is now shown that these mice are capable of developing centrally caseating necrosis in lung granulomas after approximately 4 months of infection. In contrast, mice infected intravenously with the high dose never developed this type of lesion, although bacterial counts in their lungs reached levels comparable to those attained by aerosol-infected mice (10¹⁰ cfu). To study the relevance of events signalled by tumour necrosis factor (TNF) in this model, TNFRp55 gene-deficient and syngeneic C57BL/6 immunocompetent mice were infected with l0⁵ cfu M. avium via aerosol. In gene-deficient mice, newly formed pulmonary granulomas acutely disintegrated, showing signs of apoptotic cell death and neutrophil influx, and TNFRp55 knock-out mice all succumbed to infection just beyond the stage of granuloma initiation. Aerogenic infection with M. avium in mice is a suitable model to study the immunopathogenesis of granuloma necrosis because it closely mimicks the histopathology of mycobacterial infections in humans, including TB. Furthermore, the use of TNFRp55 gene-deficient mice in this model establishes a role for TNF in maintaining the integrity of a developing pulmonary granuloma.