Apoptosis in Mycobacterium tuberculosis infection in mice exhibiting varied immunopathology

This study examined mechanisms contributing to pulmonary immunopathology following acute Mycobacterium tuberculosis (MTB) infection in vivo in a murine model. A/J and C57BL/6 mice were intravenously infected with MTB (Erdman). Pathological differences were found between strains, unrelated to pulmonary load of bacilli. A/J mice developed progressive interstitial pneumonitis, while C57BL/6 mice maintained granuloma formation. The contribution of FAS and FAS ligand‐mediated apoptosis was assessed via bioluminescent reverse transcription‐polymerase chain reaction (RT‐PCR), immunohistochemical staining, and TUNEL assessment of DNA fragmentation. Cytokine messages for pulmonary tumour necrosis factor‐α (TNF‐α) and interferon‐γ (IFN‐γ), as well as for the lytic molecules perforin and granzyme B, were quantified. Immunohistochemical staining for CD3 receptor was performed to monitor lymphocytic lung infiltration. Soon after infection, A/J mice exhibited increased pulmonary IFN‐γ message, concurrent with the appearance of CD3+ lymphocytes distributed throughout the lung. C57BL/6 mice exhibited perivascular cuffing, with no accompanying increase in IFN‐γ message. A/J mice also had elevated levels of FAS and FAS ligand message and protein early after infection, while the C57BL/6 mice had no increased expression of these molecules. Both strains exhibited qualitatively similar numbers of TUNEL‐positive cells throughout infection, with a marked increase on day 7. Apoptotic cells appeared to co‐localize with acid fast bacilli. It is therefore proposed that apoptosis during initial granuloma formation following MTB infection may occur through a FAS/FAS ligand‐independent pathway. Moreover, a failure of completion of the FAS/FAS ligand‐mediated apoptosis pathway in the A/J mice may contribute to inefficient elimination of lymphocytes, thus further aggravating pulmonary pathology. Copyright © 2000 John Wiley & Sons, Ltd.