INFECTION BY MYCOBACTERIUM-TUBERCULOSIS PROMOTES HUMAN ALVEOLAR MACROPHAGE APOPTOSIS

The effect of Mycobacterium tuberculosis infection on the viability of healthy (control) human alveolar macrophages was evaluated by stainin g with ethidium homodimer and calcein to discriminate live from dead c ells. Infection with M. tuberculosis H37Ra or H37Rv increased macropha ge mortality at 6 days from the control level of 3.8% +/- 0.7% to 28.7 % +/- 6.9% or 12.6% a 3.1%, respectively (P < 0.001 for comparisons of all conditions). A role for tumor necrosis factor alpha (TNF-alpha) i n the M. tuberculosis-induced cytolysis of alveolar macrophages was de monstrated by increased cytotoxicity following the addition of exogeno us TNF-alpha to the cultures and bg enhancement of macrophage survival when M. tuberculosis-infected alveolar macrophages were treated with pentoxifylline or anti-TNF-alpha antibody, The cytolytic mechanism was determined to he apoptosis by the demonstration of a characteristic i nternucleosomal ladder of genomic DNA by agarose gel electrophoresis, by finding nuclear fragmentation and condensation by electron microsco py, and by in situ terminal transferase-mediated nick end labeling of fragmented DNA in alveolar macrophages infected with M. tuberculosis i n vitro. The latter technique was employed to reveal extensive apoptos is within caseating granulomas from lung tissue samples from clinical tuberculosis cases. The induction of apoptosis in alveolar macrophages by M. tuberculosis may play a role in the macrophage-pathogen interac tion of tuberculosis in vivo.