Although Mycobacterium tuberculosis is assumed to infect primarily alveolar macrophages after being aspirated into the lung in aerosol form, it is plausible to hypothesize that M. tuberculosis can come in contact with alveolar epithelial cells upon arrival into the alveolar space. Therefore, as a first step toward investigation of the interaction between M. tuberculosis and alveolar epithelial cells, we examined the ability of M. tuberculosis to bind to and invade alveolar epithelial cells in

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... ithelial cells in vitro. The H37Rv and H37Ra strains of M. tuberculosis were cultured to mid-log phase and used in both adherence and invasion assays. The A549 human type II alveolar cell line was cultured to confluence in RPMI 1640 supplemented with 5% fetal bovine serum, L-glutamine, and nonessential amino acids. H37Rv was more efficient in entering A549 cells than H37Ra, Mycobacterium avium, and Escherichia coli HB101, a nonpiliated strain (4.7% ؎ 1.0% of the initial inoculum in 2 h compared with 3.1% ؎ 0.8%, 2.1% ؎ 0.9%, and 0.03% ؎ 0.0%, respectively). The invasion was more efficient at 37؇C than at 30؇C (4.7% ؎ 1.0% compared with 2.3% ؎ 0.8%). H37Rv and H37Ra were both capable of multiplying intracellularly at a similar ratio over 4 days. Binding was inhibited up to 55.7% by anti-CD51 antibody (antivitronectin receptor), up to 55% with anti-CD29 antibody (␤ 1 integrin), and 79% with both antibodies used together. Uptake of M. tuberculosis H37Rv was microtubule and microfilament dependent. It was inhibited by 61.4% in the presence of 10 M colchicine and by 72.3% in the presence of 3 M cytochalasin D, suggesting two separate pathways for uptake. Our results show that M. tuberculosis is capable of invading type II alveolar epithelial cells and raise the possibility that invasion of alveolar epithelial cells is associated with the pathogenesis of lung infection.