Comparison of in vitro models for the study of Mycobacterium tuberculosis invasion and intracellular replication

We recently evaluated several tissue culture model systems for the study of invasion and intracellular multiplication of Mycobacterium tuberculosis. These model systems include a human alveolar pneumocyte epithelial cell line, a murine macrophage cell line (J774), and fresh human peripheral blood-derived macrophages. Our data indicated that the initial level of association of M. tuberculosis with human alveolar pneumocyte cells (2%) was less than that observed with fresh human peripheral blood macrophages (9%) or J774 murine macrophages (13%) within 6 h of the addition of the bacteria. M. tuberculosis replicated in association with the pneumocyte cells by more than 55-fold by day 7 postinfection. In contrast, total bacterial growth in the J774 cells and human macrophages was considerably less, with increases of only fourfold and threefold, respectively, over the same 7-day period. Amikacin, an aminoglycoside antimicrobial agent, was added to inhibit the growth of extracellular bacteria after the initial 6-h infection period. Decreases in viable counts were observed in all three cell cultures within the first 3 days after infection. However, unlike the case with either macrophage culture, intracellular bacterial CFU obtained from the infected pneumocytes increased by fourfold by day 7 after the addition of amikacin. These data indicate that M. tuberculosis infects and multiplies intracellularly in human lung epithelial cells and that these cells may be an alternative in vitro model for the study of intracellular multiplication of M. tuberculosis in the human lung.