Pulmonary Epithelial Cells are a Source of IL-8 in the Response to Mycobacterium tuberculosis: Essential Role of IL-1 from Infected Monocytes in a NF-κB-Dependent Network

Abstract

Abstract Pulmonary epithelial cells, covering a 70-m2 surface area, have not previously been considered an important source of chemokines in pulmonary tuberculosis. We analyzed IL-8 secretion from A549 cells and primary normal human bronchial epithelial cells (NHBE) infected by Mycobacterium tuberculosis. Direct infection of A549 cells by M. tuberculosis caused IL-8 secretion of 7720 ± 1610 pg/106 cells, but no IL-8 secretion from NHBE after 24 h. In contrast, conditioned media from M. tuberculosis-infected human monocytes (CoMTB) induced a much greater IL-8 secretion of 92,635 ± 13,180 pg/106 A549 cells and 13,416 ± 3,529 pg/106 NHBE after 24 h. CoMTB induced rapid IL-8 mRNA accumulation, which was stable over 24 h, compared with TNF-α-induced transcripts. CoMTB stimulated nuclear binding of p65, p50, and c-Rel subunits of NF-κB to IL-8 promoter sequences. Transient transfections with IL-8 promoter reporter constructs showed NF-κB binding-site mutations abolished IL-8 promoter activity while NF-IL-6 binding-site mutations decreased promoter activity to 50.2 ± 6.3% of wild-type activity. IL-1R antagonist but not neutralizing anti-TNF-α inhibited epithelial cell IL-8 secretion, mRNA accumulation, and NF-κB binding. Recombinant IL-1β (2 ng/ml) induced similar levels of IL-8 secretion to CoMTB in both A549 cells and NHBE. Pulmonary epithelial cells are a major source of IL-8 in the initial host response to pulmonary tuberculosis. Such IL-8 secretion is NF-κB dependent, NF-IL-6 requiring, and activated by an IL-1-mediated pathway as a consequence of phagocytosis of M. tuberculosis by monocytes.