When human lung cells are exposed to the BCG vaccine, they quickly produce reactive oxygen species (ROS). This ROS burst is needed for the cells to turn on the antimicrobial peptide LL‑37. Blocking ROS with antioxidants or NADPH‑oxidase inhibitors stops the LL‑37 increase.

The human cathelicidin LL-37 is one of the major antimicrobial peptides of the non-specific innate immune system in Mycobacterium tuberculosis infection. Its expression has been reported in epithelial cells infected with mycobacteria. However, the underlying molecular mechanisms by which Mycobacterium bovis bacillus Calmette-Guérin (BCG) triggers gene transcription of cathelicidin have not been elucidated. The objective of this study was to investigate the role of reactive oxygen species (ROS) in the M. bovis BCG-mediated up-regulation of the antimicrobial peptide cathelicidin LL-37 in human epithelial cells. Infection of A549 cells with M. bovis BCG led to a rapid ROS production. Importantly, blockade of ROS by preincubation of cells with the general ROS scavenger N-acetyl-l-cysteine (NAC) or the NADPH oxidase inhibitor DPI significantly reduced M. bovis BCG-induced up-regulation of cathelicidin LL-37 mRNA expression as determined by semi-quantitative RT-PCR or real-time PCR. In contrast, the xanthine oxidase inhibitor allopurinol did not affect M. bovis BCG-mediated up-regulation of cathelicidin LL-37 mRNA. Moreover, M. bovis BCG-mediated cathelicidin LL-37 mRNA expression was significantly blocked by the effect of the mitochondrial electron transfer chain subunit I inhibitor rotenone and H(2)O(2) scavenging enzyme catalase. In addition, M. bovis BCG-induced cathelicidin LL-37 protein secretion was inhibited by the addition of NAC, DPI, and the selective inhibitor of NADPH oxidase apocynin. Our results collectively indicate that M. bovis BCG-mediated up-regulation of cathelicidin is influenced by NADPH/ROS signaling pathways. In conclusion, these findings demonstrate a novel regulatory mechanism for the expression of cathelicidin LL-37 in human epithelial cells stimulated with M. bovis BCG.