The study shows that exposing lung cells to the BCG vaccine triggers the cells to make more of the natural antimicrobial peptide LL‑37, and that this boost depends on specific cell signaling pathways (MEK1/2 and p38 MAPK). Blocking those pathways or stopping new RNA production stops the increase.

The antimicrobial cathelicidin LL-37 is considered to play an important role in the innate immune response to tuberculosis infection. However, little is known about the induction and secretion of this antimicrobial peptide in A549 epithelial cells after infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG), the world's most widely used tuberculosis vaccine. In this study, we investigated the effect of M. bovis BCG on LL-37 mRNA levels in A549 cells by real-time PCR and on protein levels by Western blotting. Treatment of cells with M. bovis BCG upregulates LL-37 mRNA expression in a dose- and time-dependent manner. The quantitative analysis of LL-37 gene expression correlated with our Western blotting results. Moreover, our results demonstrated that treatment of cells with the transcriptional inhibitor actinomycin D effectively inhibited in a concentration-dependent manner the ability of M. bovis BCG to induce LL-37 mRNA expression. Finally, inhibition of the MEK1/2 and p38 mitogen-activated protein kinase (MAPK) signaling pathways reduced M. bovis BCG-mediated LL-37 mRNA expression, a reduction that correlated with the observed high level of downregulation of LL-37 protein induction. Thus, these results indicate that the MEK1/2 and p38 MAPK signaling pathways play a critical role in the regulation of inducible LL-37 gene expression in A549 cells infected with M. bovis BCG.