Role of lipopolysaccharide in pulmonary inflammation and fibrosis : an in vitro study

Abstract

The contribution of airway epithelial cells to the innate immune response in the lung has been a focus of recent interest. A human mixed co-culture system of lung epithelial cell line A549 and primary peripheral blood mononuclear cells (PBMC) allowed us to study modulation of IL-1b, TNF-a, IL-6 and IL-8 expression triggered by lipopolysaccharide (LPS), staphylococcal enterotoxin B (SEB) and peptidoglycan (PGN) in comparison to respective monocultures. All immune stimuli induced significantly less TNF-a and much higher IL-6 release in co-cultures compared to PBMC alone. IL-1b release was stimulus-dependent: down-regulated upon LPS, up-regulated upon PGN and not affected upon SEB stimulation. IL-8 amounts were significantly increased in LPS and PGN-stimulated co-cultures and slightly increased upon SEB stimulation. Transwell experiments showed that LPS-induced cytokine modulation in co-culture is triggered by soluble factors. Using neutralizing IL-1b antibody we demonstrated that PBMC-derived IL-1b mediates augmented IL-6 and IL-8 production in LPS-stimulated co-cultures. A still unknown soluble factor in conditioned supernatant of resting A549 cells was shown to down-regulate TNF-a release by PBMC on mRNA level. Characterization of this anti-inflammatory compound suggests a peptide nature and opens an interesting area of future research. Here we could show that lung epithelial cells modulate the inflammatory response by down-regulating TNF-a and increasing IL-6 and IL-8 in co-cultures and must therefore be considered actors in shaping lung inflammation.Recent investigations postulate that lung inflammation due to environmental pollutants, including lipopolysaccharide (LPS) plays an important role in the development and progression of different chronic lung diseases, including idiopathic pulmonary fibrosis. Advances in understanding the molecular mechanisms of this disease point to epithelial to mesenchymal transition (EMT) as a link between inflammation and fibrosis, but so far the role of LPS in the process of EMT has not been investigated. Therefore, we hypothesized that conditioned supernatant from LPS stimulated peripheral blood mononuclear cells (PBMC) induces an inflammatory cascade and subsequent EMT in lung epithelial type II A549 cell line. EMT in epithelial cells was evidenced after 3 days by alteration in cell morphology assessed by phase contrast microscopy and analysis of Western blot marker expression. A549 cells alone stimulated with LPS did not undergo EMT. Conditioned supernatant from LPS stimulated PBMC activated A549 cells to undergo EMT, evidenced by acquisition of mesenchymal morphology, down-regulation of epithelial cytokeratin and up-regulation of mesenchymal vimentin expression. Direct stimulation with IL-1b and TNF-a, but not IL-6, IL-8, MCP-1, MIP-1a and IL-10, shown to be produced from PBMC upon LPS-stimulation, induced fibroblast-like morphology, decreased cytokeratin and increased vimentin expression in A549 cells. Using neutralizing antibodies for IL-1b and TNF-a added to conditioned supernatants from LPS-stimulated PBMC we showed that IL-1b and TNF-a cytokines are responsible for the observed EMT effect. Thus, our findings indicate a central role of IL-1b and TNF-a in the in vitro LPS-driven EMT process in A549 cells.