Mycobacteria encounter many different cells during infection within their hosts. Although alveolar epithelial cells play an essential role in host defense as the first cells to be challenged upon contact with mycobacteria, they may contribute to the acquisition of mycobacterial virulence by increasing the expression of virulence or adaptation factors prior to being ingested by macrophages on the side of pathogens. From this aspect, the enhanced virulence of nonpathogenic Mycobacterium smegmatis (MSM) passed through human alveolar A549 epithelial cells (A-MSM) was compared to the direct infection of MSM (D-MSM) in THP-1 macrophages and mouse models. The intracellular growth rate and cytotoxicity of A-MSM were significantly increased in THP-1 macrophages. In addition, compared to D-MSM, A-MSM induced relatively greater interleukin (IL)-1β, IL-6, IL-8, IL-12, TNF-α, MIP-1α, and MCP-1 in THP-1 macrophages. As a next step, a more persistent A-MSM infection was observed in a murine infection model with the development of granulomatous inflammation. Finally, 58 genes induced specifically in A-MSM were partially identified by differential expression using a customized amplification library. These gene expressions were simultaneously maintained in THP-1 infection but no changes were observed in D-MSM. Bioinformatic analysis revealed that these genes are involved mainly in bacterial metabolism including energy production and conversion, carbohydrate, amino acid, and lipid transport, and metabolisms. Conclusively, alveolar epithelial cells promoted the conversion of MSM to the virulent phenotype prior to encountering macrophages by activating the genes required for intracellular survival and presenting its pathogenicity.
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This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (2010-0008728) and Basic Science Research Program through the NRF (R13-2007-020-00000-0).
Su-Young Kim, Hosung Sohn, and Go-Eun Choi contributed equally to the work.
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Supplementary Fig. 1 Spot density of dot blot hybridization of PCR probes with the Mycobacterium smegmatis library. Dot blots were hybridized with radio-labeled PCR probes under various conditions. Lane 1, M. smegmatis control; lane 2, M. smegmatis from THP-1; lane 3, M. smegmatis from A549; lane 4, M. smegmatis by passing A549 in THP-1 infection. The original results of dot blot hybridization are shown in the upper panel (a). The dot blot hybridization results were analyzed using ArrayGauge software. Spot density was indicated numerically and by color. The highest and lowest levels of expression are represented by red and blue, respectively (b). (PPT 869 kb)
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Kim, S., Sohn, H., Choi, G. et al. Conversion of Mycobacterium smegmatis to a pathogenic phenotype via passage of epithelial cells during macrophage infection. Med Microbiol Immunol 200, 177–191 (2011). https://doi.org/10.1007/s00430-011-0190-5
- Mycobacterium smegmatis
- Enhanced virulence
- Epithelial cell
- Induced gene