Interleukin-1beta responses to Mycoplasma pneumoniae infection are cell-type specific

Interleukin-1beta (IL-1beta) is a major proinflammatory cytokine that is involved in many important cellular functions such as proliferation, differentiation, and activation of different cell types. Its mature form is released from the cells in response to various bacterial and viral infections, and it plays a significant role in host defense. Mycoplasma pneumoniae is a small bacterium without a cell wall that causes tracheobronchitis and atypical pneumonia in humans following attachment to respiratory epithelium, as well as extrapulmonary infections. Very little is known about the role of cytokines in pathogenesis or the response of target cells to M.pneumoniae attachment. The purpose of this study was to investigate the ability of M. pneumoniae to induce IL-1beta in human lung epithelial carcinoma A549 and in human monocytic U937 cell lines. Following M. pneumoniae infection, both IL-1beta mRNA and protein were induced in A549 cells vs. no induction in uninfected cells; however, the protein remained inside the A549 cells. Similarly, M. pneumoniae infection strongly increased mRNA and extracellular protein levels in U937 cells, which unlike A549 cells did exhibit baseline constitutive levels. De novo IL-1beta protein expression was verified by cycloheximide studies. M. pneumoniae infection did not affect constitutive caspase-1 mRNA or protein levels in either cell line. Reduced caspase-1 activity in A549 cell lysates suggests the presence of an endogenous caspase-1 inhibitory component in the A549 cells. These collective data confirm previous studies that show that M. pneumoniae is a potent inducer of cytokines following adherence to host target cells, and establish that IL-1beta release in response to M. pneumoniae infection is cell-type specific, thus emphasizing the importance of carefully considering multiple cell types in M. pneumoniae pathogenesis studies involving both immune cells and cytokine release patterns.