Live Legionella pneumophila induces MUC5AC production by airway epithelial cells independently of intracellular invasion.

The airway epithelium is the initial barrier against airborne pathogens, and it plays many roles in host airway defense. Legionella pneumophila is an intracellular pathogen that causes rapidly advancing pneumonia and is sometimes life-threatening. Here, we evaluated the role of the airway epithelial cells in the defense against L. pneumophila by examining mucus production in vitro. The production of MUC5AC, a major mucin protein, was not induced by formalin- or ultraviolet-killed L. pneumophila, but it was induced by live L. pneumophila. Similarly, nuclear factor-kappaB (NF-κB) was activated only by live L. pneumophila. Inhibitors of ERK and JNK, but not p38, dose-dependently inhibited the induction of MUC5AC by live L. pneumophila. Inhibition of intracellular invasion by cytochalasin D did not affect MUC5AC production. Taken together, the results suggest that live L. pneumophila induces MUC5AC production via the ERK-JNK and NF-κB pathways without internalization of bacteria and that the airway epithelium produces mucin as part of the immune response against L. pneumophila.