PKCa and PKCe differentially regulate Legionella pneumophila-induced GM-CSF

Legionella pneumophila is an important causative agent of severe pneumonia in humans. The human alveolar epithelium is an effective barrier for inhaled microorganisms and actively participates in the initiation of innate host defense. Although secretion of granulocytemacrophage colony-stimulating factor (GM-CSF) is essential for the elimination of invading Legionella spp., mechanisms of Legionella pneumophila-induced release of this cytokine are widely unknown. In this study, we have demonstrated a toll-like receptor (TLR)2and TLR5-dependent release of GM-CSF in L. pneumophila-infected human alveolar epithelial cells. GM-CSF secretion was not dependent on the bacteria type II or type IV secretion system. Furthermore, an increase in protein kinase C (PKC) activity, particularly PKCa and PKCe, was noted. Blocking of PKCa and PKCe activity or expression, but not of PKCb, PKCd, PKCg, PKCh, and PKCf, significantly reduced the synthesis of GM-CSF in infected cells. While PKCa was critical for the initiation of a nuclear factorkB-mediated GM-CSF expression, PKCe regulated GM-CSF production via activator protein 1. Thus, differential regulation of GM-CSF, production by PKC isoforms, contributes to the host response in Legionnaires’ disease.