GM-CSF engages multiple signaling pathways to enhance pro-inflammatory cytokine responses in human monocytes during Legionella infection.

The proinflammatory cytokine granulocyte-macrophage colony-stimulating factor (GM-CSF) is required for host defense against a wide range of pathogens. During infection with the intracellular bacterial pathogen Legionella pneumophila, we previously found that GM-CSF enhances inflammatory cytokine production in murine monocytes and is required for in vivo control of Legionella. It is unclear whether GM-CSF similarly augments cytokine production in human monocytes during bacterial infection. Here, we find that GM-CSF enhances inflammatory cytokine expression in Legionella-infected human monocytes by engaging multiple signaling pathways. Legionella- and Toll-like receptor-dependent NF-[Formula: see text]B signaling is a prerequisite signal for GM-CSF to promote cytokine expression. Then, GM-CSF-driven Janus kinase 2/signal transducer and activator of transcription 5 signaling is required to augment cytokine expression in Legionella-infected human monocytes. We also found a role for phosphatidylinositol-3-kinase/Akt/mTORC1 signaling in GM-CSF-dependent upregulation of cytokine expression. Finally, glycolysis and amino acid metabolism are also critical for GM-CSF to boost cytokine gene expression. Our findings show that GM-CSF-mediated enhancement of cytokine expression in infected human monocytes is regulated by multiple signaling pathways, thereby allowing the host to fine-tune antibacterial immunity.