Macrophages render alveolar epithelial cells hypo-responsive to Legionella pneumophila

Pneumonia is a leading cause of mortality worldwide. To secure organ function, pulmonary innate immune response has to be tightly regulated. Here, we analyze whether macrophages influence immune reactivity of type II alveolar epithelial cells during infection with an intracellular bacterial pathogen.

For this purpose, human macrophage-like differentiated THP-1 cells were co-cultured with human alveolar epithelial A549 cells in a transwell-setting. Infection of THP-1 cells with the important respiratory pathogen Legionella pneumophila (L. pneumophila) resulted in the release of pro-inflammatory cytokines, e.g. IL-8, by co-cultured, non-infected epithelial cells. This effect was synergistically blocked by an IL-1 receptor antagonist (IL-1ra) and a TNF-α neutralizing antibody (anti-TNF-α). Furthermore, co-culture with infected THP-1 cells reduced cytokine expression by epithelial cells following direct encounter with L. pneumophila. This epithelial hypo-responsiveness could be mimicked by stimulation with IL-1β. It was accompanied by an accelerated pro-inflammatory mRNA decay, long-lasting degradation of interleukin-1 receptor-associated kinase 1 (IRAK-1), and reduced nuclear factor of kappa light polypeptide gene enhancer in B-cells inhibitor alpha (IκBα)-degradation and less nuclear factor kappa-light-chain-enhancer of activated B-cells (NF-κB) translocation into the nucleus. miR-146a was induced, but did not mimic epithelial hypo-responsiveness in physiological concentrations. Likewise, histone modifying enzymes did not reproduce the effect.

Corroborated by in silico simulations, our results demonstrate that macrophages can negatively regulate the responsiveness of lung epithelial cells to bacterial infection by the release of IL-1β resulting in a downregulation of IRAK-1. This intercellular communication may be critical for avoiding overwhelming inflammatory response in the lung.