TLR4-Upregulated IL-1β and IL-1RI Promote Alveolar Macrophage Pyroptosis and Lung Inflammation through an Autocrine Mechanism

Acute lung injury (ALI) is a major component of multiple organ dysfunction syndrome (MODS) following pulmonary infection. Alveolar macrophages (AM) are at the center of the pathogenesis of the development of ALI. Interleukin-1 beta (IL-1 beta) is one of the key pro-inflammatory mediators, and its maturation is tightly controlled by the formation and activation of the inflammasome. The biological effects of IL-1 beta are mediated through IL-1 receptor (IL-1R). In this study, we investigated the influence of LPS-induced IL-1 beta release and IL-1RI upregulation on the development of lung inflammation. We demonstrated that in AM, LPS-TLR4 signaling not only activates Nlrp3 inflammasome activation and subsequent release of IL-1 beta, but also up-regulates IL-1RI expression on AM surface through MyD88 and NF-kappa B dependent signaling. The upregulated IL-1RI, therefore, sensitizes AM to IL-1 beta and results in pyroptosome formation, which in turn leads to AM pyroptosis, a type of caspase-1-dependent inflammatory cell death. We further showed that AM pyroptosis exaggerates lung inflammation. The present study demonstrates a novel mechanism underlying LPS-induced innate immunity; that is, a secondary upregulation of IL-1 beta-IL-1RI signaling is responsible for AM pyroptosis and augmented lung injury in response to LPS.