Mycoplasma pneumoniae lipids license TLR4 for activation of NLRP3 inflammasome and autophagy to evoke a proinflammatory response.

Luo, Haodang; He, Jun; Qin, Lianmei; Chen, Yiwen; Chen, Liesong; Li, Ranhui; Zeng, Yanhua; Zhu, Cuiming; You, Xiaoxing; Wu, Yimou

Mycoplasma pneumoniae is an obligate pathogen that causes pneumonia, tracheobronchitis, pharyngitis, and asthma in humans. It is well recognized that membrane lipoproteins are immunostimulants exerting as LPS and play a crucial role in the pathogenesis of inflammatory responses upon M. pneumoniae infection. Here, we report that the M. pneumoniae-derived lipids are another proinflammatory agents. Using an antibody-neutralizing assay, RNA interference, or specific inhibitors, we found that TLR4 is essential for M. pneumoniae lipid-induced TNF-α and IL-1β production. We also demonstrated that NLRP3 inflammasome, autophagy, and NF-κB-dependent pathways are critical for the secretion of proinflammatory cytokines, while inhibition of TLR4 significantly abrogates these events. Further characterisation revealed that autophagy-mediated inflammatory responses involved the activation of NF-κB. In addition, the activation of NF-κB also promoted lipid-induced autophagosome formation, as revealed by assays using pharmacological inhibitors, 3-MA and Bay 11-7082, or silencing of atg5 and beclin-1. These findings suggest that, unlike the response to lipoprotein stimulation, the inflammation in response to M. pneumoniae lipids is mediated by the TLR4 pathway, which subsequently initiates the activation of NLRP3 inflammasome and formation of a positive feedback loop between autophagy and NF-κB signalling cascade, ultimately promoting TNF-α and Il-1β production in macrophages.