TLR4 and acute respiratory distress syndrome: Meanwhile, TNF receptor-associated factor 6 (TRAF6) is a downstream signaling molecule of TLR4-myeloid differentiation primary response gene 88, the ubiquitination of which can promote the activation of the NF-κB and MAPK signaling pathways, while the loss of TRAF6 can induce the M2 polarization of macrophages and alleviate the inflammatory response in ALI [37,61,71,72].