Incomplete Knockdown of MyD88 Inhibits LPS-Induced Lung Injury and Lung Fibrosis in a Mouse Model

Acute lung injury (ALI) is a life-threatening disorder stemmed mainly from an uncontrolled inflammatory response. Lipopolysaccharide (LPS) is commonly used to induce ALI animal models. Toll-like receptor 4 (TLR4) is the main receptor for LPS, and myeloid differentiation factor 88 (MyD88) is a key adaptor protein molecule in the Toll-like receptor (TLR) signaling pathway. Thus, MyD88 knockdown heterozygous mice (MyD88^+/−) were used to investigate the effect of incomplete knockout of the MyD88 gene on indirect LPS-induced ALI through intraperitoneal injection of LPS. The LPS-induced ALI significantly upregulated MyD88 expression, and heterozygous mice with incomplete knockout of the MyD88 gene (MyD88^+/−) ameliorated LPS-induced histopathological injury and collagen fiber deposition. Heterozygous mice with incomplete knockout of the MyD88 gene (MyD88^+/−) inhibited LPS-induced nuclear factor-κB (NF-κB) pathway activation, but TLR-4 expression tended to be upregulated. Incomplete knockdown of the MyD88 gene also downregulated LPS-induced expression of IL1-β, IL-6, TNF-α, TGF-β, SMAD2, and α-SMA. The transcriptome sequencing also revealed significant changes in LPS-regulated genes (such as IL-17 signaling pathway genes) after the incomplete knockdown of MyD88. In conclusion, this paper clarified that LPS activates the downstream NF-κB pathway depending on the MyD88 signaling pathway, which induces the secretion of inflammatory cytokines such as IL-1β/IL-6/TNF-α and ultimately triggers ALI. Incomplete knockdown of the MyD88 reverses LPS-induced lung fibrosis, which confirmed the vital role of MyD88 in LPS-induced ALI.