TLR4 and chronic obstructive pulmonary disease: LPS can cross the intestinal barrier into the body circulation and reach the lungs, where it exacerbates COPD lung inflammation through the toll-like receptor 4/ nuclear transcription factor-κB (TLR4/ NF-κB) signaling pathway mediating the production of IL-1β, IL-6, IL-17 TNF-α (70).