TLR4 and acute respiratory distress syndrome: Moreover, inactivation of TLR4/NF-κB signaling was accompanied by down-regulation of a key protease in the apoptotic cascade, caspase-3, suggesting that protection by MSC-derived exosomes against ALI may involve inhibition of apoptosis in pulmonary microvascular epithelial cells through TLR4/NF-κB signaling 36.