TNF and acute respiratory distress syndrome: MSCs are not exclusively activated by mechanical forces (e.g., mechanical stretch and shear stress) but are also modulated by a variety of other stimuli that are altered in ALI/ARDS lungs, including changes in pH or temperature, inflammatory cytokines (e.g., TNF-α), drugs (e.g., volatile anesthetics), biological ligands (e.g., ATP and lipids), and changes in the membrane potential (voltage dependency) [29, 33, 34].