In Part 2, Dr. Matthay provides the rationale behind treating ARDS patients with MSC. Initial studies in a mouse model of ARDS, showed that treatment with MSCs increased levels of anti-inflammatory cytokines and antimicrobial peptides in the lung, and increased phagocytosis of bacteria by monocytes. Further studies in ex vivo perfused human lungs and in sheep with severe lung injury showed that treatment with MSCs improved oxygenation and reduced pulmonary edema. An NIH/NHLBI supported phase 1 clinical trial for safety has been completed and a randomized, blinded phase 2 trial has now been initiated to test the safety and efficacy of MSC treatment in human patients with ARDS.