Human Mesenchymal Stromal Cells For Acute Respiratory Distress Syndrome (START)

Inclusion Criteria: Patients will be eligible for inclusion if they meet all of the below criteria. Criteria 1-3 must all be present within a 24-hour time period and at the time of enrollment: Acute onset (defined below) of: 1. A need for positive pressure ventilation by an endotracheal or tracheal tube with a PaO2/FiO2 ratio \< 200 with at least 8 cm H2O positive end-expiratory airway pressure (PEEP) 2. Bilateral infiltrates consistent with pulmonary edema on frontal chest radiograph 3. No clinical evidence of left atrial hypertension for bilateral pulmonary infiltrates. Exclusion Criteria: 1. Age less than 18 years 2. Greater than 96 hours since first meeting ARDS criteria per the Berlin definition of ARDS 3. Pregnant or breast-feeding 4. Prisoner 5. Presence of any active malignancy (other than non-melanoma skin cancer) that required treatment within the last 2 years 6. Any other irreversible disease or condition for which 6-month mortality is estimated to be greater than 50% 7. Moderate to severe liver failure (Childs-Pugh Score \> 12) 8. Severe chronic respiratory disease with a PaCO2 \> 50 mm Hg or the use of home oxygen 9. Patient, surrogate, or physician not committed to full support (exception: a patient will not be excluded if he/she would receive all supportive care except for attempts at resuscitation from cardiac arrest) 10. Major trauma in the prior 5 days 11. Lung transplant patient 12. No consent/inability to obtain consent 13. Moribund patient not expected to survive 24 hours 14. World Health Organization (WHO) Class III or IV pulmonary hypertension 15. Documented deep venous thrombosis or pulmonary embolism within past 3 months 16. No arterial line/no intent to place an arterial line 17. No intent/unwillingness to follow lung protective ventilation strategy or fluid management protocol 18. Currently receiving extracorporeal life support (ECLS) or high-frequency oscillatory ventilation (HFOV)