<qa>  What is the primary endpoint for this study?The primary endpoint for this study will be the duration of mechanical ventilation. Additional important endpoints will include changes in the severity of physiologic derangements of respiratory gas exchange, non-respiratory organ failure, and incidence of ventilator-associated pneumonia. Additional assessments designed to determine the mechanism of benefit of GM-CSF treatment will include measures of lung epithelial cell integrity and measures of alveolar macrophage lung inflammatory cell function. 