- patients who are already on supplemental oxygen of fio2 ≥ 0.4 - patients with known history of serious allergic reactions, including anaphylaxis, to human gm-csf such as leukine®, yeast-derived products, or any component of the product. - mechanical ventilation before start of study - patients enrolled in another investigational drug study - pregnant or breastfeeding females (all female subjects of childbearing potential status must have negative pregnancy test at screening) - patients with peripheral white blood cell count above 25,000 per microliter and/or active myeloid malignancy - patients on high dose systemic steroids (> 20mg methylprednisolone or equivalent) - patients on lithium carbonate therapy - patients with serum ferritin >2000 mcg/ml (which will exclude ongoing hlh)

- patients who are already on supplemental oxygen of fio2 ≥ 0.4 - patients with known history of serious allergic reactions, including anaphylaxis, to human gm-csf such as leukine®, yeast-derived products, or any component of the product. - mechanical ventilation before start of study - patients enrolled in another investigational drug study - pregnant or breastfeeding females (all female subjects of childbearing potential status must have negative pregnancy test at screening) - patients with peripheral white blood cell count above 25,000 per microliter and/or active myeloid malignancy - patients on high dose systemic steroids (> 20mg methylprednisolone or equivalent) - patients on lithium carbonate therapy - patients with serum ferritin >2000 mcg/ml (which will exclude ongoing hlh)