GM-CSF is produced by endothelial, epithelial, hematopoietic, and other cell types.202 Under physiological conditions, low levels of GM-CSF can regulate the homeostasis of alveolar macrophages to maintain their antimicrobial functions.203,204 Under hyperinflammatory conditions such as CS, GM-CSF drives emergency myelopoiesis and recruits myeloid cells to the inflammatory sites to perpetuate inflammatory reactions.205 Increased levels of GM-CSF have been observed in SARS,206 ARDS,207 and CRS.208 A recent study also reported elevated GM-CSF levels in both severe and mild COVID-19.9 Here, CSF2 is linked to acute respiratory distress syndrome.