CSF2 and cystic fibrosis: GM-CSF was also used to treat macrophages because: (i) it has an activating effect on myeloid cell function [28,29]; (ii) it has therapeutic potential in CF [30]; (iii) high serum GM-CSF levels correlate with increased IFN-γ expression and better pulmonary function in CF patients [6], and (iv) GM-CSF complements IFN-γ in macrophage resistance to mycobacterial infection [31], strongly suggesting that the two factors may co-operate in host defence.