IL-3, IL-5, and GNI-CSF exert overlapping functions in eosinophils via a shared receptor beta-chain, and IL-3Ralpha transcript expression is the weakest in blood eosinophils. We investigated the long-term regulation of surface expression of IL-3Ralpha. IL-3 was the most potent inducer of CD69 expression after 24-h stimulation, but not after 1-h stimulation. Expression of IL-5Ralpha and GM-CSFRalpha was significantly downregulated by culturing with their respective ligands, while IL-3Ralpha expression was not. IL-3 at 30 pM significantly increased IL-3Ralpha expression and IL-3Ralpha expression was also upregulated by both IL-5 and GM-CSF. In parallel with the surface protein expression, IL-3Ralpha mRNA was also upregulated by IL-3, IL-5, and GM-CSF. These results demonstrated that long-term culturing of eosinophils with CSFs induced a change in the potency order of CSFs, with IL-3 coming to exert the strongest effect. They thus suggest that IL-3 plays more important roles in local eosinophil activation than previously recognized. (C) 2003 Elsevier Inc. All rights reserved.