Regulatory effect of cytokines on eosinophil degranulation

We tested the effects of different cytokines on IgA- and IgG-induced eosinophil degranulation in vitro to determine the potential interaction between eosinophils and mononuclear cells. Purified normodense eosinophils were incubated with cytokines (including rIL-1, rIL-2, rIL-3, rIL-4, rIL-5, rIL-6, IFN-gamma, granulocyte-macrophage CSF stimulating factor (GM-CSF), and TNF) for 1 to 3 h after which Ig-coupled Sepharose 4B beads were added as targets and the mixtures were incubated with the eosinophils at 37 degrees C for 4 h. The Ig used were secretory IgA (sIgA), serum IgA and IgG, and myeloma IgA and IgG. The release of eosinophil-derived neurotoxin (EDN) was measured by RIA as an index of degranulation. rIL-5 was the most potent enhancer of Ig-induced degranulation and increased EDN release by 48% for sIgA and 136% for IgG. The effect of rIL-5 appeared as quickly as 15 min after incubation of eosinophils, sIgA beads and IL-5. GM-CSF and rIL-3 also enhanced Ig-induced EDN release but less potently than rIL-5. GM-CSF and rIL-5 by themselves induced a small but significant release of EDN from eosinophils in the absence of Ig-coated beads; rIL-3 did not. However, IFN-gamma suppressed sIgA-induced EDN release by 23%. The other cytokines did not have any effect on eosinophil degranulation. These results suggest that cytokines which induce eosinophil differentiation and proliferation during hematopoiesis also enhance the effector function of mature eosinophils and that IFN-gamma partially down-regulates eosinophil degranulation.