Gene expression of interleukin-2 in purified human peripheral blood eosinophils

To verify the hypothesis that eosinophils produce interleukin-2 (IL-2), a cytokine essential for lymphocyte activation, the expression of IL-2 was examined in peripheral blood eosinophils obtained from normal, atopic, asthmatic and hypereosinophilic subjects. Purified blood cell preparations were > 95% eosinophils, the remaining cells being neutrophils. Based on morphological observations and on CD3 expression, no lymphocytes were detected in these eosinophil preparations. The expression of IL-2 mRNA was detected by reverse transcriptase polymerase chain reaction (RT-PCR) in total RNA extracted from purified eosinophils stimulated with granulocyte-macrophage colony-stimulating factor (GM-CSF), with or without calcium ionophore (A23187). In-cell RT-PCR combined with in situ hybridization further confirmed that it was the eosinophils that expressed IL-2 mRNA. Moreover, in this experiment IL-2 mRNA expression increased upon costimulation with A23187 and GM-CSF suggesting that a steady-state level of IL-2 mRNA was inducible. Finally, IL-2 was detected in purified eosinophils by immunochemistry. These data, obtained by different techniques, demonstrate that eosinophils can express IL-2. An IL-2-mediated eosinophil-lymphocyte interaction could contribute to the chronic state of cell activation in inflamed tissues where these cells are implicated.