The ability of recombinant human interleukin-5 (rhIL-5) to bind to human eosinophils (HEOS), and to eosinophil-like erythroleukemic cells (butyric acid differentiated HL-60 cells), and also to prime the phorbol myristate acetate ester (PMA)-induced respiratory burst of these cells was assessed. Both cell types expressed a high affinity (KD∼ 19 pM as measured ar 37 °C) IL-5 receptor and could be significantly primed by short (1 h) preincubation with 1 nM rhIL-5 as shown by leftward shifts of the PMA dose-response curves, and increased maximal responses (PMA EC50 without IL-5, 0.85 ± 0.14 nM; with 1 nM IL-5, 0.47 ± 0.15 nM, n = 6 donors; p 50 of 17 ± 11 pM and could be neutralised by an anti-interleukin-5 antibody (IC50∼ 10 nM for the priming effect of 1 nM rhIL-5). A longer (24 h) incubation of HEOS with IL-5 appeared to potentiate the effect of the cytokine, which was greatest in cells from subjects with modest eosinophilia. These data suggest that interleukin-5 is a potent and rapidly acting priming agent for eosinophils, and that release of IL-5 during an allergic asthmatic reaction may contribute to the observed activation of these granu
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