AbstractHuman eosinophilic cell line EoL‐1 was studied using luminol‐dependent chemiluminescence (CL) for the ability to produce a respiratory burst upon stimulation with 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) and N‐formyl‐L‐methionyl‐L‐leucyl‐L‐phenylalanine (fMLP). TPA, a potent activator of protein kinase C, induced a biphasic CL response in EoL‐1. Treatment of EoL‐1 with interferon‐γ (IFN‐γ) for 5 days dramatically enhanced TPA‐inducible CL, and IFN‐αA had a similar effect. Neither IFN‐γ nor IFN‐αA strongly inhibited EoL‐1 cell growth. Tumor necrosis factor (TNF) also enhanced TPA‐inducible CL response of EoL‐1 and, furthermore, was quite inhibitory to EoL‐1 cell growth. The effects of IFN‐γ and TNF were synergistic, whereas those of IFN‐αA and TNF were additive. Superoxide dismutase completely abrogated TPA‐induced CL, but sodium azide suppressed only the late phase of CL. EoL‐1 pretreated with IFN‐7, IFN‐αA, or TNF also became capable of producing CL response to a chemotactic peptide (fMLP). The effects of IFN‐γ and TNF were again synergistic. EoL‐1 cells treated with IFN‐γ, IFN‐αA, or TNF had abundant cytoplasm, but only TNF increased cells having distinct eosinophilic granules. IFN‐γ but not IFN‐αA enhanced the cytological effect of TNF. It was further demonstrated that treatment of EoL‐1 with IFN‐γ and TNF strongly increased the binding sites for phorbol diesters and also dramatically induced the surface expression of fMLP receptors. IFN‐γ had, however, little effect on the number or the ligand‐binding affinity of TNF receptors on EoL‐1. Thus, EoL‐1 may provide a useful experimental model for the study of differentiation and regulation of human eosinophils.