Replenishment of RANTES mRNA expression in activated eosinophils fromatopic asthmatics

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Abstract
Eosinophils have been shown to express the gene encoding regulated upon activation, normal T-cell expressed and secreted (RANTES), a potent eosinophilotactic chemokine. RANTES protein expression in eosinophils has previously been shown to be up-regulated by a number of agonists, including complement-dependent factors (C3b/iC3b) and interferon-γ (IFN-γ). We hypothesized that gene expression of RANTES is regulated in these cells by eosinophil-specific agonists. We analysed RANTES mRNA expression by reverse-transcription polymerase chain reaction (RT-PCR) in human peripheral blood eosinophils obtained from mild atopic asthmatics following stimulation over time. In resting eosinophils, a low level of RANTES mRNA was found to be constitutively expressed in all the atopic donors tested in this study (n = 6). Following stimulation with C3b/iC3b (serum-coated surfaces), eosinophils released measurable levels of RANTES, while sustained transcript expression was detected for up to 24 hr of stimulation. In contrast, IFN-γ (5 ng/ml) transiently and significantly (P < 0·05, n = 3) depleted relative amounts of RANTES PCR product (compared with β2-microglobulin) after 1–4 hr of stimulation. RANTES transcript was again detectable after 24 hr of IFN-γ incubation, suggesting that the pool of RANTES mRNA had been replenished. Other eosinophil-active cytokines, interleukin-3 (IL-3), IL-4, IL-5 and granulocyte–macrophage colony-stimulating factor, did not appear to modulate RANTES mRNA expression after 1 hr of incubation. The effect of IFN-γ on RANTES mRNA was reversed by cycloheximide, suggesting that IFN-γ may act by increasing the rate of translation of RANTES mRNA. These findings indicate that IFN-γ may induce a rapid and transient effect on the translation and replenishment of RANTES mRNA in eosinophils. This novel observation supports the notion that eosinophils have the potential to replenish their stored and released bioactive proteins.