Background: IL-5 is crucially involved in eosinophilic inflammation. We have previously reported that IL-5 syn thesis by atopic and nonatopic asthmatics is significantly enhanced compared to control subjects. T cell IL-5 synthesis is regulated at the transcriptional level. The proximal human IL-5 promoter (-62 to -46) homologous to the conserved lymphokine element 0 (CLE0) is essential for activation-induced gene transcription by Th cells,
Methods: Luciferase reporter analysis and gel shift analysis were performed.
Results: The CLE0 homologous element is the overlapping binding site for a constitutive and an inducible binding factor. Site-directed mutagenesis successfully differentiated the two bindings, and revealed that the transcriptional induction was ascribed to the inducible binding, while the constitutive binding was rather inhibitory. A mutant element which lost the constitutive binding, but retained the inducible binding, exerted 3 times more transcriptional activity compared to the wild-type element. In contrast, another mutant element which lost the inducible binding and retained the constitutive binding exhibited no transcriptional induction. Gel shift analysis was performed to clarify that the inducible binding was more prominent and the constitutive binding was less in IL-5 producing Th clones compared to IL-5-nonproducing clones.
Conclusion: The ratio of the inducible/constitutive binding to the CLE0 homologous element may determine the capacity of human Th cells to transcribe the IL-5 gene.