Induction and maintenance of IL-4 expression are regulated differently by the 3' enhancer in CD4 T cells.

IL-4 expression is known to be activated in CD4 T cells when they are differentiated to Th2 but not Th1 cells. However, CD4 T cells selected by MH class II-expressing thymocytes, named thymocyte-selected CD4 T cells (T-CD4 T cells), express IL-4 under both Th1 and Th2 conditions. In this study, we investigated molecular mechanisms by which IL-4 gene expression is regulated in T-CD4 T cells. We found that T-CD4 T cells express IL-4 soon after selection in the thymus. Deficiency of DNase I hypersensitive (HS) sites HS5a and HS5 at the 3'-enhancer region in the IL-4 gene decreased IL-4 production, but T-CD4 T cells were able to make IL-4 under the Th1-inducing condition. Consistent with this, IL-4 was expressed in Th1 differentiated T-CD4 T cells in the absence of recombination signal binding protein-J that interacts with HS5. When HS5 was examined separately from other endogenous regulatory elements using a reporter system, CD4 T cells that are selected by thymic epithelial cells cannot transcribe the IL-4 reporter gene with HS5 alone. However, HS5 was able to induce the expression of the IL-4 reporter gene in T-CD4 T cells. Interestingly, the Th1 differentiating signal led to deacetylation at HS5 of the IL-4 endogenous gene, whereas the Th2-inducing environment had no effect. Therefore, in T-CD4 T cells, HS5 plays an essential role during the induction phase of IL-4 expression, but the maintenance of IL-4 expression in Th1 cells requires additional regulatory elements.