CD28 is a glycoprotein expressed as a homodimer on the surface of a major subset of human T cells. Previous studies have shown that proliferation of peripheral blood T cells involving the CD28 pathway is associated with cyclosporine A (CsA) resistant IL-2 gene expression. This pathway was shown to specifically regulate the stability of mRNA for several lymphokines including IL-2. We have investigated the expression of the IL-2 gene in the Jurkat cell line, J32 clone, induced by CD28 stimulation. Cross-linked anti-CD28 mAb alone was sufficient to induce the release of small amounts of IL-2 (256 U/ml). The CD28-mediated IL-2 release was enhanced with simultaneous engagement of CD28 and CD2 or CD28 and CD3 molecules. Hybrid constructs in which the human IL-2 gene 5' flanking region drives luciferase expression (pIL-2-Luc) were used to help delineate whether the CD28 pathway activates the IL-2 gene transcriptionally. Costimulation of cells with CD28 mAb and either PHA or CD2 mAb induced a 20- to 90-fold increase in the expression of pIL-2-Luc as well as IL-2 release. Costimulation with CD28 mAb plus PMA gave only five- to sevenfold increase in enhancer activity. In contrast, no enhancer activity was detected after stimulation with CD28 or CD2 mAb alone. Both IL-2 release and pIL-2-Luc activity were inhibited by CsA in J32 cells. The degree of CsA inhibition was concentration dependent and was similar in cells stimulated with either CD28 mAb or CD3 mAb. Maximum inhibition was achieved with 1 microgram/ml of CsA. Studies with internal deletion mutations of the IL-2 gene 5' flanking sequence revealed that as with stimulation through the TCR pathway, the CD28 pathway requires 5' flanking sequences located within 500 bp of the transcription start site. These results are the first direct evidence that the triggering of CD28 molecule is sufficient to induce IL-2 release in J32 cells. Furthermore these studies strongly indicate that IL-2 gene expression induced by CD28 stimulation occurs, in part, transcriptionally and is CsA sensitive in these cells.

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