Molecular mechanisms of IL-2 gene regulation following costimulation through LFA-1

The integrin LFA-1 serves as an accessory molecule in T cell activation. In addition to its well-known role as an adhesion molecule, LFA-1 can contrib ute to T cell activation and up-regulation of IL-2 gene expression. However , the specific mechanisms by which LFA-1 influences T cell activation have not been elucidated. Therefore, we examined the impact of LFA-1:ICAM-1 inte ractions on transcriptional and posttranscriptional IL-2 gene regulation, u sing a costimulation-negative cell line transfected with MHC class II alone , or in combination with ICAM-1 or B7-1. IL-2 transcription was assessed ut ilizing transgenic mice expressing an IL-2 promoter luciferase reporter con struct crossed to DO11.10 TCR-transgenic mice, and IL-2 mRNA stability was evaluated by real-time RT-PCR. Comparison of naive and previously activated T cells demonstrates a dramatic increase in IL-2-luciferase transcription in activated T cells that can, in part, be attributed to downstream signali ng events. Costimulation through LFA-1 enhances transcription of the transg enic reporter construct across a wide Ag dose range, but does not affect IL -2 mRNA stability. In contrast, CD28 costimulation is clearly mediated thro ugh up-regulation of IL-2 transcription and through enhancement of mRNA sta bility. These results indicate that the primary pathway whereby engagement of LFA-1 through its ligand ICAM-1 up-regulates IL-2 gene expression is thr ough enhanced IL-2 transcription, in the absence of any effect on IL-2 mRNA stabilization.