Mutational analysis of CD28-mediated costimulation of Jun-N-terminal kinase and IL-2 production.

The accessory molecule CD28 delivers a costimulus that acts in concert with TCR signals to promote T cell activation. Activation of Jun-N-terminal kinases (JNK) requires simultaneous stimulation of the TCR and CD28 and, therefore, likely plays an important role in signal integration during costimulation. We investigated the effects of mutations in the 41-amino acid cytoplasmic domain of murine CD28 on its ability to deliver costimuli for JNK activation and IL-2 production when expressed in Jurkat T cells. Our results indicate that the costimulus for JNK activation requires the membrane-proximal 24 amino acids of the CD28 cytoplasmic domain and is not mediated by the tyrosine-based recruitment of signaling molecules, including phosphatidylinositol 3-kinase. Deletion of the carboxyl-terminal 17 amino acids does not affect the ability of CD28 to augment JNK activation but impairs its ability to enhance TCR-mediated production of IL-2, demonstrating that optimal costimulation of IL-2 production requires CD28 signals in addition to the activation of JNK.