Anergy is the state in which T cells are viable but incapable of responding to antigen. However, they proliferate in response to phorbol ester (PMA) and ionomycin that activate protein kinase C (PKC) and calcium flux, respectively. Intracellular cyclic AMP (cAMP) is increased in anergic cells and correlates with induction of p27kipl and inhibition of antigen-specific proliferation and IL-2 production in T cell clones. Members of the cAMP response element family can bind to IL-2 promoter and may inhibit IL-2 transcription in anergic cells. These observations prompted us to determine whether artificial increase of endogenous cAMP in primary T cells may recapitulate biochemical and functional characteristics of anergy. Increase of intracellular cAMP by forskolin that activates adenylate cyclase, and the phosphodiesterase inhibitor IBMX, completely inhibited TCR+CD28mediated proliferation. At a molecular level, cAMP led to Rapl activation but inhibition of TCR+CD28 mediated ERK activation.Analysis of the cell cycle showed that cAMP allowed entry of T cells into the Gl phase, synthesis of cyclin D2, cdk4 and cdk6 but also upregulated p27k'p' preventing hyperphosphorylation of Rb, Gl-S transition and synthesis of cyclin A. cAMP inhibited expression of activation markers CD69, CD71, IL-2 receptor a, βand y chain and prevented downregulation of CD62L. cAMP has been proposed to regulate the balance of Thl v/s Th2 type cytokines. Analysis of IL-2, IFNy, IL-4, IL-5 and IL-10 production during TCR+CD28 stimulation showed a dramatic inhibitory effect of cAMP on all cytokines. To determine a mechanism by which cAMP inhibited IL-2 transcription we examined the expression and DNA binding of Jun and Fos transcription factors, that have been implicated in the defective AP-1 transactivation in anergic T cells. Although cAMP did not.affect induction of c-Jun, it induced delayed induction of c-Fos, delayed and diminished phosphorylation of c-Jun and an altered pattern of protein binding on the consensus AP-1 sequence as determined by electrophoretic mobility shift assay. Importantly, the functional and biochemical alterations that cAMP induced upon TCR+CD28 mediated stimulation were not observed when PMA+CD28 was used. This suggests that cAMP exerted its effects on substrate(s) distal to the TCR and proximal to the PKC mediated signals. These results show that multiple functional and biochemical events observed in anergic cells are recapitulated by the induced increase of intracellular cAMP and suggest that signals regulating the expression of cAMP after T cell activation may determine its fate towards anergy of immunity.
|Original language||English (US)|
|Issue number||11 PART I|
|State||Published - Dec 1 2000|
ASJC Scopus subject areas
- Cell Biology