Inhibition of cAMP degradation improves regulatory T cell-mediated suppression.

Naturally occurring regulatory T cells (nTreg cells) are crucial for the maintenance of peripheral tolerance. We have previously shown that a key mechanism of their suppressive action is based on a contact-dependent transfer of cAMP from nTreg cells to responder T cells. Herein, we further elucidate the important role of cAMP for the suppressive properties of nTreg cells. Prevention of cAMP degradation by application of the phosphodiesterase 4 inhibitor rolipram led to strongly increased suppressive potency of nTreg cells for Th2 cells in vitro and in vivo. Detailed analyses revealed that rolipram caused, in the presence of nTreg cells, a synergistic increase of cAMP in responder Th2 cells. In vivo, the application of nTreg cells in a strictly Th2-dependent preclinical model of asthma had only a marginal effect. However, the additional treatment with rolipram led to a considerable reduction of airway hyperresponsiveness and inflammation in a prophylactic as well as in a therapeutic model. This amelioration was correlated with enhanced cAMP-levels in lung Th2 cells in vivo. Collectively, these data support our observation that cAMP has a key function for nTreg cell-based suppression and they clearly demonstrate that the effect of cAMP on T responder cells can be greatly enhanced upon application of phosphodiesterase 4 inhibitors.