The overall hypothesis tested in this proposal is that cyclic nucleotide signaling and phosphodiesterases (PDEs) play a critical role in modulating the inflammatory responses in asthma. Although the immunomodulatory functions of cAMP signaling have been widely explored, the exact physiological role of this regulatory pathway in inflammatory cells remains controversial. We have generated novel mouse models in which cAMP signaling is disrupted but not interrupted by genetic ablation of three genes coding for PDE4 enzymes that degrade and inactivate cAMP. Although ablation of each of the three genes protects against airway hyperreactivity, only ablation of PDE4B causes disruption in inflammatory responses. The expression of the PDE4B gene is regulated by activation of TLR signaling, and TNF production is greatly reduced in PDE4B-null mice but not in PDE4A or PDE4D-null mice. In addition, replication, migration, and cytokine production by T cells are affected after ablation of PDE4B but not PDE4A or PDE4D. On the basis of these preliminary findings, we hypothesize that the PDE4B induction is an indispensable component of the inflammatory cell response and required to maintain cell homeostasis. Under steady state, the cAMP tone present prevents inflammatory cells from activation, whereas antigen stimulation of TLR or TCR signaling is associated with induction of PDE4B, which in turn is indispensable to remove the cAMP inhibitory constraint. The experimental plan organized along three specific aims is designed to verify this hypothesis. With the first Specific Aim, we propose to study the function of dendritic cells deficient in PDE4B in in vivo and in vitro. Experiments described in the second Specific Aim will assess the function of PDE4B in T cells in vitro and in vivo using adoptive transfer strategies. The third Specific Aim will be devoted to understanding the molecular basis of PDE4B function. Macromolecular complexes involving PDE4B will be identified and their function explored using dominantnegative or overexpression strategies. Understanding how this regulatory circuit functions will have two major implications. Data demonstrating that the PDE4B function is distinct from that of the cognate PDE4 genes will be proof of concept for the development of PDE4B selective inhibitors as anti-inflammatory drugs, a strategy already pursued by the pharmaceutical industry. In addition, the concept that PDE4B plays a critical role in immunomodulation will pave the way for studies searching for PDE4B alleles associated with heritable states of hyper or hyporesponsiveness of the immune system.