T Lymphocyte activation involves a substantial reorganization of the membranous and intracellular compartments. Signaling complexes assemble and dismantle in a highly ordered fashion in both compartments and orchestrate the activation of T cells with high sensitivity and specificity. TCR ligation leads to a short burst of cAMP production, which is centrally required for T cell activation; however, sustained elevations in intracellular cAMP concentrations are immunosuppressive. Emerging evidence of the existence of local cAMP pools gleaned from studies on other cell types suggests that cAMP compartmentalization may account, in part, for these opposing effects. Whereas cAMP compartmentalization has been identified as a central factor in the control of the cAMP-dependent processes in other cell types, this has, as yet, not been addressed in T lymphocytes. In this review, we discuss the role of cAMP in T cell activation and differentiation, with an emphasis on the effects mediated by the cAMP effectors, protein kinase A (PKA) and exchange protein activated by cAMP (EPAC)1, and on the regulatory proteins that may control the generation of local cAMP pools in T cells. We also present an overview of the available tools to image cAMP production at the subcellular level and discuss how bacterial adenylate cyclase (AC) toxins that are known to generate local cAMP pools can be exploited to address the role of cAMP compartmentalization in T cell activation.

A central feature of the immune synapse (IS) is the tight compartmentalization of membrane receptors and signaling mediators that is functional for its ability to coordinate T cell activation. Second messengers centrally implicated in this process, such as Ca2+ and diacyl glycerol, also undergo compartmentalization at the IS. Current evidence suggests a more complex scenario for cyclic AMP (cAMP), which acts both as positive and as negative regulator of T-cell antigen receptor (TCR) signaling and which, as such, must be subjected to a tight spatiotemporal control to allow for signaling at the IS. Here, we have used two bacterial adenylate cyclase toxins that produce cAMP at different subcellular localizations as the result of their distinct routes of cell invasion, namely Bordetella pertussis CyaA and Bacillus anthracis edema toxin (ET), to address the ability of the T cell to confine cAMP to the site of production and to address the impact of compartmentalized cAMP production on IS assembly and function. We show that CyaA, which produces cAMP close to the synaptic membrane, affects IS stability by modulating not only the distribution of LFA-1 and its partners talin and L-plastin, as previously partly reported but also by promoting the sustained synaptic accumulation of the A-kinase adaptor protein ezrin and protein kinase A while suppressing the β-arrestin-mediated recruitment of phosphodiesterase 4B. These effects are dependent on the catalytic activity of the toxin and can be reproduced by treatment with a non-hydrolyzable cAMP analog. Remarkably, none of these effects are elicited by ET, which produces cAMP at a perinuclear localization, despite its ability to suppress TCR signaling and T cell activation through its cAMP-elevating activity. These results show that the IS responds solely to local elevations of cAMP and provide evidence that potent compartmentalization mechanisms are operational in T cells to contain cAMP close to the site of production, even when produced at supraphysiological levels.