TCR- and CD28-Mediated Recruitment of Phosphodiesterase 4 to Lipid Rafts Potentiates TCR Signaling

Abstract

Ligation of the TCR along with the coreceptor CD28 is necessary to elicit T cell activation in vivo, whereas TCR triggering alone does not allow a full T cell response. Upon T cell activation of human peripheral blood T cells, we found that the majority of cAMP was generated in T cell lipid rafts followed by activation of protein kinase A. However, upon TCR and CD28 coligation, β-arrestin in complex with cAMP-specific phosphodiesterase 4 (PDE4) was recruited to lipid rafts which down-regulated cAMP levels. Whereas inhibition of protein kinase A increased TCR-induced immune responses, inhibition of PDE4 blunted T cell cytokine production. Conversely, overexpression of either PDE4 or β-arrestin augmented TCR/CD28-stimulated cytokine production. We show here for the first time that the T cell immune response is potentiated by TCR/CD28-mediated recruitment of PDE4 to lipid rafts, which counteracts the local, TCR-induced production of cAMP. The specific recruitment of PDE4 thus serves to abrogate the negative feedback by cAMP which is elicited in the absence of a coreceptor stimulus.

Cancer-Induced Signaling Defects in Antitumor T Cells

“…In addition, as mentioned previously, cAMP levels are increased in T cells upon activation (Ledbetter et al, 1986) which if unopposed leads to PKA activation and inhibition of TCR signaling (Abrahamsen et al, 2004). Elevated cAMP, and thus PKA activity, may be part of the normal homeostatic mechanism to dampen signaling and therefore restrict T-cell activation, especially in the absence of costimula-tion (see below).…”

“…Elevated cAMP, and thus PKA activity, may be part of the normal homeostatic mechanism to dampen signaling and therefore restrict T-cell activation, especially in the absence of costimula-tion (see below). A counter-regulatory mechanism exists which functions to limit cAMP-induced PKA-mediated inactivation of p56 lck and is envisioned to be operative under conditions of robust T-cell activation, i.e., TCR plus CD28 costimulation (Abrahamsen, 2004). As mentioned above, lipid raftassociated PDE4 activity is enhanced upon concomitant ligation of the TCR and CD28.…”

“…In this regard, Tasken and colleagues have hypothesized that signals generated by TCR ligation appear to result in a constitutively active "default pathway" of increased cAMP levels leading to inhibition of signaling (Abrahamsen, 2004). Only when TCR ligation is accompanied by CD28 costimulation are cAMP levels reduced therein relieving PKA/Csk activity and permitting sustained T-cell activation.…”

“…Conversely, activation of T cells by crosslinking both the TCR and the costimu-latory receptor CD28 results in a reduction of cAMP levels and concomitant diminished activation of PKA, therein avoiding inactivation of p56 lck since Csk is not robustly activated (or is sequestered away from its substrate, see below) (Abrahamsen, 2004). Inhibition of cAMP is mediated by phosphodiesterases (PDE), which in T cells is largely accounted for by the PDE type 4 family ("PDE4", containing four genes and multiple isoforms) (Oh and Schnitzer, 2001).…”

Analyses of PDE-regulated phosphoproteomes reveal unique and specific cAMP-signaling modules in T cells

“…For example, PDE3B is localized predominately at membrane surfaces and is thought to regulate cAMP in regions proximal to these surfaces (11). Similarly, in activated T cells, PDE4 is recruited to the T-cell receptor in a β-arrestin-dependent manner, thus lowering cAMP in the vicinity of this part of the T-cell activation pathway (12).…”