Immunosuppressive effects of extracellular adenosine on immune cells: Implications for the pathogenesis of ADA SCID and immunomodulation

Antithetical effects of corticosterone and dibutyryl cAMP on adenosine deaminase in the gastrointestinal tract of chicken during postnatal development

Adenosine deaminase (ADA; EC 3.5.4.4) is a purine catabolic enzyme causing hydrolytic deamination of adenosine and 2'-deoxyadenosine to inosine and 2'-deoxyinosine. In the present study, the normal endogenous activity level of ADA was investigated in different regions of the gastrointestinal tract (GIT) during postnatal development of chicken. The effects of corticosterone and dibutyryl cAMP (Bt(2)-cAMP) were studied at two selected postnatal ages. The results indicated a significantly high level of ADA at day 1 in all the regions of GIT, which then declined (-34% in esophagus, -35% in crop, and -48% in small intestine) at day 10 and remained fairly constant till day 90. While in the proventiculus, the activity of ADA decreased (-30%) at day 30 and showed further decline (-52%) at day 90 as compared to day 1. Corticosterone was seen to significantly decrease (-23 to 79%) the activity level, depending on the regions of GIT studied except proventriculus. The magnitude of decline was more pronounced at day 60 compared to day 10. Bt(2)-cAMP, on the other hand, caused a significant increase (+21 to 67%) in the activity level of ADA again depending on the regions of GIT studied except crop. Western blot analyses also depicted that the decrease and/or increase, respectively, of ADA activity by corticosterone and Bt(2)-cAMP was at the ADA protein level. In conclusion, the study suggests that the ADA activity level is highest at day 1 in all the regions of chicken GIT and could be reduced or enhanced by corticosterone and dibutyryl cAMP, respectively, in an age-specific manner.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Modification of cytokine milieu by A2A adenosine receptor signaling--possible application for inflammatory diseases

Pro-inflammatory cytokine TNF-alpha (TNF) production from in vitro lipopolysaccharide (LPS)-stimulated human peripheral blood CD14+ cells (PB-CD14) was inhibited by A2A adenosine receptor (AdoR) (A2AR) or beta2 adrenergic receptor (ADR) (beta2R) signaling in a concentration-dependent manner. These inhibitory effects were presumably mediated by the increase in intracellular cAMP. Furthermore A2AR agonist and beta2R agonist synergistically inhibited the TNF production of LPS-stimulated PB-CD14 cells. These results suggest that the anti-inflammatory effect of extracellular adenosine is, at least in part, due to the modification of the cytokine milieu via A2A signaling, and that the targeting of both A2AR and beta2R may have strong therapeutic potential for the inflammatory diseases.

Targeting G protein-coupled A2a adenosine receptors to engineer inflammation in vivo

G protein-coupled adenosine receptors are the subject of intense study as immunomodulators of inflammation especially since the recent demonstration that the A2a receptor acts to down-regulate inflammation and inhibit tissue damage in vivo [Nature 414 (6866) (2001) 916]. The adverse effects of overactive inflammation are evident in diseases e.g. sepsis, rheumatoid arthritis, and multiple sclerosis underscoring the importance of inhibiting inflammation or selectively enhancing inflammatory processes. It has been shown recently that the A2a adenosine receptor is a critical component of an endogenous "immunosuppressive loop" in which extracellular adenosine that accumulates due to local hypoxia caused by inflammatory insult signals through cAMP-elevating A2a receptors in a delayed negative feedback manner. Understanding how tissues regulate inflammation will provide the information necessary to allow for the engineering, or selective targeting, of endogenous inflammatory pathways. Recognition of A2a receptors as "natural" or endogenous brakes of inflammation provides the intellectual scaffolding needed to pursue these goals.