Search for adenosine A_2A spare receptors on peripheral human lymphocytes

Abstract

Some ligand–receptor couples involve spare receptors, which are apparent when a maximal response is achieved with only a small fraction of the receptor population occupied. This situation favours cross-reactions with low-affinity ligands, which may be detrimental for cell signaling. In the case of the adenosine A2A receptors (A2AR), which have an immunosuppressive effect on lymphocytes through cAMP production, the presence of spare A2AR remains to be established. We examined the situation using patients over-expressing lymphocyte A2AR and an agonist-like mAb to A2AR. We found that maximal mAb binding and functional response varied among the patients whereas the dissociation constant and half-maximal effective concentration had similar mean values (0.19 and 0.18 μM, respectively). Lymphocyte A2AR expression was correlated to plasma adenosine level and A2AR occupation but not to A2AR response. These results are consistent with a lack of a reserve of functional A2AR on human lymphocytes as a general rule and suggest that the amount and functional state of the expressed A2AR determine the maximal level of the lymphocyte response to adenosine.

Targeting Spare CC Chemokine Receptor 5 (CCR5) as a Principle to Inhibit HIV-1 Entry

“…In other words, this indicates that not all CCR5 are occupied by native chemokines to produce full G-protein activation. This property of an agonist is also observed in other receptor systems (34) where reserve receptors (also named spare receptors) exist and are not needed for the agonists to evoke a maximal functional response. Here we provide evidence indicating that a population of CCR5 is poorly accessible to native chemokines while being targeted by PSC-RANTES, supporting the concept of spare CCR5.…”

Spare Adenosine A2a Receptors Are Associated with Positive Exercise Stress Test in Coronary Artery Disease

“…While the K D for A 2A R of the agonist used here to mimic adenosine was similar in patients and controls, the EC 50 value (related to the biological effect triggered by the Spare A 2A R was first observed in guinea pig cardiac tissue using an irreversible A 2A R antagonist to block response to various agonists (13). Here, we used an agonist-like monoclonal antibody and provided further evidence for the presence of spare A 2A R on the surface of human mononuclear cells in relation to cardiac dysfunction, spare A 2A R having already been observed in a context of neurocardiogenic syncope, but not in healthy subjects (14,15,18).…”

“…Seventeen control patients (10 men and (14,15), with only the kappa light chain (25 kDa) being stained here using specific peroxidase-labeled antibodies and chemiluminescent substrate. The staining intensity was measured using densitometry analysis and expressed as AU (that is, the ratio of pixels generated by the light chain band to pixels generated by the background signal).…”

“…The procedure has been described previously (14,15). In brief, blood samples Western blot analysis of cell lysates and probing with the anti-A 2A R monoclonal antibody Adonis prior to densitometry analysis of the 45-kDa band.…”

“…The presence of spare A 2A R is evidenced when activation of only a weak fraction of A 2A R (evaluated using the K D variable) results in maximal cAMP production (evaluated using the EC 50 variable), and hence in maximal coronary vasodilation (13)(14)(15). Thus, the presence of spare A 2A R allows for rapid, transient responses that are sensitive to low agonist concentrations.…”