Adenosine A2A receptor modulates the oxidative stress response of primed polymorphonuclear leukocytes after parabolic flight

Space flight and gravitational stress can alter innate immune function. Parabolic flights (PFs) as a model for short-term gravitational changes prime the cytotoxic capability of polymorphonuclear leukocytes (PMNs). In view of the emerging role of adenosine in the regulation of innate immune responses, we examined the potency of adenosine to control the release of cytotoxic H2O2 by primed PMNs via the adenosine receptor system. During PFs, microgravity conditions (<10−2 G) are generated for approximately 22 seconds, followed by a hypergravity (1.8 G) phase resulting in gravitational stress. We studied the ex vivo effects of adenosine on the production of H2O2 by stimulated PMNs and determined adenosine plasma levels and adenosine A2A receptor transcripts of leukocytes of PF participants (n = 15). Increasing concentrations of adenosine dose dependently reduced tissue-toxic H2O2 production by PMNs with a half-maximal inhibitory concentration (IC50) of 19.5 nM before takeoff and 7.6 nM at 48 hours after PF. This increase in the adenosine-mediated inhibition of PMNs' H2O2 production was completely reversed by addition of the A2A receptor antagonist ZM241385. PF induced a nonsignificant elevation in adenosine plasma levels; A2A receptor mRNA from leukocytes remained almost unchanged. Adenosine limits the oxidative stress response of PMNs after PFs through an upregulation of the adenosine A2A receptor function. This stop signal on inflammation is stronger than that under normal physiologic states and may limit further cytotoxic damage. Pharmacologic manipulation of the adenosine A2A receptor pathway could be a potential target for control of unwanted exacerbations of cytotoxic PMN functions.