G2A Protects Mice against Sepsis by Modulating Kupffer Cell Activation: Cooperativity with Adenosine Receptor 2b [INNATE IMMUNITY AND INFLAMMATION]

G2A is a GPCR abundantly expressed in immune cells. G2A^–/– mice showed higher lethality, higher plasma cytokines, and an impaired bacterial clearance in response to a murine model of sepsis (cecal ligation and puncture), which were blocked by GdCl₃, an inhibitor of Kupffer cells. Anti–IL-10 Ab reversed the impaired bacterial clearance in G2A^–/– mice. Indomethacin effectively blocked both the increased i.p. IL-10 levels and the impaired bacterial clearance, indicating that disturbed PG system is the proximal cause of these phenomena. Stimulation with LPS/C5a induced an increase in Escherichia coli phagocytosis and intracellular cAMP levels in G2A^+/+ peritoneal macrophages but not G2A^–/– cells, which showed more PGE₂/nitrite release and intracellular reactive oxygen species levels. Heterologous coexpression of G2A and adenosine receptor type 2b (A2bAR) induced a synergistic increase in cAMP signaling in a ligand-independent manner, with the evidence of physical interaction of G2A with A2bAR. BAY 60-6583, a specific agonist for A2bAR, increased intracellular cAMP levels in Kupffer cells from G2A^+/+ but not from G2A^–/– mice. Both G2A and A2bAR were required for antiseptic action of lysophosphatidylcholine. These results show inappropriate activation of G2A^–/– Kupffer cells to septic insults due to an impaired cAMP signaling possibly by lack of interaction with A2bAR.

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