The complement anaphylatoxin 3a (C3a) and adenosine receptors (AR) are implicated in the inflammatory process in allergic rhinitis although a direct interaction between these pathways has not been demonstrated. To investigate the interaction between these pathways, primary human cultured mast cells (HuMC) and human mast cell line (LAD2) were stimulated with C3a with or without AR agonists and antagonists. The non-selective AR agonist, 5'-N-ethylcarboxamidoadenosine (NECA; 30 uM), inhibited C3a-mediated HuMC and LAD cell degranulation (43 + 4 and 40 + 5 % inhibition respectively compared to control). NECA also blocked C3a-activated production of MCP-1 (57 + 6 %) and C3a-activated chemotaxis (46.7 + 10 %). A selective A2aR agonist, CGS 21680, inhibited C3a-mediated degranulation but the A3R agonist, IB-MECA, had no effect suggesting that inhibition of degranulation was mediated by A2aR. An A2aR antagonist, SCH 58261, blocked the inhibitory effect of NECA but an A2bR and A3R antagonist had no effect. Real-time PCR analysis showed that LAD2 and HuMC expressed mRNA for A2aR, A2bR and A3R but not A1R. Measurements of intracellular cAMP showed that NECA elevated [cAMP]i levels by at least 30% even in C3a-activated cells. Pertussis toxin blocked C3a-activated degranulation. The adenylyl cyclase inhibitor, SQ 22536, had no effect on C3a-activated degranulation but blocked the effect of NECA demonstrating that C3a and NECA mediate their effects through Gαi and Gαs proteins respectively. These results show that adenosine inhibits complement activation of human mast cells through a Gαs-protein pathway.

Funded by a grant from the Ernest S. Bazely Trust and intramural NRC funds