Adenosine provokes bronchoconstriction in asthmatics through acute activation of mast cells, but its potential role in chronic inflammation has not been adequately characterized. We hypothesized that adenosine up-regulates Th2 cytokines in mast cells, thus promoting IgE synthesis by B lymphocytes. We tested this hypothesis in human mast cells (HMC-1) expressing A 2A, A 2B, and A 3 adenosine receptors. The adenosine analog 5′-N-ethylcarboxamidoadenosine (NECA)(10 μM) increased mRNA expression of IL-1β, IL-3, IL-4, IL-8, and IL-13, but not IL-2 and IFN-γ. Up-regulation of IL-4 and IL-13 was verified using RT-PCR and ELISA; 10 μM NECA increased IL-13 concentrations in HMC-1 conditioned medium 28-fold, from 7.6±0.3 to 215±4 pg/ml, and increased IL-4 concentrations 6-fold, from 19.2±0.1 to 117±2 pg/ml. This effect was mediated by A 2B receptors because neither the selective A 2A agonist 2-p-(2-carboxyethyl) phenethylamino-NECA nor the selective A 3 agonist N 6-(3-iodobenzyl)-N-methyl-5′-carbamoyladenosine reproduced it, and the selective A 2B antagonist 3-isobutyl-8-pyrrolidinoxanthine prevented it. Constitutive expression of CD40 ligand on HMC-1 surface was not altered by NECA. Human B lymphocytes cocultured for 12 days with NECA-stimulated HMC-1 produced 870±33 pg IgE per 10 6 B cells, whereas lymphocytes cocultured with nonstimulated HMC-1, or cultured alone in the absence or in the presence of NECA, produced no IgE. Thus, we demonstrated induction of IgE synthesis by the interaction between adenosine-stimulated mast cells and B lymphocytes, and suggest that this mechanism is involved in the amplification of the allergic inflammatory responses associated with asthma.