Pharmacological studies suggest that A 2B adenosine receptors mediate proinflammatory effects of adenosine. This concept was recently challenged by the finding that A 2B adenosine receptor knockout (A 2B KO) mice had moderate inflammation due to elevated basal plasma tumor necrosis factor (TNF)-␣ and an exaggerated response to lipopolysaccharide (LPS) challenge. However, it is unclear whether this phenomenon actually reflects the loss of putative taming of proinflammatory cytokine production via activation of A 2B receptors by endogenous adenosine. In this report, we examined adenosine receptor-dependent regulation of interleukin (IL)-6 and TNF-␣ blood plasma levels in A 2B KO and wild-type mice in vivo and their release from peritoneal macrophages ex vivo. Stimulation of adenosine receptors with 5Ј-N-ethylcarboxamidoadenosine (NECA) up-regulated IL-6 and suppressed LPS-induced TNF-␣ in wild-type mice. The selective A 2B antagonists 3-isobutyl-8-pyrrolidinoxanthine and 8-[4-[((4-cyanophenyl)carbamoylmethyl)oxy]phenyl]-1,3-di(npropyl)xanthine (MRS 1754) inhibited NECA-induced IL-6 release but not the suppression of LPS-induced TNF-␣ secretion from macrophages. Genetic ablation of A 2B receptors abrogated NECA-induced increases in IL-6 release from mouse peritoneal macrophages and dramatically reduced the ability of NECA to raise IL-6 plasma levels in vivo. In contrast, the absence of A 2B adenosine receptors did not affect NECA-induced suppression of LPS-activated TNF-␣ release in macrophages, nor did it reduce the ability of NECA to suppress LPS-induced increase in TNF-␣ plasma levels in vivo. Thus, our results indicate that stimulation of A 2B receptors up-regulates the proinflammatory cytokine IL-6 and argue against the recently suggested anti-inflammatory role of A 2B receptors in suppression of LPS-stimulated TNF-␣ production by adenosine.There is growing evidence that adenosine plays an important role in the regulation of inflammation. Adenosine is an intermediate product of adenine nucleotide metabolism, and it also serves as a signaling molecule that can modulate cellular functions via binding to cell surface G-proteincoupled receptors of the P1 purinergic family comprising A 1 , A 2A , A 2B , and A 3 adenosine receptor subtypes. Among adenosine receptors, the A 2B subtype has the lowest affinity to adenosine. Although A 2B receptors are likely to remain silent under normal physiological conditions (Fredholm et al., 2001b), they can be activated during inflammation when interstitial adenosine concentrations are increased as a result of cell stress, injury, and tissue hypoxia (Fredholm et al., 2001a).Many studies suggest that A 2B receptors are involved in adenosine-dependent regulation of proinflammatory paracrine factors. We have shown previously that stimulation of A 2B receptors in human mast cell line HMC-1 increased production of proinflammatory cytokines interleukin (IL)-1, IL-3, IL-4, IL-8, and IL-13 (Feoktistov and Biaggioni, 1995;Ryzhov et al., 2004). Studies in human primary cell cultur...