Suppression of inflammation by low-dose methotrexate is mediated by adenosine A2A receptor but not A3 receptor activation in thioglycollate-induced peritonitis

Low-dose weekly methotrexate has become the mainstay treatment of rheumatoid arthritis and psoriasis, and it is the gold standard by which other systemic medications are measured in both disorders [1,2]. Methotrexate has been used to treat other inflammatory diseases including ankylosing spondylitis, multiple sclerosis and inflammatory bowel disease, but its efficacy in the therapy of these conditions is far less impressive [3-7].An increasing body of evidence indicates that adenosine mediates, at least in part, the anti-inflammatory effects of methotrexate [8-13]. All known adenosine cell surface receptors (A1, A2A, A2B and A3) contribute to the modulation of inflammation, as demonstrated by many in vitro and in vivo pharmacologic studies (reviewed in [14,15]). We have previously demonstrated pharmacologically, using nonselective antagonists, that the anti-inflammatory effect of methotrexate is mediated by more than one subtype of adenosine receptor in the adjuvant arthritis model in the rat [16], and, using mice rendered deficient in A2A or A3 adenosine receptors, we found that both receptor subtypes are critical for the anti-inflammatory effects of methotrexate in the murine air pouch model of inflammation [17]. Since inflammation at different loci may be regulated by different cellular mechanisms, we determined whether the A2A and A3 receptors played similar roles in regulating inflammation in the peritoneum.We examined the pharmacologic mechanism by which methotrexate diminishes inflammation in the thioglycollate-induced peritoneal inflammation model of acute inflammation in the mouse. We report here that, similar to the air pouch, methotrexate treatment increases peritoneal exudate adenosine concentrations in wild-type mice, A2A receptor knockout mice and A3 receptor knockout mice but, in contrast to the air pouch model, diminishes leukocyte accumulation only in the peritoneal exudates of A3 receptor knockout and wild-type mice, not of A2A knockout mice. Simil