Adenosine deaminase (ADA; EC 3.5.4.4) activity is elevated in the synovial fluid (SF) of patients with rheumatoid arthritis (RA). Since the antiinflammatory effect of methotrexate is reportedly associated with increased levels of extracellular adenosine, the present study was undertaken to clarify the role of 2 ADA isozymes, ADA1 and ADA2, in the pathogenesis of RA.

The activities of ADA1 and ADA2 were measured in SF from RA and osteoarthritis (OA) patients, in sera from RA patients, and in lysates prepared from mononuclear and polymorphonuclear cells from SF from RA patients, peripheral blood from RA patients, and fibroblast‐like synoviocytes (FLS) from RA and OA patients. Also measured were the effects of proinflammatory cytokines on ADA1 activity and ADA messenger RNA (mRNA) expression in RA FLS, as determined using real‐time polymerase chain reaction. The adenosine concentration in RA SF was measured by radioimmunoassay.

The adenosine concentration in RA SF ranged from 0.027 μM to 0.508 μM (mean ± SD 0.156 ± 0.132 μM). At those concentrations, ADA1 would be expected to be functionally dominant due to its higher affinity for adenosine. ADA1 activity in RA SF (mean ± SD 14.4 ± 8.5 IU/liter) was significantly higher than that in OA SF (3.0 ± 1.1 IU/liter) or RA sera (3.0 ± 0.6 IU/liter); moreover, ADA1 activity in RA FLS lysate was the highest among the cell lysates tested. Proinflammatory cytokines did not affect ADA1 activity or ADA mRNA expression in RA FLS.

Elevated ADA1 activity is an intrinsic characteristic of RA FLS, which likely contributes to the pathogenesis of RA by neutralizing the antirheumatic properties of endogenous adenosine.