AbstractSince abnormal regulation of interleukin‐2 (IL‐2) has been demonstrated in rheumatoid arthritis (RA), the functional role of low‐affinity soluble IL‐2 receptors (sIL‐2R) purified from RA synovial fluids (SF) was studied. Picomolar levels of sIL‐2R were detected in RA SF using an enzyme‐linked immunosorbent assay. Levels were higher in serum and SF from RA patients than in controls (P<0.001) and higher in RA SF than in paired RA serum (P<0.01). Soluble IL‐2R from RA SF had estimated molecular weights of 40‐50 kd and 80–100 kd by gel filtration analysis. The 80–100‐kd peak is likely to be a dimer of the 40–50‐kd peak, since a single 45‐kd peak was found after elution from sodium dodecyl sulfate‐polyacrylamide gels. Since inhibitory activity for lymphocyte proliferation was found in the 80–100‐kd range, the sIL‐2R were purified with an anti‐CD25 affinity column and further analyzed. The purified fractions did not interfere with the proliferation of mitogen‐stimulated lymphocytes or with the binding of radiolabeled IL‐2 to CTLL‐2 cells, although direct binding of IL‐2 was demonstrated. The affinity of sIL‐2R from RA SF for binding IL‐2 was in the range of 25 nM, which is similar to the affinity of sIL‐2R purified from a human T cell clone, indicating that both sIL‐2R are low‐affinity receptors for IL‐2. We conclude that the concentration and binding affinity of low‐affinity sIL‐2R purified from RA
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