A radioimmunoassay with the use of soluble 125I-Fab monoclonal anti-CR1 and rabbit IgG anti-CR1 bound to Staphylococcus aureus particles was employed to detect and quantitate CR1 antigen in human plasma. Among 16 normal individuals the concentration of soluble CR1 in plasma ranged from 13 to 81 ng/ml, and a similar range of concentration was found in plasma from 15 patients having systemic lupus erythematosus (SLE). The amount of plasma CR1 in normal donors, but not in SLE patients, significantly correlated with the number of CR1 sites on erythrocytes (r = 0.90, p less than 0.001), and was 7.1% of the amount of receptor that was present on erythrocytes in blood. The concentration of soluble CR1 was not diminished by ultracentrifugation or ultrafiltration of plasma, was not affected by various modes of anti-coagulation or even by clotting of blood, and did not change during incubation of blood at 4 degrees C for up to 4 hr. On sucrose density gradient ultracentrifugation of plasma the CR1 was distributed as a broad peak that overlapped the plasma protein profile. The Mr of plasma CR1 was identical to that of erythrocyte CR1 when assessed by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate and immunoblotting. In addition, the plasma form of CR1 exhibited the same structural phenotype as did receptor from erythrocytes of the same individual. CR1 antigen purified from plasma was as active as CR1 from erythrocytes in promoting the cleavage by factor I of C3b to iC3b, C3c, and C3dg. Therefore, a functionally and structurally intact form of soluble CR1 resides in plasma.

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