Low number of complement C3b/C4b receptors (CR1) on erythrocytes from patients with essential mixed cryoglobulinemia, systemic lupus erythematosus and rheumatoid arthritis: relationship with disease activity, anticardiolipin antibodies, complement activation and therapy.

Our aim was to assess whether the amount of complement C3b/C4b receptors (CR1) on erythrocytes shows a correlation to disease activity in various connective tissue diseases such as systemic lupus erythematosus (SLE), rheumatoid arthritis (RA) and essential mixed cryoglobulinemia (EMC). Using an anti-CR1 monoclonal antibody, 26 patients with SLE, 34 with RA and 22 patients with EMC were investigated for erythrocyte CR1 expression. The control group consisted of 30 healthy individuals. The mean number of CR1/erythrocyte in the control group was 568 +/- 197 (range 174-1060), significantly higher than studied (EMC:379 +/- 248; p = 0.0005;SLE 147 +/- 56, p less than 0.0001; RA 298 +/- 177, p less than 0.0001). In patients with RA and in SLE, but not in patients with EMC, the number of CR1 numbers and anticardiolipin antibody (aCl) titers (r2 = 0.493; p = 0.034). A statistically significant correlation between CR1 numbers and CH50 values was found in patients with SLE, while in 3 patients with RA 4 months of therapy with cyclosporine A led to a further 30% reduction in CR1 number. Our conclusions are that (a) the decreased expression of erythrocyte CR1 is apparently a common feature of patients with various connective tissue diseases; (b) several acquired factors such as disease activity, complement activation, aCl and drugs may contribute to the loss of CR1 from erythrocytes; (c) in patients with RA and SLE, but not in patients with EMC, CR1 enumeration on erythrocytes may serve as a variable for clinical monitoring.