Autoantibodies Against Complement Receptor 1 (CD35) in SLE, Liver Cirrhosis and HIV-infected Patients

The acquired loss of CR1 (CD35) on erythrocytes in specific autoimmune diseases and chronic infections may be due to autoAb against CR1. An ELISA using rCR1 was established to measure antiCR1 IgG autoAb. Plasma containing alloAb to polymorphism on CR1 (Knops blood group Ab) reacted strongly against rCR1 and were used as positive controls. AntiCR1 Ab was found in 3/90 (3.5%) plasma samples from healthy blood donors. The binding of these Ab was not inhibited by high salt concentrations. AntiCR1 Ab were present in the IgG fractions of plasma, and they bound to rCR1 on Western Blot. Affinity chromatography on rCR1-sepharose depleted the plasma of antiCR1, and the acid-eluted fractions contained the antiCR1 Ab. An increased frequency of antiCR1 autoAb was found in patients with SLE (36/78; 46%), liver cirrhosis (15/41; 36%), HIV infection (23/76; 30%) (all P < 0.0001), and in patients with anticardiolipin Ab (4/21; 19%, P < 0.01) multiple sclerosis (7/50; 14%, P < 0.02), and myeloma (autoAb (8/56; 14%, P < 0.02), but not in those with acute poststreptococcal glomerulonephritis (1:32; 3%). Because C1q binds to CR1, antiC1q Ab were analysed in the same patients. There was no correlation between levels of antiC1q and antiCR1 autoAb. In HIV patients, levels of antiCR1 did not correlate with low CR1 levels expressed on erythrocytes or soluble CR1 in plasma. The binding of antiCR1 autoAb to rCR1 fixed on ELISA plates was not inhibited by soluble rCR1 or by human erythrocyte CR1, in contrast to alloAb and one SLE serum, which induced partial blockade. Thus, antiCR1 autoAb recognize mostly CR1 epitope(s) not present on the native molecule, suggesting that they are not directly involved in the loss of CR1. Rather antiCR1 autoAb might indicate a specific immune response to denatured CR1.