Autoantibodies against complement receptor 1 (CD35) in SLE, liver cirrhosis and HIV‐infected patients

SUMMARY 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