Loss of complement receptor type 1 (CR1) on ageing of erythrocytes. Studies of proteolytic release of the receptor.

Complement receptor type 1 (CR1) is a glycoprotein of Mr about 250 000 present on erythrocytes and other cell types. CR1 acts as a cofactor in the factor I-mediated breakdown of complement fragment C3b to form iC3b. Using an assay of cofactor activity, a wide variation in mean CR1 levels between erythrocytes from individual donors is observed. CR1 levels also decrease on ageing of erythrocytes in vivo, and again the rate of loss is widely variable between individuals. However, variable loss of CR1 during ageing of erythrocytes is likely to make only a minor contribution to the observed variation in mean CR1 levels. CR1 is very sensitive to proteolysis, and random proteolytic removal of CR1 from erythrocytes is likely to be an important factor in loss of CR1 on ageing of red cells in vivo. In vitro, mild trypsin treatment, plasmin or thrombin digestion of erythrocytes results in the loss of the factor I cofactor activity from the cell surface, and appearance of this activity in the supernatant. We conclude that an active fragment of CR1 is released from the cell surface on proteolysis. Subsequent prolonged trypsin treatment destroys most of the activity of this fragment. Proteolytic removal of CR1 from red cells may account not only for loss on ageing of cells, but also for the acquired CR1 deficiencies observed by others in systemic lupus erythematosus.