CFHR1 is a regulator of the alternative pathway of complement. (A) Hemolysis of sheep erythrocytes in the presence of CFHR1- and CFH-depleted normal human plasma (HP_ΔCFH). Sheep erythrocytes represent nonactivator surfaces when incubated in complement active human plasma. However, when the same cells are incubated in HP_ΔCFH, these cells represent activator surfaces and are lysed. Factor H acts as a surface protector and reverts the effect (). Addition of CFHR1 (5-160 μg/mL) results in a reduction of erythrocytes lysis, which indicates a regulatory effect of this protein in complement control (▲). Similar inhibition of hemolysis is observed with vitronectin (). HSA has no effect on hemolysis (■). Data show 1 representative of 3 experiments. A₄₁₅ indicates absorbance at 415 nm. (B) CFHR1 does not affect C3a generation of an in vitro–assembled C3 convertase. C3 was incubated with factor D and factor B, and C3 convertase activity was analyzed by comparing C3a before (column 3) and after (column 4) addition of C3. Addition of CFHR1 (25 or 50 μg/mL) did not significantly effect C3a generation (columns 5-6). CFH (50 μg/mL) strongly effected C3 convertase activity (column 7). Addition of human serum albumin (HSA) did not affect C3a generation (column 8). C3 mAb, which reacts with C3a (1 μg/mL, standard; column 2) did not react with an empty well (co). A representative result of 3 independent experiments is shown and SDs are given. A₄₉₀ indicates absorbance at 490 nm.