To test fH dependent decay accelerating activity, washed E-C3b cells were resuspended to 2% (v/v) in AP buffer (5 mM sodium barbitone pH 7.4, 150 mM NaCl, 7 mM MgCl2, 10 mM EGTA) and AP convertase was formed on the cell surface by incubating with fB 42μg/ml (0.46μM) and fD (0.4μg/ml) at 37°C for 15 minutes. 1/25 volume of PBS/0.25M EDTA was added to prevent further enzyme formation and cells (50μl) were mixed and incubated with 50μl of fH (serial dilution from 15.4μg/ml (99nM)) in PBS/10mM EDTA for 12 minutes. Lysis was developed by adding 50μl NHSΔBΔH (4%, v/v) in PBS/EDTA and incubating at 37°C for 20 minutes. To calculate lysis, cells were pelleted by centrifugation, and hemoglobin release was measured by absorbance at 415 nm. Control incubations included 0%lysis (buffer only) and 100%lysis (0.1% Nonidet-P40). Percentage lysis 100*(A415 test sample-A415 0% control)/(A415 100% control-A415 0% control).
To test fH cofactor activity, washed EA-C3b cells were resuspended to 2% in AP buffer and incubated with an equal volume of different concentrations of fH as indicated and constant fI (2.5μg/mL) for 7 minutes at 22°C. After three washes in AP buffer, 50μl cells (2%) were mixed with 50μl of 70μg/ml fB (0.75μM; fB32R or fB32Q) and fD (0.4μg/ml) and incubated for 10 minutes at 22°C to form convertase on residual C3b (EA-C3bBb). Lysis was developed by adding 50μl NHSΔBΔH (4%, v/v) in PBS/EDTA and incubating at 37°C for 20 minutes. Percentage lysis was calculated as described above. To assess the effect on lysis by combining different polymorphic variants of fB and fH, the above two assays were combined and modified as follows. EA-C3b cells were incubated with 80ng/ml (0.5nM) fH-Ile62 or fH-Val62 variant and 2.5μg/ml fI for 7 minutes at 22°C. Washed cells were incubated as described above with different concentrations of fBArg32 or fBGln32, fD and properdin (1μg/ml) and lysis was developed using NHSΔBΔH.