TT30 inhibits CAP-mediated hemolysis of rabbit RBC and this inhibition is reversed by anti-CR2 antibody. (A) TT30 inhibits CAP-mediated lysis of rabbit RBC by human complement serum. Rabbit RBC were incubated with human complement-preserved serum containing serial dilutions of TT30 under experimental conditions promoting CAP activity and subsequent hemolysis. The amount of free hemoglobin correlated with the color intensity and was measured as absorbance at 415 nm. Percent lysis was calculated as (A415_{ser ×}- A415_{ser × bkgd})/(A415_water) × 100 and the data were graphed using Excel. TT30 showed high potency (IC₅₀ = 0.9 ± 0.008μM). Results represent mean ± SEM of duplicate values from 5 independent experiments. (B) Reversal of TT30-mediated inhibition of CAP-induced hemolysis of rabbit RBC by anti-CR2 monoclonal antibody (clone 1048). TT30 was serially diluted and spiked into human serum in presence of excess anti-CR2 antibody (1 mg/mL) and added to rabbit RBC under conditions promoting CAP activation in the ELISA assay format. Results represent mean ± SEM of duplicates from 3 independent experiments.