ELISAs of LPS-induced AP complement activation. (A) ELISA detection of LPS on LPS-coated plates. (B) AP complement activation by plate-bound S typhosa LPS in wild-type (WT) or properdin knockout (P^−/−) mouse serum. To reconstitute AP activity in properdin^−/− mouse serum, C3^−/− serum or purified human properdin (hP) was premixed with properdin^−/− mouse serum. Alternatively, LPS-coated plates were incubated with hP and washed (hP coat) before exposure to properdin^−/− serum. Similar assays were performed with plate-bound LPS from S minnesota (S) (C) or E coli (D). (E,F) ELISAs of hP interaction with plate-bound LPS. Plates were first coated with different concentrations of LPS and then incubated with a fixed concentration of purified hP (62.5 ng/well; E). (F) Plates were first coated with a fixed concentration of LPS (5 μg/mL) and then incubated with increasing concentrations of purified hP. After washing, the amount of plate-bound properdin was detected by antiproperdin antibodies. OD, optical density.