Properdin is well-known as enhancer of the alternative complement amplification loop when C3 is activated, whereas its role as recognition molecule of exogenous pathogen-associated molecular patterns and initiator of complement activation is less understood. We therefore studied the role of properdin in activation of complement in normal human serum (NHS) by zymosan and various Escherichia coli strains. In ELISA, microtiter plates coated with zymosan induced efficient complement activation with deposition of C4b and TCC on the solid phase. Virtually no deposition of C4b or TCC was observed with MBL deficient serum. Reconstitution with purified MBL showed distinct activation in both readouts. In ELISA, NHS-induced deposition of properdin by zymosan was abolished by the C3 inhibiting peptide compstatin. Flow cytometry was used to further explore whether properdin acts as initial recognition molecule reacting directly with zymosan and three E. coli strains. Experiments reported by other authors were made with EGTA Mg++ buffer permitting auto-activation of C3. We found inhibition by compstatin on these substrates indicating that properdin deposition depended on initial C3b deposition followed by properdin in a second step. Properdin released from human polymorphonuclear cells (PMN) stimulated with phorbol-myristate acetate did not bind to zymosan or E. coli, but when incubated in properdin-depleded serum this form of properdin bound efficiently to both substrates in a strictly C3-dependent manner, since the binding was abolished by compstatin. Collectively, these data indicate that properdin in serum as well as PMN-released properdin is unable to bind and initiate direct alternative pathway activation on these substrates.
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