Initial properdin binding contributes to alternative pathway activation at the surface of viable and necrotic cells

Properdin, the only known positive regulator of the complement system, stabilizes the C3 convertase, thereby increasing its half‐life. In contrast to most other complement factors, properdin is mainly produced extrahepatically by myeloid cells. Recent data suggest a role for properdin as a pattern recognition molecule. Here, we confirmed previous findings of properdin binding to different necrotic cells including Jurkat T cells. Binding can occur independent of C3, as demonstrated by HAP‐1 C3 KO cells, excluding a role for endogenous C3. In view of the cellular source of properdin, interaction with myeloid cells was examined. Properdin bound to the surface of viable monocyte‐derived pro‐ and anti‐inflammatory macrophages, but not to DCs. Binding was demonstrated for purified properdin as well as fractionated P2, P3, and P4 properdin oligomers. Binding contributed to local complement activation as determined by C3 and C5b‐9 deposition on the cell surfaces and seems a prerequisite for alternative pathway activation. Interaction of properdin with cell surfaces could be inhibited with the tick protein Salp20 and by different polysaccharides, depending on sulfation and chain length. These data identify properdin as a factor interacting with different cell surfaces, being either dead or alive, contributing to the local stimulation of complement activation.