Properdin binding to complement activating surfaces depends on initial C3b deposition

Abstract

Two functions have been assigned to properdin; stabilization of the alternative convertase, C3bBb, is well accepted, whereas the role of properdin as pattern recognition molecule is controversial. The presence of nonphysiological aggregates in purified properdin preparations and experimental models that do not allow discrimination between the initial binding of properdin and binding secondary to C3b deposition is a critical factor contributing to this controversy. In previous work, by inhibiting C3, we showed that properdin binding to zymosan and Escherichia coli is not a primary event, but rather is solely dependent on initial C3 deposition. In the present study, we found that properdin in human serum bound dose-dependently to solid-phase myeloperoxidase. This binding was dependent on C3 activation, as demonstrated by the lack of binding in human serum with the C3-inhibitor compstatin Cp40, in C3-depleted human serum, or when purified properdin is applied in buffer. Similarly, binding of properdin to the surface of human umbilical vein endothelial cells or Neisseria meningitidis after incubation with human serum was completely C3-dependent, as detected by flow cytometry. Properdin, which lacks the structural homology shared by other complement pattern recognition molecules and has its major function in stabilizing the C3bBb convertase, was found to bind both exogenous and endogenous molecular patterns in a completely C3-dependent manner. We therefore challenge the view of properdin as a pattern recognition molecule, and argue that the experimental conditions used to test this hypothesis should be carefully considered, with emphasis on controlling initial C3 activation under physiological conditions.

Functional and structural insight into properdin control of complement alternative pathway amplification

“…Values in parentheses are for highest resolution shell. The EMBO Journal The structure and function of properdin Dennis V Pedersen et al been raised concerning whether FP actually performs C3b-independent pattern recognition (Harboe et al, 2012(Harboe et al, , 2017. Potentially FPc enables detailed structural analysis of FP and its complexes with small molecule ligands mimicking patterns recruiting FP independent of prior C3b deposition.…”

Properdin binds independent of complement activation in an in vivo model of anti–glomerular basement membrane disease

“…8,9,13,15,29 Conversely, in other cases properdin has been shown to act only as a positive regulator, requiring preceding C3b deposition to bind. 30,31 To better understand the mechanism of properdin binding and function in vivo, we explored an experimental model of anti-GBM disease in WT, properdin KO, and C3 KO mice.…”

Complement and Bacterial Infections: From Molecular Mechanisms to Therapeutic Applications

“…These results should be interpreted carefully, given that the experiments were all done in a serum environment in the presence of intact C3. Others have shown that, in serum, properdin binding to multiple targets is fully dependent on C3 [32].…”