Complement activation by phospholipids: the interplay of factor H and C1q

Abstract

Complement proteins in blood recognize charged particles. The anionic phospholipid (aPL) cardiolipin binds both complement proteins C1q and factor H. C1q is an activator of the complement classical pathway, while factor H is an inhibitor of the alternative pathway. To examine opposing effects of C1q and factor H on complement activation by aPL, we surveyed C1q and factor H binding, and complement activation by aPL, either coated on microtitre plates or in liposomes. Both C1q and factor H bound to all aPL tested, and competed directly with each other for binding. All the aPL activated the complement classical pathway, but negligibly the alternative pathway, consistent with accepted roles of C1q and factor H. However, in this system, factor H, by competing directly with C1q for binding to aPL, acts as a direct regulator of the complement classical pathway. This regulatory mechanism is distinct from its action on the alternative pathway. Regulation of classical pathway activation by factor H was confirmed by measuring C4 activation by aPL in human sera in which the C1q:factor H molar ratio was adjusted over a wide range. Thus factor H, which is regarded as a down-regulator only of the alternative pathway, has a distinct role in downregulating activation of the classical complement pathway by aPL. A factor H homologue, β2-glycoprotein-1, also strongly inhibits C1q binding to cardiolipin. Recombinant globular domains of C1q A, B and C chains bound aPL similarly to native C1q, confirming that C1q binds aPL via its globular heads.

Interactions of complement proteins C1q and factor H with lipid A and Escherichia coli: further evidence that factor H regulates the classical complement pathway

“…These results show that C1q and factor H bind to lipid A and that the recombinant globular head regions of C1q bind differentially to lipid A. As noted previously (Tan et al, 2010), the oligomerization state of the recombinant gHA, gHB, gHC proteins varies, although they are mostly in trimer form. These materials may exhibit higher, but more usually lower, binding affinity than C1q, depending on polymerization state.…”

“…Lowest binding to lipid A was seen with ghA and ghB but this was still above the background levels observed with negative control wells coated with phosphatidylcholine (PC). Cardiolipin (CL)-coated wells were used as a positive control (Tan et al, 2010).…”

“…We have recently found that C1q and factor H bound to immobilized as well as liposomal anionic phospholipids, most notably to cardiolipin. Factor H strongly inhibited C1q binding to solid phase cardiolipin and cardiolipin liposomes, suggesting a role for factor H in regulating activation of the classical complement pathway by anionic phospholipids (Tan et al, 2010). In the work presented here, we investigated whether lipid A, a well-established classical pathway activator, also interacts with factor H as well as C1q, and followed this by tests of binding to a laboratory strain of E. coli as a source of lipid A.…”

“…Complement activation by the classical pathway occurs when the first component of complement, C1, binds via charge interactions to immune complexes containing IgG or IgM or directly to non-immunoglobulin target surfaces such as anionic phospholipids and Gram-negative bacteria (Sim and Malhotra, 1994;Gaboriaud et al, 2003;Kojouharova et al, 2004;Kishore et al, 2004a: Roumenina et al, 2006Tan et al, 2010). C1 is made up of three glycoproteins, C1q, C1r and C1s.…”

Human c1q induces apoptosis in an ovarian cancer cell line via tumor necrosis factor pathway

“…Human C1q was purified as published earlier (12). Briefly, freshly thawed human plasma was made 5 mM EDTA, centrifuged at 5,000 × g for 10 min, and any aggregated lipids were removed using Whatmann filter paper (GE Healthcare, UK).…”