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.

beta 2-Glycoprotein I-cardiolipin complexes are reported to be a target antigen for the binding of a subset of anti-phospholipid antibodies. The characteristics of binding of beta 2-glycoprotein I to cardiolipin are reported in this paper. Binding at neutral pH is specific, saturable, dependent on ionic strength and independent of bivalent cation. Binding at low pH is qualitatively different from that at neutral pH, and is not dependent on ionic strength. Denaturation of beta 2-glycoprotein I by heat inactivation and reduction/alkylation indicates that beta 2-glycoprotein I-cardiolipin interaction does not require the native three-dimensional structure of beta 2-glycoprotein I, implying that a linear sequence motif may be responsible. Modification of amino acid residues by KCNO treatment completely destroys binding capacity, indicating crucial involvement of lysine residues in binding of beta 2-glycoprotein I to cardiolipin. Complement factor H, which has some similar highly charged linear sequence motifs to beta 2-glycoprotein I and is composed of the same type of protein module, was found to bind to cardiolipin and inhibit the binding of beta 2-glycoprotein I to cardiolipin. Three different lysine-rich segments of the fifth domain of beta 2-glycoprotein I may be involved in binding to cardiolipin.

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