Heme Interacts with C1q and Inhibits the Classical Complement Pathway

Abstract

C1q is the recognition subunit of the first component of the classical complement pathway. It participates in clearance of immune complexes and apoptotic cells as well as in defense against pathogens. Inappropriate activation of the complement contributes to cellular and tissue damage in different pathologies, urging the need for the development of therapeutic agents that are able to inhibit the complement system. In this study, we report heme as an inhibitor of C1q. Exposure of C1q to heme significantly reduced the activation of the classical complement pathway, mediated by C-reactive protein (CRP) and IgG. Interaction analyses revealed that heme reduces the binding of C1q to CRP and IgG. Furthermore, we demonstrated that the inhibition of C1q interactions results from a direct binding of heme to C1q. Formation of complex of heme with C1q caused changes in the mechanism of recognition of IgG and CRP. Taken together, our data suggest that heme is a natural negative regulator of the classical complement pathway at the level of C1q. Heme may play a role at sites of excessive tissue damage and hemolysis where large amounts of free heme are released.

Non-Canonical Heme-Binding Proteins

“…Further, formation of a C1q-heme complex changed the mechanisms of recognition of IgG and CRP. 200 Binding of ferric heme to C1q was accompanied by an increase in the absorption at 400-405 nm ( Figure 13). Direct titration with hemin showed that the binding stoichiometry was 12 hemes per C1q molecule.…”

“…200 Based on these results, it was proposed that heme could function as a natural inhibitor of the complement pathway, an activity that will be exacerbated under conditions of excessive tissue damage and hemolysis, where large amounts of heme are released into the circulation. 200 Molecular communication between proteins and their interacting partners underlies the rich biochemistry of living organisms. 201 About 20% of human antibodies bind heme as an interface cofactor.…”

“…From Ref. 200 resonance experiments determined that heme binds to C1q with a binding affinity of 1-2 µM. The interaction of heme with C1q featured slow association (k on = 3.2 × 10 −3 M −1 s −1 ) and relatively fast dissociation (k off = 3.44 × 10 −3 s −1 ) ( Figure 13).…”

Endothelial cells: source, barrier, and target of defensive mediators

“…Heme induces an oxidative stress and stimulates the EC via TLR4, leading to vWF and P‐selectin expression . Heme is also a direct activator of the C3 convertase and can stimulates the alternative but inhibits the classical pathway in vitro and in vivo in a mouse model of hemolysis (N. Merle and L. Roumenina, unpublished). Aside from inducing a pro‐thrombotic phenotype and activating complement heme inhibits ADAMTS‐13 activity, or activation of coagulation cascade .…”

Regulatory FeII/III Heme: The Reconstruction of a Molecule's Biography

“…Upon binding of Fe III heme to C1q the mechanism of recognition between immunoglobulin G and C‐reactive protein changes. This leads to reduced binding and thus prevents activation of further complement compounds 42a. In vitro studies of the interaction of recombinant blood coagulation factor VIII and Fe II/III heme revealed an inhibitory effect 43.…”