The role of the complement system in cancer
Vahid Afshar-Kharghan
Vahid Afshar-Kharghan
Published March 1, 2017
Citation Information: J Clin Invest. 2017;127(3):780-789. https://doi.org/10.1172/JCI90962.
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Review

The role of the complement system in cancer

Abstract

In addition to being a component of innate immunity and an ancient defense mechanism against invading pathogens, complement activation also participates in the adaptive immune response, inflammation, hemostasis, embryogenesis, and organ repair and development. Activation of the complement system via classical, lectin, or alternative pathways generates anaphylatoxins (C3a and C5a) and membrane attack complex (C5b-9) and opsonizes targeted cells. Complement activation end products and their receptors mediate cell-cell interactions that regulate several biological functions in the extravascular tissue. Signaling of anaphylatoxin receptors or assembly of membrane attack complex promotes cell dedifferentiation, proliferation, and migration in addition to reducing apoptosis. As a result, complement activation in the tumor microenvironment enhances tumor growth and increases metastasis. In this Review, I discuss immune and nonimmune functions of complement proteins and the tumor-promoting effect of complement activation.

Authors

Vahid Afshar-Kharghan

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Figure 1

Complement activation.

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Complement activation. (A) The classical pathway is initiated by a compl...
(A) The classical pathway is initiated by a complement-fixing antibody binding to an antigen on targeted cells. C1q binds to the antibody’s Fc domain in the antibody-antigen complex. C1r and C1s assemble on C1q, C1r cleaves and activates C1s, and activated C1s cleaves C4 and C2 into C4b and C2a, respectively. C4b and C2a form the C3 convertase C4bC2a. (B) In the lectin pathway, MBL binds to repetitive sugar moieties such as mannose. MBL and MASP2 then form a C1-like complex. Activated MASP2 in MBL-MASP2 complex cleaves C4 and C2 and generates C3 convertase (C4bC2a). (C) In the alternative pathway, small amounts of hydrolyzed plasma C3 [C3(H2O)] bind to factor B, which forms the C3(H2O)Bb complex with help from factor D. C3(H2O)Bb cleaves additional plasma C3 to generate highly active C3b, which binds to cell the surface. On a complement-activating surface, C3b binds Bb (produced by factor D–mediated cleavage of factor B) and generates C3bBb (the alternative pathway’s C3 convertase). (D) Regardless of the initiation steps, C3 convertase deposits additional C3b molecules and generates C3a. If it remains intact, C3 convertase binds to additional C3b to generate C5 convertase. C5 convertase cleaves C5 to generate C5b. (E) C5b binds to C6, C7, and C8, forming a C5b-8 complex, which polymerizes several C9 molecules, forming the cytolytic MAC.