Introduction:
The complement system is made up of serum proteins that become activated in response to danger or infection, resulting in a cascade-like reaction that generates activatory signals to host cells (Kohl, 2006). Soluble pattern recognition receptors of the complement system such as C1q, mannose-binding lectin and ficolins recognize pathogen- or danger-associated molecular patterns (PAMPs and DAMPs) (Reis, Mastellos, Hajishengallis, & Lambris, 2019), triggering cleavage of complement components 4 and 2 (C4 and C2), the cleaved forms of which form together a C3 convertase, capable of cleaving the central complement component, C3, into C3b and C3a. While many copies of C3 can be cleaved by the C3 convertase before it decays, C3b can also bind to the C3 convertase and in turn form the C5 convertase, which then cleaves C5 into C5b and C5a. C5b in turn then binds to soluble proteins C6, C7 and C8, forming a complex that is then able to insert into lipid bilayers. This then catalyses recruitment of multiple copies of C9, which insert into the membrane, forming a pore known as the membrane attack complex (MAC), which is capable of lysing gram-negative bacteria (Doorduijn, Rooijakkers, & Heesterbeek, 2019).
Complement proteins are synthesized in large amounts by liver hepatocytes but also other cell types throughout the body, which ensures that complement is present not only at high concentrations in blood but also locally in tissues. Once activated by PAMPs or DAMPs, complement has 3 major outcomes: transmitting danger signals to the host, directly attacking pathogens, and covalently labeling pathogens or waste material for clearance (Kohl, 2006; Reis et al., 2019). When C3 or C5 are cleaved, the smaller split products, C3a and C5a, also known as the anaphylatoxins, can signal via specific receptors, C3aR and C5aR, as well as a second C5a receptor, C5L2. These receptors often stimulate responses linked to inflammation, including chemotaxis, cell activation, vasodilation, and smooth muscle contraction. The MAC is capable of directly lysing gram-negative bacteria, causing rapid death on exposure to serum, unless the bacteria express resistance mechanisms to escape complement attack (Lambris, Ricklin, & Geisbrecht, 2008). Finally, C3 itself covalently labels target material once cleaved to C3b, marking it for clearance by professional phagocytes.
The role of C3 in clearing material from the extracellular environment is long established. C3 is well-known as an opsonizing factor of extracellular material, involved in detection and clearance of pathogens as well as apoptotic cells and self-material (Ricklin, Reis, Mastellos, Gros, & Lambris, 2016). On cleavage to C3b, C3 changes conformation and reveals a reactive thioester group that forms a covalent bond with amine or carboxyl groups on target surfaces, leading to irreversible covalent binding of C3b. This activated C3b in turn is cleaved by serum factor I in combination with one of several cofactors to inactive C3b (iC3b), which is recognized by complement receptors 3 and 4 found on the surface of phagocytes, and ligation of which stimulates phagocytosis of iC3b-opsonised material. Additionally, material coated with deposited C3b is transported away by red blood cells expressing complement receptor 1, and is taken to either the liver or to secondary lymphatic organs for disposal by Kupffer cells or presentation to the adaptive immune system (Gonzalez et al., 2010), respectively. In this way, bacteria and virus particles are identified and removed, limiting infection, and self-material such as apoptotic cells (Trouw, Blom, & Gasque, 2008) is also cleared, limiting immune autoreactivity. Further final cleavage of iC3b forms C3d, which remains covalently bound via the thioester group, and is the ligand for complement receptor 2 (CR2), found on the surface of B-cells. CR2 ligation lowers the threshold for B-cell activation at least 1000-fold (Dempsey, Allison, Akkaraju, Goodnow, & Fearon, 1996), and C3 opsonisation of antigen is therefore an important molecular adjuvant for adaptive humoral immune responses. Complement therefore represents a sophisticated pathogen and danger detection system, able to mediate multiple responses in many host cell types due to production of a myriad of activation products. In particular, C3 activation products take several forms, with multiple specific receptors and interaction partners (Ricklin et al., 2016), and C3 is therefore involved in many effector functions within immune defence, as well as tissue homeostasis and development (Ricklin, Hajishengallis, Yang, & Lambris, 2010).
For further details of complement activation in pathogen detection and homeostasis in the extracellular environment, readers are referred to more comprehensive reviews (Reis et al., 2019; Ricklin et al., 2010). This article will however focus on more recent discoveries that complement and C3 may also have roles in detection and disposal of pathogens and unwanted material within the intracellularenvironment, via the process of autophagy. This review will therefore summarize the current evidence for involvement of complement in autophagy induction, both by transducing signals across the cell membrane, as well as potential roles within the cellular environment.