As early as 1888, a young Belgian microbiologist working at the Pasteur Institute on toxins identified a potential thermostable bactericidal factor. Working on various bacteria, Jules Bordet (1) had just found the source of the innate immune response through the identification of alexin. This element, present in the blood and "completing" the cells of the immune system, was later renamed the “complement system” by Paul Ehrlich (2).

After many years of investigation, the scientific community figured out that the complement system is actually composed of 60 different biological entities, divided into:

• 31 effector proteins; nine central components of the cascade (C1 to C9), multiple activation products (such as C3a and C3b), proteases and newly assembled enzymes (e.g. C4b2a and Factor B),
• 11 regulators and inhibitors factors (e.g. Factor H and C4BP) of the different activation pathways,
• 18 membrane receptors for the different parts of the complement system, including C3aR, C5aR anaphylatoxin receptors, member of the G protein-coupled receptors (GPCRs) family and well known for major immune-regulatory effects.

Effective destruction of unwanted bacteria requires a cascade of complement protein activation leading to the release of hidden active sites on proteins prior to their degradation. A kind of natural marking, contributing among other things to the elimination of pathogens, but also to pro-inflammatory activity, regulation of cytokine production, and removing of dead or modified cells.

There are three routes of activation cascade of the complement system: the classical pathway, the alternative pathway and the lectin pathway. Although they have different starting points, they eventually converge to the common process that leads to the assembly of the membrane attack complex (MAC). Each pathway is divided into four steps:

• Initiation,
• C3 convertase creation,
• C5 convertase activation,
• MAC formation.

• Activation starts with the meeting between immune complex and the C1q subunit. The latter interacts with the Fc fragments of antibodies (IgM or IgG) and can also bind to apoptotic cells, polyanonic molecules, C reactive protein and viral surface proteins.
• This leads to the activation of C1r that cleaves C1s to generate the C1 esterase: C1s.
• C1s cleaves C4 into C4a and C4b. While the C4a fragment is released in the medium, the C4b fragment binds to the surface of the antigen.
• C2 then binds to the C4b fragment and is cleaved by C1s into C2a and C2b fragment that remains bound to C4b to form the C3 convertase.
• The C3 convertase cleaves C3 molecules into two fragments C3a and C3b. C3a, with significant anaphylatoxic activity, is released into the environment. The large C3b fragment joins the C3 convertase to form the C5 convertase.
• The latter splits the C5 molecules into a small chemotactic C5a fragment and C5b, which binds to the surface of the activator.
• The C5b thus attached to the activator membrane allows the recruitment of C6 molecules and thus forms a C5b-6 complex. This complex recruits a C7 molecule, then C8 to form a multimolecular complex C5b-8, which serves as a receptor for several C9 molecules. The C5b-9n complex generates pores in the membrane of the activator and thus leads to its lysis. This is the membrane attack complex (MAC).

In 1954, Louis Pillemer discovered a new pathway, independent of any interaction with antibodies, and operating by direct contact with the pathogen (3). The discovery of the new serum protein "properdin" and its involvement in the stabilization of the convertase was strongly contested at that time, and eventually led to Pillemer's suicide.

Today, Pillemer's work is considered as a major breakthrough in immunology, and his details are summarized below.

• In the liquid phase, and in the absence of any activation, the C3 molecule hydrolyses spontaneously to form C3(H2O). In the presence of activator, C3(H2O) binds to it, and initiates the biochemical cascade of the alternative pathway.
• C3H2O attached to the activator binds factor B, which is cleaved into two fragments, Ba and Bb, by factor D. The Ba fragment is released into the medium, and the Bb fragment constitutes with C3H2O the alternate C3 convertase (C3H2OBb).
• The C3 convertase cleaves other C3 molecules into C3a and C3b. Newly generated C3b molecules then bind to the cell surface to form more stable
• C3 convertase in association with Bb (C3bBb) by an amplification loop.
• The C3 convertase C3bBb cleaves other C3 molecules, and the C3b fragments come to attach C3bBb to form the alternate C5 convertase ((C3b)nBb).
• The alternate C5 convertase, like the classical one, cleaves the C5 molecules into C5a and C5b molecules, and from this moment on, the biochemical cascade leading to the formation of MAC is identical to that of the classical pathway.

In 1978, Kawasaki et al stumbled upon a major band on polyacrylamide gel electrophoresis after the isolation of a specific protein from the liver of rabbits, mannan binding lectin or MBP (4). Correlation between this serum MBP level and the generation of C3b opsonins was proved by Super et al. (5) and since 1992, Matsushita et al. have dedicated their work to the formation of the classical C3 convertase through MBL-associated proteases (MASP) to reveal the complete picture of the lectin pathway (6-8).

•  This pathway is initiated by the interaction of MBL (mannan binding lectin) with mannose or N-acetylglucosamine residues on microorganisms.
• The MBL is here the equivalent of the C1q molecule; it is associated with MASP1 and MASP2, equivalent to the molecules C1r and C1s respectively.
• The activation of the MBL/MASP complex leads to the formation of the classical C3 convertase (C4b2a) and then the activation is identical to that of the classical pathway.

But apart from the three previously mentioned humoral pathways, other pathways have been uncovered more recently.

In 2002, Markus Huber-Lang et al (9) demonstrated that phagocytic cells, especially lung macrophages, could convert biologically active C5a from C5 when serine protease inhibitors are involved. Four years later, the same team (10) proved that complement and coagulation pathway are linked. In fact, without the presence of C3, thrombin is able to replace C5 convertase and to cleave C5.

Complement system acts as a protection against non-self antigens but also controls homeostasis, removing unwanted cells from the host. Sometimes, complement can get out of control and start to trigger inflammation and destroy healthy cells. Given its parallel involvement in infectious diseases, but also in autoimmune and neurodegenerative pathologies, complement factors have rapidly become a target of choice for the biotechnology and pharmaceutical industries.

Complement should be considered the bridge between the innate and adaptive immune response, interacting with many immune cells. And regarding this cornerstone role, interacting with complement elements through therapeutic molecules is not without risk to the patient's immunity.

How to effectively protect individuals without altering their immunomodulatory functions?

Thus, complement druggability has been considered at all levels of the cascade, as well as on all activation pathways described above. Nevertheless, it seems that drugs related to serine proteases and anaphylatoxin receptors remain the only acceptable ones to date.

For the targets and their related current drug candidates in preclinical development, in clinical trials or already approved, we have collected the therapeutic inhibitors of:

• Proteins Cinryze (Shire), Berinert (CSL Behring), Cetor (Sanquin) and Ruconest (Pharming)

• Monoclonal antibody ANX005 (Annexon Biosciences)

• Monoclonal antibody Sutimlimab from Sanofi (originally BIVV009, developed by Bioverativ and previously by True North)

• Monoclonal antibody Narsoplimab from Omeros (Phase III)

• Monoclonal antibody OMS906 from Omeros (Phase I)

• Fab CLG561/NOV7 developed by Novartis & Morphosys

• Synthetic cyclic peptide AMY-101 from Amyndas Pharmaceuticals (Phase II),
• Pegylated peptide APL-9 from Apellis Pharmaceuticals (Phase I)

• Antisense IONIS-FB-LRx from Ionis Pharma & Roche (Phase II)

• Small molecule Danicopan/ACH-4471 from Alexion (Phase III, originally developed by Achillion)

• Famous approved antibody Soliris (Eculizumab, from Alexion),
• Monoclonal antibodies Ravulizumab (ALXN1210, from Alexion), Tesidolumab from Novartis/Morphosys, Crovalimab (SKY59/RG6107 from Chugai/Roche), Pozelimab (REGN3918 from Regeneron), ABP959 (biosimilar eculizumab) from Amgen, MUBODINA from Adienne
• Small molecule Coversin (Nomacopan) from Akari Therapeutics
• Pegylated RNA aptamer (Ivura) from IVERIC Bio
• siRNA (Cemdisiran) from Alnylam

• Monoclonal antibody Olendalizumab (ALXN 1007, from Alexion)
• Monoclonal antibody vilobelimab (IFX-1, InflaRx)

• Small molecule Avacopan (CCX168, from ChemoCentryx),
• Monoclonal antibody IPH5401 Innate Pharma (originally developed by Novo Nordisk),
• Cyclic peptide ALS205 from Alsonex,
• Cyclic peptidomimetic DF2593A from Dompé.

(1) Bordet J. (1895) Les leukocytes et les propriétés actives du serum chez les vaccines. Ann. Inst. Pasteur. 9:462–506.

(2) Ehrlich P, Morgenroth J. (1899) Zur Theorie der Lysenwirkung. Berlin Klin. Wchsr. 36:6.

(3) Pillemer L, Blum L, Lepow I, Ross O, Todd E, Wardlaw A. The properdin system and immunity. I. Demonstration and isolation of a new serum protein, properdin, and its role in immune phenomena. Science. 1954 Aug 20; 120(3112):279-85

(4) Kawasaki T, Etoh R, Yamashina I. Isolation and characterization of a mannan-binding protein from rabbit liver. Biochem Biophys Res Commun. 1978 Apr 14;81(3):1018-24. doi: 10.1016/0006-291x(78)91452-3. PMID: 666781.

(5) Super M, Thiel S, Lu J, Levinsky RJ, Turner MW. (1989) Association of low levels of mannan-binding protein with a common defect of opsonisation. Lancet 2:1236–9.

(6) Matsushita M, Fujita T. (1992) Activation of the classical complement pathway by mannose-binding protein in association with a novel C1s-like serine protease. J. Exp. Med. 176:1497–502.

(7) Matsushita M, Endo Y, Fujita T. (2000) Cutting edge: complement-activating complex of ficolin and mannose-binding lectin-associated serine protease. J. Immunol. 164:2281–4.

(8) Matsushita M, et al. (2002) Activation of the lectin complement pathway by H-ficolin (Hakata antigen). J. Immunol. 168:3502–6.

(9) Huber-Lang M, Younkin EM, Sarma JV, Riedemann N, McGuire SR, Lu KT, Kunkel R, Younger JG, Zetoune FS, Ward PA. Generation of C5a by phagocytic cells. Am J Pathol. 2002 Nov;161(5):1849-59. doi: 10.1016/S0002-9440(10)64461-6. PMID: 12414531; PMCID: PMC1850785.

(10) Huber-Lang M, Sarma JV, Zetoune FS, Rittirsch D, Neff TA, McGuire SR, Lambris JD, Warner RL, Flierl MA, Hoesel LM, Gebhard F, Younger JG, Drouin SM, Wetsel RA, Ward PA. Generation of C5a in the absence of C3: a new complement activation pathway. Nat Med. 2006 Jun;12(6):682-7. doi: 10.1038/nm1419. Epub 2006 May 21. PMID: 16715088.