The complement system, or complement cascade, is a part of the innate immune system that assists in defense against pathogens (1-3). Complement C3, also called C3 or C3 protein, is one of nine complement proteins and is the main component of the complement system which is composed of over 30 soluble and membrane-bound proteins (1,4). The complement cascade consists of three main pathways: the classical, the lectin, and the alternative, all of which converge into a common pathway involving C3 cleavage by C3-convertases (1-6). Human Complement C3 is synthesized as a protein of 1663 amino acids (aa) in length with a theoretical molecular weight of ~185 kDa (5). Complement C3 is the most prevalent human complement protein in the serum, with a concentration of 1.2 mg/mL, and is predominantly produced by hepatocytes in the liver, but is also synthesized by blood cells and epithelial cells (3,5). Furthermore, the structure of C3 is comprised of an alpha-chain (110 kDa) and a beta-chain (75 kDa) linked by a disulfide bond (5). Cleavage of inactive C3 by C3-convertases produces active C3a, which functions as a mediator of inflammation, and C3b, which is an opsonin (1-4). In addition to amplification of complement response, C3 fragments serve multiple additional functions including chemotaxis, phagocytosis, adhesion, and immune modulation (3). Complement C3 serves dual purposes where it is involved in pathogenesis and immunity but, conversely, cellular damage results from unregulated C3 activation (5).
Both elevated levels and reduced levels of Complement C3 has been implicated in diseases pathologies (6). Deficiency in Complement proteins can result in autoimmune disorders including systemic lupus erythematosus, which is more often associated with C1 or C4 deficiency and only rarely with C3 deficiency (6). However, C3 deficiency typically results in increased risk of recurrent bacterial infections and glomerulonephritis, characterized by inflammation of the filtering glomeruli in the kidneys (6). Additionally, elevated levels of C3a and C4a is seen in patients with antiphospholipid antibody syndrome (6). Serum levels of C3 are also higher in rheumatoid arthritis cases (6). The complement system has become a target for drugs and therapeutics aimed at modulating innate immunity (7). For instance, compstatin is a peptide that binds to C3, inhibiting convertase activity and cleavage and can be used to treat diseases associated with uncontrolled C3 activation (7). C3-inhibitors and other complement inhibitors are a promising drug candidate for treatment of many diseases (7).
1. Mathern, D. R., & Heeger, P. S. (2015). Molecules Great and Small: The Complement System. Clinical Journal of the American Society of Nephrology: CJASN. https://doi.org/10.2215/CJN.06230614
2. Merle, N. S., Church, S. E., Fremeaux-Bacchi, V., & Roumenina, L. T. (2015). Complement System Part I - Molecular Mechanisms of Activation and Regulation. Frontiers in Immunology. https://doi.org/10.3389/fimmu.2015.00262
3. Ricklin, D., Reis, E. S., Mastellos, D. C., Gros, P., & Lambris, J. D. (2016). Complement component C3 - The "Swiss Army Knife" of innate immunity and host defense. Immunological Reviews. https://doi.org/10.1111/imr.12500
4. Merle, N. S., Noe, R., Halbwachs-Mecarelli, L., Fremeaux-Bacchi, V., & Roumenina, L. T. (2015). Complement System Part II: Role in Immunity. Frontiers in Immunology. https://doi.org/10.3389/fimmu.2015.00257
5. Sahu, A., & Lambris, J. D. (2001). Structure and biology of complement protein C3, a connecting link between innate and acquired immunity. Immunological Reviews. https://doi.org/10.1034/j.1600-065x.2001.1800103.x
6. Vignesh, P., Rawat, A., Sharma, M., & Singh, S. (2017). Complement in autoimmune diseases. Clinica Chimica Acta; International Journal of Clinical Chemistry. https://doi.org/10.1016/j.cca.2016.12.017
7. Mastellos, D. C., Yancopoulou, D., Kokkinos, P., Huber-Lang, M., Hajishengallis, G., Biglarnia, A. R., Lupu, F., Nilsson, B., Risitano, A. M., Ricklin, D., & Lambris, J. D. (2015). Compstatin: a C3-targeted complement inhibitor reaching its prime for bedside intervention. European Journal of Clinical Investigation. https://doi.org/10.1111/eci.12419