Introduction to C3

  • Firstly, let us know an antibody—— recombinant anti-C3 antibody

    Recombinant Human Antibody (S77) is capable of binding to C3, expressed in HEK 293 cells. Expressed as the combination of a heavy chain (HC) containing VH from anti-C3 mAb and CH1-3 region of human IgG and a light chain (LC) encoding VL from anti- C3 proteins mAb and CL of human kappa light chain. Exists as a disulfide linked dimer of the HC and LC hetero-dimer under non-reducing condition. S77 blocks binding of factor B to C3b inhibiting the first step in the formation of the alternative pathway C3 convertase. In addition, S77 inhibits C5 binding to C3b.  

    What is C3?

    Complement is a kind of serum protein, which exists in human and vertebrate serum and tissue fluid. It is not heat-resistant. It has enzyme activity after activation and can mediate immune response and inflammatory reaction. It can be activated by antigen-antibody complexes or microorganisms, leading to the lysis or phagocytosis of pathogenic microorganisms. It can be activated through three independent and intersecting pathways: classical pathway, bypass pathway and agglutinin pathway.

    The third component of complement (C3) is the core of complement system. It was discovered in 1912 when researchers treated serum with snake venom. The serum level of C3 is the highest among the components of complement and C3 is also at the central position in function. It is not only the intersection of several activation pathways, but also the basis of C3b-dependent positive feedback loop. At the same time, C3 cleavage fragments and their binding proteins are complex and diverse, and play an important role in immune defense, immune regulation and immunopathology.

    C3 gene

    The human C3 gene is located on the short arm of chromosome 19 and consists of 41 exons. The full length of the coding sequence of the human complement C3 gene is about 5052 bp. Human C3 has genetic polymorphism, and more than 30 rare types have been found, but not C3S and C3F are more common.It has been confirmed that the genetic polymorphism of C3S and C3F is based on the mutation of nucleotide sequence: the 364th nucleotide of exon 3 changes from cytosine (C) to guanidine (G), resulting in the transformation of arginine of C3S to glycine of C3F, and the change of 314th nucleotide of exon 9 leads to the transformation of leucine into proline, thus forming the structural difference between them.At the same time, studies have shown that there are significant differences in C3 genetic polymorphism among different races and populations.

    Human C3 molecule is mainly produced by hepatocytes and macrophages. When C3 was transferred to the endoplasmic reticulum, 22 amino acid-inducing peptides were cut off and alkaline conjugated peptides were discarded to secrete cells in the form of two chains.

    Binding sites of C3 molecule and its fragments

    There are many binding sites on C3 molecule with other molecules, which are the molecular basis of complement biological activity.Thioester bond is the key site to determine C3 activity.C3 covalently binds with hydroxyl and amino groups on the surface of target cells to adhere to pathogens, tumor cells and immune complexes.Many laboratories abroad have carried out in-depth research on thioester bond. Essentially, thioester bond is an unstable binding site formed by the interaction of space effect and charge effect.Under physiological conditions, the binding of complement C3 and fragments to receptor molecules triggers the biological effects of complement system, but some small molecule binding sites and complement receptors also facilitate the process of many viral infections.The binding of C3 with these sites regulates the activation and decay of complements, the recognition of self and self, and is the precondition of activating the immune defense system and preventing immunopathological damage.

    Bioactivity of C3

    1. Cytotoxicity, bacteriolysis and bactericidal effects: the activation of complement C3 and the formation of MAC lead to perforation and dissolution of target cells. This C3-mediated cytolytic effect is an important defense mechanism against microbial infection.In some pathological cases, complement C3 also causes cell lysis in the body itself, leading to tissue damage and disease.

    2. Regulation: C3b and iC3b are important conditioners. They quickly adhere to bacteria or other particles and bind to corresponding receptors on the surface of neutrophils, monocytes or macrophages, thus promoting the phagocytosis of pathogens.

    3. Scavenging immune complexes: C3b binds to antibodies, interfering with the cross-linking between Fc segments, thus accelerating IC dissociation.More importantly, C3b mediates TC to bind to a large number of blood cells in the body fluid and transport them to the liver for clearance through blood flow, which is the main way of clearing IC in vivo.

    4. Inflammatory mediators: In a series of reactions activated by complement C3, allergen toxins C3a, C4a and C5a are produced. They bind to the corresponding receptors on mast cells, basophils and smooth muscle cells, which stimulate cell degranulation and cause vasodilation and smooth muscle contraction.

    5. Immunomodulation: Complement C3 plays a regulatory role in the presentation and proliferation of immune response. C3 participates in capturing and fixing antigens, making antigens easy to be processed and presented by antigen-presenting cells; C3b binds to CR2 on the surface of B cells, making B cells proliferate and differentiate into plasma cells; NK cells bind to C3b enhances the cytotoxicity mediated by antibody-dependent cells to target cells.In addition, complement C3 binds erythrocyte through CRI, promotes erythrocyte to release peroxidase and oxidase, and directly kills microorganisms adhering to the surface.

    Lack of C3 and its clinical significance

    1. Lack of C3: At present, 17 of the more than 30 plasma complement components in human body have been reported to be deficient.Hundreds of patients with congenital deficiency of complement components were found to be more susceptible to vascular diseases than to primary diseases, and about half of those with deficiency of C2, C3 and H factors were susceptible to this disease. Those with deficiency of C3 were susceptible to widespread and severe pyogenic infections, pneumonia, meningitis and sepsis.The reason may be due to the decrease of C3 content, the decrease of anti-infection ability, and the loss of C3-mediated circulating immune complex dissolution and clearance function.Clinically, the secondary complement C3 deficiency is far more than the congenital deficiency. C3 levels were significantly reduced in E.coli and meningococcal sepsis.In chronic compensatory nephritis caused by C3 nephritis factor, a severe suppurative infection occurs when the C3 level is lower than the normal value of 10%. C3 deficiency often combines with other inhibitory factors of the body's defense function, increasing the risk of infection, such as severe burns and hemolytic anemia.

    2. Application of C3 and its fragments: Detection of C3 and fragment C3d plays an important role in judging the level of complement activation, understanding the occurrence and development of diseases and making correct diagnosis and treatment.It is reported that the determination of serum complement C3 and C3d levels can help to judge the potential activity of SLE and guide the rational use of corticosteroids in clinical practice.

    C3 and its fragments are the core of complement system. They have long been considered as one of the non-specific immune factors acquired by human beings in the long-term evolution of species, and play a complementary and auxiliary role.With the in-depth research in recent years, especially the breakthroughs in C3 binding sites, C3 receptors and small C3 inhibitors, we have re-recognized the central molecule of the complement family.

    In conclusion, C3 and its fragments not only have non-specific immune defense function, but also participate in specific immune response.They are associated with a variety of effector cells, play an immune regulatory role, and are widely used in clinical testing and vaccine preparation, which is of great significance for the diagnosis and prevention of diseases.