Complement component 3, often simply called C3, is a protein of the immune system that is found primarily in the blood. It plays a central role in the complement system of vertebrate animals and contributes to innate immunity. In humans it is encoded on chromosome 19 by a gene called C3. [5] [6]
Deficiencies and defects of C3 result in the affected person being immunocompromised and particularly vulnerable to bacterial infections.
C3 plays a central role in the activation of the complement system. [7] Its activation is required for both classical and alternative complement activation pathways. People with C3 deficiency are susceptible to bacterial infection. [8] [9]
One form of C3-convertase, also known as C4b2a, is formed by a heterodimer of activated forms of C4 and C2. It catalyzes the proteolytic cleavage of C3 into C3a and C3b, generated during activation through the classical pathway as well as the lectin pathway. C3a is an anaphylotoxin and the precursor of some cytokines such as ASP, and C3b serves as an opsonizing agent. Factor I can cleave C3b into C3c and C3d, the latter of which plays a role in enhancing B cell responses. In the alternative complement pathway, C3 is cleaved by C3bBb, another form of C3-convertase composed of activated forms of C3 (C3b) and factor B (Bb). Once C3 is activated to C3b, it exposes a reactive thioester that allows the peptide to covalently attach to any surface that can provide a nucleophile such as a primary amine or a hydroxyl group. Activated C3 can then interact with factor B. Factor B is then activated by factor D, to form Bb. The resultant complex, C3bBb, is called the alternative pathway (AP) C3 convertase.
C3bBb is deactivated in steps. First, the proteolytic component of the convertase, Bb, is removed by complement regulatory proteins having decay-accelerating factor (DAF) activity. Next, C3b is broken down progressively to first iC3b, then C3c + C3dg, and then finally C3d. Factor I is the protease cleaves C3b but requires a cofactor (e.g Factor H, CR1, MCP or C4BP) for activity.
Several crystallographic structures of C3 have been determined [10] and reveal that this protein contains 13 domains. [11] [12] [13] [14]
The C3 precursor protein is first processed by the removal of 4 Arginine residues, forming two chains, beta and alpha, linked by a disulfide bond. The C3 convertase activates C3 by cleaving the alpha chain, releasing C3a anaphylatoxin and generating C3b (beta chain + alpha' (alpha prime) chain).
In humans, C3 is predominantly synthesised by liver hepatocytes [5] and to some degree by epidermis keratinocytes. [15]
| C4 (C) | FB (A) | C3 | CH50 | Conditions |
|---|---|---|---|---|
| · | ↓ | ↓ | ↓ | PSG, C3 NeF AA |
| ↓ | · | ↓ | · | HAE, C4D |
| · | · | · | ↓ | TCPD |
| ↓ | ·/↓ | ↓ | ↓ | SLE |
| ↑ | ↑ | ↑ | ↑ | inflammation |
Levels of C3 in the blood may be measured to support or refute a particular medical diagnosis. For example, low C3 levels are associated with Systemic Lupus Erythematosus (SLE) [16] and some types of kidney disease such as post-infectious glomerulonephritis, membranoproliferative glomerulonephritis, and shunt nephritis.
Factor H is the primary regulator of C3. Deficiency of Factor H may lead to uncontrolled C3 activity through the alternative pathway of the complement system. [17]
Deficiency of C3 results in the affected person being immunocompromised. Specifically, they are vulnerable to bacterial pathogens, including repeat infections by the same organism, but are not susceptible to viruses. This vulnerability also occurs in an individual deficient in C1, C2, C4, or any of their required components or associated proteins, and the clinical effects are very similar regardless of the specific deficiency. This is because all of these must work with C3 for the complement system to function. [18]
Affected people are particularly vulnerable to infections with Gram-negative organisms such as pathogenic E. coli or Salmonella enterica . [19] Additionally, C3 and other complement deficiencies are associated with frequent and severe respiratory infections, as well as other infections that invade and penetrate tissue layers. [18]
Some data shows that acquired C3 deficiency, including when this is intentionally done for medical immunosuppression purposes, may not significantly impact a person's immune function long-term. [20] However, by contrast, congenital C3 deficiency is known to cause chronic illness. [18]
Additionally, several forms of C3 deficiency contribute to the development of systemic lupus erythematosus and other autoimmune diseases. [18]
The complement system, also known as complement cascade, is a part of the immune system that enhances (complements) the ability of antibodies and phagocytic cells to clear microbes and damaged cells from an organism, promote inflammation, and attack the pathogen's cell membrane. It is part of the innate immune system, which is not adaptable and does not change during an individual's lifetime. The complement system can, however, be recruited and brought into action by antibodies generated by the adaptive immune system.
The classical complement pathway is one of three pathways which activate the complement system, which is part of the immune system. The classical complement pathway is initiated by antigen-antibody complexes with the antibody isotypes IgG and IgM.
The alternative pathway is a type of cascade reaction of the complement system and is a component of the innate immune system, a natural defense against infections.
C3 convertase belongs to family of serine proteases and is necessary in innate immunity as a part of the complement system which eventuate in opsonisation of particles, release of inflammatory peptides, C5 convertase formation and cell lysis.
C5 convertase is an enzyme belonging to a family of serine proteases that play key role in the innate immunity. It participates in the complement system ending with cell death.
Properdin is a protein that in humans is encoded by the CFP gene.
The lectin pathway or MBL pathway is a type of cascade reaction in the complement system, similar in structure to the classical complement pathway, in that, after activation, it proceeds through the action of C4 and C2 to produce activated complement proteins further down the cascade. In contrast to the classical complement pathway, the lectin pathway does not recognize an antibody bound to its target. The lectin pathway starts with mannose-binding lectin (MBL) or ficolin binding to certain sugars.
Alternative-complement-pathway C3/C5 convertase is an enzyme. This enzyme catalyses the following chemical reaction
Complement C2 is a protein that in humans is encoded by the C2 gene. The protein encoded by this gene is part of the classical pathway of the complement system, acting as a multi-domain serine protease. Deficiency of C2 has been associated with certain autoimmune diseases.
Mannan-binding lectin serine protease 1 also known as mannose-associated serine protease 1 (MASP-1) is an enzyme that in humans is encoded by the MASP1 gene.
Factor D is a protein which in humans is encoded by the CFD gene. Factor D is involved in the alternative complement pathway of the complement system where it cleaves factor B.
Complement factor B is a protein that in humans is encoded by the CFB gene.
Complement decay-accelerating factor, also known as CD55 or DAF, is a protein that, in humans, is encoded by the CD55 gene.
Complement control proteins are proteins that interact with components of the complement system.
Factor H (FH) is a member of the regulators of complement activation family and is a complement control protein. It is a large, soluble glycoprotein that circulates in human plasma. Its principal function is to regulate the alternative pathway of the complement system, ensuring that the complement system is directed towards pathogens or other dangerous material and does not damage host tissue. Factor H regulates complement activation on self cells and surfaces by possessing both cofactor activity for Factor I–mediated C3b cleavage, and decay accelerating activity against the alternative pathway C3-convertase, C3bBb. Factor H exerts its protective action on self cells and self surfaces but not on the surfaces of bacteria or viruses. There are however, important exceptions, such as for example the bacterial pathogen, Neisseria meningitidis. This human pathogen has evolved mechanisms to recruit human FH and down-regulate the alternative pathway. Binding of FH permits the bacteria to proliferate in the bloodstream and cause disease.
C3b is the larger of two elements formed by the cleavage of complement component 3, and is considered an important part of the innate immune system. C3b is potent in opsonization: tagging pathogens, immune complexes (antigen-antibody), and apoptotic cells for phagocytosis. Additionally, C3b plays a role in forming a C3 convertase when bound to Factor B, or a C5 convertase when bound to C4b and C2b or when an additional C3b molecule binds to the C3bBb complex.
Complement factor H-related protein 5 is a protein that in humans is encoded by the CFHR5 gene.
C3a is one of the proteins formed by the cleavage of complement component 3; the other is C3b. C3a is a 77 residue anaphylatoxin that binds to the C3a receptor (C3aR), a class A G protein-coupled receptor. It plays a large role in the immune response.
Complement 3 deficiency is a genetic condition affecting complement component 3 (C3). People can suffer from either primary or secondary C3 deficiency. Primary C3 deficiency refers to an inherited autosomal-recessive disorder that involves mutations in the gene for C3. Secondary C3 deficiency results from a lack of factor I or factor H, two proteins that are key for the regulation of C3. Both primary and secondary C3 deficiency are characterized by low levels or absence of C3.
Aureolysin is an extracellular metalloprotease expressed by Staphylococcus aureus. This protease is a major contributor to the bacterium's virulence, or ability to cause disease, by cleaving host factors of the innate immune system as well as regulating S. aureus secreted toxins and cell wall proteins. To catalyze its enzymatic activities, aureolysin requires zinc and calcium which it obtains from the extracellular environment within the host.
This article incorporates text from the United States National Library of Medicine, which is in the public domain.
