OVERVIEW:
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Overview of complement pathways (yellow = classical, blue = alternative) |
The complement system is a complicated enzyme cascade made up of numerous serum glycoproteins that normally exist in in-active, proenzyme form. The system has two distinct pathways:
While the stimulating factors for each pathway are distinct, each one has a similar terminal sequence which creates the membrane attack complex (MAC), an enzyme complex which punches a hole in various cell surfaces. In addition, both the alternative and classical pathways have as their by-products a number of anaphylatoxins - small peptides which contribute to an inflammatory response.
Most molecules involved in the complement system are given the name "C" and then a number, for example "C1". The numbers are not indicative of the order in which they act within the cascade, but rather refer to the order in which they were discovered. As mentioned, complement proteins normally exist in proenzyme form. They are activated sequentially by successive cleavages of the various molecules. When a complement protein is split, two fragments are formed, generally referred to as "a" and "b." C5, for example is cleaved into fragments C5a and C5b. While these generalizations are not true for every complement component, they should prove a useful guide to the description that follows.
THE CLASSICAL PATHWAY:
The classical pathway of the complement system is a major effector of the humoral branch of the human immune response. The trigger for the classical pathway is either IgG or IgM antibody bound to antigen. Binding of antibody to antigen exposes a site on the antibody which is a binding site for the first complement component, C1.
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Adapted from http://www.imonline.gsm.com/freedemo/ |
C1 is a macromolecular complex made up of the molecules C1q, C1r, and C1s. By itself, C1q is a hexameric molecule comprised of "stalks" and "knobs". The protruding knobs of the C1q molecule bind to exposed sites on antigen-bound antibody molecules. Pairs of C1r and C1s molecules associate with one another to make a figure-8 shape. This figure-8 fits over the knobs of the C1q molecule to make a complete, intact C1 molecule
When the intact macromolecular C1 binds to the exposed regions of at least two antigen-bound antibodies, the C1r and C1s subunits are activated. Activated C1s is responsible for the cleavage of the next two involved complement components, C4 and C2. (Remember, the numbers indicate the order in which the components were discovered, not the order in which they activate in the cascade.) C4 is cleaved into two fragments. The larger C4b molecule attaches to the target membrane nearby while the small C4a molecule floats away. An exposed site on deposited C4b is available to interact with the next complement component, C2. Again, activated C1s cleaves the C2 molecule into two pieces. In this case, the fragment that remains is C2a. The smaller C2b fragment floats away. What remains bound to the membrane is C4b2a, also known as the C3 convertase because its role is to convert the next complement component, C3, into its active form.
The C3 convertase of the classical pathway splits C3 into two fragments, C3a and C3b. The convertase has the ability to cleave multiple C3 molecules, forming hundreds of C3a and C3b fragments. The C3a fragments float away and have a role in inducing an inflammatory response (more on this later). Of critical importance, some of the C3b binds to the C4b2a to form C4b2a3b - a.k.a. the C5 convertase. The C5 convertase, much like the C3 convertase before it, catalyzes the cleavage of hundreds to thousands of C5 complement component into C5a and C5b before it reverts to inactivity. C5a floats away and contributes to inflammation while the C5b fragment binds to the antigen surface. This binding of C5b is the initial step in the formation of the membrane attack complex (MAC).
THE MEMBRANE ATTACK COMPLEX:
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Adapted from http://www.imonline.gsm.com/freedemo/ |
The membrane-bound complement component C5b is bound by the next complement molecule, C6. The resulting bimolecular complex next binds C7 and then C8. The C5b-8 complex acts as a receptor for a variable number of membrane-disrupting C9 molecules. The resultant C5b-8 complex and poly-C9 is given the name "membrane attack complex." The MAC creates a transmembrane pore leading to the lysis of the target cell.
Click here to
watch a movie of the classical pathway and MAC Formation.
(Movie is from http://www.imonline.gsm.com/freedemo/
)
THE ALTERNATIVE PATHWAY:
The alternative complement pathway does not require antibody for its activation. Rather, a variety of antigens such as bacterial lipopolysaccharide and components of viruses and other pathogens have the ability to activate this pathway. It is thought to have evolved earlier than the classical pathway, which depends on the relatively recently evolved antibody molecule. Like the classical pathway, the alternative pathway produces both a C3 and a C5 convertase which leads to the production of C5b and then to the formation of the MAC. The specific molecular players and the path followed along the way are, however, different.
The complement component C3 is spontaneously cleaved at low levels. This means that there are C3a and C3b fragments freely floating in serum. The C3b component can attach to a number of different surfaces, both foreign and host cells alike. C3b is quickly inactivated by the sialic acid found on most mammalian cell surfaces. Microbes, most of which lack sialic acid, are stable sites for C3b deposition. (Sialic acid inactivation of C3b is one type of complement evasion strategy utilized by certain pathogens, discussed below.) Membrane-bound C3b fragments are bound by Factor B which is, in turn, cleaved by Factor D. The fragment Ba floats away, while Bb stays associated with C3b. The resulting C3bBb molecule is the alternative pathway C3 convertase. The C3 convertase of the alternative pathway is, however, not particularly stable. In order to effectively split a relevant number of C3 molecules, the C3 convertase requires the stabilization of another molecule, properdin (P), which binds to the C3bBb complex and extends the half-life of its activity.
The alternative pathway C3 convertase acts just like the classical pathway enzyme of the same name and cleaves hundreds of C3 molecules into C3a and C3b. The C3b molecule remains attached to form the alternative pathway C5 convertase, C3bBb3b. This enzyme cleaves C5 into C5a and C5b. The C5b molecule remains associated with the membrane and associates with C6 through C9 to form the MAC, as described above.
Click here to
watch a movie of the alternative pathway. (FYI: The shark
= factor D.)
(Movie obtained from http://www.imonline.gsm.com/freedemo/)
CLASSICAL VS. ALTERNATIVE PATHWAYS: A SUMMARY
| Classical Pathway | Alternative Pathway | |
| Initiated by: | Antibody bound to Antigen which activates C1 |
Microbial Surface Molecules bind C3b |
| C3 Convertase: | C4b2a |
C3bBb |
| C5 Convertase: | C4b2a3b |
C3bBb3b |
HOW COMPLEMENT DEFEATS PATHOGENS:
As already indicated, the formation of the MAC is one of the primary ways in which complement mediates its damage on foreign invaders. The MAC can be deposited on many viruses, fungi, bacteria, and other cells. The alternative pathway, which does not require an antibody-antigen interaction for its action, is an important line of innate defense in the human immune system. Through the formation of the MAC via the classsical pathway, complement also mediates destruction of foreign bodies that are recognized by host antibodies. The lysis of cells via the complement pathways is thus critical for both innate and specific immunity.
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Adapted from http://www.imonline.gsm.com/freedemo/ |
The MAC is not the only way in which complement mediates its pathogen-fighting effects. Many of the enzymatic split products of both complement pathways, hitherto mentioned only in passing, promote host immunity via their promotion of the inflammatory response. The split products C3a, C4a, and C5a are all anaphylatoxins. These low-molecular weight peptides have the ability to bind to mast cells and basophils. When the appropriate receptors are bound, these cells release histamine and other highly active peptides into the local environment. These peptides increase the permeability of the vascular walls allowing neutrophils to migrate into the area. Neutrophils are further encouraged to migrate to the site of complement activation due to the potent chemotactic (attractant) effect of C5a. The neutrophils phagocytose (from the Latin, "to eat cells") invading pathogens and also release mediators which attract macrophages to the site of infection. These cells also have the ability to phagocytose invading cells and further promote the inflammatory response, which is effective at eliminating many invading microorganisms.
Certain complement components, most notably C4b and C5b, act as opsonins. Many phagocytic cells have receptors for these complement split products. Antigens coated with either of these molecules are said to be opsonized, meaning that they are more likely to be ingested by phagocytes.
The complement system has many other important roles in mediating and enhancing the immune response against a wide variety of invaders. For viruses, these effector mechanisms include clumping viruses together or coating them with complement fragments to reduce their ability to infect cells. Viruses are also destroyed by complement-mediated lysis as well. A full exploration of all of the roles of complement is beyond the scope of this site. For more details on the complement system, see References.
REVIEW:
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From http://129.109.112.248/microbook/toc.htmhttp://129.109.112.248/microbook/toc.htm |
