Complement

Three major functions of complement

1. *Opsonization* 2. *Direct lysis of target cells* or usually pathogens Student question response: "When the membrane attack complex is fully engaged, it acts as a roter rooter and puts holes into cells directly." 3. *Chemoattraction* and activation of *inflammatory cells* So the complement cascade itself is *pro-inflammatory*

paroxysmal nocturnal hemoglobinuria

A disease called paroxysmal nocturnal hemoglobinuria results from the acquired deficiency in hematopoietic stem cells of an enzyme that synthesizes the glycolipid anchor for several cell surface proteins, including the complement regulatory protein DAF and CD59. In these patients, unregulated complement activation occurs on erythrocytes, leading to their lysis. Deficiency of the regulatory protein factors H and I results in increased complement activation and reduced levels of C3, causing increased susceptibility to infection.

lectin pathway

A pathway of complement activation triggered, in the absence of antibody, by the binding of microbial polysaccharides to circulating lectins such as mannose-binding protein (MBL) complement activation is initiated not by antibodies but by the attachment of plasma mannose-binding lectin (MBL) to microbes. MBL is structurally similar to a component of C1 of the classical pathway and serves to activate C4. The subsequent steps are essentially the same as in the classical pathway.

C1 inhibitor (C1 INH)

A regulatory protein called C1 inhibitor (C1 INH) stops complement activation early, at the stage of C1 activation.

Decay-accelerating factor (DAF)

Decay-accelerating factor (DAF) is a lipid-linked cell surface protein that disrupts the binding of Bb to C3b and the binding of C4b to C2a, thus blocking C3 convertase formation and terminating complement activation by both the alternative and the classical pathways.

Deficiencies of C9 and MAC formation

Deficiencies of C9 and MAC formation result in increased susceptibility to Neisseria infections. Some individuals inherit polymorphisms in the gene encoding MBL, leading to production of a protein that is functionally defective; such defects are associated with increased susceptibility to infections.

Deficiencies of the early proteins of the classical pathway, C2 and C4

Deficiencies of the early proteins of the classical pathway, C2 and C4, may have no clinical consequence, or may result in increased susceptibility to infections, or are associated with an increased incidence of systemic lupus erythematosus, an immune complex disease. The increased incidence of lupus may be because the classical pathway functions to eliminate immune complexes from the circulation and this process is impaired in individuals lacking C2 and C4.

Deficiency of C3

Deficiency of C3 results in profound susceptibility to infections and usually is fatal early in life.

Factor H

Factor H counteracts properdin; the "antithesis" of properdin. Facilitates cleavage of C3b to iC3b. iC3b cannot form the C3 convertase, but it does it indirectly by facilitating the cleavage of C3b to iC3b by plasma factor I. Factor H and plasma factor I work together to take C3b and change it into iC3b, which then is cleaved and destroyed can't make more C3 convertase.

Factor I deficiency

Formation of the C3 convertase keeps going until depletes C3. Never a good thing, because without C3→ a lot of bacterial infections occur. If you have no factor I→keep making C3 convertase until there is no more C3 to create it When faced with bacterial infection: less C3b deposited Individuals are more susceptible to ear infections and abscesses caused by encapsulated bacteria because the complement cascade is normally particularly activated on this type of bacteria in general

In addition to its antimicrobial effector functions what does the complement system do?

In addition to its antimicrobial effector functions, the complement system provides stimuli for the development of humoral immune responses. When C3 is activated by a microbe by the alternative pathway, one of its breakdown products, C3d, is recognized by complement receptor type 2 (CR2) on B lymphocytes. Signals delivered by this receptor stimulate B cell responses against the microbe. This is an example of an innate immune response to a microbe (complement activation) enhancing an adaptive immune response to the same microbe (B cell activation and antibody production). Complement proteins bound to antigen-antibody complexes are recognized by follicular dendritic cells in germinal centers, allowing the antigens to be displayed for further B cell activation and selection of high-affinity B cells. This complement-dependent antigen display is another way in which the complement system promotes antibody production.

hereditary angioneurotic edema

Inherited deficiencies of regulatory proteins cause uncontrolled and pathologic complement activation. Deficiency of C1 INH is the cause of a disease called hereditary angioneurotic edema, in which excessive C1 activation and the production of vasoactive protein fragments lead to leakage of fluid (edema) in the larynx and many other tissues.

Inherited deficiency of the alternative pathway

Inherited deficiency of the alternative pathway protein properdin also causes increased susceptibility to bacterial infection.

What regulates the complement system

Mammalian cells express regulatory proteins that inhibit complement activation, thus preventing complement-mediated damage to host cells.

Where are complement proteins found?

Many complement components exist in the *plasma* in an *inactive or zymogen form*. Activated usually by proteolytic cleavage by the protease component ahead of it in the cascade. Leads to the deposition or "fixing" of complement components to the surface of a pathogen.

Membrane cofactor protein (MCP)

Membrane cofactor protein (MCP) aka CR1 serves as a cofactor for the proteolysis of C3b into inactive fragments, a process mediated by a plasma enzyme called factor I. CR1 may displace C3b and promote its degradation.

Three pathways of activating complement system

Of the three major pathways of complement activation, two, called the alternative and lectin pathways, are initiated by microbes in the absence of antibody, and the third, called the classical pathway, is initiated by certain isotypes of antibodies attached to antigens

Functions of the Complement System: stimulation of inflammatory reactions

Small peptide fragments of C3, C4, and especially C5, which are produced by proteolysis, are chemotactic for neutrophils, stimulate the release of inflammatory mediators from various leukocytes, and act on endothelial cells to enhance movement of leukocytes and plasma proteins into tissues. In this way, complement fragments induce inflammatory reactions that also serve to eliminate microbes.

How does the complement system amplify its effect and at the same time keep it to the right places?

The activation of complement proteins involves sequential *proteolytic cleavage* of these proteins, leading to the generation of effector molecules that participate in eliminating microbes in different ways. This cascade of complement protein activation, as with all enzymatic cascades, is capable of achieving tremendous amplification; therefore an initially small number of activated complement molecules produced early in the cascade may generate a large number of effector molecules. Activated complement proteins become covalently attached to the cell surfaces where the activation occurs, ensuring that complement effector functions are limited to the correct sites. The complement system is tightly regulated by molecules present on normal host cells, and this regulation prevents uncontrolled and potentially harmful complement activation.

The alternative pathway

The alternative pathway is a component of the innate immune system and mediates inflammatory responses to infection as well as direct lysis of microbes. 1. complement activation is triggered when a breakdown product of C3 hydrolysis, called C3b, is deposited on the surface of a microbe. Here, the C3b forms stable covalent bonds with microbial proteins or polysaccharides and is thus protected from further degradation. (As described later, C3b is prevented from binding stably to normal host cells by several regulatory proteins present on host cells but absent from microbes.) 2. The microbe-bound C3b binds another protein called factor B, which is then broken down by a plasma protease to generate the Bb fragment. This fragment remains attached to C3b, forming the C3bBb complex. 3. The C3bBb complex enzymatically breaks down more C3, functioning as the alternative pathway C3 convertase. As a result of this convertase activity, many more C3b and C3bBb molecules are produced and become attached to the microbe. 4. Some of the C3bBb molecules bind an additional C3b molecule, and the C3bBb3b complex functions as a C5 convertase, to break down the complement protein C5 and initiate the late steps of complement activation.

Properdin

The alternative pathway is an unstable process. In the face of infection, a factor which is made by activated neutrophils (in response to the infection) release properdin from secondary granules. Increases the speed and power of complement activation by binding the C3 convertase C3bBb on the pathogen surface. Prevents its' degradation by other proteases. Alternative pathway can be referred to as the properdin pathway because this pathway becomes stable when properdin is around (which comes from activated neutrophils)

The classical pathway

The classical pathway is an effector arm of the humoral immune system that generates inflammatory mediators, opsonins for phagocytosis of antigens, and lytic complexes that destroy cells. 1. complement activation is triggered when IgM or certain subclasses of IgG (IgG1, IgG2, and IgG3 in humans) bind to antigens (e.g., on a microbial cell surface). 2. As a result of this binding, adjacent Fc regions of the antibodies become accessible to and bind the C1 complement protein (which is made up of a binding component called C1q and two proteases called C1r and C1s). 3. The attached C1 becomes enzymatically active, resulting in the binding and sequential cleavage of two proteins, C4 and C2. C4b (one of the C4 fragments) becomes covalently attached to the antibody and to the microbial surface where the antibody is bound, then binds C2, which is cleaved by active C1 to yield the C4b2a complex. 4. This complex is the classical pathway C3 convertase, which functions to break down C3, and the C3b that is generated again becomes attached to the microbe. 5. Some of the C3b binds to the C4b2a complex, and the resultant C4b2a3b complex functions as a C5 convertase, which cleaves the C5 complement protein.

In general, what is the complement system?

The complement system is a collection of circulating and cell membrane proteins (30) that play important roles in host defense against microbes and in antibody-mediated tissue injury. The term complement refers to the ability of these proteins to assist, or complement, the antimicrobial activity of antibodies. The complement system may be activated by microbes in the absence of antibody, as part of the innate immune response to infection, and by antibodies attached to microbes, as part of adaptive immunity.

Functions of the Complement System: complement mediated cytolysis

The complement system plays an important role in the elimination of microbes during innate and adaptive immune responses. The MAC can induce osmotic lysis of cells, including microbes. MAC-induced lysis is effective only against microbes that have thin cell walls and little or no glycocalyx, such as the Neisseria species of bacteria.

Functions of the Complement System: opsonization

The complement system plays an important role in the elimination of microbes during innate and adaptive immune responses. Microbes coated with C3b are phagocytosed by virtue of C3b being recognized by complement receptor type 1 (CR1, or CD35), which is expressed on phagocytes. Thus, C3b functions as an opsonin. Opsonization is probably the most important function of complement in defense against microbes.

What is the most abundant complement protein in plasma?

The most abundant complement protein in the plasma, C3, plays a central role in all three pathways. C3 is spontaneously hydrolyzed in plasma at a low level, but its products are unstable, rapidly broken down, and lost.

The net result of these early steps of activation

The net result of these early steps of complement activation is that *microbes acquire a coat of covalently attached C3b*. Note that the alternative and lectin pathways are effector mechanisms of innate immunity, whereas the classical pathway is a mechanism of adaptive humoral immunity. These pathways differ in their initiation, but once triggered, their late steps are the same. The late steps of complement activation are initiated by the binding of C5 to the C5 convertase and subsequent proteolysis of C5, generating C5b (Fig. 8-8). The remaining components, C6, C7, C8, and C9, bind sequentially to a complex nucleated by C5b. The final protein in the pathway, C9, polymerizes to form a pore in the cell membrane through which water and ions can enter, causing death of the microbe. This poly-C9 is the key component of the membrane attack complex (MAC), and its formation is the end result of complement activation.

Hypersensitivity diseases

The presence of these regulatory proteins is an adaptation of mammals. Microbes lack the regulatory proteins and are therefore susceptible to complement. Even in mammalian cells, the regulation can be overwhelmed by too much complement activation. Therefore, mammalian cells can become targets of complement if they are coated with large amounts of antibodies, as in some hypersensitivity diseases