The Complement system The complement system is an alarm and a weapon against infection, especially bacterial infection. activated directly by bacteria and bacterial products binding to sugars on the bacterial cell surface, or by complexes of antibody and antigen

The complement system consists of a series of heat-labile serum proteins that are activated in turn. The components normally exist as soluble inactive precursors; once activated, a complement component may then act as an enzyme cleaves several molecules of the next component in the sequence (rather like the clotting cascade). Each precursor is cleaved into two or more fragments. The major fragment has two biologically active sites: -one for binding to cell membranes or the triggering complex and -other for enzymatic cleavage of the next complement component

Complement pathways The two complement pathways differs but both form C3 and C5 converses and ultimately generate common membrane –attack complex (MAC) 1. Alternative pathway (properdin system) most commonly is activated by microbial surface and cell-surface components (eg LPS) generate earl, innate response that dies not require antibody for activation 2. Classical pathway: is activated primarily by antigen antibody complexes containing IgM or IgG. This constitute a major effect mechanism of Humoral Immunity.

Biological activities of Complement Product 1. Membrane attack complex (MAC) act as a molecule drill to puncture cell membranes Formation of MAC begins with cleavage of C5by C5 convetases formed in both pathway. Sequential additional of C6.7 and C8 to C5byiles C5b678 a complex that inserts stably into cell membranes but has limited cytotoxic ability binding of multiple C9 molecules produces ahighly cytotoxic MAC (C5b6789) that forms holes in the cell membrane killing the cell.

2. Complement cleavage products promote inflammatory response, opsonization and other effect. Some of the effect depend the presence complement receptors on specific target cells.

Regulation of complement CI inhibitor: prevent inappropriate activation of the classical pathway Inactivation of C3 and C5 convertases include decal: accelerating factor (DAF) factor H, and factor I. Anaphylotoxin inhibitor blocks anaphylactic activity of C3a and C5a.

The complement system Control of the sequence involves spontaneous decay of any expose attachment sites and specific inactivation by complement inhibitors. Minor fragments (usually prefixed ‘a’) generated by cleavage of components have important biological properties in the fluid phase, such as the chemotactic activity of C5a.

Complement functions Host benefit: Host detriment: opsonization to enhance phagocytosis phagocyte attraction and activation lysis of bacteria and infected cells regulation of antibody responses clearance of immune complexes clearance of apoptotic cells Host detriment: Inflammation, anaphylaxis

The complement system The major purpose of the complement pathways is to provide a means of removing or destroying antigen, regardless of whether or not it has become coated with antibody The key function of complement is probably the opsonization of microorganisms and immune complexes; microorganisms coated (i.e. opsonized) with immunoglobulin and/or complement are more easily recognized by macrophages and more readily bound and phagocytosed through IgG: Fc and C3b receptors

Pathways of complement activation CLASSICAL PATHWAY LECTIN PATHWAY ALTERNATIVE PATHWAY antibody Dependent antibody independent Activation of C3 and generation of C5 convertase activation of C5 LYTIC ATTACK PATHWAY

The complement system Similarly, immune complexes are opsonized by their activation of the classical complement pathway Individuals who lack one of the classical pathway components suffer from immune complex diseases . Soluble complexes are transported in the circulation from the inflammatory site by erythrocytes bearing CR1 which bind to the activated C3 (C3b) in the immune complex. Once in the spleen or liver, these complexes are removed from the red cells, which are then recycled role of C3 in complement activation is underlined by patients with a deficiency of C3, who cannot opsonize pathogens or immune complexes, predisposing them to bacterial infection as well as immune complex diseases.

Complement… Activation by either pathway initiates a cascade of proteolytic events that cleave the proteins into “a” and “b” subunits. The “a” subunits (C3a, C5a) attract (chemotactic factors) phagocytic and inflammatory cells to the site Allow access to soluble molecules and cells by increasing vascular permeability (anaphylactic C3a, C4a, C5a) activate responses. The “b” subunits are bigger bind to the agent to promote their phagocytosis (opsonization) and elimination build a molecular drill that can directly kill the infecting agent

classical pathway The first to be described. It is activated by a number of substances, the most widely recognized being antigen–antibody complexes where the antibody is either IgM or IgG . The reaction of IgM or IgG with its antigen causes a conformational change in the Fc region of the antibody to reveal a binding site for the first component in the classical pathway, C1q. IgA, IgD and IgE do not activate the classical pathway

classical pathway… Once C1q is activated, C1r and C1s are sequentially bound to generate enzyme activity (C1 esterase) for C4 and C2 , splitting both molecules into “a” and “b” fragments. The complex C4b2b is the classical pathway C3 convertase. Other fragments released are C4a, C2a and a vasoactive peptide released from C2. C4b2b cleaves C3 into two fragments, C3a possessing anaphylotoxic and chemotactic activity and C3b that binds to the initiating complex and promotes many of the biological properties of complement. The C4b2b3b complex so generated is an enzyme, C5 convertase, which initiates the final lytic pathway (the ‘attack’ sequence).

Alternate Pathway The alternate pathway is activated directly by bacterial cell surfaces and their components (e.g., endotoxin, microbial polysaccharides), as well as other factors. This pathway can be activated before the establishment of an immune response to the infecting bacteria because it does not depend on antibody and does not involve the early complement components (C1, C2, and C4). The initial activation of the alternate pathway is mediated by properdin factor B binding to C3b and then with properdin factor D, which splits factor B in the complex to yield the Bb active fragment that remains linked to C3b (activation unit). The C3b sticks to the cell surface and anchors the complex. The complement cascade then continues in a manner analogous to the classical pathway

Lectin Pathway The lectin pathway is also a bacterial and fungal defense mechanism. Mannose-binding protein is a large serum protein that binds to nonreduced mannose, frucose, and glucosamine on bacterial, fungal, and other cell surfaces. Mannose-binding protein resembles and replaces the C1q component of the classical pathways and on binding to bacterial surfaces, activates the cleavage of the mannose binding protein–associated serine protease. Mannose binding protein–associated serine protease cleaves the C4 and C2 components to produce the C3 convertase, the junction point of the complement cascade.

The complement system…. The three activation pathways of complement coalesce at a common junction point, the activation of the C3 component

Membrane Attack Complex The terminal stage of the classical pathway involves creation of the membrane attack complex (MAC), which is also called the lytic unit. The five terminal complement proteins (C5 through C9) assemble into an MAC on target cell membranes to mediate injury. Initiation of the MAC assembly begins with C5 cleavage into C5a and C5b fragments. A (C5b,6,7,8)1(C9)n complex forms and drills a hole in the membrane, leading to apoptosis or the hypotonic lysis of cells. Neisseria bacteria are very sensitive to this manner of killing, while gram-positive bacteria are relatively insensitive. The C9 component is similar to perforin, which is produced by cytolytic T cells and NK cells

Biologic Activities of Complement Components Cleavage of the C3 and C5 components produces important factors that enhance clearance of the infectious agent by promoting access to the infection site and reactions. C3b is an opsonin that promotes clearance of bacteria by binding directly to the cell membrane to make the cell more attractive to phagocytic cells, such as neutrophils and macrophages, which have receptors for C3b. C3b can be cleaved further to generate C3d,which is an activator of B lymphocytes. Complement fragments C3a, C4a, and C5a serve as powerful anaphylatoxins that stimulate mast cells to release histamine and tumor necrosis factor-α (TNF-α), which enhances vascular permeability and smooth muscle contraction

Bio.. Activities of Complement …… C3a and C5a also act as attractants (chemotactic factors) for neutrophils and macrophages by increasing adhesion protein expression of the capillary lining near the infection. These proteins are powerful promoters of inflammatory reactions. For many infections, these responses provide the major antimicrobial function of the complement system. The complement system also interacts with the clotting cascade. Activated coagulation factors can cleave C5a, and a protease of the lectin pathway can cleave prothrombin to result in production of fibrin and activation of the clotting cascade.

Regulation of Complement Activation Humans have several mechanisms for preventing generation of the C3 convertase to protect against inappropriate complement activation. These include C1 inhibitor, C4 binding protein, factor H, factor I, and the cell surface proteins, which are decay- accelerating factor (DAF) and membrane cofactor protein. In addition, CD59 (protectin) prevents formation of the MAC. Most infectious agents lack these protective mechanisms and remain susceptible to complement. A genetic deficiency in these protection systems can result in disease.

control of any cascade sequence is extremely important The control of any cascade sequence is extremely important, particularly when it results in the production of potentially self- damaging mediators of inflammation. The complement pathway is controlled by three mechanisms These mechanisms ensure that the potentially harmful effects of complement activation remain confined to the initiating antigen without damaging autologous (host) cells.