The Complement System

Immunology THE COMPLEMENT SYSTEM Complement (written as c') is one of the protective cascading systems in blood. A cascading system comprises of a large number of components or subsystems which activate each other in a sequential manner to produce a specified action. Four cascade systems of molecules are present in the plasma. These are clotting, kinin, fibrinolytic and complement systems. Of these complement system is most important in relation to infections. It is also the most complex comprising of about 25 proteins. It can be activated in two different ways which are called as classical pathway and alternate pathway of complement activation. The activated complement cascade culminates in three useful resultsphagocytosis, lysis and inflammation. General properties of complement · Present in all normal sera · Does not increase on immunization · Non-specific serologic reagent · Destroyed at 56°C in 30 mts. · Not a single substance -complex · In classical pathway; complex is of 9 proteins · In alternate pathway; complex is of 13 proteins · IgM, IgG1, IgG2, IgG3 react with complement · Activation by antigen-antibody complex: · Alternate pathway activated by polysaccharide/enzymes · Produces cytolytic destruction by specific antibody · Inactivators and inhibitors present in serum. Classical Activation Pathway As mentioned in the general properties, there are 9 components of protein complex in complement activation by classical pathway; CI to C9. The CI molecule is activated by certain antigen-antibody reactions and then a cascading effect results into activation of other components of complement. The nine components (units) of complement can be grouped into three: · Recognition Unit: CI (Clq, Clr, CIs) · Activation Complex: C4, C2, C3 · Membrane Attack Complex: C5, C6, C7, C8, C9 Activators of the human blood complement system Activator

Classical pathway

Alternative pathway

Immunologic activators

IgM > IgGl, IgG3 > IgG2

IgA, IgE and some IgG subclasses

Nonimmunologic activators

Trypsin like proteases (plasmin,streptokinase, and lysosomal proteases) Deoxyribonucleic acid polyinosinic acid

Trypsin like proteases (Plasmin, streptokinase, etc)

Staphylococcal protein A C-reactive protein, Polyanion/ polycation complexes

Lipopolysaccharides, damaged mammalian cell walls Plant or bacterial polysaccharides Cobra venom factor

Alternate Pathway for C Activation It is probably the combination of one of the activators with C3b which is obtained from endogenous production from classic pathway. The actual amount of C3b needed for alternate pathway is trivial. Proteins in Alternate Pathway: These include factor B, factor D and properdin (P) as well as C3 convertase. Factor B, the C3 proactivator, is a normal serum protein. The enzyme nature of factor B is not expressed until it is bound to C3b and acted on by a protease. This protease is factor D. Factor D also called as C3 pro activator convertase since it enzymatically converts C3 pro activator (factor B) and generates two fragments- Ba and Bb. Ba is released but Bb remains bound to C3b. The complex of C3b, Bb now displays the typical features of a serine easterase and is able to hydrolyze C3. It hydrolyses C3 at the same peptide bond which is cleaved by C4b, 2a enzyme of the classic pathway. However, the C3 convertase in alternate pathway is labile and decays rapidly. It is stabilized by the addition of properdin. Properdin is composed of four peptide subunits, each with a molecular weight of 46000 held together by hydrogen and ionic bonds. With the generation of stable C3 and C5 convertases, the remainder of the pathway from C6 to C9 is ensured. Since the alternate pathway usually occurs in solution rather than on cell surface, cell lysis is not thought of as a part of this system. Only when the neighbouring cell density is high will the C5-C9 complex associate with cells in so called bystander lysis. Immunologic and Biologic Activities of C' Biological activities of complement components Complement

Activity

C2b (Fluid phase)

Kinin like, increased vasodilatation

C4a

Anaphylatoxin-histamine release

C3a

Anaphylatoxin

C5a

Anaphylatoxin-strong chemotactic factor

C4b(membrane)

Immune adherence, opsonization

C3b (membrane)

Immune adherence, opsonization

C567

Weak chemotactic factor

C5b-C9

Membrane disruption

C4,C2

Virus neutralisation

Immunoconglutinins These are the antibodies that display a specificity towards antigenic determinants that are exposed by fixed complement but which are unavailable in free complement. These are auto-antibodies since these are produced by an animal against its own complement. These antibodies are produced during activation of complement, Le. in infections and after immunization. In the course of complement fixation, new antigenic determinants are exposed. These new sites appear to be in C3b, although some experiments have suggested that they appear in C4b. Conglutinin is a beta globulin and is activated by conglutinin activating factor (KAF) and combines with haemolytically inactive C3b to cause haemagglutination of erythrocytes that have combined with non-agglutinating quantities of antibody. This conglutinin-complement fixation test is a sensitive indicator of a serologic reaction. Regulators of Complement Mediated Functions These can be grouped into two. I. Inactivators which are mostly enzymes that destroy the primary structure of complement protein. These include factor I, factor H, Ana INH, C4 binding protein and C6 INAC. II. Inhibitors which combine with complement molecule to halt its further function. These include C1 sINH and 5 protein. Complement Receptors Four types are: CR1: these are found on a variety of cells including erythrocytes, neutrophils, monocytes, macrophages, B cells and T cells. Important roles assigned to the receptors are phagocytosis, clearance of immune complexes, converting B cells into antibody secretors and may serve in soluble form as a cofactor for Factor 1. CR2: It binds to C3b, C3d and C3d-g. Apart from acting as receptor for E-B virus, no function of CR2 has been ascertained. CR3: Like CR1, it has a wide cell distribution that includes macrophages, LGL, neutrophils and erythrocytes. Miscellaneous receptors for C protiens Physiological roles of C' receptors • Facilitates phagocytosis • Moderation of immune response • Receptors for viruses • Surface markers

Complement Fixation Test (CFT) The complement fixation test (CFT) is an important immunological test which is based upon two properties of complement viz. - C' combines with all Ag-Ab complexes whether or not it is required for that reaction - C' is needed in immunolytic reactions. Source of Complement The main source for the complement proteins are the hepatocytes, epithelial cells of the gut, blood monocytes and tissue macrophages. More than 90% of plasma C3, C6, C8 and C9 is synthesised in liver. Deficiency of Complement Complement activity can be impaired by the absence of one or more of its protein components. Impaired complement activity causes various diseases. Acquired diseases result from temporary depletion of a complement protein; which subside when cells again become able to synthesise that protein. Congenital complement deficiencies are due to permanent genetic defects that prevent synthesis of one or more complement components. The most significant effect of complement deficiencies is lack of resistance to infection. In patients with C3 deficiencies, chemotaxis, opsonization and cell lysis are impaired. Disease states related to complement deficiency Disease state

Complement deficiency

Severe recurrent infections

C3

Recurrent infections of lesser severity

CI, C2, C5

Systemic lupus erythematosus

CI,C2,C4,C5,C8

Glomerulonephritis

CI,C8

Gonococcal infections

C6,C8

Meningococcal infections

C6