IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 4. Complement

BIOLOGICAL EFFECTS OF COMPLEMENT

The biological activities of The Complement System can be divided into those that are beneficial to the host and those that are harmful.

The main beneficial effects of complement are:

✵ assisting in the destruction of microorganisms;

✵ efficient clearance of immune complexes;

✵ induction and enhancement of the HUMORAL Immune Response.

The complement system can cause damage to the host's own Cells and Tissues in the following situations:

✵ when generalized massive activation occurs, for instance in Gram-negative bacterial septicemia;

✵ when activation takes place within a focus of tissue necrosis, particularly in myocardial infarction;

✵ when activation occurs during an autoimmune reaction in tissues.

Complement promotes the destruction of microorganisms. Enhanced elimination of microbes is achieved through several pathways, including:

✵ the generation of anaphylatoxins, which increase vascular permeability, thereby facilitating the influx of other protective factors of the inflammatory response into the site of infection;

✵ opsonization of microbes to enhance phagocytosis;

✵ insertion of the membrane-attack complex into the membrane of microbial cells.

Anaphylatoxins are potent Inducers of inflammation. Activation of the complement system leads to The formation of the anaphylatoxins C3a and C5a, whose physiological role is to recruit inflammatory exudate cells to the site of inflammation and to activate their effector mechanisms.

Systemic administration of C5a or generalized intravascular complement activation (e.g., in Gram-negative Sepsis) can lead to cardiovascular collapse and bronchospasm — i.e., a condition resembling anaphylaxis (hence the name anaphylatoxins).

Activities of C5a. C5a acts as a potent activator of all myeloid Cell types (Fig. 4.18). This anaphylatoxin induces chemokinesis and chemotaxis of neutrophils, their degranulation, and a respiratory burst accompanied by The production of oxygen radicals. In addition, C5a stimulates the METABOLISM of membrane-bound arachidonic acid to yield Prostaglandins and Eicosanoids. This is accompanied by an upregulation of Cell Adhesion molecules on The Cell surface, promoting cellular attachment to the vascular endothelium (Fig. 4.18). In monocytes and macrophages, C5a elicits similar responses as well as the secretion of IL-1 and IL-6, whereas in basophils and mast cells, it induces degranulation with the release of histamine and other vasoactive mediators.

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Fig. 4.18. The anaphylatoxin C5a causes 1) neutrophil activation, 2) increased expression of cell adhesion molecules, 3) neutrophil emigration and chemotaxis, 4) monocyte activation, and 5) mast cell degranulation, resulting in smooth Muscle contraction and increased vascular permeability.

By activating these cells, C5a indirectly affects Blood Vessels, increasing their permeability, and smooth muscle, causing contraction. Furthermore, C5a can act synergistically with other inflammatory mediators, for instance, stimulating IL-1 secretion by monocytes together with IFNγ or endotoxin.

Half-life of C5a. The presence of C5a in the Circulation is very brief, as would be expected for such a potent inflammatory mediator. The blood-borne enzyme carboxypeptidase N removes the C-terminal Arginine residue from C5a, substantially diminishing all biological activities of this effector except its chemotactic activity. This is followed by binding to the C5a receptor, internalization, and rapid intracellular degradation by proteases into inactive fragments.

Activity of C3a. Compared to C5a, this complement subcomponent exhibits much lower activity and binds to a distinct cellular receptor. It induces weak neutrophil aggregation and a respiratory burst, but unlike C5a, it lacks chemotactic activity.

It should be noted that anaphylatoxins are generated not only through complement activation, but also via other enzymatic systems that directly cleave C3, C4, and C5. Such Enzymes include plasmin, kallikrein, tissue and leukocyte (lysosomal) proteases (specifically neutrophil Elastase), as well as Proteolytic Enzymes of microbial origin, such as gingipain-1 from the bacterium Porphyromonas gingivalis, which is associated with periodontal disease.

Fixed C3b and C4b act as opsonins, enhancing phagocytosis. By binding covalently to bacterial surfaces and immune complexes, C3b and C4b convert them into ligands for complement receptors on phagocytic cells. Thus, they ensure the clearance of Bacteria and immune complexes from the bloodstream. The binding of C3b and C4b to complement receptors On the surface of neutrophils, monocytes, and macrophages can induce, apart from phagocytosis stimulation, the exocytosis of granules containing proteolytic enzymes and the generation of free oxygen radicals As a result of the respiratory burst (Fig. 4.19).

Fig. 4.19. The C3 component bound to a bacterial cell as C3b or iC3b: 1) interacts with erythrocyte CR1, through which bacteria are transported via the bloodstream; 2) serves as a "docking site" for the membrane-attack complex on The surface of bacterial cells; 3) "crosslinks" complement receptors on phagocytes; 4) activates phagocytes by stimulating phagocytosis, the respiratory burst, and bactericidal activity.

Complement deficiency is associated with Susceptibility to infectious diseases. The Physiological Role of complement in opsonization and bacteriolysis becomes entirely clear when analyzing two forms of its inherited deficiency. Deficiencies in the classical pathway components and C3, as well as deficiencies in the CR3/CR4/LFA-1 receptor family, are frequently associated with Infections caused by pyogenic bacteria. The fact that a deficiency in either opsonins or receptors leads to identical consequences strongly suggests a critical role for complement in eliminating these bacteria via phagocytosis and intracellular destruction.

In contrast, deficiencies in MAC components are associated almost exclusively with an increased susceptibility to Neisseria meningitidis infection. It can be hypothesized that host resistance against this bacterium, which is capable of surviving inside phagocytes, relies on complement-dependent bacteriolysis in Blood Plasma.

Complement appears to play a less significant role in antiviral defense, where T cells are of decisive importance. Complement deficiency is typically not accompanied by an increased susceptibility to viral infections.

The membrane-attack complex also participates in the inflammatory response. According to the traditional view, the MAC destroys any cell by forming Pores in the membrane and inducing its lysis. However, it has more recently been established that nucleated cells, such as Cells of the body's immune system, are relatively resistant to the lytic action of the MAC. This is partly due to the presence of regulatory molecules like CD59 on the membrane, but also to the ability of these cells to eliminate—via endocytosis or exocytosis—The Plasma Membrane patches where the MAC has inserted itself. Even in the case of sublethal MAC exposure, the resulting alteration in the membrane bilayer Structure can stimulate immune cells (depending on their tissue origin) to release and metabolize arachidonic acid, enhance oxidative metabolism, undergo degranulation, or secrete cytokines. These reactions may be important for amplifying inflammation at sites of complement activation.

Pathogenic microorganisms counteract complement effects

The interaction between the complement system and microbes can be viewed as a factor in an ongoing evolutionary interspecies struggle. As the complement system evolved, likely under selective pressure primarily driven by infectious diseases, microbes in turn developed mechanisms to evade complement attacks and even "co-opt" this system to facilitate infection. In fact, pathogens are pathogenic precisely because of their ability to circumvent, to a certain degree, the host's defense mechanisms against infection.

Gram-negative bacteria display C3b- and MAC-binding structures that render complement bactericidal activity ineffective. The outer layer of The Cell wall in most Gram-negative bacteria contains lipopolysaccharide (LPS) with long O-specific polysaccharide side chains projecting outward from the membrane. These efficiently activate complement but restrict covalent C3 binding and MAC fixation to a distance from the bacterial cytoplasmic membrane where opsonization and lysis are impossible. In such cases, only bactericidal Antibodies can function as acquired Immunity factors. They activate complement in the immediate vicinity of those bacterial surface areas where its opsonizing and lytic effects can be realized.

Certain bacteria possess an outer coat resistant to opsonization. A number of microorganisms are resistant to complement action due to the presence of surface molecules that inhibit alternative complement activation and restrict C3 fixation. For example, strains of pathogenic Gram-positive bacteria differ from their non-pathogenic counterparts by possessing a capsule rich in sialic acid, on which C3b binds factor H rather than factor B, thereby undergoing degradation.

Microbes can express molecules that suppress complement activation. Another strategy used by microbes to bypass complement action is the expression of inhibitors similar to those of the host Organism. Bacterial surface molecules with Fc receptor-like properties are known, such as staphylococcal protein A and the Fc receptor found in many Herpesviruses. Furthermore, the expression of a complement receptor (glycoprotein C) by the Herpes simplex virus has recently been discovered. The fungus Candida albicans expresses molecules resembling CR2 and CR3, even showing antigenic Homology with human CR3. All these molecules can protect microorganisms from the typical consequences of antibody and complement binding. For instance, IgG or C3 bound to microbial surface receptors may lose their ability to interact with Fc receptors on phagocytic cells. Another strategic pathway involves the expression of regulatory molecules that suppress complement activation. For example, trypanosomes produce DAF- and CD59-like molecules, whereas schistosomes simply adsorb host DAF to achieve the same goal.

Some Viruses use the complement system to enhance their pathogenic effects

The most critical stage in the Pathogenesis of viral infections is The entry of the pathogen into host cells. It has been established that certain viruses utilize cell membrane-bound complement components as receptors to enhance cell entry. For instance, Epstein-Barr virus uses CR2, measles virus uses membrane cofactor protein (MCP, CD46), and a number of echoviruses use decay-accelerating factor (DAF, CD55).

The entry of several other viruses into cells can be facilitated by their binding to antibodies and fluid-phase C3b. Specifically, antibody interaction enhances the cellular uptake of Flaviviruses (including dengue virus) via macrophage Fc receptors, while C3 binding to Viral Particles promotes the uptake of West Nile virus (also a flavivirus) via CR3.

Complement plays an important auxiliary role in the Induction of the immune response

The complement system facilitates contact and interaction between antigen-presenting cells, B cells, and the antigen (Fig. 4.20). For example, the localization of immune complexes within germinal centers inside Lymph Nodes—which is necessary for the formation of memory B cells—is dependent on complement.

Fig. 4.20. The C3 component binds to immune complexes, thereby: 1) reducing the size of lattice-structured immune aggregates; 2) mediating the binding of circulating immune complexes to erythrocyte CR1, which transports these complexes through the bloodstream; 3) promoting the uptake of immune complexes by fixed mononuclear phagocytes, leading to antigen destruction; and 4) facilitating the localization of antigen in the form of immune complexes on B lymphocytes and antigen-presenting cells, including specialized follicular dendritic cells of the lymph nodes.

The following complement receptors have been identified on B cells and APCs:

✵ B cells: CR1, which binds C3b and iC3b, and CR2, which binds iC3b and C3dg;

✵ Monocytes and macrophages: CR1 and CR3;

✵ Follicular dendritic cells (the only cell type possessing all three receptors): CR1, CR2, and CR3.

Individuals with an inherited C3 deficiency suffer from only a moderate impairment in antibody production. However, in guinea pigs deficient in C2, C3, or C4, a marked suppression of Primary and secondary immune responses to low doses of T-dependent Antigens is observed. These facts indicate an auxiliary (though not decisive) role for complement in the efficient induction of antibody production.

Complement participates in immune complex Processing

In the 1940s, Heidelberger discovered that complement prevents the formation of a lattice-like structure in precipitating antigen-antibody complexes. The structure and size of immune complexes are influenced by numerous factors, including the following:

✵ the concentration of reactants (antibodies and antigen);

✵ the affinity of antibodies for the homologous antigen;

✵ the valence of both antibodies and antigen (the higher the valence, the larger the resulting complexes).

Complement activation via the classical pathway inhibits the formation of immune complex precipitates in blood plasma. Similarly, activation via The alternative pathway can induce the dissolution of immune complexes that have already formed precipitates in plasma as well as in tissues. This dissolution occurs as a result of the covalent incorporation of C3 into the lattice STRUCTURE OF THE immune precipitate: C3 disrupts the binding of antibodies to antigen epitopes, thereby limiting the potential for the formation of large aggregates (see Ch. 25).

Complement activation by immune complexes is normally physiologically beneficial, as C3-bound complexes are efficiently cleared from tissues and the bloodstream by monocytes and other phagocytic cells (Fig. 4.20). However, in certain cases, intensive immune complex formation persists chronically, whereupon their activation of the complement system has detrimental consequences; this occurs, in particular, in subacute bacterial endocarditis and systemic lupus erythematosus.

Complement contributes to The Development of certain diseases

Systemic complement activation leads to the generation of large quantities of anaphylatoxins. Under specific conditions, complement activation in vivo plays a deleterious rather than a protective role (Fig. 4.21). For instance, Shock in Gram-negative bacterial bacteremia is partly attributable to systemic complement activation by endotoxin. The resulting large amounts of C3a and C5a trigger the activation and degranulation of neutrophils, basophils, and mast cells. Intravascular neutrophil aggregation leads to disseminated intravascular coagulation and the trapping of microemboli in pulmonary capillaries, where leukocyte-derived products (including elastase and free radicals) can precipitate adult respiratory distress syndrome. This condition is characterized by interstitial pulmonary edema resulting from microvascular injury, the formation of a neutrophil exudate in the alveoli, and arterial hypoxemia.

Fig. 4.21. Complement activation can trigger pathological reactions as a result of 1) the systemic generation of anaphylatoxins (e.g., in Gram-negative septicemia), 2) the insertion of the membrane attack complex into the host's own cell membranes (leading to cellular activation and the release of membrane-derived arachidonic acid metabolites), and 3) the fixation of C3 (which recruits and activates tissue and circulating leukocytes) onto immune complexes localized within tissues.

Extracorporeal circulation using Heart-lung machines or cuprophan dialyzers can induce extracorporeal complement activation, accompanied by transient leukopenia much akin to that seen during neutrophil aggregation in the Lungs.

Tissue necrosis activates complement. Tissue injury resulting from ischemic necrosis can provoke local complement activation and extensive MAC deposition on The cell membrane. The potential pathophysiological role of complement activation in this Setting is underscored by experimental myocardial infarction models, in which reduced complement levels attenuate the extent of tissue damage. A similar effect has recently been demonstrated following the administration of soluble recombinant CR1.

Complement activation via in vivo immune complex formation is a potential cause of tissue injury. Complement activation plays a critical role in the pathogenesis of tissue destruction in immune complex-mediated diseases. Such complexes can form within tissues, such as in the renal glomeruli in Goodpasture's syndrome caused by autoantibodies against the glomerular basement membrane, or at the motor endplates in myasthenia gravis involving autoantibodies against acetylcholine receptors (see Ch. 24). In other instances, circulating immune complexes may deposit in the walls of blood vessels (see Ch. 25). For example, in bacterial endocarditis, the infected heart valve serves as a source of immune complexes that settle in the Kidneys or other microvascular beds.

In immune complex diseases, complement drives inflammation primarily through the following two mechanisms:

✵ leukocytes bind to C3b and C4b fixed on immune complexes, becoming activated and recruited to the sites of complex deposition by locally generated anaphylatoxins; this initiates tissue injury in Goodpasture's syndrome, and suppressing the inflammatory response in EXPERIMENTAL MODELS OF this disease requires only a reduction in blood complement or neutrophil levels;

✵ the MAC (membrane attack complex) damages the Cell Membrane and concurrently stimulates the production of prostaglandins from arachidonic acid. This underlies the tissue injury seen in membranous nephropathy, which can be experimentally induced using antibodies directed against subepithelial antigens. The inflammatory response in this scenario is not abrogated by neutrophil depletion, yet is almost entirely absent in C5-deficient animals. The basement membrane likely acts as a physical barrier to neutrophil migration, meaning that the observed marked proteinuria is driven solely by membrane attack complex deposition.

Questions for Review

■ What is the mechanism by which host tissues are protected from damage by their own complement system?

■ Where does complement activation begin?

■ How might a hereditary deficiency of factor H affect a child's complement system? What diseases are such children at risk for?

■ What are the effector mechanisms of the complement system?

■ What is the internal thioester bond in The structure of C3 and C4? What is its significance for the complement system?

■ What accounts for the selectivity whereby complement activation occurs on the surface of foreign cells rather than host cells?

■ How do the complement system and microbes interact with one another?

Introduction/47.html">Further Reading

Ahearn J.M., Fischer М.В., Croix D. et al. 1996. Disruption of the Cr2 locus results in a reduction in B-1a cells and in an impaired В cell response to T- dependent antigen. Immunity 4: 251-262.

Aulak K.S., Donaldson V.H., Coutinho M. et al. 1993. C1-inhibitor: structure/function and biologic role. Behring Inst. Mitt. 93: 204-213.

Bkakdi S., Tranum Jensen J. 1991. Complement lysis: a hole is a hole. Immunol. Today 12: 318-320.

Campbell R.D., Law S.K.A., Reid K.B.M. et al. 1988. Structure, Organization and Regulation of the complement genes. Annu Rev. Immunol. 6: 161-195.

Colten H.R., Rosen F.S. 1992. Complement deficiencies. Annu. Rev. Immunol. 10: 809-834.

Cooper N.R. 1985. The classical complement pathway: activation and regulation of the first complement component. Adv. Immunol. 37: 151-216.

Davies K.A., Schifferli J.A., Walport M.J. 1994. Complement deficiency and immune complex disease. Springer Semin. Immunopathol. 15: 397-416.

Dodds A.W., Ren X.D., Willis A.C. et al. 1996. The reaction mechanism of the internal thloester in the human complement component C4. Nature 379: 177-179.

Esser A.F. 1991. Big MAC attack: complement Proteins cause leaky-patches. Immunol. Today 12: 316-318.

Farries T.C., Atkinson J.P. 1991. Evolution of the complement system. Immunol. Today 12: 295-300.

Fearon D.T., Locksley R.M. 1996. The instructive role of innate immunity in the acquired immune response. Science 272: 50-53.

Frank M.M. 1992. The mechanism by which microorganism avoid complement attack. Curr. Opion. Immunol. 4: 14-19.

Frank M.M., Fries L.F. 1991. The Role of complement in inflammation and phagocytosis. Immunol. Today 12: 322-326.

Gerard C., Gerard N.P. 1994. C5a anaphylatoxin and its seven transmembrane-segment receptor. Annu. Rev. Immunol. 12: 775-808.

Holmskov U., Mathotra R., Sim R.B. et al. 1994. Collectins: collagenous C-type Lectins of the innate immune defense system. Immunol. Today 14: 67-74.

Hourcade D., Holers V.M., Atkinson J.P. 1989. The regulators of complement activation (RCA) Gene cluster. Adv. Immunol. 45: 381-416.

Joiner K.A. 1988. Complement evasion by bacteria and parasites. Annu. Rev. Microbiol. 42: 201-230.

Kinoshita T, Inoue N., Takeda J. 1996. Role of phos- phatidylinositol-linked proteins in paroxysmal nocturnal hemoglobinuria pathogenesis. Annu. Rev. Med. 47: 1-10.

Lachmann P.J., Walport M.J. 1987. Deficiency of the effector mechanisms of the immune response and autoimmunity. In: Whelan J. (ed.). Autoimmunity and Autoimmune Diseases. Chichester: Wiley, 149-171.

Liszewski M.K., Parries T.C., Lublin D.M. et al. 1996. Control of the complement system. Adv. Immunol. 61:201-283.

Moffitt M.C., Frank M.M. 1994. Complement resistance in microbes. Springer Semin. Immunopathol. 15: 327-344.

Morgan B.P. 1995. Complement regulatory molecules: application to therapy and transplantation. Immunol. Today 16:257-259.

Morgan B.P., Meri S. 1994. Membrane Proteins that protect against complement lysis. Springer Semin. Immunopathol. 15: 369-396.

Morgan B.P., Walport M.J. 1991. Complement deficiency and disease. Immunol. Today 12: 301-306.

Muller-Eberhard H.J. 1986. The membrane attack complex of complement. Annu. Rev. Immunol. 4:503-528.

Muller-Eberhard H.J., Schreiber R.D. 1980. Molecular biology and Chemistry of the alternative pathway of complement. Adv. Immunol. 29: 1-53.

Reid K.B.M., Day A J. 1989. Structure-function relationships of the complement components. Immunol. Today 10: 177-180.

Reid K.B.M., Porter R.R. 1981. The proteolytic activation systems of complement. Annu. Rev. Biochem. 50. 433-464.

Reid K.B.M., Turner M.W. 1994. Mammalian lectins in activation and clearance mechanisms involving the complement system. Springer Semin. Immunopathol. 15: 307-326.

Ross G.D. (ed.) 1986. Immunobiology of the Complement System. New York. Academic Press.

Smith G.L. 1994. Virus strategies for evasion of the host response to infection. Trends Microbiol. 2: 81-88.

Walport M.J. 1993. Inherited complement deficiency - clues to the physiological activity of complement in vivo. Q.J. Med. 86: 355-358.

Walport M.J., Lachmann P.J. 1993. Complement. In: Lachmann P.J., Peters D.K., Rosen F.S., Walport M.J. (eds.). Clinical Aspects of Immunology, 5th edn. Oxford: Blackwell Scientific Press., 347-375.