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Complement Determinations in Human Disease M CME review article This feature is supported by an unrestricted educational grant from AstraZeneca LP Complement determinations in human disease M. Michael Glovsky, MD*; Peter A. Ward, MD†; and Kent J. Johnson, MD† Objective: To define techniques used for complement measurements and examine the clinical relevance of alterations of complement determinations in disease. Data Sources: Data have been assembled from the authors’ research, original articles, and reviews, as well as chapters and complete books on complement. Study Selection: Studies were chosen for inclusion by the opinions of the authors, relevant complement reviews, publications, and books. Results: Complement has been shown to possess approximately 31 proteins, some of which are enzymes (C1r, C1s, C2, factor B, factor D), some cofactors, some inhibitors or inactivators, and others composed of membrane-integrated proteins. All of the complement proteins have been purified, and many of the respective genes have been identified. The complement cascade is a dual-edged sword, causing protection against bacterial and viral invasion by promoting phagocytosis and inflammation. Pathologically, complement can cause substantial damage to blood vessels (vasculitis), kidney basement membrane and attached endothelial and epithelial cells (nephritis), joint synovium (arthritis), and erythrocytes (hemolysis) if it is not adequately controlled. Conclusions: Definitive evidence is available that complement-mediated tissue destruction occurs after immune complex injury in the kidney and lung and may be important in lupus erythematosus and adult respiratory distress syndrome. Future studies on complement receptor structure and function may provide clues to treat effectively lupus, hemolytic anemias, and nephritis. In addition, gene therapy and antibody therapy need further refinement to treat immunodeficiency diseases. Ann Allergy Asthma Immunol. 2004;93:513–523. Off-label disclosure: Drs. Glovsky, Ward, and Johnson have indicated that this article does not include the discussion of unapproved/investigative use of a commercial product/device. Financial disclosure: Drs. Glovsky, Ward, and Johnson have indicated that in the last 12 months they have not had any financial relationship, affiliation, or arrangement with any corporate sponsors or commercial entities that provide financial support, eduction grants, honoraria, or research support or involvement as a consultant, speaker’s bureau member, or major stock shareholder whose products are prominently featured either in this article or with the groups who provide general financial support for this CME program. Instructions for CME credit 1. Read the CME review article in this issue carefully and complete the activity by answering the self-assessment examination questions on the form on page 524. 2. To receive CME credit, complete the entire form and submit it to the ACAAI office within 1 year after receipt of this issue of the Annals. INTRODUCTION cofactors, some inhibitors or inactivators, and others com- Since the discovery of complement by Jules Bordet more than posed of membrane-integrated proteins. All of the comple- 100 years ago, the importance of the complement system in ment proteins have been purified, and many of the respective producing lysis of bacteria and protecting humans and exper- genes have been identified.3 Other proteins of the comple- imental animals against infectious microorganisms has been ment system include the receptors for C1q, C3a/C4a, C5a, 1–9 appreciated. Within the past 30 years, complement has CR1, CR2, and CR3 (C3b, C3bi, and C3d receptors). In been shown to possess approximately 31 proteins, some of addition, membrane components (decay-accelerating factor, which are enzymes (C1r, C1s, C2, factor B, factor D), some CD55 and CD59, and membrane inhibitor of C8 and C9 insertion) are important regulating proteins. The complement cascade is a dual-edged sword, causing protection against * Huntington Medical Research Institute, Pasadena, California. bacterial and viral invasion by promoting phagocytosis and † Department of Pathology, University of Michigan Medical School, Ann inflammation. Pathologically, complement can cause sub- Arbor, Michigan. Received for publication March 1, 2004. stantial damage to blood vessels (vasculitis), kidney base- Accepted for publication in revised form May 19, 2004. ment membrane and attached endothelial and epithelial cells VOLUME 93, DECEMBER, 2004 513 (nephritis), joint synovium (arthritis), and erythrocytes (he- and C2a and C2b, respectively. C4b and C2a form C4b2a (C3 molysis) if it is not adequately controlled. This review defines convertase of the classical pathway), which splits C3 to C3a techniques used for complement measurements and examines and C3b. C3b combines with C4b2a to form C4b2a3b (C5 the clinical relevance of alterations of complement determi- convertase), which cleaves C5 to C5b and C5a. This leads to nations in disease. Data have been assembled from the au- formation of the C5b, C6, C7, C8, and C9 membrane attack thors’ research, original articles, and reviews, as well as complex (MAC). chapters and complete books on complement. Mannose-binding lectin, when bound to mannose residues on microbial surfaces, can interact with 2 serine proteases: COMPLEMENT ACTIVATION MECHANISMS MASP-2 and MASP-1. MASP-2 cleaves and inactivates C4 There are 3 currently known complement activation mecha- and C2 to form C4b2a. MASP-1 may cleave C3 directly, nisms (Fig 1): (1) the classical pathway, (2) a recently de- promoting the formation of C4b2a3b, the C5 convertase of scribed pathway (the mannose-binding lectin pathway), and the classical pathway. (3) the alternative pathway. The alternative pathway can be activated in the absence of Immune complexes, apoptotic cells, or C1q bound to its antibody by insoluble polysaccharides, yeast cell walls, and ligand can activate the classical pathway. On activation, C1r aggregated IgA and IgE at high concentrations. Factor D, a and C1s are converted from proenzymes to activated enzymes serine protease, cleaves factor B to Ba and Bb. The larger and cleave their natural substrates C4 and C2 to C4a and C4b fragment of factor B combines with the large fragment of C3, Figure 1. Complement activation pathways. 514 ANNALS OF ALLERGY, ASTHMA & IMMUNOLOGY C3b, to form the alternative pathway C3 convertase, C3bBb. CONTROL MECHANISMS OF THE COMPLEMENT Properdin stabilizes the C3 convertase by binding to the SYSTEM complex, PC3bBb. C3bBb then can further cleave C3 to form To control activation of the classical pathway, C1 inhibitor additional C3b molecules, some of which combine with binds to activated C1r and C1s and inactivates these enzymes. C3bBb to form C3bBbC3b, the C5 convertase of the alter- The binding is in a 1:1 ratio. The C4b2a complex (C3 native pathway, which cleaves C5 to C5a and C5b. Thus, the convertase) is controlled by C4 binding protein, which binds classical pathway and the mannose-binding lectin pathway, to the complex. C4b is then cleaved by factor I. Under as well as the alternative pathway, converge on C3 to initiate physiologic conditions, the classical pathway control stops the formation of C3b, the complement factor, when bound to the further breakdown of classical pathway components. The microorganisms, which initiates phagocytosis and destruction alternative pathway C3 convertase (C3bBb) is regulated by of the invading microbe. C3a, a proinflammatory anaphyla- factor H, which competes with Bb, the split product of factor toxin, is also generated. B, for the binding site on C3b, forming (C3bH).9 The C3b is The MAC complex (C5b-C9) and C5a (the most potent further degraded by factor I to smaller breakdown products. anaphylotoxin) are generated by enzymatic cleavage of C5 to Note the similar control mechanisms of the classical and C5b and C5a. Table 1 lists alterations in the complement alternative pathways.9 The classical and alternative pathway activation pathways and some of the diseases associated with convertases are broken up by C4b binding protein and factor activation mechanisms. H, respectively. C4b and C3b are then cleaved by factor I to Activation of the classical complement pathway such as inactive products. seen in immune complex disease associated with DNA–anti- DNA antibodies is often associated with low CH50, low C4, LABORATORY MEASUREMENT and low C3. Alternative pathway activation of factor B and Complement components can be measured either as proteins C3 is seen in endotoxin shock syndromes, as well as in sera or based on their functional activity.3,4,10–19 Because the pro- of patients with membranoproliferative glomerulonephritis. teins that comprise the classical and alternative complement Also, activation of the classical pathway occurs in the relative cascades have been isolated in chemically pure form, anti- absence of the C1 inhibitor as seen in hereditary angioedema bodies have been produced to these proteins in several animal (HAE). Fluid-phase activation of the alternative pathway species. These antibodies may be used to quantitate comple- occurs with factor I deficiency, when C3 convertase activity ment components through one of several immunochemical is not controlled. With chronic inflammation that occurs in procedures.3 Reiter syndrome, acute-phase reactants, such as C4, factor B, In radial immunodiffusion, antibody to a single comple- and haptoglobin, may be increased. Young children with low ment component is incorporated into an agar gel. Test serum mannan-binding lectin levels
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