COMPLEMENT
Genetic deficiencies of each of the complement compo- nents and many of the regulators have been described and provide valuable ‘experiments of nature’ illustrating the homeostatic roles of complement. In general, complement deficiencies are rare, though some deficiencies are much more common in some racial groups.
A variety of assays (Method Box 4.1) are available for detecting:
• the activity of different complement pathways; • the functional activity of individual components; • the total amount of individual components (functional
or non-functional).
The consequences of a deficiency in part of the com- plement system depend upon the pathway(s) affected (Fig. 4.16).
Complement plays important roles in adaptive immunity
Fig. 4.15 C3 fragments bound to antigen (Ag) bind complement receptors on B cells and follicular dendritic cells (FDCs), enhancing B cell development at multiple stages in the process. (1) B cells bind Ag through the B cell receptor (BCR) and bind Ag-attached C3d through CR2. The combined signals, delivered through these receptors and their co- receptors, markedly enhance positive selection of Ag-reactive B cells and subsequent maturation. (2) Binding of C3d- opsonized Ag to mature B cells in the lymphoid follicles (with appropriate T cell help) triggers B cell activation and proliferation. (3) In the spleen and bone marrow, C3- opsonized Ag binds complement receptors on FDCs to retain Ag on the FDC, where it is efficiently presented to activated B cells. Ligation of BCR and C3d on the activated B cell triggers differentiation to plasma cells and B memory cells.
mature B cell mature B cell positive selection immature B cell CR2 BCR Ag–C3d activated B cells plasma cells activation and expansion Ag–C3d 1 2 3 T cell help antigen retention and B cell selection FDCs B memory cells
METHOD BOX 4.1
A variety of assays are available for detecting: • the activity of different complement pathways; • the functional activity of individual components; • the total amount of individual components (functional or
non-functional).
Functional activity of individual components is measured in similar hemolytic assays by assessing the capacity of the test serum to restore hemolytic activity to sera depleted of indi- vidual components – for example, C6 activity would be tested using C6-depleted serum in a classical pathway assay.
The total amount of an individual component is measured using specific antisera in techniques such as nephelometry, radial immunodiffusion, and rocket immunoelectrophoresis.
Measuring classical and alternative pathway activity
Fig. 1 Classical pathway activity is measured using antibody-sensitized sheep erythrocytes in a buffer containing both Ca2+and Mg2+ions. Alternative pathway
activity is measured using rabbit erythrocytes in a buffer containing Mg2+ions but no Ca2+ions. In both assays,
standards and samples diluted in the specific buffer are incubated with the relevant target cell and the amount of hemolysis is measured. The results are converted mathematically to standardized hemolytic units (CH50 for classical pathway, AH50 for alternative pathway).
E CH50 antibody lysis⇒ classical pathway active test serum sheep Ca2+, Mg2+ E AH50 lysis⇒ alternative pathway active test serum rabbit or guinea pig Mg2+only
Classical pathway deficiencies result in tissue inflammation
Deficiency of any of the components of the classical path- way (C1, C4, and C2) predisposes to a condition that closely resembles the autoimmune disease systemic lupus erythematosus (SLE), in which immune complexes become deposited in the kidney, skin, and blood vessels.
Deficiency of any of the C1 subunits (C1q, C1r, or
C1s) invariably causes severe disease with typical SLE features including skin lesions and kidney damage. The disease usually manifests early in childhood and few patients reach adulthood.
C4 deficiency also causes severe SLE. Total deficiency
of C4 is extremely rare because C4 is encoded by two separate genes (C4A and C4B), but partial deficiencies of C4 are relatively common and are associated with an increased incidence of SLE.
C2 deficiency is the commonest complement defi-
ciency in Caucasians. Although it predisposes to SLE, the majority of C2-deficient individuals are healthy.
The large majority of cases of SLE are, however, not associated with complement deficiencies, and auto- immune SLE is discussed below and on p. 376.
The historical view of immune complex disease in classical pathway deficiency was based upon defective immune complex handling.
Q. Why would a deficiency in the classical pathway lead to impaired handling of immune complexes?
A. Classical pathway activation and opsonization of immune complexes helps prevent precipitation in tissues and aids the carriage of immune complexes on erythrocytes. Classical path- way deficiencies would therefore result in a failure to maintain solubilization and permit the resultant precipitation of immune complexes in the tissues where they drive inflammation.
Although these mechanisms of immune complex handling undoubtedly contribute, a new perspective has recently developed that takes a different view of the role of complement in waste management.
COMPLEMENT DEFICIENCIES ILLUSTRATE THE HOMEOSTATIC ROLES OF COMPLEMENT
Complement system deficiencies
Fig. 4.16 A summary of the clinical consequences of the various complement deficiencies. Black arrows denote pathway, red arrows show strong effects, and blue arrows indicate weak effects.
classical pathway MBL pathway alternative pathway
antigen–antibody immune complex C1q, C1r, C1s lupus-like disease severe, recurrent bacterial infections severe, recurrent bacterial infections P MAC
C6, C7, C8, C9 recurrent neisserialinfection neisserial infection fH, fI D B C4 C4 C2 C2 C3 C5 MBL, MASP1, MASP2 C3(H2O)
microbes with terminal mannose groups
bacteria, fungi, virus, or tumor cells
Cells continually die by apoptosis in tissues and are removed silently by tissue macrophages. Complement contributes to this essential process because the apoptotic cell binds C1q and activates the classical pathway. In C1 deficiencies, apoptotic cells accumulate in the tissues and eventually undergo necrosis, which releases toxic cell contents and causes inflammation.
This recent observation, emerging from studies in complement deficiencies, has altered the way we think of the handling of waste in the body and moved complement to center-stage in this vital housekeeping role.
Deficiencies of MBL are associated with infection in infants
MBL is a complex multi-chain collectin. Each chain comprises a collagenous stalk linked to a globular carbo- hydrate recognition domain.
The plasma level of MBL is extremely variable in the population, and governed by a series of single nucleotide polymorphisms in the MBL gene, either in the promoter region or in the first exon, encoding part of the collage- nous stalk:
• mutations in the promoter region alter the efficiency of gene transcription;
• mutations in the first exon disrupt the regular structure of the collagenous stalk, destabilizing complexes con- taining mutated chains, perhaps disrupting association with the MASP enzymes.
At least seven distinct haplotypes arise from mixing of these mutations, four of which yield very low plasma MBL levels. As a consequence, at least 10% of the population have MBL levels below 0.1μg/ml and are considered to be MBL deficient.
MBL deficiency is associated in infants with increased
susceptibility to bacterial infections. This tendency dis- appears as the individual ages and the other arms of immunity mature.
In adults, MBL deficiency is of little consequence unless there is an accompanying immunosuppression – for example, people with HIV infection who are MBL defi- cient appear to have more infections than those who have high levels of MBL.
Alternative pathway and C3 deficiencies are associated with bacterial infections
Deficiencies of either fB or fD prevent complement
amplification through the alternative pathway amplifica- tion loop, markedly reducing the efficiency of opsoniza- tion of pathogens. As a consequence, deficient individuals are susceptible to bacterial infections and present with a history of severe recurrent infections with a variety of pyogenic (pus-forming) bacteria. Only a few families with each of these deficiencies has been identified, but the severity of the condition makes it imperative to identify affected families so that prophylactic antibiotic therapy can be initiated.
C3 is the cornerstone of complement, essential for all activation pathways and for MAC assembly, and is also the source of the major opsonic fragments C3b and iC3b. Individuals with C3 deficiency present early in
childhood with a history of severe, recurrent bacterial infections affecting the respiratory system, gut, skin, and other organs. Untreated, all die before adulthood. When given broad-spectrum antibiotic prophylaxis, patients do reasonably well and survival into adulthood becomes the norm.
Terminal pathway deficiencies predispose to Gram-negative bacterial infections
Deficiencies of any of the terminal complement components (C5, C6, C7, C8, or C9) predisposes to
infections with Gram-negative bacteria, particularly those of the genus Neisseria. This genus includes the meningococci responsible for meningococcal meningitis and the gonococci responsible for gonorrhea.
Q. Why should these deficiencies be specifically associated with infection by Gram-negative bacteria and not with all bacterial infections?
A. Gram-negative bacteria have an outer phospholipid mem- brane, which may be targeted by the lytic pathway. Gram- positive bacteria have a thick bacterial cell wall on the outside.
Individuals with terminal pathway deficiencies usually present with meningitis, which is often recurrent and often accompanied by septicemia. Any patients with second or third episodes of meningococcal infection without obvious cause should be screened for complement deficiencies because prophylactic antibiotic therapy can be life-saving. Terminal pathway-deficient patients should also be intensively immunized with the best available meningococcal vaccines.
It is likely that terminal pathway deficiencies are rela- tively common and underascertained in most countries: • in Caucasians and most other groups investigated, C6
deficiency is the most common;
• in the Japanese population, C9 deficiency is very common, with an incidence of more than 1 in 500 of the population.
C1 inhibitor deficiency leads to hereditary angioedema
Deficiency of the classical pathway regulator C1inh is responsible for the syndrome hereditary angioedema
(HAE). C1inh regulates C1 and MBL in the complement
system and also controls activation in the kinin pathway that leads to the generation of bradykinin and other active kinins (Fig. 4.17).
HAE is relatively common because the disease presents even in those heterozygous for the deficiency (i.e. it is an autosomal dominant disease).
The halved C1inh synthetic capacity in those with HAE cannot maintain plasma levels in the face of con- tinuing consumption of C1inh, which is a suicide
inhibitor that is consumed as it works. As a consequence,
the plasma levels measured are often only 10–20% of normal, even in periods of health.
Episodes of angioedema are often triggered in the skin or mucous membranes by minor trauma – occasionally stress may be sufficient to induce an attack. Swelling, which may be remarkable in severity, rapidly ensues as
unregulated activation of the kinin and complement systems occurs in the affected area, inducing vascular leakiness. Swelling of mucous membranes in the mouth and throat may block the airways, leading to asphyxia (Fig. 4.18).
Episodes of angioedema are transient and usually wane over the course of a few hours without therapy. Emergency treatment for life-threatening attacks involves the infusion of a purified C1inh concentrate. Prophylactic treatment involves the induction of C1inh synthesis using anabolic steroids, or minimizing consumption of C1inh using protease inhibitors.
Although the majority of cases of HAE involve a mutation that prevents synthesis of C1inh by the defective gene (type I), in about 15% of cases the mutation results in the production of a functionally defective protein (type II). In type II HAE, the plasma levels of C1inh may be normal or even high, but its function is markedly impaired, leading to disease.
A similar syndrome to type II HAE can develop later in life. Acquired angioedema is, in most or all cases, associated with autoantibodies that:
• target C1inh;
• may arise in an otherwise healthy individual or may be
associated with other autoimmune diseases, particularly SLE.
Occasionally, acquired angioedema occurs in association with some lymphoproliferative disorders – whether autoantibodies are involved in these cases remains unclear.
Deficiencies in alternative pathway
regulators produce a secondary loss of C3
fH or fI deficiency predisposes to bacterial infections
fH and fI collaborate to control activation of the alter- native pathway amplification loop. Deficiency of either leads to uncontrolled activation of the loop and complete consumption of C3, which is the substrate of the loop. The resultant acquired C3 deficiency predisposes to bacterial infections and yields a clinical picture identical to that seen in primary C3 deficiency.
Properdin deficiency causes severe meningococcal meningitis
Properdin is a stabilizer of the alternative pathway C3 convertase that increases efficiency of the amplification loop. Properdin deficiency is inherited in an X-linked manner and is therefore seen exclusively in males. Boys deficient in properdin present with severe meningococcal meningitis, often with septicemia. The first attack is often fatal and survivors do not usually have recurrent infections because the acquisition of anti-meningococcal antibody enables a response via the classical pathway in the next encounter. Diagnosis is nevertheless important to identify affected relatives before they get disease – administration of meningococcal vaccine and antibiotic prophylaxis will prevent infection.
FURTHER READING
Barrington R, Zhang M, Fischer M, Carroll MC. The role of complement in inflammation and adaptive immunity. Immunol Rev 2001;180:5–15.
FURTHER READING
Pathogenesis of hereditary angioedema
Fig. 4.17 C1 inhibitor (C1inh) is involved in the inactivation of elements of the kinin, plasmin, complement, and clotting systems, all of which may be activated following the surface- dependent activation of factor XII. The points at which C1inh acts are shown in red. Uncontrolled activation of these pathways results in the formation of bradykinin and C2 kinin, which induce edema.
surface activation Xll Xlla kallikrein prekallikrein Xla Xl coagulation system kininogen bradykinin C1 C1 C2 C2a plasmin plasminogen C2 kinin activator proactivator Hereditary angioedema
Fig. 4.18 The clinical appearance of hereditary angioedema, showing the local transient swelling that affects mucous membranes.
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