Chapter 189. Complement Disorders

The complement system provides an important effector arm for many of the innate immune system functions, including host defense, regulation of acquired immunity, and clearance of immune complexes and other potentially dangerous material.

Complement consists of an interacting network of many different individual proteins that are present in the circulation, in most body fluids, and on many cell surfaces. Like the coagulation system, the complement system is a tightly controlled enzyme cascade in which proenzyme components are activated and a cleavage sequence is initiated by which biologically active protein fragments are produced. Figure 189-1 shows the basic schematic of the complement activation pathways. Although each pathway is activated differently, they merge at the C3 step and have a common end point of promoting inflammation, eliminating pathogens, and enhancing the immune response. When any one of the complement proteins is missing, including the control proteins and cell-associated receptors, the resulting dysfunctional system generally leads to disease.1,2 Patients with complement component deficiencies often present with pyogenic infections, particularly of encapsulated bacteria including Streptococcus pneumoniae and Haemophilus influenza type B or with autoimmune disorders as discussed for each pathway below. Outright genetic deficiencies are relatively rare, with an estimated prevalence of 0.03% in the general population, except for deficiency of the mannan-binding lectin (MBL), which may be present in the homozygous form in as many as 3–10% of the population. Subtle changes in the gene sequence that lead to enhanced or decreased function have been described for several complement proteins.

Complement abnormalities may underlie disorders, including angioedema, vasculitis, recurrent bacterial infections, impaired or improper immune responses, certain renal conditions, and increased incidence of autoimmune disease. A family history of these symptoms should increase suspicion of a potential complement deficiency. Specific complement deficiency associations with particular disease processes are discussed below.

Identifying a patient with low complement and distinguishing between an inherited or acquired deficiency can be challenging. However, understanding the reason(s) for low or absent complement guides treatment decisions, including when to use antibiotics, immunizations, or protein replacement, as well as whether genetic counseling for inherited deficiencies is needed.

Evaluation of the complement system is best accomplished using functional screening tests that are designed to evaluate the integrity of each pathway.2,3 The classical pathway is evaluated using the CH50 laboratory test. This test was initially developed as a hemolytic assay based on complement dependent reactions with a surface antigen on sheep red blood cells that led to cell lysis. Because cell lysis required the sequential action of all 9 components of the classical (C1, C4, and C2) and the terminal pathway (C3, C4, C6, C7, C9). The CH50 represents the reciprocal of the serum dilution required to lyse 50% of the cells. More convenient variants of this test are now used in most laboratories. The alternative pathway is evaluated using an analogous test that is less readily available known as the AH50. The most common approach to measurement uses rabbit red cells in combination with a buffer that blocks activation of the other complement pathways. Both the CH50 and AH50 are used to screen for complement deficiencies. If both are decreased, it is likely that there is an abnormality in the 6 terminal components (C3, C5, C6, C7, C8, C9), whereas if a classical pathway component is absent the CH50 would be decreased, and the AH50 would be normal. Determination of the specific complement deficiency requires modifications of the functional tests that focus on only one component.

Several assays for C1-INH and of the Lectin-MBL pathway are available in specialized laboratories, as our specific tests to determine levels of C1q and C1-INH autoantibodies.

The classical complement pathway was the first pathway described and then named based on its ability to “complement” the activity of antibodies. Its components were named in the order in which they were discovered not in their order of activation. Listed in the order of activation, they include: C1, C4, C2, C3, C5, C6, C7, C8, and C9. As shown in Figure 189-1, only the initiation of activation by the first three components (C1, C4, and C2) is unique to the classical pathway.

The C1 component is made up of a multimolecular complex comprised of one C1q, two C1r, and two C1s subunits, held together by calcium ions. C1q provides the recognition function, and C1r and C1s are proenzymes activated when C1q binds.

Classical Component Deficiencies

Deficiencies of the early classical components may present with recurrent pyogenic infections but those with deficiency of the early components (C1, C4, or C2) more commonly present in adolescence or older with autoimmune disease. Greater than 90% of individuals with a C1q deficiency, 75% with a C4 deficiency, and 15% of those with a C2 deficiency develop systemic lupus erythematosus (SLE). Furthermore, patients with a complement deficiency have an earlier age of onset of symptoms, greater photosensitivity, and fewer renal symptoms, and their antinuclear antibody titers are often normal.

An acquired deficiency of C1q may occur in patients with autoantibodies that react with the collagen-like region of the C1q molecule. Many of these patients have hypocomplementemic urticarial vasculitis syndrome, but anti-C1q autoantibodies are also found in about 30% of patients with SLE. Activation of complement by immune complexes can also cause a transient deficiency of C1q and other classical components. Deficiencies of C1r and C1s are rare and may be partial or combined. They predispose the patient to recurrent pyogenic infections as well as immune complex disease.

Because the pathological processes in autoimmune disorders involve activation and consumption of complement in vivo, it is difficult to discriminate between individuals with an inherited complement deficiency, versus those with a low complement due to the disease itself.

C4A and C4B, the two haplotypes of human C4, are each coded by a different gene, located in the major histocompatibility complex. The number of C4 gene copies in the population is variable. Partial deficiencies of C4 are common and usually without consequence, but total deficiency of either type can result in disease. C4A deficiency with normal C4B has a lower, but still significant (15%), incidence of lupus and rheumatoid arthritis. There is evidence that C4B deficiency is associated with a higher incidence of bacterial infections, or with Henoch-Schönlein purpura. Note that the proteins C4A and C4B are not the same as the fragments of C4 (C4a and C4b) that are generated during complement activation.

C2 deficiency is the most common complement deficiency among those of European ancestry. C2-deficient patients have increased susceptibility to infections and often present with a history of pneumonias or other bacterial infections. Type I C2 deficiency, the most common form, is due to a 28-base pair deletion in the C2 gene that results in stopping synthesis prematurely. These patients have no detectable C2 protein, no C2 function, and no CH50 activity. There is no effect on the alternative pathway (AP), but the lectin pathway (LP) would be at least partially impaired. Type II C2 deficiency results from a variety of causes, including single nucleotide polymorphisms, or small deletions or insertions that lead to low C2 production, low function, and variable amounts of C2 protein.

Hereditary Angioedema

Hereditary angioedema (HAE) results from deficiency of the classical pathway control protein, C1-inhibitor (C1-INH), which inactivates the two C1 enzymes, C1r and C1s, as well as the lectin pathway mannan-binding lectin-associated serine proteases and enzymes associated with the coagulation pathway (XIIa, kallikrein, and plasmin).4 Lack of control by this inhibitor produces bradykinin, thought to be the mediator of HAE. Two forms of C1-INH deficiency have been defined: Type I patients (70–80% of HAE patient total) generally have less than half of the normal C1-INH level and function, whereas type II (20–30%) have normal or elevated C1-INH protein that consists largely of dysfunctional C1-INH. Both forms of HAE are transmitted as an autosomal dominant trait: one allele codes for normal protein and the other produces no protein or the dysfunctional form. Edema occurs when local activation of complement or coagulation enzymes depletes the already marginal stores of C1-INH in the circulation. The result leads to production of bradykinin, which causes the symptoms of edema, further described in Chapter 193. Recurrent angioedema, a family history consistent with C1-INH deficiency, consistently low C4 levels, and low or dysfunctional C1-INH confirms the diagnosis of hereditary angioedema (HAE). The CH50 and C2 may also be low, but C3, C1q, and the late complement proteins are normal in HAE. In children, the onset of HAE is generally in the early teens, but earlier onset has been reported.

Acquired angioedema (AAE) is rare in children, but must be ruled out when the family history is negative. Type I AAE is also characterized by low levels and function of C1-INH and low C4, but unlike hereditary angioedema, acquired angioedema patients have low C1q levels. The initiation of AAE is triggered by a decrease in the inhibitor due to hypercatabolism through an underlying disease process, often a lymphoproliferative disorder. Type II AAE occurs when an autoantibody binds to the C1-INH protein and blocks its function. A third form of hereditary angioedema that is not related to C1-INH, but appears to be estrogen dependent, has been reported. This form is associated with gain-of-function mutations in the gene encoding coagulation factor XII (F12).

The lectin pathway (LP) is the most recently characterized mechanism for complement activation, yet it may be the most primitive.5 These lectins are a group of proteins that recognize patterns formed by unique sugars on the surface of bacteria. They include mannose-binding lectin and the ficolins. Figure 189-1 shows the position of the LP in the complement activation scheme. The LP is critical in infancy during the period when maternal antibodies are decreasing and the infant’s own antibody production is diminished.

The structure of mannose-binding lectin (MBL) is similar to that of C1q. MBL serum concentrations vary widely: Approximately 10% of individuals studied to date are MBL deficient. Deficiency of MBL is associated with an increased frequency of pyogenic infections and sepsis, particularly in neonates and young children. MBL deficiency is increased in patients with autoimmune disease, often identified as those who have more frequent and severe infections and worse outcomes.

Other lectins that can initiate the pathway are the ficolins, which comprise several forms that bind to different sugars. Their structures resemble the individual subcomponents of C1q: single flowers on a collagen-like stalk with a C-terminal fibrinogen domain that is thought to be the ligand-binding site for N-acetylglucosamine. Mannose-binding lectin and ficolins associate with mannan-binding lectin–associated serine proteases, which are activated when one of these proteins recognizes microorganisms and subsequently cleaves complement factors C4 and C2, thus initiating the activation of the complement system.

The alternative pathway (AP) was the second activation pathway described. The difference between the AP and either of the preceding two pathways is that the AP lacks a distinct activation step. Instead, the AP is primed to initiate a highly efficient amplification loop of C3 cleavage whenever surface-bound C3b is available. The unique proteins of the AP include factor D, factor B, C3b, and properdin (see Fig. 189-1).

Factor D is a protease that cleaves factor B to form the C3 convertase C3bBb. Only two families with factor D deficiency have been described to date. Both presented with meningococcemia from Neisseria meningitidis. Factor B is critical for alternative pathway function because it contains the enzyme site responsible for cleaving C3. There has been only one report of a B deficiency in humans. No naturally occurring deficiencies have been identified in animals, but Bf gene knock-out mice survive in pathogen-free conditions.

Properdin (P) is the only complement protein that is X linked, thus it manifests in only males. Properdin deficiency increases the susceptibility to bacterial infections, including Neisseria. Lack of properdin causes decreased alternative pathway activity when measured by an alternative pathway hemolytic assay (AH50) due to inability to form an efficient amplification loop for C3 cleavage. There are a few labs that perform tests for the gene defect responsible for properdin deficiency. The genetic screen is the best way to confirm the carrier state in females.

Factor H is a critical fluid phase control protein of the alternative pathway. Factor H dissociates the C3 convertase, C3bBb, and serves as a cofactor for factor I in the further degradation of C3b to inactive fragments. Complete deficiency of H leads to uncontrolled activation of the alternative pathway, depletion of C3, and severe pyogenic infections, including Neisseria. Recent data have been published that demonstrate the critical role for this complement control protein in maintaining health in a number of tissues.6 In addition to infections, partial deficiencies or dysfunction of factor H are associated with dense deposit disease, also called type II membranoproliferative glomerulonephritis; atypical hemolytic uremic syndrome; and age-related macular degeneration. Central to these diseases is dysregulation of C3, so factor H, factor I, and other complement proteins are implicated in their cause.

Terminal complement deficiencies (except C9) block both the alternative and the classical pathways, resulting in no CH50 or AH50 activity. Lysis of other target cells, such as bacteria, is also prevented if the terminal pathway is blocked.

Deficiency of C3, the major opsonin of complement, results in severe, recurrent pyogenic infections that begin shortly after birth, with a clinical presentation and course similar to that observed in hypogammaglobulinemia. Acquired C3 deficiency occurs as a side effect of factor H or factor I deficiency and predisposes the patient to the same risks as congenital C3 deficiency. In the presence of high levels of C3-nephritic factor, an autoantibody specific for C3bBb, C3 can become depleted, also making the patient susceptible to infections. C3-nephritic factor binds to C3bBb and stabilizes it so that factor H and factor I cannot inactivate it. This causes a prolonged half-life of the enzyme and uncontrolled cleavage of C3. C3-nephritic factor is associated with dense deposit disease (type II membranoproliferative glomerulonephritis), as well as systemic lupus erythematosus and partial lipodystrophy.

Deficiencies of C5, C6, C7, and C8 are also strongly linked with susceptibility to pyogenic infections, particularly with Neisseria. Killing these bacteria is much more efficient when complement activation ends in the membrane attack complex and lyses the bacteria. Diagnosis of C5, C6, or C7 deficiencies can be made from measurement of the amount of each protein present in the circulation, but C8 deficiency is more difficult to pinpoint since the C8 molecule is made up of three chains that are encoded by different genes. Because it requires all chains in order to function in the membrane attack complex, C8 protein assays can be misleading, but the functional C8 assay is diagnostic. Deficiency of C9, the most common complement deficiency in the Japanese population, is not as strongly linked with infections as are the other late components. C9 deficiency causes slower lysis of the cells used in the CH50 and AH50 assays and gives slightly lower results from these tests, but the deficiency can be deduced from the C9 protein level.1-5