Chapter 63: Complement

The complement system consists of approximately 20 proteins that are present in normal human (and other animal) serum. The term complement refers to the ability of these proteins to complement (i.e., augment) the effects of other components of the immune system (e.g., antibody). Complement is an important component of our innate host defenses.

There are three main effects of complement: (1) lysis of cells such as bacteria, allografts, and tumor cells; (2) generation of mediators that participate in inflammation and attract neutrophils; and (3) opsonization (i.e., enhancement of phagocytosis). Complement proteins are synthesized mainly by the liver.

Several complement components are proenzymes that must be cleaved to form active enzymes. Activation of the complement system can be initiated either by antigen–antibody complexes or by a variety of nonimmunologic molecules (e.g., endotoxin).

Sequential activation of complement components (Figure 63–1) occurs via one of three pathways: the classic pathway, the lectin pathway, and the alternative pathway (see later). Of these pathways, the lectin and the alternative pathways are more important the first time we are infected by a microorganism because the antibody required to trigger the classic pathway is not present. The lectin pathway and the alternative pathway are, therefore, participants in the innate arm of the immune system.

All three pathways lead to the production of C3b, the central molecule of the complement cascade. The presence of C3b on the surface of a microbe marks it as foreign and targets it for destruction. C3b has three important functions: (1) It combines with other complement components to generate C5 convertase, the enzyme that leads to the production of the membrane attack complex; (2) it opsonizes bacteria because phagocytes have receptors for C3b on their surface; and (3) its derivatives bind to a receptor on B cells and provide “signal 2” for T-cell–independent B-cell activation (see Chapter 61).

1. In the classic pathway, complement proteins first become fixed (bound) to an antigen–antibody complex (see Figure 63–1). Only IgM and IgG can fix complement proteins, because only the Fc regions of the γ and μ heavy chains have a C1 binding site. Complement fixation is the gathering together of bound proteins, starting a chain reaction of proteases.

   In the classic pathway, C1¹ binds and is cleaved to form an active protease, which cleaves C2 and C4 to form a C4b,2b complex. The latter is C3 convertase, which cleaves C3 molecules into two fragments, C3a and C3b. C3a, an anaphylatoxin, is discussed later. C3b forms a complex with C4b,2b, producing a new enzyme, C5 convertase (C4b,2b,3b), which cleaves C5 to form C5a and C5b. C5a is also an anaphylatoxin and a chemotactic factor (see later). C5b binds to C6 and C7 to form a complex that interacts with C8 and C9 to produce the membrane attack complex (C5b,6,7,8,9).

   This protein complex forms a pore in whichever cell membrane contained the original antigen that was bound by antibody. The pore causes leakage of water and electrolytes, leading to cytolysis. Note that for each complement protein, the “b” fragment continues in the main pathway, fixed to the target, whereas the “a” fragment is split off and has other activities.

2. In the lectin pathway, mannan-binding lectin (MBL) (also known as mannose-binding protein) binds to the surface of microbes bearing mannan (a polymer of the sugar, mannose). This activates proteases associated with MBL that cleave C2 and C4 components of complement, converging with the classic pathway (see Figure 63–1). Note that this process bypasses the antibody-requiring step and so is protective early in infection before antibody is formed.

3. In the alternative pathway, many unrelated cell surface substances (e.g., bacterial lipopolysaccharides [endotoxin], fungal cell walls, and viral envelopes) can initiate the process by binding C3(H₂O) and factor B. This complex is cleaved by a protease, factor D, to produce C3b, Bb. This acts as a C3 convertase to generate more C3b. Like the lectin pathway, the alternative pathway is antibody independent and therefore is protective before antibody is formed.

Unrestrained complement activation can cause severe tissue injury and even systemic anaphylaxis. The first regulatory step in the classic pathway is the antibody. The complement-binding site on the heavy chain of IgM and IgG is unavailable to the C1 component of complement if antigen is not bound to these antibodies. This means that complement is not activated by IgM and IgG despite being present in the blood at all times. However, when antigen binds to its specific antibody, a conformational shift occurs and the C1 component can bind and initiate the cascade.

Several serum proteins regulate the complement system at other stages.

1. C1 inhibitor is an important regulator of the classic pathway. It serves as a constant activation threshold by inactivating the protease activity of C1. This means that initiation of the classic pathway can proceed only if sufficient C1 is fixed to overwhelm the inhibitor. (See below for discussion of clinical syndrome of C1 inhibitor deficiency.)

2. Regulation of the alternative pathway is mediated by the binding of factor H to C3b and cleavage of this complex by factor I, a protease. This reduces the amount of C5 convertase available. Like the C1 inhibitor, factors H and I serve as a threshold requirement that sufficient C3b attaches to the cell membrane for the alternative pathway to proceed. Attachment of C3b to cell membranes protects it from degradation by factors H and I. Another component that enhances activation of the alternative pathway is properdin, which protects C3b and stabilizes the C3 convertase.

3. Protection of human cells from lysis by the membrane attack complex of complement is mediated by decay-accelerating factor (DAF, CD55)—a glycoprotein located on the surface of human cells. DAF acts by binding to C3b and C4b and limiting the formation of C3 convertase and C5 convertase. This prevents the formation of the membrane attack complex.

Opsonization

Microbes, such as bacteria and viruses, are phagocytized much better in the presence of C3b because there are C3b receptors on the surface of many phagocytes.

Chemotaxis

C5a and the C5,6,7 complex attract neutrophils. They migrate especially well toward C5a. C5a also enhances the adhesiveness of neutrophils to the endothelium.

Anaphylatoxin

C3a, C4a, and C5a cause degranulation of mast cells with release of mediators (e.g., histamine), leading to increased vascular permeability and smooth muscle contraction, especially contraction of the bronchioles, leading to bronchospasm. Anaphylatoxins can also bind directly to smooth muscle cells of the bronchioles and cause bronchospasm. C5a is, by far, the most potent of these anaphylatoxins. Anaphylaxis caused by these complement components is less common than anaphylaxis caused by type I (IgE-mediated) hypersensitivity (see Chapter 65).

Cytolysis

Insertion of the C5b,6,7,8,9 membrane attack complex into the cell membrane forms a “pore” in the membrane. This opening in the membrane results in the killing (lysis) of many types of cells, including erythrocytes, bacteria, and tumor cells. Gram-negative bacteria, especially Neisseria species, are very susceptible to the membrane attack complex. Cytolysis is not an enzymatic process; rather, it appears that insertion of the complex results in disruption of the membrane and the entry of water and electrolytes into the cell.

Enhancement of Antibody Production

The binding of C3b derivatives to its receptors on the surface of activated B cells (complement receptor 2 [CR2]) provides the signal 2, which greatly enhances antibody production compared with that by B cells that are activated by antigen alone (see Chapter 61). The clinical importance of this is that patients who are deficient in C3b produce significantly less antibody than do those with normal amounts of C3b. The low concentration of both antibody and C3b significantly impairs host defenses, resulting in multiple, severe pyogenic infections.

1. Inherited (or acquired) deficiency of some complement components, especially C5–C8, greatly enhances susceptibility to Neisseria bacteremia and other infections that are particularly sensitive to killing by the membrane attack complex. A deficiency of C3 leads to severe, recurrent pyogenic sinus and respiratory tract infections.

2. Deficiency of C1 esterase inhibitor results in angioedema. When the amount of inhibitor is reduced, the activation threshold for C1 esterase is also reduced. (In the absence of inhibitor, C1 continues to act on C4 to generate C4a and subsequently additional vasoactive components such as C3a and C5a.) This leads to capillary permeability and edema in several organs. Laryngeal edema can be fatal. The syndrome of C1 inhibitor deficiency can occur as an acquired disease or, rarely, as a genetic disease called hereditary angioedema.

3. Acquired or inherited deficiency of decay-accelerating factor (DAF) on the surface of cells results in an increase in membrane attack complex activity and increased complement-mediated cell lysis, particularly lysis of red blood cells (i.e., hemolysis). Clinically, this appears as the disorder paroxysmal nocturnal hemoglobinuria. This disease is characterized by episodes of brownish urine (hemoglobinuria), which reflects hemolysis, particularly early in the morning. The complement-mediated hemolysis occurs especially at night because the lower oxygen concentration (and low pH) in the blood during sleep increases the susceptibility of the red cells to lyse.

4. In transfusion mismatches (e.g., when type A blood is given by mistake to a person who has type B blood), antibody to the A antigen in the recipient binds to A antigen on the donor red cells, complement is activated, and large amounts of anaphylatoxins and membrane attack complexes are generated. The anaphylatoxins cause shock, and the membrane attack complexes cause red cell hemolysis.

5. Immune complexes formed by antigens and antibodies can bind complement, and thus, complement levels are often low in immune complex diseases (e.g., acute glomerulonephritis and systemic lupus erythematosus). Binding (fixing) of complement attracts polymorphonuclear leukocytes, which release enzymes that damage tissue.

6. Patients with severe liver disease (e.g., alcoholic cirrhosis or chronic hepatitis B), who have lost significant liver function and therefore cannot synthesize sufficient complement proteins, are predisposed to infections caused by pyogenic bacteria.