Tutorials

Tutorials   ToolBox    Abbreviations   Glossary
MIC419Home    webMIC419 Home    VSC519Home

Complement

This module will help you

• see the roles of complement in immune responses.
• learn complement vocabulary.
• understand regulation of complement activity.
• test your knowledge and immunology problem-solving skills.

Complement Functions
Complement Cascades
Regulation of Complement Function

Complement Functions

Complement Cascades

Complement activation involves three major steps: activation of a C3 convertase to cut C3 into C3a and C3b; activation of a C5 convertase to cut C5 into C5a and C5b, and formation of a membrane attack complex (MAC). The alternative and classical complement cascades produce different C3 and C5 convertases, but both produce the split products of C3 and C5 and both form MAC.

The alternative complement cascade can be activated by foreign pathogens in the absence of antibody . This gives the body a very rapid defense against certain pathogens. A small amount of C3b is always present in body fluids, due to serum and tissue protease activity. Our own cells, with high levels of membrane sialic acid, inactivate C3b if it binds, but bacteria with low external sialic acid bind C3b without inactivating it. Factor B binds to C3b and serum protease Factor D cuts Factor B to form active proteolytic Bb. C3bBb is the alternative pathway C3 convertase, rapidly producing more C3b and C3a from C3. C3b covalently attaches to the pathogen surface and acts as an opsonin, while C3a stimulates inflammation. Some C3b joins the complex to form C3bBb3b, the alternative pathway C5 convertase. Because C3bBb3b is stabilized by the plasma protein properdin; the alternative pathway is sometimes called the properdin pathway.

The classical complement cascade is activated by antigen-antibody complexes . It used complement C1, C2, and C4 which are not part of the alternative pathway. C1 is a complex of three molecules: C1q, C1r, and C1s, of which C1r and C1s are proteases. When the Fab portion (the variable region) of IgM or IgG binds antigen, the conformation of the Fc (constant) region is altered, allowing C1q to bind. C1q must bind at least 2 Fc regions to be activated, so it takes two IgG molecules to activate C1q. Serum IgM is a pentamer of five IgM molecules (see Antibody) with five Fc regions, so IgM activates complement most efficiently. IgA, IgE and IgD do not bind C1q and cannot activate complement. C1q binding to antibody activates the protease function of C1r, which cuts a peptide bond in C1s to activate its protease function.

Activated C1s activates C2 and C4 by cutting a small peptide (C2a and C4a) from each. An active thioester bond on C4b is exposed and covalently binds to a molecule on the pathogen surface. C4b associates with C2b, which has protease activity; C4b2b is the classical cascade C3 convertase. Many molecules of C3 are cut into C3b, which covalently attaches to the pathogen surface and acts as an opsonin, and C3a, which stimulates inflammation. Some C3b molecules associate with C4b2b complexes; C4b2b3b is the classical cascade C5 convertase.

The MBL (Mannose-Binding Lectin) cascade is initiated by MBL binding to pathogen surface. MBL is made by the liver in response to macrophage cytokines produced in response to pathogen binding. MBL and two serum proteases function like C1 to activate C4 and C2 to form MBL cascade C3 convertase. C3b then joins the complex to form MBL cascade C5 convertase. Like the alternative cascade, the MBL cascade is innate complement activity; like the classical cascade, the MBL cascade utilizes C4 and C2 to form C3 convertase.

C5 can be cleaved by any C5 convertase into C5a and C5b. C5b combines with C6 and C7 in solution, and the C567 complex associates with the pathogen lipid membrane via hydrophobic sites on C7. C8 and several molecules of C9, which also have hydrophobic sites, join the membrane attack complex. Poly-C9 forms a pore in the membrane through which water and solutes can pass, resulting in osmotic lysis and cell death. If complement is activated on an antigen without a lipid membrane to which the C567 can attach, the C567 complex may bind to nearby cells and initiate bystander lysis. A single MAC can lyse an erythrocyte, but nucleated cells can endocytose MAC and repair the damage unless multiple MACs are present. Gram negative bacteria with their exposed outer membrane and enveloped viruses are generally susceptible to complement-mediated lysis. Less susceptible are Gram positive bacteria, whose plasma membrane is protected by their thick peptidoglycan layer, bacteria with a capsule or slime layer around their cell wall, or viruses which have no lipid envelope.

The soluble split products C3a, C4a, and C5a are called anaphylatoxins (ANNA fla toxins) because of their inflammatory activity. C5a is the most potent. Anaphylatoxins bind to receptors on various cell types to stimulate smooth muscle contraction, increase vascular permeability, and activate mast cells to release inflammatory mediators. C5a is also chemotactic for monocytes and neutrophils, and stimulates leukocyte adherence to blood vessel walls at the site of infection, phagocytosis, and bactericidal activities. C2a can be converted to C2 kinin, which regulates blood pressure by causing blood vessels to dilate.

Regulation of Complement Activity

Complement activity is regulated by serum levels of complement components, natural decay of the activated fragments, serum protease inhibitors, and specific complement inhibitors. Serum complement levels, especially C3, often drop during infection as complement is activated faster than it is produced. The active thioester bonds on complement b fragments are short lived, inactivated by reaction with water if they do not rapidly bind the pathogen surface. To insure that complement molecules which diffuse away from the pathogen are not able to damage host cells, numerous inhibitory molecules are present in plasma and on host cell surfaces .

C1 inhibitor (C1INH) is a plasma protein which dissociates activated C1r and C1s from C1q, limiting the time the complex is active. C1INH also blocks spontaneous activation of C1 by plasma proteases. Deficiency in C1INH is associated with serious sudden edema (swelling) called Angioneurotic Edema. Several inhibitory proteins dissociate the C3 and C5 convertases and promote degradation of C4b and C3b by Factor I, a plasma protease. These include plasma proteins C4 binding protein that dissociates classical C3 convertase and Factor H that dissociates alternative C3 convertase, and membrane proteins Complement Receptor 1 (CR1), Decay Accelerating Factor (DAF), and Membrane Cofactor Protein (MCP) that inhibit the activity of both pathways. Other membrane and plasma proteins inactivate anaphylatoxins C3a, C4a, and C5a and block formation of MAC on host cells.

Complement deficiencies are linked with frequent infections and immune complex disease. Deficiencies have been identified in all of the complement factors except C9, including Factor D and properdin. Deficiencies have also been identified in the complement regulatory proteins C1INH, Factor I, Factor H, DAF, and HRF. In general, deficiencies in complement components result in increased bacterial infections,especially with Neisseria species, due to reduced opsonization and phagocytosis. Immune complex disease is caused by complement-mediated inflammation in response to persisting antigen-antibody complexes in the circulation and the tissues. Deficiencies in complement factors and in regulatory proteins result in similar symptoms. In the absence of regulatory proteins, complement proteins are depleted at accelerated rates. Deficiencies in Factor H, DAF, and HRF can also result in complement-mediated lysis of erythrocytes and appearance of hemoglobin in the urine. Complement proteins can be quantified directly by ELISA, and complement activity can be measured by the complement fixation test (see ToolBox).

Practice Quiz

Pick the one BEST answer for each question by clicking on the letter of the correct choice.

1. Complement

a. is a group of active proteolytic enzymes found in serum.
b. is secreted by macrophages and hepatocytes in response to antigen binding.
c. participates in both innate and adaptive immune responses.
d. prevents lysis of virus-infected cells.
e. All of the above statements about complement are true.

2. Complement is involved in all of the following except

a. attraction of neutrophils to an infection site.
b. increased presence of serum proteins in the infected tissues.
c. lysis of bacteria in the absence of specific antibodies.
d. opsonization of microorganisms for phagocytosis.
e. sensitization of T cells to antigen

3. Complement is

a. activated by binding to specific complement receptors.
b. antigen-specific.
c. a potent promoter of virus entry into host cells.
d. a series of intracellular proteins which work with antibody to eliminate endogenous antigen.
e. present in the circulation in an inactive form.

4. The alternative pathway of complement activation

a. causes tissue damage in the absence of C1INH.
b. occurs after the classic pathway is activated.
c. occurs only if the classical pathway is ineffective in pathogen clearance.
d. requires C3.
e. requires C4.

5. If a person is born without C2 and C4,

a. C5 can still be cleaved by the classical pathway.
b. C3b will not be able to bind to bacteria.
c. C9 will polymerize inappropriately and lyse host cells.
d. the classical pathway will be changed into the alternative pathway.
e. the amount of C3b produced during bacterial infections will be reduced.

6. Which of the following are least sensitive to complement-mediated lysis?

a. Enveloped viruses
b. Erythrocytes
c. Gram negative bacteria
d. Gram positive bacteria
e. Leukocytes

7. In the membrane attack phase of the classical complement pathway, the role of C5b is to

a. activate the C5 convertase activity.
b. attract neutrophils to lyse the pathogen.
c. initiate formation of the MAC.
d. polymerize into a membrane-spanning channel.
e. All of these are activities of C5b.

8. Complement receptors (CR)

a. activate complement on the surface of pathogens.
b. bind only activated complement proteins.
c. inhibit complement activation on the surface of host cells.
d. on erythrocytes remove immune complexes from the circulation.
e. on macrophages signal host cells to make opsonins.

9. As complement is activated by complexes of antibody-coated bacteria, bystander lysis of nearby host cells is prevented by

a. a long-lived thioester bond on active complement proteins.
b. covalent attachment of all active complement proteins to the pathogen surface.
c. plasma proteins that inactivate the anaphylatoxins.
d. proteins on host cell membranes that inhibit MAC formation.
e. the slow catalytic rates of complement proteases.

10. Complement activity is restricted by all of the following EXCEPT

a. dissociation of C3 and C5 convertases.
b. Gram positive cell walls that are resistant to MAC polymerization.
c. host cell plasma proteins that inactivate C3a, C4a, and C5a activity.
d. LPS in the outer membrane of Gram negative bacteria that inactivates C3b.
e. proteolytic cleavage of complement proteins into smaller fragments.

11. A deficiency in complement proteins or in their regulators can result in

a. blood in the urine from erythrocyte lysis.
b. decreased levels of certain complement proteins in the circulation.
c. immune complex disease.
d. increased numbers of infections.
e. All of the above can result from complement deficiencies.

Problem

Complete the table below by predicting the effects of a complete deficiency of each of the complement proteins listed at the top of the chart on the activities shown in the left hand column. Use the notation N = no inhibition; P = partial inhibition; C = complete inhibition.

Top

Tutorials   ToolBox    MIC419Home    webMIC419 Home    VSC519Home