Immunology Chapter 5: The Complement System

Describe three ways in which complement acts to protect the host during an infection

- Complement opsonizes pathogens - The membrane attack complex can induce lysis of pathogens - C3a, C4a, and C5a act as chemoattractants to bring leukocytes to the site of infection - Binding of C3 fragments by CD21 enhances B-cell activation by complement-coated antigen

Complement

A group of serum and cell membrane proteins that interact with one another and with other molecules of innate and adaptive immunity to carry out key effector functions leading to pathogen recognition and elimination

Briefly explain the mechanism of action and pathway(s) of the decay-accelerating factor (DAF)

Accelerates dissociation of C4b2a and C3bBb C3 convertases

Briefly explain the mechanism of action and pathway(s) of the C4b-binding protein (C4bBP)

Accelerates dissociation of the C4b2aC3, C3 convertase Acts as a cofactor for factor I in C4b degradation

Enzymatic mediators

Activate other enzymes that generate the central proteins of the complement cascade

Briefly explain the mechanism of action and pathway(s) of the membrane cofactor of proteolysis (MCP; CD46)

Acts as cofactor for factor I in degradation of C3b and C4b

Active complement regulation: decay of C3 convertases

Any C3 convertase complexes of either the classical and lectin pathways (C4b2a) or the alternative pathway (C3bBb) that alight on host cells are degraded by the host cell membrane protein DAF, acting in concert with cofactors that are either expressed on, or bind specifically to, host cell membranes

Where do the three complement pathways converge?

At the formation of C5 convertase and generation of the MAC

Inflammatory mediators

Bind to receptors on the endothelial cells lining blood vessels, enhancing blood flow to affected area. Also attract other cells to the site of tissue damage

Complement and APCs

Binding of complement components to antigen-presenting cells enhances their phagocytic ability and modulates cytokine secretion

Briefly explain the mechanism of action and pathway(s) of CD59 (protectin)

Binds C5b678 on host cells, blocking binding of C9 and formation of the MAC

Active complement regulation: C1 inhibition

C1INH is a serine protease inhibitor that inhibits both the C1r2s2 serine proteases of the classical pathway and the MASP-2 protease of the lectin pathway, inhibiting further activation of C4 and C2 and formation of the C3 convertase

Alternative pathway of complement activation

Can be initiated in three ways: tickover pathway, properdin binding, and with proteases. The end result of the initiating sequence is the generation of enzymes that cleave C3 into C3a and C3b, and C5 into C5a and C5b. Uses a different set of C3 and C5 convertases than the other two pathways.

Complement activation pathways

Classical, lectin, alternative

What are the consequences of an absence of C3 for clearance of immune complexes?

Clearance of immune complexes occurs only following opsonization of complexes by C3b binding, followed by phagocytosis or binding to the surface of erythrocytes via CR1 binding. Therefore, immune complex clearance is inhibited in the absence of C3

Briefly explain the mechanism of action and pathway(s) of carboxypeptidase N

Cleaves and inactivates anaphylatoxins

Opsonins

Coating proteins that promote phagocytosis

Complement and B cell-mediated immunity

Complement components enhance the B cell-mediated immune response by increasing the avidity with which a B cell binds to a complement-bound antigen

How do complements aid in the contraction phase of the immune response?

Disposal of apoptotic cells and bodies: the complement component C1q recognizes the presence of cell surface DNA and proteins on the outer membranes of apoptotic cells and bodies, and mediates phagocytosis either directly via the C1qR, or indirectly after initiation of the complement pathway Disposal of immune complexes: Antigen-antibody complexes generated during the course of an immune response are opsonized by C3b and removed from the circulation after recognition by CR1 receptors on erythrocytes

How is complement activity actively regulated?

Dissociation of C1 components, decay-accelerating activity for C3 convertase, Factor I cofactor activity, inhibition of MAC attacks via protectin (CD59), and cleavage of anaphylatoxins C3a and C5a via carboxypeptidases

Outcome of complement activation

Dramatic increase in the concentration of C3b, an important and multifunctional complement protein

Active complement regulation: Factor I activity

Factor I is a soluble, constitutively active serine protease that cleaves C3b and C4b into inactive fragments only when it is associated on host cell membranes with the necessary cofactors. The cofactor, MCP, is lost as lymphocytes enter apoptosis, thereby allowing the deposition of C3b on the dying cell surface and subsequent phagocytosis

T/F: The complement system evolved first as a component of the adaptive immune response

False: Long before the emergence of the adaptive immune system, the early complement components were already functional, although the appearance of functional components of the MAC coincides approximately with the development of the adaptive immune system

T/F: Enveloped viruses cannot be lysed by complement because their outer envelopes are resistant to pore formation by the membrane attack complex (MAC)

False: the outer membrane of a virus is derived from the outer membrane of the host cell and is therefore susceptible to complement-mediated lysis. However, some viruses have developed mechanisms that enable them to evade complement-mediated lysis

T/F: Genomic analysis has classified complement genes and the proteins they encode into five gene families on the basis of how many immunoglobulin domains they possess

False: various gene families each have uniqe domain structures

Alternative protease-activated pathway

Generation of the anaphylatoxin C5a can also be effected by thrombin cleavage of C5, linking the coagulation and complement cascades

How does the host ensure that inadvertent activation of the alternative pathway on its own healthy cells does not lead to autoimmune destruction?

Healthy cells contain a variety of complement inhibitor proteins that prevent inadvertent activation of the alternative pathway

Explain why serum IgM cannot activate complement prior to antigen binding

IgM undergoes a conformational change on binding to antigen, which enables it to be bound by the first component of complement C1q. In the absence of antigen binding, the C1q binding site in the Fc region of IgM is inaccessible

Complement and T cell-mediated immunity

Immature T cells are protected from natural antibody and complement-mediated lysis by the provision of additional sialic acid residues on their cell surface glycoproteins. Defective T cells do not have this protective layer, and so complement participates in quality control mechanisms during T-cell development. Binding of C3a, C5a, and C3b to their respective receptors on mature T cells facilitates their growth, differentiation, and survival

Explain why complement deficiencies in the early components of complement give rise to immune complex-mediated disorders such as systemic lupus erythematosus

Immune complexes are cleared from the body following opsonization by C3b. In the absence of the early components of complement, C3b convertases are not formed, and hence no C3b will be available

Briefly explain the mechanism of action and pathway(s) of the C1 inhibitor (C1INH)

Induces dissociation and inhibition of the C1 proteases C1r and C1s from C1q Serine protease inhibitor

Initiator complement components

Initiate complement cascades by binding to particular soluble or membrane-bound molecules

Classical pathway of complement activation

Initiated by antibodies of the IgM or IgG class binding to a multivalent antigen. Next, C1 binds to an antibody, activating C1-associated serine proteases that cleave the second and fourth complement components, releasing C2a and C2b and C4a and C4b fragments. C2a and C4b combine to form an active serine protease, called C3 convertase, that cleaves C3 into C3a and C3b. The C2a4b complex then combines with one molecule of C3b, forming an active serine protease enzyme called C5 convertase that cleaves C5 into C5a and C5b

Lectin pathway of complement activation

Initiated by binding of lectins such as mannose-binding lectin (MBL) to microbial surface carbohydrates. The lectin provides the recognition function and binding activates an associated serine protease (MASP-2) molecule that cleaves C4 and C2 to create C3 convertase, C2a4b. As for the classical pathway, the end result of the initiating sequence of the lectin pathway is the generation of enzymes that cleave C3 into C3a and C3b, and C5 into C5a and C5b

Alternative tickover pathway

Initiated when C3(H2O) binds to factor B, which then becomes susceptible to cleavage by factor D into Ba and Bb. The Bb fragment continues to bind to the hydrolyzed C3(H2O) and together they form a fluid-phase C3 convertase. Some of the C3b generated by this convertase adheres to microbial surfaces; there it binds factor B, which again, in the presence of factor D, is cleaved, resulting in the formation of the membrane-bound C3 convertase, C3bBb. This complex is stabilized by properdin

7 functional categories of complement

Initiators, convertase activators and enzyme mediators, opsonins, anaphylatoxins, membrane attack complex, complement receptors, regulators

What are the three main classes of complement activity in the service of host defense?

Innate defense against infection, interface between innate and adaptive immunity, and activity in the contraction phase of the immune response

What are some microbial complement evasion strategies?

Interference with antibody-complement interaction, binding and inactivation of complement proteins, protease-mediated destruction of complement proteins, and microbial mimicry of complement-regulatory proteins

Regulatory complements

Limit the effects of complement by promoting their degradation or preventing binding to host cells

What are the consequences of an absence of C3 for phagocytosis of infectious bacteria?

Phagocytosis would be diminished in the absence of C3b-mediated opsonization

Alternative properdin-activated pathway

Properdin (factor P) binds to components of microbial membranes, and stabilizes the binding of C3bBb complexes of the alternative complement pathway. The difference between this and the tickover pathway is that properdin binds first and initiates complement deposition on the membrane

Membrane attack complex (MAC)

Proteins of the MAC insert into the cell membrane of invading microorganisms and punch holes that result in the lysis of the pathogen

Zymogens

Proteins that are inactive until cleaved by proteases

Complement receptors

Receptor molecules on phagocytic cells, granulocytes, or erythrocytes

How is complement activity passively regulated?

Short protein half-lives and different host cell surface composition (vs microbial cells)

How do the three complement activation pathways differ in the substances that can initiate activation?

The classical pathway is initiated by immune complexes involving IgM or IgG; the alternate pathway is generally initiated by binding of C3b to bacterial cell wall components, and the lectin pathway is initiated by binding of lectins

What are the consequences of an absence of C3 for the initiation of the classical and alternative pathways?

The initiation of the classical pathway is mediated by the first component of complement, and C3 does not participate until after the formation of the active C3 convertase C4aC2b. The initiation of the alternate pathway begins with spontaneous cleavage of the C3 component, and therefore no part of the alternate pathway can operate in the absence of C3

Which parts of the overall complement activation sequence differ among the three pathways, and which parts are similar?

The terminal reaction sequence after C5 convertase generation is the same for all three pathways. The pathways are all initiated differently. For the classical and lectin pathways, the second step involves binding of the serine protease complexes C4b2a (classical) and MASP-1, MASP-2 (lectin). These each act as a C3 convertase in each respective pathway, and the two pathways are identical from that point on. The alternative pathway uses a different C3 convertase, C3bBb. The formation of Bb requires factor D, and the C3 convertase is stabilized on the cell surface by properdin. The alternative pathway C5 convertase is C3bBbC3b

T/F: A single molecule of bound IgM can activate the C1q component of the classical complement pathway

True

T/F: C1q genes appeared prior to immunoglobulin genes and some C1q proteins specifically bind to particular carbohydrates

True

T/F: C3a and C3b are fragments of C3 that are generated by proteolytic cleavage mediated by two different enzyme complexes

True

T/F: Gene duplication events have occurred in invertebrates in each of the five complement component gene families

True

T/F: MBL has a function in the lectin pathway analogous to that of the IgM in the classical pathway, and MASP-1 and MASP-2 take on functions analogous to C1 components

True

T/F: Nucleated cells tend to be more resistant to complement-mediated lysis than red blood cells

True

T/F: The enzymes that cleave C3 and C4 are referred to as convertases

True

Contraction phase of immune response

When most of the lymphocytes generated in the initial proliferative phase of the immune response undergo apoptosis, leaving only a few antigen-specific cells behind to provide immunological memory