Soluble Mediators of the Immune System



Soluble Mediators of the Immune System



Learning Objectives


At the conclusion of this chapter, the reader should be able to:


• Name and compare the three complement activation pathways.


• Describe the mechanisms and consequences of complement activation.


• Explain the biological functions of the complement system.


• Name and describe alterations in complement levels.


• Briefly describe the assessment of complement levels.


• Compare other types of nonspecific mediators of the immune system, including cytokines, interleukins, tumor necrosis factor, hematopoietic growth factors, and chemokines.


• Discuss the clinical applications of C-reactive protein.


• Compare acute-phase reactant methods.


• Analyze an acute-phase protein case study.


• Correctly answer case study related multiple choice questions.


• Be prepared to participate in a discussion of critical thinking questions.


• Describe the principle, reporting results, sources of error, limitations, and clinical applications of the C-reactive protein procedure.


• Correctly answer end of chapter review questions.



Key Terms



The immune system is composed of the phylogenetically oldest, highly diversified innate immune system and the adaptive immune system. Some components of the innate or natural immune system (e.g., phagocytosis) are discussed in previous chapters. This chapter discusses the other components of the innate immune system: the complement system and other circulating effector proteins of innate immunity, including cytokines and acute-phase reactants.


Regulatory mechanisms of complement are finely balanced. The activation of complement is focused on the surface of invading microorganisms, with limited complement deposited on normal cells and tissues. If the mechanisms that regulate this delicate balance malfunction, the complement system may cause injury to cells, tissues, and organs, such as destruction of the kidneys in systemic lupus erythematosus or hemolytic anemias.



The Complement System


Complement is a heat-labile series of 18 plasma proteins, many of which are enzymes or proteinases. Collectively, these proteins are a major fraction of the beta-1 and beta-2 globulins.


The complement system proteins are named with a capital C followed by a number. A small letter after the number indicates that the protein is a smaller protein resulting from the cleavage of a larger precursor by a protease. Several complement proteins are cleaved during activation of the complement system; the fragments are designated with lower case suffixes, such as C3a and C3b. Usually, the larger fragment is designated as “b” and the smaller fragment as “a.” The exception is the designation of the C2 fragments; the larger fragment is designated C2a and the smaller fragment is C2b.


Proteins of the alternative activation pathway are called factors and are symbolized by letters such as B. Control proteins include the inhibitor of C1 (C1 INH), factor I, and factor H.


The complement system displays three overarching physiologic activities (Table 5-1). These are initiated in various ways through the following three pathways (Table 5-2):





The three pathways (Fig. 5-1) converge at the point of cleavage of C3 to C3b, the central event of the common final pathway, which in turn leads to the activation of the lytic complement sequence, C5 through C9, and cell destruction (Fig. 5-2).


image
image
Figure 5-1 Early steps of complement activation.
The steps in the activation of the alternative, classical, and lectin pathways are shown. Note the sequence of events is similar in all three pathways, although they differ in their requirement for antibody and in the proteins used. (From Abbas AK, Lichtman AH: Basic immunology: functions and disorders of the immune system, updated edition, ed 3, Philadelphia, 2011, Saunders.)

image
Figure 5-2 Late steps of complement activation.
A, The late steps of complement activation start after the formation of the C5 convertase and are identical in the alternative and classical pathways. Products generated in the late steps induce inflammation (C5a) and cell lysis (the membrane attack complex [MAC]). B, The properties of the proteins of the late steps of complement activation are listed. (From Abbas AK, Lichtman AH: Basic immunology: functions and disorders of the immune system, updated edition, ed 3, Philadelphia, 2011, Saunders.)


Activation of Complement


Normally, complement components are present in the circulation in an inactive form. In addition, the control proteins C1 INH, factor I, factor H, and C4-binding protein (C4-bp) are normally present to inhibit uncontrolled complement activation. Under normal physiologic conditions, activation of one pathway probably also leads to the activation of another pathway, as follows:






Effects of Complement Activation


The activation of complement and the products formed during the complement cascade have a variety of physiologic and cellular consequences. Physiologic consequences include blood vessel dilation and increased vascular permeability. The cellular consequences include the following:



In addition to the function of complement as a major effector of antigen-antibody interaction, physiologic concentrations of complement have been found to induce profound alterations in the molecular weight, composition, and solubility of immune complexes. The activation of complement may also play a role in mediating hypersensitivity reactions. This process may occur from direct alternative pathway activation by immunoglobulin E (IgE)–antigen complexes or through a sequence initiated by the activated Hageman coagulation factor that causes the generation of plasmin, which subsequently activates the classic pathway. In either case, activation of complement components from C3 onward leads to the generation of anaphylatoxins in an immediate-hypersensitivity reaction.



Classic Pathway


The classic complement pathway is one of the major effector mechanisms of antibody-mediated immunity. The principal components of the classic pathway are C1 through C9. The sequence of component activation—C1, 4, 2, 3, 5, 6, 7, 8, and 9—does not follow the expected numeric order.


C3 is present in the plasma in the largest quantities; fixation of C3 is the major quantitative reaction of the complement cascade. Although the principal source of synthesis of complement in vivo is debatable, the majority of the plasma complement components are made in hepatic parenchymal cells, except for C1 (a calcium-dependent complex of the three glycoproteins C1q, C1r, and C1s), which is primarily synthesized in the epithelium of the gastrointestinal and urogenital tracts.


The classic pathway has three major stages:




Recognition


The recognition unit of the complement system is the C1 complex—C1q, C1r, and C1s, an interlocking enzyme system. In the classic pathway, the first step is initiation of the pathway triggered by recognition by complement factor C1 of antigen-antibody complexes on the cell surface. When C1 complex interacts with aggregates of immunoglobulin G (IgG) with antigen on a cell’s surface, two C1-associated proteases, C1r and C1s, are activated. A single IgM molecule is potentially able to fix C1, but at least two IgG molecules are required for this purpose. The amount of C1 fixed is directly proportional to the concentration of IgM antibodies, although this is not true of IgG molecules. C1s is weakly proteolytic for free intact C2, but is highly active against C2 that has complexed with C4b molecules in the presence of magnesium (Mg2+) ions. This reaction will occur only if the C4bC2 complex forms close to the C1s.


The resultant C2a fragment joins with C4b to form the new C4bC2a enzyme, or classic pathway C3 convertase. The catalytic site of the C4bC2a complex is probably in the C2a peptide. A smaller C2b fragment from the C2 component is lost to the surrounding environment.



Amplification of Proteolytic Complement Cascade


Once C1s is activated, the proteolytic complement cascade is amplified on the cell membrane through sequential cleavage of complement factors and recruitment of new factors until a cell surface complex containing C5b, C6, C7, and C8 is formed.


The complement cascade reaches its full amplitude at the C3 stage, which represents the heart of the system. The C4bC2a complex, the classic pathway C3 convertase, activates C3 molecules by splitting the peptide, C3 anaphylatoxin, from the N-terminal end of the peptide of C3. This exposes a reactive binding site on the larger fragment, C3b. Consequently, clusters of C3b molecules are activated and bound near the C4bC2a complex. Each catalytic site can bind several hundred C3b molecules, even though the reaction is very efficient because C3 is present in high concentration. Only one C3b molecule combines with C4bC2a to form the final proteolytic complex of the complement cascade.



Membrane Attack Complex


The membrane attack complex (MAC) is a unique system that builds up a lipophilic complex in cell membranes from several plasma proteins. To initiate C5b fixation and the MAC, C3b splits C5a from the alpha chain of C5. No further proteinases are generated in the classic complement sequence. Other bound C3b molecules not involved in the C4b2a3b complex form an opsonic macromolecular coat on the erythrocyte or other target, which renders it susceptible to immune adherence by C3b receptors on phagocytic cells.


When fully assembled in the correct proportions, C7, C6, C5b, and C8 form the MAC (see Fig. 5-2, inset). The C5bC6 complex is hydrophilic but, with the addition of C7, it has additional detergent and phospholipid-binding properties as well. The presence of hydrophobic and hydrophilic groups within the same complex may account for its tendency to polymerize and form small protein micelles (a packet of chain molecules in parallel arrangement). It can attach to any lipid bilayer within its effective diffusion radius, which produces the phenomenon of reactive lysis on innocent so-called bystander cells. Once membrane bound, C5bC6C7 is relatively stable and can interact with C8 and C9.


The C5bC6C7C8 complex polymerizes C9 to form a tubule (pore), which spans the membrane of the cell being attacked, allowing ions to flow freely between the cellular interior and exterior. By complexing with C9, the osmotic cytolytic reaction is accelerated. This tubule is a hollow cylinder with one end inserted into the lipid bilayer and the other projecting from the membrane. A structure of this form can be assumed to disturb the lipid bilayer sufficiently to allow the free exchange of ions and water molecules across the membrane. Ions flow out, but large molecules stay in, causing water to flood into the cell. The consequence in a living cell is that the influx of sodium (Na+) ions and H2O leads to disruption of osmotic balance, which produces cell lysis.



Alternative Pathway


The alternative pathway shows points of similarity with the classic sequence. Both pathways generate a C3 convertase that activates C3 to provide the pivotal event in the final common pathway of both systems. However, in contrast to the classic pathway, which is initiated by the formation of antigen-antibody reactions, the alternate complement pathway is predominantly a non–antibody-initiated pathway.


Microbial and mammalian cell surfaces can activate the alternative pathway in the absence of specific antigen-antibody complexes. Factors capable of activating the alternative pathway include inulin, zymosan (polysaccharide complex from surface of yeast cells), bacterial polysaccharides and endotoxins, and the aggregated IgG2, IgA, and IgE. In paroxysmal nocturnal hemoglobinuria (PNH), the patient’s erythrocytes act as an activator and result in excessive lysis of these erythrocytes. This nonspecific activation is a major physiologic advantage because host protection can be generated before the induction of a humoral immune response.


A key feature of the alternative pathway is that the first three proteins of the classic activation pathway—C1, C4, and C2—do not participate in the cascade sequence. The C3a component is considered to be the counterpart of C2a in the classic pathway. C2 of the classic pathway structurally resembles factor B of the alternative pathway. The omission of C1, C4, and C2 is possible because activators of the alternative pathway catalyze the conversion of another series of normal serum proteins, which leads to the activation of C3. It was previously believed that properdin, a normal protein of human serum, was the first protein to function in the alternative pathway; thus, the pathway was originally named after this protein.


The uptake of factor B onto C3b occurs when C3b is bound to an activator surface. However, C3b in the fluid phase or attached to a nonactivator surface will preferentially bind to and therefore prevent C3b,B formation. C3b and factor B combine to form C3b,B, which is converted into an active C3 convertase, C3b,Bb. This results from the loss of a small fragment, Ba (glycine-rich α2-globulin believed to be physiologically inert), through the action of the enzyme, factor D. The C3b,Bb complex is able to convert more C3 to C3b, which binds more factor B and the feedback cycle continues.


The major controlling event of the alternative pathway is factor H, which prevents the association between C3b and factor B. Factor H blocks the formation of C3b,Bb, the catalytically active C3 convertase of the feedback loop. Factor H (formerly β1-H) competes with factor B for its combining site on C3b, eventually leading to C3 inactivation. Factors B and H apparently occupy a common site on C3b. The factor that is preferentially bound to C3b depends on the nature of the surface to which C3b is attached. Polysaccharides are called activator surfaces and favor the uptake of factor B on the chain of C3b, with the corresponding displacement of factor H. In this situation, binding of factor H is inhibited, and consequently factor B will replace H at the common binding site. When factor H is excluded, C3b is thought to be formed continuously in small amounts. Another controlling point in the amplification loop depends on the stability of the C3b,Bb convertase. Ordinarily, C3b,Bb decays because of the loss of Bb, with a half-life of approximately 5 minutes. However, if properdin (P) binds to C3b,Bb, forming C3b,BbP, the half-life is extended to 30 minutes.


The association of numerous C3b units, factor Bb, and properdin on the surface of an aggregate of protein or the surface of a microorganism has potent activity as a C5 convertase. With the cleavage of C5, the remainder of the complement cascade continues as in the classic pathway.



Mannose-Binding Lectin Pathway


Mannose-binding lectin is a member of a family of calcium-dependent lectins, the collectins (collagenous lectins), and is homologous in structure to C1q. Mannose-binding lectin, a pattern recognition molecule of the innate immune system, binds to arrays of terminal mannose groups on a variety of bacteria.


A deficiency of mannose-binding lectin is caused by one of three point mutations in its gene, each of which reduces levels of the lectin. After the discovery that the binding of mannose-binding lectin to mannose residues can initiate complement activation, the mannose-binding lectin–associated serine protease (MASP) enzymes were discovered. MASP activates complement by interacting with two serine proteases called MASP1 and MASP2. These components make up the mannose-binding lectin pathway.



BiologicAl Functions of Complement Proteins


The biological functions of the complement system fall into the following two general categories:



The first category is the situation in which the MAC leads to osmotic lysis of a cell. The second category encompasses other effects of complement in immunity and inflammation that are mediated by the proteolytic fragments generated during complement activation. These fragments may remain bound to the same cell surfaces at which complement has been activated or may be released into the blood or extracellular fluid. In either situation, active fragments mediate their effects by binding to specific receptors expressed on various types of cells, including phagocytic leukocytes and the endothelium (Table 5-3).



In contrast, the absence of an integral component of the classic, alternative, or terminal lytic pathways can lead to decreased complement activation and a lack of complement-mediated biological functions.



Alterations in Complement Levels


The complement system can cause significant tissue damage in response to abnormal stimuli. Biological effects of complement activation can occur as a reaction to persistent infection or an autoantibody response to self antigens. In these infectious or autoimmune conditions, the inflammatory or lytic effects of complement may contribute significantly to the pathology of the disease.


Complement activation is also associated with intravascular thrombosis, which leads to ischemic injury to tissues. Complement levels may be abnormal in certain disease states (e.g., rheumatoid arthritis, systemic lupus erythematosus [SLE]) and in some genetic disorders.




Decreased Complement Levels


Low levels of complement suggest one of the following biological effects:



Specific component deficiencies are associated with a variety of disorders (Table 5-4). Deficiencies of complement account for a small percentage of primary immunodeficiencies (<2%), but depression of complement levels frequently coexists with SLE and other disorders associated with an immunopathologic process (Box 5-1).



Box 5-1   Diseases Associated With Hypocomplementemia


Rheumatic Diseases With Immune Complexes







Deficiencies in any of the protein components of complement are usually caused by a genetic defect that leads to abnormal patterns of complement activation. If regulatory components are absent, excess activation may occur at the wrong time or at the wrong site. The potential consequences of increased activation are excess inflammation and cell lysis and consumption of complement components.


Hypocomplementemia can result from the complexing of IgG or IgM antibodies capable of activating complement. Depressed values of complement are associated with diseases that give rise to circulating immune complexes. Because of the rapid normal turnover of the complement proteins—within 1 or 2 days of the cessation of complement activation by immune complexes—complement levels return to normal rapidly.


The following three types of complement deficiency can cause increased susceptibility to pyogenic infections:



Increased susceptibility to pyogenic bacteria (e.g., Haemophilus influenzae, Streptococcus pneumoniae) occurs in patients with defects of antibody production, complement proteins of the classic pathway, or phagocyte function. The sole clinical association between inherited deficiency of MAC components and infection is with neisserial infection, particularly Neisseria meningitidis. Low levels of mannose-binding lectin in young children with recurrent infections suggest that the mannose-binding lectin pathway is important during the interval between the loss of passively acquired maternal antibody and the acquisition of a mature immunologic repertoire of antigen exposure.



Diagnostic Evaluation


During immune complex reactions, certain complement proteins become physically bound to the tissue in which the immunologic reaction is occurring. These proteins can be demonstrated in tissue by appropriate immunopathologic stains. The most frequent evaluation of complement is by serum or plasma assay (Table 5-5). Complement components (e.g., C3 and C4) can be assessed by nephelometry. These assays are useful for the diagnosis and monitoring of patients.




Assessment of Complement


The procedures discussed next can be used in diagnostic immunology.










C3PA (C3 Proactivator, Properdin Factor B)


The factor B component is consumed by activation of the alternative complement pathway. Assessment of C3PA indicates whether a decreased level of C3 results from the classic or alternative pathways of complement activation. Decreased levels of C3 and C4 demonstrate activation of the classic pathway. Decreased levels of C3 and C3PA with a normal level of C4 indicate complement activation via the alternative pathway (Table 5-5).


Activation of the classic pathway (and sometimes with accompanying alternative pathway activation) is associated with disorders such as immune complex diseases, various forms of vasculitis, and acute glomerulonephritis. Activation of the alternative pathway is associated with many disorders, including chronic hypocomplementemic glomerulonephritis, disseminated intravascular coagulation (DIC), septicemia, subacute bacterial endocarditis, PNH, and sickle cell anemia.


In SLE, both the classic and alternative pathways are activated.

Related posts:

The Immune Response in Infectious Diseases A Primer on Vaccines Rubella and Rubeola Infections Immunoproliferative Disorders Systemic Lupus Erythematosus Hypersensitivity Reactions

Stay updated, free articles. Join our Telegram channel
Tags:
Jun 12, 2016 | Posted by in IMMUNOLOGY | Comments Off on Soluble Mediators of the Immune System

Full access? Get Clinical Tree
Get Clinical Tree app for offline access