http://surgerybook.net/

Activation of the alternative pathway is restricted to the cell surface of microbes, as mammalian cells

possess several regulatory proteins to rapidly degrade any C3bBb. Excessive amplification is regulated

by factor H and factor I. H accelerates the decay and delays the formation of C3bBb. It also acts as a

cofactor for I to degrade C3 into iC3b, which is unable to bind B to generate C3 convertase. Decay

acceleration factor hinders C3 convertase assembly and mediates its dissociation in all three pathways.

Also, the protein properdin, which stabilizes this alternative pathway C3 convertase, has a higher

affinity for microbial than mammalian cell surfaces.4,169,171

The classical pathway is the primary mediator of adaptive humoral immunity and is initiated by

binding of the complement protein C1 to IgG or IgM molecules engaged with antigen (Fig. 7-9). Other

substances such as lipid A in endotoxin and mitochondrial membranes may activate this pathway

independent of antigen–antibody complexes in vitro.169,173

C1 is a calcium-dependent trimeric protein consisting of C1q, which recognizes and binds the Fc

region of the immunoglobulin, and C1r and C1s, which are proteases. C1q engages the Fc portion of the

immunoglobulin μ and γ heavy chains. Each Fc region has a single C1q binding site, yet for activation,

each C1q molecule must bind to two Ig heavy chains. Hence, multiple antibodies must be approximated

for activation, which restricts activation to foci of immunoglobulin engagement. IgM exists as a

pentamer enabling it to bind to two C1q molecules, and hence it is more efficient at complement

activation.4,169,173

Interaction between C1q and the immunoglobin Fc region induces a conformational change in C1q

that activates C1r, which subsequently cleaves and activates C1s. C1s cleaves C4 to generate C4b and

C4a. The C4a anaphylatoxin possesses properties similar to those described for C3a. C4b localizes to

immune complexes on the cell surface. C2, after complexing with C4b, is cleaved by C1s, thereby

generating the classical C3 convertase C4b2a complex, which has the ability to cleave C3. The C3b

generated can bind Bb, producing more C3 convertase and amplifying the signal. The key early steps of

the alternative and classical pathways are analogous: C3 and factor B of the alternative pathway are

homologous to C4 and C2 in the classical pathway. C3b can also combine with the classical C3

convertase to generate C4b2a3b, the classical C5 convertase.4

Numerous regulatory mechanisms exist to restrict activation to sites of inflammation. Excessive

classical C3 convertase activity is prevented by the rapid decay of C2a from the complex, which renders

the complex unstable. C1 inhibitor covalently binds C1s, reducing the half-life of activated C1 to only

13 seconds. C4-binding protein enhances spontaneous dissociation of C4b2a and also acts as a cofactor

for C3b/C4b inactivator, which degrades C4b.39

The mannose-binding lectin (MBL) pathway is activated by microbial polysaccharides bound to

circulating lectins; MBL serves as the recognition unit of the MBL pathway. This pathway recognizes

polysaccharides with high mannose content and other oligosaccharides with characteristic linkages

found exclusively on pathogens and not on normal host components. Binding is calcium dependent and

results in the activation of mannose-binding lectin-associated serine proteases (MASP-1 and MASP-2).

Activated MASP-2 cleaves and activates C4 and C2 in the same fashion as the classical complement

pathway. Subsequent steps of complement activation of the MBL pathway mirror those of the classical

complement pathway.169,174,175

The C5 convertases generated during either the classical or alternative pathway initiate a cascade of

events that culminates in the formation of the cytocidal MAC. Specifically, C5 convertase cleaves C5,

yielding C5a and C5b. C6 and C7 bind to generate the C5b67 complex. This hydrophobic moiety

penetrates deeply into the lipid bilayer as a high-affinity receptor for C8. Binding of C8 forms the

complex C5b-8 that recruits numerous C9 subunits, which polymerize to form pores in the plasma

membrane of bacteria. The pores structurally resemble the membrane pores formed by perforin, the

cytolytic granule protein found in CTLs and NK cells. Their diameter may span 100 Å, which prevents

the maintenance of vital ionic gradients, induces osmotic lysis, and ultimately the death of target cells

or pathogens. Patients with deficiencies in the terminal components of C5, C6, C7, and C8 are

susceptible to meningococcal and gonococcal infections.4,176 By contrast, recent studies suggest that

aberrant induction of the complement system, specifically C5a, plays an integral role in inducing

paralysis of the innate immunity and the development of ARDS and MODS.

The effects of complement are in part mediated through several complement receptors. Opsonization

and phagocytosis are important mechanisms for pathogen destruction, and phagocytic cells express

receptors for complement factor C3 components. Complement receptor type I (CR1), the C3b receptor

for C3b and C4b, mediates engagement of and facilitates phagocytosis of complement-bound microbes

and the clearance of immune complexes from the circulation. It is expressed on a variety of cells

202

http://surgerybook.net/

including RBC, neutrophils, monocytes, eosinophils, and T and B cells. RBCs facilitate elimination by

transporting these opsonized particles to the liver and spleen where the immune complexes are removed

by phagocytes. CR2 stimulates humoral immune response by enhancing B cell activation by antigen and

by promoting the trapping of antigen–antibody complexes in germinal centers. In humans, this receptor

is the receptor for EBV. CR3 and CR4 are β2

integrins that bind the iC3b-processed fragment of C3 and

promote macrophage and neutrophil phagocytosis of iC3b-opsonized antigen.4,39

This entire cascade is under strict regulation to ensure that activation is restricted to sites of

inflammation and infection. This regulation is needed as low-level activation is always occurring, and if

not quelled, would certainly damage normal tissues. Even when locally activated, byproducts may

damage nearby cells and tissues. Several circulating proteins function to do this. C1r and C1s are

inhibited by C1 inhibitor, a serine protease inhibitor that mimics C1r and C1s. C1 inhibitor targets

activated C1qrs, and after attachment, C1r-C1s dissociates and activation of classical complement

ceases. This inhibition prevents the accumulation of active C1r-C1s, thereby limiting the duration during

which active C1r-C1s can initiate the cascade.4 C1 inhibitor also inhibits other circulating inflammatory

serine proteases including kallikrein and factor XII, both of which can activate the formation of

bradykinin.4 Hereditary angioneurotic edema is an inherited deficiency of C1 inhibitor and manifests as

acute intermittent edema of the skin and mucosa causing abdominal pain, vomiting, diarrhea, and

airway obstruction.

MCP, type 1 complement receptor (CR1) and DAF are regulatory proteins that bind to C3b and C4b

deposited on cell surfaces and competitively inhibit the binding of other components of the C3 and C5

convertases, such as Bb and C2a, and thereby block further progression of the cascade. These proteins

are only produced by mammalian cells. Deficiency of an enzyme required to form the linkages

necessary to express DAF underlies paroxysmal nocturnal hemoglobinuria, a disease characterized by

recurrent intravascular hemolysis due to unregulated complement activation on the surface of

erythrocytes. Cell-associated C3b is proteolytically degraded by a plasma serine protease called factor I

which is active only in the presence of regulatory proteins such as MCP, factor H, C4BP and CR1. MAC

formation is inhibited by CD59, a membrane protein that incorporates itself into growing MACS and

inhibits the incorporation of C9. It is not present in microbes. The function of these regulatory proteins

may be overcome by increasing amounts of complement activation.4

Lipid Mediators

Eicosanoids

7 Eicosanoids are 20-carbon lipid inflammatory mediators that are derived from membrane arachidonic

acid and are involved in numerous homeostatic processes and inflammation. These lipid mediators are

not stored in tissues, but are synthesized de novo within seconds in response to a variety of stimuli,

including mechanical trauma, specific cytokines, growth factors, and other mediators (Fig. 7-10).

Although most cells are capable of producing eicosanoids, neutrophils and macrophages are the

predominant sources. They are rapidly degraded in the circulation, which limits their role primarily to

that of autocrine and paracrine mediators of local inflammatory changes.177–179

The liberation of the precursor molecule, arachidonic acid, is the major rate-limiting step. The family

of PLA2

, in particular type IV cytosolic PLA2

, is responsible for eicosanoid production; cells lacking type

IV PLA2 are devoid of eicosanoid synthesis.180 Many PLA2 are transcriptionally regulated by IL-1 and

TNFα, whereas others are regulated by the MAP kinase pathway and by calcium-dependent

translocation to membranes. Once formed, arachidonic acid metabolism proceeds along one of the two

pathways.177–179

Cyclooxygenase Pathway (Prostaglandins)

Cyclooxygenase catalyzes the initial step of a series of reactions that converts arachidonic acid to

prostanoids (Fig. 7-10). There are two isoforms, COX1 and COX2; the former is constitutively

expressed, whereas COX2 is inducible. COX2 is considered more important during inflammatory

processes such as fever, hyperalgesia, and edema formation. Either enzyme catalyzes the conversion of

arachidonic acid to the endoperoxidase PGG2

, which is subsequently converted to PGH2

. PGH2 serves as

the precursor to numerous specific prostaglandins. All of these products possess very short half-lives and

are rapidly inactivated.180,181

Prostaglandins mediate their effects through G protein signaling (Table 7-8). Prostacyclin (PGI2

) is

produced by the endothelium as a potent vasodilator and inhibitor of platelet aggregation and adhesion;

203

http://surgerybook.net/

these characteristics enhance tissue perfusion. PGI2

inhibits neutrophil chemotaxis and activation and

interacts synergistically with PGE2

to increase vascular permeability through the bradykinin pathway.

PGE2

is the predominant anti-inflammatory prostaglandin and is produced by nearly all inflammatory

cells. It is produced in response to IL-1 and mediates the hypothalamic fever response and synergizes

with bradykinin and histamine to mediate pain. It is a bronchodilator, inhibits both IL-1 production and

T-cell responsiveness to IL-1, and at low concentrations suppresses TNFα production. It also inhibits

neutrophil chemotaxis and activation and TH1 lymphocyte proliferation.180,181 There is some suggestion

from animal studies that PGE2

, PGE1

, and PGI2 may be beneficial in response to sepsis through their

endogenous counterregulatory properties. Administration of each of these has been shown to improve

survival in several animal models of hypovolemic and traumatic shock, though clinical trials have failed

to identify benefit.181 PGD2

is a potent bronchoconstrictor that inhibits neutrophil chemotaxis and

activation. TXA2

, PGG2

, and PGH2 oppose the actions of prostacyclin by promoting platelet aggregation

and inducing bronchoconstriction. TXA2 produced by platelets and macrophages is a powerful

vasoconstrictor that induces neutrophil accumulation and increases vascular permeability.177–179,182

There is substantial evidence that TXA2 plays a significant role in early acute-phase organ injury.181

Lipoxygenase Pathway (Leukotrienes and Lipoxins)

LTs and lipoxins are leukocyte-derived molecules synthesized by the oxidation of arachidonic acid by

three LO enzymes: 5-LO, 12-LO, and 15-LO (Fig. 7-10). 5-LO associates with the perinuclear membrane

protein 5-LO-activating protein (FLAP) to catalyze the formation of 5-hydroperoxyeicosatetraenoic acid

(5-HPETE). 5-HPETE is subsequently converted to LTA4 by the combined efforts of a dehydrase and 5-

LO. LTA4 serves as the precursor for either LTB4 or LTC4

. LTC4 can in turn be successively hydrolyzed

to the dipeptide derivative LTD4 and the LTE4

. Additional LO activity results in the production of 12-

HPETE, 12-HETE, and 15-HPETE. These compounds exhibit biological activity, although they are not as

potent as the LTs or prostaglandins. The LTs are inactivated by oxidation followed by

dehydration.178–180,182,183

Figure 7-10. Eicosanoid production. 1. Phospholipases; 2. Cyclooxygenase; 3. Hydroperoxidase; 4. Thromboxane synthetase; 5.

Prostacyclin synthetase; 6. E-isomerase; 7. D-isomerase; 8. F-reductase; 9. 5-Lipoxygenase; 10. Glutathione peroxidase; 11.

Hydrolase; 12. Glutathione S-transferase; 13. γ-Glutamyl transpeptidase; 14. Cysteinyl glycinase; 15. 12-Lipoxygenase; 16.

Peroxidase; 17. 15-Lipoxygenase; 18. GSH-cis-11-trans lipoxin A4

; 19. α-Glutamyl transferase; 20. Dipeptidase.

204