Changes in Vascular Flow and Caliber

Changes in vascular flow and caliber begin early after injury and consist of the following.

Increased Vascular Permeability (Vascular Leakage)

A hallmark of acute inflammation is increased vascular permeability leading to the escape of a protein-rich exudate into the extravascular tissue, causing edema. Several mechanisms are responsible for the increased vascular permeability (Fig. 2-3):

FIGURE 2-3 Principal mechanisms of increased vascular permeability in inflammation, and their features and underlying causes. NO, nitric oxide; VEGF, vascular endothelial growth factor.

Although these mechanisms of increased vascular permeability are described separately, all probably contribute in varying degrees in responses to most stimuli. For example, at different stages of a thermal burn, leakage results from chemically mediated endothelial contraction and direct and leukocyte-dependent endothelial injury. The vascular leakage induced by all these mechanisms can cause life-threatening loss of fluid in severely burned patients.

Responses of Lymphatic Vessels

Although much of the emphasis in our discussion of inflammation is on the reactions of blood vessels, lymphatic vessels also participate in the response. The system of lymphatics and lymph nodes filters and polices the extravascular fluids. Recall that lymphatics normally drain the small amount of extravascular fluid that has seeped out of capillaries. In inflammation, lymph flow is increased and helps drain edema fluid that accumulates due to increased vascular permeability. In addition to fluid, leukocytes and cell debris, as well as microbes, may find their way into lymph. Lymphatic vessels, like blood vessels, proliferate during inflammatory reactions to handle the increased load.^11,12 The lymphatics may become secondarily inflamed (lymphangitis), as may the draining lymph nodes (lymphadenitis). Inflamed lymph nodes are often enlarged because of hyperplasia of the lymphoid follicles and increased numbers of lymphocytes and macrophages. This constellation of pathologic changes is termed reactive, or inflammatory, lymphadenitis (Chapter 13). For clinicians the presence of red streaks near a skin wound is a telltale sign of an infection in the wound. This streaking follows the course of the lymphatic channels and is diagnostic of lymphangitis; it may be accompanied by painful enlargement of the draining lymph nodes, indicating lymphadenitis.

Recruitment of Leukocytes to Sites of Infection and Injury

The journey of leukocytes from the vessel lumen to the interstitial tissue, called extravasation, can be divided into the following steps¹³ (Fig. 2-4):

FIGURE 2-4 The multistep process of leukocyte migration through blood vessels, shown here for neutrophils. The leukocytes first roll, then become activated and adhere to endothelium, then transmigrate across the endothelium, pierce the basement membrane, and migrate toward chemoattractants emanating from the source of injury. Different molecules play predominant roles in different steps of this process—selectins in rolling; chemokines (usually displayed bound to proteoglycans) in activating the neutrophils to increase avidity of integrins; integrins in firm adhesion; and CD31 (PECAM-1) in transmigration. Neutrophils express low levels of L-selectin; they bind to endothelial cells predom in antly via P- and E-selectins. ICAM-1, intercellular adhesion molecule 1; TNF, tumor necrosis factor.

Leukocyte Adhesion to Endothelium.

In normally flowing blood in venules, red cells are confined to a central axial column, displacing the leukocytes toward the wall of the vessel. Because blood flow slows early in inflammation (stasis), hemodynamic conditions change (wall shear stress decreases), and more white cells assume a peripheral position along the endothelial surface. This process of leukocyte redistribution is called margination. Subsequently, individual and then rows of leukocytes adhere transiently to the endothelium, detach and bind again, thus rolling on the vessel wall. The cells finally come to rest at some point where they adhere firmly (resembling pebbles over which a stream runs without disturbing them).

The adhesion of leukocytes to endothelial cells is mediated by complementary adhesion molecules on the two cell types whose expression is enhanced by secreted proteins called cytokines.^13,14 Cytokines are secreted by cells in tissues in response to microbes and other injurious agents, thus ensuring that leukocytes are recruited to the tissues where these stimuli are present. The initial rolling interactions are mediated by a family of proteins called selectins^15,16 (Table 2-1). There are three types of selectins: one expressed on leukocytes (L-selectin), one on endothelium (E-selectin), and one in platelets and on endothelium (P-selectin). The ligands for selectins are sialylated oligosaccharides bound to mucin-like glycoprotein backbones. The expression of selectins and their ligands is regulated by cytokines produced in response to infection and injury. Tissue macrophages, mast cells, and endothelial cells that encounter microbes and dead tissues respond by secreting several cytokines, including tumor necrosis factor (TNF),¹⁷ interleukin-1 (IL-1),¹⁸ and chemokines (chemoattractant cytokines).^19,20 (Cytokines are described in more detail below and in Chapter 6.) TNF and IL-1 act on the endothelial cells of post-capillary venules adjacent to the infection and induce the coordinate expression of numerous adhesion molecules (Fig. 2-5). Within 1 to 2 hours the endothelial cells begin to express E-selectin and the ligands for L-selectin. Other mediators such as histamine, thrombin, and platelet-activating factor (PAF), described later, stimulate the redistribution of P-selectin from its normal intracellular stores in endothelial cell granules (called Weibel-Palade bodies) to the cell surface. Leukocytes express L-selectin at the tips of their microvilli and also express ligands for E- and P-selectins, all of which bind to the complementary molecules on the endothelial cells. These are low-affinity interactions with a fast off-rate, and they are easily disrupted by the flowing blood. As a result, the bound leukocytes bind, detach, and bind again, and thus begin to roll along the endothelial surface.

TABLE 2-1 Endothelial-Leukocyte Adhesion Molecules

Endothelial Molecule	Leukocyte Molecule	Major Role
P-selectin	Sialyl-Lewis X–modified proteins	Rolling (neutrophils, monocytes, T lymphocytes)
E-selectin	Sialyl-Lewis X–modified proteins	Rolling and adhesion (neutrophils, monocytes, T lymphocytes)
GlyCam-1, CD34	L-selectin*	Rolling (neutrophils, monocytes)
ICAM-1 (immunoglobulin family)	CD11/CD18 (β2) integrins (LFA-1, Mac-1)	Adhesion, arrest, transmigration (neutrophils, monocytes, lymphocytes)
VCAM-1 (immunoglobulin family)	VLA-4 (β1) integrin	Adhesion (eosinophils, monocytes, lymphocytes)

* L-selectin is expressed weakly on neutrophils. It is involved in the binding of circulating T-lymphocytes to the high endothelial venules in lymph nodes and mucosal lymphoid tissues, and subsequent “homing” of lymphocytes to these tissues.

FIGURE 2-5 Regulation of expression of endothelial and leukocyte adhesion molecules. A, Redistribution of P-selectin from intracellular stores to the cell surface. B, Increased surface expression of selectins and ligands for integrins upon cytokine activation of endothelium. C, Increased binding avidity of integrins induced by chemokines. Clustering of integrins contributes to their increased binding avidity (not shown). IL-1, interleukin-1; TNF, tumor necrosis factor.

These weak rolling interactions slow down the leukocytes and give them the opportunity to bind more firmly to the endothelium. Firm adhesion is mediated by a family of heterodimeric leukocyte surface proteins called integrins²¹ (see Table 2-1). TNF and IL-1 induce endothelial expression of ligands for integrins, mainly vascular cell adhesion molecule 1 (VCAM-1, the ligand for the VLA-4 integrin) and intercellular adhesion molecule-1 (ICAM-1, the ligand for the LFA-1 and Mac-1 integrins). Leukocytes normally express integrins in a low-affinity state. Meanwhile, chemokines that were produced at the site of injury enter the blood vessel, bind to endothelial cell proteoglycans, and are displayed at high concentrations on the endothelial surface. These chemokines bind to and activate the rolling leukocytes. One of the consequences of activation is the conversion of VLA-4 and LFA-1 integrins on the leukocytes to a high-affinity state.²² The combination of cytokine-induced expression of integrin ligands on the endothelium and activation of integrins on the leukocytes results in firm integrin-mediated binding of the leukocytes to the endothelium at the site of inflammation. The leukocytes stop rolling, their cytoskeleton is reorganized, and they spread out on the endothelial surface.

Leukocyte Migration through Endothelium.

The next step in the process of leukocyte recruitment is migration of the leukocytes through the endothelium, called transmigration or diapedesis. Transmigration of leukocytes occurs mainly in post-capillary venules. Chemokines act on the adherent leukocytes and stimulate the cells to migrate through interendothelial spaces toward the chemical concentration gradient, that is, toward the site of injury or infection where the chemokines are being produced.²³ Several adhesion molecules present in the intercellular junctions between endothelial cells are involved in the migration of leukocytes. These molecules include a member of the immunoglobulin superfamily called PECAM-1 (platelet endothelial cell adhesion molecule) or CD31²⁴ and several junctional adhesion molecules.²⁵ After traversing the endothelium, leukocytes pierce the basement membrane, probably by secreting collagenases, and enter the extravascular tissue. The cells then migrate toward the chemotactic gradient created by chemokines and accumulate in the extravascular site. In the connective tissue, the leukocytes are able to adhere to the extracellular matrix by virtue of integrins and CD44 binding to matrix proteins. Thus, leukocytes are retained at the site where they are needed.

The most telling proof of the importance of leukocyte adhesion molecules is the existence of genetic deficiencies in these molecules, which result in recurrent bacterial infections as a consequence of impaired leukocyte adhesion and defective inflammation.²⁶ Individuals with leukocyte adhesion deficiency type 1 have a defect in the biosynthesis of the β₂ chain shared by the LFA-1 and Mac-1 integrins. Leukocyte adhesion deficiency type 2 is caused by the absence of sialyl-Lewis X, the fucose-containing ligand for E- and P-selectins, as a result of a defect in a fucosyl transferase, the enzyme that attaches fucose moieties to protein backbones.

Chemotaxis of Leukocytes.

After exiting the circulation, leukocytes emigrate in tissues toward the site of injury by a process called chemotaxis, which is defined as locomotion oriented along a chemical gradient. Both exogenous and endogenous substances can act as chemoattractants. The most common exogenous agents are bacterial products, including peptides that possess an N-formylmethionine terminal amino acid, and some lipids. Endogenous chemoattractants include several chemical mediators (described later): (1) cytokines, particularly those of the chemokine family (e.g., IL-8); (2) components of the complement system, particularly C5a; and (3) arachidonic acid (AA) metabolites, mainly leukotriene B₄ (LTB₄). All these chemotactic agents bind to specific seven-transmembrane G protein–coupled receptors on the surface of leukocytes.²⁷ Signals initiated from these receptors result in activation of second messengers that increase cytosolic calcium and activate small guanosine triphosphatases of the Rac/Rho/cdc42 family as well as numerous kinases. These signals induce polymerization of actin, resulting in increased amounts of polymerized actin at the leading edge of the cell and localization of myosin filaments at the back. The leukocyte moves by extending filopodia that pull the back of the cell in the direction of extension, much as an automobile with front-wheel drive is pulled by the wheels in front (Fig. 2-6). The net result is that leukocytes migrate toward the inflammatory stimulus in the direction of the gradient of locally produced chemoattractants.

FIGURE 2-6 Scanning electron micrograph of a moving leukocyte in culture showing a filopodium (upper left) and a trailing tail.

(Courtesy of Dr. Morris J. Karnovsky, Harvard Medical School, Boston, MA.)

The nature of the leukocyte infiltrate varies with the age of the inflammatory response and the type of stimulus. In most forms of acute inflammation neutrophils predominate in the inflammatory infiltrate during the first 6 to 24 hours and are replaced by monocytes in 24 to 48 hours (Fig. 2-7). Several reasons account for the early appearance of neutrophils: they are more numerous in the blood, they respond more rapidly to chemokines, and they may attach more firmly to the adhesion molecules that are rapidly induced on endothelial cells, such as P- and E-selectins. After entering tissues, neutrophils are short-lived; they undergo apoptosis and disappear after 24 to 48 hours. Monocytes not only survive longer but may proliferate in the tissues, and thus become the dominant population in chronic inflammatory reactions. There are, however, exceptions to this pattern of cellular infiltration. In certain infections—for example, those produced by Pseudomonas bacteria—the cellular infiltrate is dominated by continuously recruited neutrophils for several days; in viral infections, lymphocytes may be the first cells to arrive; in some hypersensitivity reactions, eosinophils may be the main cell type.

FIGURE 2-7 Nature of leukocyte infiltrates in inflammatory reactions. The photomicrographs are representative of the early (neutrophilic) (A) and later (mononuclear) cellular infiltrates (B) seen in an inflammatory reaction in the myocardium following ischemic necrosis (infarction). The kinetics of edema and cellular infiltration (C) are approximations.

The molecular understanding of leukocyte recruitment and migration has provided a large number of potential therapeutic targets for controlling harmful inflammation.¹⁴ Agents that block TNF, one of the major cytokines in leukocyte recruitment, are among the most successful therapeutics ever developed for chronic inflammatory diseases, and antagonists of leukocyte integrins (e.g. VLA-4), selectins, and chemokines are approved for inflammatory diseases or in clinical trials. Predictably, these antagonists not only have the desired effect of controlling the inflammation but can compromise the ability of treated patients to defend themselves against microbes, which, of course, is the physiologic function of the inflammatory response.

Recognition of Microbes and Dead Tissues

Once leukocytes (neutrophils and monocytes) have been recruited to a site of infection or cell death, they must be activated to perform their functions. The responses of leukocytes consist of two sequential sets of events: (1) recognition of the offending agents, which deliver signals that (2) activate the leukocytes to ingest and destroy the offending agents and amplify the inflammatory reaction.

Leukocytes express several receptors that recognize external stimuli and deliver activating signals (Fig. 2-8).

FIGURE 2-8 Leukocyte receptors and responses. Different classes of cell surface receptors of leukocytes recognize different stimuli. The receptors initiate responses that mediate the functions of the leukocytes. Only some receptors are depicted (see text for details). IFN-γ, interferon-γ; LPS, lipopolysaccharide(s).

Removal of the Offending Agents

Recognition of microbes or dead cells by the receptors described above induces several responses in leukocytes that are referred to under the rubric of leukocyte activation (see Fig. 2-8). Activation results from signaling pathways that are triggered in leukocytes, resulting in increases in cytosolic Ca²⁺ and activation of enzymes such as protein kinase C and phospholipase A₂. The functional responses that are most important for destruction of microbes and other offenders are phagocytosis and intracellular killing. Several other responses aid in the defensive functions of inflammation and may contribute to its injurious consequences.

Phagocytosis.

Phagocytosis involves three sequential steps (Fig. 2-9): (1) recognition and attachment of the particle to be ingested by the leukocyte; (2) its engulfment, with subsequent formation of a phagocytic vacuole; and (3) killing or degradation of the ingested material.³⁰

FIGURE 2-9 Phagocytosis and intracellular destruction of microbes. Phagocytosis of a particle (e.g., bacterium) involves binding to receptors on the leukocyte membrane, engulfment, and fusion of lysosomes with phagocytic vacuoles. This is followed by destruction of ingested particles within the phagolysosomes by lysosomal enzymes and by reactive oxygen and nitrogen species. The microbicidal products generated from superoxide () are hypochlorite (HOCl^•) and hydroxyl radical (^•OH), and from nitric oxide (NO) it is peroxynitrite (OONO^•). During phagocytosis, granule contents may be released into extracellular tissues (not shown). MPO, myeloperoxidase; iNOS, inducible NO synthase.

Mannose receptors, scavenger receptors, and receptors for various opsonins all function to bind and ingest microbes. The macrophage mannose receptor is a lectin that binds terminal mannose and fucose residues of glycoproteins and glycolipids. These sugars are typically part of molecules found on microbial cell walls, whereas mammalian glycoproteins and glycolipids contain terminal sialic acid or N-acetylgalactosamine. Therefore, the mannose receptor recognizes microbes and not host cells. Scavenger receptors were originally defined as molecules that bind and mediate endocytosis of oxidized or acetylated low-density lipoprotein (LDL) particles that can no longer interact with the conventional LDL receptor. Macrophage scavenger receptors bind a variety of microbes in addition to modified LDL particles. Macrophage integrins, notably Mac-1 (CD11b/CD18), may also bind microbes for phagocytosis.

The efficiency of phagocytosis is greatly enhanced when microbes are opsonized by specific proteins (opsonins) for which the phagocytes express high-affinity receptors. As described above, the major opsonins are IgG antibodies, the C3b breakdown product of complement, and certain plasma lectins, notably mannan-binding lectin, all of which are recognized by specific receptors on leukocytes.

Engulfment.

After a particle is bound to phagocyte receptors, extensions of the cytoplasm (pseudopods) flow around it, and the plasma membrane pinches off to form a vesicle (phagosome) that encloses the particle. The phagosome then fuses with a lysosomal granule, resulting in discharge of the granule’s contents into the phagolysosome (see Fig. 2-9). During this process the phagocyte may also release granule contents into the extracellular space.

The process of phagocytosis is complex and involves the integration of many receptor-initiated signals to lead to membrane remodeling and cytoskeletal changes.³⁰ Phagocytosis is dependent on polymerization of actin filaments; it is, therefore, not surprising that the signals that trigger phagocytosis are many of the same that are involved in chemotaxis. (In contrast, fluid-phase pinocytosis and receptor-mediated endocytosis of small particles involve internalization into clathrin-coated pits and vesicles and are not dependent on the actin cytoskeleton.)

Killing and Degradation.

The final step in the elimination of infectious agents and necrotic cells is their killing and degradation within neutrophils and macrophages, which occur most efficiently after activation of the phagocytes. Microbial killing is accomplished largely by reactive oxygen species (ROS, also called reactive oxygen intermediates) and reactive nitrogen species, mainly derived from NO (see Fig. 2-9).^31,32 The generation of ROS is due to the rapid assembly and activation of a multicomponent oxidase (NADPH oxidase, also called phagocyte oxidase), which oxidizes NADPH (reduced nicotinamide-adenine dinucleotide phosphate) and, in the process, reduces oxygen to superoxide anion (). In neutrophils, this rapid oxidative reaction is triggered by activating signals and accompanies phagocytosis, and is called the respiratory burst. Phagocyte oxidase is an enzyme complex consisting of at least seven proteins.³³ In resting neutrophils, different components of the enzyme are located in the plasma membrane and the cytoplasm. In response to activating stimuli, the cytosolic protein components translocate to the phagosomal membrane, where they assemble and form the functional enzyme complex. Thus, the ROS are produced within the lysosome where the ingested substances are segregated, and the cell’s own organelles are protected from the harmful effects of the ROS. is then converted into hydrogen peroxide (H₂O₂), mostly by spontaneous dismutation. H₂O₂ is not able to efficiently kill microbes by itself. However, the azurophilic granules of neutrophils contain the enzyme myeloperoxidase (MPO), which, in the presence of a halide such as Cl⁻, converts H₂O₂ to hypochlorite (OCl•, the active ingredient in household bleach). The latter is a potent antimicrobial agent that destroys microbes by halogenation (in which the halide is bound covalently to cellular constituents) or by oxidation of proteins and lipids (lipid peroxidation). The H₂O₂-MPO-halide system is the most efficient bactericidal system of neutrophils. H₂O₂ is also converted to hydroxyl radical (^•OH), another powerful destructive agent.

NO, produced from arginine by the action of nitric oxide synthase (NOS), also participates in microbial killing.³⁴ NO reacts with superoxide () to generate the highly reactive free radical peroxynitrite (ONOO^•). These oxygen- and nitrogen-derived free radicals attack and damage the lipids, proteins, and nucleic acids of microbes as they do with host macromolecules (Chapter 1). Reactive oxygen and nitrogen species have overlapping actions, as shown by the observation that knockout mice lacking either phagocyte oxidase or inducible nitric oxide synthase (iNOS) are only mildly susceptible to infections, but mice lacking both succumb rapidly to disseminated infections by normally harmless commensal bacteria. The roles of ROS and NO as mediators of inflammation are described later in the chapter.

Microbial killing can also occur through the action of other substances in leukocyte granules. Neutrophil granules contain many enzymes, such as elastase, that contribute to microbial killing.³⁵ Other microbicidal granule contents include defensins, cationic arginine-rich granule peptides that are toxic to microbes³⁶; cathelicidins, antimicrobial proteins found in neutrophils and other cells³⁷; lysozyme, which hydrolyzes the muramic acid–N-acetylglucosamine bond, found in the glycopeptide coat of all bacteria; lactoferrin, an iron-binding protein present in specific granules; major basic protein, a cationic protein of eosinophils, which has limited bactericidal activity but is cytotoxic to many parasites; and bactericidal/permeability increasing protein, which binds bacterial endotoxin and is believed to be important in defense against some gram-negative bacteria.

Other Functional Responses of Activated Leukocytes

In addition to eliminating microbes and dead cells, activated leukocytes play several other roles in host defense. Importantly, these cells, especially macrophages, produce a number of growth factors that stimulate the proliferation of endothelial cells and fibroblasts and the synthesis of collagen, and enzymes that remodel connective tissues. These products drive the process of repair after tissue injury (Chapter 3). An emerging concept is that macrophages can be activated to perform different functions—“classically activated” macrophages respond to microbial products and T-cell cytokines such as IFN-γ and have strong microbicidal activity, whereas “alternatively activated” macrophages respond to cytokines such as IL-4 and IL-13 (typically, the products of the T_H2 subset of T-cells, see Chapter 6) and are mainly involved in tissue repair and fibrosis (Fig. 2-10).³⁸ Different stimuli activate leukocytes to secrete mediators of inflammation as well as inhibitors of the inflammatory response, and thus serve to both amplify and control the reaction. This may be another distinction between classically and alternatively activated macrophages—the former trigger inflammation and the latter function to limit inflammatory reactions.

FIGURE 2-10 Subsets of activated macrophages. Different stimuli activate monocytes/macrophages to develop into functionally distinct populations. Classically activated macrophages are induced by microbial products and cytokines, particularly IFN-γ, and are microbicidal and involved in potentially harmful inflammation. Alternatively activated macrophages are induced by other cytokines and in response to helminths (not shown), and are important in tissue repair and the resolution of inflammation (and may play a role in defense against helminthic parasites, also not shown).

Release of Leukocyte Products and Leukocyte-Mediated Tissue Injury

Leukocytes are important causes of injury to normal cells and tissues under several circumstances:

In all these situations, the mechanisms by which leukocytes damage normal tissues are the same as the mechanisms involved in antimicrobial defense, because once the leukocytes are activated, their effector mechanisms do not distinguish between offender and host. During activation and phagocytosis, neutrophils and macrophages release microbicidal and other products not only within the phagolysosome but also into the extracellular space. The most important of these substances are lysosomal enzymes, present in the granules, and reactive oxygen and nitrogen species. These released substances are capable of damaging normal cells and vascular endothelium, and may thus amplify the effects of the initial injurious agent. In fact, if unchecked or inappropriately directed against host tissues, the leukocyte infiltrate itself becomes the offender,³⁹ and indeed leukocyte-dependent tissue injury underlies many acute and chronic human diseases (Table 2-2). This fact becomes evident in the discussion of specific disorders throughout the book.

TABLE 2-2 Clinical Examples of Leukocyte-Induced Injury^*

* Listed are selected examples of diseases in which the host response plays a significant role in tissue injury, and the principal cells and molecules that cause the injury. These diseases and their pathogenesis will be discussed in detail in relevant chapters.

The contents of lysosomal granules are secreted by leukocytes into the extracellular milieu by several mechanisms.⁴⁰ Controlled secretion of granule contents is a normal response of activated leukocytes. If phagocytes encounter materials that cannot be easily ingested, such as immune complexes deposited on immovable flat surfaces (e.g., glomerular basement membrane), the inability of the leukocytes to surround and ingest these substances (frustrated phagocytosis) triggers strong activation, and the release of large amounts of lysosomal enzymes into the extracellular environment. Phagocytosis of membrane-damaging substances, such as urate crystals, may injure the membrane of the phagolysosome and also lead to the release of lysosomal granule contents.

Defects in Leukocyte Function

Because leukocytes play a central role in host defense, defects in leukocyte function, both inherited and acquired, lead to increased vulnerability to infections (Table 2-3). Impairments of virtually every phase of leukocyte function have been identified—from adherence to vascular endothelium to microbicidal activity. These include the following:

• Inherited defects in microbicidal activity. The importance of oxygen-dependent bactericidal mechanisms is shown by the existence of a group of congenital disorders called chronic granulomatous disease, which are characterized by defects in bacterial killing and render patients susceptible to recurrent bacterial infection. Chronic granulomatous disease results from inherited defects in the genes encoding components of phagocyte oxidase, which generates . The most common variants are an X-linked defect in one of the membrane-bound components (gp91phox) and autosomal recessive defects in the genes encoding two of the cytoplasmic components (p47phox and p67phox).⁴² The name of this disease comes from the macrophage-rich chronic inflammatory reaction that tries to control the infection when the initial neutrophil defense is inadequate. This often leads to collections of activated macrophages that wall off the microbes, forming aggregates called granulomas (described in more detail later in the chapter).

TABLE 2-3 Defects in Leukocyte Functions

MPO, myeloperoxidase.

Modified from Gallin JI: Disorders of phagocytic cells. In Gallin JI, et al (eds): Inflammation: Basic Principles and Clinical Correlates, 2nd ed. New York, Raven Press, 1992, pp 860, 861.

Although we have emphasized the role of leukocytes recruited from the circulation in the acute inflammatory response, cells resident in tissues also serve important functions in initiating acute inflammation. The two most important of these cell types are mast cells and tissue macrophages. These “sentinel” cells are stationed in tissues to rapidly recognize potentially injurious stimuli and initiate the host defense reaction. Mast cells react to physical trauma, breakdown products of complement, microbial products, and neuropeptides. These cells release histamine, leukotrienes, enzymes, and many cytokines (including TNF, IL-1, and chemokines), all of which contribute to inflammation. The functions of mast cells are discussed in more detail in Chapter 6. Macrophages recognize microbial products and secrete most of the cytokines important in acute inflammation. We will return to a discussion of the role of macrophages in inflammation later in the chapter.

Vasoactive Amines: Histamine and Serotonin

The two major vasoactive amines, so named because they have important actions on blood vessels, are histamine and serotonin. They are stored as preformed molecules in cells and are therefore among the first mediators to be released during inflammation. The richest sources of histamine are the mast cells that are normally present in the connective tissue adjacent to blood vessels. It is also found in blood basophils and platelets. Histamine is present in mast cell granules and is released by mast cell degranulation in response to a variety of stimuli, including (1) physical injury such as trauma, cold, or heat; (2) binding of antibodies to mast cells, which underlies allergic reactions (Chapter 6); (3) fragments of complement called anaphylatoxins (C3a and C5a); (4) histamine-releasing proteins derived from leukocytes; (5) neuropeptides (e.g., substance P); and (6) cytokines (IL-1, IL-8).

Histamine causes dilation of arterioles and increases the permeability of venules. It is considered to be the principal mediator of the immediate transient phase of increased vascular permeability, producing interendothelial gaps in venules, as we have seen. Its vasoactive effects are mediated mainly via binding to H₁ receptors on microvascular endothelial cells.⁴⁷

Serotonin (5-hydroxytryptamine) is a preformed vasoactive mediator with actions similar to those of histamine. It is present in platelets and certain neuroendocrine cells, e.g. in the gastrointestinal tract, and in mast cells in rodents but not humans. Release of serotonin (and histamine) from platelets is stimulated when platelets aggregate after contact with collagen, thrombin, adenosine diphosphate, and antigenantibody complexes. Thus, the platelet release reaction, which is a key component of coagulation, also results in increased vascular permeability. This is one of several links between clotting and inflammation.

Arachidonic Acid (AA) Metabolites: Prostaglandins, Leukotrienes, and Lipoxins

When cells are activated by diverse stimuli, such as microbial products and various mediators of inflammation, membrane AA is rapidly converted by the actions of enzymes to produce prostaglandins and leukotrienes. These biologically active lipid mediators serve as intracellular or extracellular signals to affect a variety of biologic processes, including inflammation and hemostasis.^48-50

AA is a 20-carbon polyunsaturated fatty acid (5,8,11,14-eicosatetraenoic acid) that is derived from dietary sources or by conversion from the essential fatty acid linoleic acid. It does not occur free in the cell but is normally esterified in membrane phospholipids. Mechanical, chemical, and physical stimuli or other mediators (e.g., C5a) release AA from membrane phospholipids through the action of cellular phospholipases, mainly phospholipase A₂. The biochemical signals involved in the activation of phospholipase A₂ include an increase in cytoplasmic Ca²⁺ and activation of various kinases in response to external stimuli.⁵¹ AA-derived mediators, also called eicosanoids, are synthesized by two major classes of enzymes: cyclooxygenases (which generate prostaglandins) and lipoxygenases (which produce leukotrienes and lipoxins) (Fig. 2-11). Eicosanoids bind to G protein–coupled receptors on many cell types and can mediate virtually every step of inflammation (Table 2-5).

• Prostaglandins (PGs) are produced by mast cells, macrophages, endothelial cells, and many other cell types, and are involved in the vascular and systemic reactions of inflammation. They are produced by the actions of two cyclooxgenases, the constitutively expressed COX-1 and the inducible enzyme COX-2. Prostaglandins are divided into series based on structural features as coded by a letter (PGD, PGE, PGF, PGG, and PGH) and a subscript numeral (e.g., 1, 2), which indicates the number of double bonds in the compound. The most important ones in inflammation are PGE₂, PGD₂, PGF_2α, PGI₂ (prostacyclin), and TxA₂ (thromboxane), each of which is derived by the action of a specific enzyme on an intermediate in the pathway. Some of these enzymes have restricted tissue distribution. For example, platelets contain the enzyme thromboxane synthetase, and hence TxA₂ is the major product in these cells. TxA₂, a potent platelet-aggregating agent and vasoconstrictor, is itself unstable and rapidly converted to its inactive form TxB₂. Vascular endothelium lacks thromboxane synthetase but possesses prostacyclin synthetase, which leads to the formation of prostacyclin (PGI₂) and its stable end product PGF_1α. Prostacyclin is a vasodilator, a potent inhibitor of platelet aggregation, and also markedly potentiates the permeability-increasing and chemotactic effects of other mediators. A thromboxane-prostacyclin imbalance has been implicated as an early event in thrombus formation in coronary and cerebral blood vessels (Chapter 4). PGD₂ is the major prostaglandin made by mast cells; along with PGE₂ (which is more widely distributed), it causes vasodilation and increases the permeability of post-capillary venules, thus potentiating edema formation. PGF_2α stimulates the contraction of uterine and bronchial smooth muscle and small arterioles, and PGD₂ is a chemoattractant for neutrophils.

FIGURE 2-11 Generation of arachidonic acid metabolites and their roles in inflammation. The molecular targets of action of some anti-inflammatory drugs are indicated by a red X. Not shown are agents that inhibit leukotriene production by inhibition of 5-lipoxygenase (e.g., Zileuton) or block leukotriene receptors (e.g., Monteleukast). COX, cyclooxygenase; HETE, hydroxyeicosatetraenoic acid; HPETE, hydroperoxyeicosatetraenoic acid.

TABLE 2-5 Principal Inflammatory Actions of Arachidonic Acid Metabolites (Eicosanoids)

Action	Eicosanoid
Vasodilation	PGI2 (prostacyclin), PGE1, PGE2, PGD2
Vasoconstriction	Thromboxane A2, leukotrienes C4, D4, E4
Increased vascular permeability	Leukotrienes C4, D4, E4
Chemotaxis, leukocyte adhesion	Leukotriene B4, HETE

HETE, hydroxyeicosatetraenoic acid; PGI₂, etc., prostaglandin I₂, etc.

In addition to their local effects, the prostaglandins are involved in the pathogenesis of pain and fever in inflammation. PGE₂ is hyperalgesic and makes the skin hypersensitive to painful stimuli, such as intradermal injection of suboptimal concentrations of histamine and bradykinin. It is involved in cytokine-induced fever during infections (described later).

Many anti-inflammatory drugs work by inhibiting the synthesis of eicosanoids:

Platelet-Activating Factor (PAF)

PAF is another phospholipid-derived mediator.⁵⁴ Its name comes from its discovery as a factor that causes platelet aggregation, but it is now known to have multiple inflammatory effects. A variety of cell types, including platelets themselves, basophils, mast cells, neutrophils, macrophages, and endothelial cells, can elaborate PAF, in both secreted and cell-bound forms. In addition to platelet aggregation, PAF causes vasoconstriction and bronchoconstriction, and at extremely low concentrations it induces vasodilation and increased venular permeability with a potency 100 to 10,000 times greater than that of histamine. PAF also causes increased leukocyte adhesion to endothelium (by enhancing integrin-mediated leukocyte binding), chemotaxis, degranulation, and the oxidative burst. Thus, PAF can elicit most of the vascular and cellular reactions of inflammation. PAF also boosts the synthesis of other mediators, particularly eicosanoids, by leukocytes and other cells. A role for PAF in vivo is supported by the ability of synthetic PAF receptor antagonists to inhibit inflammation in some experimental models.

Reactive Oxygen Species

Oxygen-derived free radicals may be released extracellularly from leukocytes after exposure to microbes, chemokines, and immune complexes, or following a phagocytic challenge.⁵⁵ Their production is dependent, as we have seen, on the activation of the NADPH oxidase system. Superoxide anion (), hydrogen peroxide (H₂O₂), and hydroxyl radical (^•OH) are the major species produced within cells, and O can combine with NO to form reactive nitrogen species. Extracellular release of low levels of these potent mediators can increase the expression of chemokines (e.g., IL-8), cytokines, and endothelial leukocyte adhesion molecules, amplifying the inflammatory response. As mentioned earlier, the physiologic function of these ROS in leukocytes is to destroy phagocytosed microbes, but release of these potent mediators can be damaging to the host (Chapter 1). They are implicated in the following responses in inflammation:

Serum, tissue fluids, and host cells possess antioxidant mechanisms that protect against these potentially harmful oxygen-derived radicals. These antioxidants were discussed in Chapter 1; they include (1) the enzyme superoxide dismutase, which is found in or can be activated in a variety of cell types; (2) the enzyme catalase, which detoxifies H₂O₂; (3) glutathione peroxidase, another powerful H₂O₂ detoxifier; (4) the copper-containing serum protein ceruloplasmin; and (5) the iron-free fraction of serum transferrin. Thus, the influence of oxygen-derived free radicals in any given inflammatory reaction depends on the balance between production and inactivation of these metabolites by cells and tissues.

Nitric Oxide (NO)

NO was discovered as a factor released from endothelial cells that caused vasodilation and was therefore called endothelium-derived relaxing factor. NO is a soluble gas that is produced not only by endothelial cells but also by macrophages and some neurons in the brain. It acts in a paracrine manner on target cells through induction of cyclic guanosine monophosphate, which, in turn, initiates a series of intracellular events leading to a response, such as the relaxation of vascular smooth muscle cells. Because the in vivo half-life of NO is only seconds, the gas acts only on cells in close proximity to where it is produced.

NO is synthesized from l-arginine by the enzyme nitric oxide synthase (NOS). There are three different types of NOS: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS) (Fig. 2-12). eNOS and nNOS are constitutively expressed at low levels and can be activated rapidly by an increase in cytoplasmic Ca²⁺. iNOS, in contrast, is induced when macrophages and other cells are activated by cytokines (e.g., TNF, IFN-γ) or microbial products.

FIGURE 2-12 Functions of nitric oxide (NO) in blood vessels and macrophages. NO is produced by two NO synthase (NOS) enzymes. It causes vasodilation, and NO-derived free radicals are toxic to microbial and mammalian cells.

NO has dual actions in inflammation: it relaxes vascular smooth muscle and promotes vasodilation, thus contributing to the vascular reaction, but it is also an inhibitor of the cellular component of inflammatory responses.^56,57 NO reduces platelet aggregation and adhesion (Chapter 4), inhibits several features of mast cell–induced inflammation, and inhibits leukocyte recruitment. Because of these inhibitory actions, production of NO is thought to be an endogenous mechanism for controlling inflammatory responses.

NO and its derivatives are microbicidal, and thus NO is a mediator of host defense against infection (discussed earlier). High levels of iNOS-induced NO are produced by leukocytes, mainly neutrophils and macrophages, in response to microbes.

Cytokines and Chemokines

Cytokines are proteins produced by many cell types (principally activated lymphocytes and macrophages, but also endothelial, epithelial, and connective tissue cells) that modulate the functions of other cell types. Long known to be involved in cellular immune responses, these products have additional effects that play important roles in both acute and chronic inflammation. Their general properties and functions are discussed in Chapter 6. Here we review the pro-perties of cytokines that are involved in acute inflammation (Table 2-6).

TABLE 2-6 Cytokines in Inflammation

IFN-γ, interferon-γ; IL-1, interleukin-1; NK cells, natural killer cells; TNF, tumor necrosis factor.

The most important cytokines involved in inflammatory reactions are listed. Many other cytokines may play lesser roles in inflammation. There is also considerable overlap between the cytokines involved in acute and chronic inflammation. Specifically, all the cytokines listed under acute inflammation may also contribute to chronic inflammatory reactions.

Tumor Necrosis Factor and Interleukin-1.

TNF and IL-1 are two of the major cytokines that mediate inflammation. They are produced mainly by activated macrophages. The secretion of TNF and IL-1 can be stimulated by endotoxin and other microbial products, immune complexes, physical injury, and a variety of inflammatory stimuli. Their most important actions in inflammation are their effects on endothelium, leukocytes, and fibroblasts, and induction of systemic acute-phase reactions (Fig. 2-13). In endothelium they induce a spectrum of changes referred to as endothelial activation.⁵⁸ In particular, they induce the expression of endothelial adhesion molecules; synthesis of chemical mediators, including other cytokines, chemokines, growth factors, eicosanoids, and NO; production of enzymes associated with matrix remodeling; and increases in the surface thrombogenicity of the endothelium.⁵⁹ TNF also augments responses of neutrophils to other stimuli such as bacterial endotoxin.

FIGURE 2-13 Principal local and systemic actions of tumor necrosis factor (TNF) and interleukin-1 (IL-1).

The production of IL-1 is controlled by a multi-protein cellular complex, sometimes called the “inflammasome,” that responds to stimuli from microbes and dead cells. This complex activates proteases that are members of the caspase family, which function to cleave the newly synthesized inactive precursor of IL-1 into the biologically active cytokine. Mutations in genes encoding members of this protein complex are the cause of inherited autoinflammatory syndromes, the best known of which is familial Mediterranean fever.⁶⁰ The mutant proteins either constitutively activate the inflammatory caspases or interfere with the negative regulation of this enzymatic process. The net result is unregulated IL-1 production.^61,62 Affected patients present with fever and other systemic manifestations of inflammation without overt provocation. Over time, some of these patients also develop amyloidosis, a disease of extracellular protein deposition that is often the result of persistent inflammation (Chapter 6). IL-1 antagonists are effective for treating these disorders, an excellent example of molecularly targeted rational therapy. The same inflammasome complex may be activated by urate crystals in the disease called gout, and the inflammation in this disease also seems to be, at least partly, mediated by IL-1 (Chapter 26).

IL-1 and TNF (as well as IL-6) induce the systemic acute-phase responses associated with infection or injury (described later in the chapter). TNF also regulates energy balance by promoting lipid and protein mobilization and by suppressing appetite. Therefore, sustained production of TNF contributes to cachexia, a pathologic state characterized by weight loss and anorexia that accompanies some chronic infections and neoplastic diseases (Chapter 9).

Chemokines.

Chemokines are a family of small (8 to 10 kD) proteins that act primarily as chemoattractants for specific types of leukocytes.⁶³ About 40 different chemokines and 20 different receptors for chemokines have been identified. They are classified into four major groups, according to the arrangement of the conserved cysteine (C) residues in the mature proteins^64,65:

• C-X-C chemokines (also called α chemokines) have one amino acid residue separating the first two conserved cysteine residues. The C-X-C chemokines act primarily on neutrophils. IL-8 is typical of this group. It is secreted by activated macrophages, endothelial cells, and other cell types, and causes activation and chemotaxis of neutrophils, with limited activity on monocytes and eosinophils. Its most important inducers are microbial products and other cytokines, mainly IL-1 and TNF.

• C-C chemokines (also called β chemokines) have the first two conserved cysteine residues adjacent. The C-C chemokines, which include monocyte chemoattractant protein (MCP-1), eotaxin, macrophage inflammatory protein-1α (MIP-1α), and RANTES (regulated and normal T-cell expressed and secreted), generally attract monocytes, eosinophils, basophils, and lymphocytes but not neutrophils. Although most of the chemokines in this class have overlapping actions, eotaxin selectively recruits eosinophils.

• C chemokines (also called γ chemokines) lack two (the first and third) of the four conserved cysteines. The C chemokines (e.g., lymphotactin) are relatively specific for lymphocytes.

• CX3C chemokines contain three amino acids between the two cysteines. The only known member of this class is called fractalkine. This chemokine exists in two forms: the cell surface-bound protein can be induced on endothelial cells by inflammatory cytokines and promotes strong adhesion of monocytes and T cells, and a soluble form, derived by proteolysis of the membrane-bound protein, has potent chemoattractant activity for the same cells.

Chemokines mediate their activities by binding to seven-transmembrane G protein–coupled receptors. These receptors (called CXCR or CCR, for C-X-C or C-C chemokine receptors) usually exhibit overlapping ligand specificities, and leukocytes generally express more than one receptor type. As discussed in Chapter 6, certain chemokine receptors (CXCR-4, CCR-5) act as coreceptors for a viral envelope glycoprotein of human immunodeficiency virus 1 and are thus involved in binding and entry of the virus into cells.

Chemokines have two main functions: they stimulate leukocyte recruitment in inflammation and control the normal migration of cells through various tissues.^20,65 Some chemokines are produced transiently in response to inflammatory stimuli and promote the recruitment of leukocytes to the sites of inflammation. Other chemokines are produced constitutively in tissues and function to organize different cell types in different anatomic regions of the tissues. In both situations, chemokines may be displayed at high concentrations attached to proteoglycans on the surface of endothelial cells and in the extracellular matrix.

Other Cytokines in Acute Inflammation.

The list of cytokines implicated in inflammation is huge and constantly growing.⁶⁶ Two that have received considerable recent interest are: IL-6, made by macrophages and other cells, which is involved in local and systemic reactions⁶⁷; and IL-17, produced mainly by T lymphocytes, which promotes neutrophil recruitment.⁶⁸ Antagonists against both are in clinical trials for inflammatory diseases. Cytokines also play central roles in chronic inflammation; these are described later in the chapter.

Lysosomal Constituents of Leukocytes

Neutrophils and monocytes contain lysosomal granules, which, when released, may contribute to the inflammatory response. Neutrophils have two main types of granules. The smaller specific (or secondary) granules contain lysozyme, collagenase, gelatinase, lactoferrin, plasminogen activator, histaminase, and alkaline phosphatase. The larger azurophil (or primary) granules contain myeloperoxidase, bactericidal factors (lysozyme, defensins), acid hydrolases, and a variety of neutral proteases (elastase, cathepsin G, nonspecific collagenases, proteinase 3).⁴⁰ Both types of granules can fuse with phagocytic vacuoles containing engulfed material, or the granule contents can be released into the extracellular space.

Different granule enzymes serve different functions. Acid proteases degrade bacteria and debris within the phagolysosomes, in which an acid pH is readily reached. Neutral proteases are capable of degrading various extracellular components, such as collagen, basement membrane, fibrin, elastin, and cartilage, resulting in the tissue destruction that accompanies inflammatory processes. Neutral proteases can also cleave C3 and C5 complement proteins directly, releasing anaphylatoxins, and release a kinin-like peptide from kininogen. Neutrophil elastase has been shown to degrade virulence factors of bacteria and thus combat bacterial infections. Monocytes and macrophages also contain acid hydrolases, collagenase, elastase, phospholipase, and plasminogen activator. These may be particularly active in chronic inflammatory reactions.

Because of the destructive effects of lysosomal enzymes, the initial leukocytic infiltration, if unchecked, can potentiate further inflammation and tissue damage. These harmful proteases, however, are held in check by a system of antiproteases in the serum and tissue fluids. Foremost among these is α₁-antitrypsin, which is the major inhibitor of neutrophil elastase. A deficiency of these inhibitors may lead to sustained action of leukocyte proteases, as is the case in patients with α₁-antitrypsin deficiency (Chapter 15). α₂-Macroglobulin is another antiprotease found in serum and various secretions.

Neuropeptides

Neuropeptides are secreted by sensory nerves and various leukocytes, and play a role in the initiation and propagation of an inflammatory response. The small peptides, such as substance P and neurokinin A, belong to a family of tachykinin neuropeptides produced in the central and peripheral nervous systems.⁶⁹ Nerve fibers containing substance P are prominent in the lung and gastrointestinal tract. Substance P has many biologic functions, including the transmission of pain signals, regulation of blood pressure, stimulation of secretion by endocrine cells, and increasing vascular permeability. Sensory neurons can also produce other pro-inflammatory molecules, such as calcitonin-related gene product, which are thought to link the sensing of painful stimuli to the development of protective host responses.⁷⁰

Complement System

The complement system consists of more than 20 proteins, some of which are numbered C1 through C9. This system functions in both innate and adaptive immunity for defense against microbial pathogens.^71-73 In the process of complement activation several cleavage products of complement proteins are elaborated that cause increased vascular permeability, chemotaxis, and opsonization. The activation and functions of the complement system are outlined in Figure 2-14.

FIGURE 2-14 The activation and functions of the complement system. Activation of complement by different pathways leads to cleavage of C3. The functions of the complement system are mediated by breakdown products of C3 and other complement proteins, and by the membrane attack complex (MAC).

Complement proteins are present in inactive forms in the plasma, and many of them are activated to become proteolytic enzymes that degrade other complement proteins, thus forming an enzymatic cascade capable of tremendous amplification. The critical step in complement activation is the proteolysis of the third (and most abundant) component, C3. Cleavage of C3 can occur by one of three pathways: the classical pathway, which is triggered by fixation of C1 to antibody (IgM or IgG) that has combined with antigen; the alternative pathway, which can be triggered by microbial surface molecules (e.g., endotoxin, or LPS), complex polysaccharides, cobra venom, and other substances, in the absence of antibody; and the lectin pathway, in which plasma mannose-binding lectin binds to carbohydrates on microbes and directly activates C1. Whichever pathway is involved in the early steps of complement activation, they all lead to the formation of an active enzyme called the C3 convertase, which splits C3 into two functionally distinct fragments, C3a and C3b. C3a is released, and C3b becomes covalently attached to the cell or molecule where complement is being activated. More C3b then binds to the previously generated fragments to form C5 convertase, which cleaves C5 to release C5a and leave C5b attached to the cell surface. C5b binds the late components (C6–C9), culminating in the formation of the membrane attack complex (MAC, composed of multiple C9 molecules).

The biologic functions of the complement system fall into three general categories (see Fig. 2-14):

Among the complement components, C3a and C5a are the most important inflammatory mediators. In addition to the mechanisms already discussed, C3 and C5 can be cleaved by several proteolytic enzymes present within the inflammatory exudate. These include plasmin and lysosomal enzymes released from neutrophils (discussed earlier). Thus, the chemotactic actions of complement and the complement-activating effects of neutrophils can initiate a self-perpetuating cycle of neutrophil recruitment.

The activation of complement is tightly controlled by cell-associated and circulating regulatory proteins. Different regulatory proteins inhibit the production of active complement fragments or remove fragments that deposit on cells. These regulators are expressed on normal host cells and are thus designed to prevent healthy tissues from being injured at sites of complement activation. Regulatory proteins can be overwhelmed when large amounts of complement are deposited on host cells and tissues, as happens in autoimmune diseases, in which individuals produce complement-fixing antibodies against their own tissue antigens (Chapter 6).

Coagulation and Kinin Systems

Inflammation and blood clotting are often intertwined, with each promoting the other.⁷⁴ The clotting system is divided into two pathways that converge, culminating in the activation of thrombin and the formation of fibrin (Fig. 2-15) (Chapter 4). The intrinsic clotting pathway is a series of plasma proteins that can be activated by Hageman factor (factor XII), a protein synthesized by the liver that circulates in an inactive form. Factor XII is activated upon contact with negatively charged surfaces, for instance when vascular permeability increases and plasma proteins leak into the extravascular space and come into contact with collagen, or when it comes into contact with basement membranes exposed as a result of endothelial damage. Factor XII then undergoes a conformational change (becoming factor XIIa), exposing an active serine center that can subsequently cleave protein substrates and activate a variety of mediator systems (see later). Inflammation increases the production of several coagulation factors, makes the endothelial surface pro-thrombogenic, and inhibits anticoagulation mechanisms, thus promoting clotting. Conversely, thrombin, a product of clotting, promotes inflammation by engaging receptors that are called protease-activated receptors (PARs) because they bind multiple trypsin-like serine proteases in addition to thrombin.⁷⁵ These receptors are seven-transmembrane G protein–coupled receptors that are expressed on platelets, endothelial and smooth muscle cells, and many other cell types. Engagement of the so-called type 1 receptor (PAR-1) by proteases, particularly thrombin, triggers several responses that induce inflammation. They include mobilization of P-selectin; production of chemokines and other cytokines; expression of endothelial adhesion molecules for leukocyte integrins; induction of cyclooxygenase-2 and production of prostaglandins; production of PAF and NO; and changes in endothelial shape.⁷⁵ As we have seen, these responses promote the recruitment of leukocytes and many other reactions of inflammation. Because coagulation and inflammation can initiate a vicious cycle of amplification, interfering with clotting is a potential therapeutic strategy for the systemic inflammatory disease seen with severe, disseminated bacterial infections. This is the rationale for treating this disorder with the anticoagulant, activated protein C, which may benefit a subset of the patients (Chapter 4).⁷⁶

FIGURE 2-15 Interrelationships between the four plasma mediator systems triggered by activation of factor XII (Hageman factor). Note that thrombin induces inflammation by binding to protease-activated receptors (principally PAR-1) on platelets, endothelium, smooth muscle cells, and other cells. HMWK, high molecular weight kininogen.

Kinins are vasoactive peptides derived from plasma proteins, called kininogens, by the action of specific proteases called kallikreins. The kinin and coagulation systems are also intimately connected. The active form of factor XII, factor XIIa, converts plasma prekallikrein into an active proteolytic form, the enzyme kallikrein, which cleaves a plasma glycoprotein precursor, high-molecular-weight kininogen, to produce bradykinin (see Fig. 2-15).⁷⁷ Bradykinin increases vascular permeability and causes contraction of smooth muscle, dilation of blood vessels, and pain when injected into the skin. These effects are similar to those of histamine. The action of bradykinin is short-lived, because it is quickly inactivated by an enzyme called kininase. Any remaining kinin is inactivated during passage of plasma through the lung by angiotensin-converting enzyme. Kallikrein itself is a potent activator of Hageman factor, allowing for autocatalytic amplification of the initial stimulus. Kallikrein has chemotactic activity, and it also directly converts C5 to the chemoattractant product C5a.

At the same time that factor XIIa is inducing fibrin clot formation, it activates the fibrinolytic system. This cascade counterbalances clotting by cleaving fibrin, thereby solubilizing the clot. Kallikrein, as well as plasminogen activator (released from endothelium, leukocytes, and other tissues), cleaves plasminogen, a plasma protein that binds to the evolving fibrin clot to generate plasmin, a multifunctional protease. The fibrinolytic system contributes to the vascular phenomena of inflammation in several ways. Although the primary function of plasmin is to lyse fibrin clots, during inflammation it also cleaves the complement protein C3 to produce C3 fragments, and it degrades fibrin to form fibrin split products, which may have permeability-inducing properties. Plasmin can also activate Hageman factor, which can trigger multiple cascades (see Fig. 2-15), amplifying the response.

From this discussion of the plasma proteases activated by the complement, kinin, and clotting systems, a few general conclusions can be drawn:

When Lewis discovered the role of histamine in inflammation, one mediator was thought to be enough. Now, we are wallowing in them! Yet, from this large compendium, it is likely that a few mediators are most important for the reactions of acute inflammation in vivo, and these are summarized in Table 2-7. The redundancy of the mediators and their actions ensures that this protective response remains robust and is not easy to perturb.

TABLE 2-7 Role of Mediators in Different Reactions of Inflammation

IL-1, interleukin-1; PAF, platelet-activating factor; TNF, tumor necrosis factor.