Robbins Pathology PDF - Inflammation and Repair

2 Inflammation and Repair OUTLINE General Features of Inflammation, 25 Purulent (Suppurative) Inflammation, Abscess, 40 Causes of Inflammation, 27...

2 Inflammation and Repair OUTLINE General Features of Inflammation, 25 Purulent (Suppurative) Inflammation, Abscess, 40 Causes of Inflammation, 27 Ulcers, 41 Recognition of Microbes and Damaged Cells, 27 Outcomes of Acute Inflammation, 41 Acute Inflammation, 27 Chronic Inflammation, 41 Vascular Reactions in Acute Inflammation, 27 Causes of Chronic Inflammation, 41 Leukocyte Recruitment to Sites of Inflammation, 28 Morphologic Features, 42 Phagocytosis and Clearance of the Offending Agent, 31 Cells and Mediators of Chronic Inflammation, 42 Phagocytosis, 31 Role of Macrophages, 42 Intracellular Destruction of Microbes and Debris, 31 Role of Lymphocytes, 44 Leukocyte-Mediated Tissue Injury, 33 Other Cells in Chronic Inflammation, 45 Mediators of Inflammation, 33 Granulomatous Inflammation, 45 Vasoactive Amines: Histamine and Serotonin, 33 Systemic Effects of Inflammation, 46 Arachidonic Acid Metabolites, 34 Tissue Repair, 47 Prostaglandins, 34 Cell and Tissue Regeneration, 47 Leukotrienes, 34 Liver Regeneration, 49 Other Arachidonic AcideDerived Mediators, 34 Repair by Scarring, 49 Pharmacologic Inhibitors of Prostaglandins and Steps in Scar Formation, 49 Leukotrienes, 35 Angiogenesis, 50 Cytokines and Chemokines, 36 Activation of Fibroblasts and Deposition of Connective Tumor Necrosis Factor (TNF) and Interleukin-1 (IL-1), 36 Tissue, 50 Chemokines, 37 Remodeling of Connective Tissue, 52 Other Cytokines in Acute Inflammation, 37 Factors That Interfere With Tissue Repair, 52 Complement System, 37 Clinical Examples of Abnormal Wound Healing and Other Mediators of Inflammation, 39 Scarring, 53 Morphologic Patterns of Acute Inflammation, 39 Defects in Healing: Chronic Wounds, 53 Serous Inflammation, 39 Excessive Scarring, 54 Fibrinous Inflammation, 39 Fibrosis in Parenchymal Organs, 54 Inflammation is a response of vascularized tissues to infections and then remove the harmful or unwanted substances. Without inflamma- tissue damage that brings cells and molecules of host defense from the tion, infections would go unchecked, wounds would never heal, and circulation to the sites where they are needed, in order to eliminate the injured tissues might remain permanent festering sores. offending agents. Although in common medical and lay parlance We start with an overview of some of the important general fea- inflammation suggests a harmful reaction, it is actually a protective tures of inflammation, then discuss the major reactions of acute response that is essential for survival. It serves to rid the host of both the inflammation and the chemicals that mediate these reactions. We initial cause of cell injury (e.g., microbes and toxins) and the conse- continue with a discussion of chronic inflammation and close with the quences of such injury (e.g., necrotic cells and tissues) and initiates the process of tissue repair. repair of damaged tissues. The mediators of defense include phagocytic leukocytes, antibodies, and complement proteins (Fig. 2.1). Most of these GENERAL FEATURES OF INFLAMMATION normally circulate in the blood, where they are sequestered from tissues and unable to cause damage. Infections and dead cells are typically in the Inflammation may be acute or chronic (Table 2.1). The initial, rapid tissues, outside the vessels. The process of inflammation delivers leuko- response to infections and tissue damage is called acute inflammation. cytes and proteins to foreign invaders, such as microbes, and to damaged It develops within minutes to hours and lasts for several hours to a few or necrotic tissues and activates the recruited cells and molecules, which days. Its main characteristics are the leakage of fluid and plasma proteins 25 26 CHAPTER 2 Inflammation and Repair (edema) and the accumulation of leukocytes, predominantly neutrophils STIMULUS Microbes Necrotic tissue (also called polymorphonuclear leukocytes). When acute inflammation achieves its desired goal of eliminating the offenders, the reaction quickly subsides, but if the response fails to clear the stimulus, it can progress to a protracted phase that is called chronic inflammation. Chronic inflam- mation is of longer duration and is associated with continuing tissue OF MEDIATORS PRODUCTION Recognition by sentinel cells destruction and fibrosis (the deposition of connective tissue). in tissues The external manifestations of inflammation, often called its Macrophage Dendritic cell Mast cell cardinals signs, are heat (calor in Latin), redness (rubor), swelling (tumor), pain (dolor), and loss of function ( function laesa). The first Mediators (amines, cytokines) four of these were described more than 2000 years ago by a Roman encyclopedist named Celsus, who wrote the then-famous text De Medicina; the fifth was added in the late 19th century by Rudolf Virchow, known as the “father of modern pathology.” These mani- festations occur as consequences of the vascular changes and leukocyte Vasodilation, INFLUX OF LEUKOCYTES, increased recruitment and activation, as will be evident from the discussion that follows. PLASMA PROTEINS vascular Recruitment permeability Neutrophil Monocyte Inflammatory reactions develop in steps (which can be summa- of leukocytes rized as the five R’s): (1) recognition of the offending agent; (2) recruitment of blood cells and proteins to the tissue site; (3) removal of the offending agent; (4) regulation of the reaction; and (5) repair of Elimination injured tissue. Each of these steps is described in detail in this chapter. of microbes, While normally protective, in some situations, the inflammatory Edema reaction becomes the cause of disease, and the damage it produces is dead tissue its dominant feature. Inflammatory reactions to infections are often Neutrophil Macrophage accompanied by local tissue damage and pain. Typically, however, these Cytokines, harmful consequences resolve as the inflammation abates, leaving little growth factors or no permanent damage. By contrast, there are many diseases in which the inflammatory reaction is misdirected (e.g., against self tissues in autoimmune diseases), occurs against usually harmless environmental Resolution Repair substances (e.g., in allergies), or is excessively prolonged (e.g., in in- fections by microbes that resist eradication such as Mycobacterium tuberculosis). These abnormal reactions underlie many common chronic diseases, such as rheumatoid arthritis, asthma and lung fibrosis Extracellular (Table 2.2). Inflammation may also contribute to diseases that are matrix proteins Fibroblasts thought to be primarily metabolic, degenerative, or genetic, such as FIG. 2.1 Sequence of events in an inflammatory reaction. Macro- atherosclerosis, type 2 diabetes, and Alzheimer disease. In recognition of phages and other cells in tissues recognize microbes and damaged cells the wide-ranging harmful consequences of inflammation, the lay press and liberate mediators, which trigger the vascular and cellular reactions has rather melodramatically referred to it as “the silent killer.” of inflammation. Influx of plasma proteins from the blood (not shown) accompanies edema. Table 2.2 Disorders Caused by Inflammatory Reactions Cells and Molecules Disorders Involved in Injury Acute Acute respiratory distress Neutrophils Table 2.1 Features of Acute and Chronic Inflammation syndrome Acute Glomerulonephritis, vasculitis Antibodies and complement; Feature Inflammation Chronic Inflammation neutrophils Onset Fast: minutes to Slow: days Septic shock Cytokines hours Chronic Cellular infiltrate Mainly Monocytes/macrophages Rheumatoid arthritis Lymphocytes, macrophages; neutrophils and lymphocytes antibodies? Tissue injury Usually mild and May be significant Asthma Eosinophils; IgE antibodies self-limited Fibrosis None May be severe and Pulmonary fibrosis Macrophages; fibroblasts progressive Listed are selected examples of diseases in which the inflammatory response plays a significant role in tissue injury. Some, such as asthma, can present as Local and Prominent Variable, usually modest a chronic illness with repeated bouts of acute exacerbation. These diseases systemic signs and their pathogenesis are discussed in relevant chapters.  CHAPTER 2 Inflammation and Repair 27 Inadequate inflammation is typically manifested by increased sus- proteins, such as antibodies and members of the complement system. ceptibility to infections. Impairment of inflammation is caused by These proteins can destroy circulating microbes and are recruited to reduced production of leukocytes resulting from replacement of the tissue sites of infection, where they stimulate inflammatory reactions. bone marrow by cancers (e.g., leukemias), immunosuppressive agents used to treat graft rejection and autoimmune disorders, and many other With this background, we proceed to a discussion of acute conditions such as malnutrition. Inherited genetic disorders of leuko- inflammation, its underlying mechanisms, and how it functions to cyte function are rare, but they provide valuable information about the eliminate microbes and dead cells. mechanisms of leukocyte responses. These conditions are described in Chapter 5 in the context of immunodeficiency diseases. ACUTE INFLAMMATION Once inflammation has eliminated the offending agents, it subsides and also sets into motion the process of tissue repair. In this process, Acute inflammation has three major components: (1) dilation of the injured tissue is replaced through regeneration of surviving cells small vessels; (2) increased permeability of the microvasculature; and filling of residual defects with connective tissue (scarring). and (3) emigration of the leukocytes from the microcirculation (see Fig. 2.1). Most of these changes happen in postcapillary venules at the site of infection or tissue injury. The walls of these vessels are capable CAUSES OF INFLAMMATION of reacting to stimuli and are sufficiently thin to allow passage of fluid Of the myriad causes of inflammation, the following are the most and proteins. Vasodilation slows down blood flow and sets the stage frequent: for the subsequent reactions, while increased vascular permeability Infections, in which the products of microbes are recognized by the enables plasma proteins to enter the tissue site. Transmigration moves host and elicit different types of inflammatory reactions. leukocytes from their peaceful home inside the vessels into the Tissue necrosis, which may be caused by ischemia (reduced blood maelstrom of infection or necrosis, where the cells perform their flow, the cause of infarction in the heart, brain, and other tissues), function of destroying noxious agents and cleaning up the damage. All trauma, and physical and chemical injury (e.g., thermal injury, irra- these reactions are induced by cytokines and other molecules diation, and exposure to toxins). Molecules released from necrotic (collectively called inflammatory mediators) produced at the site of cells trigger inflammation even in the absence of infection (so- infection or necrosis (described later). called “sterile inflammation”). Foreign bodies, such as sutures and tissue implants, also elicit sterile Vascular Reactions in Acute Inflammation inflammation. Vasodilation is one of the earliest reactions of acute inflammation and Immune reactions (also called hypersensitivity) are reactions in is responsible for the externally visible redness (erythema) and warmth which the normally protective immune system damages the indi- that accompany most acute inflammatory reactions. The most impor- vidual’s own tissues. As mentioned earlier, autoimmune diseases tant chemical mediator of vasodilation is histamine, discussed later. and allergies are diseases caused by immune responses; in both, Vasodilation is quickly followed by increased permeability of the inflammation is a major contributor to tissue injury (Chapter 5). microvasculature and the outpouring of protein-rich fluid into the extravascular tissues. The escape of fluid, proteins, and blood cells from the vascular system into the interstitial tissue or body cavities is RECOGNITION OF MICROBES AND DAMAGED CELLS known as exudation (Fig. 2.2). An exudate is an extravascular fluid The first step in inflammatory responses is the recognition of that has a high protein concentration and contains cellular debris. Its microbes and necrotic cells by cellular receptors and circulating presence implies that there is an increase in the permeability of small proteins. All tissues contain resident cells whose primary function is blood vessels, typically during an inflammatory reaction. By contrast, a to detect the presence of foreign invaders or dead cells, to ingest and transudate is a fluid with low protein content (most of which is al- destroy these potential causes of harm, and to elicit the inflammatory bumin), little or no cellular material, and low specific gravity. A reaction that recruits cells and proteins from the blood to complete transudate is essentially an ultrafiltrate of blood plasma that is pro- the elimination process. The most important of these sentinel cells duced as a result of osmotic or hydrostatic imbalance across the vessel are tissue-resident macrophages and dendritic cells. These cells ex- wall without an increase in vascular permeability and is usually not press receptors for microbial products in multiple cell compartments: associated with inflammation (Chapter 3). Edema denotes an excess of on their surface, where they recognize microbes in the extracellular fluid in the interstitial tissue or serous cavities; it can be either an space; in endosomes, into which microbes are ingested; and in the exudate or a transudate. Pus, a purulent exudate, is an inflammatory cytosol where certain microbes may survive. The best known of these exudate rich in leukocytes (mostly neutrophils), the debris of dead receptors are the Toll-like receptors (TLRs) (Chapter 5). Activation of cells, and, in many cases, microbes. TLRs leads to the production of cytokines that trigger inflammation The principal mechanism of increased vascular permeability is (discussed later). A different sensor system consists of cytosolic the contraction of endothelial cells, which creates interendothelial NOD-like receptors (NLRs) that, upon activation, recruit and activate openings. It is elicited by histamine, bradykinin, leukotrienes, and a multiprotein complex (the inflammasome, Chapter 5) which gen- other chemical mediators. It occurs rapidly after exposure to the erates the biologically active cytokine interleukin-1 (IL-1). NLRs mediator (within 15 to 30 minutes) and is usually short lived. In recognize a wide range of stimuli, including microbial products and unusual cases (e.g., in burns), increased vascular permeability may indicators of cell damage such as leaked DNA and decreased cyto- result from direct endothelial injury. In these instances, leakage starts solic potassium levels. The cytokine-induced inflammation then immediately after injury and is sustained for several hours until the eliminates the stimulus that elicited the reaction (microbes and dead damaged vessels become thrombosed or are repaired. cell debris). The loss of fluid and increased vessel diameter lead to slower blood If microbes navigate the gauntlet of sentinels in tissues and enter flow and higher concentration of red cells in small vessels, raising the the circulation, they are then recognized by a number of plasma viscosity of the blood. Involved small vessels become engorged with 28 CHAPTER 2 Inflammation and Repair Hydrostatic pressure Colloid osmotic pressure A. HEALTHY Plasma proteins No net fluid or protein leakage Inflammation Opening of interendothelial spaces B. EXUDATE (high protein content; Vasodilation and stasis may contain white and red cells) Fluid and protein leakage C. TRANSUDATE (low protein content, few cells) Increased hydrostatic pressure Decreased colloid osmotic pressure (venous outflow obstruction, (decreased protein synthesis [e.g., liver disease]; [e.g., congestive heart failure]) increased protein loss [e.g., kidney disease]; protein malnutrition [e.g., kwashiorkor]) Fluid leakage FIG. 2.2 Formation of exudates and transudates. (A) Normal hydrostatic pressure (blue arrow) is about 32 mm Hg at the arterial end of a capillary bed and 12 mm Hg at the venous end; the mean colloid osmotic pressure of tissues is approximately 25 mm Hg (green arrow). Therefore, there is virtually no net flow of fluid across the vascular bed in the steady state. (B) An exudate is formed in inflammation because vascular permeability increases as a result of retraction of endothelial cells, creating spaces through which fluid and proteins can pass. (C) A transudate is formed when fluid leaks out because of increased hydrostatic pressure or decreased osmotic pressure. red cells, a condition termed stasis, which is seen histologically as peripheral position along the endothelial surface, a process called vascular congestion and externally as localized redness of the affected margination. By moving close to the vessel wall, leukocytes are able to tissue. detect and react to changes in the endothelium. When endothelial cells In addition to the reactions of blood vessels, lymph flow is are activated by cytokines and other mediators produced locally, they increased and helps drain edema fluid that accumulates because of express adhesion molecules to which the leukocytes attach loosely. increased vascular permeability. The lymphatics may become These cells bind and detach and thus begin to tumble on the endo- secondarily inflamed (lymphangitis), appearing clinically as red streaks thelial surface, a process called rolling. The cells finally come to rest at extending from an inflammatory focus along the course of lymphatic some point where they adhere firmly (resembling pebbles over which a channels. Involvement of the draining lymph nodes may lead to stream runs without disturbing them). enlargement (because of increased cellularity) and pain. The associated The initial weak binding of leukocytes and their rolling on the constellation of pathologic changes is termed reactive or inflammatory endothelium are mediated by a family of proteins called selectins lymphadenitis (Chapter 10). (Table 2.3). Selectins are receptors expressed on leukocytes and endothelium that have an extracellular domain that binds carbohy- Leukocyte Recruitment to Sites of Inflammation drates (hence the lectin part of the name). The ligands for selectins are The journey of leukocytes from the vessel lumen to the tissue is a sialic acidecontaining oligosaccharides attached to glycoprotein multistep process that is mediated and controlled by adhesion backbones; some are expressed on leukocytes and others on endo- molecules and cytokines. Leukocytes normally transit rapidly through thelial cells. Endothelial cells express two selectins, E- and P-selectins, small vessels. In inflammation, they have to be stopped and then as well as the ligand for L-selectin, whereas leukocytes express brought to the offending agent or the site of tissue damage, outside the L-selectin. E- and P-selectins are typically expressed at low levels or vessels. This process can be divided into phases, consisting first of not at all on nonactivated endothelium but are upregulated after adhesion of leukocytes to endothelium at the site of inflammation, stimulation by cytokines and other mediators. Therefore, binding of then transmigration of the leukocytes through the vessel wall, and leukocytes is largely restricted to endothelium at sites of infection or finally, movement of the cells toward the offending agent (Fig. 2.3). tissue injury (where the mediators are produced). For example, in When blood flows from capillaries into postcapillary venules, un- nonactivated endothelial cells, P-selectin is found primarily in intra- der conditions of normal laminar flow, red cells are concentrated in cellular membrane-bound vesicles called Weibel-Palade bodies; how- the center of the vessel, displacing leukocytes toward the vessel wall. ever, within minutes of exposure to mediators such as histamine or As the rate of flow slows early in inflammation (stasis), leukocytes, thrombin, P-selectin traffics to the cell surface. Similarly, E-selectin being larger than red cells, slow down more and assume a more and the ligand for L-selectin, which are not expressed on normal  CHAPTER 2 Inflammation and Repair 29 ROLLING INTEGRIN ACTIVATION BY CHEMOKINES Leukocyte Selectin ligand STABLE ADHESION Integrin (low-affinity state) MIGRATION THROUGH ENDOTHELIUM Integrin (high-affinity state) Chemokine P-selectin E-selectin Proteoglycan PECAM-1 (CD31) Integrin ligand (ICAM-1) Cytokines (TNF, IL-1) Chemokines Macrophage Fibrin and fibronectin with microbes Microbes (extracellular matrix) FIG. 2.3 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 endo- thelium, pierce the basement membrane, and move toward chemoattractants emanating from the source of injury. Different molecules play predominant roles at each step of this process: selectins in rolling; chemokines (displayed bound to proteoglycans) in activating the neutrophils to increase avidity of integrins; integrins in firm adhesion; and CD31 (PECAM-1) in transmigration. E- and P-selectins are expressed on endothelial cells; L-selectin is expressed on leukocytes (not shown). ICAM-1, Intercellular adhesion molecule-1; IL-1, interleukin-1; PECAM-1 (CD31), platelet endothelial cell adhesion molecule-1; TNF, tumor necrosis factor. Table 2.3 Endothelial and Leukocyte Adhesion Molecules Family Adhesion Molecule Major Cell Type Principal Ligands Selectin L-selectin Leukocytes Sialyl-Lewis X on various glycoproteins expressed on endothelium E-selectin Activated endothelium Sialyl-Lewis X on glycoproteins expressed on neutrophils, monocytes, T lymphocytes P-selectin Activated endothelium Sialyl-Lewis X on glycoproteins expressed on neutrophils, monocytes, T lymphocytes Integrin LFA-1 T lymphocytes, other leukocytes ICAM-1 expressed on activated endothelium MAC-1 Monocytes, other leukocytes ICAM-1 expressed on activated endothelium VLA-4 T lymphocytes, other leukocytes VCAM-1 expressed on activated endothelium a4b7 Lymphocytes, monocytes MAdCAM-1 expressed on endothelium in gut and gut-associated lymphoid tissues ICAM, Intercellular adhesion molecule; LFA, lymphocyte function-associated antigen; MAC-1, macrophage antigen 1; MAdCAM-1, mucosal addressin cell adhesion molecule-1; VCAM, vascular cell adhesion molecule; VLA, very late antigen. 30 CHAPTER 2 Inflammation and Repair endothelium, are induced after stimulation by the cytokines IL-1 and inflammatory diseases, such as multiple sclerosis and inflammatory tumor necrosis factor (TNF), which are produced by tissue macro- bowel disease. phages, dendritic cells, mast cells, and endothelial cells after encoun- After arresting on the endothelial surface, leukocytes migrate tering microbes or dead tissues. Selectin-mediated interactions have a through the vessel wall, primarily by squeezing between inter- low affinity with a fast off-rate, and they are easily disrupted by the endothelial junctions. This extravasation of leukocytes is called flowing blood. As a result, the leukocytes bind, detach, and bind again transmigration or diapedesis. Platelet endothelial cell adhesion to endothelium. These weak rolling interactions slow down the leu- molecule-1 (PECAM-1, also called CD31), a cellular adhesion mole- kocytes sufficiently for them to recognize additional adhesion mole- cule expressed on leukocytes and endothelial cells, mediates the cules on the endothelium. binding events needed for leukocytes to traverse the endothelium. Firm adhesion of leukocytes to endothelium is mediated by a After crossing the endothelium, leukocytes pierce the basement family of leukocyte surface proteins called integrins (see Table 2.3). membrane, probably by secreting collagenases, and enter the extra- Integrins are transmembrane two-chain glycoproteins that mediate the vascular tissue. The directionality of leukocyte movement within tis- adhesion of leukocytes to endothelium and of various cells to the sues is controlled by locally produced chemokines, which create a extracellular matrix. They are normally expressed on leukocyte plasma diffusion gradient that the cells migrate along. membranes in a low-affinity form and do not adhere to their specific After exiting the circulation, leukocytes move in the tissues ligands until the leukocytes are activated by chemokines. Chemokines toward the site of injury by a process called chemotaxis, defined as are chemoattractant cytokines that are secreted by many cells at sites of locomotion along a chemical gradient. Among the many chemo- inflammation, bind to endothelial cell proteoglycans, and are displayed attractants known, the most potent are bacterial products, particularly at high concentrations on the endothelial surface. When the rolling peptides with N-formylmethionine termini; cytokines, especially leukocytes encounter the displayed chemokines, the cells are activated those of the chemokine family; components of the complement sys- and their integrins undergo conformational changes and cluster tem, particularly C5a; and leukotrienes. These chemoattractants, together, thereby converting to a high-affinity form. At the same time, which are described in more detail later, are produced in response to other cytokines, notably TNF and IL-1 (also secreted at sites of infection infections and tissue damage and during immunologic reactions. All and injury), activate endothelial cells to increase their expression of li- of them bind to G proteinecoupled receptors on the surface of leu- gands for integrins. The combination of cytokine-induced expression of kocytes. Signals initiated from these receptors activate second mes- integrin ligands on the endothelium and increased affinity of integrins sengers that induce the polymerization of actin at the leading edge of on the leukocytes results in firm integrin-mediated binding of the leu- the cell and the localization of myosin filaments at the back. The kocytes to the endothelium at the site of inflammation. The leukocytes leukocyte moves by extending filopodia that pull the back of the cell stop rolling, and engagement of integrins by their ligands delivers sig- in the direction of extension, much as an automobile with front-wheel nals to the leukocytes that lead to cytoskeletal changes that arrest the drive is pulled by the front wheels. The net result is that leukocytes leukocytes and firmly attach them to the endothelium. migrate toward the inflammatory stimulus in the direction of the A telling indication of the importance of leukocyte adhesion locally produced chemoattractants. molecules is the existence of mutations affecting integrins and selectin The nature of the leukocyte infiltrate varies with the age of the ligands that result in recurrent bacterial infections as a consequence of inflammatory response and the type of stimulus. In most forms of impaired leukocyte adhesion and defective inflammation. These acute inflammation, neutrophils predominate in the inflammatory leukocyte adhesion deficiencies are described in Chapter 5. Antago- infiltrate during the first 6 to 24 hours and are replaced by monocytes nists of integrins are approved for the treatment of some chronic in 24 to 48 hours (Fig. 2.4). There are several reasons for the early Monocytes/ Edema Neutrophils Macrophages ACTIVITY B 1 2 3 A C DAYS FIG. 2.4 Nature of leukocyte infiltrates in inflammatory reactions. The photomicrographs show an inflam- matory reaction in the myocardium after ischemic necrosis (infarction). (A) Early neutrophilic infiltrates and congested blood vessels. (B) Later mononuclear cell infiltrates (mostly macrophages). (C) The approximate kinetics of edema and cellular infiltration. The kinetics and nature of the infiltrate may vary depending on the severity and cause of the reaction.  CHAPTER 2 Inflammation and Repair 31 preponderance of neutrophils: they are more numerous in the blood Phagocytosis than other leukocytes, they respond more rapidly to chemokines, and Phagocytosis is the ingestion of particulate material by cells. The they may attach more firmly to adhesion molecules that are rapidly body’s most important phagocytes are neutrophils and macrophages induced on endothelial cells, such as P- and E-selectins. After entering (Table 2.4). Neutrophils are rapid responders but relatively short- tissues, neutrophils are short lived; they undergo apoptosis and lived. In inflammatory reactions, macrophages are derived from disappear within a few days. Monocytes develop into macrophages in blood monocytes and can live for days or months. (As we will discuss tissues that not only survive longer but may also proliferate, and thus later, some long-lived tissue-resident macrophages are derived from they become the dominant population in prolonged inflammatory embryonic precursors that seed the tissues in early life and remain for reactions. years.) Macrophage responses tend to be slower but more long There are, however, exceptions to this stereotypic pattern of lasting. cellular infiltration. In certain infectionsdfor example, those pro- Neutrophils and macrophages can ingest microbes following their duced by Pseudomonas bacteriadthe cellular infiltrate is dominated recognition by phagocyte receptors, such as the mannose receptor by continuously recruited neutrophils for several days; in viral in- (which recognizes terminal mannose residues found in microbial fections, lymphocytes may be the first cells to arrive; some hyper- glycoproteins) and so-called “scavenger receptors.” The efficiency of sensitivity reactions are dominated by activated lymphocytes, this process is greatly increased if the microbes are coated (opsonized) macrophages, and plasma cells (reflecting the immune response); and with molecules called opsonins for which the phagocytes also have in allergic reactions and infections with certain parasites, eosinophils specific receptors. Opsonins include antibodies, the C3b cleavage may be the main cell type. product of complement, and certain plasma lectins. Following binding The molecular understanding of leukocyte recruitment and to phagocyte receptors the particle is ingested into a membrane-bound migration has provided a large number of therapeutic targets for vesicle called the phagosome, which then fuses with lysosomes, controlling harmful inflammation. As we discuss later, agents that resulting in discharge of lysosomal contents into the phagolysosome block TNF, one of the major cytokines in leukocyte recruitment, are (Fig. 2.5). During this process, neutrophils may also release granule extremely useful therapeutics for chronic inflammatory diseases such contents into the extracellular space. as rheumatoid arthritis. Antibodies that block integrins were mentioned earlier. Intracellular Destruction of Microbes and Debris Killing of microbes and destruction of ingested materials are Phagocytosis and Clearance of the Offending Agent accomplished by reactive oxygen species (ROS, also called reactive Neutrophils and monocytes that have been recruited to a site of oxygen intermediates), reactive nitrogen species (mainly derived infection or cell death are activated by products of microbes and from nitric oxide [NO]), and lysosomal enzymes. All these sub- necrotic cells and by locally produced cytokines. Activation induces stances are normally sequestered in lysosomes, to which phagocytosed several responses (eFig. 2.1), of which phagocytosis and intracellular materials are brought. Thus, potentially harmful substances are killing are most important for destruction of microbes and clearance segregated from the cell’s cytoplasm to avoid damage to the phagocyte of dead tissues. while it is performing its normal function. Table 2.4 Properties of Neutrophils and Macrophages Neutrophils Macrophages Origin HSCs in bone marrow HSCs in bone marrow (in inflammatory reactions) Stem cells in yolk sac or fetal liver (early in development, for some tissue-resident macrophages) Life span in tissues 1e2 days Inflammatory macrophages: days or weeks Tissue-resident macrophages: years Responses to activating stimuli Rapid, short-lived, mostly degranulation and More prolonged, slower, often dependent on new enzymatic activity gene transcription Reactive oxygen species Rapidly induced by assembly of phagocyte Less prominent oxidase (respiratory burst) Nitric oxide Low levels or none Induced following transcriptional activation of iNOS Degranulation Major response; induced by cytoskeletal Not prominent rearrangement Cytokine production Low levels or none Major functional activity, requires transcriptional activation of cytokine genes NET formation Rapidly induced, by extrusion of nuclear Little or none contents Secretion of lysosomal enzymes Prominent Less HSC, Hematopoietic stem cell; iNOS, inducible nitric oxide synthase; NET, neutrophil extracellular traps. This table lists the major differences between neutrophils and macrophages. Note that the two cell types share many features, such as phagocytosis, ability to migrate through blood vessels into tissues, and chemotaxis.  CHAPTER 2 Inflammation and Repair 31.e1 Microbe Chemokines Cytokines N-formyl- Lipid Toll-like (e.g., IFN-γ) methionyl mediators LPS receptor peptides G-protein coupled CD14 Cytokine Various Recognition receptors receptors phagocytic of microbes, receptors mediators Cellular Cytoskeletal changes, Production of Production of reactive Phagocytosis of response signal transduction mediators oxygen species (ROS); microbe into (e.g., arachidonic lysosomal enzymes phagosome acid metabolites, cytokines) Increased Chemotaxis integrin avidity Microbicidal activity of leukocytes Functional outcomes Adhesion to Migration Amplification of the Killing of microbes endothelium into tissues inflammatory reaction eFIG. 2.1 Leukocyte activation. Various types of leukocyte cell surface receptors recognize different ago- nists. Once stimulated, the receptors initiate responses that mediate leukocyte functions. Only some receptors are depicted (see text for details). LPS first binds to a circulating LPS-binding protein (not shown). IFN-g, Interferon-g; LPS, lipopolysaccharide. 32 CHAPTER 2 Inflammation and Repair A 1. RECOGNITION AND ATTACHMENT Microbes bind to phagocyte receptors Lysosome with enzymes Fusion of phagosome Phagocytic Microbe ingested with receptor in phagosome lysosome Degradation of microbes 2. ENGULFMENT by lysosomal enzymes Phagocyte membrane Phagolysosome in phagolysosome zips up around microbe Phagosome with ingested microbe 3. KILLING AND DEGRADATION Primary granule MPO MPO NADPH + Cl– O2 Phagocyte oxidase iNOS NADP+ O2 H2O2 ClO Arginine Fe++ NO OH ROS Membrane Phagocyte oxidase O2 B PHAGOCYTIC VACUOLE C FIG. 2.5 Phagocytosis and intracellular destruction of microbes. (A) Phagocytosis of a particle (e.g., a bacte- rium) involves binding to receptors on the leukocyte membrane, engulfment, and fusion of the phagocytic vacuoles with lysosomes. This is followed by destruction of ingested particles within the phagolysosomes by lysosomal enzymes and by reactive oxygen and nitrogen species. (B) In activated phagocytes, cytoplasmic components of the phagocyte oxidase enzyme assemble in the membrane
of the phagosome to form the active enzyme, which catalyzes the conversion of oxygen into superoxide O,2 and H2O2. Myeloperoxidase, present in the granules of neutrophils, converts H2O2 to hypochlorite. (C) Microbicidal reactive oxygen species (ROS) and nitric oxide (NO) kill ingested microbes. During phagocytosis, granule contents may be released into extracellular tissues (not shown). iNOS, Inducible NO synthase; MPO, myeloperoxidase; ROS, reactive oxygen species. Reactive Oxygen Species. These free radicals are produced mainly free radicals bind to and modify cellular lipids, proteins, and nucleic in the phagolysosomes of neutrophils. Upon activation of neutrophils, acids and thus destroy cells such as microbes. The production of ROS a multicomponent enzyme called phagocyte oxidase (or NADPH coupled with oxygen consumption is called the respiratory burst. oxidase) is rapidly assembled in the membrane of the phagolysosome Genetic defects in the generation of ROS are the cause of an (Fig. 2.5B). This enzyme oxidizes NADPH (reduced nicotinamide- immunodeficiency disease called chronic granulomatous disease, adenine dinucleotide phosphate)
and, in the process, reduces described in Chapter 5. oxygen to superoxide anion O,2 , which is then converted to H2O2. Nitric Oxide. NO, a soluble gas produced from arginine by the H2O2 is not able to efficiently kill microbes by itself. However, the action of nitric oxide synthase (NOS), also participates in microbial azurophilic granules of neutrophils contain the enzyme killing, especially in macrophages. Inducible NOS (iNOS) is upregu- myeloperoxidase (MPO), which, in the presence of a halide such as lated in macrophages by transcriptional activation of the gene in Cl
, converts H2O2 to hypochlorite (ClO
), a potent antimicrobial response to microbial products and cytokines
such as IFN-g agent that destroys microbes by halogenation (in which the halide (Fig. 2.5C). NO reacts with superoxide O,2 generated by phagocyte is bound covalently to cellular constituents) or by oxidation of oxidase to produce the highly reactive peroxynitrite (ONOO
). These proteins and lipids (lipid peroxidation). The H2O2-MPO-halide nitrogen-derived molecules, similar to ROS, attack and damage the system is the most efficient bactericidal system of neutrophils. lipids, proteins, and nucleic acids of microbes. H2O2 is also converted to hydroxyl radical ( OH), another powerful Leukocyte Granule Contents. Neutrophils have two main types of destructive agent. As discussed in Chapter 1, these oxygen-derived granules containing enzymes that degrade microbes and dead tissues  CHAPTER 2 Inflammation and Repair 33 and may contribute to tissue damage. The smaller specific (or sec- ondary) granules contain lysozyme, collagenase, gelatinase, lacto- MEDIATORS OF INFLAMMATION ferrin, plasminogen activator, histaminase, and alkaline phosphatase. The inflammatory reaction is initiated and regulated by chemicals that The larger azurophil (or primary) granules contain myeloperoxidase, are produced at the site of the reaction. The large number of mediators bactericidal factors (such as defensins), acid hydrolases, and a variety is daunting, but a basic understanding of the molecules is important of neutral proteases (elastase, cathepsin G, nonspecific collagenases, because their identification has been the foundation for the develop- proteinase 3). The contents of both types of granules are released ment of many widely used and effective antiinflammatory drugs. We when neutrophils are activated. Phagocytic vesicles containing begin by summarizing the general properties of the mediators of engulfed material may fuse with these granules (and with lysosomes, inflammation and then discuss some of the more important molecules. as described earlier), allowing the ingested materials to be destroyed Mediators may be produced locally by cells at the site of inflam- in the phagolysomes by the actions of the enzymes. Similarly, mation or may be derived from circulating precursors that are macrophages contain lysosomes filled with acid hydrolases, colla- activated at the site of inflammation. genase, elastase, and phospholipase, all of which can destroy ingested Cell-derived mediators are rapidly released from intracellular materials and cell debris. granules (e.g., amines) or are synthesized de novo (e.g., prosta- In addition to granule contents, activated neutrophils liberate glandins and leukotrienes, cytokines) in response to a stimulus. chromatin components, including histones, which form fibrillar The major cell types that produce mediators of acute inflam- networks called neutrophil extracellular traps (NETs) (eFig. 2.2). mation are tissue macrophages, dendritic cells, and mast These networks bind and concentrate antimicrobial peptides and cells, but platelets, neutrophils, endothelial cells, and most granule enzymes, forming extracellular sites for the destruction of epithelia also elaborate some inflammatory mediators. microbes. In the process of NET formation, the nuclei of the neu- Plasma-derived mediators (e.g., complement proteins) are pro- trophils are lost, leading to death of the cells. NETs have also been duced mainly in the liver and circulate as inactive precursors detected in the blood during sepsis, as a consequence of widespread that enter and are activated at sites of inflammation, usually neutrophil activation. by a series of proteolytic cleavages. Active mediators are produced only in response to various stim- Leukocyte-Mediated Tissue Injury uli, including microbial products and substances released from Leukocytes are important causes of injury to normal cells and tis- necrotic cells, which ensures that inflammation is triggered only sues. This happens during normal defense reactions against microbes, when and where it is needed. especially if the microbes are resistant to eradication, such as myco- Most mediators are short lived. They quickly decay or are inacti- bacteria. It is also the basis of tissue damage when the response is vated by enzymes, or they are otherwise scavenged or inhibited. inappropriately directed against self antigens (as in autoimmune dis- These built-in control mechanisms prevent excessive reactions. eases) or against normally harmless environmental antigens (as in allergic diseases). The principal mediators of acute inflammation are summarized in The mechanism of leukocyte-mediated tissue injury is release of the Table 2.5 and discussed next. contents of granules and lysosomes. To some extent, this happens normally, when activated leukocytes try to eliminate microbes and Vasoactive Amines: Histamine and Serotonin other offenders. The process is exaggerated if phagocytes encounter The major vasoactive amine is histamine, which is stored as a pre- materials that cannot be easily ingested, such as antibodies deposited formed molecule in the granules of mast cells, blood basophils, and on indigestible flat surfaces, or if phagocytosed substances, such as platelets. It is rapidly released when these cells are activated, so it is urate and silica crystals, damage the membrane of the phagolysosome. among the first mediators to be produced during inflammation. The The harmful proteases released by leukocytes are normally controlled richest source of histamine is the mast cell, which is normally present by a system of antiproteases in the blood and tissue fluids. Foremost in the connective tissue adjacent to blood vessels. Mast cell degranu- among these is a1-antitrypsin, which is the major inhibitor of lation and histamine release occur in response to a variety of stimuli, neutrophil elastase. A deficiency of these inhibitors may lead to sus- including binding of IgE antibodies to mast cells, which underlies tained protease activity, as is the case in patients with a1-antitrypsin immediate hypersensitivity (allergic) reactions (Chapter 5); products deficiency (Chapter 11). of complement called anaphylatoxins (C3a and C5a), described later; While we have emphasized the role of neutrophils and macro- and physical injury induced by trauma, cold, or heat, by unknown phages in acute inflammation, other cell types also serve important mechanisms. Antibodies and complement products bind to specific roles. Some T cells, called Th17 cells, secrete cytokines such as IL-17 receptors on mast cells and trigger signaling pathways that induce that recruit neutrophils and stimulate production of antimicrobial rapid degranulation. Neuropeptides (e.g., substance P) and cytokines peptides that directly kill microbes. In the absence of effective Th17 (IL-1, IL-8) may also trigger release of histamine. responses, individuals are susceptible to fungal and bacterial in- Histamine causes dilation of arterioles and increases the perme- fections. The skin abscesses that develop lack the classic features of ability of venules. Its effects on blood vessels are mediated mainly via acute inflammation, such as warmth and redness. Eosinophils are binding to histamine receptors called H1 receptors on microvascular especially important in reactions to helminthic parasites and in endothelial cells. Common antihistamine drugs that treat inflammatory some allergic disorders, and mast cells and basophils are critical cells reactions, such as allergies, bind to and block the H1 receptor. Hista- of allergic reactions. mine also causes contraction of some smooth muscles, but leukotrienes, Once the acute inflammatory response has eliminated the offend- described later, are much more potent and relevant for causing spasms ing stimulus, the reaction subsides because there is no further leuko- of bronchial smooth muscle, such as in asthma. cyte recruitment, mediators are short lived and decline if they are no Serotonin (5-hydroxytryptamine) is a preformed vasoactive medi- longer produced, and neutrophils have short life spans. ator present in platelets and certain neuroendocrine cells, for example,  CHAPTER 2 Inflammation and Repair 33.e1 A B C eFIG. 2.2 Neutrophil extracellular traps (NETs). (A) Healthy neutrophils with nuclei stained red and cytoplasm green. (B) Release of nuclear material from neutrophils (note that two have lost their nuclei), forming extra- cellular traps. (C) An electron micrograph of bacteria (staphylococci) trapped in NETs. (From Brinkmann V, Zychlinsky A: Beneficial suicide: why neutrophils die to make NETs. Nat Rev Microbiol 5:577, 2007.) 34 CHAPTER 2 Inflammation and Repair Table 2.5 Principal Mediators of Inflammation Mediators Sources Actions Histamine Mast cells, basophils, platelets Vasodilation, increased vascular permeability, endothelial activation Prostaglandins Mast cells, leukocytes Vasodilation, pain, fever Leukotrienes Mast cells, leukocytes Increased vascular permeability, chemotaxis, leukocyte adhesion, and activation Cytokines (e.g., TNF, IL-1, Macrophages, endothelial cells, Local: endothelial activation (expression of adhesion molecules) IL-6) mast cells Systemic: fever, metabolic abnormalities, hypotension (shock) Chemokines Leukocytes, activated macrophages Chemotaxis, leukocyte activation Platelet-activating factor Leukocytes, mast cells Vasodilation, increased vascular permeability, leukocyte adhesion, chemotaxis, degranulation, oxidative burst Complement Plasma (produced in liver) Leukocyte chemotaxis and activation, direct target killing (membrane attack complex), vasodilation (mast cell stimulation) Kinins Plasma (produced in liver) Increased vascular permeability, smooth muscle contraction, vasodilation, pain IL, Interleukin; TNF, tumor necrosis factor. in the gastrointestinal tract. It is a vasoconstrictor, but its importance in potentiating exudation and resultant edema. PGD2 is also a chemo- inflammation is unclear. attractant for neutrophils. Platelets contain the enzyme thromboxane synthase, which pro- Arachidonic Acid Metabolites duces TxA2, the major eicosanoid in these cells. TxA2 is a potent Prostaglandins and leukotrienes are lipid mediators produced from platelet-aggregating agent and vasoconstrictor. arachidonic acid (AA) present in membrane phospholipids that Vascular endothelium lacks thromboxane synthase and instead stimulate vascular and cellular reactions in acute inflammation. AA is contains prostacyclin synthase, which is responsible for the forma- a 20-carbon polyunsaturated fatty acid that is released from membrane tion of prostacyclin (PGI2) and its stable end product PGF1a. Pros- phospholipids through the action of cellular phospholipases, mainly tacyclin is a vasodilator and a potent inhibitor of platelet phospholipase A2, that are activated by inflammatory stimuli, including aggregation and thus serves to prevent thrombus formation on cytokines, complement products, and physical injury. AA-derived me- normal endothelial cells. A thromboxane-prostacyclin imbalance diators, also called eicosanoids (from the Greek, eicosa, meaning 20, as has been implicated as an early event in thrombosis in coronary they are derived from 20-carbon fatty acids), are synthesized by two and cerebral arteries (Chapter 10). major classes of enzymes, cyclooxygenases (which generate prostaglan- In addition to their local effects, prostaglandins are involved in the dins) and lipoxygenases (which produce leukotrienes and lipoxins) pathogenesis of pain and fever, two common systemic manifesta- (Fig. 2.6). Eicosanoids bind to G proteinecoupled receptors on many cell tions of inflammation (described later). types and can mediate virtually every step of inflammation (Table 2.6). Leukotrienes Prostaglandins Leukotrienes are produced by leukocytes and mast cells by the ac- Prostaglandins (PGs) are produced by mast cells, macrophages, tion of lipoxygenase and are involved in vascular and smooth endothelial cells, and many other cell types and are involved in the muscle reactions and leukocyte recruitment. The synthesis of leu- vascular and systemic reactions of inflammation. They are generated kotrienes involves multiple steps. The first generates leukotriene A4 by the actions of two cyclooxgenases, called COX-1 and COX-2, which (LTA4), which in turn gives rise to LTB4 or LTC4. LTB4 is produced by differ in where they are expressed. COX-1 is produced in response to neutrophils and some macrophages and is a potent chemotactic agent inflammatory stimuli and is also constitutively expressed in most and activator of neutrophils. LTC4 and its metabolites, LTD4 and tissues, where it may have homeostatic functions (e.g., fluid and LTE4, are produced mainly in mast cells and cause intense vasocon- electrolyte balance in the kidneys, cytoprotection in the gastrointes- striction, bronchospasm (important in asthma), and increased tinal tract). By contrast, COX-2 is induced by inflammatory stimuli permeability of venules. and thus generates prostaglandins in inflammatory reactions but is low or absent in most healthy tissues. Other Arachidonic AcideDerived Mediators Prostaglandins are named based on structural features coded by a Lipoxins are also generated from AA by the lipoxygenase pathway, letter, as in PGD, PGE, and others, and a subscript numeral (e.g., 1, 2), but unlike prostaglandins and leukotrienes, the lipoxins suppress which indicates the number of double bonds in the compound. The inflammation by inhibiting neutrophil chemotaxis and adhesion to most important prostaglandins in inflammation are PGE2, PGD2, endothelium and hence the recruitment of leukocytes. Leukocytes, PGF2a, PGI2 (prostacyclin), and TxA2 (thromboxane A2), each of particularly neutrophils, produce intermediates in the lipoxin synthesis which is derived by the action of a specific enzyme on an intermediate pathway that are converted to lipoxins by platelets interacting with the in the pathway. Some of these enzymes have restricted tissue distri- leukocytes. bution and functions. Various other antiinflammatory AA-derived mediators have been PGD2 is the major prostaglandin made by mast cells; along with described and given names such as resolvins because they resolve the PGE2 (which is more widely distributed), it causes vasodilation active phase of acute inflammation. The role of these compounds in and increases the permeability of postcapillary venules, thus the inflammatory response is a topic of active study.  CHAPTER 2 Inflammation and Repair 35 Stimulus Cell membrane Phospholipase A2 Steroids phospholipids COOH ARACHIDONIC ACID CH3 COX-1 and COX-2 Cyclooxygenase 5-Lipoxygenase Lipoxygenase inhibitors, aspirin, inhibitors indomethacin Prostaglandin G2 (PGG2) 5-HPETE 12-Lipoxygenase Prostaglandin H2 (PGH2) Leukotriene A4 (LTA4) Lipoxin A4 (LXA4) LTB4 Lipoxin B4 (LXB4) Prostacyclin Thromboxane A2 PGD2, PGE2 Precursor (PGI2) (TXA2) lipoxins Mast cells LTB4 Neutrophil Platelet PGD2, PGE2 adhesion, Active chemotaxis lipoxins Increased LIPOXINS PGI2 vascular LTC4 LTD4 LTE4 Inhibit permeability TXA2 Platelet platelet aggregation aggregation Vasodilation Chemoattractant Increased Leukotriene Vasodilation Vasoconstriction for neutrophil vascular receptor permeability antagonists Bronchospasm PROSTAGLANDINS LEUKOTRIENES FIG. 2.6 Production of arachidonic acid metabolites and their roles in inflammation. Clinically useful antag- onists of different enzymes and receptors are indicated in red. While leukotriene receptor antagonists inhibit all actions of leukotrienes, they are used in the clinic to treat asthma, as shown. COX-1, COX-2, Cyclooxygenase 1 and 2; HPETE, hydroperoxyeicosatetraenoic acid. Table 2.6 Principal Actions of Arachidonic Acid Metabolites Cyclooxygenase inhibitors include aspirin and other nonsteroidal in Inflammation antiinflammatory drugs (NSAIDs), such as ibuprofen. They inhibit Action Eicosanoids both COX-1 and COX-2 and thus inhibit prostaglandin synthesis (hence their efficacy in treating pain and fever); aspirin does this Vasodilation Prostaglandins PGI2 (prostacyclin), by irreversibly inactivating cyclooxygenases. Selective COX-2 in- PGE1, PGE2, PGD2 hibitors were developed to target prostaglandins involved solely Vasoconstriction Thromboxane A2, leukotrienes in inflammatory reactions. However, COX-2 inhibitors may in- C4, D4, E4 crease the risk of cardiovascular and cerebrovascular events, Increased vascular Leukotrienes C4, D4, E4 possibly by impairing endothelial cell production of prostacyclin permeability (PGI2), which is antithrombotic, while leaving intact the COX-1e Chemotaxis, leukocyte Leukotriene B4 mediated production by platelets of thromboxane A2 (TxA2), which adhesion promotes platelet aggregation. COX-2 inhibitors are now used mainly to treat arthritis and perioperative pain in patients who do not have cardiovascular risk factors. Pharmacologic Inhibitors of Prostaglandins and Leukotrienes Lipoxygenase inhibitors. 5-lipoxygenase is not affected by NSAIDs. The importance of eicosanoids in inflammation has driven the A pharmacologic agent that inhibits leukotriene production (zileu- development of antiinflammatory drugs, including the following: ton) is useful in the treatment of asthma. 36 CHAPTER 2 Inflammation and Repair Corticosteroids are broad-spectrum antiinflammatory agents that leukocyte integrins. These changes are critical for the recruitment reduce the transcription of genes encoding COX-2, phospholipase of leukocytes to sites of inflammation. They also stimulate produc- A2, proinflammatory cytokines (e.g., IL-1 and TNF), and iNOS. tion of various mediators, including other cytokines and chemo- Leukotriene receptor antagonists block leukotriene receptors and kines, and eicosanoids, and increase the procoagulant activity of prevent the actions of the leukotrienes (zafirlukast). These drugs the endothelium. are used in the treatment of allergic asthma and allergic rhinitis. Activation of leukocytes and other cells. TNF augments responses of neutrophils to other stimuli such as bacterial endotoxin and stim- Cytokines and Chemokines ulates the microbicidal activity of macrophages. IL-1 activates Cytokines are proteins produced by many cell types (principally fibroblasts to synthesize collagen and stimulates proliferation of activated lymphocytes, macrophages, and dendritic cells, but also synovial and other mesenchymal cells. IL-1 also stimulates Th17 re- endothelial, epithelial, and connective tissue cells) that mediate and sponses, which in turn induce acute inflammation. regulate immune and inflammatory reactions. By convention, Systemic acute-phase response. IL-1 and TNF (as well as IL-6) growth factors that act on epithelial and mesenchymal cells are not induce the systemic responses associated with infection or injury, grouped under cytokines. The general properties and functions of including fever (described later in the chapter). They are also cytokines are discussed in Chapter 5. Here the cytokines involved in implicated in the pathogenesis of the systemic inflammatory acute inflammation are reviewed (Table 2.7). response syndrome (SIRS), resulting from disseminated bacterial infection and other serious conditions, described later. At high Tumor Necrosis Factor (TNF) and Interleukin-1 (IL-1) concentrations, TNF dilates blood vessels and reduces myocar- TNF and IL-1 serve critical roles in leukocyte recruitment by dial contractility, both of which contribute to the fall in promoting adhesion of leukocytes to endothelium and their blood pressure associated with SIRS. TNF regulates energy bal- migration through vessels. These cytokines are produced mainly by ance by promoting lipid and protein mobilization and by sup- activated macrophages and dendritic cells; TNF is also produced by T pressing appetite. Therefore, sustained production of TNF lymphocytes and mast cells, and IL-1 is produced by some epithelial contributes to cachexia, a pathologic state characterized by cells as well. The secretion of TNF and IL-1 can be stimulated by weight loss and anorexia that accompanies some chronic infec- microbial products, necrotic cells, and a variety of other inflamma- tions and cancers. tory stimuli. The production of TNF is induced by signals through TLRs and other microbial sensors. The synthesis of IL-1 is stimulated TNF antagonists have been remarkably effective in the treatment by the same signals, but the generation of the biologically active form of chronic inflammatory diseases, particularly rheumatoid arthritis, of this cytokine is dependent on activation of the inflammasome psoriasis, and some types of inflammatory bowel disease. One of the (Chapter 5). complications of this therapy is that patients become susceptible to The actions of TNF and IL-1 contribute to the local and systemic mycobacterial infection, resulting from the reduced ability of macro- reactions of inflammation (Fig. 2.7). The most important roles of these phages to kill intracellular microbes. Although many of the actions of cytokines in inflammation are the following. TNF and IL-1 are overlapping, IL-1 antagonists are not as effective, for Endothelial activation and leukocyte recruitment. Both TNF and obscure reasons. Blocking either cytokine does not affect the outcome IL-1 act on endothelium to increase the expression of endothelial of sepsis (see later), perhaps because other cytokines contribute to this adhesion molecules, mostly E- and P-selectins and ligands for serious systemic inflammatory reaction. Table 2.7 Cytokines in Inflammation Cytokine Principal Sources Principal Actions in Inflammation In Acute Inflammation TNF Macrophages, mast cells, T lymphocytes Stimulates expression of endothelial adhesion molecules and secretion of other cytokines; systemic effects IL-1 Macrophages, endothelial cells, some epithelial cells Similar to TNF; greater role in fever IL-6 Macrophages, other cells Systemic effects (acute-phase response) Chemokines Macrophages, endothelial cells, T lymphocytes, mast Recruitment of leukocytes to sites of inflammation; migration of cells cells, other cell types in healthy tissues In Chronic Inflammation IL-12 Dendritic cells, macrophages Increased production of IFN-g IFN-g T lymphocytes, NK cells Activation of macrophages (increased ability to kill microbes and tumor cells) IL-17 T lymphocytes Recruitment of neutrophils and monocytes IFN-g, Interferon-g; IL, interleukin; NK cells, natural killer cells; TNF, tumor necrosis factor. Chemokines are divided into four groups based on the number of amino acids between two of the conserved cysteines in the protein. As indicated, the chemokines of these groups have somewhat different target cell specificities. The most important cytokines involved in inflammatory reactions are listed. Many other cytokines may play 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.  CHAPTER 2 Inflammation and Repair 37 LOCAL INFLAMMATION SYSTEMIC PROTECTIVE EFFECTS SYSTEMIC PATHOLOGICAL EFFECTS TNF, Increased TNF IL-1 permeability TNF, IL-1 Endothelial Heart activation Increased Brain expression of Low adhesion molecules output Fever Endothelial cells, blood vessels IL-1, chemokines IL-1, IL-6 TNF Vasodilation Activation of leukocytes and other cells Liver Acute TNF, IL-1, IL-6, phase IL-1 chemokines proteins Increased Thrombus permeability Macrophage TNF, TNF, Enhanced neutrophil IL-1, IL-1 response to other stimuli TNF, IL-6 Skeletal muscle IL-1 Neutrophil (Insulin resistance) Other Bone IL-1, Multiple IL-17 cell marrow IL-6 tissues types Loss of T cell Leukocytosis Chemokines fatty tissue FIG. 2.7 Major roles of cytokines in acute inflammation. Chemokines promotes neutrophil recruitment. Antagonists against both have Chemokines are a family of small (8 to 10 kD) proteins that act shown impressive efficacy in the treatment of inflammatory diseases. primarily as chemoattractants for specific types of leukocytes. About Type I interferons, whose normal function is to inhibit viral replica- 40 different chemokines and 20 different receptors for chemokines tion, contribute to some of the systemic manifestations of inflamma- have been identified. Different chemokines act on specific cell types tion. Cytokines also play key roles in chronic inflammation (see later). according to their expression of various chemokine receptors (see Table 2.7). Chemokines bind to proteoglycans and are thus displayed Complement System at high concentrations on the surface of endothelial cells and in the The complement system is a collection of soluble proteins and their extracellular matrix (see Fig. 2.3). They have two main functions: membrane receptors that function mainly in host defense against In inflammation. Production of inflammatory chemokines is induced microbes and in pathologic inflammatory reactions. There are more by microbes and other stimuli. These chemokines bind to leukocyte than 20 complement proteins, some of which are numbered C1 receptors and stimulate both the integrin-dependent attachment of through C9. The activation and functions of complement are outlined leukocytes to endothelium and the migration (chemotaxis) of leuko- in Fig. 2.8. cytes in tissues to sites of infection or tissue damage. Complement proteins are present as proforms that are activated Maintenance of tissue architecture. Some chemokines are produced during inflammatory reactions. They participate in a cascade of constitutively by stromal cells in tissues (homeostatic chemokines) enzymatic reactions that is capable of tremendous amplification. The and promote the localization of various cell types to specific critical step in complement activation is the proteolytic cleavage of anatomic regions. Examples include the ability of certain chemo- the third (and most abundant) component, C3, which can occur by kines to promote the localization of T and B lymphocytes to one of three pathways: discrete areas of the spleen and lymph nodes (Chapter 5). The classical pathway, which is triggered by fixation of C1 to anti- body (IgM or IgG) that has combined with antigen Although the role of chemokines in inflammation is well estab- The alternative pathway, which is triggered by microbial surface lished, it has proved difficult to develop antagonists that block the molecules (e.g., endotoxin, or lipopolysaccharide [LPS]), complex activities of these proteins. polysaccharides, and other substances, in the absence of antibody The lectin pathway, in which plasma mannose-binding lectin binds Other Cytokines in Acute Inflammation to carbohydrates on microbes and activates C1, also without a role The list of cytokines implicated in inflammation is huge and for antibody constantly growing. In addition to the ones described earlier, two that have received considerable recent interest are IL-6, which is made by All three pathways of complement activation lead to the forma- macrophages and other cells and is involved in local and systemic tion of an enzyme called the C3 convertase, which splits C3 into two reactions, and IL-17, which is produced mainly by T lymphocytes and functionally distinct fragments, C3a and C3b. C3a is released, and 38 CHAPTER 2 Inflammation and Repair COMPLEMENT ACTIVATION EFFECTOR FUNCTIONS C3 C5a, C3a: Inflammation C3b Alternative C3a pathway Recruitment and Destruction of Triggered by microbial activation of microbes by surface molecules leukocytes leukocytes C3a C3b Microbe C3b: Phagocytosis Antigen C3b Classical Antibody pathway C3 convertase C1-complex C3b is deposited Recognition of bound Phagocytosis Triggered by fixation