Path 3 Inflammation and Repair PDF

See TARGETED THERAPY available online at www.studentconsult.com C H A P T E R Inflammation and Repair 3...

See TARGETED THERAPY available online at www.studentconsult.com C H A P T E R Inflammation and Repair 3 CHAPTER CONTENTS Overview of Inflammation: Leukocyte-Mediated Tissue Injury 84 Role of Lymphocytes 98 Definitions and General Other Functional Responses of Activated Other Cells in Chronic Inflammation 99 Features 71 Leukocytes 84 Granulomatous Inflammation 100 Historical Highlights 73 Termination of the Acute Inflammatory Systemic Effects of Causes of Inflammation 73 Response 85 Inflammation 101 Recognition of Microbes and Damaged Mediators of Inflammation 85 Tissue Repair 103 Cells 74 Vasoactive Amines: Histamine and Serotonin 86 Overview of Tissue Repair 103 Acute Inflammation 75 Arachidonic Acid Metabolites 86 Cell and Tissue Regeneration 103 Reactions of Blood Vessels in Acute Cytokines and Chemokines 88 Cell Proliferation: Signals and Control Inflammation 75 Complement System 90 Mechanisms 103 Changes in Vascular Flow and Caliber 75 Other Mediators of Inflammation 92 Mechanisms of Tissue Regeneration 104 Increased Vascular Permeability (Vascular Morphologic Patterns of Acute Repair by Connective Tissue Leakage) 76 Inflammation 93 Deposition 105 Responses of Lymphatic Vessels and Lymph Serous Inflammation 93 Steps in Scar Formation 105 Nodes 76 Fibrinous Inflammation 93 Angiogenesis 106 Leukocyte Recruitment to Sites of Purulent (Suppurative) Inflammation and Deposition of Connective Tissue 107 Inflammation 77 Abscess 94 Remodeling of Connective Tissue 107 Leukocyte Adhesion to Endothelium 77 Ulcers 94 Factors That Influence Tissue Repair 108 Leukocyte Migration Through Endothelium 78 Outcomes of Acute Inflammation 95 Examples of Tissue Repair and Chemotaxis of Leukocytes 79 Summary of Acute Inflammation 95 Fibrosis 108 Phagocytosis and Clearance of the Chronic Inflammation 96 Healing of Skin Wounds 108 Offending Agent 80 Causes of Chronic Inflammation 96 Fibrosis in Parenchymal Organs 110 Phagocytosis 80 Morphologic Features 96 Abnormalities in Tissue Repair 110 Intracellular Destruction of Microbes and Cells and Mediators of Chronic Defects in Healing: Chronic Wounds 110 Debris 81 Inflammation 97 Excessive Scarring 110 Neutrophil Extracellular Traps 83 Role of Macrophages 97 OVERVIEW OF INFLAMMATION: of these normally circulate in a resting state in the blood, from where they can be rapidly recruited to any site in DEFINITIONS AND GENERAL the body. Some of the cells involved in inflammatory FEATURES responses also reside in tissues, where they function as sentinels on the lookout for threats. The process of inflam- nflammation is a response of vasculari ed tissues that mation delivers circulating cells and proteins to tissues delivers leukocytes and molecules of host defense from and activates the recruited and resident cells as well as the circulation to the sites of infection and cell damage the soluble molecules, which then function to get rid of in order to eliminate the offending agents Although in the harmful or unwanted substances. Without inflam- common medical and lay parlance, inflammation suggests mation, infections would go unchecked, wounds would a harmful reaction, it is actually a protective response that never heal, and injured tissues might remain permanent is essential for survival. It serves to rid the host of both festering sores. The suffix -itis after an organ denotes inflam- the initial cause of cell injury (e.g., microbes, toxins) and mation in that site, such as appendicitis, conjunctivitis, or the consequences of such injury (e.g., necrotic cells and meningitis. tissues). The mediators of defense include phagocytic The typical inflammatory reaction develops through a leukocytes, antibodies, and complement proteins. Most series of se uential steps (Fig. 3.1): 71 72 CHAPTER 3 Inflammation and Repair Microbes Necrotic tissue proteins from blood requires coordinated changes in blood vessels and secretion of mediators, described in detail later. Removal of the stimulus for inflammation is accomplished mainly by phagocytic cells, which ingest and destroy Recognition by microbes and dead cells. tissue-resident Regulation of the response is important for terminating sentinel cells the reaction when it has accomplished its purpose. Macrophage Dendritic cell Mast cell Repair consists of a series of events that heal damaged tissue. In this process the injured tissue is replaced through Mediators (amines, cytokines) regeneration of surviving cells and filling of residual defects with connective tissue (scarring). Recruitment of leukocytes Before discussing the mechanisms, functions, and pathol- Platelets ogy of the inflammatory response, it is useful to review Plasma proteins some of its fundamental properties. Monocyte (complement, omponents of the inflammatory response. The major par- Granulocyte Vasodilation, kinins, others) ticipants in the inflammatory reaction in tissues are blood increased vascular vessels and leukocytes (see Fig. 3.1). As will be discussed permeability in more detail later, blood vessels respond to inflammatory stimuli by dilating and by increasing their permeability, enabling selected circulating proteins to enter the site of infection or tissue damage. In addition, the endothelium Macrophage lining blood vessels also changes, such that circulating Edema leukocytes adhere and then migrate into the tissues. Elimination of microbes, dead tissue Leukocytes, once recruited, are activated and acquire the Cytokines, growth factors ability to ingest and destroy microbes and dead cells, as well as foreign bodies and other unwanted materials in Fibroblasts the tissues. armful conse uences of inflammation. Protective inflam- matory reactions to infections are often accompanied by Extracellular matrix proteins and cells local tissue damage and its associated signs and symptoms (e.g., pain and functional impairment). Typically, however, these harmful consequences are self-limited and resolve Repair as the inflammation abates, leaving little or no permanent damage. In contrast, there are many diseases in which Figure 3.1 Sequence of events in an inflammatory reaction. Sentinel cells the inflammatory reaction is misdirected (e.g., against in tissues (macrophages, dendritic cells, and other cell types) recognize microbes and damaged cells and liberate mediators, which trigger the self tissues in autoimmune diseases), occurs against vascular and cellular reactions of inflammation. normally harmless environmental substances (e.g., in allergies), or is inadequately controlled. In these cases the normally protective inflammatory reaction becomes the cause of the disease, and the damage it causes is the dominant feature. In clinical medicine, great attention is Recognition of the noxious agent that is the initiat- given to the injurious consequences of inflammation ing stimulus for inflammation. The cells involved in (Table 3.1). Inflammatory reactions underlie common inflammation (tissue-resident sentinel cells, phago- chronic diseases such as rheumatoid arthritis, athero- cytes, and others) are equipped with receptors that sclerosis, and lung fibrosis, as well as life-threatening recognize microbial products and substances released hypersensitivity reactions to insect bites, foods, drugs, from damaged cells. These receptors are described in and toxins. For this reason our pharmacies abound with more detail later. Engagement of the receptors leads antiinflammatory drugs, which ideally would control the to the production of mediators of inflammation, which harmful sequelae of inflammation yet not interfere with then trigger the subsequent steps in the inflammatory its beneficial effects. Inflammation also may contribute response. to a variety of diseases that are thought to be primarily Recruitment of leukocytes and plasma proteins into the metabolic, degenerative, or genetic, such as type 2 dia- tissues. Since blood perfuses every tissue, leukocytes and betes, Alzheimer disease, and cancer. In recognition of proteins such as complement can be delivered to any the wide-ranging harmful consequences of inflammation, site of microbial invasion or tissue injury. When patho- the lay press has rather melodramatically referred to it genic microbes invade the tissues, or tissue cells die, as “the silent killer.” leukocytes (first mainly neutrophils, later monocytes and ocal and systemic inflammation. Much of this discussion lymphocytes) and plasma proteins are rapidly recruited focuses on the inflammatory response to a localized from the circulation to the extravascular site where the infection or tissue damage. Although even local reactions offending agent is located. The exodus of cells and plasma may have systemic manifestations (e.g., fever in the setting  Overview of inflammation: definitions and general features 73 Table 3.1 Diseases Caused by Inflammatory Reactions fungi, and dead cells. It typically develops within minutes Cells and Molecules or hours and is of short duration (several hours to a few Disorders Involved in Injury days). It is characterized by the exudation of fluid and Acute plasma proteins (edema) and the emigration of leukocytes, predominantly neutrophils. If the offending stimulus is Acute respiratory distress Neutrophils eliminated, the reaction subsides, and residual injury is syndrome repaired. Asthma Eosinophils; IgE antibodies Chronic inflammation may follow acute inflamma- Glomerulonephritis Antibodies and complement; tion or arise de novo. It is a response to agents that neutrophils, monocytes are difficult to eradicate, such as some bacteria (e.g., Septic shock Cytokines tubercle bacilli) and other pathogens (such as viruses Chronic and fungi), as well as self antigens and environmental Arthritis Lymphocytes, macrophages; antigens. Chronic inflammation is of longer duration and antibodies? is associated with more tissue destruction and scarring (fibrosis). Sometimes, chronic inflammation may coexist Asthma Eosinophils; IgE antibodies with unresolved acute inflammation, as may occur in Atherosclerosis Macrophages; lymphocytes peptic ulcers. Pulmonary fibrosis Macrophages; fibroblasts IgE, Immunoglobulin E. Historical Highlights Listed are selected examples of diseases in which the inflammatory response plays a significant role in tissue injury. Some, such as asthma, can present with Although clinical features of inflammation were described in acute inflammation or a chronic illness with repeated bouts of acute exacerbation. These diseases and their pathogenesis are discussed in relevant an Egyptian papyrus dated around 3000 BC, Celsus, a Roman chapters. writer of the first century AD, first listed the four cardinal signs of inflammation: rubor (redness), tumor (swelling), calor (heat), and dolor (pain). These signs are hallmarks of of bacterial or viral pharyngitis), the inflammation is acute inflammation. A fifth clinical sign, loss of function largely confined to the site of infection or damage. In (functio laesa), was added by Rudolf Virchow in the 19th rare situations, such as some disseminated bacterial century. In 1793 the Scottish surgeon John Hunter noted infections, the inflammatory reaction is systemic and what is now considered an obvious fact: inflammation is not causes widespread pathologic abnormalities. This reaction a disease but a stereotypic response that has a salutary effect has been called sepsis, which is one form of the systemic on its host. In the 1880s Russian biologist Elie Metchnikoff inflammatory response syndrome. This serious disorder is discovered the process of phagocytosis by observing the discussed in Chapter 4. ingestion of rose thorns by amebocytes of starfish larvae ediators of inflammation. The vascular and cellular reac- and of bacteria by mammalian leukocytes. He concluded tions of inflammation are triggered by soluble factors that that the purpose of inflammation was to bring phagocytic are produced by various cells or derived from plasma cells to the injured area to engulf invading bacteria. This proteins and are generated or activated in response to the concept was satirized by George Bernard Shaw in his play inflammatory stimulus. Microbes, necrotic cells (whatever The Doctor’s Dilemma, in which one physician’s cure-all is the cause of cell death), and even hypoxia can trigger to “stimulate the phagocytes!” Sir Thomas Lewis, studying the elaboration of inflammatory mediators and thus elicit the inflammatory response in skin, established the concept inflammation. Such mediators initiate and amplify the that chemical substances, such as histamine (produced inflammatory response and determine its pattern, severity, locally in response to injury), mediate the vascular changes and clinical and pathologic manifestations. of inflammation. This fundamental concept underlies the Acute and chronic inflammation. The distinction between important discoveries of chemical mediators of inflam- acute and chronic inflammation was originally based on mation and the use of antiinflammatory drugs in clinical the duration of the reaction, but we now know that they medicine. differ in several ways (Table 3.2). Acute inflammation is a rapid, often self-limited, response to offending agents Causes of Inflammation that are readily eliminated, such as many bacteria and Inflammatory reactions may be triggered by a variety of stimuli: Table 3.2 Features of Acute and Chronic Inflammation Infections (bacterial, viral, fungal, parasitic) and microbial Feature Acute Chronic toxins are among the most common and medically important causes of inflammation. Different infectious Onset Fast: minutes or Slow: days hours pathogens elicit varied inflammatory responses, from mild acute inflammation that causes little or no lasting Cellular Mainly neutrophils Monocytes/macrophages damage and successfully eradicates the infection, to severe infiltrate and lymphocytes systemic reactions that can be fatal, to prolonged chronic Tissue injury, Usually mild and Often severe and reactions that cause extensive tissue injury. The outcomes fibrosis self-limited progressive are determined largely by the type of pathogen and the Local and Prominent Less host response and, to some extent, by other, poorly defined systemic signs characteristics of the host. 74 CHAPTER 3 Inflammation and Repair Tissue necrosis elicits inflammation regardless of the cause product of DNA breakdown), adenosine triphosphate of cell death. Cells may die because of ischemia (reduced (ATP) (released from damaged mitochondria), reduced blood flow, the cause of myocardial infarction), trauma, intracellular K+ concentrations (reflecting loss of ions and physical and chemical injury (e.g., thermal injury, because of plasma membrane injury), even DNA when as in burns or frostbite; irradiation; exposure to some it is released into the cytoplasm and not sequestered in environmental chemicals). Several molecules released nuclei, as it should be normally, and many others. These from necrotic cells are known to trigger inflammation; receptors activate a multiprotein cytosolic complex called some of these are described later. the inflammasome (Chapter 6), which induces the production Foreign bodies (splinters, dirt, sutures) may elicit inflam- of the cytokine interleukin-1 (IL-1). IL-1 recruits leukocytes mation by themselves or because they cause traumatic and thus induces inflammation (see later). Gain-of-function tissue injury or carry microbes. Even endogenous sub- mutations in the genes encoding some of the receptors are stances can be harmful if they deposit in tissues; such the cause of rare diseases grouped under autoinflammatory substances include urate crystals (in gout), cholesterol syndromes that are characterized by spontaneous IL-1 crystals (in atherosclerosis), and lipids (in obesity- production and inflammation; IL-1 antagonists are effective associated metabolic syndrome). treatments for these disorders. The inflammasome has Immune reactions (also called hypersensiti ity) are reactions also been implicated in inflammatory reactions to urate in which the normally protective immune system damages crystals (the cause of gout), lipids (in metabolic syndrome the individual’s own tissues. The injurious immune and obesity-associated type 2 diabetes), cholesterol crystals responses may be inappropriately directed against self (in atherosclerosis), and even amyloid deposits in the brain antigens, causing autoimmune diseases, or may be reac- (in Alzheimer disease). These disorders are discussed later tions against environmental substances, as in allergies, in this and other chapters. or against microbes. Inflammation is a major cause of ther cellular receptors in ol ed in inflammation. In addition tissue injury in these diseases (Chapter 6). Because the to directly recognizing microbes, many leukocytes express stimuli for the inflammatory responses (e.g., self antigens receptors for the Fc tails of antibodies and for complement and environmental antigens) cannot be eliminated, proteins. These receptors recognize microbes coated with autoimmune and allergic reactions tend to be persistent antibodies and complement (the coating process is called and difficult to cure, are often associated with chronic opsonization) and promote ingestion and destruction of inflammation, and are important causes of morbidity the microbes as well as inflammation. and mortality. The inflammation is induced largely by irculating proteins. The complement system reacts against cytokines produced by T lymphocytes and other cells of microbes and produces mediators of inflammation the immune system (Chapter 6). (discussed later). A circulating protein called mannose- binding lectin recognizes microbial sugars and promotes Recognition of Microbes and Damaged Cells ingestion of the microbes and the activation of the complement system. Other proteins called collectins also Recognition of microbial components or substances released bind to and combat microbes. from damaged cells is the initiating step in inflammatory reactions The cells and receptors that perform this function evolved to protect multicellular organisms from microbes in KEY CONCEPTS the environment, and the responses they trigger are critical GENERAL FEATURES AND CAUSES OF for the survival of the organisms. Several cellular receptors INFLAMMATION and circulating proteins are capable of recognizing microbes Inflammation is a beneficial host response to foreign invaders and products of cell damage and triggering inflammation. and necrotic tissue, but it may also cause tissue damage. ellular receptors for microbes. Cells express receptors in The main components of inflammation are a vascular reaction the plasma membrane (for extracellular microbes), the and a cellular response, both activated by mediators that are endosomes (for ingested microbes), and the cytosol (for derived from plasma proteins and various cells. intracellular microbes) that enable the cells to sense the The steps of the inflammatory response can be remembered presence of foreign invaders in any cellular compartment. as the five R’s: (1) recognition of the injurious agent, (2) recruit- The best defined of these receptors belong to the family ment of leukocytes, (3) removal of the agent, (4) regulation of Toll-like receptors (TLRs); these and other cellular recep- (control) of the response, and (5) repair (resolution). tors of innate immunity are described in Chapter 6. The Acute and chronic inflammation differ in the kinetics of the receptors are expressed on many cell types including reaction, the principal cells involved, and the degree of injury. epithelial cells (through which microbes enter from the The outcome of acute inflammation is either elimination of the external environment), dendritic cells, macrophages, and noxious stimulus followed by decline of the reaction and repair other leukocytes (which may encounter microbes in of the damaged tissue or persistent injury resulting in chronic various tissues). Engagement of these receptors triggers inflammation. production of molecules involved in inflammation includ- Causes of inflammation include infections, tissue necrosis, foreign ing adhesion molecules on endothelial cells, cytokines, bodies, trauma, and immune responses. and other mediators. Epithelial cells, tissue macrophages and dendritic cells, leukocytes, ensors of cell damage. All cells have cytosolic receptors, and other cell types express receptors that sense the presence such as NOD-like receptors (NLRs), that recognize diverse of microbes and substances released from damaged cells. Circulat- molecules that are liberated or altered as a consequence ing proteins recognize microbes that have entered the blood. of cell damage. These molecules include uric acid (a  Acute inflammation 75 implies the existence of an inflammatory process that has ACUTE INFLAMMATION increased the permeability of small blood vessels. In contrast, a transudate is a fluid with low protein content (most of which cute inflammation has three major components ( ) is albumin), little or no cellular material, and low specific dilation of small vessels leading to an increase in blood gravity. It is essentially an ultrafiltrate of blood plasma that flow; (2) increased permeability of the microvasculature is produced as a result of osmotic or hydrostatic imbalance enabling plasma proteins and leukocytes to leave the across the vessel wall without an increase in vascular perme- circulation; and ( ) emigration of leukocytes from the ability (Chapter 4). Edema denotes an excess of fluid in the microcirculation their accumulation in the focus of injury interstitial tissue or serous cavities; it can be either an exudate and their activation to eliminate the offending agent (see or a transudate. Pus, a purulent exudate, is an inflammatory Fig. 3.1). When an individual encounters an injurious agent, exudate rich in leukocytes (mostly neutrophils), the debris such as a microbe or dead cells, phagocytes that reside in of dead cells, and, in many cases, microbes. tissues try to eliminate these agents. At the same time, phagocytes and other sentinel cells in the tissues recognize Changes in Vascular Flow and Caliber the presence of the foreign or abnormal substance and react Changes in vascular flow and caliber begin early after injury by liberating cytokines, lipid messengers, and other mediators and consist of the following. of inflammation. Some of these mediators act on small blood Vasodilation is induced by the action of several mediators, vessels in the vicinity and promote the efflux of plasma and notably histamine, on vascular smooth muscle. It is one the recruitment of circulating leukocytes to the site where of the earliest manifestations of acute inflammation. the offending agent is located. Vasodilation first involves the arterioles and then leads to opening of new capillary beds in the area. The result Reactions of Blood Vessels in Acute Inflammation is increased blood flow, which is the cause of heat and redness (erythema) at the site of inflammation. The vascular reactions of acute inflammation consist of Vasodilation is quickly followed by increased permeability changes in the flow of blood and the permeability of the microvasculature, with the outpouring of protein-rich of vessels both designed to ma imi e the movement of fluid into the extravascular tissues; this process is plasma proteins and leukocytes out of the circulation and described in detail later. into the site of infection or injury The escape of fluid, The loss of fluid and increased vessel diameter lead to proteins, and blood cells from the vascular system into the slower blood flow, concentration of red cells in small interstitial tissue or body cavities is known as exudation vessels, and increased viscosity of the blood. These (Fig. 3.2). An exudate is an extravascular fluid that has a high changes result in engorgement of small vessels with slowly protein concentration and contains cellular debris. Its presence moving red cells, a condition termed stasis, which is seen Hydrostatic Colloid osmotic pressure pressure A. NORMAL Plasma proteins Fluid and protein leakage B. EXUDATE (high protein content, and Inflammation Vasodilation and stasis may contain some white and red cells) Increased interendothelial spaces Increased hydrostatic pressure Fluid leakage Decreased colloid osmotic (venous outflow obstruction, pressure (decreased protein [e.g., congestive heart failure]) synthesis [e.g.,liver disease]; increased protein loss [e.g., C. TRANSUDATE kidney disease]) (low protein content, few cells) Figure 3.2 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), which is equal to the mean capillary pressure. Therefore the net flow of fluid across the vascular bed is almost nil. (B) An exudate is formed in inflammation because vascular permeability increases as a result of increased interendothelial spaces. (C) A transudate is formed when fluid leaks out because of increased hydrostatic pressure or decreased osmotic pressure. 76 CHAPTER 3 Inflammation and Repair as vascular congestion and localized redness of the ndothelial in ury resulting in endothelial cell necrosis involved tissue. and detachment. Direct damage to the endothelium is As stasis develops, blood leukocytes, principally neutro- encountered in severe physical injuries, for example, in phils, accumulate along the vascular endothelium. At thermal burns, or is induced by the actions of microbes the same time, endothelial cells are activated by mediators and microbial toxins that damage endothelial cells. produced at sites of infection and tissue damage and Neutrophils that adhere to the endothelium during express increased levels of adhesion molecules. Leukocytes inflammation may also injure endothelial cells and thus then adhere to the endothelium, and soon afterward they amplify the reaction. In most instances leakage starts migrate through the vascular wall into the interstitial immediately after injury and is sustained for several hours tissue in a sequence that is described later. until the damaged vessels are thrombosed or repaired. Increased Vascular Permeability (Vascular Leakage) Although these mechanisms of increased vascular perme- Several mechanisms are responsible for the increased perme- ability are described separately, all probably contribute in ability of postcapillary venules, a hallmark of acute inflam- varying degrees in responses to most stimuli. For example, mation (Fig. 3.3). at different stages of a thermal burn, leakage results from Contraction of endothelial cells resulting in opening of chemically mediated endothelial contraction and direct and interendothelial gaps is the most common mechanism of leukocyte-dependent endothelial injury. The vascular leakage vascular leakage. It is elicited by histamine, bradykinin, induced by these mechanisms can cause life-threatening leukotrienes, and other chemical mediators. It is called loss of fluid in severely burned patients. the immediate transient response because it occurs rapidly after exposure to the mediator and is usually short-lived Responses of Lymphatic Vessels and Lymph Nodes (15 to 30 minutes). In some forms of mild injury (e.g., after In addition to blood vessels, lymphatic vessels also participate burns, irradiation or ultraviolet radiation, and exposure in acute inflammation. The system of lymphatics and lymph to certain bacterial toxins), vascular leakage begins after nodes filters and polices the extravascular fluids. Lymphatics a delay of 2 to 12 hours and lasts for several hours or even drain the small amount of extravascular fluid that seeps days; this delayed prolonged leakage may be caused by out of capillaries in the healthy state. In inflammation, lymph contraction of endothelial cells or mild endothelial damage. flow is increased and helps drain edema fluid that accu- Sunburn is a classic example of damage that results in mulates because of increased vascular permeability. In late-appearing vascular leakage. Often the immediate and addition to fluid, leukocytes and cell debris, as well as delayed responses occur along a continuum. microbes, may find their way into lymph. Lymphatic vessels, like blood vessels, proliferate during inflammatory reactions to handle the increased load. The lymphatics may become A. NORMAL Vessel lumen secondarily inflamed (lymphangitis), as may the draining Leukocytes lymph nodes (lymphadenitis). Inflamed lymph nodes are often enlarged because of hyperplasia of the lymphoid Plasma proteins follicles and increased numbers of lymphocytes and mac- Endothelium rophages. This constellation of pathologic changes is termed reactive, or inflammatory, lymphadenitis (Chapter 13). The presence of red streaks near a skin wound is a telltale sign Tissues of bacterial infection. The streaks represent inflamed lym- phatic channels and are diagnostic of lymphangitis; it may B. RETRACTION OF be accompanied by painful enlargement of the draining ENDOTHELIAL lymph nodes, indicating lymphadenitis. CELLS Induced by histamine, other mediators Rapid and short-lived KEY CONCEPTS (minutes) VASCULAR REACTIONS IN ACUTE INFLAMMATION asodilation is induced by chemical mediators such as histamine C. ENDOTHELIAL INJURY (described later) and is the cause of erythema and increased blood flow. Caused by thermal burns, Increased vascular permeability is induced by histamine, kinins, some microbial toxins and other mediators that produce gaps between endothelial Rapid; may be long-lived cells and by direct or leukocyte-induced endothelial injury. (hours to days) Increased vascular permeability allows plasma proteins and leukocytes, the mediators of host defense, to enter sites of infection or tissue damage. Fluid leak from blood vessels results in edema. ymphatic vessels and lymph nodes are also involved in inflam- Figure 3.3 Principal mechanisms of increased vascular permeability in mation and often show redness and swelling. inflammation and their features and underlying causes.  Acute inflammation 77 Leukocyte Recruitment to Sites of Inflammation chemokines This process can be divided into sequential phases (Fig. 3.4). The changes in blood flow and vascular permeability are 1. In the lumen: margination, rolling, and adhesion to endothe- uickly followed by an influ of leukocytes into the tissue lium. Vascular endothelium in its normal state does not These leukocytes perform the key function of eliminating bind circulating cells or allow their passage. In inflam- the offending agents. The most important leukocytes in mation the endothelium is activated and can bind leu- typical inflammatory reactions are the ones capable of kocytes as a prelude to their exit from blood vessels. phagocytosis, namely neutrophils and macrophages. They 2. Migration across the endothelium and vessel wall. ingest and destroy bacteria and other microbes, as well as 3. Migration in the tissues toward a chemotactic stimulus. necrotic tissue and foreign substances. Macrophages also produce growth factors that aid in repair. A price that is Leukocyte Adhesion to Endothelium paid for the defensive potency of leukocytes is that, when In normally flowing blood in venules, red cells are confined activated, they may induce tissue damage and prolong the to a central axial column, displacing the leukocytes toward inflammatory reaction because the leukocyte products that the wall of the vessel. Because of dilation of inflamed destroy microbes and help “clean up” necrotic tissues can postcapillary venules, blood flow slows (stasis), and more also injure normal bystander host tissues. white cells assume a peripheral position along the endothelial The journey of leukocytes from the vessel lumen to surface. This process of leukocyte redistribution is called the tissue is a multistep process that is mediated and margination. The slowed leukocytes sense signals from the controlled by adhesion molecules and cytokines called endothelium, resulting first in the cells rolling on the vessel ROLLING INTEGRIN ACTIVATION BY CHEMOKINES Leukocyte Sialyl–Lewis X–modified glycoprotein STABLE ADHESION Integrin (low-affinity state) MIGRATION THROUGH ENDOTHELIUM Integrin (high-affinity state) 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) Figure 3.4 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. ICAM-1, Intercellular adhesion molecule 1; IL-1, interleukin-1; PECAM-1, platelet endothelial cell adhesion molecule (also known as CD31); TNF, tumor necrosis factor. 78 CHAPTER 3 Inflammation and Repair wall and then recognizing adhesion molecules expressed complementary molecules on the endothelial cells. These on the endothelium that lead to the cells adhering firmly are low-affinity interactions with a fast off-rate, so they (resembling pebbles over which a stream runs without are easily disrupted by the flowing blood. As a result, disturbing them). the bound leukocytes bind, detach, and bind again and The attachment of leukocytes to endothelial cells is thus begin to roll along the endothelial surface. mediated by adhesion molecules whose e pression is Integrins. The weak rolling interactions slow down the enhanced by cytokines which are secreted by sentinel cells leukocytes and give them the opportunity to bind more in tissues in response to microbes and other injurious agents, firmly to the endothelium. Firm adhesion is mediated thus ensuring that leukocytes are recruited to the tissues by a family of heterodimeric leukocyte surface proteins where these stimuli are present. The two major families of called integrins (see Table 3.3). TNF and IL-1 induce proteins involved in leukocyte adhesion and migration are endothelial expression of ligands for integrins, mainly the selectins and integrins and their ligands (Table 3.3). vascular cell adhesion molecule 1 (VCAM-1), the ligand They are expressed on leukocytes and endothelial cells. for the β1 integrin VLA-4, and intercellular adhesion electins. The initial rolling interactions are mediated by molecule-1 (ICAM-1), the ligand for the β2 integrins LFA-1 selectins, of which there are three types: one expressed and MAC-1. Leukocytes normally express integrins in a on leukocytes (L-selectin), one on endothelium (E-selectin), low-affinity state. Chemokines that were produced at and one in platelets and on endothelium (P-selectin) (see the site of injury bind to endothelial cell proteoglycans Table 3.3). The ligands for selectins are sialylated oligosac- and are displayed at high concentrations on the endo- charides bound to mucin-like glycoproteins. The expres- thelial surface. These chemokines bind to and activate sion of selectins and their ligands is regulated by cytokines the rolling leukocytes. One of the consequences of activa- produced in response to infection and injury. Tissue tion is the conversion of VLA-4 and LFA-1 integrins on macrophages, mast cells, and endothelial cells that the leukocytes to a high-affinity state. The combination encounter microbes and dead tissues respond by secreting of cytokine-induced expression of integrin ligands on several cytokines including tumor necrosis factor (TNF), the endothelium and increased integrin affinity on the IL-1, and chemokines (chemoattractant cytokines). (Cyto- leukocytes results in firm integrin-mediated adhesion of kines are described in more detail later and in Chapter the leukocytes to the endothelium at the site of inflam- 6.) TNF and IL-1 act on the endothelial cells of postcapil- mation. The leukocytes stop rolling, their cytoskeleton lary venules adjacent to the infection and induce the is reorganized, and they spread out on the endothelial coordinate expression of numerous adhesion molecules. surface. Within 1 to 2 hours the endothelial cells begin to express E-selectin and the ligands for L-selectin. Other mediators Leukocyte Migration Through Endothelium such as histamine and thrombin, described later, stimulate The ne t step in the process of leukocyte recruitment is the redistribution of P-selectin from its normal intracellular migration of the leukocytes through intact endothelium stores in endothelial cell granules (called Weibel-Palade called transmigration or diapedesis Transmigration of bodies) to the cell surface. Leukocytes express L-selectin leukocytes occurs mainly in postcapillary venules. Chemo- at the tips of their microvilli and also express ligands for kines act on the adherent leukocytes and stimulate the cells E-selectin and P-selectin, all of which bind to the to migrate through interendothelial gaps toward the chemical Table 3.3 Endothelial and Leukocyte Adhesion Molecules Family Molecule Distribution Ligand Selectin L-selectin (CD62L) Neutrophils, monocytes Sialyl-Lewis X/PNAd on GlyCAM-1, CD34, MAdCAM- T cells (naïve and central memory) 1, others; expressed on endothelium (HEV) B cells (naïve) E-selectin (CD62E) Endothelium activated by Sialyl-Lewis X (e.g., CLA) on glycoproteins; expressed cytokines (TNF, IL-1) on neutrophils, monocytes, T cells (effector, memory) P-selectin (CD62P) Endothelium activated by Sialyl-Lewis X on PSGL-1 and other glycoproteins; cytokines (TNF, IL-1), histamine, expressed on neutrophils, monocytes, T cells or thrombin (effector, memory) Integrin LFA-1 (CD11aCD18) Neutrophils, monocytes, T cells ICAM-1 (CD54), ICAM-2 (CD102); expressed on (naïve, effector, memory) endothelium (upregulated on activated endothelium) MAC-1 Monocytes, DCs ICAM-1 (CD54), ICAM-2 (CD102); expressed on (CD11bCD18) endothelium (upregulated on activated endothelium) VLA-4 (CD49aCD29) Monocytes VCAM-1 (CD106); expressed on endothelium T cells (naïve, effector, memory) (upregulated on activated endothelium) α4β7 (CD49dCD29) Monocytes VCAM-1 (CD106), MAdCAM-1; expressed on T cells (gut homing naïve effector, endothelium in gut and gut-associated lymphoid memory) tissues Ig CD31 Endothelial cells, leukocytes CD31 (homotypic interaction) CLA, Cutaneous lymphocyte antigen-1; GlyCAM-1, glycan-bearing cell adhesion molecule-1; HEV, high endothelial venule; Ig, immunoglobulin; IL-1, interleukin-1; ICAM, intercellular adhesion molecule; MAdCAM-1, mucosal adhesion cell adhesion molecule-1; PSGL-1, P-selectin glycoprotein ligand-1; TNF, tumor necrosis factor; VCAM, vascular cell adhesion molecule.  Acute inflammation 79 concentration gradient, that is, toward the site of injury or infection where the chemokines are being produced. 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 CD31 or PECAM-1 (platelet endothelial cell adhesion molecule). After traversing the endothelium, leukocytes pierce the basement membrane, probably by secreting collagenases, and enter the extravas- cular space. After leukocytes pass through, the basement membranes become continuous again. The cells that have exited the vessel then migrate toward the chemotactic gradient created by chemokines and other chemoattractants and accumulate in the extravascular site. The most telling proof of the importance of leukocyte adhesion molecules in the host inflammatory response are genetic deficiencies in these molecules, which result in increased susceptibility to bacterial infections. These leu- kocyte adhesion deficiencies are described in Chapter 6. Chemotaxis of Leukocytes fter e iting the circulation leukocytes move in the tissues toward the site of injury by a process called che- motaxis, which is defined as locomotion along a chemical gradient. Both exogenous and endogenous substances act as chemoattractants. The most common exogenous factors are bacterial products, including peptides with Figure 3.5 Scanning electron micrograph of a moving leukocyte in culture N-formylmethionine terminal amino acids and some lipids. showing a filopodium (upper left) and a trailing tail. (Courtesy Dr. Morris J. Endogenous chemoattractants include several chemical Karnovsky, Harvard Medical School, Boston, Mass.) 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 of cellular infiltration. In certain infections—for example, (LTB4). All these chemotactic agents bind to specific seven- those produced by Pseudomonas bacteria—the cellular transmembrane G protein–coupled receptors on the surface infiltrate is dominated by continuously recruited neutrophils of leukocytes. Signals initiated from these receptors result in for several days; in viral infections, lymphocytes may be activation of second messengers that induce polymerization the first cells to arrive; some hypersensitivity reactions are of actin at the leading edge of the cell and localization of dominated by activated lymphocytes, macrophages, and myosin filaments at the back. This reorganization of the plasma cells (reflecting the immune response); and in hel- cytoskeleton allows the leading edge of the leukocyte to minthic infections and allergic reactions, eosinophils may extend filopodia that pull the back of the cell in the direc- be the main cell type. tion of extension, much as an automobile with front-wheel The molecular understanding of leukocyte recruitment drive is pulled by the wheels in front (Fig. 3.5). The net and migration has led to development of a large number of result is that leukocytes migrate in the direction of locally drugs for controlling harmful inflammation, including agents produced chemoattractants emanating from the site of the that block TNF (discussed later), and antagonists of leukocyte inflammatory stimulus. integrins that are approved for inflammatory diseases or are The nature of the leukocyte infiltrate varies with the being tested in clinical trials. Predictably, these antagonists age of the inflammatory response and the type of stimulus not only have the desired effect of controlling the inflamma- In most forms of acute inflammation, neutrophils predomi- tion but can also compromise the ability of treated patients nate in the inflammatory infiltrate during the first 6 to 24 to defend themselves against microbes, which, of course, hours and are replaced by monocytes in 24 to 48 hours (Fig. is the physiologic function of the inflammatory response. 3.6). There are several reasons for the early preponderance of neutrophils: they are more numerous than are other leukocytes, respond more rapidly to chemokines, and may attach more firmly to the adhesion molecules that are rapidly KEY CONCEPTS induced on endothelial cells such as P-selectin and E-selectin. LEUKOCYTE RECRUITMENT TO SITES OF After entering tissues, neutrophils are short-lived; most INFLAMMATION neutrophils in extravascular tissues undergo apoptosis within eukocytes are recruited from the blood into the extravascular a few days. Monocytes not only survive longer but may tissue, where infectious pathogens or damaged tissues may be also proliferate in the tissues, and thus they become the located, migrate to the site of infection or tissue injury, and dominant population in prolonged inflammatory reactions. are activated to perform their functions. There are, however, exceptions to this stereotypic pattern 80 CHAPTER 3 Inflammation and Repair Monocytes/ Edema Neutrophils Macrophages ACTIVITY B 1 2 3 A C DAYS Figure 3.6 Nature of leukocyte infiltrates in inflammatory reactions. The photomicrographs show an inflammatory reaction in the myocardium after ischemic necrosis (infarction). (A) Early (neutrophilic) infiltrates and congested blood vessels. (B) Later (mononuclear) cellular infiltrates. (C) The approximate kinetics of edema and cellular infiltration. For simplicity, edema is shown as an acute transient response, although secondary waves of delayed edema and neutrophil infiltration can also occur. eukocyte recruitment is a multistep process consisting of loose Phagocytosis attachment to and rolling on endothelium (mediated by selectins), hagocytosis involves se uential steps (Fig. 3.8): firm attachment to endothelium (mediated by integrins), and Recognition and attachment of the particle to be ingested migration through interendothelial spaces. by the leukocyte; arious cytokines promote expression of selectins and integrin Engulfment, with subsequent formation of a phagocytic ligands on endothelium (e.g., TNF, IL-1), increase the avidity of vacuole; and integrins for their ligands (e.g., chemokines), and promote Killing of the microbe and degradation of the ingested directional migration of leukocytes (also chemokines); many of material. these cytokines are produced by tissue macrophages and other cells responding to the pathogens or damaged tissues. Phagocytic Receptors. Mannose receptors, scavenger Neutrophils predominate in the early inflammatory infiltrate receptors, and receptors for various opsonins enable phago- and are later replaced by monocytes and macrophages. cytes 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 Once leukocytes (particularly neutrophils and monocytes) typically part of molecules found on microbial cell walls, are recruited to a site of infection or cell death, they must whereas mammalian glycoproteins and glycolipids contain be activated to perform their functions. The responses of terminal sialic acid or N-acetylgalactosamine. Therefore the these leukocytes consist of recognition of the offending agents mannose receptor recognizes microbes and not host cells. by TLRs and other receptors, described earlier, which deliver Scavenger receptors were originally defined as molecules signals that activate the leukocytes to phagocytose and that bind and mediate endocytosis of oxidized or acetylated destroy the offending agents. low-density lipoprotein (LDL) particles that do not interact with the conventional LDL receptor. Macrophage scavenger Phagocytosis and Clearance of the receptors bind a variety of microbes in addition to modified Offending Agent LDL particles. Macrophage integrins, notably MAC-1 (CD11b/CD18), may also bind microbes for phagocytosis. The two major phagocytes are neutrophils and macro- The efficiency of phagocytosis is greatly enhanced when phages Although these cell types share many functional microbes are coated with opsonins for which the phagocytes properties, they also differ in significant ways (Table 3.4). express high-affinity receptors. The major opsonins are Recognition of microbes or dead cells induces several immunoglobulin G (IgG) antibodies, the C3b breakdown responses in leukocytes that are collectively called leukocyte product of complement, and certain plasma lectins, notably activation (Fig. 3.7). Activation results from signaling path- mannose-binding lectin and collectins, all of which are ways that are triggered in leukocytes, resulting in increases recognized by specific receptors on leukocytes. in cytosolic Ca2+ and activation of enzymes such as protein kinase C and phospholipase A2. The functional responses that Engulfment. After a particle is bound to phagocyte receptors, are most important for destruction of microbes and other extensions of the cytoplasm flow around it, and the plasma offenders are phagocytosis and intracellular killing. Several membrane pinches off to form an intracellular vesicle other responses aid in the defensive functions of inflam- (phagosome) that encloses the particle. The phagosome then mation and may contribute to its injurious consequences. fuses with a lysosomal granule, which discharges its contents  Acute inflammation 81 Table 3.4 Properties of Neutrophils and Macrophages Neutrophils Macrophages Origin HSCs in bone marrow HSCs in bone marrow (in inflammatory reactions) Many tissue-resident macrophages: stem cells in yolk sac or fetal liver (early in development) Lifespan in tissues Several days Inflammatory macrophages: days or weeks Tissue-resident macrophages: years Responses to activating Rapid, short-lived, mostly degranulation More prolonged, slower, often dependent on new stimuli and enzymatic activity gene transcription Reactive oxygen species Rapidly induced by assembly of Less prominent phagocyte 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 No contents Secretion of lysosomal Prominent Less enzymes HSC, Hematopoietic stem cells; iNOS, inducible nitric oxide synthase; NET, neutrophil extracellular trap. This table lists the major differences between neutrophils and macrophages. The reactions summarized above are described in the text. Note that the two cell types share many features such as phagocytosis, ability to migrate through blood vessels into tissues, and chemotaxis. into the phagolysosome (see Fig. 3.8). During this process not surprising that the signals that trigger phagocytosis are the phagocyte may also release lysosome contents into the many of the same that are involved in chemotaxis. extracellular space. The process of phagocytosis is complex and involves the Intracellular Destruction of Microbes and Debris integration of many receptor-initiated signals that lead to mem- illing of microbes is accomplished by reactive o ygen brane remodeling and cytoskeletal changes. Phagocytosis is species (R ) also called reactive o ygen intermediates dependent on polymerization of actin filaments; it is therefore and reactive nitrogen species mainly derived from nitric Microbe Chemokines Cytokines N-formyl- Lipid Toll-like (e.g., IFN-γ) methionyl mediators LPS receptor peptides G-protein coupled CD14 Cytokine Recognition receptors Phagocytic receptor of microbes, receptor 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 Figure 3.7 Leukocyte activation. 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). Lipopolysaccharide (LPS) first binds to a circulating LPS-binding protein (not shown). IFN-γ, Interferon-γ. 82 CHAPTER 3 Inflammation and Repair 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 Cytoplasmic Primary oxidase granule MPO MPO + Cl– NADPH O2 Active oxidase NADP+ iNOS O2 H2O2 OCl Arginine Membrane Fe++ NO oxidase OH ROS Membrane Phagocyte oxidase O2 PHAGOCYTIC VACUOLE Figure 3.8 Phagocytosis and intracellular destruction of microbes. Phagocytosis of a particle (e.g., a bacterium) 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. Hypochlorite (HOCl˙) and hydroxyl radical (˙OH) are microbicidal products generated from superoxide (O2 ), and peroxynitrite (OONO˙) is generated from nitric oxide (NO). During phagocytosis, granule contents may be released into extracellular tissues (not shown). iNOS, Inducible nitric oxide synthase; MPO, myeloperoxidase; ROS, reactive oxygen species. o ide ( ) and these as well as lysosomal en ymes destroy enzyme complex consisting of at least seven proteins. In phagocytosed materials (see Fig. 3.8). This is the final step resting neutrophils, different components of the enzyme are in the elimination of infectious agents and necrotic cells. located in the plasma membrane and the cytoplasm. In The killing and degradation of microbes and dead cell debris response to activating stimuli, the cytosolic protein compo- within neutrophils and macrophages occur most efficiently nents translocate to the phagosomal membrane, where they after activation of the phagocytes. All these killing mecha- assemble and form the functional enzyme complex. Thus, nisms are normally sequestered in lysosomes, to which the ROS are produced within the phagolysosome, where phagocytosed materials are brought. Thus, potentially they can act on ingested particles without damaging the harmful substances are segregated from the cell’s cytoplasm host cell. O 2 is converted into hydrogen peroxide (H2O2), and nucleus to avoid damage to the phagocyte while it is mostly by spontaneous dismutation. H2O2 is not able to performing its normal function. efficiently kill microbes by itself. However, the azurophilic granules of neutrophils contain the enzyme myeloperoxidase Reactive Oxygen Species. ROS are produced by the rapid (MPO), which, in the presence of a halide such as Cl−, converts assembly and activation of a multicomponent oxidase, H2O2 to hypochlorite (HOCl ), the active ingredient in NADPH oxidase (also called phagocyte oxidase), which household bleach. The latter is a potent antimicrobial agent oxidizes reduced nicotinamide-adenine dinucleotide phos- that destroys microbes by halogenation (in which the halide phate (NADPH) and, in the process, reduces oxygen to is bound covalently to cellular constituents) or by oxidation superoxide anion (O 2 ). In neutrophils, this oxidative reaction of proteins and lipids (lipid peroxidation). The H2O2-MPO- is triggered by activating signals accompanying phagocytosis halide system is the most potent bactericidal system of and is called the respiratory burst. Phagocyte oxidase is an neutrophils. Nevertheless, inherited deficiency of MPO by  Acute inflammation 83 itself leads to minimal increase in susceptibility to infection, engulfed material, or the granule contents can be released emphasizing the redundancy of microbicidal mechanisms into the extracellular space during “frustrated phagocytosis” in leukocytes. H2O2 is also converted to hydroxyl radical (discussed later). ( OH), another powerful destructive agent. As discussed in Different granule enzymes serve different functions. Acid Chapter 2, these oxygen-derived free radicals bind to and proteases degrade bacteria and debris within the phagoly- modify cellular lipids, proteins, and nucleic acids and thus sosomes, which are acidified by membrane-bound proton destroy cells such as microbes. pumps. Neutral proteases are capable of degrading various Oxygen-derived radicals may be released extracellularly extracellular components such as collagen, basement mem- from leukocytes after exposure to microbes, chemokines, brane, fibrin, elastin, and cartilage, resulting in the tissue and antigen-antibody complexes or following a phagocytic destruction that accompanies inflammatory processes. challenge. These ROS are implicated in tissue damage Neutral proteases can also cleave C3 and C5 complement accompanying inflammation. proteins and release a kinin-like peptide from kininogen. Plasma, tissue fluids, and host cells possess antioxidant The released components of complement and kinins act as mechanisms that protect healthy cells from these potentially mediators of acute inflammation (discussed later). Neutrophil harmful oxygen-derived radicals. These antioxidants are elastase has been shown to degrade virulence factors of discussed in Chapter 2 and include (1) the enzyme superoxide bacteria and thus combat bacterial infections. Macrophages dismutase, which is found in, or can be activated in, a variety also contain acid hydrolases, collagenase, elastase, phos- of cell types; (2) the enzyme catalase, which detoxifies H2O2; pholipase, and plasminogen activator. (3) glutathione peroxidase, another powerful H2O2 detoxifier; Because of the destructive effects of lysosomal enzymes, (4) the copper-containing plasma protein ceruloplasmin; the initial leukocytic infiltration, if unchecked, can potentiate and (5) the iron-free fraction of plasma transferrin. further inflammation by damaging tissues. These harmful Inherited deficiencies of components of phagocyte oxidase proteases, however, are normally controlled by a system of cause an immunodeficiency disease called chronic granu- antiproteases in the serum and tissue fluids. Foremost among lomatous disease (CGD), which is discussed in Chapter 6. these is α1-antitrypsin, which is the major inhibitor of neutrophil elastase. A deficiency of these inhibitors may Nitric Oxide. NO, a soluble gas produced from arginine lead to sustained action of leukocyte proteases, as is the by the action of nitric oxide synthase (NOS), also participates case in patients with α1-antitrypsin deficiency, who are at in microbial killing. There are three different types of NOS: risk for emphysema due to destruction of elastic support endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). fibers in the lung because of uncontrolled elastase activity eNOS and nNOS are constitutively expressed at low levels, (Chapter 15). α2-Macroglobulin is another antiprotease found and the NO they generate functions to maintain vascular in serum and various secretions. tone and as a neurotransmitter, respectively. iNOS, the type Other microbicidal granule contents include defensins, that is involved in microbial killing, is induced when cationic arginine-rich granule peptides that are toxic to macrophages (and, to a lesser extent, neutrophils) are microbes; cathelicidins, antimicrobial proteins found in activated by cytokines (e.g., interferon-γ [IFN-γ]) or microbial neutrophils and other cells; lysozyme, which hydrolyzes products. In macrophages, NO reacts with superoxide the muramic acid-N-acetylglucosamine bond found in the (O 2 ) to generate the highly reactive free radical peroxynitrite glycopeptide coat of all bacteria; lactoferrin, an iron-binding (ONOO−). These nitrogen-derived free radicals, similar to protein present in specific granules; and major basic protein, ROS, attack and damage the lipids, proteins, and nucleic a cationic protein of eosinophils, which has limited bacte- acids of microbes (Chapter 2). Reactive oxygen and nitrogen ricidal activity but is cytotoxic to many helminthic species have overlapping actions, as shown by the observa- parasites. tion that knockout mice lacking either phagocyte oxidase or iNOS are only mildly susceptible to infections, but mice Neutrophil Extracellular Traps lacking both succumb rapidly to disseminated infections eutrophil e tracellular traps ( ETs) are e tracellular by normally harmless commensal bacteria. fibrillar networks that concentrate antimicrobial substances In addition to its role as a microbicidal substance, NO at sites of infection and trap microbes helping to prevent relaxes vascular smooth muscle and promotes vasodilation. their spread They are produced by neutrophils in response It is not clear if this action of NO plays an important role to infectious pathogens (mainly bacteria and fungi) and in the vascular reactions of acute inflammation. inflammatory mediators (e.g., chemokines, cytokines [mainly interferons], complement proteins, and ROS). The extracel- Lysosomal Enzymes and Other Lysosomal Proteins. Neu- lular traps consist of a viscous meshwork of nuclear trophils and macrophages contain lysosomal granules that chromatin that binds and concentrates granule proteins such contribute to microbial killing and, when released, may as antimicrobial peptides and enzymes (Fig. 3.9). NET forma- cause tissue damage. Neutrophils have two main types tion starts with ROS-dependent activation of an arginine of granules. The smaller specific (or secondary) granules deaminase that converts arginines to citrulline, leading to contain lysozyme, collagenase, gelatinase, lactoferrin, plas- chromatin decondensation. Other enzymes that are produced minogen activator, histaminase, and alkaline phosphatase. in activated neutrophils, such as MPO and elastase, enter The larger azurophil (or primary) granules contain MPO, the nucleus and cause further chromatin decondensation, bactericidal proteins (lysozyme, defensins), acid hydrolases, culminating in rupture of the nuclear envelope and release and a variety of neutral proteases (elastase, cathepsin G, of chromatin. In this process, the nuclei of the neutrophils nonspecific collagenases, proteinase 3). Both types of are lost, leading to death of the cells. NETs have also been granules can fuse with phagocytic vacuoles containing detected in the blood during sepsis. The nuclear chromatin 84 CHAPTER 3 Inflammation and Repair A B C Figure 3.9 Neutrophil extracellular traps (NETs). (A) Healthy neutrophils with nuclei stained red and cytoplasm stained green. (B) Release of nuclear material from neutrophils (note that two have lost their nuclei), forming extracellular traps. (C) 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, with permission.) in the NETs, which includes histones and associated DNA, released substances are capable of damaging host cells such has been postulated to be a source of nuclear antigens in as vascular endothelium and may thus amplify the effects of systemic autoimmune diseases, particularly lupus, in which the initial injurious agent. If unchecked or inappropriately individuals react against their own DNA and nucleoproteins directed against host tissues, the leukocyte infiltrate itself (Chapter 6). becomes the offender, and indeed leukocyte-dependent inflammation and tissue injury underlie many acute and Leukocyte-Mediated Tissue Injury chronic human diseases (see Table 3.1). This fact becomes eukocytes are important causes of injury to normal cells evident in the discussion of specific disorders throughout and tissues under several circumstances this book. As part of a normal defense reaction against infectious The contents of lysosomal granules are secreted by microbes, when adjacent tissues suffer collateral damage. leukocytes into the extracellular milieu by several mecha- In some infections that are difficult to eradicate, such as nisms. Controlled secretion of granule contents is a normal tuberculosis and certain viral diseases, the prolonged response of activated leukocytes. If phagocytes encounter host response contributes more to the pathology than materials that cannot be easily ingested, such as immune does the microbe itself. complexes deposited on large surfaces (e.g., glomerular When the inflammatory response is inappropriately basement membrane), the inability of the leukocytes to directed against host tissues, as in certain autoimmune surround and ingest these substances (frustrated phagocy- diseases. tosis) triggers strong activation and the release of lysosomal When the host reacts excessively against usually harmless enzymes into the extracellular environment. Some phago- environmental substances, as in allergic diseases, including cytosed substances, such as urate crystals, may damage the asthma. membrane of the phagolysosome, also leading to the release of lysosomal granule contents. In all these situations, the mechanisms by which leukocytes damage normal tissues are the same as the Other Functional Responses of Activated Leukocytes mechanisms involved in antimicrobial defense because once In addition to eliminating microbes and dead cells, activated the leukocytes are activated, their effector mechanisms do leukocytes play several other roles in host defense. Impor- not distinguish between offender and host. During activation tantly, these cells, especially macrophages, produce cytokines and phagocytosis, neutrophils and macrophages produce that can either amplify or limit inflammatory reactions, microbicidal substances (ROS, NO, and lysosomal enzymes) growth factors that stimulate the proliferation of endothelial within the phagolysosome; under some circumstances, these cells and fibroblasts and the synthesis of collagen, and substances are also released into the extracellular space. These enzymes that remodel connective tissues. Because of these