Immune System 1 - Inflammation PDF

PAGE 1 UNIVERSITÄTSMEDIZIN NEUMARKT A. M. https://edu.umch.de www.umfst.ro CAMPUS HAMBURG 2024 May The immune system. Inflammation. The immune system...

PAGE 1 UNIVERSITÄTSMEDIZIN NEUMARKT A. M. https://edu.umch.de www.umfst.ro CAMPUS HAMBURG 2024 May The immune system. Inflammation. The immune system PAGE 2 Definition: the ability to resist almost all types of offending agents that tend to damage the tissues and organs is called immunity. UNIVERSITY OF SURREY Sneezing spreads influenza virus. (Courtesy American Association for the Advancement of Science.) Immunology for Medical Students Helbert, Matthew, MBChB, FRCP, FRCPath, PhD, Elsevier The immune system PAGE 3 Classification of the immunity: 1. Innate immunity - general processes, rather than processes directed at specific disease organisms: A. humoral component B. cellular component 2. Acquired immunity that does not develop until after the body is first attacked by a bacterium, virus, or toxin; often weeks or months are required for the immunity to develop. A. humoral component B. cellular component Humoral immunity/component acts through molecules secreted in the body fluids. Cellular immunity/component kills pathogens directly by immune cells. The immune system PAGE 4 Innate immunity - also called natural, or native, immunity is mediated by cells and proteins that are always present (hence the term innate), to react against infectious pathogens. – These mechanisms are called into action immediately in response to infection, and thus provide the first line of defense. Some of these mechanisms also are involved in clearing damaged cells and tissues. – A major reaction of innate immunity is inflammation. Adaptive immunity - is normally silent and responds (or adapts) to the presence of infectious agents by generating potent mechanisms for neutralizing and eliminating the pathogens. Innate immunity PAGE 5 The major components of innate immunity are: – epithelial barriers - that block the entry of microbes – phagocytic cells (mainly neutrophils and macrophages) – dendritic cells (DCs) – natural killer (NK) cells and other innate lymphoid cells – several plasma proteins, including the proteins of the complement system. The principal components and kinetics of response of the PAGE 6 innate and adaptive immune systems Robbins Basic Pathology,Tenth edition, 2018 Innate immunity PAGE 7 Barriers include: – Physical barriers Pathogens cannot adhere to and pass the intact skin and mucous membranes due to cellular junctions and secretions – mucus and flow due to cilia. – Chemical barriers = non-neutral pH of secretions, enzymes and antimicrobial peptides present in secretions and in the tissues underlying the mucous membranes contained in the bile, gastric acid, saliva, tears, and sweat: Defensins, cathelicidins, histatins–bind to carbohydrate structures on the microbe, kill microbes by forming pores in the microbial cell membranes. – Biological barriers = commensal bacteria (normal flora, microbiome) - do not cause disease (in case of properly functioning immune system), but prevent the colonization by pathogens and their multiplication. ! To cause infection, microorganisms first must pass the barriers. Innate immunity PAGE 8 Phagocytes, dendritic cells and many other cells, such as epithelial cells, express receptors that sense the presence of infectious agents and substances released from dead cells. The microbial structures recognized by these receptors are called pathogen-associated molecular patterns; they are shared among microbes of the same type, and they are essential for the survival and infectivity of the microbes (so the microbes cannot evade innate immune recognition by mutating these molecules). The substances released from injured and necrotic cells are called damage-associated molecular patterns. The cellular receptors that recognize these molecules are often called pattern recognition receptors. It is estimated that innate immunity uses about 100 different receptors to recognize 1000 molecular patterns. Innate immunity PAGE 9 Pattern recognition receptors are located in all the cellular compartments where pathogens may be present: plasma membrane receptors detect extracellular pathogens, endosomal receptors detect ingested microbes, and cytosolic receptors detect microbes in the cytoplasm. – The best known of the pattern recognition receptors are the Toll-like receptors (TLRs). There are 10 TLRs in mammals that recognize a wide range of microbial molecules. – The plasma membrane TLRs recognize bacterial products such as lipopolysaccharide (LPS), and endosomal TLRs recognize viral and bacterial RNA and DNA. – Recognition of microbes by these receptors activates transcription factors that stimulate the production of several secreted and membrane proteins, including mediators of inflammation, anti-viral cytokines (interferons), and proteins that promote lymphocyte activation and the even more potent adaptive immune responses. Innate immunity PAGE 10 NOD-like receptors (NLRs) are cytosolic receptors. They recognize a wide variety of substances, including products of necrotic cells (e.g., uric acid and released ATP), ion disturbances (e.g., loss of K+), and some microbial products. Several of the NLRs signal via a cytosolic multiprotein complex called the inflammasome, which activates an enzyme (caspase-1) that cleaves a precursor form of the cytokine interleukin-1 (IL-1) to generate the biologically active form. IL-1 is a mediator of inflammation that recruits leukocytes and induces fever. The NLR-inflammasome pathway also may play a role in a number of chronic disorders marked by inflammation. – E.g. recognition of urate crystals by a class of NLRs underlies the inflammation associated with gout. Innate immunity PAGE 11 C-type lectin receptors (CLRs) expressed on the plasma membrane of macrophages and DCs detect fungal glycans and elicit inflammatory reactions to fungi. Several cytosolic receptors detect the nucleic acids of viruses that replicate in the cytoplasm of infected cells, and stimulate the production of anti-viral cytokines. G protein–coupled receptors on neutrophils, macrophages, and most other types of leukocytes recognize short bacterial peptides containing N-formylmethionyl residues. This receptor enables neutrophils to detect bacterial proteins and stimulates chemotactic responses. Mannose receptors recognize microbial sugars (which often contain terminal mannose residues, unlike mammalian glycoproteins) and induce phagocytosis of the microbes. Two families of cytosolic receptors, recognize microbial RNA and DNA.  PAGE 12 Cellular receptors for microbes and products of cell injury Robbins Basic Pathology,Tenth edition, 2018 Reactions of innate immunity PAGE 13 The innate immune system provides host defense by the following two main reactions: – Inflammation. Cytokines and products of complement activation, as well as other mediators, are produced during innate immune reactions and trigger the vascular and cellular components of inflammation. The recruited leukocytes destroy pathogens and ingest and eliminate damaged cells. – Anti-viral defense. Type I interferons produced in response to viruses act on infected and uninfected cells and activate enzymes that degrade viral nucleic acids and inhibit viral replication. In addition to these defensive functions, the innate immune system generates signals that stimulate the subsequent, more powerful adaptive immune response. Inflammation PAGE 14 Definition: Inflammation is a response of vascularized tissues to infections and tissue damage that brings cells and molecules of host defense from the circulation to the sites where they are needed, to eliminate the offending agents. The typical inflammatory reaction develops through a series of sequential steps: – The offending agent, which is located in extravascular tissues, is recognized by host cells and molecules; – Leukocytes and plasma proteins are recruited from the circulation to the site where the offending agent is located; – The leukocytes and proteins are activated and work together to destroy and eliminate the offending substance; – The reaction is controlled and terminated; – The damaged tissue is repaired. Overview of inflammation PAGE 15 Inflammation may be of two types, acute and chronic. Feature Acute Chronic Onset Fast: minutes or hours Slow: days Monocytes/macrophages Cellular infiltrate Mainly neutrophils and lymphocytes Usually mild and self- May be severe and Tissue injury, fibrosis limited progressive Local and systemic signs Prominent Less Inflammation PAGE 16 Its main characteristics are the exudation of fluid and plasma proteins (edema) and the emigration of leukocytes, predominantly neutrophils. When a microbe enters a tissue or the tissue is injured, the presence of the infection or damage is sensed by resident cells, including macrophages, dendritic cells, mast cells, and other cell types. These cells secrete molecules (cytokines and other mediators) that induce and regulate the subsequent inflammatory response. Inflammatory mediators are also produced from plasma proteins that react to the microbes or to products of necrotic cells. Some of these mediators promote the efflux of plasma and the recruitment of circulating leukocytes to the site where the offending agent is located. Mediators also activate the recruited leukocytes, enhancing their ability to destroy and remove the offending agent.  PAGE 17 Immunology, Male, David, BA, MA, PhD; Peebles, R. Stokes, MD; Male, Victoria, BA, MA, PhD, Published January 1, 2021. Pages 1- 13. © 2021. Basic Immunology: Functions and Disorders of the Immune System, Abbas, Abul K., MBBS; Lichtman, Andrew H., MD, PhD; Pillai, Shiv, MBBS, PhD, Published January 1, 2024. Pages 23-52. © 2024. Inflammation PAGE 18 The external manifestations of inflammation, often called its cardinal signs, are: 1. heat (calor in Latin) 2. redness (rubor) 3. swelling (tumor) 4. pain (dolor) 5. loss of function (functio laesa). The first four of these were described more than 2000 years ago by a Roman encyclopedist named Celsus. These manifestations occur as consequences of the vascular changes and leukocyte recruitment and activation. Causes of inflammation PAGE 19 Inflammatory reactions may be triggered by a variety of stimuli: 1. Infections (bacterial, viral, fungal, parasitic) and microbial toxins are among the most common and medically important causes of inflammation. 2. Tissue necrosis elicits inflammation regardless of the cause of cell death, which may include ischemia (reduced blood flow, the cause of myocardial infarction), trauma, and physical and chemical injury (e.g., thermal injury, as in burns or frostbite; irradiation; exposure to some environmental chemicals). Several molecules released from necrotic cells are known to trigger inflammation. Causes of inflammation PAGE 20 Inflammatory reactions may be triggered by a variety of stimuli: 3. Foreign bodies (splinters, dirt, sutures) may elicit inflammation by themselves or because they cause traumatic tissue injury or carry microbes. Even some endogenous substances stimulate potentially harmful inflammation if large amounts are deposited in tissues; such substances include urate crystals (in the disease gout), and cholesterol crystals (in atherosclerosis). 4. Immune reactions (also called hypersensitivity) are reactions in which the normally protective immune system damages the individual’s own tissues. The injurious immune responses may be directed against self antigens, causing autoimmune diseases, or may be inappropriate reactions against environmental substances, as in allergies, or against microbes. Acute inflammation PAGE 21 The first step in inflammatory responses is the recognition of microbes and necrotic cells by cellular receptors and circulating proteins. Then an acute inflammation is initiated. Acute inflammation has three major components: (1) dilation of small vessels, leading to an increase in blood flow; (2) increased permeability of the microvasculature, enabling plasma proteins and leukocytes to leave the circulation; (3) emigration of the leukocytes from the microcirculation, their accumulation in the focus of injury, and their activation to eliminate the offending agent. Reactions of blood vessels PAGE 22 The escape of fluid, proteins, and blood cells from the vascular system into interstitial tissues or body cavities is known as exudation. – An exudate is an extravascular fluid that has a high protein concentration and contains cellular debris. Its presence implies that there is an increase in the permeability of small blood vessels, typically during an inflammatory reaction. – In contrast, a transudate is a fluid with low protein content, little or no cellular material, and low specific gravity. It is essentially an ultrafiltrate of blood plasma that is produced as a result of osmotic or hydrostatic imbalance across vessels with normal vascular permeability. Formation of exudates and transudates PAGE 23 Edema denotes an excess of fluid in the interstitial tissue or serous cavities; it can be either an exudate or a transudate. Pus, a purulent exudate, is an inflammatory exudate rich in leukocytes (mostly neutrophils), the debris of dead cells, and, in many cases, microbes. Robbins & Kumar Basic Pathology, Kumar, Vinay, MBBS, MD, FRCPath; Abbas, Abul K., MBBS; Aster, Jon C., MD, PhD; Deyrup, Andrea T., MD, PhD; Das, Abhijit, MD, Published January 1, 2023. Pages 25-56. © 2023. Reactions of blood vessels PAGE 24 Vasodilation is induced by the action of several mediators, notably histamine, on vascular smooth muscle. It is one of the earliest manifestations of acute inflammation, and may be preceded by transient vasoconstriction. Vasodilation first involves the arterioles and then leads to the opening of new capillary beds in the area. The result is increased blood flow, which is the cause of heat and redness (erythema) at the site of inflammation. Vasodilation is quickly followed by increased permeability of the microvasculature, with the outpouring of protein-rich fluid (an exudate) into the extravascular tissues. The loss of fluid and increased vessel diameter lead to slower blood flow, concentration of red cells in small vessels, and increased viscosity of the blood. These changes result in stasis of blood flow, engorgement of small vessels jammed with slowly moving red cells, seen histologically as vascular congestion and externally as localized redness (erythema) of the involved tissue. Reactions of blood vessels PAGE 25 As stasis develops, blood leukocytes, principally neutrophils, accumulate along the vascular endothelium. At the same time endothelial cells are activated by mediators produced at sites of infection and tissue damage, and express increased levels of adhesion molecules. Leukocytes then adhere to the endothelium, and soon afterward they migrate through the vascular wall into the interstitial tissue. Reactions of blood vessels PAGE 26 Several mechanisms are responsible for increased vascular permeability in acute inflammation: Retraction of endothelial cells resulting in opening of interendothelial spaces is the most common mechanism of vascular leakage. It is elicited by histamine, bradykinin, leukotrienes, and other chemical mediators. It occurs rapidly after exposure to the mediator (within 15 to 30 minutes) and is usually short-lived. The main sites for this rapid increase in vascular permeability are postcapillary venules. Endothelial injury, resulting in endothelial cell necrosis and detachment. Direct damage to the endothelium is encountered in severe injuries (e.g. burns, microbes and microbial toxins). Neutrophils that adhere to the endothelium during inflammation may also injure the endothelial cells and thus amplify the reaction. In most instances leakage starts immediately after injury and is sustained for several hours until the damaged vessels are thrombosed or repaired. Reactions of blood vessels PAGE 27 Several mechanisms are responsible for increased vascular permeability in acute inflammation: Increased transport of fluids and proteins, called transcytosis, through the endothelial cell. This process, documented in experimental models, may involve intracellular channels that open in response to certain factors, such as vascular endothelial growth factor (VEGF), that promote vascular leakage. Its contribution to the vascular permeability seen in acute inflammation in humans is unclear.  Responses of lymphatic vessels and lymph PAGE 28 nodes Lymphatic vessels also participate in acute inflammation. The system of lymphatics and lymph nodes filters and polices the extravascular fluids. Lymphatics drain the small amount of extravascular fluid that seeps out of capillaries under normal circumstances. In inflammation, lymph flow is increased to help drain edema fluid that accumulates because of increased vascular permeability. In addition to fluid, leukocytes and cell debris, as well as microbes, may find their way into lymph. Lymphatic vessels, like blood vessels, proliferate during inflammatory reactions to handle the increased load. The lymphatics may become secondarily inflamed (lymphangitis), as may the draining lymph nodes (lymphadenitis).  Leukocyte recruitment to sites of inflammation PAGE 29 The most important leukocytes in typical inflammatory reactions are the ones capable of phagocytosis - neutrophils and macrophages. Neutrophils are rapidly recruited to sites of inflammation. Macrophages are slower responders. The principal functions of these cell types differ in subtle but important ways—neutrophils use cytoskeletal rearrangements and enzyme assembly to mount rapid, transient responses, whereas macrophages, being long-lived, make slower but more prolonged responses. Macrophages also produce growth factors that aid in repair. When strongly activated, they may induce tissue damage and prolong inflammation, because the leukocyte products that destroy microbes and help “clean up” necrotic tissues can also produce “collateral damage” of normal host tissues. When there is systemic activation of inflammation, the resulting systemic inflammatory response may even be lethal. Leukocyte recruitment to sites of inflammation PAGE 30 The journey of leukocytes from the vessel lumen to the tissue is a multistep process that is mediated and controlled by adhesion molecules and cytokines. This process can be divided into phases, consisting first of adhesion of leukocytes to endothelium at the site of inflammation, then transmigration of the leukocytes through the vessel wall, and movement of the cells toward the offending agent. Different molecules play important roles in each of these steps. Leukocyte recruitment to sites of inflammation PAGE 31 Normally red cells are confined to the central axial column, and leukocytes are pushed out toward the wall of the vessel, but the flow prevents the cells from attaching to the endothelium. As the blood flow slows early in inflammation (stasis), hemodynamic conditions change (wall shear stress decreases), and more white cells assume a peripheral position along the endothelial surface. This process of leukocyte redistribution is called margination. By moving close to the vessel wall, leukocytes are able to detect and react to changes in the endothelium. If the endothelial cells are activated by cytokines and other mediators produced locally, they express adhesion molecules to which the leukocytes attach loosely. These cells bind and detach and thus begin to tumble on the endothelial surface, a process called rolling. The cells finally come to rest at some point where they adhere firmly (resembling pebbles over which a stream runs without disturbing them). Leukocyte recruitment to sites of inflammation PAGE 32 The two major families of molecules involved in leukocyte adhesion and migration are the selectins and integrins. These molecules are expressed on leukocytes and endothelial cells. Selectins mediate the initial weak interactions between leukocytes and endothelium. Selectins are receptors expressed on leukocytes and endothelium that contain an extracellular domain that binds sugars (hence the lectin part of the name). The three members of this family are: – E-selectin, expressed on endothelial cells; – P-selectin, present on platelets and endothelium; – L-selectin, found on the surface of most leukocytes. The endothelial selectins are typically expressed at low levels or not at all on unactivated endothelium, and are upregulated after stimulation by cytokines and other mediators. Binding of leukocytes is largely restricted to the endothelium at sites of infection or tissue injury (where the mediators are produced). Leukocyte recruitment to sites of inflammation PAGE 33 Firm adhesion of leukocytes to endothelium is mediated by a family of leukocyte surface proteins called integrins. Integrins are transmembrane two-chain glycoproteins that mediate the adhesion of leukocytes to endothelium and of various cells to the extracellular matrix. They are normally expressed on leukocyte plasma membranes in a low-affinity form and do not adhere to their specific ligands until the leukocytes are activated by chemokines. Chemokines are chemoattractant cytokines that are secreted by many cells at sites of inflammation, and are displayed at high concentrations on the endothelial surface. When the rolling leukocytes encounter the displayed chemokines, the cells are activated, and their integrins undergo conformational changes and cluster together, thus converting to a high- affinity form. Leukocyte recruitment to sites of inflammation PAGE 34 At the same time, other cytokines, notably TNF and IL-1, activate endothelial cells to increase their expression of ligands for integrins. The combination of cytokine-induced expression of integrin ligands on the endothelium and increased affinity of integrins on the leukocytes results in firm integrin-mediated binding of the leukocytes to the endothelium at the site of inflammation. The leukocytes stop rolling, and engagement of integrins by their ligands delivers signals leading to cytoskeletal changes that arrest the leukocytes and firmly attach them to the endothelium. Leukocyte migration through endothelium PAGE 35 After being arrested on the endothelial surface, leukocytes migrate through the vessel wall primarily by squeezing between cells at intercellular junctions. This extravasation of leukocytes, called transmigration, occurs mainly in postcapillary venules, the site at which there is maximal retraction of endothelial cells. Further movement of leukocytes is driven by chemokines produced in extravascular tissues, which stimulate leukocytes to travel along a chemical gradient. In addition, platelet endothelial cell adhesion molecule-1 (PECAM-1), expressed on leukocytes and endothelial cells, mediates the binding events needed for leukocytes to traverse the endothelium. After traversing the endothelium, leukocytes pierce the basement membrane, probably by secreting collagenases, and they enter the extravascular tissue. Typically, the vessel wall is not injured during leukocyte transmigration. Chemotaxis of leukocytes PAGE 36 After exiting the circulation, leukocytes move in the tissues toward the site of injury by chemotaxis (locomotion along a chemical gradient). The net result is that leukocytes migrate toward the inflammatory stimulus in the direction of the locally produced chemoattractants. Both exogenous and endogenous substances can act as chemoattractants, including the following: – Bacterial products, particularly peptides with N-formylmethionine termini – Cytokines, especially those of the chemokine family – Components of the complement system, particularly C5a – Products of the lipoxygenase pathway of arachidonic acid (AA) metabolism, particularly leukotriene B4 (LTB4)  Leukocyte infiltrate PAGE 37 In most forms of acute inflammation, neutrophils predominate in the inflammatory infiltrate during the first 6 to 24 hours and are gradually replaced by monocyte-derived macrophages over 24 to 48 hours. There are several reasons for the early preponderance of neutrophils: – they are more numerous in the blood than other leukocytes; – they respond more rapidly to chemokines; – they may attach more firmly to the adhesion molecules that are rapidly induced on endothelial cells, such as P- and E-selectins. After entering tissues, neutrophils are short-lived; they undergo apoptosis and disappear within 24 to 48 hours. Macrophages survive longer and also may proliferate in the tissues, and thus they become the dominant population in prolonged inflammatory reactions. There are exceptions: in certain infections – e.g. those produced by Pseudomonas bacteria— the cellular infiltrate is dominated by neutrophils for several days; in viral infections, lymphocytes may be the first cells to arrive. The multistep process of leukocyte migration through blood vessels PAGE 38 Robbins Basic Pathology,Tenth edition, 2018 Phagocytosis and clearance of the offending PAGE 39 agent Robbins Basic Pathology,Tenth edition, 2018 Reactive oxygen species PAGE 40 Oxygen-derived radicals may be released extracellularly from leukocytes after exposure to microbes, chemokines, and antigen–antibody complexes, or following a phagocytic challenge. These ROS are implicated in tissue damage accompanying inflammation. Serum, tissue fluids, and host cells possess anti-oxidant mechanisms that protect against these potentially harmful oxygen-derived radicals. These anti-oxidants include: – (1) the enzyme superoxide dismutase, which is found in or can be activated in a variety of cell types; – (2) catalase, which detoxifies H2O2; – (3) glutathione peroxidase, another powerful H2O2 detoxifier. The role of oxygen-derived free radicals in any given inflammatory reaction depends on the balance between production and inactivation of these metabolites by cells and tissues. Nitric oxide (NO) PAGE 41 NO, a soluble gas produced from arginine by the action of nitric oxide synthase (NOS), also participates in microbial killing. There are three different types of NOS: endothelial (eNOS), neuronal (nNOS) and inducible (iNOS). eNOS and nNOS are constitutively expressed at low levels, and the NO they generate acts to maintain vascular tone and as a neurotransmitter. iNOS, the type that is involved in microbial killing, is expressed when macrophages are activated by cytokines (e.g., IFN-γ) or microbial products, and induces the production of NO. In macrophages, NO reacts with superoxide to generate the highly reactive free radical peroxynitrite. These nitrogen-derived free radicals, similar to ROS, attack and damage the lipids, proteins, and nucleic acids of microbes and host cells. Granule enzymes and other proteins PAGE 42 Neutrophils and monocytes contain granules packed with enzymes and anti-microbial proteins that degrade microbes and dead tissues and may contribute to tissue damage: Acid proteases degrade bacteria and debris within phagolysosomes; Neutral proteases are capable of degrading various extracellular components, such as collagen, basement membrane, fibrin, elastin, and cartilage, resulting in the tissue destruction that accompanies inflammatory processes; Neutrophil elastase combats infections by degrading virulence factors of bacteria; Macrophages also contain acid hydrolases, collagenase, elastase, phospholipase, and plasminogen activator; These harmful proteases are normally controlled by a system of anti-proteases in the serum and tissue fluids. Among these is α1-anti-trypsin, which is the major inhibitor of neutrophil elastase. A deficiency of these inhibitors may lead to sustained action of leukocyte proteases. Neutrophil extracellular traps PAGE 43 Neutrophil extracellular traps (NETs) are extracellular fibrillar networks that concentrate anti-microbial substances at sites of infection and prevent the spread of the microbes by trapping them in the fibrils. They are produced by neutrophils in response to infectious pathogens (mainly bacteria and fungi) and inflammatory mediators (e.g., chemokines, cytokines, and complement proteins). The extracellular traps consist of a viscous meshwork of nuclear chromatin that binds and concentrates granule proteins such as anti-microbial peptides and enzymes. Other functional responses of activated PAGE 44 leukocytes Especially macrophages, produce cytokines that can either amplify or limit inflammatory reactions, growth factors that stimulate the proliferation of endothelial cells and fibroblasts and the synthesis of collagen, and enzymes that remodel connective tissues. – Because of these activities, macrophages also have central roles in orchestrating chronic inflammation and tissue repair, after the inflammation has subsided. T lymphocytes, which are cells of adaptive immunity, also contribute to acute inflammation. – The most important of these cells are those that produce the cytokine IL-17 (so-called “TH17 cells”). – IL-17 induces the secretion of chemokines that recruit other leukocytes. In the absence of effective TH17 responses, individuals are susceptible to fungal and bacterial infections, and the skin abscesses that develop are “cold abscesses,” lacking the classic features of acute inflammation, such as warmth and redness. Other functional responses of activated PAGE 45 leukocytes There are two major pathways of macrophage activation, called classical and alternative. Which of these two pathways is taken by a given macrophage depends on the nature of the activating signals. Classical macrophage activation may be induced by microbial products such as endotoxin, which engage TLRs and other sensors, and by T cell–derived signals, importantly the cytokine IFN-γ, in immune responses. Classically activated (also called M1) macrophages produce NO and ROS and upregulate lysosomal enzymes, all of which enhance their ability to kill ingested organisms, and secrete cytokines that stimulate inflammation. These macrophages are important in host defense against microbes and in many inflammatory reactions.  PAGE 46 Other functional responses of activated leukocytes Alternative macrophage activation is induced by cytokines other than IFN-γ, such as IL-4 and IL-13, produced by T lymphocytes and other cells. These macrophages are not actively microbicidal; instead, the principal function of alternatively activated (M2) macrophages is in tissue repair. They secrete growth factors that promote angiogenesis, activate fibroblasts, and stimulate collagen synthesis. Robbins Basic Pathology,Tenth edition, 2018 Termination of the acute inflammatory response PAGE 47 The inflammatory response must be actively terminated when no longer needed to prevent unnecessary "bystander" damage to tissues. Failure to do so results in chronic inflammation, and cellular destruction. Resolution of inflammation occurs by different mechanisms in different tissues. Mechanisms which serve to terminate inflammation include: – Short half-life of inflammatory mediators in vivo; – Production and release of transforming growth factor (TGF) beta from macrophages; – Downregulation of pro-inflammatory molecules, such as leukotrienes; – Upregulation of anti-inflammatory molecules such as the antagonist of the tumor necrosis factor receptor; – Apoptosis of pro-inflammatory cells; – IL-4 and IL-10 are cytokines responsible for decreasing the production of TNF-a, IL-1, IL-6, and IL-8.  PAGE 48 Termination of the acute inflammatory response Production of anti-inflammatory lipoxins. Evidence now suggests that an active, coordinated program of resolution initiates in the first few hours after an inflammatory response begins. After entering into the tissues, granulocytes promote the switch of arachidonic acid – derived prostaglandins and leukotrienes to lipoxins, which initiate the termination sequence. Neutrophil recruitment thus ceases and programmed death by apoptosis is engaged. These events coincide with the biosynthesis, from omega-3 polyunsaturated fatty acids, of resolvins and protectins, which critically shorten the period of neutrophil infiltration by initiating apoptosis. Apoptotic neutrophils undergo phagocytosis by macrophages, leading to neutrophil clearance and release of anti-inflammatory and reparative cytokines such as transforming growth factor-β1. The anti-inflammatory program ends with the departure of macrophages through the lymphatics. Mediators of inflammation PAGE 49 Mediator Source Action Vasodilation, increased vascular permeability, Histamine Mast cells, basophils, platelets endothelial activation Prostaglandins Mast cells, leukocytes Vasodilation, pain, fever Increased vascular permeability, chemotaxis, Leukotrienes Mast cells, leukocytes leukocyte adhesion, and activation Local: endothelial activation (expression of Macrophages, endothelial cells, Cytokines (TNF, IL-1, IL-6) adhesion molecules). Systemic: fever, mast cells metabolic abnormalities, hypotension (shock) Mediators of inflammation PAGE 50 Mediator Source Action Leukocytes, activated Chemokines Chemotaxis, leukocyte activation macrophages Vasodilation, increased vascular permeability, Platelet-activating Leukocytes, mast cells leukocyte adhesion, chemotaxis, degranulation, factor oxidative burst Leukocyte chemotaxis and activation, direct Complement Plasma (produced in liver) target killing (membrane attack complex), vasodilation (mast cell stimulation) Increased vascular permeability, smooth muscle Kinins Plasma (produced in liver) contraction, vasodilation, pain Mediators of inflammation PAGE 51 The mediators of inflammation are the substances that initiate and regulate inflammatory reactions. They may be produced locally by cells at the site of inflammation, or may be derived from circulating inactive precursors that are activated at the site of inflammation. Cell-derived mediators are rapidly released from intracellular granules (e.g., amines) or are synthesized de novo (e.g., prostaglandins, leukotrienes, cytokines) in response to a stimulus. The major cell types that produce mediators of acute inflammation are tissue macrophages, dendritic cells, and mast cells, but platelets, neutrophils, endothelial cells, and most epithelia also can be induced to elaborate some of the mediators. Plasma-derived mediators (e.g., complement proteins) are present in the circulation as inactive precursors that must be activated, usually by a series of proteolytic cleavages, to acquire their biologic properties. They are produced mainly in the liver, are effective against circulating microbes, and also can be recruited into tissues. Mediators of inflammation PAGE 52 Active mediators are produced only in response to various molecules that stimulate inflammation. Most of the mediators are short-lived. They quickly decay, or are inactivated by enzymes, or they are otherwise scavenged or inhibited. There is thus a system of checks and balances that regulates mediator actions. One mediator can stimulate the release of other mediators. – products of complement activation stimulate the release of histamine – the cytokine TNF acts on endothelial cells to stimulate the production of another cytokine, IL-1, and many chemokines. The secondary mediators may have the same actions as the initial mediators but also may have different and even opposing activities, thus providing mechanisms for amplifying—or, in certain instances, counteracting—the initial action of a mediator. Vasoactive amines: histamine and serotonin PAGE 53 They are stored as preformed molecules in cells and are therefore among the first mediators to be released during inflammation. The richest sources of histamine are mast cells, which are normally present in the connective tissue adjacent to blood vessels. Histamine also is found in blood basophils and platelets. It is stored in mast cell granules and is released by degranulation in response to a variety of stimuli, including – (1) physical injury, such as trauma, cold, or heat, by unknown mechanisms – (2) binding of antibodies to mast cells, which underlies immediate hypersensitivity (allergic) reactions – (3) products of complement called anaphylatoxins (C3a and C5a) Antibodies and complement products bind to specific receptors on mast cells and trigger signaling pathways that induce rapid degranulation. Neuropeptides (e.g., substance P) and cytokines (IL-1, IL-8) also may trigger release of histamine. Vasoactive amines: histamine and serotonin PAGE 54 Histamine is considered the principal mediator of the immediate transient phase of increased vascular permeability, producing interendothelial gaps in postcapillary venules. Its vasoactive effects are mediated mainly via binding to receptors, called H1 receptors, on microvascular endothelial cells. Histamine also causes contraction of some smooth muscles. Serotonin (5-hydroxytryptamine) is a preformed vasoactive mediator present in platelets and certain neuroendocrine cells, such as in the gastrointestinal tract, and in mast cells in rodents but not humans. Its primary function is as a neurotransmitter in the gastrointestinal tract. It also is a vasoconstrictor, but the importance of this action in inflammation is unclear. Arachidonic acid metabolites PAGE 55 The lipid mediators prostaglandins and leukotrienes are produced from arachidonic acid present in membrane phospholipids, and they stimulate vascular and cellular reactions in acute inflammation. Arachidonic acid is a 20-carbon polyunsaturated fatty acid that is derived from dietary sources or by conversion from the essential fatty acid linoleic acid. Most cellular arachidonic acid is esterified and incorporated into membrane phospholipids. Mechanical, chemical, and physical stimuli or other mediators (e.g., C5a) trigger the release of arachidonic acid from membranes by activating cellular phospholipases, mainly phospholipase A2. Arachidonic acid metabolites PAGE 56 Once freed from the membrane, arachidonic acid is rapidly converted to bioactive mediators. These mediators, also called eicosanoids (because they are derived from 20-carbon fatty acids; Greek eicosa = 20), are synthesized by two major classes of enzymes: – Cyclooxygenases, which generate prostaglandins – Lipoxygenases, which produce leukotrienes and lipoxins  PAGE 57 Arachidonic acid metabolites Robbins Basic Pathology,Tenth edition, 2018 Prostaglandins PAGE 58 Prostaglandins (PGs) are produced by mast cells, macrophages, endothelial cells, and many other cell types, and are involved in the vascular and systemic reactions of inflammation. They are generated by the actions of two cyclooxygenases called COX-1 and COX-2. COX-1 is produced in response to inflammatory stimuli and also is constitutively expressed in most tissues, where it may serve a homeostatic function (e.g., fluid and electrolyte balance in the kidneys, cytoprotection in the gastrointestinal tract). In contrast, COX-2 is induced by inflammatory stimuli and thus generates the PGs that are involved in inflammatory reactions, but it is low or absent in most normal tissues. Leukotrienes PAGE 59 Leukotrienes are produced in leukocytes and mast cells by the action of lipoxygenase and are involved in vascular and smooth muscle reactions and leukocyte recruitment. The synthesis of leukotrienes involves multiple steps, the first of which generates leukotriene A4 (LTA4), which in turn gives rise to LTB4 or LTC4. LTB4 is produced by neutrophils and some macrophages, and is a potent chemotactic agent and activator of neutrophils, causing aggregation and adhesion of the cells to venular endothelium, generation of ROS, and release of lysosomal enzymes. The leukotriene LTC4 and its metabolites, LTD4 and LTE4, are produced mainly in mast cells and cause intense vasoconstriction, bronchospasm (important in asthma), and increased permeability of venules. Lipoxins PAGE 60 Lipoxins also are generated from arachidonic acid by the lipoxygenase pathway, but unlike prostaglandins and leukotrienes, the lipoxins suppress inflammation by inhibiting the recruitment of leukocytes. They inhibit neutrophil chemotaxis and adhesion to endothelium. They also are unusual in that two cell populations are required for the transcellular biosynthesis of these mediators. Leukocytes, particularly neutrophils, produce intermediates in lipoxin synthesis, and these are converted to lipoxins by platelets interacting with the leukocytes. Cytokines and chemokines PAGE 61 Cytokines are proteins secreted by many cell types (principally activated lymphocytes, macrophages, and dendritic cells, but also endothelial, epithelial, and connective tissue cells) that mediate and regulate immune and inflammatory reactions. By convention, growth factors that act on epithelial and mesenchymal cells are not grouped under cytokines. Cytokines and chemokines PAGE 62 TNF and IL-1 serve critical roles in leukocyte recruitment by promoting adhesion of leukocytes to endothelium and their migration through vessels. Activated macrophages and dendritic cells mainly produce these cytokines; TNF also is produced by T lymphocytes and mast cells, and some epithelial cells produce IL-1 as well. Microbial products, foreign bodies, necrotic cells, and a variety of other inflammatory stimuli can stimulate the secretion of TNF and IL-1.  PAGE 63 Robbins Basic Pathology,Tenth edition, 2018 Cytokines and chemokines PAGE 64 The actions of TNF and IL-1 contribute to the local and systemic reactions of inflammation: – Endothelial activation - both TNF and IL-1 act on endothelium to induce increased expression of endothelial adhesion molecules, mostly E- and P-selectins and ligands for leukocyte integrins; increased production of various mediators, including other cytokines and chemokines, and eicosanoids; and increased procoagulant activity of the endothelium. – Activation of leukocytes and other cells TNF augments responses of neutrophils to other stimuli such as bacterial endotoxin and stimulates the microbicidal activity of macrophages; IL-1 activates fibroblasts to synthesize collagen and stimulates proliferation of synovial cells and other mesenchymal cells; IL-1 and IL-6 also stimulate the generation of a subset of CD4+ helper T cells called TH17 cells. Cytokines and chemokines PAGE 65 Systemic acute-phase response: – IL-1 and TNF (as well as IL-6) induce the systemic acute-phase responses associated with infection or injury, including fever. – They also are implicated in the pathogenesis of the systemic inflammatory response syndrome (SIRS), resulting from disseminated bacterial infection (sepsis) and other serious conditions. – TNF regulates energy balance by promoting lipid and protein catabolism and by suppressing appetite. Therefore, sustained production of TNF contributes to cachexia, a pathologic state characterized by weight loss, muscle atrophy, and anorexia that accompanies some chronic infections and cancers. Complement system PAGE 66 The complement system is a collection of soluble proteins and their membrane receptors that function mainly in host defense against microbes and in pathologic inflammatory reactions. There are more than 20 complement proteins, some of which are numbered C1 through C9. They function in both innate and adaptive immunity for defense against microbial pathogens. In the process of complement activation, several cleavage products of complement proteins are elaborated that cause increased vascular permeability, chemotaxis, and opsonization. Complement system PAGE 67 Complement proteins are present in inactive forms in the plasma, and many of them are activated to become proteolytic enzymes that degrade other complement proteins, thus forming an enzymatic cascade capable of tremendous amplification. The critical step in complement activation is the proteolysis of the third (and most abundant) component, C3. Cleavage of C3 can occur by one of three pathways: – The classical pathway, which is triggered by fixation of C1 to antibody (IgM or IgG) that has combined with antigen; – The alternative pathway, which can be triggered by microbial surface molecules (e.g., endotoxin, or LPS), complex polysaccharides, and other substances, in the absence of antibody; – The lectin pathway, in which plasma mannose-binding lectin binds to carbohydrates on microbes and directly activates C1. Complement system PAGE 68 All three pathways of complement activation lead to the formation of an enzyme called the C3 convertase, which splits C3 into two functionally distinct fragments, C3a and C3b. C3a is released, and C3b becomes covalently attached to the cell or molecule where the complement is being activated. More C3b then binds to the previously generated fragments to form C5 convertase, which cleaves C5 to release C5a and leave C5b attached to the cell surface. C5b binds the late components (C6–C9), culminating in the formation of the membrane attack complex (MAC, composed of multiple C9 molecules). The enzymatic activity of complement proteins provides such tremendous amplification that millions of molecules of C3b can deposit on the surface of a microbe within 2 or 3 minutes! Complement system PAGE 69 Robbins Basic Pathology,Tenth edition, 2018 Complement system PAGE 70 The complement system has three main functions: 1. Inflammation – C5a, and, to a lesser extent, C4a and C3a, are cleavage products of the corresponding complement components that stimulate histamine release from mast cells and thereby increase vascular permeability and cause vasodilation. They are called anaphylatoxins because they have effects similar to those of mast cell mediators that are involved in the reaction called anaphylaxis. – C5a also is a chemotactic agent for neutrophils, monocytes, eosinophils, and basophils. – C5a activates the lipoxygenase pathway of arachidonic acid metabolism in neutrophils and monocytes, causing release of more inflammatory mediators. Complement system PAGE 71 The complement system has three main functions: 2. Opsonization and phagocytosis – C3b and its cleavage product iC3b (inactive C3b), when fixed to a microbial cell wall, act as opsonins and promote phagocytosis by neutrophils and macrophages, which bear cell surface receptors for these complement fragments. 3. Cell lysis – The deposition of the MAC on cells drills holes in the cell membrane, making the cells permeable to water and ions and resulting in their osmotic death (lysis). This function of complement is important mainly for the killing of microbes with thin cell walls. Complement system PAGE 72 The activation of complement is tightly controlled by cell-associated and circulating regulatory proteins: – C1 inhibitor blocks the activation of C1, the first protein of the classical complement pathway. – Decay accelerating factor (DAF) and CD59 are two proteins that are linked to plasma membranes by a glycophosphatidyl (GPI) anchor. DAF prevents formation of C3 convertases and CD59 inhibits formation of the MAC. – Other complement regulatory proteins proteolytically cleave active complement components. Factor H is a plasma protein that serves as a cofactor for the proteolysis of the C3 convertase. Platelet-activating factor (PAF) PAGE 73 PAF is a phospholipid-derived mediator that was discovered as a factor that caused platelet aggregation, but it is now known to have multiple inflammatory effects. A variety of cell types, including platelets themselves, basophils, mast cells, neutrophils, macrophages, and endothelial cells, can elaborate PAF. In addition to platelet aggregation, PAF causes vasoconstriction and bronchoconstriction, and at low concentrations it induces vasodilation and increased vascular permeability. Kinins PAGE 74 Kinins are vasoactive peptides derived from plasma proteins, called kininogens, by the action of specific proteases called kallikreins. The enzyme kallikrein cleaves a plasma glycoprotein precursor, high-molecular-weight kininogen, to produce bradykinin. Bradykinin increases vascular permeability and causes contraction of smooth muscle, dilation of blood vessels, and pain when injected into the skin. These effects are similar to those of histamine. The action of bradykinin is short-lived, because it is quickly inactivated by an enzyme called kininase. Bradykinin has been implicated as a mediator in some forms of allergic reaction, such as anaphylaxis. Neuropeptides PAGE 75 Neuropeptides are secreted by sensory nerves and various leukocytes, and may play a role in the initiation and regulation of inflammatory responses. These small peptides, including substance P and neurokinin A, are produced in the central and peripheral nervous systems. Nerve fibers containing substance P are prominent in the lung and gastrointestinal tract. Substance P has many biologic functions, including the transmission of pain signals, regulation of blood pressure, stimulation of hormone secretion by endocrine cells, and in increasing vascular permeability. Role of mediators in different reactions of inflammation PAGE 76 Reaction of Inflammation Principal Mediators Histamine Vasodilation Prostaglandins Histamine C3a and C5a (by liberating vasoactive amines from mast cells, other Increased vascular permeability cells) Leukotrienes C4, D4, E4 TNF, IL-1 Chemotaxis, leukocyte Chemokines recruitment and activation C3a, C5a Leukotriene B4 Role of mediators in different reactions of inflammation PAGE 77 Reaction of Inflammation Principal Mediators IL-1, TNF Fever Prostaglandins Prostaglandins Pain Bradykinin Lysosomal enzymes of leukocytes Tissue damage Reactive oxygen species Outcomes of acute inflammation PAGE 78 Complete resolution – The inflammatory reactions, after they have succeeded in eliminating the offending agent, end with restoration of the site of acute inflammation to normal. Healing by connective tissue replacement (scarring, or fibrosis) – This occurs after substantial tissue destruction, when the inflammatory injury involves tissues that are incapable of regeneration, or when there is abundant fibrin exudation in tissue or in serous cavities (pleura, peritoneum) that cannot be adequately cleared. In all these situations, connective tissue grows into the area of damage or exudate, converting it into a mass of fibrous tissue. Progression of the response to chronic inflammation – Acute to chronic transition occurs when the acute inflammatory response cannot be resolved, as a result of either the persistence of the injurious agent or some interference with the normal process of healing. Tissue repair PAGE 79 Chemical mediators and growth factors orchestrate the healing process. Some growth factors act as chemoattractants, enhancing the migration of white blood cells and fibroblasts to the wound site, and others act as mitogens, causing increased proliferation of cells that participate in the healing process (e.g. platelet-derived growth factor, which is released from activated platelets, attracts white blood cells and acts as a growth factor for blood vessels and fibroblasts). Reparatory processes: – Cell proliferation – Connective tissue proliferation – Blood vessels neoformation = angiogenesis – Lymphatic drainage of exudates – Phagocytosis Tissue repair PAGE 80 Injured tissues are repaired by regeneration of parenchymal cells or by connective tissue repair in which scar tissue is substituted for the parenchymal cells of the injured tissue (could lead to malfunction of organs - fibrosis). Fibroblasts and vascular endothelial cells begin proliferating to form a specialized type of soft, pink granular tissue, called granulation tissue. This tissue serves as the foundation for scar tissue development. It is fragile and bleeds easily because of the numerous, newly developed capillary. The newly formed blood vessels are leaky and allow plasma proteins and white blood cells to leak into the tissues. At approximately the same time, epithelial cells at the margin of the wound begin to regenerate and move toward the center of the wound, forming a new surface layer. Tissue repair PAGE 81 As the proliferative phase progresses, there is continued accumulation of collagen and proliferation of fibroblasts. Collagen synthesis reaches a peak within 5 to 7 days and continues for several weeks, depending on wound size. By the second week, the white blood cells have largely left the area, the edema has diminished, and the wound begins to blanch as the small blood vessels become thrombosed and degenerate. Factors that affect healing PAGE 82 Malnutrition – Protein deficiencies prolong the inflammatory phase of healing and impair fibroblast proliferation, collagen and protein matrix synthesis, angiogenesis, and wound remodeling. – Carbohydrates are needed as an energy source for white blood cells. – Fats are essential constituents of cell membranes and are needed for the synthesis of new cells. – Vitamins A and C have been shown to play an essential role in the healing process. – Vitamin C is needed for collagen synthesis. – Vitamin A functions in stimulating and supporting epithelialization, capillary formation, and collagen synthesis. The B vitamins are important cofactors in enzymatic reactions that contribute to the wound-healing process. – Vitamin K plays an indirect role in wound healing by preventing bleeding disorders. Factors that affect healing PAGE 83 Blood Flow and Oxygen Delivery – Pre-existing health problems – Arterial disease and venous pathology Molecular oxygen is required for collagen synthesis. It has been shown that even a temporary lack of oxygen can result in the formation of less stable collagen. Wounds in ischemic tissue become infected more frequently. PMNs and macrophages require oxygen for destruction of microorganisms. Chronic inflammation PAGE 84 Chronic inflammation is a response of prolonged duration (weeks or months) in which inflammation, tissue injury, and attempts at repair coexist, in varying combinations. It may follow acute inflammation, or may begin insidiously, as a smoldering, sometimes progressive, process without any signs of a preceding acute reaction. In contrast to acute inflammation, which is manifested by vascular changes, edema, and predominantly neutrophilic infiltration, chronic inflammation is characterized by the following: – Infiltration with mononuclear cells, which include macrophages, lymphocytes, and plasma cells; – Tissue destruction, induced by the persistent offending agent or by the inflammatory cells; – Attempts at healing by connective tissue replacement of damaged tissue, accomplished by angiogenesis (proliferation of small blood vessels) and, in particular, fibrosis. Chronic inflammation PAGE 85 The dominant cells in most chronic inflammatory reactions are macrophages, which contribute to the reaction by secreting cytokines and growth factors that act on various cells, by destroying foreign invaders and tissues, and by activating other cells, notably T lymphocytes. Macrophages secrete mediators of inflammation, such as cytokines (TNF, IL-1, chemokines, and others) and eicosanoids. Thus, macrophages are central to the initiation and propagation of inflammatory reactions. Macrophages display antigens to T lymphocytes and respond to signals from T cells, thus setting up a feedback loop that is essential for defense against many microbes by cell- mediated immune responses. Systemic effects of inflammation PAGE 86 Inflammation, even if it is localized, is associated with cytokine-induced systemic reactions that are collectively called the acute-phase response. Fever, characterized by an elevation of body temperature, usually by 1° to 4°C, is one of the most prominent manifestations of the acute-phase response, especially when inflammation is associated with infection. Substances that induce fever are called pyrogens. Acute-phase proteins are plasma proteins, mostly synthesized in the liver, whose plasma concentrations may increase several hundred-fold as part of the response to inflammatory stimuli – e.g. C-reactive protein (CRP), fibrinogen. – Synthesis of these molecules in hepatocytes is stimulated by cytokines. Many acute- phase proteins, bind to microbial cell walls, and they may act as opsonins and fix complement. – Fibrinogen binds to red cells and causes them to form stacks (rouleaux) that sediment more rapidly at unit gravity than do individual red cells. This is the basis for measuring the erythrocyte sedimentation rate as a simple test for an inflammatory response caused by any stimulus. Systemic effects of inflammation PAGE 87 Leukocytosis is a common feature of inflammatory reactions, especially those induced by bacterial infections. The extreme elevations are referred to as leukemoid reactions. The leukocytosis occurs initially because of accelerated release of cells from the bone marrow postmitotic reserve pool (caused by cytokines likeTNF and IL-1) and is therefore associated with a rise in the number of more immature neutrophils in the blood, referred to as a shift to the left. Prolonged infection also induces proliferation of precursors in the bone marrow, caused by increased production of colony-stimulating factors (CSFs). – Most bacterial infections induce neutrophilia. Viral infections, such as infectious mononucleosis, mumps, and German measles, cause an absolute increase in the number of lymphocytes (lymphocytosis). In some allergies and parasitic infestations, there is eosinophilia. – Certain infections (typhoid fever and infections caused by some viruses, rickettsiae, and certain protozoa) are associated with leukopenia. Systemic effects of inflammation PAGE 88 Other manifestations of the acute-phase response include: – Increased heart rate and blood pressure; – Decreased sweating, mainly because of redirection of blood flow from cutaneous to deep vascular beds, to minimize heat loss through the skin; – Rigors (shivering), chills; – Anorexia, somnolence, and malaise, probably because of the actions of cytokines on brain cells. References PAGE 89 Guyton and Hall, Textbook of Medical Physiology, 2016 Robbins Basic Pathology, Tenth edition, 2018