General Pathology

Theme 5: The Acute Inflammatory Response

1. Appreciate the role of the acute inflammatory response in the body's defence mechanisms.
2. Describe the sequence of events in the acute inflammatory response and discuss the roles of the different components of the response.
3. Appreciate the possible pathological sequelae of acute inflammation.
4. List the local and systemic clinical changes that can result from acute inflammation.

The first visible tissue change that begins immediately after an injury is the microcirculatory response, which is accompanied by mobilization of phagocytic cells – the acute inflammatory response. A complex network of chemical mediators and cellular events occur in the vascular and tissue compartments during this response.

This acute inflammatory is not a disease but should be regarded as the first line of defense against injury. It is generally beneficial to the host. The beneficial effects of localizing the damage/infection can, however, be accompanied by deleterious tissue damage

The immune response is also triggered at the time of the injury but takes several days to manifest microscopically visible changes at the site of injury.

The term chronic inflammation is applied to the complex of tissue changes that represents a combined inflammatory and immune response against an agent that persists in the tissues long enough so that the microscopic changes of the immune response can occur. Chronic inflammation also shows changes associated with tissue damage and repair

The host response is designed to inactivate and remove the injurious agents, remove any damaged tissue and accomplish repair.

General Principles of Acute Inflammation

Cardinal Features of Inflammation

Inflammation

Acute	Chronic
• short duration — from few minutes, hours or days • exudation of fluid and plasma proteins (edema) • emigration of (leukocytes) PMNs	• longer • presence of lymphocytes and macrophages • proliferation of BV and CT

Acute Inflammation

1. alteration of vascular caliber to increase blood flow
2. structural changes in microvasculature => increase permeability of plasma proteins and leukocytes to leave the circulation to produce an inflammatory exudate
3. migration of leukocytes and plasma proteins from microcirculation to site of injury

Changes in vascular flow and caliber constitute one of the three major components of the inflammatory response. They begin immediately after injury and develop at various rates depending on the severity of the injury.

• Initially, transient vasoconstriction of arterioles occurs.
• Vasodilation follows, causing increased flow; it accounts for the heat and redness.
• Eventual slowing of the circulation occurs as a result of increased vascular permeability, leading to stasis. The increased permeability is the cause of edema.
• With slowing, the larger white cells fallout of the axial stream, and leukocytic margination appears, a prelude to the cellular events.

Increased Vascular Permeability

• Normal fluid exchange depends on Starling's law and an intact endothelium. Starling's law maintains that normal fluid balance is modulated mainly by two opposing forces: (1) hydrostatic pressure causing fluid to move out of the circulation and (2) plasma colloid osmotic pressure causing fluid to move into the capillaries.
• In inflammation, there is increased hydrostatic pressure, caused by vasodilation, and decreased osmotic pressure, caused by leakage of high-protein fluid across a hyperpermeable endothelium — resulting in marked net outflow of fluid and edema formation.

There are 6 possible mechanisms of increased endothelial permeability:
1. Endothelial cell contraction in venules, leading to the formation of widened intercellular junctions, or intercellular gaps. This is the most common form elicited by chemical mediators (e.g., histamine); occurs immediately after injection of the mediator; is short-lived (immediate-transient response); and classically involves only venules 20 to 60 mM in diameter, leaving capillaries and arterioles unaffected.
2. Endothelial retraction owing to cytoskeletal and junctional reorganization, also resulting in widened interendothelial junctions. This results in a somewhat delayed response that can be long-lived and is induced by cytokine mediators, such as interleukin-l (IL-l) and tumor necrosis factor (TNF).
3. Direct endothelial injury, resulting in endothelial cell necrosis and detachment. This is caused by severe necrotizing injuries and affects all levels of the microcirculation, including venules, capillaries and arterioles. The damage usually evokes an immediate and sustained endothelial leakage.
4. Leukocyte-mediated endothelial injury resulting from leukocyte aggregation, adhesion and emigration across the endothelium. These leukocytes release toxic oxygen species and proteolytic enzymes, which cause endothelial injury or detachment, resulting in increased permeability.
5. Increased transcytosis across the endothelial cytoplasm via vesicles and vacuoles of the vesiculovacuolar organelle. Some growth factors (e.g., vascular endothelial growth factor) may cause vascular leakage by increasing the number and size of these channels.
6. Leakage from regenerating capillaries, during healing. This occurs when new capillary sprouts are leaky.

A critical function of inflammation is the delivery of leukocytes to the site of injury. The sequence of events in this journey, called extravasation, can be divided into the following steps:

1. Margination, rolling and adhesion of leukocytes in the lumen.
2. Transmigration across the endothelium (also called diapedesis).
3. Migration in interstitial tissues toward a chemotactic stimulus.

Adhesion and Transmigration

• The selectins (E, Po and L), which bind through their lectin (sugar binding) domains to oligosaccharides (e.g., sialylated Lewis X), which themselves are covalently bound to cell surface glycoproteins.
• The immunoglobulin family, which includes the endothelial ICAM-l (intercellular adhesion molecule I) and VCAM-I (vascular cell adhesion molecule I).
• The integrins, which function as receptors for some of the members of the immunoglobulin family and the extracellular matrix. The principal integrin receptors for ICAM-I are the ฿₂

1. Redistribution of preformed adhesion molecules to the cell surface. For example, after exposure to histamine or thrombin, P- selectin is rapidly translocated from the endothelial Weibel-Palade body membranes to the cell surface, where it can bind leukocytes.
2. Induction of adhesion molecules on endothelium. For example, IL-l and TNF induce the synthesis and surface expression of E-selectin and increase expression of ICAM-I and VCAM-I, rendering such activated endothelial cells more adherent to leukocytes.
3. Increased avidity of binding. This is most relevant to the binding of integrins (LFA-I and MAC-I), which are normally present on leukocytes but can be converted from a state of low-affinity binding to high-affinity binding toward their ligand ICAM-1 by chemical mediators. Such activation causes firm adhesion of the leukocytes to the endothelium and is also necessary for subsequent transmigration across endothelial cells.

• Endothelial activation: Mediators present at the inflantmatory sites increase the expression of E-selectin and P-selectin by endothelial cells.
• Leukocyte rolling: There is an initial rapid and relatively loose adhesion, resulting from interactions between the selectins and their carbohydrate ligands.
• Firm adhesion: The leukocytes are then activated by chemokines or other agents to increase the avidity of their integrins.
• Transmigration: This is mediated by interactions between platelet-endothelial cell adhesion molecule-1 (PECAM-I or CD31) on leukocytes and endothelial cells.

Chemotaxis and Leukocyte Activation

Chemotaxis involves binding of chemotactic agents to specific receptors on leukocytes and production of second messengers. This signal transduction process results in activation of phospholipase C and protein kinase C, increased intracellular calcium, and assembly of the contractile elements responsible for cell movement. The leukocyte moves by extending a pseudopod that pulls the remainder of the cell in the direction of the extension. Locomotion is controlled by the effects of calcium ions and phosphoinositols on actin regulatory proteins, such as gelsolin, filamin, and calmodulin. Chemotactic agents also cause leukocyte activation, characterized by:

• Degranulation and secretion of enzymes
• Activation of an oxidative burst
• Production of arachidonic acid metabolites
• Modulation of leukocyte adhesion molecules

Phagocytosis

1. Recognition and attachment of the particle to be ingested by the leukocyte. Many microorganisms are coated with specific factors, called opsonins, which enhance the efficiency of phagocytosis because they are recognized by receptors on the leukocytes. The two major opsonins are the Fc fragment of immunoglobulin G and a product of complement, C3b.
2. Engulfment by pseudopods encircling the phagocytosed particle, with subsequent formation of a phagocytic vacuole or phagosome. The membrane of the phagocytic vacuole then fuses with the membrane of a lysosomal granule, resulting in discharge of the granule's content into the phagolysosome.
3. Killing and degradation of bacteria. Phagocytosis stimulates a burst of oxygen consumption and production of reactive oxygen metabolites. There are two types of bactericidal mechanisms:
   • Oxygen-dependent mechanisms. Bacterial killing is accomplished largely by oxygen-dependent mechanisms. This is triggered by activation of nicotinamide-adenine dinucleotide phosphate {reduced form) (NADPH) oxidase, in the process reducing oxygen (O₂ to superoxide anion (0₂^–) and hence to hydrogen peroxide (H₂O₂). Myeloperoxidase (MPO) from lysosomal granules then converts H₂O₂, in the presence of a halide such as CI^–, to the highly bactericidal HOCl•. Although the H₂O₂-MPO-halide system is the most efficient bactericidal mechanism, the reactive oxygen species produced during an oxidative burst can kill bacteria directly.
   • Oxygen-independent mechanisms. These include bactericidal permeability increasing protein, lysozyme, lactoferrin, major basic protein (MBP) of eosinophils, and arginine-rich defensins. Killed organisms are then degraded by hydrolases and other enzymes in lysosomes.

Release of Leukocyte Products and Leukocyte Induced Tissue Injury

• Lysosomal enzymes, by regurgitation during feeding, reverse endocytosis, or cytotoxic release.
• Oxygen-derived active metabolites.
• Products of arachidonic acid metabolism, including prostaglandins and leukotrienes.

Defects in Leukocyte Function

• Defects in leukocyte adhesion, such as leukocyte adhesion deficiency type I and type II.
• Defects in phagocytosis, such as Ch้diak-Higashi syndrome. Neutrophils in patients with this syndrome have giant granules because of aberrant organelle fusion and reduced transfer of lysosomal enzymes to phagocytic vacuoles (causing susceptibility to infections).
• Defects in microbicidal activity. In chronic granulomatous disease, there are inherited defects in NADPH oxidase, leading to a defect in the respiratory burst, H2O2 production, and the MPO-H₂O₂-halide bactericidal mechanism.

Summary
The vascular phenomena in acute inflammation are characterized by increased blood flow to the injured area, resulting mainly from arteriolar dilation and opening of capillary beds. Increased vascular permeability results in the accumulation of protein-rich extravascular fluid, which forms the exudate. Plasma proteins leave the vessels mast commanly through widened interendothelial cell junctions of the venules or by direct endothelial cell injury. The leukocytes, initially predominantly neutrophils, adhere to the endothelium via adhesion molecules, transmigrate across the endothelium, and migrate to the site of injury under the influence of chemotactic agents. Phagocytosis of the offending agent follows, which may lead to the death of the microorganism. During chemotaxis and phagocytosis, activated leukocytes may release toxic metabolites and proteases extracellularly, potentially causing tissue damage.

The acute inflammatory response is aimed at neutralizing or inactivating the agent causing the injury. There are several outcomes:

1. Resolution: In uncomplicated acute inflammation, tissue returns to normal in a process of resolution, in which the exudate and cellular debris are liquefied and removed by macrophages and lymphatic flow.
2. Repair: Healing by connective tissue replacement (fibrosis) and scarring, which occurs after substantial tissue destruction, when the inflammation occurs in tissues that do not regenerate or when there is abundant fibrin exudation.
3. Suppuration: In virulent bacterial infections, (esp. pyogenic organisms) exaggerated emigration of neutrophils with liquefactive necrosis occurs (suppurative inflammation). The liquefied mass of necrotic tissue and neutrophis is called pus. When an area of suppuration becomes walled off, abscess results.
4. Chronic inflammation: When the noxious agent is not neutralized by the acute inflammatory response, the body mouns an immune response which leads to chronic inflammation.

Local Changes

1. Redness
2. Swelling
3. Heat
4. Pain
5. Possibe loss of function of that area

Systemic Clinical Changes

1. Fever: Fever may result following the entry of phrogens and prostaglandins into the circulation at the site of inflammation. These act upon the brain stem to reset body temperature.
2. Change in peripheral WBC count: The total number of neutrophis in the peripheral blood is increased (neutrophil leukocytosis); initially, this is due to accelerated release of neutrophils from bone marrow. Later, neutrophil production in the marrow is increased. Peripheral blood neutrophils tend to be the less mature forms, and they frequently contain large cytoplasmic granules (toxic granulation). The term shift to the left signifies this change. Viral infections tend to produce neutropenia (decreased number of neutrophils in the blood) an dlymphocytosis (excess of normal lymphocytes in the blood). Acute inflammation resulting from viral infection therefore represents an exception in that he microcirculatory changes and fluid exudation are accompanied by a lymphocytic rather than a neutrophil response.
3. Change in plasma protein levels: The levels of certain proteins typically increase when acute inflammation is present. These acute phase reactions include C-reactive protein, a₁-antitrypsin, fibrinogen, haptoglobin and ceruloplasmin. Increased levels of these substances in turn lead to an increased erythrocyte sedimentation rate, a simple and useful (though nonspecific) clue to the presence of inflammation.