INFLAMMATION AND REPAIR

Navid JubaerLecturer

Department of PharmacyUniversity of Asia Pacific

Definition

Inflammation is a non specific, localized complex immune reaction of the organism, which tries to localize the injurious agent and leading to the exudation and accumulation of protein-rich fluids and leucocytes, provided that the injury does not destroy the tissue.

It consist in vascular, metabolic, cellular changes, triggered by the entering of pathogen agent in healthy tissues of the body.

Inflammation: Beneficial or Harmful?

Inflammation is usually a protective response which is beneficial for human body. The purpose of inflammation is: To dilute, localize and destroy injurious

agent To limit tissue injury To restore the tissue towards normality

However, inflammation may be harmful if left untreated or the inflammation due to hypersensitivity reactions

Types of Inflammation

Acute Inflammation: It is an immediate and rapid response of living tissue to an injurious agent and lasts for minute to few days. Histologically, there is extravascular accumulation of protein-rich fluid and leucocytes, mainly neutrophils in many acute inflammation due to exudation. It is an exudative lesion

Chronic Inflammation: It is the inflammation that persists for weeks to months. Histologically, there is extravascular accumulation of lymphocytes and macrophages, tissue destruction and attempts of healing by proliferation of small blood vessels and connective tissue.

Etiology

The causes of inflammation are many and varied: Exogenous causes:

Physical agents Mechanical agents: fractures, foreign corps, sand,

etc. Thermal agents: burns, freezing

Chemical agents: toxic gases, acids, bases Biological agents: bacteria, viruses, parasites

Endogenous causes: Circulation disorders: thrombosis, infarction,

hemorrhage Enzymes activation – e.g. acute pancreatitis Metabolic products deposals – uric acid, urea Immune reactions e.g. allergic rhinitis, acute

glomerulonephritis

Cardinal Signs

Celsus described the local reaction of injury in terms that have come to be known as the cardinal signs of inflammation.

These signs are: rubor (redness) tumor (swelling) calor (heat) dolor (pain) functio laesa, or loss of function (In the second

century AD, the Greek physician Galen added this fifth cardinal sign)

Changes/Events at The Injured Site

Acute inflammatory reaction and the changes it causes is stereotyped and are grouped together as following: Vascular changesa) Changes in vascular calibre and flowb) Increased vascular permeability (vascular

leakage) Exudation of blood constituentsa) Fluid exudateb) Cellular exudate Changes in other tissue

Vascular Changes

At the site of injury, the changes occur in the microvasculature consisting of arterioles, venules and capillaries. The changes are

Changes is the calibre of blood vessel and blood flow

Structural changes that allows the plasma proteins and blood cells to leak

Changes in vascular calibre and flow

Changes occur in the following order: Transient vasoconstriction of arterioles. It

disappears within 3-5 seconds in mild injuries. In more severe injury It may last several minutes

Vasodialation: It causes opening of sphincters and capillary beds at the injured site which is temporarily shut down due to injury and persists for a short time

Vasodialation helps to increase the blood flow which leads to rubor (redness) and calor (heat)

Slowing of blood flow or stasis due to formation of exudate and increased viscosity of blood

In mild injury, it takes 15-30 minuets for these events, and with severe injury, it may occur in a few minutes

Increased vascular permeability (vascular leakage)

A hallmark of acute inflammation (escape of a protein-rich fluid).

It affects small & medium size venules, through gaps between endothelial cells

Mechanism of Vascular Leakage 1. Formation of endothelial gaps in venules

This is the most common mechanism of vascular leakage

It is elicited by histamine, bradykinin, leukotrienes, the neuropeptide substance P, and many other classes of chemical mediators.

It occurs rapidly after exposure to the mediator and is usually reversible and short-lived (15 to 30 minutes). It is known as immediate transient response.

It affects venules (20 to 60 μm in diameter), leaving capillaries and arterioles unaffected

2. Direct injury (Immediate sustained reactions) Direct endothelial injury (necrosis and detachment) Direct damage to the endothelium by the injurious

stimulus, as, severe burns or lytic bacterial infections. Neutrophils may also injure the endothelial cells.

In most instances, leakage starts immediately after injury and is sustained at a high level for several hours until the damaged vessels are thrombosed or repaired.

All levels of the microcirculation are affected: venules, capillaries, and arterioles

3. Delayed prolonged leakage Relatively common type of increased permeability that

begins after a delay of 2 to 12 hours, lasts for several hours or even days

Involves venules as well as capillaries. Such leakage is caused by mild to moderate thermal

injury, x-radiation or ultraviolet radiation, sunburn and certain bacterial toxins.

The mechanism of such leakage is unclear. It may result from the direct effect of the injurious agent, leading to delayed endothelial cell damage (perhaps by apoptosis), or the effect of cytokines causing endothelial retraction.

4. Leukocyte-mediated endothelial injury. Leukocytes adhere to endothelium relatively early in

inflammation. Such leukocytes may be activated in the process,

releasing toxic oxygen species and proteolytic enzymes, which then cause endothelial injury or detachment, resulting in increased permeability.

In acute inflammation, this form of injury is largely restricted to vascular sites, such as venules and pulmonary and glomerular capillaries.

5. Increased transcytosis across the endothelial cytoplasm

Transcytosis occurs across channels consisting of clusters of interconnected endothelial cells

This mechanism of increased permeability is induced by histamine and most chemical mediators

6.Leakage from new blood vessels It occurs during angiogenesis in early healing phases that

follow inflammation.

Exudation of Blood Constituents Exudation is the leaking of blood constituents

from blood vessels into interstitial tissue. Exudate (or inflammatory edema) contains protein and leukocytes Fluid exudate• Fluid exudate is formed by the plasma constituents-

fluid, solute and proteins. It may have the same chemical composition as that of plasma

Cellular exudate• Circulating leucocytes constitute the cellular exudate.

In most cases, the cells are neutrophils and monocytes

Leukocyte extravasation Phagocystosis

Leukocyte Extravasation

It is the sequence of events of migration of leucocytes from vessel lumen to the interstitial tissue

Leukocyte regulates the inflammatory reactions of cytokines and other arachidonic acid metabolites such as prostaglandins, thrombroxane A2 etc.

Phagocytosis

Phagocytosis is the process of engulfment of particulate matters such as microbes, immune complex, cellular debris by phagocytes.

Usually, neutrophils and macrophages are the phagocytes. Phagocytosis involves three distinct steps:

1. Recognition and attachment2. Engulfment3. Killing and degradation

Figure: Phagocytosis

Step-1(Recognition and attachment): Neutrophils and macrophages recognize and attach microbes by several membrane receptors. Opsonization further enhances this step. Opsonin is a substance capable of enhancing phagocytosis by coating the microbes and making it more active for binding to specific receptors

Step-2 (Engulfment): Pseudopods flow around the microbes and enclose it within a phagosome formed by the plasma membrane of the cell which fuses with the limiting membrane of lysosomal granule forming phagolysosome

Step-3 (Killing and degradation): It is the ultimate step in the elimination of infectious agents i.e. the microbes within the phagocytes. Microbial killing occurs by nitric oxide, peroxyonitrites, hydrogen peroxide, and hypochlorous acid

Chemical Mediators of Inflammation

Changes in inflammatory responses are due to the production of chemical mediators in and around the area. These mediators performs their activity by binding to specific receptors or by some oxidative or enzymatic activity

These mediators can be derived from cells or plasma

Chemical mediators from cells:• Histamine• Serotonin• Lysosomal enzymes• Prostaglandins• Leukotrienes and lipoxins• Platelet activating factors• Cytokines• Nitric oxide• Activated oxygen species

Chemical mediators from plasma:• Complement fragments- C3a, C5a, C3b etc• Kinins- bradykinins, kallikrein• Thrombin, fibrinopeptides etc Histamine

It is found in high concentration in platelets, basophils, and mast cells

Causes dilation and increased permeability of capillaries

Prostaglandins The prostaglandins are ubiquitous, lipid soluble

molecules derived fro arachidonic acid, a fatty acid liberated from cell membrane phospholipids, through the cyclooxygenase pathway.

Prostaglandins contribute to vasodilation, capillary permeability, and the pain and fever that accompany inflammation.

The stable prostaglandins (PGE1 and PGE2) induce inflammation and potentiate the effects of histamine and other inflammatory mediators

They cause the dilation of precapillary arterioles (edema), lower the blood pressure, modulates receptors activity and affect the phagocytic activity of leukocytes.

The prostaglandin thromboxane A2 promotes platelet aggregation and vasoconstriction.

Leukotrienes The leukotrienes are formed from arachidonic acid, but

through the lipoxygenase pathway. Histamine and leukotrienes are complementary in action

in that they have similar functions. Histamine is produced rapidly and transiently while the

more potent leukotrienes are being synthesized slowly. Leukotrienes C4 and D4 are recognized as the primary

components of the slow reacting substance of anaphylaxis (SRS-A) that causes slow and sustained constriction of the bronchioles.

The leukotrienes also have been reported to affect the permeability of the postcapillary venules, the adhesion properties of endothelial cells, and stimulates the chemotaxis and extravascularization of neutrophils, eosinophils, and monocytes.

The cyclooxygenase and lipoxygenase pathways

Platelet-activating factor (PAF) It is generated from a lipid complex stored in cell membranes It affects a variety of cell types and induces platelet aggregation It activates neutrophils and is a potent eosinophil

chemoattractant It contributes to extravascularization of plasma proteins and so,

to edema. Plasma Proteases

The plasma proteases consist of: Kinins

Bradykinin - causes increased capillary permeability (implicated in hyperthermia and redness) and pain

Clotting factors The clotting system contributes to the vascular phase of

inflammation, mainly through fibrin peptides that are formed during the final steps of the clotting process.

Regeneration and Repairing

Repair is the replacement of injured or dead cells or tissues after injury like inflammation, wounds, surgical resection by proliferation of viable cells

Repair occurs by two distinct processes: Regeneration-which restores normal tissues, and Healing-which leads to scar formation and fibrosis. Mostly, repair occurs by a combination of these two processes

Repair begins early in inflammation sometimes in 24 hours after injury

Repair involves the proliferation of different types of cells and their interaction with the ECM (extracellular matrix).

Regeneration

Regeneration involves the restitution of tissue components identical to those removed or killed. Tissue with high proliferative capacity, such as epithelia of skin, GIT and hematopoietic system, renew themselves continuously and can regenerate after injury

Based on regenerative capacity, cells are divided into three groups, such as: continuously dividing cells (Labile cells), quiescent cells (Stable cells) and non-dividing cells (Permanent cells)

1. Continuously dividing cells (labile cells) such as hematopoietic cells of the bone marrow, stratified squamous epithelium, cuboidal epithelium of excretory ducts, and gastrointestinal tract. These tissues can easily regenerate after injury as long as stem cells are intact.

2. Quiescent cells (stable cells): They are quiescent and have minimal replication activity. However, cells are able to replicate in response to injury or loss of tissue mass. Stable tissues constitute the parenchyma of most solid organs such as the liver, kidney, and pancreas, as well as endothelial cells, fibroblasts, and smooth muscle cells. Stable tissues have a limited capacity to regenerate.

3. Permanent cells: They are terminally differentiated in post-natal life. Cardiac muscle cells and most neurons are in this category. Injury to brain and heart muscle results in liquefaction, necrosis and scar formation. The liver has a great regenerative capacity that occurs after surgical removal or injury of hepatic tissue. As much as 40% to 60% of the liver may be removed in a procedure called living-donor transplantation. In this situation, replication after partial hepatectomy is initiated by the cytokines TNF and IL6 that trigger the transition of hepatocytes

Repair by Healing

Healing by connective tissue replacement occurs in chronic inflammatory process, wound, and cell necrosis incapable of regeneration

Components of healing: Healing involves a number of orderly processes. But all processes do not occur in every healing. The components are:

a) Inflammation in response to injury with removal of damaged tissues

b) Proliferation and migration of parenchymal and connective tissue cells

c) Formation of granulation tissues, scar and fibrosisd) Wound contractione) Acquisition of wound strength

Formation of Granulation Tissue, Scar and Fibrosis

Angiogenesis and Vasculogenesis Blood vessels are formed by angiogenesis

and vasculogenesis originated by angioblasts (endothelial precursor cells (EPCs)) present in the bone marrow

Angiogenesis is involved in the development of collateral circulation at sites of ischemia and allowing tumors to increase in size. EPCs may migrate from the bone marrow to areas of injury but the mechanism is not known.

Migration of fibroblasts and ECM deposition (scar formation)

Scar formation takes place on the network of the newly formed granulation tissue and loose ECM. The scar develops in two steps:a) Migration and proliferation of fibroblasts

at the injury site, and b) Deposition of ECM by fibroblasts

The migration and proliferation of fibroblasts is triggered by several growth factors synthesized by activated endothelial and inflammatory cells, especially macrophages which also clear extracellular debris and elaborate mediators that induce fibroblast proliferation and ECM components.

The fibroblast migration starts early in wound healing, and continues for several weeks, depending on the size of the wound.

As the healing progresses, there is a decrease of the number of proliferating fibroblasts and newly formed blood cells but there is an increase in the deposition of ECM, particularly collagen.

Eventually, the granulation tissue becomes a scar composed of inactive spindle-shaped fibroblasts, dense collagen, fragments of tissue, and other ECM components.

ECM tissue remodeling After scar deposition, the ECM continues to

be modified and remodeled. The outcome of the repair process depends on the balance between ECM synthesis and degradation.

The degradation of collagen and ECM is done by matrix metalloproteinases (MMPs) which are zinc dependent. ECM can also be degraded by neutrophil elastase, cathepsin, plasmin, and other serine proteases.

Scar: The richly vascularized granulation tissue is converted into a scar composed of spindle-shapped fibroblasts, dense collagen, fragments of elastic tissue and other ECM components. The scar is collagenous at first and then a pale, avascular fibrous scar is formed

Fibrosis: Refers to the heavy deposition of collagen that occurs in organs such as lungs, liver and kidney following chronic inflammatory processes or in the myocardium after extensive ischemic necrosis (infarction).

Organization: It is the replacement of damaged tissue, inflammatory exudate, thrombus or hematoma by granulation tissue and ultimately by fibrosis

http://wps.prenhall.com/wps/media/objects/1551/1589206/web_tut/13_01/nav/13_01.html