Mediators of inflammation

Dr. Bahoran Singh

Meadiators of inflammation

• Mediators are the substances thatinitiate and regulate inflammatory reactions.

• These are: cell derived or plasma protein derived• vasoactive amines,• lipid products,• cytokines,• products of complement activation

Mediators of inflammation

• Active mediators are produced only in response to various stimuli like microbial products

• life span of mediators are very short• one mediator can stimulate the release of

other like complement activation cause release of histamine and cytokines

Vasoactive Amines: Histamine and Serotonin

• Acts on blood vessels.• First mediators• Histamine• Mast cells are richest source of histamine• Histamine stored as granules and released by

degranulation in response to various stimuli • 1. physical injuries• 2.Antibodies mediated – hypersensitivity reaction• 3. Complement products- anaphyltoxin C3a, C5a.• 4. Neuropeptides (e.g., substance P) and cytokines (IL 1, IL

8)

• Histamine causes dilation of arterioles and increases the permeability of venules.

• principal mediator of the immediate transient phase it

• it increased vascular permeability, producing interendothelial gaps in venules.

• Act via H1 receptors present on microvascular endothelial cells.

Serotonin((5 -hydroxytryptamine)• preformed mediator• Present in plateletes and certain

neuroendocrinal cells like GIT.• primary function is as a neurotransmitter.• vasoconstriction

Arachidonic Acid Metabolites

• The lipid mediators prostaglandins and leukotrienes are produced from arachidonic acid (AA) present in membrane phospholipids

• Stimulate vascular and cellular reactions.• Arachidonic acid derived from dietary sources

or by conversion from the linoleic acid.• Mechanical, chemical, and physical stimuli or

other mediators (e.g., C5a) release AA from membrane phospholipids through the action of cellular phospholipases, mainly phospholipase A2

• Phospholipase A2 Activation occur via increase in cytosolic Ca2 + and kinase in response to external stimuli.

Prostaglandins

• Produced by mast cells , macrophages, endothelial cells, .

• it involved in vascular and systemic reactions of inflammation.

• COX-1 & COX-2 is involved in synthesis of prostaglandins.

• COX -1 is produced in response to inflammatory stimuli and is also constitutively expressed

• COX 2 is induced by inflammatory stimuli .

Thromboxane A2 (TxA2)

• derived by action of thromboxane synthase present in platelets.

• Functions – • platelets aggregation• Vasoconstrictions• it is unstable and converted to inactive forms

TxB2

Prostacyclin( PGI2)

• formed by action of prostacyclin synthase present in vascular endothelium.

• Functions • increase vascular permeability• inhibit platelets aggregation

• PGD2 produced by mast cells along with PGE2.• it causes vasodilation and increases the permeability

of postcapillary venules, thus potentiating edema formation.

• PGF2- stimulates contraction of uterine and bronchial smooth muscles and small arterioles.

• Prostaglandins involved in the pathogenesis of pain and fever in inflmmation.

• PGE2 is hyperalgesic makes the skin hypersensitive to painful stimuli

Leukotriene

• produced by leukocytes and mast cells by the action of lipoxygenase.

• It involved in vascular and smooth muscle reactions and leukocyte recruitment.

• Three types of lipoxygenase1. - lipoxygenase – present in neutrophils,

convert AA to 5 hydroxyeicosatetraenoic acid• Act as chemotactic for neutrophils

• LTB4- Chemotactic and neutrophil activator.• leads to aggregation and adhesion of the cells

to venular endothelium.• Generation of ROS• Release of lysosomal enzymes.

• LTC4, LTD4, and LTE4- causes• vasoconstrictions• Bronchospasm • increased permeability of venules• Leukotrienes are more potent than is

histamine in increasing vascular permeability and causing bronchospasm.

Lipoxins

• Generated from AA by the lipoxygenasepathway.

• It suppress inflammation by inhibiting recruitment of leukocytes.

• Inhibit neutrophil chemotaxis and adhesion to endothelium.

Pharmacologic Inhibitors of Prostaglandinsand Leukotrienes

• Cyclooxygenase inhibitors- aspirin and othernonsteroidal anti -inflammatory drugs (NSAIDs)

• inhibit both COX-1 and COX-2• COX 1 is responsible for the production of

prostaglandins that are involved in both inflammation and homeostatic functions (e.g., fluid and electrolyte balance in the kidneys, cytoprotection in the gastrointestinaltract)

• COX 2 generates prostaglandins that areinvolved only in inflammatory reactions.

• Lipoxygenase inhibitors- Zileuton• Corticosteroids - broad spectrum anti-

inflammatory • Reduce the transcription of genes encoding

COX 2, phospholipase A2, proinflammatory cytokines (e.g., IL 1 and TNF), and iNOS

• Leukotriene receptor antagonists- block receptor . eg Montelukast

Cytokines and Chemokines

• Cytokines are proteins produced by many cell types (activated lymphocytes, macrophages, and dendritic cells)

• mediate and regulate immune and inflammatory reactions

Tumor Necrosis Factor (TNF) and Interleukin-1 (IL-1)

• roles in leukocyte recruitment• promoting adhesion of leukocytes to

endothelium and their migration through vessels.

• Produced by – macrophages and dendritic cells.

• TNF also via T lymphocytes and mastcells

• Stimulus for secretion- microbial products, immune complexes, foreign bodies, physicalinjury, and a variety of other inflammatory stimuli.

• TNF production is induced by TLRs and other microbial sensors.

Roles of cytokines in inflammation• Endothelial activation.- by TNF and IL-1• increased expression of adhesion molecules ( P- and

E- selectins and ligands foe leukocytes• increased production of various mediators• increased procoagulant activity• Activation of leukocytes and other cells-• stimulates microbicidal activity of macrophages by

inducing production of NO. • IL 1 activates fibroblasts to synthesize collagen

and stimulates proliferation of synovial and other mesenchymal cells.

• stimulates TH17 responses- induce acute inflmmation

• Systemic acute-phase response• Induced by IL-1, TNF and IL-6 • TNF regulates energy balance by promoting

lipid and protein mobilization and by suppressing appetite leads to cachexia

• TNF antagonists have been remarkably effective in the treatment of chronic inflammatory diseases- Rheumatoid arthritis and psoriasis

Roles of cytokines

Chemokines

• Chemokines are a family of small (8 to 10 kD) proteins that act primarily as chemoattractants for specific types of leukocytes.

• About 40 different chemokines and 20 different receptors for chemokines have been identifid.

Classification of chemokines

• Four major groups• 1. C-X-C chemokines- have one amino acid

residue separating the fist two of the four conserved cysteine residues.

• Act primarily on neutrophils.• Secreted by macrophages, endothelial cells• function- Activation and chemotaxis of

neutrophils• Eg. IL-8

• 2. C-C chemokines- have the fist two conserved cysteine residues adjacent.

• It includes- monocyte chemoattractant protein (MCP 1),

• eotaxin,• macrophage inflmmatory protein 1 α (MIP 1 α),• RANTES (regulated and normal T cell expressed

and secreted)• attract monocytes, eosinophils, basophils and

lymphocytes.• eotaxin selectively recruits eosinophils.

• C chemokines -lack the first and third of the four conserved cysteines.

• e.g., lymphotactin• specific for lymphocytes.

• CX3C chemokines-contain three amino acids between the two cysteines.

• eg- fractalkine exist in two forma• cell surface bound protein- promotes strong

adhesion of monocytes and T cells,• soluble form-derived

by proteolysis of the membrane bound protein,

• potent chemo-attractant

• Chemokines mediate their activities by binding to seven- trans membrane G protein–coupled receptors.

• Functions of chemokines- • In acute inflammation-stimulate leukocyte

attachment to endothelium by acting on leukocytes to increase the affinity of integrins,

• Chemotaxis• Maintenance of tissue architecture• homeostatic chemokines-produced

constitutively in tissues

Other Cytokines in Acute Inflammation

• IL- 6 – Secreted by macrophages involved in local and systemic reactions

• IL -17 – Produced by T lymphocytes, promotes neutrophil recruitment.

• Type I interferons – some systemic manifestations

Complement System

• The complement system is a collection of soluble proteins and membrane receptors that function mainly in host defense against microbes and in pathologic inflammatory reactions.

• The system consists of more than 20 proteins, some of which are numbered C1 through C9.

• functions in both innate and adaptive immunity

The activation and functions of the complement system

• Complement proteins are present in inactive forms in the plasma.

• The critical step in complementactivation is the proteolysis of the third

• Cleavage of C3 can occur by one ofthree pathways:

• classical pathway- triggered by fixation ofC1 to antibody (IgM or IgG) that has combined with antigen

• Alternative pathway- triggered bymicrobial surface molecules (e.g., endotoxin, or LPS), complex polysaccharides, cobra venom, and other substances, in the absence of antibody

• The lectin pathway- plasma mannose bindinglectin binds to carbohydrates on microbes and directly activates C1.

Functions of complements

• Inflammation- C3a, C5a and C4a• stimulate histamine release• They are called anaphylatoxins• C5a – chemotactic to neutrophils, monocytes,

eosinophils, and basophils.• Opsonization and phagocytosis- C3b and iC3b• Cell lysis- deposition of the MAC on cells

makes these cells permeable to water and ions and results in death (lysis) of the cells.

• The activation of complement is tightly controlled by cell-associated and circulating regulatory proteins.

• C1 inhibitor (C1 INH)- block C1 activation• Inherited deficiency of this inhibitor is the cause

of hereditary angioedema• Decay accelerating factor (DAF) and CD59 • DAF- inhibit C3 convertase formation• CD59 inhibits formation of the

membrane attack complex.• An acquired deficiency leads to PNH.

Other Mediators of Inflammation

• Platelet-Activating Factor (PAF)- phospholipid derived mediator

• Secreted by platelets, basophils, mast cells, neutrophils, macrophages, and endothelial cells

• It causes- platelets aggregation• vasoconstriction• bronchoconstiction• at low concentrations it induces vasodilation

and increased venular permeability.

Products of Coagulation

• Kinins- vasoactive peptides derived from plasma proteins, called kininogens, by the action of specific proteases called kallikreins.

• Bradykinin- increases vascular permeability, and smooth muscles contractions,

• Dilatation of blood vessels and pain • Neuropeptides-secreted by sensory nerves and

various leukocytes.• substance P and neurokinin A

• Substance P- prominent in nerve fibers in lungs and GIT.

• Functions- transmission of pain signals• regulation of blood pressure• stimulation of hormone secretion by

endocrine cells• increasing vascular permeability.

Morphologic Patterns of Acute Inflammation

• The morphologic hallmarks -dilation of small blood vessels

• accumulation of leukocytes and fluid in the extravascular tissue.

• Various morphological pattern occur-• Serous inflammation• Fibrinous inflammation• Purulent (Suppurative) Inflammation, Abscess• Ulcers

Serous Inflammation

• Exudation of cell poor fluid into spaces created by cell injury or into body cavities lined by the peritoneum, pleura, or pericardium.

• not infected by destructive organisms and does not contain large numbers of leukocytes.

• accumulation of fluid in these cavities is called an effusion.

• Eg. Skin blister in burn or viral infections

cross-section of a skin blister showing the epidermis separated from the dermis by a focal collection of serous effusion

Fibrinous Inflammation

• A fibrinous exudate develops when the vascular leaks are large or there is a local procoagulant stimulus (e.g., cancer cells)

• Fibrinous exudate is characteristic of inflammation in the lining of body cavities, such as the meninges, pericardium and pleura.

• Histologically, fibrin appears as an eosinophilic meshwork of threads or sometimes as an amorphous coagulum.

• Fibrinous exudates may be dissolved by fibrinolysis and cleared by macrophages.

• If fibrin not removed it stimulates ingrowth of fibroblast and blood vessels leads to scarring.

• Conversion of the fibrinous exudateto scar tissue (organization) within the pericardial sac leads to opaque fibrous thickening of the pericardium and epicardium.

Fibrinous pericarditis. A, Deposits of fibrin on the pericardium. B, A pink meshwork of fibrin exudate (F) overlies the pericardial surface (P)

Purulent (Suppurative) Inflammation, Abscess

• characterized by the productionof pus, an exudate consisting of neutrophils, the liquefied debris of necrotic cells, and edema fluid.

• Most common cause – bacterial infections (staphylococci)

• Abscesses are localized collections of purulent inflammatory tissue caused by suppuration buried in a tissue, an organ, or a confined space.

• Abscesses have a central region that appears as a mass of necrotic leukocytes and tissue cells.

• zone of preserved neutrophils around this necrotic focus, and outside this region there may be vascular dilation and parenchymal and fibroblastic proliferation.

Purulent inflammation. A, Multiple bacterial abscesses (arrows) in the lung in a case of bronchopneumonia. B, The abscess contains neutrophils and cellular debris, and is surrounded by congested blood vessels.

Ulcers

• An ulcer is a local defect, or excavation, of the surface of an organ or tissue that is produced by the sloughing (shedding) of inflamed necrotic tissue

• Most commonly encountered in • the mucosa of the mouth, stomach, intestines,

or genitourinary tract• skin and subcutaneous tissue of the lower

extremities

A chronic duodenal ulcer. B, Low-power cross-section view of a duodenal ulcer crater with an acute inflammatory exudate in the base.

Outcomes of Acute Inflammation