seminar in pathology

1. Mediators of
inflammation
By
Ali Faris

2. Meadiators of inflammation
• Mediators are the substances that
initiate and regulate inflammatory reactions.
• These are: cell derived or plasma protein derived
• vasoactive amines,
• lipid products,
• cytokines,
• products of complement activation

3. • Mediators may be produced locally by cells at
the site of inflammation, or may be derived from
circulating inactive precursors (typically
synthesized by the liver) that are activated at
the site of inflammation

4. • Cell-derived mediators are normally sequestered
in intracellular granules and are rapidly
secreted upon cellular activation (e.g.,
histamine in mast cells) or are synthesized de
novo in response to a stimulus (e.g.,
prostaglandins and cytokines produced by
leukocytes and other cells).
• Plasma protein–derived mediators (complement
proteins, kinins) circulate in an inactive form
and typically undergo proteolytic cleavage to
acquire their biologic activities.

5. Mediators of inflammation

8. • 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

9. Cell-Derived Mediators
• Tissue macrophages, mast cells, and endothelial
cells at the site of inflammation, as well as
leukocytes that are recruited to the site from the
blood, are all capable of producing different
mediators of inflammation.

10. Vasoactive Amines: Histamine and
Serotonin
• The two vasoactive amines, histamine and
serotonin, are stored as preformed molecules in
mast cells and other cells and are among the
first mediators to be released in acute
inflammatory reactions.
• Acts on blood vessels.
• First mediators

11. 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
(IL1, IL8)

12. • 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.

13. Serotonin((5-hydroxytryptamine)
• is a preformed vasoactive mediator
• Present in plateletes and certain
neuroendocrinal cells like GIT.
• primary function is as a neurotransmitter.
• It induces vasoconstriction during clotting
• found within platelet granules that is released
during platelet aggregation .. It is produced
mainly in some neurons and enterochromaffin
cells, and is a neurotransmitter and regulates
intestinal motility.

14. Arachidonic Acid Metabolites:
Prostaglandins,Leukotrienes,
and Lipoxins
• Products derived from the metabolism of AA affect a variety of
biologic processes, including inflammation and hemostasis.
• AA metabolites, also called eicosanoids (because they are
derived from 20-carbon fatty acidscan mediate virtually every
step of inflammation
• their synthesis is increased at sites of
• inflammatory response, and agents that inhibit their synthesis
• also diminish inflammation.
• Leukocytes, mast cells, endothelial cells, and platelets are the
major sources of AA metabolites in inflammation.
• These AA-derived mediators act locally at the site of generation
• and then decay spontaneously or are enzymatically destroyed.

15. • AA is a 20-carbon polyunsaturated fatty acid (with
four double bonds)
• produced primarily from dietary linoleic acid
• and present in the body mainly in its esterified form
• as a component of cell membrane phospholipids.

17. 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
• COX2 is induced by inflammatory stimuli .

19. Thromboxane A2 (TxA2)
• derived by action of thromboxane synthase present in
platelets.
• Functions –
• platelets aggregation
• Vasoconstrictions
• it is unstable and converted to inactive formsTxB2

20. Prostacyclin( PGI2)
• formed by action of prostacyclin synthase present in
vascular endothelium.
• Functions
• increase vascular permeability
• inhibit platelets aggregation

21. • 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

22. 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 lipoxygenase
1. - lipoxygenase – present in neutrophils, convert AA to 5-
hydroxyeicosatetraenoic acid
• Act as chemotactic for neutrophils

23. • LTB4- Chemotactic and neutrophil activator.
• leads to aggregation and adhesion of the cells to venular
endothelium.
• Generation of ROS
• Release of lysosomal enzymes.

24. • 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.

25. Lipoxins
• Generated fromAA by the lipoxygenase
pathway.
• It suppress inflammation by inhibiting recruitment of
leukocytes.
• Inhibit neutrophil chemotaxis and adhesion to
endothelium.

26. Pharmacologic Inhibitors of
Prostaglandins
and Leukotrienes
• Cyclooxygenase inhibitors- aspirin and other
nonsteroidal anti-inflammatory drugs (NSAIDs)
• inhibit both COX-1 and COX-2
• COX1 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 gastrointestinal
tract)
• COX2 generates prostaglandins that are
involved only in inflammatory reactions.

27. • Lipoxygenase inhibitors- Zileuton
• Corticosteroids - broad spectrum anti-inflammatory
• Reduce the transcription of genes encoding COX2,
phospholipase A2, proinflammatory cytokines (e.g., IL1
andTNF), and iNOS
• Leukotriene receptor antagonists- block receptor . eg
Montelukast

28. Cytokines and Chemokines
• Cytokines are proteins produced by many cell types
(activated lymphocytes, macrophages, and dendritic
cells)
• mediate and regulate immune and inflammatory
reactions

29. 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 viaT lymphocytes and mast
cells

30. • Stimulus for secretion- microbial products, immune
complexes, foreign bodies, physical
injury, and a variety of other inflammatory stimuli.
• TNF production is induced byTLRs and other microbial
sensors.

31. Roles of cytokines in inflammation
• Endothelial activation.- byTNF 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.
• IL1 activates fibroblasts to synthesize collagen
and stimulates proliferation of synovial and other
mesenchymal cells.
• stimulatesTH17 responses- induce acute inflmmation

32. • 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

33. Roles of cytokines

36. 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.

37. 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

38. • 2. C-C chemokines- have the fist two conserved cysteine
residues adjacent.
• It includes- monocyte chemoattractant protein (MCP1),
• eotaxin,
• macrophage inflmmatory protein1 α (MIP1 α),
• RANTES (regulated and normalTcell expressed and secreted)
• attract monocytes, eosinophils, basophils and lymphocytes.
• eotaxin selectively recruits eosinophils.

39. • C chemokines -lack the first and third of the four
conserved cysteines.
• e.g., lymphotactin
• specific for lymphocytes.

40. • 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 andT cells,
• soluble form-derived
by proteolysis of the membrane bound protein,
• potent chemo-attractant

41. • 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

42. Other Cytokines in Acute
Inflammation
• IL- 6 – Secreted by macrophages involved in local and
systemic reactions
• IL-17 – Produced byT lymphocytes, promotes neutrophil
recruitment.
• Type I interferons – some systemic manifestations

43. Plasma Protein–Derived Mediators
• Circulating proteins of three interrelated systems—the
• complement, kinin, and coagulation systems—are
involved
• in several aspects of the inflammatory reaction.

44. 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

45. The activation and functions of the
complement system

46. • Complement proteins are present in inactive forms in the
plasma.
• The critical step in complement
activation is the proteolysis of the third
• Cleavage of C3 can occur by one of
three pathways:
• classical pathway- triggered by fixation of
C1 to antibody (IgM or IgG) that has combined with
antigen

47. • Alternative pathway- triggered by
microbial surface molecules (e.g., endotoxin, or LPS),
complex polysaccharides, cobra venom, and other
substances, in the absence of antibody
• The lectin pathway- plasma mannose binding
lectin binds to carbohydrates on microbes and directly
activates C1.

48. 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.

49. • 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.

50. 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.

51. 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

52. • 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.