33. Acute Inflammation
34. Phases of Inflammation
35. Cellular events in inflammation
Acute inflammation – the initial rapid response, developed within minutes and lasts up to days, associated with the innate
immunity of the host. Characterized by the presence of edema and neutrophils.
It has 3 major components:
1. Dilation of small vessels leading to increase blood flow
2. Increased permeability of microvasculature
3. The increased permeability enables migration of leukocytes and their activation in the focus of injury to eliminate
to offending agent.
Inflammation has 3 major phases:
1. Alteration – change – it all begins with a change caused by a damage (or infectious agent) to the tissue –
alteration of homeostatic state, alteration of normal tissue organization and components.
2. Exudation – edema formation ad immigration of WBCs.
3. Proliferation – of immune cells to deal with the damage, and later on, of endothel and fibroblasts for repair.
In reality, all these phases overlap in different manners, and we may observe all of them occurring at once.
Based on this, inflammatory response has the following steps – 5 R's:
1. Recognition of the injurious agent – the damage or microbe, based on substances released or specific features
they have.
2. Vascular Events - increase blood flow and permeability to let WBC perform diapedesis
3. Recruitment of leukocytes – mediated by chemokines and adhesion molecules
4. Removal of the agent – by leukocytes activation, performing phagocytosis.
5. Regulation – to ensure right time termination, to prevent excessive host damage.
6. Repair – endothelial and fibroblasts proliferation.
Alteration - Can be either due to tissue damage or the presence of infectious agent.
1. Recognition of injurious agent:
Recognition of microbes:
PAMP – pathogen-associated molecular pattern – a structural component on a microbial cell or virus which can act as
an antigen and be recognized by PRRs. E.g. LPS, peptidoglycans, lipoteichoic acids, mannose-rich glycans, pilin and
flagellin.
PRRs - Pattern recognition receptors – membrane-bound receptors of phagocytes that recognize PAMPs. Were first
observed in Drosophila where they are called Toll receptors, and thus in human:


TLR – Toll-like receptors – types of PRRs, each TLR recognized a specific PAMP. TLR activation results in
upregulation of NF-kB, a nuclear transcription factor that promotes inflammatory mediators
production. E.g M1 marcrophages with CD14 – recognizes microbes.
Fc receptors – for Fc region of antibodies, promote opsonization by the AB and complement
activation.
Recognition of cell damage:
All cells have cytosolic receptors that recognize molecules that were liberated or altered due to damage.
These molecules include Uric Acid, ATP from damaged mitochondria, DNA released.
These receptors activate a cytosolic complex called inflammasome, which induces release of IL-1. This, recruits leukocytes
and thus induces inflammation.
Gain of function mutation of the receptors cause auto-inflammatory syndromes – characterized by spontaneous
inflammation.
Tissue phagocytes are the first to reach the damaged cells, and release cytokines which trigger the blood vessel events and
the leukocytes recruitment.
Exudation
2. Blood vessel events:
The vascular reactions consist of changes in the blood flow and the
permeability of vessels – both designed to maximize the movement of
plasma proteins and leukocytes toward the injured tissue in a process
called exudation – extravascular fluid that has a high protein
concentration which results in edema.

Change is vascular flow:
Vasodilation – action of mediators, mainly histamine, on vascular smooth muscle. The result is increase blood flow which is
the cause of the heat and redness. Vasodilation followed by increased permeability and outpouring of exudate. Loss of
fluid and increased vessel diameter lead to slower blood flow and higher RBCs concentration  increased viscosity,
results in stasis (observe histologically as vascular congestion of acute inflammation).
As stasis developed, neutrophils accumulate along the endothelium. The endothelial cells are activated, mediated by TNFalpha and IL-1 and express increased levels of selectin. Leukocytes then adhere to endothelium and migrate through the
vascular wall.
1.
2.
3.

Increased vascular permeability:
Retraction of endothelial cells, mainly in the post-capillary venules, resulting in opening of
inter-endothelial spaces – elicited by histamine, bradykinin, leukotrienes and more.
Endothelial injury – direct damage to the endothelium, such as in burns or induced by
microbes or by neutrophils adherent, may cause injury to the endothelial cells and increase
permeability.
Transcytosis – transport through the endothelial cells, involve intracellular channels that open
and response to certain factors (VEGF).
Lymphatic vessels:
Drain edema fluids – by increasing the lymph flow.
May become secondary inflamed – lymphangitis, as well as the lymph nodes – lymphadenitis.
Proliferation
3. Recruitment of leukocytes: Margination  Rolling  Adhesion  Transmigration  Chemotaxis
The most important leukocytes for inflammation are the phagocytes – neutrophils and macrophages.
But, Th17, releasing IL-17 has been found to be very important in inducing acute inflammation. In its absent “cold
abscesses” develop on the skin, lacking the classic feature of warmth and redness.
Inflammatory cytokines released from damaged tissue and tissue macrophages, recruit leukocytes - initially mainly
neutrophils, which reach the damaged tissue in a multistep process that is mediated by adhesion molecules and cytokines.
1.
Margination – vasodilation slows the blood flow in the postcapillary venules. Cells marginate from the center of the
flow to the periphery.
2.
Rolling – a process mediated by selectin proteins – initial weak leukocytes-endothelium interaction:


E-selectin – expressed on endothelium, induced by TNF and IL-1 (from macrophages and others).
P-selectin – released from Weibel-Palade bodies of endothelium to be expressed on its surface, induced by histamine
and thrombin. (Present also on platelets.)
Leukocytes have ligands for endothelial selectins: selectins bind sialyl Lewis X on leukocytes – these interactions have low
affinity, thus they bond and easily detach by the blood flow – resulting in rolling along the vessel wall.
3.


Adhesion – mediated by integrins on leukocytes, and their ligands on the endothelium :
Ligands - Endothelial ICAM and VCAM – (vascular and intercellular cell adhesion molecule) – ligands for integrins
expressed on the endothelium, induced by TNF and IL-1.
Leukocytes Integrins – normally are found in a low affinity state, turns into high affinity state by chemokines which
resulting in firm binding at the site of inflammation  Rolling stops. Integrins are also upregulated by C5a.

Leukocyte adhesion deficiency – AR defect of integrins, proof of the importance of integrins: result in recurrent
bacterial infections.
4.
Transmigration – Diapedesis – occurs in post capillary venules – the site of maximal retraction of endothelial cells.
Movement of leukocytes is driven by chemokines produced in the extravascular tissues – stimulate leukocytes to
travel along a chemical gradient. Leukocytes then pierce the basement membrane, probably by secretion of
collagenases.
5.
Chemotaxis – locomotion along a chemical gradient from the damaged site – guides the leukocytes to the site of
injury. Chemo attractants can be:


Exogenous – bacterial products.
Endogenous –

Cytokines (of chemokine family – IL-8),

Components of the complement system (C5a)

Arachidonic acid metabolites – mainly leukotrienes.
These bind receptors on leukocytes, which initiate signal transduction that results on polymerization of actin
filaments to one pole of the cell  leading to its migration in the direction of the damage.
Predominant leukocytes:


Neutrophils – predominant in the first 6-24 hours. This is because they are the most numerous
leukocytes, they respond more rapidly to chemokines and adhere better to adhesion molecules. But,
they have short lives in the tissues – they undergo apoptosis within 24-48 hours.
Monocytes – predominant in 24-48 hours. They live longer and even proliferate within the tissue.
4. Removal of the agent:
Recognition of microbes or dead cells induces Leukocyte activation – signaling pathways (especially Ca2+ pathways) which
lead to effective destruction of the target. 4 mediators are responsible for activation:
1. LTB4 – leukotriene B4.
2. C5a
3. IL-8
4. Bacterial products.
Phagocytosis:
Involves 3 steps – recognition, engulfment and killing/degradation:
1. Recognition – by phagocytic receptors:

MBL – Mannose-binding lectin – lectin found on macrophage wall, binds mannose or fucose which are part of
molecules found on microbial cell walls (it can also activate complement).

Scavenger receptors – bind microbes and also LDL particles.

Opsonins receptors – the efficiency of phagocytosis is greatly enhanced when microbes are opsonized (coated)
by opsonins for which the phagocytes express high affinity receptors – such as IgG and C3b.
2. Engulfment – after a particle is bound to phagocyte receptors, extensions of the cytoplasm – pseudopods – flow
around it and engulf it into cytosolic vesicle called – phagosome, fused with lysosomes, release lysosomal content.
3.
Intracellular killing

Oxygen-dependent – oxidative burst –
NADPH-oxidase – on the phagolysosome membrane takes O2 and NADPH and forms a superoxide O2- radical.
Superoxide dismutase – converts it to H2O2+O2.
Myeloperoxidase – combines H2O2 with Cl- to create HOCl – hypochlorous acid, which attacks and kills the bacteria.
H2O2 is also converted to hydroxyl radical HO-, another powerful destructive agent.
Serum, tissue fluids and host cells possess anti-oxidant mechanisms that protect against leaked radicals: superoxide
dismutase, catalase and glutathione peroxidase, as well as non-enzymatic anti-oxidants (e.g. Vitamin C, E)
NO – nitric oxide, produced from arginine by the action of iNOS (inducible) – which is expressed when macrophages are
activated by cytokines (IFN-gamma) or microbial products. NO reacts with superoxide to generate the highly reactive free
radical peroxynitrite (ONOO-) – attack the microbes components.
1.
2.
3.
Associated disease - Chronic granulomatous disease (CGD) – poor O2-dependent killing due to a defective NADPH
oxidase. The patient then gets chronic granulomas. It’s X-linked or AR. This is associated with infection of organisms which
are catalase positive. (most important - Pseudomonas)

Oxygen-independent – function of phagolysosome:
H+ pumping into the phagosome – reducing the pH.
Acidic hydrolases of the lysosome kill gram-positive bacteria by disrupting their cell membrane, but they are less
effective against gram-negative bacteria, which are killed principally by the oxygen-dependent mechanism.
Defensins – penetrate the bacterial membrane.
Regulation of phagocytosis achieved by antiproteases found in the serum
and ECM, prevent excessive inflammation, e.g. - α1-antitrypsin – inhibits
neutrophil elastase.
NETs – Neutrophil Extracellular Traps: Another mechanism of killing
microbes that doesn't involve phagocytosis. NETs are extracellular fibrillary
networks that concentrate anti-microbial substances at sites of infection
and prevent the spread of the microbes by trapping them in the fibrils.
5. Regulation:
Such a powerful system sometimes causes injuries, as their mechanisms are
not specific and may also damage host cells. Termination on time is
important to prevent inflammation which isn’t necessary anymore:
Inflammation declines after removal of the offending agent – simply
because mediators of inflammation are produced as long as they have stimuli from the injurious agent.
Short half-life – neutrophils have short half-life, they die by apoptosis within a few hours in tissues.
Release of anti-inflammatory substances – as part of the process:

Switch in the type of arachidonic acid metabolites produced, from pro inflammatory leukotriens to antiinflammatory lipoxins.

Anti-inflammatory cytokines - TGF-β and IL-10 – from macrophages.

Neural impulses – cholinergic discharge, inhibit production of TNF by macrophages.
Proliferation of cells needed for repair process:
6. Repair:
Repair of damaged tissues occurs by one of two types of reactions:


Regeneration by proliferation of residual cells (that survived the injury) and maturation of
tissue stem-cells
Scar formation – deposition of connective tissue to form scar.
(More in question 36)
Mediators of inflammation:
Mediators 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 (e.g. complement proteins). The major cell types that produce
mediators of acute inflammation are tissue macrophages, dendritic cells and mast cells.
I.
Vasoactive amines – important actions on blood vessels:
1. Histamine – mainly from mast cells (also stored in basophils and platelets) – released by degranulation in
response to physical injury or by binding of IgE antibodies to mast cells Fc receptors, as well as product of
complement that bind to receptors on mast cell (anaphylatoxins – C3a, C5a). Neuropeptides (Substance P) and
cytokines (IL-1, IL-8) also trigger degranulation.
2. Serotonin – secreted from platelets. Its importance in inflammation is unclear.
Once mast cell is activated, it undergoes:
Immediate response: Release of histamine granules – inducing vasodilation of arterioles and increased permeability in the
post capillary venules. It binds H1 receptors on endothelial cells. Antihistamine drugs are commonly H 1 antagonists which
block the receptor.
Delayed response: Release of arachidonic acid metabolites:


Leukotriens – promote vasodilation and permeability.
Prostaglandins – vasodilation, permeability and pain.
These are responsible for maintenance and progression of the acute inflammatory response.
II.
Arachidonic acid metabolites:
Arachidonic acid FA found as part of the membranes, and is released due to
inflammatory stimuli by the action of phospholipase A2. Once released it can
proceed in one of 2 pathways:
I.
COX – cyclooxygenase pathway – generation of prostaglandins.
Produced by – mast cells, macrophages, endothelial cell and many others.
Major ones – PGI2 (prostacyclin – TXA2 antagonist), PGD2, PGE2 – mediate vasodilation and increase vascular permeability.
PGE2 – also mediates fever and pain
Thromboxane A2 – causes vasoconstriction and promotes platelet aggregation, while PGI2 – vasodilator and inhibitor of
platelet aggregation.
Inhibition of prostaglandin synthesis:

Cortisol – has an anti-inflammatory effect by inhibiting phospholipase A2.

NSAIDs e.g. Aspirin, inhibits COX1 and COX2.
II.
LOX – lipoxygenase pathway – generating leukotriens and lipoxins.
Produced by – leukocytes and mast cells.
Leukotriens:

LTB4 – chemoattractant - attracts and activates neutrophils, promote adhesion to endothelial cells.

LTC4, LTD4, LTE4 – mediate vasoconstriction, bronchospasm, and increase vascular permeability of venules. These
are more potent than histamine in inducing vascular
permeability (by causing contraction of the pericytes).
Lipoxins:

Theses suppress inflammation by inhibiting recruitment of
leukocytes.
III.
Cytokines and chemokines:
Mediate and regulate immune and inflammatory reactions.
TNF and IL-1 – Tumor necrosis factor –
Produced by – macrophages, mast cells and T cells, endothelium, by
signals through TLRs.
Functions 
Endothelial activation – expression of adhesion molecules, increased procoagulant activity of endothelium,
production of more cytokines.

Activation of leukocytes and other cells –
 TNF – activates leukocytes by inducing their NO synthetase activity.
 IL-1 – activates fibroblast to secrete collagen and stimulates Th17.
Systemic acute-phase response – induce fever and cachexia, hypotension (may lead to shock in cytokine strom)
IL-6 – produced by macrophages, involved in local and systemic reactions.
IL-17 – produced by Th17 and promotes neutrophil recruitment.
Chemokines – small proteins that act as chemoattractants – promote chemotaxis and leukocyte activation.
IV.
Complement system:
A weapon especially against bacterial infection. Activation by 3 possible pathways initiates a cascade of proteolytic events.
Functions:
Opsonization – bind to the agent to promote their phagocytosis – C3b.
Lysis - directly kill the infecting agent – MAC – especially important against Neisseria.
Chemotaxis and Increase vascular permeability – by stimulating histamine release from mast cells, to allow access to
the site of infection. Done by C3a and C5a.
Agglutination – causing pathogens to adhere each other.
There are 3 activation pathways of the complement, which all activate the C3 component:
I.
Classical pathway – triggered by fixation of C1 to IgM or IgG antibody that has combined with antigen
II.
Alternative pathway – triggered by microbial surface molecules in the absence of antibody
III.
Lectin pathway – plasma mannose-binding lectin binds to carbohydrates on microbes and directly activates C1
All three pathways lead to the formation of an enzyme called C3 convertase, which splits C3 into two functionally distinct
fragments – C3a and C3b. C3a is released, and C3b becomes attached to the cell/molecule where the complement is being
activated – leading to further more C3b attach, forming C5 convertase, which cleaves C5 to release C5a and C5b – attached
to the cell surface. C5b binds to the late components (C6-C9) to form MAC and cell lysis (cells permeable to water and ions
resulting in their osmotic death).


-
C5a, C4a and C3a – Inflammatory mediators stimulate histamine release from mast cells –
increase vascular permability and vasodilation. C5a
is a chemotactic agent for leukocytes, and also
activates lipoxygenase pathway of arachidonic
acid.
C3b – when fixed to microbial cell wall, act as
opsonin and promote phagocytosis.
V.
Platelet activating factor – PAF:
Produced by – platelets, basophils, mast cells, neutrophils, macrophages and endothelium.
Functions –



Platelet aggregation.
Vasoconstriction and bronchoconstriction.
Vasodilation and increased vascular permeability – in low concentrations.
VI.
Products of coagulation:
Coagulation is tightly linked with inflammation. All forms of tissue injury that lead to clotting also induce inflammation, and
inflammation causes changes in endothelial cells that increase the likelihood of cbnormal clotting (thrombosis).
Hageman factor – Factor XII:
Inactive proinflammatory protein - produced by the liver. Gets activated upon exposure to subendothelial collagen.
Plays an important role in DIC – especially in severe gram-negative sepsis, in which these organisms also have the
ability to activate Hageman factor.
-
Upon activation it turns on:

Coagulation and fibrinolytic system

Complement

Kinin system – by cleaving kininogen to bradykinin.
VII.
Kinins:
Vasoactive peptides derived from plasma proteins called kininogens, by the action of enzymes – kallikreins.
Bradykinin – functions:



-
Vasodilation and increased vascular permeability.
Contraction of smooth muscle.
Pain – when injected into the skin.
Short-lived – quickly inactivated by kininase.
VIII.
Neuropeptides:
Substance P – transmission of pain signals, and increase vascular permeability.