The Inflammatory Response:

The simple reddening of the tissues, which is seen when e.g. the skin in wounded is in fact the outcome of a very complex regulatory process.

The inflammatory response acts to

· seal the site of injury

· prevent opportunistic infections

· neutralize established infection

Overview

Tissue based “startle” reaction to trauma

Initiation of activating proteolytic cascades - “Go signals”

Molecular cues for the beckoning, instruction and dispatch of leucocytes

Killing of microbes and host cells they infect

Liquefaction of surrounding tissue to prevent microbial metastasis

Healing of tissues damaged by host response

The inflammatory response may cause more problems than any infection.

Disorders in which an important pathogenic role is assigned to inflammation

Alzherimers disease Multiple sclerosis

Asthma Psoriasis

Atherosclerosis Rheumatoid arthritis

Crohn Diseases Type I diabetes

Xenograft rejection

Diseases where inflammation may contribute to the overall pathology

Bacterial dysentery Influenza virus pneuomonia

Cystic fibrosis Leprosy (tuberculoid form)

Heliobacter pylori gastris Tuberculosis

Hepatitis C

BUT, inflammation is usually life preserving

As shown by patients which have a genetic deficenicy in a component to the inflammatory process.

e.g. inability to produce complement components predisposes to meningococcal infection.

An important concept to grasp is that the inflammatory response occurs in two phases

· a proinflammatory stage and an

· anti-inflammatory stage.

..and the same signal can have either effect…

What governs the role of a signal is its context i.e. interaction with other signals which are

derived from

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Cellular (Leukocytes) Components of Inflammation.

Polymorphonuclear cells

Click on image for larger version

Neutrophils:

· Major leukocytes present in bone marrow and blood.

· Differentiate under the influence of Granulocyte and Granulocyte-macrophage colony stimulating factors.

May have medical role in e.g. chemo and radiotherapy.

· Terminally differentiated

· Rapidly recruited to site of inflammation / infection

· In response to phagocytic stimulus undergo – an “oxidative burst”

Major antimicrobial and cytotoxic mechanism.

Produced by an NADPH oxidase. b₅₅₈ cytochrome

Uses NADPH as a reductant provide electrons across the membrane.

· Recruited cells undergo apoptosis.

· Neutrophils- the most motile of leukocytes- will accumulate first, though they have the shortest “half-life” (6-72h).

After 12-48h macrophages will predominate.

Eosinophil and basophils

· more weakly phagocytotic than neutrophils.

Their roles are less well defined.

Sentinel Cells – Mast Cells and Macrophages

Mast cells are scattered in the skin and muscosa.

Originate from a haematopoietic lineage

Circulate in the blood and lymphatic system homing into target tissues.

Triggered by

Allergens complexed to IgE

C3a and C5a anaphylabtoxins of complement system.

Perivascular mast cells release

histamine, eicosanoids,

(cause vasodilation - responsible for heat and redness- and extravasation of fluid; the cause of swelling).

pre-formed “Tumour Necrosis Factor”. IL-1. Activates neutrophils initiating small amounts of elastase.

Cleaves the anti-adhesive coat of CD43 (leukosialin) allow integrins to engage in endothelial extracellular matrix proteins,

Trytases cleave protease-activated receptors

Neotermini engage G-protein-coupled receptors the net effect of which is make endothelium sticky for leukocytes and leaky to fluid.

Mutants show that the following signals for switching from killing to healing.

Secretory leukocyte protease inhibitor (SLP1) - suppresses the release of elastase and ROI by TNF-stimulated neutrophils

TNF

Mononuclear phagocytes (macrophages).

· Made in bone marrow before entering circulatory system. (A reserve is “retained”).

· Once reaching target tissue they have a life span of several months- macrophages.

Macrophages produce

· Proteases, hydrolyases, Oxygen free radicals

· Complement

· Prostaglandins and leukotrienes

· Interleukin 1 (IL-1) and Tumour Necrosis Factor-a (TNF a)

Cellular Events in Inflammation

The major cellular event during inflammation is the movement of cells to the site of injury.

Leukocytes then become sticky, at first rolling over the inflamed endothelium following adhering tightly before dispedesis (transmigration).

The processes underlying each stage are increasingly well understood.

Chemotaxis

Local injury results in the production of chemokines especially IL-8.

Side point: What are chemokines?

Chemokines have

· Leukocyte chemoattractant

· Cytokine-like activity.

First discovered when IL-8 was a monocyte derived neutrophil-specific chemotactic factor.

No previous chemoattractant was selective for specific leukocyte subtypes.

Key directors of

(i) specific leukocyte trafficking under emergency conditions such as inflammation.

(ii) cell-type specific interactions with growth factors e.g.

· Angiogenesis – blood vessel development

· Hematopoiesis – blood cell development in bone marrow

(iii) Anti-viral agents .

Certain cytokines can suppress HIV-1 interaction with

specific chemokine

receptors of leukocytes that are important for viral entry.

Showed that some microbes act my molecular mimicry.

What makes a chemokine?

Defined primarily by structural criteria –

· Single polypeptide chain 70-100aa

· 25-95% amino acid sequence homology

· Conserved cysteines – used to distinguish four main subfamilies

C, CC, CXC and CX₃C

· Lymphotactin – only member of the C family

· Fractalkine - only member of the CX₃C family and which has a transmembrane domain allowing it to be tethered to the cell surface.

Chemokine Receptors

· 15 known chemokine receptor subtypes

· bind multiple chemokines

· in a sub-class specific manner

· Seven transmembrane G-protein coupled receptors

Other major attractants..

Complement components (mainly C5a)

Kinins

Collagen and fibrin breakdown products

Bacterial (e.g. from pathogens) products, particularly lipopolysaccharide (LPS; endotoxin).

The attractants activates the cells locomotor apparatus and amoeboid movement (by imitation of cytoskeletal movement).

Margination

Macrophages produce IL-1, TNF cytokines which induce the expression of adhesion molecules- SELECTINS.

Selectins are carbohydrate-specific molecules involved in weak-reversible binding to sugar moieties on the cell surface.

Selectins are responsible for the rolling and margination of neutrophils.

Click here for a movie showing neutrophil rolling.

Click on image for larger version.

P- selectin is a likely initiator of rolling.

In P-selectin deficient mice, no rolling is seen.

Stored in Weibel-Palade bodies of endothelial cells and is released within minutes of treatment with such proinflammatory agents as histamine and thrombin.

L-selectins – expressed in leukocytes and is involved in interactions with endothelial cells. Responsible for continued rolling.

E-selectins – are expressed in the endothelial cells and interact with neutrophil carbohydrate.

Firm binding is brought about by IL-8 induced expression of b2 integrins. These also direct the arriving neutrophils to the site of injury.

Leukocytes b2 integrin – heterodimeric transmembrane glycoprotein with a common b chain (CD18) and three a chains (CD11a,b,c).

b2 integrins also bind to the C3 component of complement.

Neutrophils express elastase cleaves the anti-adhesive coat of CD43 (leukosialin) allow integrins to engage in endothelial extracellular matrix proteins

Leucocyte activation results in their “lining up” along the endothelial surface.

Emigration

The process through which leukocytes enter the perivascular tissue by moving BETWEEN the endothelial cells.

Occurs 30-40 min after inflammatory stimulation.

Once in contact with the endothelial cells they extend pseudopodia and migrate through tissue.

The cells migrate through intercellular junctions in the endothelium using a plasma membrane glycoprotein (CD31) –

CD31 is a member of immunoglobulin superfamily which is expressed in neutrophils, monocytes as well as the endothelium.

Hence can get homophillic binding.

Results in a dispedesis without leakage.

Plasma leakage is a sign of acute inflammation.

Within the tissue IL-8, C5a complement and leukotrienes are potent chemoattractants.

Ligate to specific receptors on the neutrophil surface initiating cytoskeletal re-organisation, respiratory burst as well as chemotaxis.

Initiating the Inflammatory Responses -

Click on either image for a larger version.

1. Plasma-derived mediators

Proteins, which circulate in the plasma, usually in an “inactivated” form.

Hageman Factor (XII of the coagulation cascade)

Induced by b-globulin which is activated by contact with negatively charged surfaces –

· Collagen and basal membranes (i.e. wounded tissue).

· Antigen: Antibody complexes (e.g. infections).

Activated factor XIIa initiated three major processes…..

· Coagulation of blood proteins – fibrin formation.

· Plasmin activation

Plasminogen cleavage releases plasmin; a broad specificity protease which degrades fibrin (breakdown products increase vascular permeability) and activates complement system.

Activates more factor XII – positive feedback loop.

Generation of kinins.

Bradykinin is the major product of these reactions.

Increases vascular permeability, smooth muscle contraction and “causes pain”.

ACTIVATES MACROPHAGES

The developing Inflammatory Response : The Acute Phase

Characteristics

(i) Fever (hormonal interaction with the hypothalamus in the brain)

(ii) Increased number of circulatory neutrophils

(iii) Synthesis of acute phase proteins – synthesized predominantly in the liver.

Gene expression during the active phase has pro or anti-inflammatory roles.

Control of blood clotting and repair Function

alpha-2 antiplasmin modulation of coagulation cascade

Factor VIII clotting formation of fibrin matrix for repair

Fibrinogen clotting formation of fibrin matrix for repair

Fibronectin fibrin clot formation

Haptoglobin haemoglobin scavenger

Heme oxygenase heme degradation

Cytokine-like Function

C-reactive protein Activates complement

interaction with T-cells and B-cells

Kallikreins vascular permeability and dilatation

Complement and associated functions Function

C1 inhibitor negative control of complement cascade

C2, C4, C5 and C9 complement component

Plasminogen proteolytic activation of complement, clotting, fibrinolysis

Proteinase inhibitors Function

Act to prevent the migration of leukocyte cells preventing the establishment of a

systemic inflammatory response.

alpha-1 antichymotrypsinogen

alpha-1 antitrypsin

Plasminogen activator inhibitor-1

Many of the above pro- and anti-inflammatory proteins are regulated by cytokines which are produced by activated macrophages.