Overview of Acute and Chronic Inflammation

Acute inflammation: Vascular changes and cellular events

Events across time: Oedema first>then neutrophils>then macrophages

Vascular response to inflammation

1. Vasodilation induced by histamines on vascular smooth muscle is one of the earliest manifestations of acute inflammation. It involves mostly post-capillary venules and results in increased blood flow (heat and redness). Increased vessel diameter and loss of fluid (exudate) leads to slowing of blood flow (stasis) and increased RBC concentration, causing vascular congestion histologically.
2. Increased permeability as endothelial cells contract, opening inter-endothelial gaps. Elicited by histamine, bradykinin, leukotrienes. Usually immediate and short lived (12-30mins), except can be delayed 2-12 hours in the case of burns. Permeability can also be increased by endothelial injury (direct damage).
3. Emigration of leukocytes to the site of injury/infection

Cellular response to inflammation

1. Recognition

• PRR's (pattern recognition receptors)
  • Outside cell: Most recognition is done by the innate immune cells via Toll-like receptors which are expressed on epithelial cells (often first line of defence) as well as tissue-resident sentinel cells e.g. dendritic cells, mast cells, macrophages and other leukocytes.
  • Inside cell: NOD-like receptors are cytosolic receptors in all cells, that recognise signs of cell damage e.g. uric acid (product of DNA breakdown), ATP (released in damage mitochondria), reduced intracellular K+ (reflecting plasma membrane injury), DNA (normally sequestered in nucleus). These receptors activate the inflammasome complex in the cell, which produces IL-1, recruiting leukocytes to remove the damaged cells.
• Opsonisation: Microbes coated with antibodies or complement are recognised by leukocytes via receptors for the Fc tails of antibodies and for complement proteins. The major opsonins are IgG and C3b.
• Mannose-binding lectin recognises microbial sugars and activates the complement cascade (there are 3 complement pathways; alternate, classical and mannose-binding lectin.

2. Recruitment of leukocytes(neutrophils) and plasma proteins e.g. complement

• Margination in blood vessel
• Rolling (mediated by selectins i.e. L-selectin on leukocytes, E-selectin on endothelium, P-selectin on platelets and endothelium)
• Firm adhesion to endothelium (mediated by integrins)
• Transmigration across endothelium (also called diapedesis)
• Migration towards a chemotactic stimulus (chemo-attractants include cytokines such as IL-8, complement such as C5a, arachidonic acid metabolites such as leukotriene B)

3. Removal by phagocytic cells (e.g. neutrophils and macrophages)

• In acute inflammation neutrophils predominate in the first 6-24 hours, and are replaced by monocytes in 24-48 hours. However in certain infections e.g. Pseudomonas, neutrophils predominate for several days. In viral infections lymphocytes may be the first cells to arrive.

4. Regulation (termination of the reaction)

5. Repair of damaged tissue

• The acute inflammatory reaction subsides as the initiating stimulus is removed, mainly because the cells and mediators of inflammation have short half lives
• Anti-inflammatory cytokines include TGF-β and IL-10

Acute Inflammation: Chemical mediators

Vasodilation: histamines

Increased vascular permeability: histamine, serotonin*, C3a and C5a, Leukotrienes

Chemotaxis, leukocyte recruitment and activation: TNF, IL1, chemokines, C3a and C5a, leukotrienes

Fever: IL-1 >TNF, prostaglandins

Pain: prostaglandins, bradykinin, substance P

Tissue damage: lysosomal enzymes of leukocytes, reactive oxygen species

*Serotonin is a pre-formed vasoactive mediator in platelets and certain neuroendocrine cells. Its importance in inflammation is unclear.

Key Cytokines

ACUTE INFLAMMATION

• TNF (principal source: macrophages, dendritic cells, mast cells, T cells): stimulates expression of endothelial adhesion molecules (selectins, ligands for integrins such as VCAM-1 and ICAM-1) and other cytokines, augments responses of neutrophils and macrophages. TNF antagonists are used to treat rheumatoid arthritis and psoriasis.
• IL-1 (principal source: macrophages, dendritic cells, epithelial cells): similar to TNF, greater role in fever
• IL-6 (principal source: macrophages, other cells): acute-phase response
• Chemokines (principal source: macrophages, endothelial cells, T cells, mast cells, other cells): recruitment and  migration
• IL-17 (principal source: T cells): recruitment of neutrophils and monocytes/macrophages

CHRONIC INFLAMMATION

• IL-12 (principal source: dendritic cells, macrophages): increases IFN-y production
• IFN-y (principal source: T cells, NK cells): activation of macrophages (increases ability to kill microbes and tumour cells)
• IL-17 (principal source: T cells): recruitment of neutrophils and monocytes/macrophages

Morphological patterns of acute inflammation

Serous inflammation: accumulation of protein-rich exudate in body cavities and spaces created by tissue necrosis, typically not containing microbes or large numbers of leukocytes.

Fibrous inflammation: large molecules such as fibrinogen leak out of vessels, and fibrin deposited, usually on the surfaces of organs such as heart and lungs

Purulent inflammation: formation of pus (dead cells, neutrophils, microbes), most often caused by bacterial infection. An abscess is a localised purulent infection

Ulcer: discontinuity in an epithelium, with underlying acute and chronic inflammation. During the acute stage there is intense polymorphonuclear infiltration and vascular dilatation in the margins of the defect. With chronicity the margins and base develop fibroblastic proliferation, scarring, accumulation of lymphocytes, macrophages and plasma cells

• Venous ulcers: fail to heal due to poor oxygen delivery, deposits of hemosiderin are commonly seen + chronic inflammation
• Arterial ulcers: painful, ischaemia results in atrophy and necrosis
• Diabetic ulcers: ischaemia, neuropathy and secondary infection. Characterised by epithelial ulceration, extensive granulation tissue

What are the possible outcomes of acute inflammation?

1. Complete resolution
2. Healing by connective tissue replacement (scarring, or fibrosis)
3. Progression of the response to chronic inflammation

Chronic inflammation: Mononuclear cell infiltration

Macrophages/monocytes play a key role in chronic inflammation.

Origin: Derived from HSC in bone marrow in postnatal life and from progenitors in the embryonic yolk sac or fetal liver during early development. Circulating cells of this lineage are known as monocytes

Lifespan: Days-weeks, although Tissue Resident(e.g. Kuppfer in liver, microglia in brain): years, stable population

Response to stimuli: Prolonged, slower, dependent on new gene transcription

Main anti-microbial tool: nitric oxide + cytokines

Cytokine production: major functional activity, requires transcriptional activation of cytokine genes

- M1 (classically activated): ROS, NOS and lysosomal enzymes + TNF and IL-1, causing inflammation and tissue destruction

- M2 (alternately activated): IFN-y, IL4, IL-13 promoting tissue repair and terminating inflammation

Scar formation and fibrosis: dominant role

Define granulomatous inflammation with examples

Chronic inflammation characterised by collections of activated macrophages, often interacting bi-directionally with T cells and associated necrosis

• Epithelioid cells; activated macrophages can develop an abundant cytoplasm, and start to resemble epithelial cells
• Multinucleate giant cells(Langhans giant cells): activated macrophages may fuse, forming multinucleate giant cells
• In granulomas(tubercles) associated with TB, a combination of hypoxia and ROS/NOS mediated injury leads to a central zone on necrosis, grossly called caseous necrosis
• Granulomas associated with Crohs, sarcoidosis and foreign body reactions tend NOT to have necrotic centres, and are noncaseating.

There are 2 types of granulomas

1. Foreign body granulomas: inflammation is induced in the absense of T cell-mediated responses e.g. to Talc in IVDU's
2. Immune granulomas: persistent T cell mediated response e.g. Th-1-->macrophages, Th-2-->eosinophils (schistosomiasis)

Diseases with granulomatous inflammation

• Tuberculosis
• Leprosy (mycobacterium leprae)
• Syphilis(treponema pallidum): gumma's (soft growths)
• Cat-scratch disease (bartonella henselae)
• Sarcoidosis: noncaseating granuloma, abundant macrophages
• Crohns disease

What are the systemic effects of inflammation?

• FEVER: pyrogens-->IL-1 and TNF released by immune cells-->COX enzymes up-regulated + hypothalamus releases prostaglandins (PGE2)-->production of neurotransmitters by hypothalamus that resets the body's steady state temperature by vasoconstriction (reduces heat loss) and increased heat generation through effects on brown fat and skeletal muscle
• LEUKOCYTOSIS: happens initially because of the accelerated release of granulocytes from the bone marrow (caused by TNF and IL-1), leading to a rise in both mature and immature neutrophils in the blood ('left shift'). Increased production of Colony stimulating factors by macrophages induces proliferation of leukocyte precursors in the bone marrow. EBV/glange, mumps and german measles causes increased lymphocyte count. Allergies and helminth infections cause eosinophilia. Note, typhoid, rickettsia and protozoal infections cause leukopenia, partly because of sequestration of activated leukocytes in vascular spaces and tissues.
• ACUTE PHASE proteins are mostly produced in the liver: CRP, fibrinogen and serum amyloid(SAA). CRP and SAA act as opsonins, whilst fibrinogen causes stacking of red cells meaning they sediment more rapidly (increase ESR). Hepcidin(regulates iron uptake) and thrombopoietin(growth factor for platelet precursors) also increase as part of the acute phase response.
• SEPSIS; large quantities of TNF and IL-1 released

What activates the complement pathways

Complement

• Classical pathway: C1 binds to IgM or IgG
• Alternative pathway: triggered by microbial surface molecules
• Lectin pathway: plasma mannose-binding lectin binds to carbohydrates on microbes

All 3 pathways of complement activation lead to the formation of C3 convertase which splits C3 into C3a and C3b(opsonin)

Three main functions

• Inflammation
• Opsonisation and phagocytosis
• Cell lysis (MAC). Particularly important in Neisseria infection (cells with thin walls)

Describe phagocytosis in detail

Three key steps:

1. Recognition and attachment of the particle to be ingested by the leukocyte. Easier recognition if antigen is opsonised.
2. Engulfment, with the plasma membrane pinching off to form of a phagocytic vacuole (phagosome). The phagosome fuses with the lysosome.
3. Killing and degradation of the microbe. The phagolysosome contains ROS, NOS and lysosomal granules such as elastase, collagenase, alkaline phosphatase and other proteases.

What does TNF do?