8. inflammation

NOT comprehensive! focused on main points + key molecules

what are the 5 cardinal signs of inflammation?

• pain

• heat

• redness

• swelling

• loss of function

very generally, how does cell membrane injury trigger inflammation via the arachidonic acid cascade?

1. cell membrane injury

2. phospholipase A₂ liberated when membrane is damaged

3. phospholipase A₂ generates arachidonic acid

4. enzymes convert AA into inflammatory mediators

5. enzymes act on each AA metabolite in a cascading reaction

leukotrienes and prostaglandins are examples of what type of molecule?

eicosanoids (arachidonic acid derivatives) → produced in the AA cascade following cell membrane injury

what are the major components of acute inflammation? (3)

1. dilation of small vessels

2. increased permeability of the microvasculature

3. emigration of leukocytes from circulation

exudation

fluid leaking from blood vessels

edema

• excess fluid in interstitial tissue or serous cavities

• can be an exudate or a transudate

transudate

• no increase in vascular permeability

  • low protein content

• non-inflammatory

exudate

• increased vascular permeability

• protein-rich fluid

• diagnostic for inflammation

what are the main mediators of vasodilation in an inflammatory response?

prostaglandins (+ histamine and other signals such as nitric oxide, endotoxins, LPS)

what is the purpose of vasodilation?

increases blood flow  → enhances delivery of immune cells

• results in redness and heat

active vasodilation happens in what type of vessels?

arterioles (venules have little/no smooth muscle)

slowing of blood/stasis follows what processes?

vasodilation & permeability changes

what is the purpose of stasis?

• promotes leukocyte margination

  • disruption of laminar flow → leukocytes contact vessel walls

• leads to vascular congestion and erythema

what major chemical mediators drive increased vascular permeability?

leukotrienes

what is the purpose of increased vascular permeability?

allows plasma proteins and leukocytes to exit vessels

• leads to fluid leakage and edema

how can vascular permeability increase without damage to the endothelial cells?

relaxation of adhesion molecules (between endothelial cells) & endothelial cell contraction

where does increased vascular permeability mainly occur?

post-capillary venules (low pressure, low velocity flow)

how does the lymphatic system respond to the exudate?

lymph flow increases to drain excess fluid & transport antigens to lymph node to start adaptive immune response

lymphangitis

inflamed lymph vessels (seen as red streaks) → suggests insult is spreading from primary site

reactive lymphadenitis

swollen, painful lymph nodes as a result of increased drainage & accumulation of immune cells and debris stretching tissue

• may involve lymphoid hyperplasia

what are the key phases of leukocyte recruitment? (4)

margination, adhesion, transmigration, chemotaxis

what are selectins?

adhesion molecules expressed on leukocytes and endothelium that facilitate rolling phase interactions (low-affinity → weak)

what selectin is expressed on leukocytes?

L-selectin

what selectins are expressed on endothelial cells?

E-, P-selectins

what activates the endothelium during the rolling phase?

cytokines (TNF, IL-1)

what activates leukocytes during the rolling phase?

chemokines

how are different leukocytes recruited during rolling?

different patterns of selectin expression recruit different leukocytes

what are integrins?

• adhesion molecules expressed on leukocytes that facilitate firm adhesion to endothelium

• switch to high-affinity state via chemokines

• bind to endothelial ligands (ex. VCAM-1, ICAM)

transmigration (diapedesis) involves what kind of interaction?

leukocyte-PECAM-1 (aka CD31)

where is PECAM/CD31 expressed?

• normally intrinsically expressed in vessels (always expressed on endothelium)

• not always expressed on leukocytes

how do leukocytes breach the basement membrane?

• use collagenases → work on contact with collagen

• use adhesion molecules to grab onto things on other side of cell → fibrinogen & fibronectin

what is chemotaxis?

• directed migration towards chemical signals

• involved receptor-mediated signaling

general mechanism of chemotaxis

chemoattractants bind leukocyte receptors → activate intracellular signaling pathways → induce cytoskeletal rearrangement → promote directional movement & ensure efficient immune targeting

what chemokine attracts neutrophils?

IL-8

what chemokines are broad chemoattractants?

• C5a → from complement

• leukotriene B4 (LTB4) → from AA cascade

characteristics of neutrophils in inflammation

• first responders from circulation

• abundant and fast-acting

• short-lived in tissues

• do not recirculate or divide

• undergo apoptosis after action

(aka polymorphonuclear leukocytes — PMNs)

neutrophil functions in inflammation

main role is to phagocytize and kill microbes → release enzymes and ROS

characteristics of macrophages in inflammation

• long-lived & present at low levels in most tissues (increase during inflammation)

• can proliferate

• differentiate from monocytes that leave circulation and migrate into tissues

• key players in chronic inflammation and tissue repair

macrophage functions in inflammation

• phagocytose microbes and debris

• produce LOTS of cytokines

  • direct leukocytes and lymphocytes inside tissues

• support tissue repair

• key players in chronic inflammation and resolution

where are mast cells found?

in tissues, especially near blood vessels and nerves (pre-positioned to respond)

mast cell functions in inflammation

• local cytokine release → release histamine and other mediators upon activation (act as phagocytes in primates)

• key players in allergic reactions and defense against parasites

• contribute to vascular changes and leukocyte recruitment

innate lymphocyte (ILC) & natural killer cell functions

circulating and tissue-resident → recognize danger

• sentinels, release cytokines, cytotoxic

timing of leukocyte infiltration

• 6-24 hrs → neutrophils dominate

• 24-48 hrs → monocytes/macrophages take over

exceptions to typical inflammatory timing

• lymphocytes: dominate in viral infection

• eosinophils: parasites, fungi, allergies

how do phagocytes detect their targets?

surface receptors (not cytokine-mediated)

what types of receptors do phagocytes use to detect targets?

• peptidoglycan receptors (NOD-like)

• mannose receptors (PRRs)

what are examples of opsonins?

• IgG (from plasma cells/B cells)

• C3b (from complement)

• plasma lectins (complex carbohydrates floating around in blood)

what receptors do phagocytes use to bind to opsonins?

• complement-R 3 → C3b

• Fcγ-R → IgG

• scavenger-R → bind many different molecules (ex. lipids, glycoproteins, peptides, etc.)

what are the most important neutrophil granule contents?

• myeloperoxidase (mammals; absent in birds/reptiles)

• lysozymes

• acid hydrolases

• defensins

myeloperoxidase

forms hypochlorous acid (not bleach)

NADPH oxidase

generates

• O₂^- (superoxide anion)

• H₂O₂ (hydrogen peroxide)

• OH (hydroxyl radical)

what antioxidants counter ROS?

catalase and superoxide dismutases

nitric oxide

• severe vasodilation

• potent antimicrobial and cytotoxic effects

• synthesized by nitric oxide synthase

what is the key activator of the classical complement pathway?

• C1 complex

• triggered by antibodies bound to pathogens

what is the key activator of the lectin pathway?

• mannose-binding lectins (MBL)

• triggered by sugar molecules (like mannose) on microbes

what is the key activator of the alternative pathway?

• C3b binding microbes

• triggered directly by microbes or damaged cells

• spontaneous cleavage of C3

what 2 major chemokines are produced during complement activation?

C3a and C5a

frustrated phagocytosis

target is too large to engulf → incomplete phagosome formed → lysosomal enzymes/ROS/NO released into open space

how can frustrated phagocytosis cause glomerulonephritis?

1. antigen-antibody complexes attach to/become trapped in basement membranes

2. complement fixation recruits leukocytes

3. leukocytes are unable to phagocytize basement membranes

4. release of lysosome contents

5. tissue damage

phagolysosome rupture

phagolysosome membrane damaged by particles, cholesterol crystals, or pathogens → releases lysosomal enzymes into cytoplasm

extracellular enzyme release

ex. mannheimia haemolytica leukotoxin

• endocytosis of leukotoxin triggers “explosion” that kills macrophage and releases enzymes

• leukotoxin recruits neutrophils, then kills them → enzymes destroy surrounding tissue

mechanisms of regurgitation during phagocytosis (3)

• incomplete phagosome fusion → during engulfment, lysosomes join forming phagosome while partially open to cell’s exterior

• premature degranulation → primary lysosomes release enzymes into phagosome while it remains partially open

• extrusion of contents → if phagosome continues engulfing or fails to fully fuse with the membrane, lysosomal enzymes may be released outside the cell

what are the 2 main protective mechanisms against leukocyte-mediated tissue injury?

• α1-antitrypsin: blocks neutrophil elastase

• tissue inhibitors of metallopreteinases: counteract activity of matrix metalloproteinases (enzymes that break down the ECM)

termination of inflammation

when the inciting stimulus is removed:

1. vascular leakage slows and stops

2. inflammatory mediator production ceases

3. edema is resolved by lymphatic drainage

4. neutrophils undergo apoptosis

5. macrophages clear apoptotic cells/debris

6. resolution signals shift mediator profile

7. new mediators shift macrophage phenotypes

inflammation resolution signals

• lipoxins replace leukotrienes

• TGF-β and IL-10 promote healing

• shift M1 macrophages to M2 macrophages

M1 macrophages

• pro-inflammatory

• activated by cytokines/chemokines during inflammatory response

• express PRRs

• involved in killing

M2 macrophages

• anti-inflammatory

• decrease expression of PRRs

• involved in healing

what molecules induce M1?

TNF-α, INF-γ, LPS

what molecules are produced by M1?

TNF-α, IL-1β, IL-6

what molecules induce M2?

IL-4, IL-13, IL-10

what molecules are produced by M2?

IL-10, TGF-β

prostaglandins (mediator of acute inflammation)

• eicosanoid produced by cyclooxygenase pathway

• normally synthesized by many of the body’s cells → part of normal cell physiology & signaling 

• increased production → mediates inflammation

leukotrienes (mediators of acute inflammation)

• eicosanoid produced by lipoxygenase pathway

• mostly synthesized by neutrophils in response to injury

COX-1

• continuously expressed in many tissues and helps produce PG’s for initial inflammation

• required for normal homeostatic processes

COX-2

• low expression, increases during inflammation

• support vascular and kidney functions

• expression boosted by various cytokines

how do corticosteroids decrease inflammation?

increase the expression of annexin A1 (phospholipase A2 antagonist) → block entire AA cascade

how do aspirin/NSAIDs decrease inflammation?

block COX-1 & COX-2 (nonspecific) → block PG’s but not other AA pathways

how do COXIBS decrease inflammation?

selectively blocks COX-2 → spare constitutive COX-1 expression

where are plasma-derived inflammatory mediators produced? where are they found?

• produced by the liver

• circulate as inactive precursors

• include complement proteins and kinins

what are some causes of chronic inflammation?

• persistent infections

• hypersensitivity diseases

• prolonged exposure to toxic agents

• foreign bodies

• inciting cause of inflammation is not removed; may follow unresolved acute inflammation

what are gross morphologic features of chronic inflammation?

• tissue injury & loss*

• fibrosis* may distort/impair organs

• granuloma formation in some cases

• granulation tissue (not inflammatory cells; macrophagic)

• ulceration

• (fibrous) adhesions

*characteristic for chronic inflammation

what are microscopic features of chronic inflammation?

• mononuclear cell infiltrate

  • macrophages

  • lymphocytes

  • plasma cells

• polymorphonuclear cells (neutrophils) if ongoing tissue damage

• fibroblasts → collagen deposition → fibrosis

• capillary proliferation

what is the dominant cell type in chronic inflammation?

macrophages

how do macrophages interact with T cells?

present antigens and secrete IL-12 → IL-12 activates T cells → T cells recruit and activate macrophages

granulomas

organized aggregates of activated macrophages and T cells

granulomatous inflammation

has all the same features of a granuloma, but disorganized

microscopic features of granulomas

• central necrosis

• **epithelioid macrophages with abundant cytoplasm

• **multi-nucleated macrophages from macrophage fusion

• **lymphocytes & plasma cells

**indicators of chronicity

how do granulomas heal?

via fibrosis → normal physiological structure destroyed

systemic effects of inflammation

• fever

• increased hepatic acute phase proteins

• leukocytosis

• other systemic signs

  • elevated pulse/BP

  • decreased sweating

  • chills and rigors

  • anorexia, malaise, somnolence

what is an example of an exogenous pyrogen?

bacterial products

what are examples of endogenous pyrogens?

IL-1 and TNF

how do pyrogens produce their effects?

• activate the hypothalamic AA cascade via COX-1 → increases PGE2 synthesis in thalamus → raises temperature set-point

• thalamic changes lead to vasoconstriction, heat production, somnolence, and malaise

• COX-2 NOT involved

how do NSAIDs and corticosteroids reduce fever?

inhibit COX-1 and PGE2 production, returning temp. set-point to normal

(note: COX-2 inhibitors would not affect fever, as COX-2 is not involved in inducing fever)

where are acute phase proteins (APPs) generated? what stimulates their production?

1. by the liver during inflammation

2. induced by acute inflammatory mediators