Module 28 - Acute Inflammation

Question 1

Define inflammation.

Answer 1

A protective response intended to eliminate the cause and consequence (necrotic cells and tissue) of cell injury. It is induced by chemical mediators that are produced by host cells in response injurous stimuli.

Question 2

Mention the fundamental signs of inflammation.

Answer 2

These manifestations occur as consequences of the vascular changes and leukocyte recruitment and activation.

• Heat (calor): Vasodilation; increased blood flow to the injured region
• Redness (rubor): Vasodilation and stasis (congestion/ hyperemia/ engorgement)
• Swelling (tumour): Vasodilation & vascular permeability leading to extravasation of fluid (transudate/ exudate/ edema)
• Pain (dolor): Compression of tissues or Direct effect of inflammatory mediators
• Loss of function (functio laesa): direct effect of injury, or due to pain/swelling

Question 3

Differ between acute and chronic inflammation.

Answer 3

Acute:

• Rapid onset, short duration
• Fluid and plasma protein exudation
• Neutrophil (PMN) accumulation

Chronic:

• Insidious onset lasting days to years
• Lymphocytes and macrophages
• Scarring (more tissue destruction)

Question 4

Mention the 5 essential steps in inflammation.

Answer 4

• Recognition
• Recruitment
• Removal
• Regulation
• Resolution

Question 5

What happens during the recognition phase of inflammation?

Answer 5

Phagocytes and dendritic cells (cells that reside in the connective tissue of organs) and other cells (epithelial cells) express receptors that recognise microbes and necrotic cells.

These Pattern Recognition Receptors (PRR) includes:

• Toll-like receptors: recognize PAMPs - microbe receptors
• Inflammasome: recognize products of dead cells and microbial products DAMPs - cell damage sensors

Question 6

What happens during the recruitment phase of inflammation with respect to vascular change?

Answer 6

Designed to initiate rapid delivery of leukocytes and plasma proteins from the circulation to the site of the injury.

Vascular changes includes:

• Transient vasoconstriction followed by arteriolar vasodilation: due to chemical mediators (histamines) -> increased blood flow -> engorgement of capillary beds -> formation of transudate
• Increased Vascular Permeability -> formation of exudate
  
  • Either due to contraction or direct injury of endothelial
• Stasis: Increases RBC concentration, slowed blood flow -> accumulation of neutrophils

The lymphatic system then helps drain the edema fluid accumulated (increased lymph flow).

Question 7

Describe the transudate mechanism.

Answer 7

Increased hydrostatic pressure or decreased colloid osmotic pressure leads to accumulation of interstitial fluid. However, the only ultrafiltrate can pass through the vessels, not a lot of protein or cells.

Question 8

___________ line the entire blood and lymphatic vascular system and control the passage of materials into and out of the bloodstream.

Answer 8

Endothelial cells

Question 9

Differ between vasoactive mediators that are responsible for the immediate/transient and slow/prolonged change in permeability.

Answer 9

Histamine, bradykinin, leukotrienes are responsible for the immediate and short-lived increase in vascular permeability through initiating endothelial cell contraction (gaps between endothelial cells)

Changes in cytoskeleton induced by IL-1 and TNF leads to prolonged permeability increase (larger gaps)

Question 10

Explain how endothelial injury can also initiate vascular permeability.

Answer 10

Direct injury to the endothelium allows leakage and formation of exudate to start immediately and is sustained for several hours until the damaged blood vessels are thrombosed and repaired.

Question 11

Explain how increased vascular permeability leads to exudate.

Answer 11

Due to the increase in vascular permeability, bigger things such as protein and cells are able to pass through, forming exudate (protein-rich fluid + cells) in the extravascular space.

This leads to a change in osmotic pressure causing an outflow of water and ions to extravascular space -> edema-fluid accumulation (swelling).

Question 12

What happens during the recruitment phase of inflammation with respect to cellular change?

Answer 12

• Margination: slowed blood flow-stasis causes leukocytes to be pushed to the margins of the blood vessels
• Rolling: weak, transient adhesion between selectins and sialylated oligosaccharides (upregulated by immune mediators) in endothelial cells, reduce rolling velocity
• Adhesion: mediated by integrins, low affinity until activated by chemokines (also stimulate expression of integrin ligands I-CAM/V-CAM in endothelial cells)
• Transmigration: occurs in the post-capillary venules through diapedesis, in response to chemical gradient produces at the site of inflammation (PECAM1 - Platelet endothelial cell adhesion molecule 1) - Collagenase degrade basement membrane
• Migration/Chemotaxis: a chemical gradient produced by exogenous and endogenous sources, which binds to the surface receptors (chemokines) and initiates leukocyte movement

Question 13

Describe the morphological types of acute inflammation.

Answer 13

Serous Inflammation: exudation of protein-poor fluid, located in body cavities (peritoneal, pleural, or pericardial)

Fibrinous inflammation: exudate contains large molecules (fibrinogen) forming fibrin, characteristic of inflammation of meninges, pericardium, and pleura

Suppurative (PUS): exudate consisting of edema fluid, prominent cellular components (neutrophils); associated with pyogenic bacterial infection, if occur inside tissue called abscess

Ulceration: necrosis and inflammation on or near the surface, shedding of tissue

Question 14

What happens during the removal phase of inflammation?

Answer 14

1. Recognition and Leukocyte Activation: engagement of receptors expressed on leukocyte surfaces by microbial products and mediators, PAMP/DAMP, cytokines, and opsonized particles
2. Opsonization
3. Phagocytosis: A form of endocytosis in which specialized endocytic vesicles (phagosome) are used to ingest
4. Killing & Degradation

Question 15

Describe the phagocytic receptors.

Answer 15

• Mannose Receptor - a lectin that binds glycolipids and glycoproteins that are unique to microbial cell walls
• Scavenger Receptors - recognize cellular debris (ie. acetylated LDL) and opsonized microbes.
• Opsonic receptors

Question 16

Describe the mechanism of opsonization.

Answer 16

Below are examples of opsonins:

• Bacterial LPS activates complement pathway, generating C3b (deposited in the bacterial surface) with opsonizing properties.
• IgG antibodies binding to antigen also activates complement (acts as opsonins).
• Collectins (PRR) which bind to microbial cell wall sugar group.
• Mannose-binding lectins

Question 17

Describe the mechanism of phagocytosis.

Answer 17

It involves the recognition of the particles to be ingested by the phagocytic receptors, which falls under two categories:

1. Non-opsonic: Mannose receptor, Scavenger Receptor
2. Opsonic: Complement receptor, FcR
   
   • Requires opsonins coating the particle

This triggers engulfment, through receptor-initiated signalling, membrane remodelling, and cytoskeletal changes. Actin assembly to form phagosome.

It then fuses with lysosomes, resulting in the discharge of lysosomal contents into the phagolysosome. During this, the phagocyte also may release some granule contents into the extracellular space, damaging normal cells.

Question 18

_________ occurs when phagolysosome is formed, leading to an increase in ROS. This process is mediated by _______.

Answer 18

Respiratory burst

Phagocyte oxidase/NADPH oxidase

Question 19

What compounds are primarily used to destroy ingested particle in oxygen-dependent process? How is it manufactured inside lysosome?

Answer 19

Hypochlorous radical (HOCl•) - powerful oxidant and microbicidal. It is obtained through a reaction of H₂O₂ and Cl^- in the presence of myeloperoxidase (MPO), which is present in the lysosome of neutrophils.

Nitric oxide is a short-lived, free radical gas. When it reacts with superoxide, it forms peroxynitrite (ONOO), a cytotoxic agent in macrophages - microbicidal and promotes vasodilation

Note: NADPH-oxidase activated in phagolysosome to produce superoxide ion -> H₂O₂

Question 20

What compounds are primarily used to destroy ingested particle in oxygen-independent process?

Answer 20

Leukocyte granules that contain:

• Bactericidal permeability-increasing enzyme: Phospholipase mediated membrane degradation
• Lysozyme: Oligosaccharide coat degradation
• Major basic protein: Cytotoxic to parasites
• Defensins: Create holes in microbe membranes

Question 21

Mention the compounds that are responsible for degradation.

Answer 21

• Lysosomal acid hydrolases (function in low pH inside lysosome)
• Neutral proteases, elastase, collagenase, cathepsins ( regulated by antiproteases in ECM)

Question 22

Describe how leukocyte may induce tissue injury.

Answer 22

Effector mechanisms don’t distinguish between offender and host, which leads to bystander tissue injury.

Activated leukocytes may release toxic products into extracellular space. This may be caused by:

• Regurgitation during feeding (neutrophils/macrophage)
• Frustrated phagocytosis
• Ingestion of substance (urate crystals) damage phagolysosome

Question 23

In regulation phase of acute inflammation, regulation is done by __________ or ___________ mediators.

Answer 23

Cell-derived or Plasma-derived

Question 24

Mention the systems at which plasma-derived mediators are obtained. Explain the characteristics and advantages of these type of mediators.

Answer 24

Complement, Kinin, Coagulation, and Fibrinolytic systems are enzymatic cascades that produce plasma-derived mediators.

They are present as inactive precursors circulating in the plasma that needs to be activated at the site of inflammation. They are:

• Safer than active mediators
• Act to amplify the response
• A larger number of possible regulators can modulate the response
• Each step produces end products that can have different activities

Question 25

Describe the mechanism of complement system producing plasma-derived mediator

Answer 25

Question 26

Cell-derived chemical mediators may be produced by _________ (4). It can exist ________ or \_\_\_\_\_\_\_\_\_\_\_\_\_.

Answer 26

* Tissue macrophages * Mast cells * Endothelial cells * Leukocytes Preformed or newly synthesized

Question 27

Give an example of preformed and newly synthesized cellular mediators.

Answer 27

Preformed: **Histamine -\>** released by a variety of stimuli (activation of complement. physical injury, binding of IgE etc) leading to vascular changes (dilation and increased permeability) Newly Synthesized: **Arachidonic Acid metabolites** (prostaglandin and leukotrienes) -\> AA cleaved from phospholipid, mediate most inflammatory step, short range and act locally (decay/enzymatic degradation)

Question 28

\_\_\_\_\_\_ and _______ target AA metabolite formation to reduce pain and fever.

Answer 28

Aspirin, NSAIDs (Non-steroidal inflammatory drugs)

Question 29

Briefly recall endogenous inflammatory mediators and their common responses.

Answer 29

Question 30

Describe what occurs during the resolution stage of acute inflammation.

Answer 30

Occurs when **DAMP/PAMP is no longer present** at the site of injury. Achieved through the following **passive processes**: * Clearance of mediators and acute inflammatory cells (apoptosis) * Regeneration of damaged tissue * Normalization of vascular permeability * Clearance of exudate (lymphatic drainage, macrophages ingestion) **Active process -** Inhibitors of inflammation (Lipoxins): AA metabolite produced by leukocyte when entering tissue inhibits neutrophil (chemotaxis and adhesion) - negative regulators of leukotrienes

Question 31

Mention the possible outcomes of acute inflammation.

Answer 31

* **Complete Resolution** -\> functional normalcy * **Progression to chronic inflammation** * **Scarring and Fibrosis** -\> in case of substantial tissue destruction

Question 32

Mention the systemic effects of inflammation. What is the underlying cause?

Answer 32

Inflammation is associated with cytokine-induced systemic reactions that are collectively called the **acute-phase response.** * Fever (+1-4ºC) - due to pyrogens * Myalgia (muscle pain) * Arthralgia (joint pain) * Anorexia (loss of appetite) * Somnolence (sleepy) A result of the **systemic release of cytokines (IL-1, IL-6 and TNF)** induced by inflammatory stimuli (also called acute-phase response)

Question 33

Define pyrogenes and its mechanism of action.

Answer 33

It refers to endogenous or exogenous molecules the cause fever. It achieves this by **facilitating prostaglandin synthesis**, which goes to the hypothalamus and *_reset (increase) the temp. set point._* Aim: ward of infection

Question 34

Mention the clinical manifestations of acute phase response.

Answer 34

* Increased heart rate * Increased blood pressure * Decreased sweating * Rigors and chills (temperature set point) * Anorexia, somnolence, malaise (action of cytokines on brain) * Cachexia (wasting due to TNF mediated appetite suppression and mobilization of fat stores)

Question 35

Mention the pathological manifestations of acute phase response.

Answer 35

**Acute-phase proteins:** plasma proteins, mostly synthesized in the liver, whose concentrations increase several hundred-fold in response to inflammatory stimuli. * **C-reactive protein (CRP) & Serum Amyloid A (SAA)**: bind to the microbial cell wall (opsonization) * **Fibrinoge**n (binds RBC, basis of erythrocyte sedimentation rate) **Increased leukocyte count (Leukocytosis):** * Neutrophilia (bacteria) * Lymphocytosis (viral infection) * Eosinophilia (hypersensitivities and parasitic infections)

Question 36

Another example of an acute phase response is sepsis. How does this occur and what are the correlating characteristics?

Answer 36

It refers to the stimulation of the enormous production of cytokines **(TNF, IL-12, 1L-1)** in response to severe microbial infection (in the blood), leading to: * Disseminated **intravascular coagulation** * **Hyperglycemia** (sugar dysregulation) * **Hypotensive shock** (low blood pressure)

Question 37

Why are neutrophils more prominent in acute inflammation rather than macrophages, even though both are capable of phagocytosis?

Answer 37

Neutrophils can be recruited rapidly to the site of inflammation, the whike macrophages are slow responders. This has to do with the fact that PMN uses cytoskeletal rearrangements to mount a rapid and transient response, while macrophages use a long-lived, and slower response of gene transcription.

Question 38

What are inflammatory mediators?

Answer 38

They are substances that initiate and regulate inflammatory reactions.