L50 - Inflammation 3: Inflammatory Mediators

What are Cytokines?

Small signaling proteins that regulate immune responses by initiating, modifying, or terminating inflammation

• Can be pro-inflammatory or anti-inflammatory

What are Chemokines?

A subset of cytokines that direct immune cell migration toward specific locations, especially sites of inflammation

• Function as chemoattractants

What is the Main Difference between Cytokines and Chemokines?

• Cytokines → Tell immune cells what to do

• Chemokines → Tell immune cells where to go

What are the Three Mechanisms of Cytokine Signaling?

1. Autocrine → Acts on the same cell that secretes it

2. Paracrine → Acts on nearby cells

3. Endocrine → Acts on distant cells through circulation.

What Cells produce Cytokines during Inflammation?

• Many nucleated cells can produce cytokines

• Major sources include leukocytes, endothelial cells, and fibroblasts

• T lymphocytes are especially important producers.

What does it mean that Cytokines are Pleiotropic?

One cytokine can produce different effects on different target cells

• The response depends on the target cell and its receptors

What is Functional Redundancy in Cytokine Signaling?

Different cytokines can produce overlapping or similar biological effects

Why are Cytokines described as a Network?

Multiple cytokines are produced simultaneously and interact to regulate the immune response

• No single cytokine acts independently.

What are the Major Classes of Cytokines?

1. Interleukins (IL)

2. Interferons (IFN)

3. Tumor necrosis factors (TNF)

4. Chemokines

What are Growth Factors, and how are they involved in Inflammation?

Signaling molecules that promote cell proliferation and differentiation

• Especially important in tissue repair and wound healing.

What are the Four Cardinal Signs of Inflammation?

1. Rubor → Redness

2. Calor → Heat

3. Tumor → Swelling

4. Dolor → Pain

What causes Rubor (Redness) during Inflammation?

Vasodilation increases blood flow to inflamed tissue

• Important mediators include nitric oxide and prostaglandins

What causes Calor (Heat) during Inflammation?

Increased blood flow due to vasodilation

• Mediators include nitric oxide and prostaglandins

What causes Tumor (Swelling) during Inflammation?

Increased vascular permeability allows fluid and proteins to leave blood vessels

• Mediators include histamine, leukotrienes, and prostaglandins

What causes Dolor (Pain) during Inflammation?

Bradykinin is a major mediator of inflammatory pain

• Prostaglandins also contribute to pain.

How do Inflammatory Cytokines contribute to Fever?

Pro-inflammatory cytokines, including IL-1, IL-6, and TNF-α, promote systemic inflammatory signaling

• They can influence the hypothalamus, increasing the body's temperature set point.

What are the Major Pro-inflammatory Cytokines?

• IL-1

• TNF-α

• IFN-γ

What are the Two Major Anti-inflammatory Cytokines?

• IL-10

• TGF-β

What is the Primary Function of Pro-inflammatory Cytokines?

1. Activate and amplify inflammatory responses

2. Promote immune cell activation and defense against pathogens

What is the Primary Function of Anti-inflammatory Cytokines?

1. Suppress excessive immune activation

2. Help prevent inappropriate inflammatory responses against normal tissues

What determines whether an Immune Response becomes Pro-inflammatory or Anti-inflammatory?

The balance between pro-inflammatory and anti-inflammatory cytokine signaling

• Excessive pro-inflammatory signaling → tissue-damaging inflammation

• Excessive anti-inflammatory signaling → inadequate immune response

How does a Dendritic Cell initiate a Pro-inflammatory T-cell Response to E. coli?

1. Dendritic cell encounters E. coli and recognizes antigen and PAMPs

2. PAMP recognition activates the dendritic cell

3. Dendritic cell travels to a lymph node

4. Presents antigen and produces pro-inflammatory cytokines (IL-1, TNF)

5. Cytokines help instruct T cells to mount a pro-inflammatory response

6. Chemokines guide responding T cells to the inflammatory site

How does a Dendritic Cell promote an Anti-inflammatory Response to Normal Collagen?

1. Dendritic cell encounters normal collagen without a danger signal

2. Acquires antigen without inflammatory activation

3. Travels to the lymph node and presents antigen

4. Anti-inflammatory signaling, including IL-10, promotes a regulatory response

5. IL-10 and TGF-β help suppress inappropriate inflammation

Why is Antigen Presentation alone Insufficient to determine the T-cell Response?

Cytokines provide context about whether the antigen represents a threat

• Pro-inflammatory signals promote an immune response

• Anti-inflammatory signals support tolerance and immune regulation

What is the Role of IL-10?

Major anti-inflammatory cytokine

• Suppresses excessive immune activation and helps maintain immune tolerance

What are the Two Major Functions of TGF-β?

1. Suppresses inflammation

2. Promotes fibrosis during wound healing

Why is TGF-β called the Wound-healing Cytokine?

It promotes fibrosis and connective tissue formation during tissue repair

What is a Major Disadvantage of Blocking Pro-inflammatory Cytokines Therapeutically?

Immunosuppression

• Decreased inflammation may also reduce the ability to fight infections

What are the Three Major Cytokines associated with Acute Inflammation?

1. TNF-α

2. IL-1

3. IL-6

What are the Two Major Cytokines associated with Chronic Inflammation?

1. IFN-γ

2. IL-12

What are the Major Functions of TNF-α during Acute Inflammation?

1. Promotes endothelial activation

2. Increases expression of adhesion molecules

3. Stimulates additional inflammatory cytokine production

4. Contributes to systemic inflammation

What are the Major Functions of IL-1 during Acute Inflammation?

1. Promotes endothelial activation

2. Contributes to inflammatory signaling

3. Helps induce fever

What is the Major Role of IL-6 during Acute Inflammation?

1. Promotes systemic inflammatory responses

2. Stimulates the acute-phase response

What is the Major Role of IFN-γ during Chronic Inflammation?

1. Activates macrophages

2. Enhances their ability to eliminate microbes

3. Promotes sustained pro-inflammatory responses

What is the Major Role of IL-12 during Chronic Inflammation?

1. Promotes IFN-γ production

2. Supports cell-mediated immune responses

How do the Cytokines associated with Acute and Chronic Inflammation differ?

• ACUTE → TNF-α, IL-1, IL-6 → Promote rapid inflammatory responses

• CHRONIC → IFN-γ, IL-12 → Support sustained immune activation and macrophage responses

What are the Major Type I Interferons?

1. IFN-α

2. IFN-β

What is the Primary Function of Type I Interferons?

1. Antiviral defense

2. Activate genes that interfere with viral replication

3. Promote natural killer (NK) cell activity

What is the Only Type II Interferon?

1. IFN-γ

What is the Primary Function of Type II Interferon (IFN-γ)?

1. Strong pro-inflammatory signaling

2. Activates macrophages

3. Contributes to chronic inflammation and endothelial activation

Compare Type I and Type II Interferons.

• Type I (IFN-α, IFN-β) → Primarily antiviral

• Type II (IFN-γ) → Primarily pro-inflammatory and macrophage activating.

What are Eicosanoids?

Potent lipid-derived inflammatory mediators produced from arachidonic acid

• Regulate vascular and other inflammatory responses

What are the Four Major Families of Eicosanoids?

1. Prostaglandins

2. Leukotrienes

3. Thromboxanes

4. Prostacyclins

What are the Three Major Vascular Effects of Eicosanoids?

1. Vasoconstriction

2. Vasodilation

3. Increased vascular permeability

What are the Major Cellular Sources of Eicosanoids?

• Leukocytes

• Mast cells

• Endothelial cells

Where does Arachidonic Acid come from?

Directly from dietary sources such as meat and eggs

• Can also be synthesized from linoleic acid

What Enzyme releases Arachidonic Acid from Cell Membrane Phospholipids?

Phospholipase A₂ (PLA₂)

What are the Two Major Pathways of Arachidonic Acid Metabolism?

1. Cyclooxygenase (COX) pathway → Prostaglandins and related mediators

2. Lipoxygenase (LOX) pathway → Leukotrienes.

Diagram the Arachidonic Acid Pathway.

1. Cell membrane phospholipids

2. Phospholipase A₂ (PLA₂)

3. Arachidonic acid →Two pathways:

   • COX → Prostaglandins, prostacyclins, thromboxanes

   • LOX → Leukotrienes

What is the Major Function of Prostaglandins during Inflammation?

Promote vasodilatation

• Certain prostaglandins increase vascular permeability and contribute to pain.

What is the Major Function of Thromboxane A₂ (TXA₂)?

Promotes vasoconstriction

• Promotes platelet aggregation

What is the Major Function of Prostacyclin (PGI₂)?

Promotes vasodilation

• Inhibits platelet aggregation.

What are the Major Inflammatory Effects of Leukotrienes?

Promote leukocyte recruitment

• Increase vascular permeability

• Certain leukotrienes cause bronchoconstriction.

Which Arachidonic Acid Pathway is Primarily Responsible for Producing Prostaglandins versus Leukotrienes?

• COX → Prostaglandins

• LOX → Leukotrienes

Where do Corticosteroids such as Prednisone act in the Arachidonic Acid Pathway?

Inhibit phospholipase A₂ activity

• Reduce arachidonic acid release from membrane phospholipids

Why are Corticosteroids such Potent Anti-inflammatory and Immunosuppressive Drugs?

They inhibit an early step in the arachidonic acid pathway

• This reduces the production of multiple downstream inflammatory mediators from both COX and LOX pathways

Where do NSAIDs act in the Arachidonic Acid Pathway?

Inhibit cyclooxygenase (COX)

• Reduce production of prostaglandins and other COX-derived mediators.

Why are Corticosteroids more Broadly Anti-inflammatory than NSAIDs?

• Corticosteroids act upstream of both COX and LOX pathways

• NSAIDs primarily inhibit the COX pathway

• Therefore, corticosteroids suppress a broader range of inflammatory mediator production.

What is the Primary Target of Anti-leukotriene Medications?

The leukotriene pathway or leukotriene receptors

• Used to reduce leukotriene-mediated inflammatory effects, including airway inflammation

What is Bradykinin?

A potent peptide inflammatory mediator

• Produced through the kallikrein-kinin system

• Particularly important in mediating inflammatory pain

What are the FOUR Major Effects of Bradykinin in Inflammation?

1. Pain

2. Vasodilation

3. Increased vascular permeability

4. Increased phospholipase A₂ (PLA₂) activity

Why is Bradykinin Important in Inflammatory Pain?

It stimulates pain-sensitive nerve endings

• Contributes to dolor, one of the cardinal signs of inflammation

How does Bradykinin amplify Inflammation through Phospholipase A₂?

1. Bradykinin increases PLA₂ activity

2. PLA₂ releases arachidonic acid from membrane phospholipids

3. Arachidonic acid is converted into inflammatory eicosanoids

What are the Major Vasoactive Amines, and why can they act Rapidly?

Histamine and Serotonin

• Stored as preformed mediators in cells such as mast cells, basophils, and platelets

• Can be rapidly released, contributing to allergic and anaphylactic reactions

What are the Major Differences between Endothelial Nitric Oxide (eNOS) and Inducible Nitric Oxide (iNOS)?

• eNOS → Constitutively produced by endothelial cells; maintains vasodilation and inhibits platelet adhesion

• iNOS → Induced during inflammation; increases vasodilation and contributes to microbial killing.

Cardinal Signs of Inflammation SUMMARY

Cardinal sign	Mechanism	Important mediators
Rubor (redness)	Vasodilation → ↑ blood flow	NO, prostaglandins
Calor (heat)	Vasodilation → ↑ blood flow	NO, prostaglandins
Tumor (swelling)	↑ Vascular permeability → fluid leakage	Histamine, Leukotrienes, Prostaglandins
Dolor (pain)	Stimulation of pain receptors	Bradykinin, Prostaglandins

Remember:

• Rubor + Calor = Increased blood flow

• Tumor = Increased permeability

• Dolor = pain mediators

Major Cytokines and Their Functions SUMMARY

Category	Major cytokines	Primary function
Pro-inflammatory	IL-1, TNF-α, IFN-γ	Promote inflammation
Anti-inflammatory	IL-10, TGF-β	Suppress inflammation
Acute inflammation	TNF-α, IL-1, IL-6	Rapid inflammatory response
Chronic inflammation	IFN-γ, IL-12	Sustained immune activation
Type I interferons	IFN-α, IFN-β	Antiviral defense
Type II interferon	IFN-γ	Pro-inflammatory; macrophage activation
Wound healing	TGF-β	Promotes fibrosis

Important: These categories overlap. For example, IFN-γ is both pro-inflammatory and associated with chronic inflammation.

Pro-inflammatory vs. Anti-inflammatory Antigen Response SUMMARY

Feature	Pro-inflammatory	Anti-inflammatory
Antigen example	E. coli	Normal collagen
Danger signal	PAMPs present	No danger signal
Dendritic cell	Activated	Not inflammatory-activated
Cytokines	IL-1, TNF-α	IL-10, TGF-β
T-cell response	Pro-inflammatory activation	Regulatory/tolerant response
Outcome	Eliminate pathogen	Prevent inflammation against self

Key concept: Antigen presentation + cytokine environment determines the type of immune response.

Major Eicosanoids and Their Functions SUMMARY

Eicosanoid	Pathway	Major function
Prostaglandins	COX	Vasodilation, permeability, pain
Thromboxane A₂	COX	Vasoconstriction, platelet aggregation
Prostacyclin (PGI₂)	COX	Vasodilation, inhibits platelet aggregation
Leukotriene B₄	LOX	Leukocyte chemotaxis
Leukotrienes C₄, D₄, E₄	LOX	Bronchoconstriction, ↑ permeability

Memory tip: 

• COX → Prostaglandins, thromboxanes, prostacyclins

• LOX → Leukotrienes

Corticosteroids vs. NSAIDs vs. Anti-leukotriene Drugs SUMMARY

Drug	Target	Result
Corticosteroids (prednisone)	PLA₂ pathway	↓ Arachidonic acid release → ↓ COX and LOX products
NSAIDs	COX	↓ Prostaglandins and other COX products
Anti-leukotriene drugs	Leukotriene pathway/signaling	↓ Leukotriene effects

Why corticosteroids are more potent: They suppress mediator production upstream of both COX and LOX pathways. They also have broader effects on inflammatory gene expression.

Other Important Inflammatory Mediators SUMMARY

Mediator	Main function
Histamine	Rapid vasodilation and ↑ permeability
Serotonin	Vasoactive signaling
Endothelial NO (eNOS)	Maintains vascular dilation
Inducible NO (iNOS)	Inflammatory vasodilation, microbial killing
Bradykinin	Pain, vasodilation, ↑ permeability, ↑ PLA₂
TGF-β	Suppresses inflammation; promotes fibrosis

Pro-inflammatory vs. Anti-inflammatory T-cell Activation SUMMARY

Pro-inflammatory response (E. coli)	Anti-inflammatory response (normal collagen)
E. coli (PAMPs + antigen) ↓ Dendritic cell recognizes PAMPs ↓ Dendritic cell ACTIVATED ↓ Travels to lymph node ↓ Presents antigen + IL-1, TNF-α ↓ Pro-inflammatory T-cell response ↓ Chemokines guide T cells to infection ↓ Inflammation + pathogen defense	Normal collagen (self-antigen) ↓ Dendritic cell acquires antigen ↓ NO inflammatory activation ↓ Travels to lymph node ↓ Presents antigen + IL-10 signaling ↓ Regulatory/tolerant T-cell response ↓ IL-10 + TGF-β suppress inflammation ↓ Prevents inappropriate immune response

SUMMARY

1. Cell membrane phospholipids

↓

2. Phospholipase A₂ (PLA₂)

↓

3. Arachidonic acid

↓

4. TWO PATHWAYS:

COX (Cyclooxygenase)

  ↓

• Prostaglandins → Vasodilation, pain

• Prostacyclin → Vasodilation

• Thromboxane A₂ → Vasoconstriction

LOX (Lipoxygenase)

  ↓

• Leukotrienes → Chemotaxis, bronchoconstriction, ↑ permeability

Where Anti-inflammatory Drugs Act SUMMARY

1. Cell membrane phospholipids

↓

2. PLA₂ ← CORTICOSTEROIDS inhibit upstream

↓

3. Arachidonic acid

↓

TWO PATHWAYS:

COX ← NSAIDs inhibit

↓

• Prostaglandins, thromboxanes, prostacyclins

LOX → Leukotrienes

       ↑

• Anti-leukotriene drugs target this pathway

Bradykinin and Amplification of Inflammation SUMMARY

1. Kallikrein-kinin system

↓

2. Bradykinin produced

↓

FOUR MAJOR EFFECTS:

• Pain (DOLOR)

• Vasodilation

• ↑ Vascular permeability

• ↑ PLA₂ activity

    ↓

• ↑ Arachidonic acid release

    ↓

• ↑ Eicosanoid production

    ↓

• Amplified inflammation

Main Questions

Main question	What you need to know
1. Who controls inflammation?	Cytokines and chemokines
2. What determines the type of inflammation?	Pro- vs. anti-inflammatory; acute vs. chronic cytokines
3. What physically causes inflammation?	Eicosanoids, histamine, NO, bradykinin
4. How can inflammation be suppressed?	Corticosteroids vs. NSAIDs

The Most Important Distinction

• Cytokines

  • Tell immune cells what to do

    • Examples: IL-1, TNF-α, IL-10

• Chemokines

  • Tell immune cells where to go

    • Guide cell migration

• Eicosanoids

  • Produce inflammatory effects

    • Vascular changes, pain, recruitment

• Bradykinin

  • Major pain mediator

    • Also promotes vascular changes and PLA₂

      
      

Memorize These Cytokines Before Anything Else

Category	Memorize	Think
Pro-inflammatory	IL-1, TNF-α, IFN-γ	Turn inflammation ON
Anti-inflammatory	IL-10, TGF-β	Turn inflammation DOWN
Acute	TNF-α, IL-1, IL-6	Early inflammatory response
Chronic	IFN-γ, IL-12	Sustained immune response
Type I IFN	IFN-α, IFN-β	Antiviral
Type II IFN	IFN-γ	Macrophage activation

• IL-1 and TNF-α = Pro-inflammatory AND acute

• IFN-γ = Pro-inflammatory AND Chronic AND Type II interferon

• TGF-β = Anti-inflammatory AND Promotes fibrosis

Learn the Arachidonic Acid Pathway as One Story

• Corticosteroids → Reduce PLA₂ activity and broader inflammatory signaling → Both branches affected.

• NSAIDs → Inhibit COX → COX-derived mediators reduced.