L10 - Innate immunity I

What are the Four Functions of the Immune System?

1. Clear damaged tissue

2. Manage beneficial microbes

3. Defend against pathogens

4. Destroy abnormal or cancerous cells

What is the Difference between Commensals, Symbionts, and Pathogens?

• Commensals are generally harmless

• Symbionts provide benefits

• Pathogens can cause disease

What is the Colonizing Microbiota, and Why is It Important?

Microbes inhabiting host surfaces that:

• Support nutrient metabolism

• Immune development

• Immunological balance

How do Pathogens Manipulate Host Physiology?

They exploit host processes for:

• Adhesion, invasion, replication, and spread

• Some manipulate the actin cytoskeleton

What are the Common Routes of Infection and their Examples?

• Respiratory: influenza A

• GI: parvovirus

• Reproductive: T. foetus

• Skin: S. pseudintermedius

• Vectors: West Nile virus

What are the Two Perspectives on Immune System Evolution?

• Prevailing view: Control pathogens

• Paradigm Shifting Idea: Also shape and maintain beneficial resident microbes

What are the Three Major Components of Innate Immunity?

1. Epithelial barriers

2. Innate immune cells

3. Soluble components

   • Complement

   • Acute-phase proteins

QUESTION: What Organisms have an Innate Immune System?

a) All Animals, but not plants

b) Invertebrates only (Jellyfish…tunicates)

c) Vertebrates only (lamprey and sharks on figure)

d) All plants and animals

ANSWER → All organisms have an innate immune system, including plants, invertebrates, and vertebrates

1. Plants

2. Fungi

3. Invertebrates

4. Vertebrates

What Organisms have Both Innate and Adaptive Immunity?

Vertebrates, including lampreys, sharks, and mammals

What are the Key Features of Innate Immunity?

1. Present before pathogen exposure

   • Germline-encoded, not acquired in lifetime of the host

   • No long-term memory after exposure

2. Rapid response (vs adaptive immunity)

   • Defenses in place or rapidly induced (minutes-days)

3. Recognizes conserved categories of microbial molecules

   • Broad specificity compared to high specificity of antibodies

   • Diversity is limited to conserved microbial molecules

How do Innate and Adaptive Immunity Differ?

• Innate: Rapid, germline-encoded PRRs recognize conserved motifs

• Adaptive: Delayed primary response, diverse B/T receptors, memory

QUESTION: Who has an innate AND and adaptive immune system?

a) 4

b) 3 + 4

c) 2 + 3 +4

d) 1 + 2 + 3 +4

ANSWER → b) 3 + 4

Why does Innate Immunity use Pattern Recognition?

Recognizing conserved microbial motifs allows rapid detection of multiple microbes without prior exposure

What are Pattern Recognition Receptors (PRRs)?

Germline-encoded receptors that recognize conserved microbial molecular patterns

What is the Sequence of Host Defenses during Infection?

1. Barrier containment

2. Local phagocyte response

3. Induced inflammation and leukocyte recruitment

4. Adaptive immunity

What is the Primary Function of Epithelial Barriers?

Contain resident and environmental microbes, preventing entry into underlying tissues

What are the Three Categories of Epithelial Defense?

1. Mechanical/physical

   • Tight junctions

2. Chemical

   • Antimicrobial peptides

3. Microbiological defenses (competition)

   • Microbiota

What are the Three Major Epithelial Barrier Sites?

1. Skin

2. Respiratory tract

3. Gastrointestinal tract

What Protective Features are Shared by Major Epithelial Surfaces?

1. Tight junctions

2. Antimicrobial peptides

3. Resident microbiota

What are the Major Protective Features of the Skin?

• Multiple epithelial layers

• Keratinization

• Secreted lipids

• Lamellar bodies

What are the Major Protective Features of the Respiratory Tract?

• Thin epithelium

• Mucus

• Cilia

• Antimicrobial surfactant

What are the Major Protective Features of the Gastrointestinal Tract?

• Single-layer epithelium

• Mucus

• Peristalsis

• Antimicrobial substances

How do Cilia and Peristalsis Protect Epithelial Surfaces?

Cilia

• Move respiratory mucus and trapped microbes

Peristalsis

• Moves intestinal contents and microorganisms

What are the Major Chemical Defenses of Epithelial Barriers?

1. Low pH

2. Lysozyme

3. Defensins, cathelicidins, fatty acids, and surfactant

What is the Antimicrobial Function of Lysozyme?

Breaks down bacterial cell walls

• Found in tears, saliva, and other secretions

How do Defensins and Cathelicidins Protect against Microbes?

Cationic, amphipathic peptides that disrupt microbial membranes, including through pore formation

How does the Normal Microbiota Protect Epithelial Surfaces?

Competes with pathogens for nutrients and colonization sites

QUESTION: What Happens When Epithelial Barriers are Breached?

Resident macrophages and dendritic cells detect invading microbes and initiate local defenses

QUESTION: Which Cells are the Main Tissue-Resident Sentinels?

Macrophages and dendritic cells; neutrophils are recruited after inflammation begins

What are the Origins of Tissue-Resident Macrophages?

• Fetal: Some arise from embryonic yolk sac or fetal liver progenitors and self-renew

• Adult: Others derive from adult monocytes

Where are Sentinel Macrophages Located near Epithelial Barriers?

• Skin & GI: Macrophages are located BEHIND epithelial barrier

• Lung (lower airways): No great physical barrier present, alveolar macrophages in in FRONT of epithelium/in lumen of alveoli

What are Kupffer Cells and Microglia?

• Kupffer cells: liver macrophages that intercept gut-derived microbes

• Microglia: CNS macrophages involved in tissue maintenance

What are the Major Functions of Macrophages?

1. Phagocytose

2. Clear Apoptotic Cells (healthy tissues)

3. Many Microbicidal Mechanisms

4. Antigen Presentation

5. Cytokine Production

What are the Major Functions of Neutrophils?

Rapidly recruited phagocytes that kill microbes through antimicrobial mechanisms and contribute to pus formation

• Recruited during inflammation

What are the Major Functions of Dendritic Cells?

1. Antigen sampling

2. Phagocytose

3. Cytokine Production

4. Activate Innate and Adaptive Immunity

5. Bridge both via Antigen Presentation

How do Macrophages and Neutrophils Differ?

Macrophages:

• Found in healthy tissues → Resident

• Slightly increased during inflammation

• Long-lived, survive after phagocytosis

• Slowly form granuloma with help from T cells

Neutrophils:

• Found only in inflamed tissues → Recruited

• Rapid increase in numbers during acute phase response

• Short-lived, die after phagocytosis

• Rapidly form pus

What are the Major Phagocytic Receptors and their Targets?

1. Mannose receptors/dectins → Microbial sugars

2. Complement receptors → Complement-coated microbes

3. Fc receptors → Antibody-coated microbes

What is the Sequence of Phagocytosis?

1. Recognition

2. Engulfment

3. Phagosome formation

4. Lysosome fusion

5. Phagolysosome-mediated microbial destruction

What are the Major Antimicrobial Mechanisms of Phagocytes?

1. Acidification

2. Reactive oxygen species

3. Nitric oxide

4. Antimicrobial peptides

5. Lysozyme

6. Iron sequestration

What is the Difference between Bacteriostatic and Bactericidal?

• Bacteriostatic inhibits bacterial growth

• Bactericidal kills bacteria

Why is Lactoferrin Important for Antimicrobial Defense?

Sequesters iron required for microbial metabolism, restricting microbial growth

What is the Respiratory Burst?

Rapid production of antimicrobial reactive oxygen species by phagocytes, initiated by NADPH oxidase

What is the Sequence of Reactive Oxygen Species Production?

1. NADPH oxidase generates superoxide from oxygen

2. Superoxide dismutase produces hydrogen peroxide

3. Myeloperoxidase uses peroxide and chloride to produce hypochlorite

NADPH oxidase: superoxide → superoxide dismutase: hydrogen peroxide → myeloperoxidase: hypochlorous acid

What are Neutrophil Extracellular Traps (NETs)?

Extracellular DNA coated with histones and antimicrobial proteins that traps microbes and aids their clearance

When is NETosis particularly useful?

When microbes are too large or clustered for efficient phagocytosis

• ROS may trigger NET formation

How is Inflammation Initiated after Pathogen Recognition?

Resident immune cells recognize microbes and release pro-inflammatory cytokines and lipid mediators

What are the Major Effects of Inflammatory Mediators?

• Vasodilation

• Increased vascular permeability

• Endothelial activation

• Leukocyte recruitment

What are the Major Steps of Neutrophil Extravasation?

1. Rolling

2. Chemokine activation

3. Firm adhesion

4. Transmigration

5. Migration toward infection

Which Molecules Mediate Neutrophil Rolling and Firm Adhesion?

• Rolling: selectins and sialyl Lewis X

• Firm adhesion: leukocyte integrins (LFA-1/Mac-1) bind endothelial ICAMs.

What is the Role of Chemokines in Leukocyte Recruitment?

They attract immune cells and activate adhesion mechanisms that support migration into inflamed tissues

Which Chemokines Recruit Neutrophils and Monocytes?

• CXCL8 recruits neutrophils

• CCL2 recruits inflammatory monocytes

How do Dendritic Cells Bridge Innate and Adaptive Immunity?

They sample antigens, detect pathogens, migrate to lymph nodes, and present antigens to T cells

What are the Major Antigen-Sampling Mechanisms of Dendritic Cells?

Phagocytosis and macropinocytosis, including sampling across epithelial barriers

What are the Main Stages of the Innate Response to Infection?

1. Epithelial containment

2. Resident phagocyte defense

3. Cytokine-driven inflammation

4. Neutrophil/monocyte recruitment and DC activation

Innate vs. Adaptive Immunity SUMMARY

Feature	Innate Immunity	Adaptive Immunity
Response time	Immediate (minutes–days)	Delayed during initial exposure
Receptors	Pattern recognition receptors (PRRs)	B-cell and T-cell receptors
Specificity	Conserved microbial patterns	Highly specific epitopes
Receptor diversity	Limited	Extensive
Receptor origin	Germline-encoded	Somatic gene recombination
Memory	No classical long-term memory	Long-term immunological memory
Main cells	Macrophages, neutrophils, dendritic cells, NK cells	B cells and T cells
Distribution	Plants, fungi, and animals	Vertebrates

Key takeaway: Innate immunity recognizes conserved microbial patterns rapidly, whereas adaptive immunity provides highly specific responses and immunological memory.

Epithelial Barriers: Skin vs. Respiratory vs. GI SUMMARY

Feature	Skin	Respiratory Tract	Gastrointestinal Tract
Epithelium	Multiple layers	Thin epithelial barrier	Single epithelial layer
Primary function	Physical protection	Gas exchange	Nutrient absorption
Mechanical defenses	Keratinization, tight junctions	Cilia, mucus, tight junctions	Peristalsis, mucus, tight junctions
Chemical defenses	Fatty acids, defensins, cathelicidins	Surfactant, antimicrobial peptides	Low stomach pH, lysozyme, antimicrobial peptides
Microbiological defenses	Normal microbiota	Normal microbiota	Normal microbiota
Unique feature	Lamellar bodies secrete lipids and antimicrobial proteins	Mucociliary clearance	Peristalsis

Key takeaway: All three epithelial surfaces use mechanical, chemical, and microbiological defenses, but their protective mechanisms differ according to their physiological functions.

Macrophages vs. Neutrophils vs. Dendritic Cells SUMMARY

Feature	Macrophages	Neutrophils	Dendritic Cells
Location	Healthy tissues	Primarily inflamed tissues	Healthy and inflamed tissues
Lifespan	Long-lived	Short-lived	Not emphasized
Phagocytosis	Yes	Yes	Yes
Microbial killing	Strong	Strong	Not primary function
Cytokine production	Yes	Not emphasized	Yes
Antigen presentation	Yes	Not emphasized	Major function
Primary role	Microbial killing and tissue maintenance	Rapid microbial killing	Connect innate and adaptive immunity
Special feature	Granuloma formation	Pus formation and NETs	Migration to lymph nodes

Key takeaway:

• Macrophages: Resident defenders and tissue maintenance.

• Neutrophils: Rapidly recruited microbial killers.

• Dendritic cells: Antigen presentation and adaptive immune activation.

Phagocyte Antimicrobial Mechanisms SUMMARY

Mechanism	Important Molecule/Process	Function
Acidification	Low phagolysosomal pH	Inhibits microbial growth
Reactive oxygen species	Superoxide, H₂O₂, hypochlorous acid	Damages and kills microbes
Reactive nitrogen species	Nitric oxide (NO)	Toxic to microorganisms
Antimicrobial peptides	Defensins, cathelicidins	Disrupt microbial membranes
Enzymatic degradation	Lysozyme	Degrades bacterial cell walls
Nutrient restriction	Lactoferrin	Sequesters iron needed for microbial metabolism
Extracellular trapping	NETs	Trap microorganisms and facilitate clearance

Key takeaway: Phagocytes use multiple antimicrobial mechanisms, including chemical toxicity, enzymatic degradation, and nutrient restriction.

Respiratory Burst Pathway SUMMARY

Step	Enzyme	Reaction/Product	Function
1	NADPH oxidase	O₂ → Superoxide (O₂⁻)	Generates reactive oxygen species
2	Superoxide dismutase	Superoxide → H₂O₂	Produces hydrogen peroxide
3	Myeloperoxidase	H₂O₂ + Cl⁻ → HOCl	Produces a highly antimicrobial compound

Respiratory Burst Sequence

NADPH oxidase → Superoxide → Superoxide dismutase → Hydrogen peroxide → Myeloperoxidase → Hypochlorous acid

Key takeaway: The respiratory burst produces reactive oxygen species that help neutrophils and macrophages destroy microorganisms

Neutrophil Recruitment and Extravasation SUMMARY

Step	Process	Important Molecules	Function
1	Vascular changes	Cytokines, lipid mediators	Vasodilation and increased permeability
2	Rolling	Selectins, sialyl Lewis X	Slow neutrophils along endothelium
3	Activation	CXCL8	Activates neutrophil integrins
4	Firm adhesion	Integrins (LFA-1, Mac-1), ICAMs	Firmly attaches neutrophils to endothelium
5	Transmigration	Endothelial adhesion machinery	Allows neutrophils to cross into tissues
6	Chemotaxis	CXCL8	Directs neutrophils toward infection

Key takeaway:

Rolling → Activation → Firm Adhesion → Transmigration → Chemotaxis