Comprehensive Notes – Innate Immune System

Page 1

• Course: Innate Immune System (Edward Via College of Osteopathic Medicine – Carolinas)
• Lecturer: Amy M. Hicks, PhD, MPH
• Assigned reading: Abbas, Lichtman, Pillai – Chapter 4

Page 2 – Objectives

• Two broad components of innate immunity:
– Surface (natural) barriers
– Internal defenses
• Recognize components and mechanisms of:
– Body’s natural barriers (physical, chemical, biological)
– Sentinel cells & functions
– Inflammation (definition, sequence of events)
– Cells performing phagocytosis
– Complement system (function & pathways)
– Soluble pattern-recognition proteins (pentraxins, collectins, ficolins)
– Natural killer (NK) cells & other innate lymphoid cells (ILCs)
– Antiviral defenses during innate response
• Compare & contrast locations and roles of key innate components

Page 3 – Anatomical Barriers to Infection

• Physical: skin, epithelial layers, peristalsis, cough reflex, ciliary escalator, mucus, low skin temperature
• Chemical: acidic $pH$, high salt, sebaceous secretions, lysozyme, saliva
• Biological: normal flora (competition + bactericidal factors), anti-bacterial peptides, innate immune cells

Page 4 – Physical Barriers (detail)

A. Tight junctions between epithelial cells – block microbial passage
B. Stratum corneum of skin – continual desquamation removes attached pathogens
C. Mucus entraps pathogens → removal by:
• Ciliary action, sneezing, coughing (respiratory tree)
• Peristalsis, vomiting, diarrhea (GI tract)
D. Lower skin temperature (≈$33^{\circ}$C) inhibits microbial growth

Page 5 – Chemical Barriers

• Lysozyme: present in saliva, tears, sweat, phagocyte granules; hydrolyzes $\text{N-acetylmuramic acid–N-acetylglucosamine}$ bond in peptidoglycan
• Acidic environments:
– Sebum keeps skin $pH\;3$–$5$
– Gastric acid + enzymes – antimicrobial
– Cervical mucus + low $pH$ in female GU tract
• Prostatic fluid – antibacterial via free $Zn^{2+}$
• Urine – bactericidal due to high urea, not $NaCl$ or ammonium

Page 6 – Biological Barriers

• Normal flora (microbiome):
– Space and nutrient competition
– Antibacterial secretions
– Continual immune alerting
• Antimicrobial peptides: defensins, etc.
• Intraepithelial lymphocytes (IELs):
– Natural (resident) IELs
– Induced (rapidly recruited) IELs

Page 7 – PRRs, PAMPs, DAMPs

• When barriers fail, innate cells use pattern-recognition receptors (PRRs)
• DAMPs (damage-associated): released by stressed self cells – e.g., heat-shock proteins, complement fragments, altered self DNA/CHOs
• PAMPs (pathogen-associated): microbial molecules – peptidoglycan, LPS, flagellin, unmethylated CpG DNA, ds/ss viral RNA, glycolipids

Page 8 – Coordination of Innate & Adaptive Immunity

• PRR engagement on immature dendritic cells (DCs) → maturation, ↑MHC & costimulatory molecules (CM), cytokine release
• Adjuvants mimic PAMPs/Tissue Factors → enhance APC recruitment, antigen retention, slow release
• Threshold concept: sufficient antigen + PRR signals needed to initiate adaptive response; innate phase reduces pathogen load while adaptive ramps up → immunological memory

Page 9 – Clinical Importance of Innate Immunity

• First-line defense; also primes adaptive immunity
• Defects → severe, disseminated infections
– Signaling (MyD88, IRAK-4)
– PAMP recognition (TLR3)
– Effector responses (NK cell, $IFN\gamma$, $IL-12/23$)

Page 10 – Innate Immune Cell Categories

• By origin/phenotype:
– Granulocytes: neutrophils/PMN, eosinophils, basophils, mast cells
– Lymphocytes: NK cells, IELs
– Mononuclear: monocytes → macrophages (Mac), dendritic cells (DCs)
• By function:
– Professional phagocytes: PMN, Mac, DC
– Antigen-presenting cells (APC): DC, Mac, B cells
– Sentinel cells: Mac, DC, IELs, (mast)

Page 11 – Basophils & Mast Cells

• Myeloid granulocytes; granules stain with basic dyes
• Location: basophils circulate; mast cells reside in tissues around vessels & nerves
• Granule contents: heparin, histamine → vasodilation
• Functions: allergy/hypersensitivity (baso), acute inflammation (mast)
• Express high-affinity $IgE$ receptors ($Fc\epsilon R$)

Page 12 – Eosinophils

• Bi-lobed nucleus; eosin-positive granules containing cytokines, chemokines, cationic proteins, proteases
• Receptors for $IgE$, $IgG$, complement, PRRs
• Roles: allergy, asthma, anti-helminth defense
• Killing strategies:
– Phagocytosis (if small)
– Degranulation on large parasites or allergen-coated surfaces (via Fc, CR, PRR)

Page 13 – Neutrophils / PMNs

• Abundant, short-lived granulocytes; azurophilic granules with lysozyme, proteases, peroxidases, lactoferrin, acid hydrolases, cationic proteins
• Key antibacterial phagocytes; also clear necrotic cells
• Express $Fc\gamma R$ (bind $IgG$), $Fc\mu R$ (bind $IgM$), and complement receptors → opsonization
• Chemokine receptors for rapid chemotaxis to infection sites

Page 14 – FcR-Mediated Degranulation

1. Granulocytes or NK cells express Fc receptors (FcR)
2. Antibody ($IgE$ or $IgG$) pre-bound to FcR
3. Antigen cross-links FcR → cell activation
4. Release of histamine, proteases, etc. → symptoms OR target killing

Page 15 – Monocyte Life Cycle

• Circulating monocytes sense danger → start differentiating
• Up-regulate adhesion molecules → extravasate
• In tissues, further differentiate into resident macrophages (Kupffer cells, osteoclasts, microglia) or inflammatory DCs based on microenvironmental cues

Page 16 – Macrophages

• Tissue-resident sentinels or recruited effectors
• PRRs detect PAMPs/DAMPs; cytokine/chemokine receptors respond to danger signals
• Activation → enlarged “foamy” cytoplasm filled with vacuoles & granules

Page 17 – Macrophage Functions

• Pathogen destruction: phagolysosomal killing + secretion of reactive oxygen/nitrogen species (ROS/RNS)
• Antigen presentation: processed peptides displayed on MHC to activate T/B cells
• Amplify immunity: secrete cytokines $TNF\alpha$, $IFN\alpha$, $IL-1$, $IL-10$, $IL-12$

Page 18 – Dendritic Cells (DC)

• Tissue sentinels in immature state
• Upon PRR triggering:
– Form dendrites
– ↑MHC & costimulatory molecules
– ↑Chemokine receptors & adhesion molecules for migration
– Secrete cytokines that prime lymphocytes
• Mature DCs = professional APCs for naïve T/B cells

Page 19 – Lymphocyte Overview

• All derived from common lymphoid progenitor; single-lobed nucleus
• Adaptive lymphocytes:
– Conventional: T, B, plasma cells, some IELs
– Non-conventional: NKT, $\gamma\delta$ T, Treg
• Innate lymphocytes: NK cells, IELs – rapid, non-specific, no prior priming required

Page 20 – NK Cell Receptors & Actions

• Activation cytokines from Macs/DCs → NK activation
• Effector functions:
– Cytolysis of targets
– Cytokine secretion (e.g., $IFN\gamma$) → amplifies Mac/DC/T cell activity
• Targets: virus-infected cells, transformed tumor cells, over-activated immune cells (homeostasis reset)

Page 21 – Key Functions of Innate Immunity

Sequence of inflammation (local):

1. Injury/barrier breach → microbial entry
2. Sentinel cell activation
3. Inflammatory mediator secretion
4. Vasodilation & ↑vascular permeability → plasma proteins enter tissue
5. Complement, antibodies, antimicrobial proteins attack microbes
6. Endothelial adhesion molecules + chemokines guide leukocyte extravasation
7. Phagocytosis & killing of microbes

Page 22 – Local vs Systemic Inflammation

• Local cytokines ($IL-1$, $TNF\alpha$, chemokines) on endothelium → adhesion, permeability
• Systemic protective:
– Fever (brain) – $IL-1$, $TNF\alpha$, $IL-6$
– Acute-phase protein production (liver)
– Leukocyte production (bone marrow)
• Systemic pathologic (excess $TNF\alpha$):
– ↓Cardiac output
– Thrombosis, vascular leak → shock
– Insulin resistance in muscle

Page 23 – Chemotaxis I

• Chemotaxis = cell attraction to chemical gradients
• Chemokines released by injured/infected cells
• Leukocytes express specific chemokine receptors

Page 24 – Chemotaxis II

• Professional phagocytes (PMN, Mac, DC) highly responsive
• Chemokine-receptor binding polarizes cell → directed movement along gradient from blood → endothelium → tissue → focus of infection

Page 25 – Diapedesis Overview

Steps:

1. Margination / rolling (selectins)
2. Activation (chemokines)
3. Adhesion (integrins)
4. Transmigration through endothelium
5. Chemotaxis within tissue

Page 26 – Diapedesis Detail Table

0. Endothelium activation: $IL-1$, $TNF\alpha$ ↑selectins
1. Rolling: sialyl-Lewis X on leukocyte ↔ selectin
   2a. Leukocyte activation: chemokines ($IL-8$, PAF, complement) trigger integrin high-affinity state
   2b. Stable adhesion: CD11b–ICAM-1/3; VLA-4–VCAM-1
2. Transmigration: bind endothelial tight-junction proteins
3. Chemotaxis through interstitium

Page 27 – Phagocytosis

• Professional: PMN, Mac, DC; Secondary: B cells, eosinophils
• Recognition:
– Direct via PRR
– Indirect via opsonins: complement $C3b$ (CR1) or antibody Fc portion (FcR)
• Steps: binding → engulfment → phagolysosomal destruction (ROS, enzymes)

Page 28 – Complement-Mediated Cytotoxicity

Three enzymatic cascades:

1. Alternative pathway – spontaneous $C3$ hydrolysis & pathogen surface binding
2. MB-lectin pathway – mannose-binding lectin binds pathogen carbohydrates
3. Classical pathway – $C1q$ binds $Ag:Ab$ complexes
   → Formation of membrane-attack complex (MAC) $C5b6789$ → lysis

Page 29 – NK Receptor Balance

• Activation receptors detect stress/foreign ligands
• Inhibitory KIRs recognize $MHC\,I$ → block killing
• Net signal determines cytolysis

Page 30 – NK Cytotoxic Mechanisms

• ADCC: $IgG$ opsonized target → NK $Fc\gamma R$ cross-linking → degranulation
• Necrosis: perforin channels disrupt membrane
• Apoptosis:
– $FasL$ (CD95L) on NK binds Fas on target
– Granzymes enter via perforin pores → caspase activation

Page 31 – Antigen Presentation (Bridge Function)

• Professional APCs: DC, Mac, B
• Steps: phagocytosis → processing → MHC display → migrate to lymph node/spleen via lymphatics → activate naïve T/B
• Critical link between innate & adaptive immunity

Page 32 – Complete Innate Response (Sequence)

1. Acute phase initiation: damaged epithelium → pro-inflammatory proteins, vasoactives
2. Resident Macs/DCs activated → phagocytose Ag; release cytokines/chemokines; start Ag processing
3. Recruitment: endothelial activation (selectins) → PMN, NK, mast, eos, new Mac/DC extravasation (integrins)
4. Local effector action: killing/degranulation within tissue
5. Distal: lymphatic endothelium activated; activated APCs traffic to lymph node
6. Adaptive activation: APCs present Ag to naïve lymphocytes

Page 33 – Innate Response to Viruses (Intracellular)

• PRR binding + DAMPs → Macs, DCs, NK recruited
• Cytokines:
– Early: $IFN\alpha/\beta$, $TNF\alpha$ from infected cells
– Induced: $IL-12$, $IFN\gamma$, more $IFN\alpha/\beta$ from Macs/DCs/NK
• Type I IFNs: antiviral state in neighbors, trigger apoptosis, inhibit replication
• NK cells: kill infected host cells lacking $MHC\,I$ or expressing stress ligands

Page 34 – Board-Style Question (Opsonization)

Scenario: child with recurrent encapsulated bacterial infections; normal pro-B, ↓mature B → opsonization defect most likely
(Answer: opsonization)

Page 36 – Focusing Innate Immunity

Different PAMP/PRR signals tailor mediator & cell recruitment:
• Bacteria → $IL-8, IL-17$ → PMN
• Helminths → $IL-4, IL-5$ → Eosinophils
• Allergens → $IL-4, IL-5$ → Mast cells, eos
• Viruses → Type I IFN → NK cells

Page 37 – Innate Response to Extracellular Pathogens

• Barrier breach → PRR activation, DAMP release
• Granulocyte recruitment depends on PRR signals (Mac, PMN, Mast, Eos)
• APC activation; cytokine timeline:
– Immediate: $IL-1, IL-6, TNF\alpha$
– Later: $IL-8, IL-17$
• Complement: activation (lectin or Ab) → MAC lysis
• Opsonization (C3b or Ab)
• Phagocytosis/degranulation for pathogen clearance

Page 38 – Success vs Failure

• If acute response eliminates danger → repair & homeostasis
• If not, chronic inflammation ensues due to persistent PAMP/DAMP signals

Page 39 – Acute vs Chronic Inflammation

Acute: PMN, Mac; mediators complement, kinins, $IL-1$, $TNF\alpha$, eicosanoids → pus, abscess
Chronic: Mac, T cells; cytokines dominate → fibrosis, granulomas

Page 40 – IEL “Light & Dark”

• Protective: regulate barrier immunity
• Pathogenic: misrecognition of commensals → chronic inflammation (IBD, Crohn’s, sinusitis hypothesis)

Page 41 – Summary I (Barriers)

Components: epithelium, cilia, defensins, sebum, acidic $pH$, bile, lysozyme, IELs, microbiome

Page 42 – Summary II (Post-Breach)

• Soluble mediators: kinins, prostaglandins, leukotrienes, complement, histamine
• PRR engagement → ILCs, Macs, DCs activation; cytokine secretion; Ag processing
• Chemokines $IL-8$ etc. recruit leukocytes; $IL-1$, $TNF\alpha$ activate endothelium → diapedesis
• Phagocytes kill pathogen
• Know diapedesis steps and phagocytosis process

Page 43 – Summary III (APCs & Ongoing Response)

• Activated APCs migrate via lymph → secondary lymphoid organs → present Ag to naïve T/B
• Adaptive lymphocytes return to infection site
• New cycles of chemotaxis, cytolysis, antigen presentation continue until clearance
• Know chemotaxis sequence & NK cytolysis mechanisms

Page 44 – Immunoglobulin Board Question Examples

• IgG: directly participates in opsonization
• IgE: binds surface of basophils (and mast cells)

Page 45 – Suggested Videos (optional study aids)

• Initiation of innate immunity, macrophage functions, cross-talk with adaptive immunity

Page 46 – References

• Cellular & Molecular Immunology (Abbas et al.)
• Primary literature cited in slides
• TrueLearn/ComBank

Page 47 – Medical Impact of Innate Immunity

Genetic defects → clinical syndromes:

1. Leukocyte adhesion deficiency (LAD I/II): β2-integrin/E- & P-selectin defects → impaired PMN adhesion → recurrent infections
2. Chronic granulomatous disease (CGD): NADPH oxidase defect → ↓ROS → bacterial/fungal infections
3. Rheumatoid arthritis, gout: immune cell degranulation/cytokines in joints → cartilage damage
4. Complement regulatory defects (hereditary angioedema, PNH): uncontrolled complement → edema, hemolysis, ↑infection/SLE risk
5. Chédiak-Higashi: lysosomal trafficking defect → recurrent infections, neuropathy, partial albinism
   • Clinical takeaway: Understanding innate pathways identifies points of failure and therapeutic targets (e.g., anti-cytokine biologics such as etanercept or adalimumab).