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Innate Immunity

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Definition

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Components of Innate Immunity

1. Epithelial Barriers (First Line)

• Skin, respiratory, and GI epithelia act as mechanical barriers to microbial entry
• Epithelial cells secrete antimicrobial peptides such as defensins
• Intraepithelial lymphocytes (especially in the gut) add a cellular defense at these surfaces

2. Phagocytic Cells

• Reactive oxygen species (respiratory burst via NADPH oxidase)
• Lysosomal enzymes
• Antimicrobial peptides

• Kupffer cells (liver)
• Microglia (brain)
• Alveolar macrophages (lungs)

3. Dendritic Cells (DCs)

• Capture and present antigens to T lymphocytes (linking innate to adaptive immunity)
• Express pattern recognition receptors (PRRs) to detect danger signals
• Secrete cytokines that direct adaptive immune responses
• Are not directly involved in microbial killing (unlike macrophages)

4. Natural Killer (NK) Cells

• Virus-infected cells (which downregulate MHC-I to evade T cells - NK cells detect this "missing self")
• Tumor cells

• Perforin/granzyme pathway
• Fas-FasL interactions
• Antibody-dependent cellular cytotoxicity (ADCC)

5. Innate Lymphoid Cells (ILCs)

• ILC1 - produces IFN-gamma (mirrors Th1 function)
• ILC2 - produces IL-4, IL-5, IL-13 (mirrors Th2; important in allergy and parasitic defense)
• ILC3 - produces IL-17, IL-22 (mirrors Th17; important at mucosal barriers)

6. Plasma Proteins

• Complement: Activated via the alternative pathway (spontaneous C3 hydrolysis) or lectin pathway (MBL binds mannose on microbes) - both are part of innate immunity
• Mannose-binding lectin (MBL): Binds mannose-rich carbohydrates on bacteria/fungi, acts as an opsonin and activates complement
• C-reactive protein (CRP): Acute-phase protein that coats microbes (opsonization) and activates complement
• Lung surfactant: Protects against inhaled microbes

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Pattern Recognition Receptors (PRRs)

• Shared among related microbes
• Essential for microbial survival - so microbes cannot easily mutate these structures to escape immunity
• Not found in mammalian cells

a. Toll-Like Receptors (TLRs)

• Plasma membrane TLRs: Detect bacterial products (e.g., LPS detected by TLR4, flagellin by TLR5, lipoteichoic acids by TLR2)
• Endosomal TLRs: Detect viral/bacterial nucleic acids after phagocytosis (e.g., dsRNA by TLR3, ssRNA by TLR7/8, CpG DNA by TLR9)

1. NF-kB - stimulates cytokine synthesis and adhesion molecule expression (drives inflammation)
2. Interferon Regulatory Factors (IRFs) - stimulate type I interferon (IFN-alpha/beta) production (antiviral defense)

b. NOD-Like Receptors (NLRs) and the Inflammasome

• Bacterial peptidoglycan fragments (NOD1, NOD2)
• Products of necrotic cells (uric acid, ATP)
• Ion disturbances (K+ loss)
• Various PAMPs

1. Activates caspase-1
2. Caspase-1 cleaves pro-IL-1beta and pro-IL-18 into their active forms
3. Can also trigger pyroptosis (inflammatory cell death)

• Gain-of-function NLR mutations cause autoinflammatory syndromes - respond to IL-1 antagonists
• NLRP3 activation by urate crystals underlies inflammation in gout
• NLRP3 activation by cholesterol crystals may contribute to atherosclerosis

c. RIG-Like Receptors (RLRs)

d. Cytosolic DNA Sensors (cGAS-STING Pathway)

• cGAS (cyclic GMP-AMP synthase) detects cytosolic dsDNA
• Produces cGAMP, which activates STING (Stimulator of Interferon Genes)
• Leads to type I IFN production
• Activated by viral DNA, bacterial DNA, and aberrant self-DNA (nuclear damage)
• Defects in this regulation cause interferonopathies (e.g., Aicardi-Goutieres syndrome)

e. C-Type Lectin Receptors (CLRs)

• On macrophages and DCs
• Recognize microbial polysaccharides (bacterial and fungal cell walls)
• Stimulate phagocytosis and inflammatory responses
• Important in defense against fungi (e.g., Dectin-1 recognizes beta-glucan on Candida)

f. G Protein-Coupled Receptors

• On neutrophils and macrophages
• Recognize N-formylmethionyl peptides (fMLP) from bacterial proteins (bacteria initiate protein synthesis with fMet; mammalian cells do not)
• Drive leukocyte chemotaxis toward bacteria

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Reactions of Innate Immunity

1. Inflammation

• Cytokine release (TNF, IL-1, IL-6, IL-12, CXCL8/IL-8)
• Complement activation (C3a, C5a - anaphylatoxins)
• Recruitment and activation of neutrophils and macrophages
• Vascular changes (vasodilation, increased permeability)
• Fever (via IL-1, IL-6, TNF acting on hypothalamus)

2. Antiviral Defense

• Act on infected and uninfected cells
• Upregulate antiviral enzymes (e.g., RNase L, protein kinase R)
• Degrade viral nucleic acids and inhibit viral replication
• Upregulate MHC-I expression (enhancing cytotoxic T cell recognition)

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Innate Immunity and the Link to Adaptive Immunity

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Clinical Correlations

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• Robbins, Cotran & Kumar Pathologic Basis of Disease, Chapter 6
• Robbins & Kumar Basic Pathology, Chapter 5
• Cellular and Molecular Immunology (Abbas, Lichtman & Pillai)
• Janeway's Immunobiology, 10e

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Innate Immunity - Immunology

Overview: Innate vs. Adaptive Immunity

"Innate immunity is essential for defending against microbes in the first few hours or days after infection, before adaptive immune responses have developed." - Cellular and Molecular Immunology

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Cellular Origin: From Bone Marrow to Effector

• Common Myeloid Progenitor (CMP): Gives rise to macrophages, neutrophils, eosinophils, basophils, mast cells, and dendritic cells - the dominant cells of innate immunity
• Common Lymphoid Progenitor (CLP): Gives rise to NK cells and ILCs (innate lymphoid lineages), alongside adaptive B and T cells

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Components of Innate Immunity

1. Physical and Chemical Barriers

• Skin epithelium: Physical barrier; low pH; fatty acids on surface
• Mucosal epithelia (GI, respiratory, urogenital): Tight junctions; mucus trapping; ciliary clearance (mucociliary escalator)
• Antimicrobial molecules:
  • Defensins: Small cationic peptides that disrupt microbial membranes. Produced by epithelial cells and neutrophils (alpha-defensins in crypts of Lieberkuhn)
  • Lysozyme: Cleaves bacterial peptidoglycan (present in saliva, tears, mucus)
  • Lactoferrin: Sequesters iron needed for bacterial growth
  • Secretory IgA (bridges innate structure with adaptive product): Prevents microbial attachment at mucosal surfaces

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2. Phagocytic Cells

Macrophages

• Embryonic precursors (yolk sac / fetal liver) - populate tissues before birth
• Adult bone marrow monocytes - circulate in blood and differentiate into macrophages upon entering tissues

1. Phagocytosis and killing of microbes (respiratory burst, lysosomal enzymes)
2. Production of inflammatory cytokines: TNF, IL-1, IL-6, IL-12, IL-23, CXCL8
3. Activation of complement via secreted pattern recognition molecules
4. Antigen presentation to T cells (link to adaptive immunity)
5. Tissue repair and resolution of inflammation

Neutrophils (Polymorphonuclear Leukocytes)

• Respiratory burst: NADPH oxidase generates superoxide → H₂O₂, hypochlorous acid (HOCl via myeloperoxidase)
• Degranulation: Azurophilic granules release elastase, cathepsin G, defensins
• Neutrophil Extracellular Traps (NETs): Chromatin + antimicrobial proteins expelled to trap and kill extracellular pathogens

Monocytes

• Classical monocytes (CD14hi CD16-): Patrolling, highly phagocytic
• Non-classical monocytes (CD14lo CD16+): More inflammatory, patrolling vasculature

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3. Dendritic Cells (DCs)

• Immature DCs in tissues: Phagocytic, sample their environment constantly via macropinocytosis
• Mature DCs after pathogen encounter: Migrate to lymph nodes, downregulate phagocytosis, upregulate MHC-II and costimulatory molecules (CD80, CD86) to activate naïve T cells
• Rich expression of PRRs - prime detectors of PAMPs and DAMPs

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4. Mast Cells and Basophils

• Express FcεRI (high-affinity IgE receptor) - central to allergic responses
• Innate activation: Directly by complement (C3a, C5a), PAMPs via TLRs, and physical stimuli
• Release histamine, tryptase, prostaglandins, leukotrienes, TNF upon degranulation
• Important in defense against parasites and venoms
• Produce cytokines that activate ILC2 (IL-4, IL-13, IL-33)

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5. Natural Killer (NK) Cells

• Normal healthy cells express MHC class I → delivers inhibitory signal to NK cells via KIR (Killer Immunoglobulin-like Receptors) and CD94/NKG2A → NK cell is inhibited
• Virus-infected or tumor cells downregulate MHC-I → loss of inhibitory signal → NK cell becomes activated
• Stressed cells upregulate NKG2D ligands (MICA, MICB, ULBP) → activating signal to NK cells

• Perforin/Granzyme pathway: Perforin forms pores; granzymes (serine proteases) enter and trigger apoptosis
• Fas-FasL interaction → apoptosis of target cell
• ADCC (Antibody-Dependent Cell Cytotoxicity): NK cells express CD16 (FcγRIII); bind antibody-coated target cells and kill

• NK cells produce large amounts of IFN-gamma → activates macrophages → enhanced killing of intracellular pathogens

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6. Innate Lymphoid Cells (ILCs)

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7. Soluble Components

Complement System

• C3b: Opsonin - coats microbes for phagocytosis (via CR1/CR3)
• C3a/C5a: Anaphylatoxins - mast cell degranulation, neutrophil chemotaxis (C5a is more potent)
• C5b-9 (MAC): Membrane Attack Complex - directly lyses gram-negative bacteria

Acute Phase Proteins

Cytokines of Innate Immunity

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Pattern Recognition Receptors (PRRs)

• PAMPs are evolutionarily conserved - essential for microbial survival, so microbes cannot easily mutate them
• ~100 different PRRs recognize thousands of molecular patterns
• PRRs are germline-encoded (non-rearranging) - unlike adaptive immune receptors

PRR Families and Their Locations:

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A. Toll-Like Receptors (TLRs)

• All TLRs signal via adaptor proteins containing TIR domains
• MyD88 (used by all TLRs except TLR3) → NF-κB → pro-inflammatory cytokines (TNF, IL-6, IL-12)
• TRIF (used by TLR3, TLR4) → IRF3/IRF7 → Type I IFN (IFN-α/β) production

• TLR4 mutations: Increased susceptibility to gram-negative sepsis
• TLR loss-of-function: Rare but serious immunodeficiency syndromes
• LPS signaling through TLR4 is the key driver of septic shock

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B. NOD-Like Receptors (NLRs) and the Inflammasome

• NOD1: Detects DAP (diaminopimelic acid) from gram-negative bacteria
• NOD2: Detects MDP (muramyl dipeptide) from both gram+ and gram- bacteria
• Signal via RIPK2 → NF-κB activation
• NOD2 mutations: Associated with Crohn's disease (impaired barrier immunity)

NLRP3 + ASC + pro-caspase-1 → INFLAMMASOME
                                      ↓
                               Caspase-1 (active)
                              ↙              ↘
              pro-IL-1β → IL-1β        Gasdermin D cleavage
              pro-IL-18 → IL-18              ↓
                                        PYROPTOSIS
                                  (inflammatory cell death)

• Crystals: Monosodium urate (gout), calcium pyrophosphate (pseudogout), cholesterol crystals (atherosclerosis)
• Particulates: Silica, asbestos, amyloid fibrils (Alzheimer's)
• Metabolic signals: Fatty acids (obesity, metabolic syndrome)
• Potassium efflux, ROS, lysosomal damage

• Gain-of-function NLRP3 mutations: Cryopyrin-associated periodic syndromes (CAPS) - treated with IL-1 antagonists (anakinra, canakinumab)
• Gout: NLRP3 activated by urate crystals
• Type 2 diabetes: NLRP3 activation by islet amyloid polypeptide (IAPP)
• Atherosclerosis: Cholesterol crystal activation of NLRP3

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C. RIG-I-Like Receptors (RLRs)

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D. cGAS-STING Pathway

Cytosolic dsDNA → cGAS activated
                      ↓
              cGAMP synthesis (2'3'-cGAMP)
                      ↓
              STING (on ER membrane) activated
                      ↓
              TBK1 → IRF3 → IFN-α/β
              also → NF-κB → inflammation

• Viral DNA (HSV, HIV, CMV)
• Bacterial DNA (Mycobacterium tuberculosis, Listeria)
• Mitochondrial DNA released by stressed cells
• Nuclear DNA from cells with DNA damage

• Interferonopathies (Aicardi-Goutieres syndrome): Mutations in DNases that clear cytosolic self-DNA → chronic cGAS-STING activation → excessive IFN production
• STING gain-of-function: STING-associated vasculopathy with onset in infancy (SAVI)
• cGAS-STING is a therapeutic target for autoimmunity, cancer immunotherapy, and antiviral drugs

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E. C-Type Lectin Receptors (CLRs)

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F. Other Cytosolic Sensors

• AIM2 (Absent in Melanoma 2): Detects cytosolic dsDNA → forms AIM2 inflammasome → caspase-1 → IL-1β/IL-18. Activated by Francisella tularensis and vaccinia virus
• G protein-coupled receptors (GPCRs): Detect fMLP (N-formylmethionyl peptides from bacterial proteins); drive neutrophil chemotaxis (bacteria use fMet to initiate proteins; mammalian cytoplasmic translation does not)
• Scavenger receptors: SR-A, CD36 - recognize oxidized lipids, apoptotic cells, bacterial LTA; promote phagocytosis

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Reactions of Innate Immunity

1. Inflammation

• Vasodilation → increased blood flow (redness, heat)
• Increased vascular permeability → edema (swelling)
• Driven by: histamine (mast cells), C3a/C5a, bradykinin, prostaglandins

• Neutrophil recruitment (minutes-hours): CXCL8, C5a, LTB4
• Monocyte/macrophage recruitment (hours-days): CCL2 (MCP-1)
• Adhesion cascade: Selectins → rolling; integrins (LFA-1/ICAM-1) → firm adhesion → transmigration

• Fever: IL-1β, IL-6, TNF act on hypothalamus → PGE2 → elevated set point
• Acute phase proteins: CRP, MBL, fibrinogen, SAA - opsonins, complement activators
• Leukocytosis: IL-1β, G-CSF release neutrophils from bone marrow
• Hepatocyte production of acute phase proteins (upregulation of CRP, fibrinogen; downregulation of albumin, transferrin)

2. Antiviral Defense (Type I Interferon Response)

• Upregulate ISGs (Interferon-Stimulated Genes)
• Activate OAS/RNase L → degrade viral RNA
• Activate PKR (protein kinase R) → phosphorylate eIF2α → halt protein translation
• Upregulate MHC class I → better recognition by cytotoxic T cells
• Activate NK cells → enhanced killing of infected cells

3. Activation of Adaptive Immunity

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Innate Immune Evasion by Pathogens

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Clinical Correlations and Immunodeficiencies

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Summary: The Innate Immune System as an Integrated Network

MICROBE INVADES
       ↓
Epithelial barriers breached
       ↓
PAMPs/DAMPs recognized by PRRs on:
• Macrophages / DCs (TLRs, CLRs, NLRs, cGAS)
• Neutrophils (TLRs, GPCRs)
• Epithelial cells (TLRs, RLRs)
       ↓
         ┌──────────────────────────────┐
         ↓                              ↓
   NF-κB activation             IRF3/IRF7 activation
         ↓                              ↓
   Pro-inflammatory           Type I IFN production
   cytokines (TNF, IL-1,          (IFN-α/β)
   IL-6, IL-12, CXCL8)               ↓
         ↓                     Antiviral state
   Inflammation                NK cell activation
   (neutrophil/monocyte             ↓
   recruitment)              Infected cell killing
         ↓
NK cells, ILCs → effector killing / cytokine amplification
         ↓
DC maturation + migration to lymph nodes
         ↓
   ADAPTIVE IMMUNITY ACTIVATED

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• Janeway's Immunobiology 10e - Chapters 1 & 3 (Garland Science/Norton)
• Cellular and Molecular Immunology, Abbas, Lichtman & Pillai - Chapters 1 & 4
• Robbins, Cotran & Kumar Pathologic Basis of Disease - Chapter 6
• Robbins & Kumar Basic Pathology - Chapter 5