14.6 Innate Immunity: Barriers, Complement & Cellular Effectors

14.6 Innate Immunity: Barriers, Complement & Cellular Effectors

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The Architectural Dichotomy: Innate vs. Adaptive Immunity

Host defense is organized into two cooperating arms differing in speed, specificity, and immunological memory:

• Innate Immunity: Immediate to early response (operational within 0–12 hours of exposure). It is antigen-non-specific, does not require prior sensitization, and does not alter its kinetics or magnitude upon repeated exposures (lacks immunological memory). It utilizes germline-encoded Pattern Recognition Receptors (PRRs), such as Toll-Like Receptors (TLRs), to detect conserved microbial structural motifs known as Pathogen-Associated Molecular Patterns (PAMPs) (e.g., bacterial lipopolysaccharide [LPS], flagellin, peptidoglycan, viral double-stranded RNA) and host-derived Damage-Associated Molecular Patterns (DAMPs) (e.g., HMGB1, extracellular ATP, uric acid crystals).
• Adaptive Immunity: Delayed initial response (takes 3–7 days for clonal expansion and differentiation). It is exquisitely antigen-specific, generates a diverse repertoire of antigen receptors via somatic gene rearrangement, and develops long-lived memory cells that mediate a rapid, high-affinity secondary response upon re-exposure.

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Innate Defenses: Anatomical Barriers, Chemical Factors & Complement

Innate Physical & Chemical Defenses:
- Keratinized Stratified Squamous Epidermis: Physical impermeable barrier; continuous desquamation.
- Mucociliary Escalator: Respiratory goblet cells trap particles in mucus; ciliated cells sweep them upward.
- Lysozyme (Tears, Saliva, Mucus): Hydrolyzes beta-1,4-glycosidic bonds between NAM and NAG in bacterial peptidoglycan.
- Acidic Secretions: Gastric hydrochloric acid (pH 1.5-2.0) and vaginal lactic acid (pH ~4.0) kill swallowed/colonizing microbes.
- Antimicrobial Peptides: Defensins and cathelicidins insert into microbial membranes, forming lethal pores.

The Complement Cascade

The complement system comprises over 30 heat-labile plasma proteins synthesized primarily by the liver. They circulate as inactive zymogens that activate through an amplifying proteolytic cascade along three distinct initiation pathways:

1. Classical Pathway: Initiated when the C1 complex ($C1q, C1r, C1s$) binds directly to the Fc region of antibody-antigen immune complexes containing either IgM (most efficient; a single pentameric IgM can bind C1q) or aggregated IgG (requires at least two adjacent IgG monomers).
2. Lectin Pathway: Initiated when mannose-binding lectin (MBL) or ficolins bind conserved terminal mannose residues on foreign bacterial or fungal cell surfaces, activating MBL-associated serine proteases (MASPs).
3. Alternative Pathway: Initiated spontaneously at low basal rates by the continuous "tickover" (hydrolysis) of plasma C3 into $C3(H_2O)$, which binds Factor B and Factor D on microbial surfaces lacking host regulatory proteins (such as CD55/DAF and CD59).

Complement Cascade Convergence & Effector Outputs:
All three pathways assemble a C3 Convertase:
  - Classical/Lectin: C4b2a
  - Alternative: C3bBb
C3 Convertase cleaves abundant plasma C3 into:
  -> C3a: Small peptide acting as a potent Anaphylatoxin (induces mast cell degranulation, histamine release, vasodilation).
  -> C3b: Binds covalently to pathogen surfaces, acting as a master Opsonin recognized by CR1 receptors on macrophages.
C3b joins the convertase to assemble C5 Convertase (C4b2a3b or C3bBbC3b), cleaving C5 into:
  -> C5a: Potent Anaphylatoxin and the strongest chemoattractant for neutrophil recruitment and extravasation.
  -> C5b: Initiates Membrane Attack Complex (MAC) assembly: sequentially recruits C6, C7, C8, and 10-16 C9 monomers.
Polymerized C9 forms a 10 nm transmembrane aqueous pore: Membrane Attack Complex (MAC, C5b-C9).
Fluid rushes in down the osmotic gradient, causing cell swelling and osmotic lysis of Gram-negative bacteria and enveloped viruses.

Type I Interferons (IFN-α and IFN-β)

When viral pattern molecules (e.g., intracellular dsRNA) engage cytosolic RIG-I-like receptors or endosomal TLR3/TLR7, virally infected host cells synthesize and secrete Type I Interferons (IFN-$\alpha$ and IFN-$\beta$). These cytokines act in an autocrine and paracrine manner, binding IFNAR receptors on adjacent uninfected cells to activate the JAK-STAT pathway, transcriptionally upregulating hundreds of interferon-stimulated genes (ISGs):

• Protein Kinase R (PKR): Phosphorylates elongation factor eIF-2$\alpha$, halting all host and viral mRNA translation.
• 2'-5'-Oligoadenylate Synthetase (OAS) and RNase L: Degrades viral and host single-stranded RNA.
• MHC Class I Upregulation: Increases surface MHC I display on uninfected and infected cells, enhancing recognition by $CD8^+$ cytotoxic T cells.

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Innate Cellular Effectors

1. Macrophages: Derived from circulating blood monocytes that extravasate into tissues, differentiating into specialized resident phagocytes (e.g., Kupffer cells in the liver, microglia in the CNS, alveolar macrophages in the lungs, osteoclasts in bone). Phagocytose opsonized microbes into phagosomes, which fuse with lysosomes to form phagolysosomes. Inside, they execute an oxygen-dependent respiratory burst: $$\text{O}_2 \xrightarrow{\text{NADPH Oxidase}} \cdot\text{O}_2^- \text{ (Superoxide)} \xrightarrow{\text{Superoxide Dismutase}} \text{H}_2\text{O}_2 \xrightarrow{\text{Myeloperoxidase (MPO)}} \text{HOCl (Hypochlorous Acid / Bleach)}$$ Macrophages also secrete key pro-inflammatory cytokines: IL-1 (fever, osteoclast activation), TNF-$\alpha$ (vascular permeability, cachexia), IL-6 (acute-phase reactant synthesis by hepatocytes), IL-8 (neutrophil chemotaxis), and IL-12 ($T_h1$ differentiation).
2. Neutrophils (Polymorphonuclear Leukocytes / PMNs): Most abundant circulating white blood cells (~60%–70%). Rapid first responders to acute inflammation. They extravasate across inflamed post-capillary venules via a multi-step adhesion cascade: Margination $\to$ Rolling (endothelial E-selectin and P-selectin bind neutrophil Sialyl Lewis X) $\to$ Tight Adhesion (endothelial ICAM-1 binds activated neutrophil $\beta_2$-integrins [LFA-1/Mac-1]) $\to$ Diapedesis (transmigration via PECAM-1/CD31) $\to$ Chemotaxis (migrating toward C5a, LTB4, IL-8, or bacterial fMLP). Neutrophils phagocytose, degranulate antimicrobial enzymes, and cast Neutrophil Extracellular Traps (NETs)—webs of extracellular chromatin laced with granule proteins that trap and kill extracellular bacteria.
3. Eosinophils: Bilobed granulocytes specialized in defending against large multicellular parasites (helminths) that cannot be phagocytosed. Activated by IL-5 from $T_h2$ cells; bind IgE-coated parasites via Fc receptors and release Major Basic Protein (MBP) and eosinophil cationic protein to induce tegument damage. Prominent in allergic and asthmatic reactions.
4. Basophils and Mast Cells: Tissue-resident mast cells (skin, mucosa) and circulating basophils express high-affinity $Fc\epsilon RI$ receptors pre-loaded with monomeric IgE. When multivalent allergen cross-links adjacent surface-bound IgE molecules, it triggers explosive degranulation, releasing pre-formed histamine (vasodilation, bronchoconstriction, increased vascular permeability), heparin, and newly synthesized lipid mediators (Leukotrienes $LTC_4, LTD_4, LTE_4$).
5. Natural Killer (NK) Cells: Innate lymphocytes (CD56+, CD16+, CD3-) that survey host cells for viral infection and malignant transformation without prior clonal expansion. Regulated by the "Missing Self" Hypothesis:
   • Healthy host cells express high levels of surface MHC Class I. This binds NK cell Killer-cell Inhibitory Receptors (KIRs), delivering a dominant inhibitory signal that overrides activating signals.
   • Virally infected cells or neoplastic cells frequently downregulate surface MHC Class I to hide from $CD8^+$ cytotoxic T cells. In the absence of MHC Class I engagement, the inhibitory signal is abolished, and activating receptors trigger NK degranulation.
   • Cytotoxic Mechanism: NK cells release perforin (forms transmembrane pores in the target membrane) and granzymes (serine proteases that enter through perforin pores to cleave and activate initiator caspase-3 and BID, triggering caspase-dependent apoptosis).
   • Antibody-Dependent Cellular Cytotoxicity (ADCC): NK cells express CD16 (an $Fc\gamma RIII$ receptor). When IgG antibodies bind foreign antigens on an infected target cell, CD16 binds the exposed IgG Fc domains, triggering targeted perforin/granzyme degranulation.
6. Dendritic Cells (DCs): The quintessential professional Antigen-Presenting Cells (APCs) bridging innate and adaptive immunity. Reside as immature sentinels in peripheral tissues (e.g., cutaneous Langerhans cells). Upon engulfing pathogens via phagocytosis or macropinocytosis, they mature, upregulate costimulatory molecules (CD80/CD86) and CCR7 chemokine receptors, and migrate via afferent lymphatics to regional lymph nodes to present processed peptides on MHC molecules to naive T cells.

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Innate Cellular Effectors

Each innate leukocyte is defined by what it kills and how, and several double as the bridge into adaptive immunity.