Lecture 12

Overview of Immunity and Innate Defenses

• The First Line of Defense: Encompasses prevention, passive resistance, and non-specific cell responses. This corresponds to Chapters 2 and 3 of the source material.

• Prevention and Passive Defenses:     * Behavior: Behavioral adaptations to avoid infection.     * Physical and Chemical Barriers:         * Epithelia: Includes the skin and mucosal surfaces.         * Chemical Barriers: Mucus, Lysozyme, and Keratin.         * Antimicrobial Proteins/Peptides: Lactoferrins, defensins, and Cathelicidin Antimicrobial Peptides (CAMPs).

• Innate Defenses: Includes the blood system, lymphatics, phagocytes, and non-specific cytotoxic cells.

• Cell Distribution: These cells are distributed throughout the body and utilize specific signaling to leave the blood and travel to infection sites.

Cellular Components of the Innate Immune System

• Stem Cell Progenitors:     * Common Myeloid Progenitor: Gives rise to megakaryocytes (platelets), monocytes (which differentiate into macrophages and dendritic cells), PMNs (neutrophils), eosinophils, basophils, and mast cells.     * Common Lymphoid Progenitor: Gives rise to B cells (plasma and memory), T cells (effector, cytotoxic, and memory), and Natural Killer (NK) cells.

• Phagocytes: Primarily include monocytes, macrophages, dendritic cells (DCs), and Polymorphonuclear leukocytes (PMNs).

• Polymorphonuclear Leukocytes (PMNs): Also known as Neutrophils or "Polys."

• Auxiliary Cells: Include platelets, mast cells, and basophils.

Role of Mast Cells and Basophils in Innate Immunity

• Phagocytic Status: Usually not phagocytic.

• Location: They congregate around blood vessels.

• Stimulation: Triggered by cytokines and the complement cascade upon detecting foreign material or pathogens.

• Granule Release: They release granules containing histamine.     * Histamine Functions: Acts as a vasodilator.     * Vascular Effects: Causes blood vessels to become "leaky."

• Cellular Recruitment: Histamine-induced leakiness facilitates the movement of PMNs and monocytes out of the bloodstream to the site of infection. This process involves:     * Transmigration     * Diapedesis     * Extravasation

Natural Killer (NK) Cells

• Primary Function: Kill infected human host cells. This releases sequestered bacteria into the extracellular environment where phagocytes can access them.

• Target Pathogens: Critical for controlling viral infections and bacterial pathogens that "hide out" inside human cells.

• Production: Produced in the bone marrow, similar to phagocytes.

• Mechanism of Action:     * Toxic Compounds: NK cells store toxic compounds in internal granules, released upon binding to a target cell.     * Perforins: Induce holes in the host cell membrane.     * Granzymes: Injected into the host cell to induce apoptosis (programmed cell death).

Cell Navigation and Recruitment Signals

• Complement System: Activated by contact with invading bacteria or antibodies. Components like $C3a$ and $C5a$ serve as recruitment signals.

• Cytokines:     * Structure: Proteins or glycoproteins weighing between $8 \text{ to } 30†kDa$.     * Role: Mediate inflammatory responses to antigens and tissue damage; activate specific immune system cells.

• Chemokines:     * Structure: Subgroup of cytokines; small glycopeptides weighing between $8 \text{ to } 10†kDa$.     * Role: Specifically guide phagocytes to infection sites; orchestrate the immune response.

• Producers: Cytokines and chemokines are produced by monocytes, macrophages, endothelial cells, PMNs, DCs, T cells, B cells, mast cells, and fibroblasts.

Comprehensive Cytokine and Chemokine Activity Table

• GM-CSF: Produced by Th cells. Targets progenitor cells. Function: Growth and differentiation of monocytes and DCs.

• IL-1a: Produced by monocytes. Targets Th cells. Function: Costimulation.

• IL-1b: Produced by macrophages, B cells, and DCs. Targets B cells, NK cells, and others. Function: Maturation, proliferation, activation, inflammation, acute-phase response, and fever.

• IL-2: Produced by Th1 cells. Targets activated T/B cells and NK cells. Function: Growth, proliferation, and activation.

• IL-3: Produced by Th and NK cells. Targets stem cells and mast cells. Function: Growth, differentiation, and histamine release.

• IL-4: Produced by Th2 cells. Targets activated B cells, Macrophages (polarizing to M2 to dampen immune response), and T cells. Function: Proliferation, differentiation, $IgG1$ and $IgE$ synthesis, and MHC-II expression.

• IL-5: Produced by Th2 cells. Targets activated B cells. Function: Proliferation, differentiation, and $IgA$ synthesis.

• IL-6: Produced by monocytes, macrophages, Th2 cells, and stromal cells. Targets activated B cells, plasma cells, and stem cells. Function: Differentiation, antibody secretion, and acute phase response.

• IL-7: Produced by marrow and thymus stroma. Targets stem cells. Function: Differentiation into progenitor B and T cells.

• IL-8: Produced by macrophages and endothelial cells. Targets neutrophils. Function: Chemotaxis.

• IL-10: Produced by Th2 cells. Targets macrophages and B cells. Function: Inhibits cytokine production; activation.

• IL-12: Produced by macrophages and B cells. Targets Tc cells and NK cells. Function: Differentiation into CTLs (with IL-2) and activation.

• IL-13: Produced by Kupffer cells, lung macrophages, and kidney epithelial cells. Targets macrophages. Function: Inhibits inflammatory cytokine production.

• IFN-alpha: Produced by leukocytes. Function: Inhibits viral replication and increases MHC-I expression.

• IFN-beta: Produced by fibroblasts. Function: Inhibits viral replication and increases MHC-I expression.

• IFN-gamma: Produced by Th1, Tc, and NK cells. Targets various cells. Function: Inhibits viral replication, activates pathogen elimination, and induces Ig class switch to $IgG2a$.

• MIP-1a: Produced by macrophages. Function: Chemotaxis for monocytes and T cells.

• MIP-1b: Produced by lymphocytes. Function: Chemotaxis for monocytes and T cells.

• TGF-beta: Produced by T cells and monocytes. Function: Chemotaxis, $IL-1$ synthesis, $IgA$ synthesis, and inhibition of proliferation.

• TNF-alpha: Produced by macrophages, mast cells, and NK cells. Function: Expression of ICAM, cytokine expression, cell death, and acute phase response.

• TNF-beta: Produced by Th1 and Tc cells. Function: Phagocytosis and $NO$ production.

Pathogen Recognition: PAMPs and TLRs

• TLRs (Toll-like Receptors):     * Distribution: Found on phagocytes and endothelial cells.     * Conservation: Evolutionarily conserved in vertebrates and invertebrates.     * Quantity: 13 mammalian TLRs total; 10 identified in humans.

• TIR Domain: A conserved domain in TLRs that transduces signals to cytoplasmic adapters.

• Signal Adapters: Relay signals from the TLR to the nucleus. Examples include MyD88, IRAKs, TAK1, TAB1, TAB2, TRAF6, TIRAP, TRAM, and Trif.

• Outcomes of Signaling: Activation of cytokine production and PMN functions, such as the oxidative burst. This cascade ultimately triggers PMN apoptosis.

• Lipopolysaccharide (LPS) Mechanism:     1. LPS (the PAMP) interacts with LBP (LPS-binding protein).     2. CD14 transmits the signal through TLR4 and MD2.     3. Internal signaling involves MyD88, IRAK, TRAF6, and NF-KB.     4. Result: Production of inflammatory cytokines and Type 1 IFN.

Intracellular Pathogen Sensing (NLRs)

• NOD-like Receptors (NLRs): High-sensitivity sensors for intracellular pathogens.     * NOD1: Detects Gram-negative bacterial products.     * NOD2: Detects Gram-positive bacterial products.

• Inflammasomes: Complexes formed when NLRs sense PAMPs and oligomerize.

• Caspase 1: Activation occurs via inflammasomes through cleavage of pro-caspase 1.

• Pyroptosis: A specific type of programmed cell death caused by NLR activation.

• Cytokine Production: Caspase 1 signaling also leads to proinflammatory cytokines that stimulate local macrophages and NK cells.

Phagocyte Killing Mechanisms

• The Phagosome: An endocytic vesicle characterized by an ATPase pump in its membrane and an internal pH of approximately $5.0$.

• Lysosomal Granules: Contain antibacterial compounds in an inactive state.

• Phagolysosome: Formed when the phagosome and lysosome fuse. Low pH activates the antibacterial compounds.

• Degradative Enzymes: Proteases and lysozyme that destroy bacterial surface components.

• Defensins: Antimicrobial compounds that create physical pores in bacterial membranes.

• Oxidative vs. Non-Oxidative Killing: Two primary distinct pathways.

The Oxidative Burst and Reactive Oxygen Species (ROS)

• Myeloperoxidase (MPO):     * Location: Primarily found in the granulosomes of Neutrophils.     * Activation: Occurs only upon contact with NADPH oxidase (located in the phagosomal membrane) and when pH drops.     * Color: MPO is green, contributing to the green color of pus and the buffy coat of blood.

• Chemical Reactions in Oxidative Burst:     * $NADPH + 2O_2 \rightarrow 2O_2^- + NADP^+ + H^+$     * Superoxide ($O_2^-$): Highly reactive; oxidizes sulfur groups in cysteine and methionine, creating disulfide bonds that inactivate proteins.     * Fenton’s Reaction: Interaction of $H_2O_2$ with $Fe^{2+}$ leading to nucleic acid damage via hydroxyl radicals.     * Toxic Ions: MPO catalyzes the reaction of $H_2O_2$ with halides ($Cl^-$, $Br^-$, $I^-$) or thiocyanate ($SCN^-$) to produce ions like hypochlorite ($HOCl$), which are toxic to proteins.

• Nitric Oxide (NO): Produced by phagocytes and epithelial cells. Can combine with superoxide to form Peroxynitrite ($ONOO^-$), a highly reactive oxidant.

The Complement System

• Definition: A set of soluble proteins produced by the liver that circulate in an inactive state.

• Activation: Occurs through proteolytic cleavage (e.g., $C3 \rightarrow C3a + C3b$).

• Three Activation Pathways:     1. Collectin Mediated Pathway: Mannose-binding lectins (produced by the liver in the acute phase response) bind to bacterial mannose surfaces, inducing clumping.     2. Classical Pathway: Mediated by antibodies of the specific defense system binding to bacteria. Links innate and specific immunity.     3. Alternative Pathway: Triggered directly by bacterial molecules such as LPS or Teichoic Acid (TA). Involves $C3b$ binding directly to the bacterial surface.

• Functional Components of Complement:     * C3a and C5a: Stimulate mast cells to release histamine; guide cells to infection sites.     * C3b: Acts as an Opsonin to facilitate ingestion by phagocytes.     * C5b: Triggers the synthesis of the Membrane Attack Complex (MAC).

• Pathology: Extreme or systemic activation of these systems can lead to septic shock.