Innate Immunity

1. Innate Immunity

2. Innate Immunity • Mechanisms that do not depend on prior exposure to the pathogen • Have evolved over time to protect against groups of organisms that have distinctive molecular features • Some elements have also been co-opted for use in the specific acquired immune response

3. Elements of Innate Immunity • External Barriers • Phagocytic cells • Complement • Humoral mechanisms • Extracellular Killing

4. External Barriers • Skin • Very effective barrier to infection – cuts, burns etc, infection is major complication • Mucus and mucosal surfaces • Mucus can trap and move foreign things along • Some secretions have bacteriocidal components • Normal bacterial flora can play a protective fole • Environment (e.g: vaginal pH) • Colicins

5. Phagocytic Cells • Polymorphonuclear Leukocytes (a.k.a PMNs or “Polys” • Macrophages (lit: “big eater” Gr.)

6. Phagocytic Cells • Polymorphonuclear neutrophils: PMNs • White cells (leukocytes), multi-lobed nuclei (look like many nuclei in section) • Neutrophil: does not take up the major dyes • Short-lived; lots of glycogen (energy from glycolysis even if O2 is not available) • Macrophages • Mononuclear cells • Long lived, significant RER • Do not circulate, but hang out in lymph nodes and other interesting places

7. Recognition by Phagocytic cells • Phagocytes have evolved a system of receptors that can recognize molecular patterns on the surface of pathogen that are • Conserved • Shared by many infections agents • Different from “self”-patterns • Examples: • The lipopolysaccharide of certain bactreia • Yeast cell wall mannans • Mycobacterial glycolipids • But not glactose and sialic acid group that are typical the ends of mammalian surface polysaccharides

8. Phagocytosis • Engagement of receptors generates internal signal to start phagocytosis • An actin—myosin contractile sytsem for putting pseudopods around the particle • Lysosomes discharge their contents into the phagosome • Several biochemical pathways ensue to degrade content

9. Complement • A system of 20 or so different proteins that are “set off” in a cascade of events. • Cascade: one molecule, when activated, catalyzes a change in the next, makes it active, so that it catalyzes a change in the next etc. Such protein cascades allow a profound response to a trigger. • Complement proteins modify each other, and the fragments may have other effects as well

10. Complement has 2 pathways: the classical and the “alternative” • The classical pathway was figured out first, but may be more recent in evolution, because it depends acquired immune response (antibodies) • So we will talk about that later! • The “alternative” pathway is activated by microbial polysaccharides, and is part of innate immunity

11. Complement: Act I, Scene 1Complement activation by bacterial cell walls • C3 is routinely cleaved at slow rates to C3b • Factor B is cleaved by a plasma enzyme, Factor D to Bb • C3b and Bb bind together to form an enzyme, C3bBb, or C3 convertase. • This convertase very actively cleaves C3 to C3a and C3b • C3b is the central molecule – sets off complement-mediated killing ….. so how is this prevented from running amok in the uninfected state? • A protein H binds to C3bBb, and then another protein I binds to the C3bBbH complex, inhibiting it. However ….

12. Complement: Act I, Scene 2If certain bacteria are present, the cascade is not inhibited. • Some mico-organisms can stabilize the convertase C3bBb, so that it does not bind Factor H • If the convertase does not bind H, it also does not bind I, so it is not inhibited, so…. • The reaction does get amplified, and much more • C3b is made

13. Complement: Act II • The C3b can for a short time, react covalently with local hydroxyl or amino groups at the microbial cell surface • C3b bound to the surface makes the cell “tasty to macrophages” – the surface is “opsonized” • In the presence of extra C3b, the C3bBb changes its enzymatic specificity such that it now cleave C5 into C5a and C5b • C5b stays bound to C3b at the surface of the invading cell. • Seeing C5b bound, C6, C7, C8 and C9 bind, and form a membrane pore (called the Membrane Attack Complex) • The cell lyses

14. Complement: Act III • The smaller fragments C3a and C5a stimulate • Phagocytosis • Mast cell degranulation • Histamine (causes vasodialation, capillary permeability, bronchioconstriction) • Proteases and other degradative enzymes • Chemotactic factors (recruit phagocytes) • Interleukins further activation of macrophage activity , and other things = basic inflammatory response

15. Overview --complement alternative pathway

16. Humoral Mechanisms provide a second defensive strategy • The term “humoral” means soluble in serum of other fluids; non involving cells • Antibodies are humoral defenses (but specific acquired immunity, so discussed later) • Among the innate humoral immune defenses: • Complement • Clotting Factors (cascade, fibrin and fibinogen) • Lysozyme (in many secretions, attacks bacterial cell wall) • Acute phase proteins • Intereferons

17. Complement

18. Clotting Cascade

19. Acute Phase Proteins • Concentration increases in response to alarm proteins, such as IL-1 • IL-1 is released by macrophages when they are activated (by bacterial endotoxins, foreign molecular patterns, complement etc) • Increase in C-Reactive protein, and mannan binding proteins (as much as 1000-fold). • These bind to bacteria, and activate the classical pathway of complement (take the place of the C1 proteins that are normally activated by Ab) • C3b opsonizes the foreign cells

20. Extracellular Killing • NK (Natural Killer) cells kill virally- infected cells using perforin • Induce apoptosis • Apoptosis = Programmed cell death • A series of reactions, cascade of proteolytic enzymes (capases); the end result is rapid degradation of the nucleus, and cutting DNA in to nucleosome size pieces