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MICR 304 Immunology & Serology

MICR 304 Immunology & Serology. Lecture 3 Complement Chapter 2.11-2.22; 9.19, 9.20 Primary Literature. Overview of Today’s Lecture. What is Complement? Complement activation Results of complement activation Control of complement Ancestral complement systems. Key Players in Immunity.

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MICR 304 Immunology & Serology

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  1. MICR 304 Immunology & Serology Lecture 3 Complement Chapter 2.11-2.22; 9.19, 9.20 Primary Literature

  2. Overview of Today’s Lecture • What is Complement? • Complement activation • Results of complement activation • Control of complement • Ancestral complement systems

  3. Key Players in Immunity

  4. Key Players in Immunology

  5. General Description of Complement • System of plasma proteins • Activates a cascade of proteolytic reactions and subsequent protein aggregation on the microbial surface but not on host cell surface • Coat microbes with a substance that is bound by phagocytes • Form pores on microbial surfaces triggering killing • Release small peptides that contribute to inflammation

  6. Complement Is a Complex System • Consists of > 20 proteins • Includes active proteins (C proteins), stabilizers and inhibitors, and receptors • Some components are zymogens • Inactive precursors (CX) active local acting serine proteases (CXb) distinct acting bioactive (CXa) molecule • Some are structural proteins • Form a pore

  7. Functional Protein Classes in the Complement System

  8. Distribution of Complement Factors in the Body • Widely distributed in body • Many cells can synthesis complement factors • Major producers: • Monocytes/macrophages • Fibroblasts • Hepatocytes

  9. Complement System Important for Innate Immunity Killing Inflammation Opsonization

  10. Inflammation • Tumor (swelling) • Rubor (reddening) • Calor (heat) • Dolor (pain)

  11. Phagocytosis

  12. Killing via Pore Formation Proc Natl Acad Sci U S A. 1985 July; 82(14): 4808–4812.

  13. A Closer Look at the Complement System

  14. Classical Pathway of Complement Activation C3 Convertase

  15. Amplification loop

  16. Lectin Pathway of Complement Activation C3 Convertase

  17. Compare: Lectin Pathway of Complement Activation • Lectin: a protein that binds sugars • MBL • Collagen-like tails + lectin • Binds to mannose • In host, mannose residues are covered by e.g. sialic acid • Ficolin • Collagen like domain + fibrinogen like domain • Binds to N-acetylglucosamine • Higher concentrated in plasma than MBL

  18. Alternative Pathway of Complement Activation _________

  19. The Amplification Loop of the Alternative Pathway

  20. Complement Activation Summarized • Classical pathway • C1  C4  C2  C3 • Lectin pathway • MBL or ficolin  C4  C2  C3 • Alternative pathway • C3(H2O)  B cleaved by D  C3

  21. Activation of Complement Factor C5 • C3b initiates C5 cleavage • Classical and lectin pathway: C4b2a3b • Alternative pathway: C3b2Bb

  22. Terminal Complement Components Protein Aggregation

  23. Complement Pore Formation(C5b – C9) Last steps of complement activation are protein aggregation C8 and C9(10-16) insert into the membrane

  24. The Complement Pore

  25. Complement Mediated Killing • In vitro well documented, better against gram-negative bacteria (serum sensitive strains) • In vivo only Neisseria infections increased with deficiencies in C5-9

  26. , C4a

  27. Concept Map Enzymatic activation Factor D C5b-9 Pore C1q C3b C1r, C1 s Protein aggregation

  28. C2a

  29. Selected Complement Receptors

  30. Local Inflammatory Responses Induced by Complement C5a > C3a > C4a • Increased permeability • Fluid leakage • Transmigration of leukocytes • Phagocyte attraction and activation

  31. Enhanced Phagocytosis • Opsonization via C3b and CR1 • Activation of phagocytes via C5a required

  32. Activated Complement can Cause Anaphylactic Shock • C5a, C3a are anaphylaxins • Action on blood vessels • Activation of mast cells that release histamine • Massive increase in vascular permeability • Severe drop in blood pressure • Airway constriction • Swelling of epiglottis

  33. Complement Control Proteins • To protect host cells • To prevent excessive activation

  34. Selected Complement Regulators

  35. Ancestral Complement Systems Phagocytic cell inhibitory

  36. Today’s Take Home Message • Complement is a complex system of a controlled proteolytic cascade (C1-C5b) and subsequent protein aggregation (C6-C9n). • Activation is triggered by at least 3 major pathways (classical, lectin, alternative) but the outcome is identical. • Complement has wide reaching effects that result in inflammation (C5a, C3b, C4a: action on endothelial cells and recruitment of defense cells), enhanced phagocytosis (C3b: microbial uptake), and direct killing (C5b – C9n) through pore formation.

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