The Complement System and the Integration of Innate and Acquired Immunity

1. The Complement System and the Integration of Innate and Acquired Immunity David J. Kusner, M.D., Ph.D.

2. The Integrated Immune Response • Objectives • 1. To understand the mechanisms by which the distinctive characteristics of innate and acquired immunity are integrated into a diverse and multifunctional immune system • 2. To understand the complement system as the primary soluble component of innate immunity and characterize its interaction with phagocytes and the adaptive immune system.

3. The Diversity of Microbial Challenges • Classes of Microbes: Viruses, Bacteria, Fungi, Parasites (Protozoa, Helminths) • Size - 20 nm (Parvoviruses) • 25 meters (Tapeworms) • Composition - Protein (Prions), Nucleic acid + protein (Viruses) ® Eukaryotes

4. The Diversity of Microbial Challenges • Presence/Absence of reservoirs/vectors –Tuberculosis, Malaria, Lyme disease, • Mode of Transmission: aerosol (Measles), blood and body fluids (HIV) • Extracellular / Intracellular Pathogenesis: Staph. aureus, M. tuberculosis

5. The Diversity of Microbial Challenges • Rapidity of disease onset: Meningococcal meningitis, Prion-induced neurodegenerative disease • Severity of disease: Localized vs. Systemic inflammation

6. Epithelial barrier defenses • Epithelial barriers • - Skin and mucous membranes (GI, Respiratory, Urogenital tracts) • - Physical barrier: inhibit establishment of infection, block spread through tissues • - Clearance mechanisms – mucus, cilia

7. Epithelial barrier defenses • - Secretions • Acid, enzymes – stomach • Antimicrobial peptides – Defensins • - Normal (non-pathogenic) flora • Competition for nutrients • Production of antimicrobial compounds • - Epithelial turnover and cell sloughing

8. Disruption of epithelial defenses • - Extremely common feature of infection. • - Provides microbes with access to tissues, blood and lymphatic vessels ® potential for systemic spread of pathogen/ products – wounds, burns • - The consequences of disruption of epithelial barrier defenses can often be prevented or minimized.

9. Infection of an Epithelial Surface and the Integrated Immune Response

10. The Complement System • A group of > 30 plasma proteins which comprise the primary soluble component of innate immunity. • Rapidly activated in response to infection, without induction or recall of adaptive immunity.

11. The Complement System • However, in the presence of an adaptive immune response, complement proteins interact with both its soluble and cellular components (antibodies, lymphocytes, activated macrophages, dendritic cells). • Complement proteins circulate in plasma as inactive precursors.

12. The Complement System • Infection results in activation of complement proteins via a series of proteolytic reactions that yield biologically active fragments. • These coupled proteolytic reactions result in an amplification cascade, in which limited stimulation of proximal complement components results in massive activation of distal complement proteins.

13. Complement System: Overview

16. Complement proteins opsonize microbes and promote their phagocytosis.

17. Complement receptors on the surface of leukocytes bind complement proteins • Phagocytic receptors: CR1, CR3, CR4 - present on neutrophils, monocytes, macrophages, dendritic cells. • - Bind C3b / C3bi that are attached to microbial surfaces (as opsonins) ® phagocytosis

18. Complement receptors on the surface of leukocytes bind complement proteins • CR2: surface of B cells and dendritic cells. Binding of activated complement fragments to CR2 amplifies antigen-induced cellular activation - provides a link between the innate and acquired immune systems.

20. The Complement System kills microbes via the Membrane Attack Complex (MAC).

21. Activation of the Complement System • 2 activation pathways are components of innate immunity : Alternative pathway, Lectin pathway • The Classical pathway of complement activation is part of the adaptive immune response - dependent on Abs.

23. Activation of the Complement System • All 3 activation pathways result in generation of an enzyme complex which can cleave C3, called the C3convertase. • C3 ® C3a + C3b • - Integration point of the complement system • (Innate and Adaptive immunity) • - Major amplification point

24. Activation of the Classical Pathway • Requires Antigen - Antibody complexes • Complement component C1 binds to the Fc region of the Ab (IgM- most potent activator) • C1 is a protease that cleaves C2 and C4 to form the C3 convertase of the classical path.

26. Activation of the Alternative Pathway • Soluble proteins of the Alternative Pathway (B,D,P) bind to repetitive structures on microbial surfaces, such as components of the cell wall. • The complex of B, D and P forms the C3 convertase of the Alternative Pathway. • Recognition is selective for microbes, but not highly specific (pattern recognition).

27. Activation of the Lectin Pathway • The soluble plasma protein, Mannan-Binding Protein (MBP or MBL) binds to the sugar mannan which is restricted to the surface of certain microbes (not on vertebrate cells) • This leads to attachment of other complement proteins (C4,C2) to form the C3 convertase of the Lectin Pathway.

29. Regulation of Complement System • Due to its potential for rapid amplification, and the ability of activated complement proteins to damage host tissue as well as microbes, strict regulation is needed.

30. Soluble Inhibitors of Complement • C1 inhibitor (C1 INH, C1 esterase inhibitor) - binds and inactivates C1 ® inhibition of Classical Pathway. • Lack of C1 INH (hereditary angioneurotic edema) - spontaneous activation of complement and kinin systems, inflammation of the skin and critical organs. • - Symptoms are completely reversed by • administration of C1 INH.

31. Cellular Inhibitors of Complement • Decay accelerating factor (DAF) and CD59 are proteins present on mammalian cells, inhibit the assembly of the MAC on host cells. • Deficiencies of DAF and CD59 occur in the disease Paroxysmal Nocturnal Hemoglobinuria (PNH) - intermittent hemolysis of RBCs due to unregulated deposition of activated complement components on the cell surface.

32. Complement Deficiencies • Congenital: Deficiencies of C7, C8, C9 - “terminal complement components”. Markedly increased incidence of Neisserial infections. • - N. meningitidis: meningitis, bacteremia • - N. gonorrhoeae: STDs, bacteremia • Acquired: consumption of complement proteins during extensive activation of the complement system.

33. Complement System - Summary • Plasma proteins + Complement Receptors on leukocytes + Regulatory proteins • Integral to innate immunity, also functions with the adaptive immune system. • Amplification cascade that proceeds via coupled proteolytic reactions.

34. Complement System - Summary • Functions: • Recruit inflammatory cells: C3a, C5a • Opsonizes pathogens: C3b, C3bi • Directly microbicidal: MAC

35. General Biomedical Principles • In addition to its intrinsic biochemical beauty and physiologic importance, the complement system illustrates several principles which are fundamental to the biomedical sciences. • 1. Amplification - coagulation, fibrinolytic, kinin systems • 2. Multiple levels of compensatory regulation • 3. Receptor - ligand interactions determine specificity • 4. Inflammation- both beneficial / deleterious

36. Cytokines • Cytokines are essential soluble mediators of both the innate and adaptive immune systems. In fact, cytokines are one of the major integrators which bridge these complementary systems. • 1. recruitment of leukocytes (chemokines) • 2. activation of phagocytes ® augment innate immunity • 3. absolutely required for the maturation and activation of lymphocytes

37. Activated Macrophages Secrete Cytokines that Stimulate both Innate and Acquired Immunity

38. Cytokines Exhibit Both • Immunoprotective and Tissue- • Damaging Effects

39. Integrators of Immunity • Components which are responsible for the interdependence of innate and acquired immunity • 1. Complement system • 2. Cytokines/chemokines • 3. Macrophages • 4. Dendritic cells • 5. Antibodies

40. The Integrated Immune Response