Complement. Material in this document is given for internal use only and exclusively for teaching purposes

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1 Complement Material in this document is given for internal use only and exclusively for teaching purposes

2 Jules Bordet Discovery in 1890s due the complement activity of a heat-labile component of normal plasma that augments the opsonization and killing of bacteria by antibodies. Nobel Prize was awarded in 1919 for his discoveries relating to immunity, particularly complement-mediated lysis Complement originally evolved as part of innate immune system providing protection early in infection

3 Definition Composed of more than 30 different plasma proteins produced mainly by the liver. In absence of infection, proteins circulate in inactive form: proteases of the complement are synthesized as inactive pro-enzymes, the zymogens. Zymogens become enzymatically active only after proteolytic cleavage, usually by another complement protein, triggering a proteolytic cascade. Final effector complement involved in the removal of the pathogen Small number of pathogen produces a rapid response greatly amplified at each step.

4 Nomenclature Three pathways to activate: classical, alternative, lectin First proteins discovered for the classical pathways: C1, C2, Named in the order of discovery rather than sequence of reactions Cleavage of fragments (C3) produces a small fragment called a (C3a) and a bigger fragment called b (C3b) except for C2 because the larger fragment C2a was enzymatically active and this name has survived

5 Nomenclature Another exception to the rule: C1q, C1r and C1s which are not cleavage products but distinct proteins together comprising C1 Proteins of alternative pathway were discovered later and designed by capital letters, for example factor B and D anaphylatoxins Cleavage products also called Bb the larger fragment and Ba the small fragment.

6 The complement system recognizes features of microbial surfaces and marks them for destruction by the deposition of C3b

7 Complement is a system of soluble patternrecognition receptors and effector molecules that detect and destroy microorganisms. anaphylatoxins

8 Pathways of complement activation anaphylatoxin anaphylatoxin Three main functions: Opsonization and phagocytosis Stimulation of inflammation Complement-mediated cytolysis

9 Complement pathways

10 The alternative pathway can be activated by spontaneous cleavage of C3 2 different ways of activation: Tick-over of C3 C3b produced from lectin or classical pathway Alternative pathway can act as an amplification loop to increase C3b production

11 The alternative pathway can be activated by spontaneous cleavage of C3

12 Properdin stabilizes the alternative pathway C3 convertase on pathogen surfaces Properdin-deficient patients are particularly susceptible to infections with Neisseria meningitidis

13 The alternative pathway

14 Complement pathways

15 Mannose-binding lectin (MBL) and Ficolins form complexes with serine proteases and recognize particular carbohydrates on microbial surfaces Lectin pathway triggered by 4 different pattern recognition molecules in blood and extracellular fluids MBL produced in liver and circulate in the blood MASP: MBL-associated serine protease Human ficolins: ü M-ficolin (1) synthesized and secreted by lung and blood cells ü L-ficolin (2) synhesized by liver and circulate in the blood ü H-ficolin (3) synhesized by liver and circulate in the blood

16 The actions of C3 convertase result in the binding of large numbers of C3b molecules to the pathogen surface Individuals deficient with MBL or MASP-2 experience substantially more respiratory infections by common extracellular bacteria during early childhood

17 Complement activation

18 Only antibodies bound to antigens can initiate complement activation C H 3 C H 2

19 The classical pathway C4a C2 is cleaved in small soluble fragment and larger fragment For historical reasons: large fragment that takes part of the active enzyme C3 convertase is called C2a C3a C5a

20 C4a: anaphylatoxin of the classical and lectin pathways

21 MAC formation

22 Complement system (classical pathway)

23 Pathways leading to potent inflammatory and destructive effects must be tightly regulated One important safeguard is that key activated components are rapidly inactivated unless they bind pathogen surface Complement can be activated by dying cells (sites of ischemic injury, apoptosis). Complement coating helps phagocytes to dispose of the dead and dying cells limiting the risk of cell contents being released and triggering an autoimmune response Presence of regulatory proteins to prevent activation of complement on the surfaces of healthy host cells

24 Complement activation is regulated by a series of proteins that serve to protect host cells from accidental damage

25 Complement activation is regulated by a series of proteins that serve to protect host cells from accidental damage

26 Complement activation is regulated by a series of proteins that serve to protect host cells from accidental damage

27 Complement activation is regulated by a series of proteins that serve to protect host cells from accidental damage

28 Complement activation spares host cells which are protected by complement regulatory proteins

29 Complement components

30 Ingestion of complement-tagged pathogens by phagocytes is mediated by receptors for the bound complement proteins

31 The cleavage products of C3b are recognized by different complement receptors ic3b: CR2, CR3, CR4 C3dg: CR2

32 The anaphylatoxin C5a can enhance the phagocytosis of microorganisms opsonized in innate immune response

33 Local inflammatory responses can be induced by small complement fragments C5a > C3a > C4a

34 Functions of complement

35 Close evolutionary relationship

36 Difetti delle proteine di controllo del complemento sono associati con diverse malattie

37 Deficit in proteine regolatorie del complemento causa malattie patologia Angioedema ereditario (HAE) HAE = hereditary angioedema

38 Edema angioneurotico ereditario o angioedema ereditario C1-INH si lega anche a MASP2 attivata - L origine dell Angioedema Ereditario è quindi un difetto genetico, responsabile di una insufficiente quantità o di un deficit funzionale del C1 esterasi-inibitore (C1-INH) - Malattia autosomica dominante, incidenza 1: caratterizzata da angioedemi ricorrenti

39 Edema angioneurotico ereditario o angioedema ereditario

40 Deficit in proteine regolatorie del complemento causa malattie patologia Emoglobinuria parossistica notturna

41 Deficit di DAF e CD59: Emoglobinuria parossistica notturna (PNH: Paroxysmal Nocturnal Hemoglobinuria) Anemia emolitica accompagnata dall emissione ricorrente di urine scure (eliminazione di emoglobina per via renale), trombosi venosa

42 Deficit di DAF e CD59: Emoglobinuria parossistica notturna (PNH: Paroxysmal Nocturnal Hemoglobinuria)

43 Deficit in proteine regolatorie del complemento causa malattie patologia Sindrome uremica emolitica (HUS, Hemolytic-Uremic Syndrome ) Condizione caratterizzata da: - un danno alle piastrine e globuli rossi - glomerulonefrite provocata da inadeguata eliminazione degli immunocomplessi che tendono a depositarsi a livello renale

44 I difetti dei componenti del complemento determinano una deficienza nella funzione dell immunità umorale 12

IMMUNOLOGY. Done By: Banan Dabousi. Dr. Hassan Abu Al-Ragheb

IMMUNOLOGY. Done By: Banan Dabousi. Dr. Hassan Abu Al-Ragheb IMMUNOLOGY Done By: # 6 Dr. Hassan Abu Al-Ragheb Immunity lecture #6 #Functions of complement system: 1-opsonin. 2-lysis. 3- Production of inflammation. 4-chemotaxis. 5-clearance of immune complexes. 6-