ComplementMaterial in this document is given for internal

use only and exclusively for teaching purposes

Jules Bordet

� Discovery in 1890s due to the “complement activity” of a heat-labile component of normal plasma thataugments the opsonization and killing of bacteria by antibodies.

� Nobel Prize was awarded in 1919 for his discoveriesrelating to immunity, particularly complement-mediatedlysis

� Complement originally evolved as part of innate immune system providing protection early in infection

Definition� Composed of more than 30 different plasma proteins

produced mainly by the liver.

� In absence of infection, proteins circulate in inactiveform: proteases of the complement are synthesized asinactive pro-enzymes, the zymogens.

� Zymogens become enzymatically active only afterproteolytic cleavage, usually by another complementprotein, triggering a proteolytic cascade.

� Final effector complement involved in the removal of the pathogen

� Small number of pathogen produces a rapid responsegreatly amplified at each step.

Nomenclature� Three pathways to activate: classical,

alternative, lectin

� First proteins discovered for the classical pathways: C1, C2, …

� Named in the order of discoveryrather than sequence of reactions

� Cleavage of fragments (C3) producesa small fragment called a (C3a) and a bigger fragment called b (C3b) exceptfor C2 because the larger fragmentC2a was enzymatically active and thisname has survived

Nomenclature� Another exception to the rule:

C1q, C1r and C1s which are notcleavage products but distinctproteins together comprising C1

� Proteins of alternative pathwaywere discovered later and designed by capital letters, for example factor B and D

� Cleavage products also calledBb the larger fragment and Bathe small fragment.

anaphylatoxins

Pathways of complement activation

anaphylatoxin anaphylatoxin

Three main functions:� Opsonization and phagocytosis� Stimulation of inflammation� Complement-mediated cytolysis

CRP, SAP, PTX3

MBL, ficolins

The complement system recognizes features of microbial surfaces and marks them for destruction by

the deposition of C3b

Complement is a system of soluble pattern-recognition receptors and effector molecules that

detect and destroy microorganisms.

Stimulation of inflammation

Opsonization and phagocytosis Cytolysis

Anaphylatoxins: C3a, C4a, C5a

Complementpathways

The alternative pathwaycan be activated by

spontaneous cleavageof C3

� 2 different ways of activation:� Tickover of C3 (cleavage of

1-2% total plasma C3/hour)� C3b produced from lectin or

classical pathway

� Alternative pathway can actas an amplification loop to increase C3b production

The alternative pathway

C3a

Anaphylatoxins:

The alternative pathway can be activated by spontaneous cleavage of C3

C3 convertase: C3bBb

Properdin stabilizes the alternative pathway C3

convertase on pathogen surfaces

Properdin-deficient patients are particularlysusceptible to infections with Neisseria meningitidis

The alternative pathway

C3a

C5a

Anaphylatoxins:

Complementpathways

Mannose-binding lectin (MBL) and Ficolins form complexes with serine proteases and recognize particular carbohydrates on

microbial surfaces

MASP: MBL-associatedserine protease

MBLproduced in liver and circulate in the blood

Lectin pathway triggered by 4 different pattern recognition molecules in blood and extracellular fluids

Human ficolins:ü M-ficolin (1)

secreted by activatedmacrophagesin tissues

ü L-ficolin (2) synhesized by liver and circulate in the blood

ü H-ficolin (3)synhesized by liver and circulate in the blood

MASP-3 MASP-3

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

Complement activation

Only antibodies bound to antigens can initiate

complement activation

CH2

CH3

The classicalpathway

C2 is cleaved in small solublefragment and larger fragment

For historical reasons: large fragmentis named C2a and is a serine protease, active enzyme of C3 convertase

C4a

C3a

C5a

CRP, SAP, PTX3 can also bind to C1q

Anaphylatoxins:

C4a: anaphylatoxin of the classical and lectinpathways

MAC formation

Pathways leading to potent inflammatory and destructive effects must be tightly regulated

� One important safeguard is that key activatedcomponents are rapidly inactivated unless they bindpathogen surface

� Complement can be activated by dying cells (sites of ischemic injury, apoptosis). Complement coating helpsphagocytes to dispose of the dead and dying cellslimiting 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

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

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

DAF: Decay Accelerating Factor / C4BP: C4 Binding Protein / CR1: Complement Receptor 1/ MCP: Membrane Cofactor Protein

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

DAF: Decay Accelerating FactorCR1: Complement Receptor 1MCP: Membrane Cofactor ProteinFactor HFactor I = Protease I

C3bBb

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

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

Protectina =CD59

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

Complementcomponents

C4bC4a

Ingestion of complement-tagged pathogens by phagocytes ismediated by receptors for the bound complement proteins

Integrins:

The cleavage products of C3b are recognized by different complement receptors

iC3b: CR2, CR3, CR4C3dg, C3d: CR2

The anaphylatoxin C5a can enhance the phagocytosis of microorganisms opsonized in

innate immune response

Local inflammatory responses can be induced by small complement fragments

C5a > C3a > C4a

Functions of complement

� 4.Clearance of immune complexes

� 5. B cell activationthrough binding of fragments to CR2 (co-receptor),

� 6. selection of high-affinity B cells withingerminal centers by exposure of Ag throughCR1, CR2, and CR3

Close evolutionary relationship

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

Deficit in proteine regolatorie del complemento causa malattie

patologia

Angioedema ereditario (HAE)

HAE = hereditary angioedema

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 1:50.000 caratterizzata da angioedemi ricorrenti

Edema angioneurotico ereditario o angioedema ereditario

Trattamento con C1 INH ricombinante

Deficit in proteine regolatorie del complemento causa malattie

patologia

Emoglobinuria parossistica notturna

CD55

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

CD55 = DAFCD59 = protectin

Deficit di DAF (CD55) e CD59: Emoglobinuria parossistica notturna

(PNH: Paroxysmal NocturnalHemoglobinuria)

CD55 = DAFCD59 = protectina

Deficit in proteine regolatorie del complemento causa malattie

patologia

Sindrome uremica emolitica

(HUS, Hemolytic-UremicSyndrome )

Condizione caratterizzata da:- un danno alle piastrine e globuli rossi- glomerulonefrite provocata da inadeguata eliminazione degli immunocomplessi che tendono a depositarsi a livello renale

12

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