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