Le Complément

Dr Alessandra RosenthalLaboratoire Central d’Immunologie

(Prof. A. Bernard)CHU de Nice

Système Immunitaire

Immunité naturelle Immunité acquise

Composants cellulaires :

• Cellules phagocytaires (neutro, mono, macrophages)• Cellules relarguant des médiateurs (baso, eosino, mast

cells)• Cellules NK

Composants moléculaires :

• Complément• Protéines de la phase aïgue• Cytokines (IFN etc)

Immunité Naturelle

Voie Initiateurs

Classique Immuno-complexesCellules apoptotiquesCertains virus et bacteries Gram-CRP liée au ligand

des lectines Microorganismes avec groupementsmannoses terminaux

Alterne Bactéries, champignons, virus,cellules tumorales

Les trois voies d’activation du Complément

Walport NEJM 2001

Activation du Complément

Assembly of C1 during the Classical Complement Pathway

The Fab of IgG or IgM bind to epitopes on an antigen. C1q, C1r, and C1s then assembles on the Fc portion of the antibodies to form C1, the first enzyme of the classical complement pathway. The enzyme C1 is able to cleave C4 into C4a and C4b, as well as C2 into C2a and C2b.

Formation of C3 Convertase during the Classical Complement Pathway

The enzyme C1 is able to cleave C4 into C4a and C4b. The C4b binds to adjacent proteins and carbohydrates on the surface of the antigen. C2 then binds to the C4b and C1 cleaves C2 into C2a and C2b. The C4a2b functions as a C3 convertase that can subsequently cleave hundreds of molecules of C3 and C3b.

Formation of C5 Convertase during the Classical Complement Pathway

The C4a2b functions as a C3 convertase that can subsequently cleave hundreds of molecules of C3 into C3a and C3b. Much of the C3b binds to adjacent proteins and carbohydrates on the antigen to participate in opsonization while C3a can stimulate inflammatory responses. Some of the C3b binds to C4b2a to form C4b2a3b, a C5 convertase that can cleave C5 into C5a and C5b.

The Membrane Attack Complex (MAC) Causing Cell Lysis

This C5b6789 membrane attack complex (MAC), puts pores into lipid bilayer membranes of human cells to which antibodies have bound. This results in cell lysis. MAC can also damage the envelope of enveloped viruses and put pores in the outer membrane and cytoplasmic membrane of gram-negative bacteria causing their lysis

Activation of the alternative complement pathway

and formation of C3 convertase

Activation of the alternative complement pathway begins when C3b (or C3i) binds to the cell wall and other surface components of microbes. Alternative pathway protein Factor B then combines with the cell-bound C3b to form C3bB. Factor D then splits the bound Factor B into Bb and Ba, forming C3bBb. A serum protein called properdin then binds to the Bb to form C3bBbP that functions as a C3 convertase capable of enzymatically splitting hundreds of molecules of C3 into C3a and C3b.

                                          

Some of the C3b subsequently binds to some of the C3bBb to form C3bBb3b, a C5 convertase capable of enzymatically splitting hundreds of molecules of C5 into C5a and C5b

Formation of C5 convertase during the alternative complement pathway    

Régulation du clivage of C3 par le Factor H et Factor I

Walport NEJM 2001

Les trois principales activités physiologiques du système complémentaire

Activité Protéines responsables

Défense contre les infections :Opsonisation C3 et C4Chimiotactisme et activation Anaphylotoxines (C5a, C3a et C4a)Lyse des bactéries et des cellules MAC (C5b-C9)

Interface entre immunité naturelles et spécifique :Augmentation de la réponse Ab C3b et C4b liés aux CI; C3R sur B et APCPotentialisation de la mémoire Imm. C3b et C4b liés aux CI; C3R sur FDC

Élimination des cellules endommagées:Clearance des CI dans les tissus C1q; fragments de C3 et C4Clearance des cellules apoptotiques C1q; fragments de C3 et C4

The Fab portion of IgG binds to epitopes of an antigen. The Fc portion can now attach the antigen to Fc receptors on phagocytes for enhanced attachment. This is especially important against encapsulated microbes. C3b and C4b from the complement pathways can also attach antigens to phagocytes. Once attached to the phagocyte by way of IgG, C3b, or C4b, the microbe can be engulfed more efficiently and placed in a phagosome. As lysosomes fuse with the phagosome the microbe is digested.

Opsonization of a Bacterium

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Extracellular Killing After Opsonization

The Fab portions of IgG or C3b bind to epitopes on human cells recognized as foreign, such as virus-infected cells, tumor cells, or cells involved in autoimmune reactions. This enables phagocytes to attach to the cell by way of their Fc and C3b receptors. The phagocytes now destroy the cell by discharging their lysosomal contents

              

                    

Chemotactict effect of C5a and C3b 

Most C3b binds to antigens on the microbial surface. Some C3b combines with C2a and C4b to form the third enzyme of the complement pathway that is able to split C5 into C5a and C5b. C5a stimulates mast cells to release histamine for inflammation and diapedesis. It also functions as a chemoattractant for phagocytes. The phagocytes are then able to bind to the C3b attached to the surface of the microorganism allowing for opsonization (enhanced attachement)

Récepteurs du complément

Nom Ligand(s) Distribution Fonction

CR1 (CD35)  iC3b, C4b    B, phagocytes, GR, CFD       élimination des CI liés aux GR                                                     opsonisation                                                                         activation B

CR2 (CD21) iC3b, C3dg     B, cellules epitheliales         activation Bréc. EBV

                                                                                                                    

CR3 (CD11b/CD18) iC3b, ICAM-1      Phagocytes, NK CFD             Opsonine, lectine,                                                      adhesion molecule

CR4 (CD11c/CD18) iC3b   Phagocytes          molécule d’adhésionopsonisation

Effets cliniques des principaux déficits héréditaires du Complément

Déficit Conséquences sur l’act. du C’ Associations cliniques

C3 incapacité d’opsonisation Infections à pyogènesincapacité à activer le MAC GN membrano-proliférative

C3Properdine incapacité à former le MAC Infections à NeisseriaMAC

C1q, C1r, incapacité à activer la voie LESC4, C2 classique

Anomalie Clinique Ethiopathogénèse

C3 nef - Glomérulonéphrite Auto-Ab qui stabilise le C3bBb

membrano-proliférative

Déficit en facteur H, I - Glomérulonéphrite Défaut de régulation de l’activation membrano-proliférative de C3, avec inflammation chronique - Syndr. Hémolyt. Urém. dans le glomérule et déficit secondaire en C

Déficit en C1 inhibiteur - Oedème angioneurotique Défaut de la sérine-protéase qui inactive le C1 et le système de

kinines-bradikinines

Hémoglobinurie paroxystique - Hémolyse intravasculaire Déficit de régulation du MAC par nocturne défaut d ’expression des molécules GPI (CD55 et CD59)

Pathologie du complément - anomalies de régulation