Complement System Notes

LE SYSTEME DU COMPLEMENT

• Introduction
• Composition/nomenclature
• Activation
• Récepteurs cellulaires
• Rôle biologique
• Méthodes d'études
• Variations du complément

INTRODUCTION

• Discovered in the early 20th century (Ehrlich /Dausset)
• Substances that complement the action of antibodies =
  << complément »>
• About thirty soluble and membrane proteins
• 5% of total serum proteins
• Synthesized by the liver
• Part of the humoral immune system, but there are cellular receptors for the complement on:
  • Macrophages
  • Polynuclears
  • Lymphocytes
  • Red blood cells

ROLE

• Important role in:
  • Defense of the organism against infectious agents: bacteria, viruses, parasites
  • Inflammatory reaction
  • Metabolism of immune complexes
  • Cellular interactions
  • Pathological processes leading to cell or tissue damage
• Used as a laboratory reagent

ACTIVATION

• Three pathways:
  • Classical pathway
  • Alternative pathway
  • Lectin pathway
• All lead to the formation of a complex that lyses the cell membrane.

COMPOSITION / NOMENCLATURE

• Classical pathway: 11 proteins
  • C1q, C1r, C1s, C4, C2, C3, C5, C6, C7, C8, C9
• Alternative pathway:
  • Factors B, D, P
• Lectin pathway:
  • MBL (Mannose Binding Lectin)
  • MASP1 and MASP2 (MBL-Associated Serine protease)
• Regulatory proteins:
  • C1-inh, C4-bp, Factor I, Factor H, DAF, HRF, MCP…

ACTIVATION PROCESS

• Complement components activate in a chain reaction after cleavage:
  • A fragment capable of binding to cell membranes
  • A small fragment that is not bound but has a defined biological activity

NOMENCLATURE

• Native compounds: C1 to C9, B, D, P
• Cleavage fragments: lowercase letters, e.g., C3a or C3b
• "i" indicates an inactivated protein, e.g., C3bi
• A bar above indicates an activated protein, e.g., C3bBb

ACTIVATION PROCESS DETAILS

• Native components have no biological effects.
• A stimulus is needed to trigger activation.
• C3 is the central element of the system.

ACTIVATION PATHWAYS

• Classical Pathway
• Lectin Pathway
• Alternative Pathway
• All converge on C3, then C5-C9 leading to lysis

C3 ACTIVATION

• C3 is activated to C3b, leading to the formation of a membrane attack complex (C5-C9).

CLASSIC PATHWAY ACTIVATORS

• Antigen-antibody complexes (IgM, IgG)
• Aggregated Immunoglobulins
• Certain viruses
• Certain proteins:
  • Plasmin
  • Thrombin
  • C-reactive protein

CLASSICAL PATHWAY ACTIVATION

• C1 is the first molecule activated.
• Macromolecular complex including C1q and a tetramer (C1r)2+(C1s)2
• C1q is the recognition unit.
• C1r and C1s are serine proteases.

ALTERNATIVE PATHWAY ACTIVATORS

• Cell surfaces
• Gram + or - bacteria
• Infected cells/viruses
• Yeasts
• Parasites
• Polysaccharides
• Bacterial endotoxins
• Various substances
  • Gluten, dust
  • Radiological contrast products
  • Immune complexes (IgA)

LECTIN PATHWAY ACTIVATORS

• Macromolecular complex of 3 proteins
  • MBL
  • MASP1
  • MASP2
• Homologous to the C1 complex
• MBL is the recognition protein, binds to carbohydrates of microorganisms.
• Has serine esterase activity and cleaves C2, C4.

C3 LYSE & C5 CONVERTASE FORMATION

• Classical/Lectin pathway: (C4b2a) converts C3 to C3b.
• Alternative pathway: (C3bBb) converts C3 to C3b.
• Both generate C5 convertase.
• C5 convertase cleaves C5 into C5a (anaphylatoxin) and C5b.

TERMINAL PATHWAY

• The C5b-9 complex forms a transmembrane hydrophilic tunnel, causing osmotic lysis of cells, bacteria, and viruses.
• Important in defense against Neisseria bacteria.

ACTIVATION REGULATION

• Intrinsic: lability of activated components
• Extrinsic: action of regulatory proteins
  • In solution (e.g., C1 inhibitor)
  • On membranes (e.g., CR1 receptor)

CELLULAR RECEPTORS

• Regulate the complement system.
• Interpret messages that the complement sends to cells.
  • Mast cell receptors (release vasoactive amines)
  • Macrophage receptors (phagocytosis)
  • Polynucleotide receptors (chemotaxis)
  • Red blood cell receptors (transport immune complexes to macrophages)
  • B lymphocyte receptors (differentiation, proliferation…)

RECEPTORS FOR ACTIVATION FRAGMENTS

OPSONIZATION OF ANTIGEN

• Complement activation and IgG opsonize bacteria, facilitating phagocytosis by phagocytes via CR1 and Fc receptors.

ELIMINATION OF IMMUNE COMPLEXES

• Soluble immune complexes activate complement.
• Erythrocytes transport immune complexes, facilitating phagocytosis.

CAPTURE OF ANTIGEN BY FOLLICULAR DENDRITIC CELLS

• Capture is mediated by CR3.

BIOLOGICAL ROLE

• First line of defense against infectious agents, in the absence of antibodies.
• Effects:
  • Lysis of foreign cells
  • Lysis of bacteria
  • Activation and chemotaxis of neutrophils
  • Opsonization and phagocytosis of bacteria
  • Degranulation of mast cells
  • Increased vascular permeability
  • Contraction of smooth muscles

METHODS OF STUDY

• Dosage of hemolytic activity of the complement
  • CH50 = search for hemolytic activity of a serum against sheep red blood cells sensitized by an antibody
• Dosage of complement components (C3, C4, B…)
  • Immunochemical methods
  • Hemolytic methods
• Search for complement degradation products
  • C3a, C4a, C5a
• Complement deposits (tissues or cells)

VARIATIONS IN COMPLEMENT

• Increase due to increased synthesis
  • Inflammation
  • Cancer
• Decrease
  • Synthesis defect
  • Excessive consumption
  • Loss
  • Genetic deficits

PATHOLOGICAL CONSEQUENCES

• Deficiencies in complement proteins:
  • Classical pathway proteins (C1q, C1r/C1s, C4, C2): lupus
  • C1 inhibitor deficiency: angioneurotic edema
  • Alternative pathway proteins: recurrent bacterial infections, Neisseria meningitis (factor P)
• Diseases with excessive complement consumption
  • Lupus
  • Vasculitis
  • Cryoglobulinemia
  • Post-infectious glomerulonephritis