Complement Flashcards
Outline what complement is and 3 main functions
Around 30 different proteins that are soluble or membrane associated. Some components are cleaved and form products with enzymatic activity. Mainly made in the liver as inactive zymogens. Components are named C1, C2, factor B, and when cleaved are usually C2a, C2b, factor Ba, factor Bb etc. The b fragment is usually larger
Functions: i) opsonisation ii) lysis of target cells iii) inflammation. Also participates in the clearance of immune complexes and apoptotic cells and stimulates adaptive immune responses
a) C3 and why it’s important
b) How is C3b recognised
a) Can attach to pathogens and mark them for phagocytosis (opsonisation). Has an unusual internal thioester bond that can react with hydroxyl or amino groups. When cleaved, the larger fragment C3b binds to the pathogen and the smaller fragment C3a stimulates inflammation (anaphylotoxin)
b) Recognised by complement receptors (eg CR1, CR3) on macrophages. CR3 and CR1 are very important for dealing with C3b tagged bacteria. CR1 binds to C3b, but macrophage needs an additional signal (C5a) to induce phagocytosis . (see image). CR3 doesn’t need an additional signal.
Diagram showing 3 major pathways of complement activation
(see image)
The alternative pathway
First to act when there is no previous knowledge of the pathogen. Spontaneous conformational change in C3 exposes the thioester group. This reacts with water to generate C3H₂O. Factor B bings to C3H₂O, which opens up factor B. Factor D then cleaves factor B into factor Ba (leaves) and factor Bb (still bound), creating C3H₂OBb (the fluid phase C3 convertase). When another C3 binds to this, it is cleaved into C3a (leaves) and C3b. Factor B then binds to this C3b, and factor D again cleaves factor B, resulting in C3bBb attached to the pathogen (the alternative pathway C3 convertase). There is a feedback loop so more C3b are generated, marking the pathogen for opsonisation.
Formation of the membrane attack complex
The cleaved C3b may remain attached to the C3bBb formed in the alternative pathway, which forms C3b₂Bb - the alternative pathway C5 convertase. C5 binds and is cleaved to C5a and C5b. C5b binds to C6 and C7, which attaches to the pathogen surface. This complex binds transmembrane C8 and many C9, forming a pore through the pathogen surface. This is especially effective against gram negative bacteria and enveloped viruses
The lectin pathway
Activated by Mannose Binding Lectin and Ficolins. MBL binds mannose and fucose residues (on pathogen, which is different from mannose and fucose on host cells). MBL is a soluble PRR.
C4 binds to MBL and is cleaved to C4a (leaves) and C4b (remains bound to MBL). C2 then binds to C4b and is cleaved into C2b (leaves) and C2a (remains bound to C4b) - different as C2a is the larger fragment!. C4bC2a is the classical C3 convertase. C3b generated can then use alternative pathway to amplify.
The classical pathway
Activated by binding antibody or C-reactive protein. C1q binds to the Fc region of multiple antibodies present on the pathogen surface (or C1q binds to CRP). C4 binds and is cleaved to C4a (leaves) and C4b (remains bound to MBL). C2 then binds to C4b and is cleaved into C2b (leaves) and C2a (remains bound to C4b) - different as C2a is the larger fragment!. C4bC2a is the classical C3 convertase. C3b generated can then use alternative pathway to amplify.
a) Positive regulation of complement activity
b) Negative regulation of complement activity
a) Properdin (factor P) - only known positive regulatory factor. Stabilises C3bBb (alternative pathway C3 convertase), which otherwise has a very short half life
b) Factor I - a constitutively active serine protease that degrades C3b and C4b (otherwise could bind to host cell surface). Requires cofactors MCP and factor H to work
Membrane cofactor protein (MCP) - protects host surfaces from complement. MCP binds to C3bBb and causes Bb to dissociate. Factor I then cleaves C3b so it is no longer functional
Factor H - is very important as it’s the main factor regulating complement acitvation in solution (which MCP cannot do). It also helps protect host cells from complement attack. Factor H binds host sialic acid and C3bBb, Bb dissociates. Factor I then cleaves C3b.
Decay accelerating factor (DAF) - present on host cells. Binds to C3bBb causing Bb to dissociate (no factor I)
Protectin - a host cell surface proteun that prevents MAC formation. Prevents C8 from inserting into the membrane and prevents C9 from binding to C8.
Complement deficiencies
a) C1, C2, C4
b) C3
c) C5-9
d) Factor D, Properdin
e) Factor I, Factor H
f) DAF, Protectin
a) immune-complex disease, infection
b) infection with encapsulated bacteria (pyogenic bacteria)
c) susceptibility to Neisseria infections
d) susceptibility to pyogenic bacteria, especially Neisseria infections
e) uncontrolled alternative pathway activation resulting in C3 depletion. Susceptibility to infection with pyogenic bacteria
f) autoimmune-like conditions
Complement evasion strategies (+ example of Staphylococcus aureus
Many pathogens evolved evasion mechanisms that target the complement pathway. These include: interference of antibody-complement interaction, binding and inactivation of a complement component, destruction of complement components by protease, mimicry of inhibitory regulators or recruitment of host inhibitors
Staph. aureus (gram positive). Cell wall is protected by a polysaccharide capsule to prevent opsonisation. If C3 convertases do assemble on bacteria surface, they are bound and inactivated by Staphylococcus Complement Inhibitor (SCIN), preventing the generation of opsonins. It also secretes proteins that bind, degrade and inactivate C3. Staph. proteinA binds Fc region of IgG, blocking complement recruitment and activation. Clumping factor A recruits factor I to the bacterial surface to inactivate complement. Chemotaxis inhibitory protein blocks the C5a chemotactic receptor on neutrophils, limiting their recruitment to infection sites.
