Complement Flashcards
1
Q
What does the complement system do?
A
- Activation involves a cascade of cleavage enzymes
- Amplication = each activated protease can generate multiple activated proteases in the following step
- Products of complement proteolysis covalently bond to microbial surfaces (or Abs bound to microbes/ other Ags)
- Stabilisation = complement proteins are inactive when in fluid form, then stably activate when on microbes
- Regulatory proteins inhibit complement activation on healthy host cells, pathogens dont have these = specificity
2
Q
What are the 5 main complement roles?
A
- To eliminate microbes
- To eliminate apoptotic cells/ debris (so dont cause inflammation)
- To promote clearance of immune complexes
- To promote B cell activation
- Also involved in disease pathogenesis - excess complement causes disease
3
Q
How can complement kill microbes?
A
- Opsonisaion of pathogens (facilitates phagocytosis)
- Inflammation - recruitment and activation of neutrophils and monocytes
- Lysis of microbes (MAC)
4
Q
What are the 3 complement activation pathways?
A
- Classical pathway - C1 interacts with IgM/G bound to microbes to be activated (adaptive)
- Alternative pathway - triggered by direct recognition of microbial structures (innate)
- Lectin pathway - Mannose binding lectin regonises terminal mannose residues on microbes (innate); ficolin recognises residues on microbes (innate)
- All have different triggers for initiation, but all generate C3 convertase (C3 to C3a/b), which can generate C5 convertase.
5
Q
What happens in the alternative pathway to make C3 convertase?
A
- Spontaneous cleavage of C3 into C3b and C3a
- C3 cleaves causes a conformational change in C3b, exposing a thioester bond, which can covalently bind to microbial surface (otherwise it is hydrolysed) = stabilisation
- C3b then binds factor B, which is then cleaved by factor D - leaving Bb (big) and Ba (small)
- This C3bBb complex is the alternative pathway C3 convertase - which cleaves more C3 molecules = activation
6
Q
How is C5 convertase made in alternative pathway?
A
- C3 convertase will cleave more C3
- These new C3b molecules bind to the microbial surface to survive
- Some C3b molecules will bind to the C3 convertase
- This generates C5 convertase (C3bBbC3b)
- C5 convertase cleaves C5 and initiates the late steps of complement activation
7
Q
What is the late stage of the alternative pathway?
A
- C5 convertase recruits and cleaves C5 -> C5a and C5b
- C5a is soluble - has biological activities
- C5b remains bound to C5 convertase
- This complex recruits C6,7,8 - builds a complex
- C8 binds into the membrane of the molecule
- This then recruits C9, which forms a pore into the microbial surface, allowing water and ions to enter the microbe and causes lysis after swelling
- C5-9 = Membrane attack complex
8
Q
What initiates the classical pathway?
A
- C1 binds to antigen-bound IgG/M
- C1 is multimeric - C1a, C1r and C1s
- C1q binds to the Abs (6 globular heads connected to central stalk)
- C1r/s are proteases - cleave other complement components (2x each, forms tetramer)
- C1 can only bind to Ab molecules if they are bound to a microbe
9
Q
How does the C1 bind to the Fc regions of IgM/G?
A
- C1q globular heads must bind to at least 2 Fc regions in order to activate pathway
- Globular heads can accomodate for different angles as are flexible
- All the heads are in the same plane, but changes conformation, bending its heads to bind to the Ag, exposing the domain that can interact with C1q
10
Q
How is C3 convertase made in classical pathway?
A
- Binding of 2 or more globular heads of C1q to Ag-bound abs -> activates C1r
- C1r cleaves and activates C1s
- C1s cleaves C4-> C4b/a
- C4b binds covalently to microbe surface to survive
- this recruits C2, which is cleaved by C1s into C2a (larger) and C2b (smaller)
- This is now the C3 convertase = C4bC2a complex
11
Q
How does the classical pathway generate C5 convertase?
A
- C3 covertase cleaves more C3
- New C3b deposits on microbial surface
- Some C3b binds to C3 convertase complex -> C5 convertase (C4b2a3b)
- C5 convertase then cleaves C5 and initiates late steps
12
Q
What is the late phase of classical pathway?
A
- Identical
- C5 convertase cleaves C5
- C5b binds to C5 convertase
- This C5b recruits C6,7,8. 8 can then bind to the surface and with C9, form the MAC -> lysis
13
Q
How does the lectin pathway activate complement?
A
- Doesnt need Ab presence - triggered by microbial carbohydrate recognition by PRRs
- PRRs are mannose binding lectin and ficolin - have similar structure to C1q
- Binding of MBL to mannose/ ficolin to N-acetylglucosamine residues -> activates MASP1 and 2
- MASPS leave C4 to generate C4b and then C2
- Then proceeds identically to classical
14
Q
C1 vs MBL and ficolin
A
- Have globular heads with collagen stems
- Also contain proteases called MASP (similar to C1r/s)
- Mannose residues are much more common on microbial surfaces than our cells, and are arranged differently, allowing MBL to distinguish between them
- same for ficolin
15
Q
What are the 6 ways that cleavage of C3 and C5 can help us?
A
- Microbe lysis via MAC
- C3b will bind to old RBCs and allow their degradation
- C3b,C4b - opsonisation on microbe promoting phagocytosis
- C3a,C4a,C5a are important in activating phagocytes and mast cells in inflammation
- C5a is involved in chemotaxis and increased permeability of enodethlium - help microbe destruction by leukocytes and ROS
- B cell activation
