Complement Flashcards
What are the three complement pathways?
What activates each one?
Classical- activated by antigen:antibody complexes (antibody need to be either soluble or in a complex with surface).
Lectin- activated by lectin binding to pathogen surfaces (patterns of carbohydrates on fungi and bacteria).
Alternative- is active all the time, but allowed to proceed by pathogen surfaces.
These all activate complement.
What happens once complement is activated?
Recruitment of inflammatory and immunocompetent cells.
Opsonisation of pathogens (red flag recognised by phagocytes.
Killing of pathogens.
What is the first molecule in the classical pathway?
C1q- molecule with globular heads that actively bind to antigen:antibody complexes. It is always complexed with the enzymes C1r and C1s.
Describe the classical complement pathway.
C1s is the first enzyme involved. Activated C1s cleaves C4 in circulation, to C4a and C4b. C4a floats off, C4b bind to the same microbial surface as C1q.
C1s then cleaves C2 into C2a and C2b. C2b floats away, C2a binds with C4b to form the C4b2a complex.
This complex is an active C3 convertase (conservative to all three pathways). It cleaves C3 into C3a and C3b. C3a floats off, and C3b bind to the microbial surface or the convertase.
The complex is a cascade- one molecule of C4b2a can cleave up to 1000 molecules of C3 to C3b. Many C3b molecules bind to the surface.
Describe the native components, active forms and functions of the proteins of the classical pathway?
Native: C1 (C1q:C1s2:C1r2).
Active: C1q- binds directly to pathogen surfaces or indirectly to antibody bound to pathogens, thus allowing autoactivation of C1r.
C1r- cleaves C1s to active protease.
C1s- cleaves C4 and C2.
Native: C4
Active: C4b- covalently binds to pathogen and opsonises it. Binds C2 for cleavage by C1s.
C4a- Peptide mediator of inflammation (weak activity).
Native: C2
Active: C2a- active enzyme of classical pathway C3/C5 converatse: cleaves C3 and C5.
C2b- precursor of vasoactive C2 kinin.
Native: C3
Active: C3b- many molecules of C3b bind to pathogen surfaces and act as opsonins (one of the main ones). Initiates amplification via the alternative pathway.
Binds C5 for cleavage by C2b.
C3a- Peptide mediator of inflamamtion (intermediate activity).
What is the first molecule in the classical pathway?
C1q- molecule with globular heads that actively bind to antigen:antibody complexes. It is always complexed with the enzymes C1r and C1s.
The globular heads/arms are flexible and can move independently, so they can bind to a range of antibody surfaces.
What is mannose binding lectin (MBL) and what does it do?
Molecule with two to six clusters of carbohydrate-recognition domains. Within each of the clusters, the carbohydrate-binding sites have a fixed orientation. Molecule is not as flexible as C1q.
MBL binds with high affinity to mannose and fucose residues with the correct spacing. Mannose and fucose residues that don’t have the correct spacing are not bound by MBL- needs to see mannose and fucose resiues in a particular orientation.
MBL is a soluble pattern recognition molecule that deals with a specific set of pathogens due to the specific binding motif.
What is mannose binding lectin (MBL) and what does it do?
Molecule with two to six clusters of carbohydrate-recognition domains. Within each of the clusters, the carbohydrate-binding sites have a fixed orientation. Molecule is not as flexible as C1q.
MBL binds with high affinity to mannose and fucose residues with the correct spacing. Mannose and fucose residues that don’t have the correct spacing are not bound by MBL- needs to see mannose and fucose resiues in a particular orientation.
MBL is a soluble pattern recognition molecule that deals with a specific set of pathogens due to the specific binding motif.
MBLs and ficolins are related.
The actiivated pathway after recognition is the same as the classical pathway.
How is the alternative pathway different?
Does “tick over”.
Pathway is active all the time. C3 is always activated in solution for a very short time. Bond (C3(H2O)) is activated in aqueous environment. If activated bond doesn’t interact with permissive surface within milliseconds of activation, it is deactivated.
What are the native components, active forms and functions of the proteins of the alternative pathway?
Native: C3
Active: C3b- Binds to pathogen surfaces, binds B for cleavage by D, C3bBb is C3 convertase and C3b2Bb is C5 convertase.
Native: Factor B (B)
Active: Ba- small fragment of B, unknown function.
Bb- active enzyme of C3 convertase and C5 convertase.
Native: Factor D (D)
Active: D- plasma serine protease, cleaves B when it is bound to C3b to Ba and Bb.
Native: Properdin (P)
Active: P- plasma protein that stabilises the C3bBb convertase on bacterial cells.
Describe the alternative pathway.
C3 undergoes spontaneous hydrolysis to C3(H2O), which binds to factor B allowing it to be cleaved by factor D into Ba and Bb.
The C3(H2O)Bb complex is a C3 convertase, cleaving more C3 into C3a and C3b. C3b is rapidly inactivated unless it binds to cell surface.
Factor B binds non-covalently to C3b on a cell surface and is cleaved to Bb by factor D.
There are different outcomes on host cells and pathogens.
Describe the alternative pathway.
C3 undergoes spontaneous hydrolysis to C3(H2O), which binds to factor B allowing it to be cleaved by factor D into Ba and Bb.
The C3(H2O)Bb complex is a C3 convertase, cleaving more C3 into C3a and C3b. C3b is rapidly inactivated unless it binds to cell surface.
Factor B binds non-covalently to C3b on a cell surface and is cleaved to Bb by factor D.
There are different outcomes on host cells and pathogens.
Pathway doesn’t have initiator like the others.
Alternative will bind to lots of different things, including host cells. Will destroy anything if not controlled.
What happens to the alternative pathway on host cells?
On host cells, complement-regulatory proteins CR1, H, MCP, and DAF bind to C3b. CR1, H, DAF displace Bb.
C3b bound to H, CR1, and MCP is cleaved by factor I to yield inactive C3b (iC3b).
No activation of complement on host cell surfaces.
What happens to the alternative pathway on pathogens?
Pathogens lack complementary-regulatory proteins. Binding of properdin (factor P) stabilises the C3bBb complex.
(Most pathogens don’t have control molecules, but some bacteria etc. have acquired genes to make control proteins, so can evade complement.)
C3bBb complex is a C3 convertase and deposits many molecules of C3b on the pathogen surface.
Opsonisation, activation of terminal complement components.
How does the alternative pathway amplify the other pathways?
Amplification is one of the most important jobs of alternative.
Pathways can work together to build up deposition.
C3b deposited by classical or lectin pathway C3 convertase.
C3b binds factor B.
Bound factor B is cleaved by plasma protease factor D into Ba and Bb.
C3bBb complex is a C3 convertase, cleaving many C3 molecules to C3a and C3b.
