Complement Flashcards
What is the complement system? (2)
Made up of plasma proteins that react with one another to eliminate pathogens and induce a series of inflammatory responses that help to fight infection
Inactivated, circulating components are activated by what? (1)
The action of specific proteases
How many components are there and where are they produced? (2)
Approximately 30
Produced by the liver
How does the complement cascade amplify as it is activated? (2)
Proteases cleave and further activate other components of the pathway resulting in activation of the complement cascade, that amplifies as it is activated
What are the 3 pathways of complement activation? (3)
Classical
Lectin
Alternative
What is the classical pathway of complement activation and describe it? (1)
Commonly generated antibody mediated
Antibody binds to the antigen
C1 binds to Fc regions of these antibodies
C1r and C1s are then recruited
C1q activates C1r and C1r activates C1s
This becomes a protease
C1s cleaves C4 and C2 to produce the C3 convertase C4bC2a and anaphylotoxin (C3a)
What is the lectin pathway of complement activation? (4)
Requires carbohydrate recognition by complement proteins
Initiated by mannose binding lectin, a serum protein, binds to mannose-containing carbohydrate on bacteria or viruses in the cell wall
Forms a complex with two protease zymogens, MASP-1 and MASP-2, activating MASP-1 and MASP-2
This cleaves C4 and C2 to produce the C3 convertase C4bC2a and
anaphylatoxin
What is the alternative pathway of complement activation? (1)
Produces the C3 convertase C3bBb
C3 spontaneously makes C3b
C3b + Factor B = C3bB
C3bB + factor D = C3bBb this is the C3 convertase
C3 is continuously activated in serum however unless it meets a surface to form a thioester bond with, it is spontaneously hydrolysed
What do all 3 complement activation pathways result in the formation of? (1)
Results in the formation of C3 convertase
Which enzyme does classical/lectin pathway generate? (1)
C2aC4b
Which enzyme does alternative pathway generate? (1)
C3bBb
What are the outcomes of complement activation? (3)
Inflammation
Phagocytosis
Membrane attack - pathogen lysis
What is the most abundant complement protein in plasma? (1)
C3
How is C3 cleaved? (3)
Cleaved by C3 convertase into C3a and C3b
What are the two forms of C3 convertase? (2)
C2aC4b
C3bBb
How is C5 cleaved? (2)
By C5 convertase into C5a and C5b
What is C5 convertase made from? (1)
C3 convertase
What are the two forms of C5 convertase? (2)
C2aC4bC3b
C3bBbC3b
How does C3b initiate membrane attack complement system? (4)
Has activated thioester carbonyl inside it
Once this reaches a hydroxyl group on surface of microorganism, it binds to it and forms covalent ester linkage
This linkage starts initiating interaction with other complement proteins to target the microorganism
What does C1s cleave and produce? (3)
Cleaves C4 and C2 to produce the C3 convertase C4bC2a and anaphylatoxin
Cleaves C4 to produce C4b that binds to pathogen surface, and C4a a weak inducer of inflammation
Cleaves C2 to produce C2a that binds to pathogen surface, and C2b an inactive small fragment
What is C4b2a? (2)
A C3 convertase that produces C3b and C3a mediator of inflammation
Why is the complement cascade enzymatic? (2)
Each enzyme can cleave many molecules of its substrate
Hence, hundreds of MACs are deposited into the lipid bilayer membrane of cellular antigens and therefore, kill target efficiently
What is the membrane attack complex (MAC)? (2)
Forms pores in lipid bilayer of microorganisms
Kills gram-negative bacteria, enveloped viruses and some protozoan viruses
Why can MAC not kill gram positive bacteria? (1)
Gram positive bacteria protect themselves from complement mediated lysis by their thick peptidoglycan walls
Why can MAC not kill fungi? (1)
Fungi protect themselves from complement mediated lysis by their thick chitin walls
What are the non-lytic effects of complement? (3)
C3a and C5a - these are anaphylatoxins which activate mast cells to degranulate
C3a and C5a - these are also chemotaxins and attract phagocytes
C3b - this is an opsonin which enables phagocytes to recognise and bind to antigens and phagocytose them
How does C3b enhance phagocytosis? (2)
C3b receptor on the surface recognises C3b bound to the microbe
Able to take this up and phagocytose it
How does factor I and factor H regulate complement? (3)
Factor I is a highly specific proteinase which is concerned with the regulation of the C3 convertase (C3bBb) of the alternative pathway
With the help of co-factor H; factor I splits the alpha chain of C3b into C3c and C3d
This destroys the C3 convertase activity
How do DAF and MCP regulate complement? (4)
Found on surface of normal human cells
Both bind strongly bind to C3b and remove Bb
Therefore C3 convertase cannot be produced
MCP subsequently inactivated C3b by recruiting factor I
How does factor P regulate complement? (3)
Able to make complex with C3 and C5 convertase
Protects it from enzymes that inactivate it
Stabilised C3 convertase on surface and allows amplification step
What are the functional outputs of complement? (2)
Opsonisation
Signal augmentation and cross-talk
What is opsonisation? (3)
C3b is deposited on bacteria
Recognised by complement receptors on surface of phagocytes, allowing phagocytosis
Enhanced by C5a
What is signal augmentation and cross-talk? (3)
Factor I cleaves C3b to produce C3d that is deposited on surface of antigen
C3d remains coated to antigen following cleavage by factor I
slg recognises antigen and the co-receptor CR2 binds C3d
This dual recognition/signal increases B-cell sensitivity to an antigen by 1000-1000
What does a genetic deficiency of factor I do? (3)
Continually use up the C3 available in the serum through spontaneous unchecked C3bBb activity
What can deficiencies in immune complex clearance lead to? (2)
Systemic lupus
Glomerulonephritis
How do inflammatory thrombotic conditions occur? (1)
Uncontrolled C5a in an antiphospholipid syndorme
What does failure of regulation of complement lead to? (1)
Atypical Haemmolytic Uremic Syndrome (aHUS)
What is aHUS? (4)
Failure to regulate complement:
Mutations in factor H, I, CD46, thrombomodulin, C3
Chronic activation of complement
Vascular injury and inflammation
Thrombotic microangiopathy - platelets and leukocytes form aggregates in small blood vessels
What are the functions of complement? (7)
Lysis via MAC (destroys foes)
Chemotactic (summons helps, escalates response)
Bind antibody/lectins
Tick-over mechanisms (surveillance)
Binds apoptotic cells/immune complexes (clears up)
Opsonises (marks foes)
Enhances response of B cells and granularocytes
What makes the membrane attack complex? (3)
C4b2a3B is a C5 convertase that produces C5b and C5a mediator of inflammation
C5b joins with C6, C7, C8 and then recruit C9 which makes MAC.
How does mannose binding protein work? (3)
Bind to mannose in cell
mannose recruited
C4 and C2 cleaved to produce C2aC4b C3 convertase
Cut C3 to add C3 to C5 convertase
MAC
How are C5a and C3a controlled - anaphylatoxin Inactivator (1)
Carboxypeptidase N cleaves C5a and C3a into inactive forms
How is MAC regulated? (3)
C5b, C6, C7, C8 make a complex and connect to C9
CD59 (found on red blood cells) bind to that complex so it can’t interact with C9 and stop membrane attack complex from happening
What are the manifestations of Systemic Lupus Erythamatosus (SLE)? (6)
Skin rashes
Chronic fatigue
Arthritis
More severe:
- glomerulonephritis
- serositis
- neurological involvement
What is Systemic Lupus Erythamatosus (SLE)? (4)
Autoantibodies directed against DNA and histones induce complement activation and a general Fc gamma receptor mediated immune reaction
Autoantibodies form immune complexes that deposit in the kidney glomeruli and activate the classical complement pathway causing tissue damage
What is Systemic Lupus Erythamatosus (SLE) in simpler terms? (1)
Immune complexes made and immune system unable to clear it out
What is Paroxysmal Nocturnal
Haemoglobinuria (PNH)? (4)
Failure of regulation of complement:
Don’t have a particular type of signalling complex
Any protein that uses that signalling complex is not expressed
Cd55 and Cd59 are not expressed which are normally expressed on all of our cells
Cannot control C3 convertase and cannot inhibit insertion of C9 into red blood cell membrane
Complement Inhibitor (Eculizumab) (3)
Has a human constant region and VH and VL domain are derived from mice antibody
Won’t be recognised by human immune system and portion at end will bind the target
Can’t initiate membrane attack complex
