Greg Lecture 1 Flashcards
Define The Complement System
A biological cascade made up of multiple serum proteins that is part of the innate immune system (however does link to the adaptive immune system).
Where is complement located?
Within the b globulins in serum (-5% of globulin composition) and elevates during immune response
What are the functions of complement?
- Formation of MAC leads to bacteria lysis
- Increased vascular permeability and phagocyte recruiting
- Opsonisation
- Assists in disposal of apoptotic cells/immune complexes
What problems can complement activation cause?
Inappropriate activation causes unwanted inflammation (immune response)
What synthesises complement proteins?
hepatocytes, monocytes, macrophages and epithelial cells of the gastrointestinal and genitourinary tracts
How are complement proteins activated?
By proteolytic cleavage; they are acute phase proteins (zymogens) whose plasma concentrations increase in response to inflammation
Explain complement nomenclature
Numbered C1 - C9
Factor B, D, H and P (involved in alternative pathway)
After cleavage, divided into an and b components (e.g. C3 => C3a (smaller fragment) and C3b (larger, membrane binding fragment))
The exception is C2 in which the fragments are labelled the opposite
Complexes with proteolytic activity are called convertases (e.g. C3 convertase cleaves C3)
How are activated components written?
They are over-lined e.g. C1qrs
What are the three complement pathways?
- Classical pathway
- Alternative pathway
- MBL pathway (mannose binding lectin)
What is the common endpoint of all complement pathways?
Formation of C3 convertase to cleave C3 into C3a and C3b, which ultimately leads to MAC formation
How is the classical complement pathway activated?
- Antibody bound pathogen (IgM and IgG (1, 2, 3))
- C1q binds two or more Fc portions of antigen-bound antibody (C1q is made of 6 subunits and associates with two C1r and C1s molecules
- A conformational change occurs; C1r activates auto-catalytically activating the second C1r; both activate C1s
How does C1q bind?
C1q binds the CH2 domain of immunoglobulins and requires at least 2 adjacent Fc regions
What does this process depend on?
Ca2+ dependent process
- EDTA anti-coagulant will interfere with this
What immunoglobulin is better at activating the complement cascade?
IgM is better than IgG at activating
the complement cascade
How does C3 convertase form?
C1s cleaves C4 and C2; C4 cleavage exposes binding site for C2; C4 binds C1 surface and C2 binds C4, forming C3 convertase as a C4b2a complex
How does C5 convertase form?
C3 convertases hydrolyses C3 molecules; some complex with C3 convertase to form C5 convertase
How is C5 cleaved?
The C3b component of C5 convertase binds C5; permits C3 convertase to cleave C5
How is the MAC formed?
C5a binds C6 which initiates it’s formation.
How is the MBL pathway activated?
MBL attaches to mannose-containing polysaccharides on bacteria; then MBL bound by MASP-1 and MASP-2 (MBL associated serine proteins)
(similar to C1q, C1r and C1s)
How does MBL bind?
It has two to six clusters of carbohydrate recognition domains (each with fixed binding sites); mannose and fucose residues with correct spacing are bound with high affinity
What is the difference between the Classical and the MBL pathways?
Classical requires Abs and is delayed until they are produced; MBL does not and is activated immediately - it also only works on mannose-/fucose-rich pathogens
What are the functions of the MAC?
Disruption of bi-layer results in; loss of cellular homeostasis; disruption of proton gradient across membrane; penetration of enzymes e.g. lysozyme into cell; cell death
Kills: gram-ve bacteria (some exceptions); viruses including most envelope viruses; parasites; erythrocytes
What is the function of C2b?
Prokinin; cleaved by plasmin to yield kinin, which results in edema
What is the function of C3b? (1)
Anaphylotoxin; can activate basophils and mast cells to degranulate resulting in increased vascular permeability and contraction of smooth muscle cells, which may lead to anaphylaxis
What is the function of C3b? (2)
Opsonin; promotes phagocytosis by binding to complement receptors
Activation of phagocytic cells
What is the function of C4a?
Anaphyltoxin (weaker than C3a)
What is the function of C4b?
Opsonin; promotes phagocytosis by binding to complement receptors
What are the anaphylatoxins?
C5a, C3a, C4a
What do anaphylatoxins cause?
- Degranulation of mast cells
- Smooth muscle contraction
- Increased vascular permeability and cell adhesion molecules;
- allows increased fluid leakage from vessels
- allows extravasation of Ig and complement molecules
- increased migration of macrophages, PMNs and lymphocytes
- increased microbicidal activity - Inflammation
What is the process of opsonisation?
- Bacterium coated with complement and IgG Ab
- C3b binds to CR1 and Ab binds to Fc receptor
- Bacteria are phagocytosed
- Lysosomes fuse with vesicles; deliver enzymes that degrade bacteria
What diseases are caused by Early complement component deficiencies?
Deficiencies in C1q, C1r, C1s, C4 and C2 can cause;
- Immune complex diseases e.g. SLE
- Recurrent infections by pyogenic bacteria
What diseases are caused by C3 deficiencies?
- Severe clinical manifestations
- Recurrent bacterial infections and immune complex disorders
What diseases are caused by MAC deficiencies?
Recurrent meningococcal and gonococcal infections
What diseases are caused by Regulatory Protein deficiencies?
- Hereditary angiodema
- Paroxysmal Nocturnal Haemoglobinuria
How is complement activity tested in the lab?
The CH50 Assay: demonstrates the ability of serum complement to lyse sheep RBCs coated with antibody
What is a successful CH50 result?
A high CH50 = poor complement activation; red cell lysis results in the release of haemoglobin which is measured at 541nm on a spectrophotometer
