Complement Flashcards
What is Complement?
A group (~30) of small (25-570 KDa), soluble, heat
sensitive proteins that can combine to create complexes with proteolytic activity (zymogens) which can activate other members of the complement family in a cascade like manner.
one of the major immune defense systems of the body
it is central to the development of inflammatory reactions
one of the links between the innate and adaptive arms of the immune system
What are the 3 pathways of the compliment?
- classical pathway,
- alternative pathway
- lectin pathway
What does Complement do?
Recognises and tags the target
*Attaches to ‘non-self’ patterns (Innate recognition)
*Recognises ‘fixed’ antibody (Humoral recognition)
*Recognes apoptotic cells for removal without inducing inflammation
Brings in the troops
*Initiates chemotaxis and activation of phagocytic cells
*Increases blood vessel permeability and adhesion of inflammatory cells
*Contracts smooth muscle cells (Anaphylatoxic effect)
*Releases inflammatory mediators from Mast cells (degranulation)
Attack and remove the threat
*Lyse microbes by hole formation in complement targeted membranes
*Strengthen (adjuvant) T and B cell responses
How is the classical pathway activated?
The classical pathway is activated by antibody
bound to antigen and requires Ca2+
Only surface-bound IgG and IgM antibodies can activate complement, and they do so via the classical pathway:
* IgM is the most efficient activator, but unbound IgM in plasma does not activate complement;
* among IgG subclasses, IgG1 and IgG3 are strong
complement activators, whereas IgG4 does not activate because it is unable to bind the first component of the classical pathway.
Describe the first component of the classical pathway
The first component of the pathway, C1, is a complex molecule comprising a large, 6-headed recognition unit termed C1q and two molecules each of C1r and C1s, the enzymatic units of the complex.
Assembly of the C1 complex is Ca2+-dependent, and the classical pathway is therefore inactive if Ca2+ ions are absent
Describe the activation of the first component of the classical pathway
C1 activation occurs only when several of the
head groups of C1q are bound to antibody
C1q in the C1 complex binds through its globular head groups to the Fc regions of the immobilized antibody and undergoes changes in shape that trigger autocatalytic activation of the enzymatic unit C1r. Activated C1r then cleaves C1s at a single site in the protein to activate it.
Since C1 activation occurs only when several of the six head groups of C1q are bound to antibody, only surfaces that are densely coated with antibody will trigger the process. This limitation reduces the risk of inappropriate activation on host tissues.
Describe what follows the activation of the first component of the classical pathway
C1s enzyme cleaves C4 and C2The C1s enzyme has two substrates – C4 and C2 – which are the next two proteins in the classical pathway sequence.C1s cleaves the abundant plasma protein C4 at a single site in the molecule:
* releasing a small fragment, C4a; and
* exposing a labile thioester group in the large fragment C4b.
Through the highly reactive thioester, C4b becomes covalently linked to the activating surface. C4b binds the next component, C2, in a Mg2+ -dependent complex and presents it for cleavage by C1s in an adjacent C1 complex:
* the fragment C2b is released; and
* C2a remains associated with C4b on the surface.
Describe the role of C4b2a as an activation enzyme in the classical pathway.
C4b2a is the classical pathway C3 convertase
The complex of C4b and C2a (termed C4b2a – the classical pathway C3 convertase) is the next activation enzyme. C2a in the C4b2a complex cleaves C3, the most abundant of the
complement proteins:
* releasing a small fragment, C3a; and
* exposing a labile thioester group in the large
fragment C3b.
As described above for C4b, C3b covalently binds the activating surface.
Multiple copies of covalently bound C3b creates a shell around particle C3b is an opsonin but further
modification to be a target needed for phagocytosis by neutrophils
Describe the role of C4b2a3b as an activation enzyme in the classical pathway.
C4b2a3b is the classical pathway C5 convertase
Some of the C3b formed will bind directly to C4b2a, and the trimolecular complex formed, C4b2a3b (the classical pathway C5 convertase), can bind C5 and present it for cleavage by C2a:
* a small fragment, C5a, is released; and
* the large fragment, C5b, remains associated with the C4b2a3b complex.
Cleavage of C5 is the final enzymatic step in the classical pathway
How does the complement activate macrophages?
Macrophage requires C3b coating AND C5a to activate phagocytosis
Describe the membrane attack pathway
a group of five globular plasma proteins associate with one another to form the membrane attack complex (MAC).
The MAC is a transmembrane pore. C5b binds first C6 then C7 from the plasma. Conformational changes occurring during assembly of this trimolecular C5b67 complex:
* cause release from the convertase; and
* expose a labile hydrophobic site
The complex can stably associate with a membrane through the labile hydrophobic site
Membrane-bound C5b67 recruits C8 from the plasma, and, finally, multiple copies of C9 are incorporated in the complex to form the MAC. by major conformational changes in the components with globular hydrophilic plasma proteins unfolding to reveal amphipathic regions that penetrate into and through the lipid bilayer.
The fully formed MAC creates a rigid pore in the membrane, the walls of which are formed from multiple copies of C9
How does the MAC kill cells?
The pore has an inner diameter approaching 10 nm:
* allowing the free flow of solutes and electrolytes across the cell membrane; and
* because of the high internal osmotic pressure, causing the cell to swell and sometimes burst.
Metabolically inert targets such as aged erythrocytes are readily lysed by even a small number of MAC lesions, whereas viable nucleated cells resist killing through a combination of ion pump activities and recovery processes that
remove MAC lesions and plug membrane leaks.
What are the signals that initiate the Classical pathway?
- Binding Fc fragment of Ag-Ab complexes
- Binding Lipid A in bacterial lipopolysaccharides (LPS)
- Binding to Pentraxins which have bound bacterial phospholipid
How does the lectin pathway differ from the classical pathway
The lectin pathway differs from the classical pathway only in the initial recognition and
activation steps.
The C1 complex is replaced by a structurally similar complex, comprising a C1q-like recognition unit, either mannan-binding lectin (MBL) or ficolin and several MBLassociated serine proteases (MASPs). MASP-2 provides enzymatic activity. As in the classical pathway, assembly of this initiating complex is Ca2+-dependent.
Describe the structure of C1q, MBL and Ficolin
C1q and MBL are members of the collectin family of proteins characterized by globular head regions with binding activities and long collagenous tail regions with diverse roles. Ficolins are structurally similar but the head regions comprise fibrinogen-like domains.
Describe the activation of the alternative pathway
This pathway is in a constant state of low-level activation (termed ‘tickover’).
C3 is hydrolyzed at a slow rate in plasma and the product, C3(H2O), has many of the properties of C3b, including the capacity to bind a plasma protein factor B (fB), which is a close relative of the classical pathway protein C2. Formation of the complex between C3b (or C3(H2O)) and fB is Mg2+-dependent, so the alternative pathway is inactive in the absence of Mg2+ ions.
Activation may occur in plasma or, more efficiently, on surfaces
Describe the formation of the C3bBb in the alternative pathway
Once bound to C3(H2O) or C3b, fB becomes a substrate for an intrinsically active plasma enzyme termed factorD (fD).fD cuts fB in the C3bB complex:
* releasing a fragment, Ba;
* while the residual portion, Bb, becomes an active
protease.
The C3bBb complex is the C3 cleaving enzyme (C3 convertase) of the alternative pathway. Properdin stabilises the binding of C3b/Bb to microbes such
as yeasts and Neisseria gonorrhoeae but NOT host cells. C3b generated by this convertase can be fed back into the pathway to create more C3 convertases, thus forming a positive feedback amplification loop
On an activating surface such as a bacterial membrane, amplification will occur unimpeded and the surface will rapidly become coated with C3bC3b molecules binding to the C3 convertase will change the substrate specificity of the complex, creating a C5 cleaving enzyme, C3bBbC3b
