Lecture 26 - Complement Activation 2 Flashcards
Describe the mechanism of Opsonisation and Phagocytosis in the C’ cascade
- C3b and C4b binds to the surface of microbes
- Macrophages bind C3b through CR1
- Macrophages bind Ab on microbe surface through FcγR
- Receptor mediated endocytosis
- Destruction of microbe in phagosome with hydrolytic enzymes
How are the recognition molecules (responsible for initiation of C’ cascade) like pattern recognition receptors?
They recognise PAMPs on the surfaces of pathogens:
MBL: sugars (mannose)
CRP: phosphocholine
C3b: amino acids and hydroxyl groups
What is an opsonin?
Material that coats pathogens to promote opsonisation
What are the various complement receptors?
Which cells are they expressed on?
CR1 • aka CD35 • Macrophages, neutrophils, RBCs, FDCs • Bind C3b, C4b, iC3b • Important role in C3 convertase regulation
CR3
• aka Mac-1
• Macrophages
CR2:
• B cells, FDCs
CR4:
• DCs
• similar function to CR3
Expression on:
Phagocytes:
• macrophages
• neutrophils
CR1 also on:
• FDCs
• RBCs
What is iC3b?
Which CRs will bind it?
A breakdown product of C3b
Forms on cell membranes
Binds C’ receptors, especially CR3
How do macrophages recognise opsonised microbes most efficiently?
Recognition through:
- CR1 : C3b
- FcγR : IgG
Describe RBC use of C’ receptors
RBC express CR1
- CR1 binds Ab-Ag complexes in blood
- RBCs circulate (with immune complexes bound) to the spleen
- Complexes stripped from the RBC in the spleen
- RBCs survive and recirculate
What is the function of CR2?
Forms complex with CD19 and CD81 (also present in membrane of B cell)
Provides ‘second’ signal for B cell activation
Describe the role of C’ in B cell activation
The following are expressed on the surface of B cells:
• BCR (e.g. IgM)
• CR2
• CD19, CD81
- Microbe bound by C3dg
- Ag on microbes bind CDRs on BCR
- C3dg on microbe binds CR2
- Triggering of transduction pathway, leading to B cell activation
Which C’ receptors do FDCs express?
CR1
CR2
Which C’ receptors do DCs express?
CR4
Describe the interaction between EBV and B cells
CR2 present on B cells
EBV uses this receptor to get into B cells
Remains latent in B cells
What is C3dg?
What is its receptor?
C3dg is another breakdown product of C3b
C3dg is a ligand for CR2
What is an anaphylatoxin?
Give some examples
Molecules that can cause systemic inflammation, and possibly lead to anaphylactic shock
Leads to a massive drop in BP (through systemic vasodilation)
Examples:
• C3a
• C5a
Describe triggering of inflammation through the C’ cascade
Mediated by C3a and C5a
- Cleavage of C3 and C5 through the C’ cascade to deliver:
• C3a
• C5a - C3a and C5a bind to their receptors expressed on:
• Mast cells
• Endothelial cells
• Phagocytes - Release of TNF and histamine from these cells
- TNF and Histamine, as C3a, C4a and C5a:
→ Increase in vascular permeability - Migration of leukocytes into tissues → tissue inflammation
- C5a acts as a chemoattractant for Neutoprhils and monocytes
Where are the receptors for C5a and C3a located?
Mast cells
Endothelial cells
Phagocytes
Describe chemoattraction through the C’ cascade
Mediated by C5a
- C5a generated through cleavage of C5 in the C’ cascade
- C5a binds to C5aR (GPCR) expressed on:
• Neutrophils
• Monocytes - Triggering of transduction cascade in the cells
→ Motility towards the source of C5a
What is the structure of C3aR and C5aR?
GPCRs
Which sort of pathogens are targeted by the C’ cascade?
Bacteria
Free virions
Parasites
Which factors regulate C3 convertase production?
DAF
CR1
MCP
Which molecule cleaves and inactivates C3b?
Factor I
and Factor H assisting
Which molecule inhibits MAC formation?
CD59
What are some of the host regulatory proteins that prevent C’ damage of our own cells?
CD59 DAF CR1 MCP Factor I Factor H
Describe the function of: • MCP • CD59 • Factor H • DAF • CR1 • Factor I
Host cell regulatory proteins
CD59:
• Inhibition of MAC formation
DAF:
• Regulation of C3 convertase production
• Classical and Alternative pathways
CR1
• Regulation of C3 convertase production
• Classical and Alternative pathways
MCP
• Regulation of C3 convertase production
• Classical pathway only
Factor I:
• Degradation of C3b
Factor H
• Degradation of C3b
Describe inhibition of C3 convertase
Mediated by:
• DAF
• CR1
• MCP
– Classical pathway –
- Formation of C4bC2a on Ig bound to Ag on host cells
- DAF/CR1/MCP expressed in the cell membrane of host cells
- DAF/CR1/MCP displaces C2a from C4b
– Alternative pathway –
- C3bBb forms on host cell surface
- DAF/CR1 (expressed on host cells) displaces Bb from C3b
Describe the function of Factor I
What type of molecule is it?
It is a serine protease
- C3b present in the fluid phase, bound covalently to cells
- Factor I binds to C3b
MCP/CR1 (present in host cell membrane) associate with C3b - Proteolysis of C3b
C3b → iC3b / C3d / C3dg - Degradation products bind to CR on phagocytes and B cells (clearance)
Describe regulation of MAC formation
Mediated by CD59
- Activation of C5bC6C7C8 in late stages of C’ cascade
- CD59 (expressed on host cells) interferes with polymerisation of C9 pore
→ no MAC formed in host cells
Which co-factors are required for Factor I function?
Factor H
MCP
CR1
Which cells does Factor H bind?
Cell surfaces rich in sialic acid (mammalian cells)
→ helps clear C3b on host cells
Describe the role of C’ components in the following processes:
• Solubilisation of immune complexes
• Clearance of immune complexes
Immune complexes form all the time
Solubilisation:
C’ deposited on immune complexes makes them more soluble
Clearance:
Immune complexes with C’ attached will bind to CR1 on RBCs
When these circulate to the spleen, they are stripped off and phagocytosed
The RBCs survive and recirculate
How can microbes evade the complement system?
Expression of homologues of the host complement regulatory proteins:
- Expression of sialic acid
- Expression of proteins that recruit Factor H
• e.g. HIV gp41 recruits Factor H - Virus budding incorporating the host regulatory proteins DAF and CD59
- Production of specific proteins that mimic C’ proteins
• E. coli produce C1q - Inhibition of C’ mediated inflammation
• S. aureus produce CHIPS
What is the basis of ABO blood groups?
Glycans (complex sugars) present on the cell surface
A allele: terminal N-acetylgalactosamine
B allele: terminal galactose
O allele: enzyme not present, so no terminal sugars
Why is there rejection of blood in transfusion?
Outline the process
What are the symptoms of mis-matched blood transfusion?
“Natural IgM” antibodies are present against sugars in the other blood group
NB Rh blood groups can also activate C’
e.g. A individual:
• IgM against terminal galactose residues
- Blood transfusion from B individual to A individual
- anti-B Ab from host bind to terminal galactose sugar on donor RBCs
- Activation of C’
4. Lysis of RBCs Fever Nausea Vomiting Pain Shock
From whom can the following individuals receive blood: • A • B • AB • O?
Which Ab do they respectively have?
A:
• A
• O
• Have anti-B Ab
B:
• B
• O
• Have anti-A Ab
AB: • A • B • O • Have no Ab
O:
• O
• Have anti-A and anti-B Ab
What are some C’ deficiency diseases?
What does it leave the people susceptible to?
Classical pathway:
• C1, C2, C4 deficiency
→ SLE
Alternative pathway:
• Factors B, D and properdin deficiency
→ Increase infection susceptibility: Neisseria
Alternative and classical pathway:
• C3 deficiency
→ Increased infection susceptibility to S. pneumoniae, Neisseria, H. influenzae
What are the receptors for the following:
• C3a
• C5a
C3a: C3aR
C5a: C5aR
Describe the products of Factor I proteolysis of C3b
C3B → iC3b + C3dg
Also: C3d (binds to CR-2)
Which C' components do the following receptors bind: • CR1 • CR2 • CR3 • CR4
CR1:
• C3b
• C4b
CR2: • C3d • C3dg • iC3b i.e. the breakdown products of C3b
CR3:
• iC3b
CR4:
• iC3b
