Lecture 26 - Complement Activation 2 Flashcards by Eleanor Nicklason

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

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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

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What is an opsonin?

Material that coats pathogens to promote opsonisation

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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

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What is iC3b?

Which CRs will bind it?

A breakdown product of C3b
Forms on cell membranes
Binds C’ receptors, especially CR3

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How do macrophages recognise opsonised microbes most efficiently?

Recognition through:

CR1 : C3b
FcγR : IgG

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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

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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

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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

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Which C’ receptors do FDCs express?

CR1

CR2

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Which C’ receptors do DCs express?

CR4

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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

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What is C3dg?

What is its receptor?

C3dg is another breakdown product of C3b

C3dg is a ligand for CR2

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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

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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

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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 -- 1. Formation of C4bC2a on Ig bound to Ag on host cells 2. DAF/CR1/MCP expressed in the cell membrane of host cells 3. DAF/CR1/MCP displaces C2a from C4b -- Alternative pathway -- 1. C3bBb forms on host cell surface 2. 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 1. C3b present in the fluid phase, bound covalently to cells 2. Factor I binds to C3b MCP/CR1 (present in host cell membrane) associate with C3b 3. Proteolysis of C3b C3b → iC3b / C3d / C3dg 4. Degradation products bind to CR on phagocytes and B cells (clearance)

Describe regulation of MAC formation

Mediated by CD59 1. Activation of C5bC6C7C8 in late stages of C' cascade 2. 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: 1. Expression of sialic acid 2. Expression of proteins that recruit Factor H • e.g. HIV gp41 recruits Factor H 3. Virus budding incorporating the host regulatory proteins DAF and CD59 4. Production of specific proteins that mimic C' proteins • E. coli produce C1q 5. 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 1. Blood transfusion from B individual to A individual 2. anti-B Ab from host bind to terminal galactose sugar on donor RBCs 3. 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