Lecture 26 - Complement Proteins II Flashcards
Complement components that act as opsonins
C3b, C4b
Complement receptors involved in phagocytosis
CR1, CR3
Another name for CR1
CD35
Another name for CR3
Mac-1
CD11bCD18
Cells that express CR1 and CR3 1) 2) 3) 4)
1) Macrophages
2) Neutrophils
3) Follicular dendritic cells
4) Erythrocytes
What do CR1 and CR3 have a high affinity for?
C3b, iC3b, C4b
What does CR3 bind?
iC3b
iC3b
Breakdown product of C3b that forms on cell membranes
inactive C3b
What enhances C3b phagocytosis?
Specific IgG also binding to microbe
Role of CR1 on erythrocytes
Binds to C3b/C4b opsonised microbes, transports them to the spleen, where they are destroyed
Erythrocyte is not destroyed in this process.
CR2 1) 2) 3) 4)
1) CD21
2) Complement receptor on B cells
3) Complexes with CD19 and CD81
4) Provides a second signal for B cell activation
Which cells express CR2?
B cells
Follicular dendritic cells
What does CR2 bind?
iC3b, C3dg antigen/antibody complexes
What is C3dg?
A C3 breakdown product
CR4
1)
2)
3)
1) Dimer of CD11c and CD18
2) Present on dendritic cells
3) Similar function to CR3
Role of complement in B cell activation
1)
2)
3)
1) Antigens coated by C3dg bind IgM and CR2
2) Boosts phosphorylation of ITAM residues on Igalpha and Igbeta
3) This amplifies BCR signalling
Which complement components are anaphylatoxins?
C3a, C5a
Effects of C3a and C5a
Anaphylatoxins
Cause systemic inflammation, which in extreme cases resembles anaphylactic shock
Where are C3a and C5a receptors found?
1)
2)
3)
1) Mast cells
2) Endothelial cells
3) Phagocytes
How do C3a and C5a cause systemic inflammation?
1)
2)
3)
1) Bind to mast cells, cause them to release TNFa, histamine
2) Bind to endothelial cells, induce vascular leakage
3) C5a is a chemotactant. Attract neutrophils, monocytes
C3aR and C5aR structure
Seven-pass transmembrane proteins
GPCR
Agents targeted by the complement cascade
Extracellular bacteria, free virions, parasites
Bacterial factors that activate C’
Peptidoglycan and LPS activate the alternative pathway
Results of activation of C' 1) 2) 3) 4) 5)
1) Direct lysis
2) Inflammation
3) Chemotaxis
4) B cell activation
5) Opsonisation
DAF, CR1, MCP roles
Regulate C3 convertase production
Membrane-bound
Factor I and factor H roles
Cleave C3b (to inactivate)
CD59 role
Inhibit MAC formation
Self cells susceptible to C’ lysis
Erythrocytes
Have low levels of regulatory proteins on surface
How can C3 convertase be inhibited?
1)
2)
1) Classical pathway
DAF, CR1, MCP bind to C4b, displace C2a from C3 convertase
2) Alternative pathway
DAF, CR1 bind to C3b, displace Bb from C3 convertase complex
DAF
1)
2)
3)
1) Decay accelerating factor
2) Membrane-bound
3) Bind to C4b or C3b, displace C2a or Bb from C3 convertase
MCP 1) 2) 3) 4)
1) Membrane cofactor protein
2) Membrane-bound
3) Bind to C4b, displace C2a from C3 convertase
4) Can also act as a cofactor for factor I cleaving membrane-bound C3b
CR1 roles
1)
2)
3)
1) Bind C3b, C4b when these act as opsonins
2) Break down C3 convertase complex by displacing either C2a from C4b, or Bb from C3a
3) Can act as a cofactor for factor I cleaving membrane-bound C3b
Factor I
1)
2)
3) a,b
1) Serine protease
2) Inactivates fluid-phase C3b
3) Requires additional cofactors to cleave cell-membrane-bound C3b
a) Factor H
b) MCP or CR1
Factor H
1)
2)
1) Cofactor for factor I cleavage of cell-membrane-bound C3b
2) Binds to sialic acid, which is highly expressed on mammalian cells.
Products of factor I cleavage of C3b
iC3b, C3d, C3dg (which bind CR on B cells and phagocytes)
CD59
1)
2)
1) Cell-membrane-bound protein
2) Binds to C5a on C5b/C6/C7/C8 complex, inhibits C9 from forming pore
Effect of C’ on immune complexes
Promotes solubilisation of antibody-antigen complexes
What determines the ABO blood groups?
Allelic difference in enzymes that modify glycans on cell surfaces
A allele in blood groups
Encodes enzyme that transfers an N-terminal acetylgalatosamine onto glycans
B allele in blood groups
Encodes an enzyme that transfers a terminal galactose residue onto glycans
O allele in blood groups
Doesn’t encode an enzyme to modify cell-surface glycans
Immune component enforcing blood groups
Natural IgM produced against missing allele
Classical pathway deficiencies
Missing C1, C2, C4
Leads to immune complex disease (EG: systemic lupus erythematosus)
Alternative pathway deficiencies
Missing B, D, properdin
Increased risk of disease from Neisseria gonorrhoeae or meningitidis
Alternative and classical pathway deficiencies
Deficiencies in C3 or factors I or H
Increased infections with Strep pneumoniae, Neisseria spp, Haemophilus influenzae
What do deficiencies in factors I and H do?
Mimic C3 deficiencies
How can microbes evade C’?
1)
2)
3)
1) Recruit host C’ regulatory proteins
2) Mimic human C’ proteins
3) Inhibit C’-mediated inflammation
How do microbes recruit host C’ regulatory proteins?
1)
2)
3)
1) Express or scavenge sialic acid (this recruits factor H)
2) Synthesise or recruit factor H (EG: gp41 of HIV)
3) Viruses incorporate host regulatory proteins into envelope when budding (EG: HIV recruits DAF, CD59)
Microbe that mimics human C’ proteins
E coli has a C1q binding protein
Microbe that inhibits C’-mediated inflammation
Staph aureus, using CHIPS (chemotaxis inhibitory protein of Staph aureus)
When do C’ deficiencies normally present?
~6-9 months after birth
