13 - Innate Immunity Components Flashcards
Ways that commensals protect against infection 1) 2) 3) 4)
1) Production of toxic metabolites
2) Production of bacteriocins, antibiotics
3) Competition for binding sites on GIT epithelium
4) Stimulation of antibacterial compound production by epithelial cells through PAMP recognition
MBL (mannan binding lectin) ligand
Mannan on bacterial cell wall
Proportion of blood proteins that are complement
~10%
Effects of complement activation 1) 2) 3) 4)
1) Inflammation
2) Opsonisation
3) Chemotaxis
4) Lysis of microbes
What are complement proteins?
Inactive proteins (often pro-enzymes, zymogens) in serum activated by proteolysis to carry out a range of immune functions
Number of C’ proteins
Over 30
What produce C’ proteins?1)2)3)4)
1) Hepatocytes
2) Macrophages/monocytes
3) Some epithelial cells
4) Neutrophils (less commonly)
Globin plasma
Protein component of plasma
How are C’ activated?
Enzymatic cascade
Significance of ‘a’ and ‘b’ fragments of a C’
‘a’ is smaller fragment, ‘b’ is larger fragment.Exception is C2, where ‘a’ is larger
Where does activation of C’ often occur?
Surface of pathogen
How do host cells minimise self damage by C’?
Self cells have regulatory factors on their surfaces for reducing C’ activity.
Pathogens lack these
Activity of soluble/fluid phase C’
Often transiently active, inactive.
Which C’ pathway is an effector of humoral immunity?
Classical pathway
Alternative pathway origins
Evolutionarily older than the lectin or classical pathways
Antibody-independent pathways
Lectin, alternative
Steps in C’ cascade1)2)3)
1) Initiation
2) Early
3) Late
Initiation 1)2)3)
1) 3 pathways for activation
2) Different pathways use different, but homologous components
3) Result in formation of different, but homologous C3 convertases
Early stages1)2)
1) Cleavage of C3
2) Formation of C5 convertase
How can C3 be cleaved?1)2)
1) C3 convertase (C4b/C2a, C3b/Bb)
2) Spontaneous hydrolysis of C3 (tickover)
Types of C3 convertase1)2)
1) Classical/lectin - C4b/C2a
2) Alternative - C3b/Bb
Late steps (effector phase)1)2)
1) After C3 cleavage, C5 convertases are formed
2) C5 activation results in pore formation, inflammation, cell lysis
Common steps in complement activation1)2)3)
1) C3 convertase cleaves C3 (C4b/C2a or C3b/Bb)
2) C3 is cleaved, C3a is an inflammatory mediator, C3b binds to the surface of microbe (acts as an opsonin)
3) C3 convertases form the C5 convertases (C4b/C2a/C3b or C3b/Bb/C3b)
How does C3b bind to microbial surface?
Cleavage exposes reactive thioester groups on C3b
Reactive thioester groups bind amino and hydroxyl groups on microbial surface
Two types of C5 convertase
1) Classical/lectin - C4b/C2a/C3b
2) Alternative - C3b/Bb/C3b
Alternative pathway initiation1)2)3)4)5)6)
1) Low-levels of C3 hydrolysis initiate formation of active intermediates2) Intermediates cleave C3 to C3a and C3b. In fluid, C3b is short-lived3) C3b thioester group is revealed, C3b binds to microbial surface4) B cleaved by factor D to Bb5) C3b and Bb form C3 convertase on microbial surface6) Properdin binds and stabilises C3 convertase on microbial surface
What cleaves B into Bb?
Factor D
What stabilises C3 convertase on microbial surface in alternative pathway?
Properdin
How is C5 convertase formed in the alternative pathway?
When C3 convertase cleaves C3, C3b joins C3b/Bb on cell surface, forms C5 convertase
Factor Bb1)2)3)
1) Active form of factor B
2) Cleaved by factor D
3) Forms alternative C3 convertase with C3b, and alternative C5 convertase with two C3b’s
Factor D1)2)
1) Serine protease
2) Cleaves factor B when bound to C3b
How is alternative initial phase regulated?1)2)
1) Factors I and H rapidly degrade C3b in fluid phase
2) C3b can bind host-cell surfaces, but is rapidly inactivated by complement regulatory proteins on these cells
Factor I and factor H role
Inactivate C3b in fluid phase
What initiates the classical C’ pathway?
C1q binding to an antibody or C-reactive protein, bound to a pathogen
C1 structure
A compex of C1q (6x collagen-like tails, 6x globular heads), C1r and C1s (serine proteases)
What does C reactive protein bind?
Phosphocholine residues on bacterial surfaces
How is C1 activated?
C1q binding activates C1r to cleave C1s, which forms an active serine protease
IgGs that best activate classical pathway
IgG1, IgG3
Antibody that best activates classical pathway
IgM
Minimum number of Ig heavy chains that C1q must bind to activate classical pathway
2
Can soluble IgM activate classical pathway?
No
How does IgM/IgG activate classical pathway?
Binds to pathogen, this results in a conformational change.IgM Fc region involved in C1q binding is exposed
Classical complement pathway formation of C3 convertase1)2)3)4)5)6)7)8)
1) C1q binds antibody
2) C1r2, C1s2 are activated
3) C4 binds activated C1q
4) Activated C1s cleaves C4 into C4a, C4b
5) C4b binds covalently to cell surface
6) C4b binds C2
7) C2 is cleaved by C1s8) C4b/C2a C3 convertase formed
Classical pathway equivalent to alternative factor B
C2
Classical pathway equivalent to alternative C3b
C4b
Classical and alternative formation of C5 convertase1)2)
1) C3 convertase cleaves C3 into C3a, C3b
2) C3b binds to C3 convertase, forms C5 convertase
C5a
Powerful inflammatory mediator
C3a
Inflammatory mediator
Lectin pathway formation of C3 convertase1)2)3)4)5)6)7)
1) MBL binds to sugars on microbial surface
2) MBL-associated serine protease (MASP) binds to collagen-like region of MBL
3) MASP cleaves C4 into C4a, C4b (from here on, the same as classical pathway)
4) C4b covalently binds to cell surface
5) C4b binds C2
6) C2 cleaved by MASP into C2a, C2b
7) C4b and C2a form C3 convertase
What is MBL equivalent to?
C1q
Alternative pathway amplification loop1)2)3)4)
1) C3b produced by any pathway can interact with factors B and D
2) C3b binds to cell surface with thioester region
3) Factor B is cleaved by factor D
4) Bb binds to C3b, forms C3 convertase
Formation of membrane attack complex1)2)3)4)5)6)7)
1) C5b bound to cell membrane
2) C5b recruits C6, C7 (hydrophobic)
3) C5b/C6/C7 insert into cell membrane, recruit C8
4) C8 is a trimer. 1 unit inserts into cell
5) C5b/C6/C7/C8 capable of transiently lysing cells
6) C5b/C6/C7/C8 casuses polymerisation of C9
7) Polymerised C9 forms a 100 Angstrom hole
Bacteria that is controlled with C9 pore formation
Neisseria spp
Factors involved in forming classical C3 convertase
C1(q, r, s), C2a, C4b
Factors involved in forming lectin C3 convertase
MBL, MASP, C2a, C4b
Factors involved in forming alternative C3 convertase
C3b, Bb, factor D (cleaves B)
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-1CD11bCD18
Cells that express CR1 and CR31)2)3)4)
1) Macrophages2) Neutrophils3) Follicular dendritic cells4) 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 destroyedErythrocyte 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 cellsFollicular dendritic cells
What does CR2 bind?
iC3b, C3dg antigen/antibody complexes
What is C3dg?
A C3 breakdown product
CR41)2)3)
1) Dimer of CD11c and CD182) Present on dendritic cells3) Similar function to CR3
Role of complement in B cell activation1)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
AnaphylatoxinsCause 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 proteinsGPCR
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 lysis2) Inflammation3) Chemotaxis4) B cell activation5) Opsonisation
DAF, CR1, MCP roles
Regulate C3 convertase productionMembrane-bound
Factor I and factor H roles
Cleave C3b (to inactivate)
CD59 role
Inhibit MAC formation
Self cells susceptible to C’ lysis
ErythrocytesHave 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 pathwayDAF, CR1 bind to C3b, displace Bb from C3 convertase complex
DAF1)2)3)
1) Decay accelerating factor
2) Membrane-bound
3) Bind to C4b or C3b, displace C2a or Bb from C3 convertase
MCP1)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 roles1)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 I1)2)3) a,b
1) Serine protease
2) Inactivates fluid-phase C3b
3) Requires additional cofactors to cleave cell-membrane-bound C3ba) Factor H b) MCP or CR1
Factor H1)2)
1) Cofactor for factor I cleavage of cell-membrane-bound C3b2) 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)
CD591)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, properdinIncreased risk of disease from Neisseria gonorrhoeae or meningitidis
Alternative and classical pathway deficiencies
Deficiencies in C3 or factors I or HIncreased 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
