Innate immunity - barriers & complement Flashcards
1
Q
Name 5 general principles of innate immunity
A
- Oldest form of immunity
- Elements shared by plants, insects, mammals
- Changes on an evolutionary timescale
- Always available
- Prior exposure not required
- Little/no ‘memory’
- Major form of immunity in young children
- Period between loss of maternal antibodies/formation of their own
- Crucial in initiating and directing adaptive immune responses.
- Adaptive immunity takes ~ 4-6 days to develop. Innate immunity is critical in controlling infections before this.
2
Q
What are the 3 elements of innate immunity?
A
Barriers
- prevent establishment of infection
Pre-formed mediators
- proteins/peptides with broad specificity that damage pathogens or induce inflammation or help recruit and activate innate immune cells
Innate immune cells
- recognise and are activated by pathogen
- eliminate pathogen
- communicate with other cells
- activate and “steer” adaptive immune response
3
Q
What are the 4 regions in which we have barriers
A
Skin, Gi tract, UG tract & Respiratory tract
4
Q
What are the 3 types of barriers?- describe them briefly
A
-
Physical
- epithelial cells joined by tight junctions, flow of air or fluid, cilia (move mucus)
-
Chemical
- Fatty acids, low pH enzymes (pepsin), Lysozyme, antibacterial peptides (defensins)
-
Microbiological
- commensals (microbiota)
5
Q
Which regions would you find the following barriers
-
Physical
- epithelial cells joined by tight junctions
- flow of air or fluid
- cilia (move mucus)
-
Chemical
- Fatty acids
- low pH
- enzymes (pepsin),
- Lysozyme,
- antibacterial peptides (defensins)
-
Microbiological
- commensals (microbiota)
A
-
Physical
- epithelial cells joined by tight junctions: S + GI + UG + Respt tract
- flow of air or fluid: Skin + GI + UG + Resp tract
- cilia (move mucus): Resp tract
-
Chemical
- Fatty acids: Skin
- low pH: GI + UG
- enzymes (pepsin): GI + UG
- Lysozyme: Resp tract
- antibacterial peptides (defensins): Skin + GI + UG + Resp tract
-
Microbiological
- commensals (microbiota): Skin + GI + UG + Resp tract
6
Q
What are the 3 physicalbarriers that create the 3 types of physical barriers
A
-
Keratinized skin generally impermeable (in brown)
- keratinocytes produce keratin (hard substance), sebum -contains fatty acids, defensins
- Shedding skin, frees microbes
- Commensals on skin
-
Mucous membranes largest interface with environment, semi-permeable
- GI, UI tracts, respiratory tract
-
mucous, cilia (respiratory tract), secreted enzymes (e.g. lysozyme in tears and saliva)
- CF patients more prone to respiratory and GI infections (mucus effected)
- in mucus → enzymes
- low pH (gut, vagina), peristalsis (gut) (movement of the gut)
- Shedding of epithelia (particularly in infection increase in shedding as well)
- Commensals
7
Q
Name 3 examples of pre-formed mediators
A
Lysozyme, Antimicrobial peptides (e.g. defensins), Complement
8
Q
Describe Lysozyme function i.e. where are they found, what do they do
A
- present in secretions (tears, saliva, mucous etc.)
- Breaks a bond in peptidoglycan. More active against G+ve bacteria, where peptidoglycan is exposed.
- cleaves peptidoglycan bonds → destabilises → Gram positive bacteria therefor more susceptible to lysozymes
9
Q
Describe antimicrobial peptides such as defensins: structure, function, who produces
A
- 29-47 AA in length, found in very primitive organisms
- Beta-sheet core structure stabilised by 3 conserved intramolecular disulphide bonds. 2 subfamilies (alpha and beta) in humans (not needed to learn)
- alfa form are pre-formed in granules of innate immune cells.
- Beta form have to be synthesised de-novo upon infection
- produced by many epithelial cells and neutrophils
- way of function:
- cationic, disrupt lipid bilayers in bacteria, fungi and enveloped viruses
- can insert into bacterial membranes to disrupt lipid bilayer
10
Q
Describe the basic principles of complement
A
- > 20 soluble proteins found in blood and other body fluids
- Components normally inert, but “activated” by presence of pathogens or antibody bound to pathogen
- pro-enzyme (inert) acted upon stimulus → turns into enzyme
- cascade of reactions → amplification of enzymes
- Originally evolved as part of the innate immune response
- Provides protection early in infection in the absence of antibodies through other “older” activation pathways
11
Q
Describe the principles of the complement cascade: Central event, cleavage of c3 does what?
A
- Many activated complement components are proteases; act on one another to generate a large and a smaller fragment:
- proteases are enzymes that cleave proteins
- C3 → C3b + C3a
- Cleavage of C3 exposes a reactive thioester in C3b, which can bind covalently to adjacent proteins/carbohydrates e.g. on the surface of a pathogen.
- (rapidly inactivated in the fluid phase). i.e. C3b can be made without infection by random chance but quickly inactivated if it doesn’t bind to anything
- C3b (bigger fragment) C3a (smaller fragment)
- The central event of complement activation is cleavage of C3 by a “C3 convertase”.
- 3 pathways of complement activation, initiated in different ways, but all generate a C3 convertase.
12
Q
What 3 ways can complement be activated? and how are they similar?
A
- The central event of complement activation is cleavage of C3 by a “C3 convertase”.
- 3 pathways of complement activation, initiated in different ways, but all generate a C3 convertase.
1. Classical pathway
2. Mannose-binding lectin (MBL) pathway
3. Alternative pathway
13
Q
Describe the classical pathway of activation of complement
A
Antibody binds bacterial surface → C1 interacts with antibbody Fc region → activates C1 → serine protease which acts on C4 which acts on C2 → C3 convertase
C4b + C2a (is the C3 convertase) (from cleavage of C4 and C2 by C1)
NB! dependent on adaptive immune system antibodies!
C1 -> C4 -> C2 -> C3 -> C5 -> C6 -> C7 -> C8 -> C9
14
Q
Describe the Mannose-binding lectin pathway of activation of complement
A
- binding of lectin to bacterial surface acts MASP1 → acts on MASP2 → C3 convertase
C4b + C2a (is the C3 convertase) (from cleavage of C4 and C2 by MASP2) - MASP = MBL associated serine protease
- Lectin (Mannose-binding lectin) to bacterial surfaces
- Lectin is a molecule that loves to bind to carbohydrate. MBL loves to bind to mannose residues which are often found on surface of bacteria. Other lectins discovered but MBL is the main one. (research this!)
- MASP = MBL associated serine protease
15
Q
Describe the alternative pathway of activation of complement
A
-
some C3b is generated spontaneously in body fluids by a “tickover” mechanism
- tickover mechanism = even without infection
- during infection, some C3b generated can bind to bacterial membranes to LPS in particular (through reactive thioesters in C3b
- C3b + Bb
- If C3b generated binds to LPS on a pathogen surface → factor B binds
- Factor B is cleaved by Factor D → generates Bb which is connected to C3b → C3bBb → acts as a C3 convertase
- Normally C3bBb is quiet easily inactivated → Factor P (properdin) stabilises C3bBb enzyme (allows it to work longer on the bacterial surface)
- C3b generated by the Classical or MBL pathway can also bind factor B → amplification of complement activation!
- complement interacts with LPS on bacterial surface → factors which are found at high concentration in body fluids called factor B and Factor D, which are stabilised by factor P → c3 convertase made
- C3 convertase = C3b + Bb
- C3b: Cleavage of C3 exposes a reactive thioester in C3b, which can bind covalently to adjacent proteins/carbohydrates e.g. on the surface of a pathogen.
(rapidly inactivated in the fluid phase). i.e. C3b can be made without infection by random chance but quickly inactivated if it doesn’t bind to anything
C3b (bigger fragment) C3a (smaller fragment)
