Development and Activation of B cells Flashcards
1
Q
what are the advantages of the adaptive immune system?
A
- diversity/specificity
2. memory
2
Q
why is it important that the adaptive immune system has diversity/specificity?
A
- immune system has a large repertoire of lymphocytes, which express uniquely different receptors
- innate causes collateral damage but we only want to kill the infected
- very specific and can target exactly
- has a large variety of T and B cells which can recognise any pathogen
3
Q
why is it important that the adaptive immune system has memory?
A
- innate has no memory
- memory allows a more rapid and effective response on a secondary encounter with an antigen
- keeps a reserve of immune cells
- second time the response is more aggressive and quick
4
Q
what are the 5 principles of clonal selection?
A
- each lymphocyte has a single type of receptor with a unique specificity
- each lymphocyte bears multiple of the same receptor
- cells that have receptors for self-molecules are deleted from the repertoire
- interaction between an antigen and a receptor leads to activation of that lymphocyte
- the derived daughter cells are identical to the activated parent cell
5
Q
how is clonal expansion initiated?
A
- each receptor has a different shape
- A B cell has the right receptor for a pathogen
- will the one cell clonally expand
6
Q
what do B cells produce?
A
antibodies
7
Q
what are the roles of B cells?
A
- neutralise toxins and viruses
- help in the phagocytosis of pathogens
- destruction of bacteria and virus
- act as an antigen presenting cell to activate T cells
8
Q
what are the 2 forms of immunoglobulins?
A
- as a membrane receptor that binds an antigen
2. as a secreted fluid phase molecule that directs a specific immune function
9
Q
what immune functions can immunoglobulins direct?
A
- complement fixation
- opsonisation
- neutralisation
10
Q
what is the hinge region of the immunoglobulin?
A
gives flexibility, helps to bind
11
Q
what is the variable region of the immunoglobulin?
A
amino sequence is variable from one B cell to another
- antigen binding happens at the top at the variable region
12
Q
what are the 5 types of heavy chains?
A
IgM IgG IgD IgA IgE
13
Q
what are the 2 types of light chains?
A
kappa (K) and lambda (y)
14
Q
how is the immonglobulin isotype defined?
A
types of heavy and light chain recombine
15
Q
what is the hinge region susceptible to?
A
cleavage by 2 enzymes:
papain
pepsin
16
Q
what is the role of the enzyme papain?
A
- cleaves at the hinge releasing the arms
- Fab fragments
- Fab fragments can bind antigen but lack any function
17
Q
what is the role of the enzyme pepsin?
A
- cleaves just underneath the hinge region
- Fab2 fragment
- could bind an antigen but no function
18
Q
what did the Fab and Fc regions show?
A
- showed that the top was for recognition
- the bottom, the Fc region, determines the immunoglobulin domain
19
Q
what do B cells recognise?
A
- recognise extracellular molecules/antigens
- look for the shape of the molecules on the surface of the pathogen
- match to the B cell receptor
- lock and key recognition
20
Q
what is the hyper variable region (HV)?
A
- within the variable region
- high levels of amino acid variability
- amino acids in the HV regions determine the shape of the antigen binding site
21
Q
what is the size of the adult immunoglobulin repertoire?
A
about 10^11 different immunoglobulins
- however, the human genome only encodes for ~30, 000 genes
22
Q
what are HV regions the result of?
A
- result of random rearrangement/combinations of defined gene segments
23
Q
what are the heavy chain gene segments?
A
V, D and J
24
Q
what are the light chain gene segments?
A
V and J
25
which heavy chain is rearranged first?
- IgM heavy chain is rearranged first
- followed by the light chian
- IgM is the first BcR made
26
what does splicing result in?
brings the constant region next to the variable region resulting in a complete light chain
27
how does the heavy chain recombine?
- D recombines with J first
- then with V
(ordered recombination)
28
what is combinatorial diversity?
can produce many different immunoglobulins
29
what is the 23-12 rule?
- you have a recombination signal sequence (RSS)
- 2 types: 23 base pair spacer and 12 base pair spacer
- a 23 spacer can only recombine with a 12 spacer
- means orientation will be in the correct way to get a functioning protein
- gives diversity
30
why does recombination only occur in lymphocytes?
B and T cells have recombination activation genes 1 and 2 (RAG 1 and RAG 2)
- allow the recombination of RSS upstream of the gene segment
31
what is the role of RAG 1 and RAG 2?
- RAG proteins bind to the RSS
- brings them close together and forms a hair pin loop
- has endonuclease complex which cuts at the RSS segments leaving free ends
32
what is the signal joing?
extrachromosomal circular piece that is lost from the genome when the cell divides
- DNA we dont want forms a circle and gets secreted away
33
what is the coding joint?
brings the gene segments together
34
what is the result of the 'nick' in the DNA strand left by the activtiy of endonuclease?
- leaves free ends
| - enables random addition/removal of nucleotides
35
what enzyme facilitates the addition and removal of nucleotides during recombination?
- enzyme terminal deoxynucleotide transferase (TdT) adds N nucelotides to the D-T pairing of the heavy chain
36
what happens once nucleotides have been added to the free ends during recombination?
DNA is repaired with DNA ligase
37
how is there combinatorial diversity?
heavy and light chain pairing
38
how is there junctional diversity?
TdT
39
what is tolerance?
- immune system has evolved strategies to protect us from itself
40
how does autoimmunity occur?
- failure to tolerate the adaptive immune system
| - need some quality control
41
where are B cells assessed for auto reactivity?
- when they develop in the bone marrow and completed in the spleen
42
Describe the HSC stage in B cell development?
- develop into common lymphoid progenitor
- makes it responsive to a chemokine CXCL12
- holds a potential B cell in the bone marrow
43
what is the CLP stage in B cell development?
- transcription factors
| - heavy chains recombine first
44
what is the pro B stage in B cell development?
- recombines again
| - you have the heavy chain in side the cell not on the surface
45
what is the quality control that happens at the PreB stage in B cell development?
- does the heavy chain have the capability to bind a light chain
- make a surrogate light chain
- if the heavy chain can complex with the surrogate light chain it will progress
- if not it will die
- get the rearrangement of the light chain
46
what is the quality control that happens at the immature B cell stage in B cell development?
- the receptor is on the surface
- expressed with the proteins in our body
- presented to the B cell
47
what happens if the B cell recognises its own molecule at the second quality control stage?
- re-initiate rearrangement of the light chain V region segments to form a new shape in the antigen binding site
- called receptor editing
- die if the new receptor still has auto-reactivity for self-antigens
48
what happens if the B cell doesn't recognise its own molecule at the second quality control stage?
- immature B cell leaves the bone marrow and moves to the spleen to complete development
- transition into the type of B cell it will become
49
what form of B cells secrete immunoglobulins?
- B cells themselves do not secrete antibodies
| - plasma cells that secrete antibodies
50
what is the transmembrane receptor of the B cell?
- transmembrane and a small cytoplasmic component
- held on the surface of the B cell
- membrane BcR
- B cells express Ig heavy chain transcripts
51
what does the Fc region define?
- defines its function in the body
- Fab gives antibody specificity
- Fc gives it its function
52
what happens if you swap the Fc region for one class of antibody to another?
- can change the function
53
how do antibodies facilitate eradication of microbes?
- neutralisation of microbes and toxins
- opsonoization and phagocytosis of microbes
- antibody dependent cellular cytotoxicity
- phagocytosis of microbes opsonized with complement fragments
- inflammation
- lysis of microbes
54
how do antibodies facilitate the neutralisation of microbes and tozxins?
- antibodies bind surface molecules needed for microbe to bind to the host cell
- most microbes need to get inside to use the host cell
- if you block this then they can't infect the cell
55
how do antibodies facilitate the opsonization of microbes?
- antibodies bind microbe then bind to FCyR on phagocytes
- can trigger phagocytes
- common for dealing with bacteria
56
what is antibody dependent cellular cytotoxicity?
- antibodies binds microbes then binds FCyR on NK cells which then kill the microbe
57
how do antibodies facilitate the phagocytosis of microbes?
- antibodies bind microbe activate complement which targets microbe to phagocytosis
- through C3b - phagocytes have C3b receptor and can form the immune complex
58
how do antibodies facilitate inflammation?
- antibodies bind microbe activate complement that recruit other immune cells
59
how do antibodies facilitate lysis of microbes?
- antibodies bind microbe and activate complement to kill the microbe
60
what does the B cell recognise?
- lock and key approach. can recognise
1. continuous epitope (amino acids in one long line)
2. discontinuous epitope (shape made when the molecule is folded)
61
why is B cell recognition important to consider in experiments?
- for instance if the proteins been denatured
- if the antibody doesn't recognize it this could be because it recognizes a discontinuous epitope
- therefore the experiment would fail
62
what molecules does the BcR have?
- Iga and IgB molecules
63
what happens when an antigen binds to the BcR?
- causes a change in the cytoplasmic region which trigers the phosphorylation of co-receptors Iga and IgB and transmission of signals
64
how can BcR signal be amplified through co-stimulatory molecules?
- complex of CD19/CD81 and a complement receptor
- works with BcR for activation
- provides costimulatory amplification
65
what is complement through BcR activation?
- C3 can be cleaved into C3d
| - combine with complement receptor 2 which amplifies signals into the cell
66
what happens once an antigen has bound to the BcR?
gets internalised and turned into peptides
67
why are CD4 T cells needed for activation?
- vital for activation and differentiation of B cells into plasma cells
- peptides are put on the surface of the B cell wil MHC II
- activates the CD4 T cell which releases cytokines
- causes B cells to proliferate, class switch and differentiate into plasma cells
68
what is the role of CD4+ Tfh cells?
unique cells essential for activation, class switching and differentiation of activated B cells into plasma cells
69
what cytokines are involved into CD4 Tfh?
- IL-6 causes naive CD4 T cells to differentiate in Tfh
- IL-6 - STAT3 - activates TF Bcl-6
- produces the cytokine IL-21
70
where does B cell activation occur?
in secondary lymphoid tissue
71
how does B cell activation occur?
- initiated at junction of the B cell follicle zone and T cell zone
- B cells receiving activation signals enter specilaised germinal centres within B cell follicles
- very few B cells enter the germinal centre
72
what are germinal centres?
- dynamic environments
- clonal expansion of desired B cell (need massive numbers)
- need to change the BcR shape slightly
- need a reassessment of BcR
- class swithcing
- recieves signals to differentiate to plasma cells
- generate memory B cells which lie in wait for re-infection
73
how is the shape of BcR changed slightly?
- refining of antigen-binding site (somatic hyper mutation)
- first B cells triggered - not as high affinity
- when it changes slightly you could change specificity for your own cells/tissues
74
what happens if the BcR recognises your own peptides after re-assessment?
- no second chance if it is autoreactive there is no further rearrangmeent
75
what happens to short term plasma cells?
go into the bloodstream and secrete lots of antibodies and then dies
76
what happens long term plasma cells?
- small proprotion
| - go to the bone marrow and secrete small amounts of antibodies
77
what occurs in the dark zone?
expansion
78
what occurs in the light zone?
selection of B cells, the honing and class switching
79
what is the follicular dendritic cell?
central to the B cell response - they act as depots of the antigen the BcR is specific for
80
what is the role of follicular DCs?
- produce IL-6 to help maintain Tfh and IL-21 that acts on B cells to help differentiation and proliferation
- continuously stimulating to BcR to see if its specific for the microbial antigen
81
what is the role of IL-6 in B cell activation?
- binds to CD4 T cells
82
what is the role of IL-21?
- causes upregulation of the chemokine receptor CXCR5
- reacts specifically with the chemokine in the germinal centre
- CD4 T cell activated will now move into the germinal centre
83
what is somatic hyper mutation?
- enables antigen binding site to become more specific with each encounter with the same antigen
84
what does somatic hyper mutation enable?
mutations in the hypervariable region enable a tighter fit between the BcR and
antigen
- hones the antigen binding site further
85
what happens to B cells that have undergone somatic hyper mutation?
have BcRs re-screened to determine if the mutations generated lead to an autoreactive BcR if so the B cell dies
86
what is Activation Induced Deamnase (AID)?
- important in initiating somatic hyper mutation
| - de-animates cytosine to uracil
87
what happens when you transcribe the BcR with RNA polymerase II?
- end up with free ssDNA
| - AID capable of binding the ssDNA
88
how can uracil be processed?
- replication (basically ignores it and causes transition mutations
- basic residues to form and then edit (processed by uracil DNA glycosylase)
- cause mutations to occur later on in the AT sections closer to the uracil
89
what does the Fc region lead to?
lead to very different antibody structures and function, each class has their own function
90
what are the different classes of antibodies
- IgM
- IgD
- IgG
- IgE
- IgA
91
how does the immune system decide which class of antibdoy?
- what is the infection
| - where is the infection
92
why is AID active in the germinal centre?
to facilitate class switching
93
what are the switch regions?
- mature B cells express IgM and IgD first
- when rearranging and transcribing you get ssDNA
- AID binds to switch regions
- AID makes it a better antibody and switches the antibdoy
94
how does AID facilitate class swithcing?
```
causes a hair pin loop and brings the switch region from IgM and the switch region form whatever class of antibdoy you want to produce close togehter
- joins them
slashes out all the intervening sequencing
```
95
what does the FcYRIIB do?
- assess how much free IgG you have in your body
96
how does eradication work when the antigen levels begin to drop?
- start having too much free IgG specific for the pathogen that has not got saturation for its antigen binding site
- signal for the B cell to start sucking up the IgG#- - binds to the receptor, negative transmission to de-phosphorylate everything thats switched on
- effectively switches the B cell back off
97
what happens if theres a mutation in FcYRIIB?
- linked to autoimmune production of auto-antibodies to host cells
- cant switch off activated B cells
- have a strong response to own cells
98
describe primary v secondary antibody response?
- maturation of humoral immune response is characterised by Ig with increased affinity for its cognate antigen and switching of the Ig class
- secondary - B memory cells - much quicker
99
what is IgM?
- first youll produce in an immune response
- a low affinity antibody
- key role in protection during an immune response
100
what is the structure of IgM?
- forms a penatmeric structure with 5 immunoglobulins held together by a protein called the J chain
101
how does IgM make up for its low affinity?
has 10 antigen binding sites which generates a strong signal
102
what is IgG?
- high affinity antibody that has a key role in memory immune responses
- most common immunoglobulin in serum
- can cross the placenta (protect developing baby)
- generated following class switching of IgM
103
what is IgA?
- small amounts in the blood
- found in mucosa tissues, tears and breast mile
- neutralises toxins - in food and in drink
104
what is the structure IgA?
exists as a dimer two monomeric immunoglobulin units held together by a J chain
105
what is IgE?
- least common in serum
- binds to sepcilaised Fc epsilon receptors on mast cells, basophils and eosinophils
- strongly involved in allergic reactions and clearance of parasitic infections
106
what is the structure of IgE?
- a monomer
| - has an extra constant domain C4
107
how do you get allergic reactions?
- influenced by genes
- individuals that are pre-disposed to developing allergies are atopic
- have higher circulating levels of IgE
- many genes thought to play a role in susceptibility to allergies inherited
- causes degranulation of mast cells and basophils that have epsilon receptors
108
how does IgE affect asthma?
IgE activates mast cells to release inflammatory mediators
109
what are the features of asthma?
- bronchospasm, swelling and airflow obstruction
- get inflammation, obstruction and hyper responsivenss
- can be a genetic pre-disposition
110
what are the to hypothesis of asthma?
1. hygiene hypothesis: the cleaner we are the more susceptible to allergies we are
2. infectious-asthma hypothesis: infections/microbiome is different
