Adaptive Immune System Flashcards
what are different agents of pathogens
viruses, bacteria, fungi and parasites (protozoa and helminths)
What is a BCR
B cell receptor - the surface receptor of a B lymphocyte
What do BCRs recgonise importantly
molecules in their natural conformation
where are BCRs generated
Bone marrow
what happens once BCRs are activated
the clones differentiate into plasma cells which secrete antibody or immunoglobulin
what is immunoglobulin
a soluble version of the BCR
define antigen
What do they interact with
anything that can bind to an Ab
epitopes - small parts of molecules
what are linear epitopes
When antibodies recognise
consecutive amino acids within a protein or peptide
will conformational epitopes always be apparent
they may only appear in the molecules native 3D form
describe the variable and constant regions of an antibody (briefly)
variable: N terminal - interacts a with antigen
constant: recruits effector function
what can the constant region of an antibody recruit/ interact with
receptors on cells such as dendritic cells, macrophages, neutrophils, basophils
and mast cells and can interact with complement.
What are the 3 ways that a antibody works
neutralisation
opsonization
complement activation
Describe the neutralisation caused by an antibody
Blocking the biological
activity of a target molecule e.g. a toxin to
its receptor.
Describe the opsonisation activity of an antibody
Antibody coated antigen interacts with specific receptors (Fc receptors) on various cells, including macrophages and neutrophils allowing them to "recognise" antigen more efficiently. Antibody functions as an opsonin and phagocytosis is greatly enhanced.
If the antibody activates complement, this will add to the opsonisation as well as directly lysing the molecules
Where are T cells generated
generated in the bone marrow but mature in the thymus
Give an overview of what happens to T cells in the thymus
the genes
encoding the T cell receptor (TCR) rearrange to generate clones of T cells with different
receptor specificity.
Can T cells recognise native proteins
No - while being structurally related to Immunoglobulins, T cells can only recognise degraded proteins when they are complexed with MHC molecules on neighbouring cells
what does MHC stand for
major histocompatability complex
what does antigen processing do
generates the peptides for display by antigen
presenting cells
What are the 2 classes of T cell
class I restricted and class II restricted
What do Class I Restricted T cells do
What do they mature into
What are they important for
recognise MHC Class I:peptide complexes and express the co-receptor CD8
Cytotoxic T cells
viruses and tumour cells
What do class II restricted T cells express
What do they do
Where are they predominantly expressed
CD4
Provide help for cytotoxic T cells and B cells
antigen presenting cells (APCs)
Name some APCs
dendritic cells, macrophages and B cells
When can Class II molecules be presented on non-APCs
They can be induced on other cells in the presence of IFN gamma
What are lymphocytes derived from ?
hematopoietic stem cells via a common lymphoid progenitor
What are naive cells
Why are they called this
T and B cells with successfully
rearranged receptors
they have not yet been exposed to their particular antigen and been activated
What do lymphocytes do in primary lymphoid organs
rearrange their receptors
What role do naive lymphocytes take on once released from the primary lymphoid organs
patrol and respond
How many lymphocytes pass through a lymph node each day
2.5x10^10
What % of the lymphocyte pool is recirculated each hour
1-2%
What are the lymph nodes and spleen designed to do
optimise interaction between APC and lymphocytes
Describe the course of lymphatic fluid
lymphatics drain fluid (containing dendritic cells and antigens) from tissue into lymph nodes
Lymph is collected by afferent lymphatics to pass through macrophage lined sinuses - here antigens can be captured and prevented from passing on into the blood
Why is the spleen different from lymph nodes
no direct connections to the lymphatic system
What does the spleen do
collects antigen from the blood and deals with immune responses to blood-borne pathogens
How do lymphocytes enter lymph nodes from the blood
through specialised endothelia (high
endothelial venules)
What do dendritic cells when they arrive at the lymph node from the periphery
enter the T cell area and present the antigen they have captured on MHC
Where do some B cells congregate
in B cell areas around follicular dendritic cells
How do FDCs trap antigens
What happens once the antigen is trapped?
in the form of antigen/antibody/C3b
complexes on their surface
hold it for long periods of time for B cells to screen - B cells with the highest affinity are induced to undergo affinity maturation
What does localisation of
APCs, B cells and T cells in the lymph node do
maximises the chance of successful B-T cell cooperation.
Do all B cells produce antibodies?
only produced by B cells that differentiate into plasma cells
Are Ab always bound to cells
They may be free in plasma, bound to cells by specific (Fc) receptors, or present in secretions such as milk, mucus and sweat
What does the constant domain of the Ab do
allow the
antibody to interact with other immune system effector cells and molecules such as macrophages
and complement and are said to recruit effector function
How are the heavy and light chains of antibodies held together
by covalent (disulphide bridges) and non-covalent forces
What are both heavy and light chains of Ab constructed of
Ig fold domains
light: 1 constant and 1 variable
Heavy: 1 variable, and three (IgG, IgA and IgD) or four (IgM and IgE) constant domains.
How can the main classes of Ig be distinguished
by their heavy chains denoted the Greek version of their Ig letter
Which part determines the function of the Ig
heavy chain
What gives the Ab flexibility
hinge region
V-C junction
What is the VC junction often compared to
a ball and socket joint
What is the antigen binding site formed by
how is variation concentrated
interaction between heavy and light chain variable
domains
three complementarity determining
regions or CDRs
Which is the most variable CDR
CDR3
How do CDRs appear
as 3 loops at the surface, forming a variable surface for interaction with the antigen
What part of the loops of the CDRs do antigens bind to
Antigens can bind in tight pockets, grooves or on extended surfaces which form the antigen
binding site of the molecule.
How many different antibody specificities do humans have?
10^11
What is the first stage of diversity generation in Ab
What is this
somatic recombination
gene rearrangement in B cell development
How many variable gene segments does the human heavy chain locus encode
What are these segments responsible for?
How many gene segments for diversity and for junctional
38-46
variation in CDR1 and 2
D: 23
J: 6
What generates variation in CDR3
rearrangements of VDJ segments
How are heavy chains arranged
1st: D->J
2nd: V–>DJ
What is allelic exclusion
Heavy chain rearrangement can occur on both chromosomes but if a functional heavy chain is
generated then rearrangement on the other chromosome is prevented
How many heavy chains can a single B cell express
only 1
How many diversity segments do light chains have
None
Light chains have no diversity segments. How do they recombine to generate diversity
rearrange kappa chain first then if unproductive rearrange delta chain
What enzymes are involved in somatic recombination
RAG1 and 2
What sequences are conserved in rearrangement during diversity generation in b cells
conserved heptamer, nonamer and spacer sequences located adjacent to
V, D and J segments.
What are the 4 main processes to generate Ab diversity
- Different heavy and light chain combinations
- Selection of different heavy V, D and J segments.
- junctional diversity
- somatic hypermutation
What is junctional diversity
Variable addition and loss of nucleotides at VDJ junctions
What order does junctional diversity occur in
Addition by TdT (to N terminal)
Addition due to the recombination mechanism (P-nucleotide addition).
Deletion of nucleotides
What is TdT
terminal deoxynucleotidyl transferase
enzyme that performs N-nucleotide addition in junctional diversity
What is somatic hypermutation
Point mutations are introduced into heavy and light chain variable regions. This involves deamination of cytosine to uracil by the enzyme Activation Induced (Cytidine) Deaminase
What is AID used in
deamination of cytosine to uracil in somatic hypermutation
What happens to Ab with increased affinity after hypermutation
selected for by affinity maturation
What is the mutation rate of somatic hypermutation
1 per 1000 base pairs
per cell division.
What do T helper cells do generally?
s activate the B cells to produce antibody,
generally of relatively low affinity (IgM)
What is one model for the interaction between B cells and T follicular helper cells?
B cells that by chance have higher affinity for antigen will interact with FDCs bearing that antigen.
They can capture and present the antigen to a T follicular helper cell (TFH). It is these B cells that will receive help from the TFH cell and be stimulated to survive and predominate
mature B cells usually present which Ig isotypes?
How are these generated?
Can they change their isotype?
IgM and IgD
IgM and IgD are coexpressed and are generated by alternative splicing
yes via class/isotype switching
What choice determines whether an Ig will be soluble or membrane bound while still having the same VDJ specificity?
different poly-A sites
What are the 5 human Ig isotypes?
IgM, IgG, IgA, IgD and IgE,
Which Ig isotypes are split into subclasses?
What are the subclasses?
IgG and IgA
IgG1, IgG2, IgG3, IgG4, IgA1 and IgA2
What determines most of the the effector function of an antibody?
Fc receptors
What do Fc receptors do
deliver the antibody to different anatomical
sites and link antigen to molecules or cells that effect its destruction.
What controls recruitment of effector cells
Differential expression
of Fc receptors by the effector cells
What are 2 key differences between Fc receptors
affinity
valency
How do affinity and valency affect Fc receptors
some are of high affinity and bind monovalent
antibody/antigen complexes while others are low affinity receptors and only bind multivalent antibody/antigen complexes.
What is the Kd of a high and low affinity Fc receptor
high: 10^-9
low: 10^-6
Give 2 monovalent Fc receptors
Fc (gamma)RII and III
Give 1 multivalent fc receptor
Fc (gamma) RI
What do the differences in affinity and valency provide for different Fc receptors
different levels of sensitivity to cells expressing the different receptors.
What do macrophages and neutrophils express to aid opsonisation
Fcg receptors which efficiently
recognise the Fc region of IgG antibodies,
particularly IgG1
What happens when the Fc gamma receptors on macrophages and neutrophils interact with antibodies?
leads to
phagocyte activation and results in enhanced
antigen uptake and degradation
Which Ig activates complement very effectively?
Name a receptor that recognises it efficiently?
IgM
the C3b receptor
What is the purpose of binding of antigen/antibody
complexes by C3b receptors present on red blood
cells
allows delivery of the complex to the liver and spleen for removal by macrophages
What is ADCC
Antibody dependent cell mediated cytotoxicity
Name 3 cells which express Fc (gamma) RIII
Natural killer cells,
neutrophils,
eosinophils
(also phagocytes)
What does Fc (gamma) RIII recognise?
it is a low affinity receptor that recognises only Ab/antigen complexes
What does ligation of the Fc (gamma) RIII on NK cells do
triggers release of cytoplasmic granules
containing lytic enzymes.
Can free Ig trigger ADCC
no - requires complexes of
antibody and antigen to provide multiple Fc
regions for recognition.
Which granulocyte mediates a unique type of ADCC?
Describe
eosinophils - to fight helminths
IgE binds to the surface of
worms. It signals through FceRI on the eosinophil to
release granules containing proteins that
are toxic to helminths.
What receptors do mast cells and basophils express and what do they recognise
FceRI and FcgRIII for IgE and IgG.
Why do mast cells and basophils have 2 types of Fc receptor
When is a similar response also elicited
Crosslinking of these receptors signals
release of inflammatory mediators into
surrounding tissues.
during allergic reactions
when allergens bind to IgE on mast cells
How long does degranulation of basophils and mast cells take?
What is released?
seconds
vasoactive amines such as
histamine and lipid and cytokine mediators
of inflammation such as, prostaglandins,
leukotrienes and TNF-a
Which 2 Ig protect mucosal surfaces from infection
IgA and to a lesser extent IgM
How does IgA cross the epithelial layer on mucosal surfaces?
A receptor for
polymeric Ig recognises the J chain region of IgA
and transports it across the epithelial cell.
Which Ig is important in milk for infants
IgA (this is the enzyme that crosses mucosal surfaces)
What do antibodies do in the gut of infants
IgA from breast milk remains in the gut and prevents
attachment of bacteria and toxins to gut epithelia.
Which antibody is important for fetal protection?
IgG
How does IgG cross from the mother to the fetus?
neonatal Fc receptor transports maternal IgG
across the placenta directly into the blood stream of
the fetus and provides high-level protection against
pathogens common in the environment at birth
How do ruminants (NOT humans) acquire IgG
postnatally from colostrum via an IgG-specific
receptor in the gut
How do B cells shut down Ab responses?
Ab produced in response to antigen
binds and forms immune complexes. B cells specific for the same antigen can bind
these immune complexes with both
Fc(gamma)RIIB and the BCR.
This acts as a
negative signal and terminates B cell
responses
Which Fc is important in terminating an Ab immune response?
Fc(gamma)RIIB
How do Fc(gamma) RIIB receptors help control inflammatory responses?
by antagonising the activity of activatory Fc receptors
Which cells express FcgRIIB
many including macrophages, neutrophils, eosinophils and mast cells.
What does the affinity of the Ab refer to ?
e interaction between a single antibody binding site
and a single monovalent epitope on an antigen
How many antigen binding sites does an Ab have
What do they do with these sites?
at least 2
often bind to two epitopes on the same antigen
What is avidity
a measure of the strength
of interaction due to recognition of polyvalent epitopes
Describe the affinity and avidity of IgM
low
affinity but bind with high avidity
What is the Major Histocompatibility complex (MHC)?
a large genetic locus on chromosome 6 that
codes for MHC class I and class II molecules and many other proteins involved in the
processing and presentation of antigens to T cells.
What are MHC class I and II molecules?
cell surface glycoproteins whose function is to present
peptides to T cells
Give an overview of the structure of MHC class I molecules
made from a transmembrane heavy or alpha
chain, which is non-covalently linked to b2-microglobulin.
Describe the heavy chain of the MHC class I molecule
has three extracellular domain a1, a 2 and a 3.
The membrane distal domains (a1 and a 2)
form a peptide binding groove. The base of the groove consists of a beta pleated sheet and two alpha helices form the sides.
What can the peptide binding groove of the heavy chain of MHC class I molecules accomodate?
peptides that are often 8-9 amino
acids in length
What is the size of the heavy chain and the beta2 microglobulin in MHC Class I molecules?
heavy chain: 45kDa
beta2 microglobulin: 12kDa
How does the structure of MHC class II relate to class I molecules?
MHC class II molecules have a very similar structure but are made from two similar sized transmembrane chains (a 33kD and b 30kD).
The peptide binding groove is formed by the a1
and b 1 domains and is supported by the membrane proximal a2 and b2 domain
True or false:
Antibodies and T cell receptors show great binding specificity
How does this compare to MHC molecules?
True
A single MHC molecule in
contrast is able to bind a broad range of peptides composed of different amino acid
combinations
What do the pockets in an MHC class I binding groove do?
What is the point?
interact with amino acid side chains of the
peptide.
It makes important interactions with the amino and carboxy terminus of the peptide
and this places restrictions on the length of peptide that can be accommodated
Can peptides longer than 8-9 amino acids fit into the binding groove in MHC Class I molecules
yes if bent in the middle
How does the MHC class II binding groove differ from that of the class I ?
The class II peptide groove is open ended and can bind longer peptides 13-25 amino acids, in an extended conformation.
Generally, what is MHC diversity due to
polygeny and polymorphism
What are MHC molecules also known as?
Human Leukocyte Antigens (HLA)
What is polygeny in MHC diversity?
the expression of multiple independent loci encoding class I and class II genes
What are the different isotype of each class of MHC molecule?
3 of each
class I :HLA-A, HLA-B and HLA-C
class II: HLA-DP, HLA-DQ and HLA-DR
What are the most polymorphic genes known
MHC genes
How many Class I MHC alleles are there?
How many proteins does this refer to?
13,680 Class I alleles corresponding to 9,341
protein variants
By how many amino acids can MHC alleles differ by?
between 1 and 50
MHC genes are the most polymorphic genes known. Is this normal
This extensive allelic polymorphism is
thought to be pathogen driven and is unique to the MHC.
Describe MHC inheritance
MHC loci are closely linked genetically and, in most cases (97%), are inherited together as sets
of alleles or haplotypes
alleles are co-dominantly expressed
What does it mean to say MHC alleles are co-dominantly expressed
t both maternal and
paternal haplotypes are expressed together on the same cell
How common is it to be homozygous for MHC alleles
In outbred populations
polymorphism at each loci makes it virtually impossible for two individuals to express the same
combination of MHC alleles
Why is it better to be heterozygous for MHC alleles
Heterozygotes can present more peptides and activate more T cells than homozygotes.
Where is polymorphism concentrated in MHC molecules
in the peptide binding domain and the domain that interacts with the TCR
What will the TCR see of the MHC
a combination of a
particular MHC molecule associated with a particular peptide (MHC restriction)
Can a TCR respond to the same peptide presented by different MHC molecules
this is very unusual due to MHC restriction
What are anchor residues (MHC)
conserved amino acids at a certain position of a peptide
they anchor the peptide to the binding groove
What do anchor residues determine
the peptide binding motif of a particular MHC
Why can a single MHC molecule present a range of peptides?
Amino acids at non-anchor positions are not under the same strict constraints and may vary
considerably
What are MHC molecules occupied by in the absence of infection?
self peptides
Why does peptide presentation require a balance in binding characteristics? (2)
- If a large number of different peptides can bind only a few copies of any particular peptide-MHC combination will be presented at the cell surface.
- If a small number of different peptides can bind, many copies of the same peptide-MHC will be present at the surface. However pathogens with a small genome may not have a suitable peptide for presentation by the host’s MHC molecules
Typically a cell with 100,000 MHC class I molecules of a single allotype presents how many different peptides?
1000
What fraction of random peptides are able to bind a particular MHC allele?
Roughly 1/1,000-1/10,000
An individual peptide-MHC complex can present how many molecules per cell?
What is the mean/
roughly 1-5,000 molecules per
cell
mean=100
What is the range of MHC-peptide complexes that can activate a T Cell
from a single complex up to a few thousand
Describe the genetic associations of MHCs with autoimmune diseases and with infectious diseases
many strong genetic associations with autoimmune disease but
associations with infectious disease are much weaker
Why do we use chickens to explore the development of MHC molecules?
they have a single dominantly expressed MHC class I molecule
Compare promiscuous and fastidious MHC class I molecules in chickens
Promiscuous: bind a wide range of peptides and give more or less protection against a wide range of pathogens.
Fastidious: bind fewer peptides and confer either resistance or
susceptibility to a given pathogen
What happens to promiscuous and fastidious chicken MCHI alleles after infection
After infection with Marek’s Disease Virus (MDV) the percentage of promiscuous chicken MHCI haplotypes (B21 and B2) in the flock increases substantially.
Fastidious chicken MHCI haplotypes (B12-B19) are greatly reduced or eliminated
Why would fastidious chicken MHCI haplotypes be reduced after MDV infection?
they were unable to find viral peptides to
present
How have MHC moleules developed to counter both intra and extra cellular infections?
MHC class I molecules present peptides from endogenous proteins e.g. viral proteins. MHC class II molecules present peptides from exogenous proteins e.g. bacterial proteins.
What are MHC CI molecules designed to do?
present peptides from within the cell to CD8 cytotoxic T
cells
What primarily degrades proteins within the cytosol
What happens to the products?
proteasome
moved to ER via TAP
How are peptides from the proteasome moved to the ER
What happens to the peptides in the ER
via the Transporter associated with Antigen Processing (TAP)
Peptides of suitable length and sequence are loaded onto partially folded class I molecules in a complex process that is assisted by a range of chaperone proteins assembled into a Peptide Loading Complex
What happens to fully loaded MHC peptide complexes in the ER
they are released from the chaperones, pass through the golgi, and follow
the secretory pathway to the cell surface.
How can the Peptide Loading complex (PLC) be visualised practically/
by single-particle electron cryo-microscopy
Describe the MHC Class I antigen processing pathway
virus infects cell and antigen enters proteasome
- -> peptides from proteasome go to ER via TAP
- -> Suitable peptides are loaded on to partially folded MHC CI by PLC
- -> fully loaded MHC CI released and passes through golgi to cell surface
True or false
MHC Class I molecules have to pass through the ER during biosynthesis
False
MHC class II (NOT 1) molecules have to pass through the ER during their
biosynthesis.
What prevents MHC CII binding to peptides as it passes through the ER
a/b chains associate with a third
component invariant chain (Ii)
Give the 3 actions of the invariant chain of the MHC CII molecule as it passes through the ER
a) blocks the peptide binding groove
b) acts as a folding chaperone
c) targets class II/Ii complexes into the endocytic pathway.
Where do the MHC CII molecules pass after the ER
Golgi to be sorted un to the endocytic pathway
What happens to MHC CII in the endocytic pathway
Invariant chain is partially removed, leaving CLIP in the binding groove.
within the MIIC compartment CLIP is removed and the appropriate antigen derived peptides are loaded on to empty MCH CII with help from HLA-DM
What helps loading of appropriate antigen derived peptides onto MHC CII in the endocytic pathway
the class II related molecule HLA-DM.
What is the path of the MCH CII molecule after it has been loaded in the MIIC compartment during the endocytic pathway?
MHC class II/peptide complexes are transported to the cell surface for presentation to CD4 T cells, but the pathway used is unknown
Does the MHC locus only have Class I and II genes
no the MHC encodes other components of the antigen processing machinery: HLA-DM HLA-DO 2 LMP components of the proteasome TAP1 and 2 and TAPBP
What are the 7 genes included in the MHC that are not the classical Class I and II genes
HLA-DM
HLA-DO
2 LMP components of the proteasome
TAP1 and 2 and TAPBP
What are HLA-DM and -DO involved in?
regulating peptide presentation by MHC class II molecules.
What is TAPBP
TAP Binding Protein (aka Tapasin) -Like TAP1 and 2, is involved in peptide generation, peptide transport and peptide loading onto MHC class I molecules.
The genes for TAP1, TAP2, and TAPBP are located near which MHC class region?
Expand
MHC Class II
This is surprising because they are involved in peptide generation, peptide transport and peptide loading onto MHC class I molecules (NOT CLASS II)
Are T cells important in humoural or cell mediated adaptive immunity?
both
How are the main 2 subsets of t cells distinguished?
d by
expression of different co-receptor molecules CD4 and CD8, their effector function and MHC
restriction
What is the TCR encoded by
wo polypeptide chains each containing a constant and a
variable region
What are individual domains of a TCR related to in the Ig?
the Ig fold domain
What is the immunoglobulin superfamily?
Molecules containing Ig like domains are members of the immunoglobulin superfamily.
Give the 4 key ways TCRs differ from BCRs
the TCR:
a) is monovalent
b) is membrane bound and does not have a secreted counterpart
c) does not undergo somatic hypermutation
d) is used solely for antigen recognition and is not linked to effector
function
What are the stages of TCR rearrangement
1) Germline rearrangement
2) beta chain rearrangement
3) alpha chain rearrangement
4) non-template P and N nucleotides added to both alpha and beta chains
describe germline reshuffling in TCR diversity generation
RAG1/2 stimulated by thymosin etc in thymus to shuffle DNA for TCR (different VDJ arrangement in α and β chains) and for CD8 and CD4
What are the enzymes stimulated to reshuffle TCR DNA? What molecule stimulates them?
RAG1/2 stimulated by thymosin etc in thymus to shuffle DNA
What happens in the beta chain reshuffle stage of TCR diversity generation
Dβ->Jβ, then Vβ->DJβ.
This is then presented on the surface with the surrogate α. This is double negative stage when both CD4- and CD8- are expressed
What happens with non template nucleotides in TCR diversity generation?
final stage
o Nontemplate P and N nucleotides are added to both β-chain (VDJ) and α-chain (VJ) junction and αβ chains are displayed = double positive – selection can now begin
Are RAG1 and 2 used in BCR diversity?
Yes used in both TCR and BCR diversity generation
After rearrangement of the TCR DNA what will the exons now code for
a leader sequence, a VDJ region, a constant region
and a transmembrane region
What is the most variable region of the TCR
What does it do
Where is it focussed?
CDR3
CDR3 makes
the major contact with peptide present in the
MHC groove.
VDJ junction
What are the 3 things T Cells are selected for?
1) successful β chain rearrangement
2) positive selection
3) negative selection
What is the aim of T cell selection
to “eliminate the harmful and reject the useless”.
What % of double positive T cells survive
2%
What does negative selection of T cells do
prevent autoimmunity by deleting autoreactive cells
What does positive selection of T cells do
ensures that peripheral
T cells will be useful since cells that cannot recognise self MHC cannot become activated.
Do B Cells undergo positive selection
no
Positive selection is unique to T cells.
How does positive selection of T cells work (3 stages)
Newly rearranged T-cell receptors are tested against self
peptide/MHC complexes expressed on cortical epithelial cells.
TCRs with a “moderate
affinity” for self MHC/peptide receive a positive signal to continue maturation. Lack of
interaction results in death by neglect.
Expression of CD4 and CD8 co-receptors is altered to match MHC restriction. Cells that survive will be CD4+ single positive if selected against MHC class II/peptide or CD8+ single positive if selected against MHC class I/peptide.
What happens to thymocytes
expressing TCRs with a “high” affinity for
MHC/self peptide
eliminated from the
repertoire by apoptosis
Which T cells are not affected by negative selection
Negative selection can not
eliminate T cells with receptors recognising
MHC/peptide combinations that are not
expressed in the thymus
Negative selection can not
eliminate T cells with receptors recognising
MHC/peptide combinations that are not
expressed in the thymus. How are these dealt with?
by mechanisms that operate in the periphery
How do T cell precursors progress through the thymus?
bone marrow precursors
enter the outer sub-capsular region and progress towards the medulla
through the cortex.
What cells mediate positive selection of T cells in the thymus
Where are they found
cortical epithelial cells
cortex of the thymus
Describe the cortex of the thymus
densely packed with immature rapidly
dividing T cells (thymocytes). These cells continue rearranging their α chain gene segments
giving multiple opportunities for positive selection
What is the major cause of thymocyte death?
lack of positive selection
Name 3 cell types present in the medulla of the thymus
Medullary epithelial cells
bone marrow derived dendritic cells
macrophages
Which cells are most efficient at triggering negative selection in T cells
Bone marrow derived cells (dendritic cells and
macrophages)
What are macrophages important for in the medulla of the thymus
removing thymocytes that fail to mature
What happens to t cells that survive selection in the thymus
enter the circulation as mature naïve T cells and take up a life of
patrol and respond. They must be activated by a professional APC to become functional.
Can mature naive t cells recognise MHC/peptide complexes?
yes
Mature naïve T cells have TCRs capable of recognising MHC/peptide complexes but require
additional stimulation to become activated
What is the 2 signal hypothesis
signal 1 is
delivered by TCR engagement and co-stimulatory molecules deliver signal 2
What does CD28 do
molecule on t cell which interacts with molecules on the APC
delivers co-stimulation to t cell needed for t cell activation
What is CD40L
a molecule on the t cell that would interact with CD40 on the APC
What is a defining feature of professional APCs?
Expression of costimulatory molecules
name 3 professional APCs
DCs,
Macrophages and activated B cells
Where are dendritic cells found
What do they do
in most tissues
, take up antigen in the infected tissues and
are activated by TLRs and CD40 receptors. They increase MHC class II synthesis and begin to
express co-stimulatory molecules (B7.1 and B7.2) at the cell surface.
What do DCs do once they have their antigen cargo?
migrate to draining lymph nodes where they remain and present antigen to
circulating T cell
How do naive t cells enter the lymphoid organs?
through specialised endothelial cells
high endothelial venules
Do naive t cells remain in a lymph node once they reach one?
no
They continually circulate through lymph nodes and back to the
blood, making contact with thousands of APCs every day
What induces a naive T cell to proliferate
What do they proliferate and differentiate into ?
Binding of
MHC II:peptide complexes by the TCR and CD28 by B7
an expanded population of armed effector T helper cells.
How does a B cell become fully active?
must interact with armed effector T helper cells
Describe the interaction between b cell and helper t cell
B cell binds and internalizes antigen via surface Ig (signal 1)
antigen is processed and presented as MHC class II/peptide complex (this complex is identical to original presented by DC)
CD28 and CD40L on the T cell to bind B7 and
CD40 on the B cell.
Cross-linking of CD40 on the B cell (B cell activation signal 2) promotes B cell proliferation, provides B cell survival signals and promotes immunoglobulin class switching.
Is Interaction between B cell and T cell co-stimulatory molecules important
yes
it is essential and leads to mutual activation (B-T cell co-operation).
What are the co stimulatory elements expressed on b and t cells ?
Which binds to which?
T: CD28/CD40L
B:B7/CD40
CD28-B7
CD40L-CD40
What is the 3rd cell-cell interaction of the primary antibody response
the binding of activated B cells to follicular dendritic
cells
What are FDCs (3)
follicular dendritic
cells
specialised stromal cells that hold intact, i.e. unprocessed, antigen on their
surface in the form of long-lived immune complexes. They are only present in B cell follicles
and enable the activated B cell to form germinal centres.
What has the intrinsic ability to stimulate naïve B cells without the need for
B cell/T cell interaction
thymus independent
antigens (TI antigens)
What are TI antigens typically?
microbial products composed of repetitive
elements (polysaccharides and lipopolysaccharides) which cross-link membrane Ig and induce
B cell proliferation.
What are CD8 T cells destined to become?
What about CD4 t cells?
cytotoxic T cells
Naïve
CD4 (Th0) cells can differentiate into distinct subsets (originally described as Th1 or
Th2), depending upon the nature of the antigenic challenge and the cytokines present during
proliferation
What do Th1 cells do
produce cytokines that support inflammation and cell mediated responses
What is the signature TF of Th1 cells
T-BET
What infections are Th1 cells important for
intracellular pathogens (e.g. mycobacteria).
What cells do Th1 cells mainly activate?
macrophages,
NK cells and CTLs
What do Th2 cells do
activate immune responses that assist in the elimination of
parasitic infections.
What is the transcription factor for Th2
GATA3
What are Th17 cells involved in
transcription factor?
defence against extracellular bacteria and fungi
RORγT
What are Tfh cells?
Follicular helper T cells
essential promoting antibody responses
What is the TF for Follicular helper T cells
BCL-6
What are Tregs
T cell suppressors -suppress the activity of other T cells
Transcription factor for Tregs
FOXP3
When does the differentiation into Th1 or 2 subsets occur?
Why is this important to note
early in immune response
, cytokines
produced by cells of the innate immune system (including dendritic cells, macrophages, ILCs
and NK cells) play a vital role in focusing the immune response
Does amount of antigen affect immune response?
abundance of antigen,
MHC/peptide concentration and T cell receptor affinity also influence the response. Abundant
antigen and high affinity TCR interactions favour TH1 responses and low peptide abundance or
with weak affinity for TCR favour TH2 responses
Why is it important to get Th1/2 decisions right early
bias towards one or the other becomes self reinforcing
Which t cell pathway leads to cell mediated immunity
What does it do
The TH1 pathway
It
increases inflammation to defend against
intracellular bacterial, viral and protozoan infections
What does the Th2 pathway do
humoral immunity
promotes production of pathogenspecific IgE antibodies that work with basophils,
mast cells and eosinophils to control parasite
infections.
What does the Th2 pathway promote compared to Th1
tissue repair
rather than inflammation.
When is t cell lineage decided
initial priming - when pathogen is first detected
What plays a major role in t cell lineage decisions
Cytokines secreted by antigen-presenting
cells and other innate cells (signal 3)
What happens to cytokines involved in t cell lineage decisions
recognised by receptors that activate particular STAT proteins.
The STAT proteins induce the expression of additional lineage defining transcription factors such as T-bet and GATA-3 that reprogram the cell.
What do STAT proteins do
induce the expression of additional lineage defining transcription factors such as T-bet and GATA-3 that reprogram the cell.
Can T cells produce cytokines?
yes - produce cytokines that act to regulate other T cell subsets
What do Treg cells release
What does this cytokine do
TGF-β
maintains homeostasis by inhibiting TH1 and TH2
cell differentiation
What do Tregs do during inflammation in the presence of IL-6
Tregs continue to produce TGF-β.
IL-6 can drive TH17 commitment whilst TGF-β
prevents TH1 and TH2 cells from producing IL-4 or
IFN-γ that would otherwise inhibit the TH17
response.
What cytokine does Th1 produce
What does it do
IFN-γ
blocks both TH2 and
TH17 cell differentiation
Name an IL that blocks T cell commitment for 2 subsets
IL-4 blocks both TH1 and TH17 cell commitment
Discuss relation of ILCs and T cells
ILC1, ILC2, and ILC3 innate lymphocytes show parallels with TH1, TH2 and TH17 helper
cells respectively.
What do ILCs respond to
cytokines produced by other innate cells such as DCs,
macrophages and epithelial cell
Compare ILCs and their T cell counterpart (3)
share the same signature
transcription factors
often produce the same effector cytokines.
ILCs are ready to act
before the T cells.
What do Th1 and its ILC counterpart do
what do they secrete?
ILC1 and TH1 cells help control infections with intracellular bacteria or viruses
IFN-γ
What do Th2 and its ILC counterpart do
what do they secrete?
ILC2 and TH2 cells help deal with parasite and helminth infections
IL-5 and IL-13
What do Th17 and its ILC counterpart do
what do they secrete?
ILC3 and TH17 cells deal with extracellular bacteria and fungi
IL-17 and IL-22
