Lecture 1- 6 Flashcards
What must the immune system do for an effective response?
- be able to recognise and respond to any invading organism
- not over react to benign or self
- be able to direct different effector mechanisms against different pathogens
What are the features of specific/adaptive immunity?
- mediated by lymphocytes (B/T cells)
- clonally distributed receptors = recognise different pathogens
- response takes time but memory cells are produced
What do antibodies do?
- activate the complement system = opsonisation -> classical pathway activation -> activation of effector cells
What is the role of TCR ( T-cell receptors)?
recognizes peptide fragments of antigen bound to MHC expressed by APC
What is the role of BCR (B-cell receptor)?
membrane form of Ig that binds free antigens = secreted when B cell is activated
What is the structure of antibodies?
- formed from 4 polypeptides
- 2 identical heavy chains and light chains held together by covalent and non-covalent bonds
- 2 types of light chain = lamda and kappa
- each chain has a variable region and a constant region
What is the function of the V and C region in antibodies?
- paired variable region (V) = form 2 identical antigen-binding sites
- constant (C) region = responsible for Ab structure and interacting with other molecules
What are the 5 antibody classes and how is the isotope determined?
- IgM
- IgD
- IgA
- IgG
- IgE
isotype is determined by the heavy chain (C region)
How many domains does the H and L chain on an antibody have?
L chain = 2 domains
H chain = 4 or 5 domains
What are antibody domains and what do the comprise of?
domains = patterns present in many other proteins in the immune system
comprises of two beta sheets linked by disulphide bridge
What is an epitote?
the piece of antigen that antibodies recognise
What are the characteristics of MHC class I?
- expressed on all nucleated cells
- heterodimer: alpha chain (42 kD) and b2-microglobulin (12 kD)
- 3 different types of MHC class I molecules
What are the 3 MHC class I molecules?
HLA-A
HLA-B
HLA-C
What are the characteristics of MHC class II?
- expressed only on APC and cytokine-activated cells
- heterodimer: alpha chain and and beta chain
What are the 3 MHC class II molecules?
HLA-DP
HLA-DQ
HLA-DR
What is the process by which enough cells expressing receptors for the correct pathogen is known as?
clonal selection
What are the 2 chains of an antibody called?
heavy chain and light chain
Which part of an antibody interacts with the epitope of an antigen?
two identical antigen binding sites mainly CDRs
Define the Fab and the Fc regions of an antibody?
Fab is (VH/CH1+ VL/CL1) domains (artificially generated with papain)
Fc is ((CH2/CH3/+ (CH4 sometimes)) x2 regions of antibody that binds to FcRs;
(also artificially generated with papain)
What is the antigen binding site of an antibody made up of?
variable domain of H and L chains
The lamda and kappa chains form what chain of the antibody?
light chain
How many hypervariable regions are involved with antigen binding in a single antibody molecule?
12
What type of proteins are TCR?
heterodimeric proteins
What are the most common T cells?
T cells that express gamma and delta TCRs
What class of MHC binds smaller processed peptides?
Class I MHC in the endoplasmic recticulum
What are the H chain and TCRb encoded by?
V region encoded by 3 gene segments = V, D and J
What are the L chain and TCRa encoded by?
V region encoded by two gene segments: V and J
When and why do genes rearrange?
genes rearrange during B cell development to form a functional gene
What is the gremline organisation of L and H loci in the human genome?
lamda light chain = one of the variable genes on the light chain locus randomly joins to the J region and then joins to constant region
kappa light chain = V region genes and J region genes randomly join together
heavy chain = take a D and J and then a gene segment and bind randomly with a B region and constant region
Which gene segments encode the variable region of the Ig heavy chain?
VDJ
Which gene segments encode the variable region of the light chain of an
antibody?
VJ
How are the heavy (H) chain of an antibody and the beta (b) chain of the TCR
similar?
Similar gene fragments (VDJ)
When does rearrangements of Ig genes occur?
during their development in the BM when they are termed pro and pre-B cells
Does the heavy or light chain constant region determine an antibody’s isotype?
heavy
What guides DNA rearrangement?
guided by special sequences flanking the V, D and J regions = recombination signal sequences
What do the recombination activating gene encode?
RAG-1 and RAG-2 genes encode lymphoid specific components of the recombinase
What are the 5 distinct mechanisms that allow for antibody diversity?
- Families of gene segments that re-arrange i.e. combinatorial
- junctional diversity (not precise joining)
- N region addition
- heavy and light chain pairing
- somatic hypermutation
What is allelic exclusion?
In a single B cell, only one allele of H and L chain is expressed to produce a cell with specificity to a certain antibody
What is somatic hypermutation?
AID acts on DNA to de-aminate cytosine to uracil = is recognised by error-prone DNA repair pathways leading to mutation
What is the difference between the arrangement of B and T cells?
T cells = occurs in thymus
B cells = occurs in bone marrow
What are the most abundant T cell receptors?
TCR-alpha and TCR-beta
How does MHC polymorphism affect their function?
occur in the domains that fold to form
the peptide-binding groove so will increase the no. of possible peptides that are suitable for
binding, allowing the human population to be able to bind as many different peptides as
possible
How do peptides end up on the surface of cells bound to MHC class I and/or class II?
- peptides from protein Ag synthesized in cytoplasm = presented by MCH class I
- peptides derived from exogenous Ag = presented by MCH class II
What is TAP?
a component of a multi-protein assembly = peptide loading complex
How is an antigen (Ag) processed and presented by an MHC class I molecule?
1) Ag (e.g. viral protein) synthesised in cytoplasm
2) protein cleaved to peptides by proteasome
3) peptides transported to endoplasmic reticulum by TAPtransporter
4) peptides bind to MHC class I molecules
5) MHC-I/peptide complex then transported to cell surface
How is an antigen processed and presented by a MHC class II molecule?
1) Ag (e.g. bacteria) endocytosed into intracellular vesicles inside the cell
2) protein cleaved to peptides by acid proteases in vesicles
3) vesicles fuse with vesicles containing MHC class II molecules
4) peptides bind MHC class II molecules
5) MHC-II/peptide complex then transported inside vesicles to cell surface
Which gene segments encode the variable region of the TCRa chain?
TCRVa/TCRJa
What does the CLIP protein do?
prevents peptides in ER from binding class II
What is the role of the invariant chain?
prevents peptides in ER binding class II
AND targets class II to endocytic compartment
Is the proteosome involved with MHC class I or MHC class II peptide processing?
MHC class I
Do MHC class I molecules present peptides from exogenous sources?
class I MHC molecules present peptides derived from endogenous sources (self-peptides or viral antigens).
What function does the protein encoded by the HLA-DM genes perform?
removes CLIP from MHC class II molecules allowing peptides from Ags present in the endocytic pathways to bind class II
What type of MHC molecule do HLA-A, B and C encode?
MHC class I molecule
Can a single individual express more than one type of MHC molecule on the surface of their macrophages?
Yes. MHC genes are co-dominantly
expressed = an APC such as a macrophage found in an MHC hetereozygous individual can express up to 12 MHC molecules (6 class I and 6 class II) at the
same time
What is the process of B cell development?
develop from haematopoietic stem cells in bone marrow that express PAX5 transcription factor which involves re-arrangemnt and expression of Ig genes
What are the steps for the formation of pre-B cell receptor?
- rearrange heavy chain first and then checked out by Vprebeta and lamda products to make sure it is a proper heavy chain = pre B cell receptor
- Light chain is rearranged and binds to heavy chain
- Becomes a proper B cell
What is the role of the pre-B cell receptor?
Delivers signals to pre-B cell with no antigen requires
What are the steps for pre-B cell receptor signals?
- signal turns off RAG-1 & RAG-2 genes
- 5-6 rounds of cell division
- surrogate light chain expression stops
- RAG-1 and RAG-2 turned on again
- L chain rearrangement starts
Most predominant surface Ig (BCR) expressed by immature B cells?
IgM
What happens to immature B cells that bind multivalent self-antigens?
- clonal deletion = cell dies by apoptosis
or
- receptor editing = further light chain gene rearrangements of variable regions
If BCR expressed by immature B cell binds to self cell-surface antigen, the B cell is removed
by which type of selection?
deletion/negative selection
Activated B cells in the periphery can develop into?
plasma cells, memory B cells
Which Ig chain is re-arranged first?
heavy chain
What is the surrogate light chain?
non-variable light chain-like molecule that binds
rearranged heavy chain to check heavy chain has assembled correctly
Which gene segments get re-arranged in the heavy and light chains?
VDJ and VJ
What is the difference between T-cell development and B-cell development?
T cell
- undergo development/selection in thymus
- alternative lineages
- rearrange ab TCR genes or gd TCR genes
What do the RAG1 and 2 genes do and when?
form a complex to break B/T cell DNA to
allow gene segments to be rearranged during B/T cell development in the bone marrow (B
cells) and thymus (T cells)
What are the steps for alpha/beta T cell development?
- re-arrange TCR genes (beta first) and express TCR
- acquire other markers e.g CD3, CD4 and CD8
- undergo positive and negative selection
What are the Cells other than thymocytes that are present in the thymus?
epithelial cells and APC
What is the pre-TCR?
re-TCRa protein bound to rearranged TCRb = Checks assembly of b chain
What does the CD3 molecule do and why is it important?
Multi-component signalling part of TCR = leads to T cell activation following peptide/MHC antigen binding
What type of T cells do not express CD4 or CD8 markers?
gamma delta T cells
What is positive and negative selection of T cells?
Positive selection = TCR must recognize self MHC alleles in thymic cortex
Negative selection = TCR binds
self MHC and peptide too strongly in thymic medulla will die
If a T cell recognises peptide and MHC I which marker does it have on its cell surface?
CD8
Are the criteria for positive and negative selection of immature T cells the same?
Similar; both involve
ability to recognize self MHC molecules, but the affinity of TCR for peptide/MHC forms the basis of the differences
Where do T cells and APCs contact each other?
T cell zone of secondary lymphoid tissues eg. LNs
What are the two types of T cells and what are their roles?
- cytotoxic effect T cells (CD8+) = kill infected cells
- helper effector T cells (CD4+) = secrete cytokines
How are APC (antigen presenting cell) activated?
Following the binding of PAMPs present in pathogens PRR expressed by APC will signal to induce the activation of APCs
How do T cells get to where they need to be?
- enter the lymph node from blood via high endothelial venules
- move into T cell area which is rich in dendritic cells and macrophages
- APC present antigen and deliver other activation signals
What is the effector role of a CD8+ T cell?
Cytotoxic T cell
What are high endothelial venules?
Specialized vascular pathways into secondary lymphoid tissues from the blood
What happens to a naïve T cell that is not activated in a lymph node?
It will leave and re-enter the blood and then re-enter the Lymph Node again
What is a CAM?
surface molecule on a cell that binds another CAM on another cell to allow the cells to transiently adhere to each other
List 3 types of cells that a T cell might interact with via a CAM
HEV cell, APC, infected target cell
How do T cells contact antigen presenting cells (APCs)?
T cell contact APCs using CAMs
- TCR scans APC peptide/MHC complexes
- recognition = signal from TCR complex
- increases affinity of CAM interactions
- T cell divides
- progeny differentiate to effector cells and leave lymph node
How many stimulatory signals are needed to activate a T cell?
3 (TCR, CD28, cytokine receptor)
What is CTLA-4?
Co-inhibitory molecule expressed on activated T cells
What does B7 on an APC bind to on a T cell?
CD28 and CTLA-4
How are APC activated?
via PAMPs binding to APC’s PRR
Why is the cytokine IL-2 important?
induces both CD4+ and
CD8+ T cell proliferation
Do activated T cells express a high or low affinity IL-2 receptor on their surface?
high
What are the 3 antibody functions?
- neutralisation = antibody prevents bacterial adherence
- opsonization = antibody promotes phagocytosis
- complement activation = antibody activates complement = enhance opsonization and lyses some bacteria
What is signal 1 that induces B cell activation?
Ag binding BCR
Define a TI Ag B cell response?
Non-switched (IgM), low affinity response usually induced by an antigen that
binds BCR and another B cell surface receptor (e.g a TLR)
How does the delivery of signal 2 in a TD response differ from that delivered by a TI Ag?
TD signal (via T cell produced cytokines or their expression of CD40L) leads to B cell undergoing
somatic hypermutation and class switching
Why are MHC class II molecules essential for a B cell
response to a TD Ag?
TD B cell response require CD4+ T cells which are activated via MHC class II
What is CD40, and which cell type is it usually expressed by?
Signalling molecule that can provide signal 2 to a B cell (or activate a macrophage) via interaction with T cell CD40L
What are the roles of Iga and Igb once B cells leave the bone-marrow?
Delivers signal 1 (Ag binding the BCR) to B cell
What does ITAM stand for?
immunoreceptor tyrosine-based activation motif
What is a centroblast and where is it found?
proliferating Ag-specific B cell found in dark zone of germinal centre in secondary lymphoid tissue
What is a centrocyte and where is it found?
non-proliferating Ag-specific B cell found in light zone of germinal centre in secondary lymphoid tissue
How can signal 1 be increased/enhanced?
- if an antigen has activated complement cascade
- lots of C3b
- complement receptor 2 on B cell surface
What are follicular T helper cells (TFH) and where are they usually found?
specialized CD4+ T cell sub-set found in light zone of GC that recognize peptides/MHC that centrocytes compete to present.
How do follicular dendritic cells (FDC) differ to DC1, 2 and DC3?
FDC are only found in secondary lymphoid tissues and do not express MHC class II molecules = do not process and present antigen to CD4+ T cells
Which BCR isotypes can be co-expressed by a single B cell?
IgM and IgD
