Cell and Molecular Biology of the Immune System Flashcards
What is specific/adaptive immunity?
-Induced by exposure to an infection
-Shows a high degree of specificity
-Exhibits ‘memory’
Effective immune response must…
-Not overreact to benign or self
-Direct different effector mechanisms against different pathogens
-Recognise and respond to any invading organism
Features of specific immunity
-Mediated by lymphocytes (B/T cells)
-Clonally distributed receptors
-Large repertoire - low freq. of cells specific for any antigen
-Response takes time to develop, memory cells produced
2 phases of an immune response
- Innate response
- Adaptive response
What is BCR?
-B cell receptor - antibody
-Membrane form of immunoglobulin binds ‘free’ antigen
-Subsequently secreted when B cell is activated now known as antibody
What is TCR
-T cell receptor
-a chains and B chain
-membrane form only
-doesn’t bind free antibody, recognises peptide fragment of antigen bound to MHC expressed by APC
Antibody structure
-Basic 4 chain structure - 2 heavy chains and 2 light chains held together by covalent and non-covalent bonds.
-2 types of L chains (lambda and kappa)
-Each chain has a variable and constant region
-Bivalent
How many classes of antibody are there
5 - IgM, D, A, G and E
Called isotypes - determined by heavy chain
What is the V region encoded by in H chain and TCRB?
3 gene segments (V, D and J - V is biggest)
What is the V region encoded by in L chain and TCRa?
2 gene segments (V and J)
How do gene segments come together when functional immunoglobulins are produced?
After DNA breaks, a single V and a
single J gene segment are joined
together to encode the V region of
the light chain. This process is random - there are two loci containing L chain genes: lambda and kappa).
Similarly, a single random V, D
and a J gene segment are joined
together in a single B cell
to encode the V region of the
heavy chain
Occur in the same B cell to end up with an antibody with randomly generated chain regions.
What order are Igs rearranged during B cell development?
-H chain gene segments rearrange (D-J, then V-DJ)(greater variability in H chain as V, D and J segments)
-Light chain gene segments rearrange - kappa segments (V-J) first
-If this is unsuccessful, then lambda segments (V-J) rearrange
What is the recombination process?
-Ig gene segment rearrangement is guided by special sequences flanking each of the V, D and J gene segments = recombination signal sequences
-Rearrangement involves a complex of enzymes = V(D)J recombinase
-(recombination activating gene)
-RAG-1 and RAG-2 genes encode lymphoid-specific components of the recombinase
-Mutations in RAG genes result in immunodeficiency
What is allelic exclusion?
-In each B cell, only 1 rearranged H chains gene from one chromosome is expressed.
-Similarly, only one rearranged L chain from one chromosome is expressed by each individual
-Light chain isotope exclusion - each B cell expresses either a rearranged kappa or lambda light chain - never both.
-These mechanisms ensure that each individual B cell produces one randomly generated antibody.
Mechanisms for generation of antibody diversity
- Multiple gene segments for each chain.
- Combinatorial diversity - different V,D, J segments recombine to produce different sequences
- Combinations of heavy and light chains
- Junctional diversity increases diversity further - imprecise joining, N regions
- Somatic hypermutation (SHM)
What is SHM?
-Performed by the enzyme, activation-induced deaminase (AID)
-AID acts on DNA to deaminate cytosine to uracil
-Uracil is then recognised by error-prone DNA repair pathways leading to mutations
What are the antibody classes respective heavy chains?
IgM - mu
IgD - delta
IgG - gamma
IgA - alpha
IgE - epsilon
Membrane (BCR) vs secreted antibody
-Following Ag recognition as each B cell differentiates, it will start to secrete its unique BCR as an antibody
-The secreted form made by each B cell has an alternative constant region that lacks a transmembrane region. As the original re-arranged VDJ regions are not altered, the secreted antibody has the same antigen specificity as the membrane BCR.
-The membrane and secreted forms are produced by alternative RNA processing
Describe heavy chain constant regions
-Encoded by a different C region gene segment (i.e. Cm, Cd, Cg, Ce , Ca)
-4 gamma chain gene segments correspond to the four IgG subclasses; similarly two a chain gene segments
-At the the heavy chain locus, the Cm segment is physically closest to the V, D and J gene segments and so IgM is the first antibody expressed by each developing B cell
What happens after B cells have assembled a random BCR?
-They leave the bone marrow, and their V region genes may undergo SHM in specialized areas of secondary lymph organs following the binding of Ag. and the activation of the B cell
-This leads to further BCR diversity being generated
-These activated B cells start to secrete their BCR (as an antibody)
-Class-switch recombination may then occur, as a result of the cytokines that may be present in this environment
Generation of diversity of TCR
Similar to that seen in BCR:
-multiple V,D,J gene segments
-combinatorial diversity
-junctional diversity
Unlike BCR, TCR is never secreted and no SHM occurs in TCR genes
Why are there such high levels of MHC polymorphism?
-Allows the binding of many peptides that can be presented to T cells - provides a clear evolutionary advantage to the population as can respond to almost unlimited no. of different pathogens
However,
-Increases risk of immune-mediated disease
-Reduces pool of available donor organs
Summarise MHC Class 1 and 2
-Highly polymorphic (diverse at population level)
-Individual has a limited number of different forms
-Influence which peptides can be presented
Ag processing and presentation by MHC class 1 molecules
1) Ag. (e.g. viral protein) synthesised in cytoplasm
2) protein cleaved to peptides by proteasome
3) peptides transported to endoplasmic reticulum by TAP transporter
4) peptides bind to MHC class I molecules
5) MHC-I/peptide complex then transported to cell surface
Ag processing and presentation by MHC class 2 molecules
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
What do MHC class 2 molecules bind to?
Invariant chain in ER - prevents peptides binding in the groove
