L7 - Genetics of antigen receptors Flashcards
Recognition of pathogens: what two types are there and what do they produce?
Pattern recognition - PAMPS/MAMPS/DAMPS
* receptors are encoded in the germline (inherited)
BCR/TCR - rearrangement of genes within B-cells and T-cells to recognise more specific epitopes
* Somatically generated
* Needs no prior exposure to pathogens
* Great for achieving broad recognition
* Difficult for pathogens to predict and evolve to avoid
* potential for Autoimmunity
B-cell receptor: how is it???
Recruited by FcR interactions of different soluble antibody isotypes
(“whatever that means”)
T-cell receptor: how is it????
Mediated by cell-cell contact or local secretion of cytokines
(“whatever that means”)
T-helper cell 1: what is it used for and what example is there?
Th1 Intracellular pathogens
- Increase macrophage killing
T-helper cell 2: what is it used for and what example is there?
Th2 Extracellular parasites
- Eosinophils, mast cells, IgE (activation (??))
T-helper cell 17: what is it used for and what example is there?
Th17 (Inflammatory) extracellular bacteria/fungi
- Recruit neutrophils
T-follicular helper cell 1: what is it used for?
Localise to B cell follicles to support them
T-regulatory cell: what is it used for?
Tolerance/Immune regulation
T-cytotoxic cell 1: what is it used for?
Killer T cells
Antibody typical structure
2 heavy chains and 2 light chains
Each heavy site forms both the constant region and the hypervariable region, with the two regions separated by a hinge and the two chains joined by disulfide bridges
The light chains only form the hypervariable region and work with the heavy chain to form antigen-binding sites
Antibody/TCR/BCR genes: why are they strange?
They contain V-gene segments and J-gene segments which can recombine to give rise to various manufactured proteins
A typical protein contains a Vₗ chain and a Cₗ chain
Light chains of antibodies: what type of segments in their genes do they have?
- V-segments
- J-segments
Heavy chains of antibodies: what type of segments in their genes do they have, how does rearranging occur, what is the exon, where are CDR1/2/3 located, and what gene is the extra gene segment found?
V, D, and J segments
Two gene arrangements occur sequentially,
D → J and then V → DJ
VDJ = V exon
CDR1 and CDR2 are contained in V and V, D and J all contribute to CDR3
IgH gene
Combinatorial diversity: how are the high levels of diversity established?
V/D/J (and sometimes H) differential joining results in some of the diversity
Junctional diversity - imprecise joining of gene segments causes lots more diversity
RSS: what are they, what do they do, and what use do they have?
Recognition signal sequences
Border gene segments and enforce the 12/23 rule (recombination can only occur between segments with different spacer lengths - only either 12/23 or 23/12 may be formed)
Allows controlled diversity - helps keep the immune system functional
Molecular mechanics of V(D)J recombination
1 - Initiation:
* RAG1/RAG2 - bind to RSS and initiate a cleavage event, generating a DSB in the DNA, and allowing recombination to occur
2 - DNA-damage recognition and hairpin opening:
* The cleavage results in hairpin structures at the ends of the gene segments which are opened by enzymes and other cellular components and generate free ends which can be ligated
3 - DNA repair:
* TdT and ligase activity repair DNA and finish the recombination
TdT: what is it and what does it do?
Terminal deoxynucleotidyl transferase
Adds nucleotides to hairpins on VDJ recombinations
Combinatorial and junctional diversity: the difference between the two?
Combinatorial Diversity multiple gene segments
any H with any L
Junctional Diversity P-nucleotide addition
N-nucleotide addition exonuclease activity
T cells: what types are there?
Gamma/delta - play roles in immune responses at epithelial surfaces (innate)
Alpha/beta - most abundant (~95%)
B cells/T cells: where do they develop?
B cells - bone marrow
T cells - thymus
T cells
Follow the same recombinant machinery as BCRs
Immunoglobulin vs TCR recombination diversity
Both are assembled by somatic recombination of multiple gene segments - Similar levels of combinatorial diversity
> Greater junctional diversity in TcR;
- D segments read in all reading frames
- TdT activity at all junctions
- TCR diversity is focused on CDR3, which contacts the antigenic peptide
> More J gene segments
- Footprint of αβTCRs docking down onto MHC class I and II peptide complexes
Mature activated b cell gene expression
Only productively rearranged genes are expressed, everything else is excluded
Clonality: what is required?
- Only one antigen specificity to be expressed
