31 - B cell: Generation of TCR diversity, Ig isotopes Flashcards
TCR structure
Composed of ⍺& β chain
variable region contain Ag binding site
Each has a variable & a constant region
* ⍺ chain locus – multiple V & J segments
* β chain locus – multiple V, D, J segments
- Combinatorial diversity of TCR:
o VJ in ⍺ chain
o VDJ in β chain
D-J recombination 1st
V-DJ recombination 2nd
o Combination of ⍺& β chain
Somatic recombination takes place in… & is…
thymus & irreversible
Combinatorial diversity of TCR:
VJ in ⍺ chain
VDJ in β chain
-D-J recombination 1st
-V-DJ recombination 2nd
Combination of ⍺& β chain
V(D)J recombination of TCR similariety steps to BCR
- V, D, J segments flanked by RSS
- RAG-1/2 recognize the sequences & bring them tgt
-singla & coding join - Artemis cuts the DNA hairpins
- TdT adds non-coded nucleotides in joining regions
V(D)J recombination of TCR differences steps to BCR
BCR: Ig heavy chain = D segment surrounded by 2 RSS
-Both with 12-bp spacing
TCR β chain = D segments have 5’end 12 bp RSS & a 3’end 23 bp RSS
12-23 base apir rule still apply
TCR CDRs
3 CDRs par chain
Sites with most diversity
CDR3 = particularly important source of diversity
-Encoded in D & J segments of β chain & between V & J segments of ⍺ chain
CDR 1&2 = encoded within v segments of ⍺& β chain
allelic exclusion definition
ensures that each B cell synthesised only 1 allele for a heavy & 1 allele for a light chain
reason why only 1 allele of heavy chains & 1 allele of light chain is expressed on each B cell
All BCR of B cell =
Each B cell =
- same
-has its own specificity
how does allelic exclusion really work
Once a BCR is expressed on surface of developing cell = send signal to silence the part of gene that codes for the other chromosome
Other chromosome becomes methylated & inaccessible to transcription machinery
Genomic silencing of the chromosome ensures each B cell only express same copy of BCRs that have same specificity
So while B cells and T cells differ distinctly in their functions during an immune response, the two lymphocyte subsets share the enzymatic machinery and overall scheme for generating the antigen receptor diversity.
This is because…
B cells and T cells both need enormous antigen receptor diversity to provide protection against the diversity of pathogens.
Ig isotypes:
Mature naïve B cells
Express transmembrane IgM & IgD
IgM & IgD = part of 1st wave of secreted Ab
Initially= newly formed B cells express IgM as their primary BCR
Later, some B cells switch to expressing IgD
-Result in IgM & IgD being co-expressed on B cell surface
Upon activation by signals 1 &2 = some form primary focus & becomes plasmablast = capable of secreting Ab
How does B cell start secreting Ab & how does it change the production of Ig subtypes/isotypes?
How does B cell start secreting Ab & how does it change the production of Ig subtypes/isotypes?
alternative RNA splicing
Alternative RNA splicing
Remove some of constant regions
B cells produce long primary mRNA transcript differentially spliced to yield either of 2 distinct mRNA molecules
Secreted & transmembrane mRNA = result of alternative RNA splicing as well
Ig classes
5 isotypes of heavy chains
o M – µ
o D – δ
o G – 𝜸
o A - ⍺
o E – ε
Depending on H chain of (constant region) isotype = 5 different classes of Igs
o IgM
o IgD
o IgG
o IgA
o IgE
There are subtypes of IgG & IgA classes
How do we switch from producing the standard IgM & IgD to other subtypes?
Secondary lymphoid follicle
(germinal centre)
what happens in germinal centre
B cells undergo processes to plasma cells to produce Ab that are more effective, because of :
o Somatic hypermutation
o Affinity maturation
o Class switching
secondary diversification
In germinal centre – after B cells has received signal 1 & 2 AGAIN
Somatic hypermutation
Class switching
Mechanisms act on already rearranged Ig genes
-V(D)J recombination has occurred in the variable region
-Primary diversification already taken place
Irreversible
secondary diversification:
Somatic hypermutation:
-Higher affinity for tis Ag
-Binding = better/effective
-Specificity is same
Operated on activated B cells in peripheral lymphoid organs
-In germinal center of lymph node
High rate of point mutations in V gene sequences that improve Ag binding
-Not specific
Affinity maturation – select for survival of mutated B cells that have high affinitiy for the Ag
-Survival of the fittest = only B cell with most affinity to Ag will survive
Occurs during secondary/tertiary responses (faster/effective)
-Get higher affinity Ab
secondary diversification:
Class switching
process that replaces 1 heavy chain constant region with one of a different isotype
Only occurs after B cell activation
Irreversible
Class switch recombination guided by switch regions located upstream of each C gene
Cytokines secreted by TFH in germinal center will inform class switching
-Cytokines secreted doesn’t go inside cell, It binds to receptor - intracellular signaling
example: Type 2 response cytokine IL-4 induces IgE
diversity in molecules of human adaptive immunity:
HLA molecules
Two main classes:
* Polymorphism
* Polygeny
diversity in molecules of human adaptive immunity:
TCRs
Combinatorial diversity
Junctional diversity
Paring (alpha & beta)
diversity in molecules of human adaptive immunity:
BCRs
Primary diversification
* Combinatorial diversity
* Junctional diversity
* Combination of H & L chains
Secondary diversification
* Somatic hypermutation
* Class switching
highlights of class switching
- C gene in between nicked switch regions are removed
-constant region encoding IgG, IgA, IgE are downstream of C regions encoding IgM and IgD
-Aid protein play a role in class switching
