Antibody Diversity and T Cell-APC Interactions Flashcards
B-CELL-RECEPTOR:
What are the 5 isotypes of the Constant region of an Ig chain?
µ, δ, γ, ε, α
B-CELL-RECEPTOR:
How can constant and variable regions of heavy and light chains be combined?
-The same C(H) and C(L) region can be connected to millions of V(H) and V(L) regions
-The same V(H) can be connected to different C(H) regions
B-CELL-RECEPTOR:
How are heavy and light chains encoded?
-The heavy chains are encoded together
-The light chains are encoded on different chromosomes and separately from heavy chains
B-CELL-RECEPTOR:
What are the different region gene segments?
V segment - Variable
D segment - Diversity -> only in heavy chains
J segment - Joining
C segment - Constant
B-CELL-RECEPTOR:
What is the composition of an Ig?
-Variable and constant region
-2 identical heavy chains [IgM (μ), IgD (δ), IgG (γ),
IgE (ε), IgA (α)] -> C regions: μ, δ, γ, ε, α
-2 identical light chains, kappa (κ) or lambda (λ)
B-CELL-RECEPTOR:
How is the Kappa light chain locus structured?
Light chains include V, J, and C segments
1820 kb on Chromosome 2
-Multiple V(kappa) gene families separated by noncoding gaps
-Downstream of Vkappa genes are four J(kappa) segments
-The C(kappa) segments only have structural functions
B-CELL-RECEPTOR:
How is the Lamda light chain locus structured?
Light chains include V, J, and C segments
1050 kb on Chromosome 22
-Multiple V(Lamda) gene families with noncoding gaps
-each J segment with a particular C segment
-loss of most of the Vλ segments in - only 5% of mouse Igs are lamda
-In humans 40% of light chains are Lamda
B-CELL-RECEPTOR:
How is the heavy chain locus structured?
1250 kb on Chromosome 14
-Isotypes determine the type of Ig -> [IgM (μ), IgD (δ), IgG (γ),
IgE (ε), IgA (α)]
-V, D, J, and C segments -> C regions encode for the isotypes
B-CELL-RECEPTOR:
How are gene segments recognized upon recombination?
-The gene segments are flanked by recombination signal sequences (RSS) -> RAG 1 and 2 slides through the DNA and recognize, cut, and join V, D, J, and C
RAG1 and RAG2 (recombination activating gene encode for RAG1 and 2 -> only in lymphocytes)
-other proteins are required too (Heptamer, Nonamer,..) -> they are not restricted to lymphocytes
B-CELL-RECEPTOR:
What is the difference between the Kappa and Lambda light chains?
They are very similar, slightly different structure
-It provides variability
-Lambda serves as backup, in case Kappa doesn’t work (2 chances for Kappa bc of 2 chromosomes)
TCR:
What are the chains that form T-Cell-Receptrors?
-αβ chains or gamma-delta (γδ) TCR
-αβ is preferred and attempted first, if it doesn’t work it will try gamma-delta
-most T-Cells have αβ chains; gamma-delta (γδ) TCR are less variable and often have other specific roles
TCR:
For TCRs which one is the heavy and which is the light chain?
-αβ: β is the heavy chain with the D segment, α is light
-γδ: δ is the heavy chain with the D segment, γ is light
What is the structural difference between BCR and TCR?
In terms of heavy, light chains, and the epitope binding site:
-BCR has 2 heavy chains and 2 light chains
-BCR has two binding sites that bind the epitope
-TCR has 1 heavy chain combined with 1 light chain
-TCR has 1 binding site for the epitope
-Recombination step (RAG1 and 2) is pretty much the same
INCLASS QUIZ1:
Which TCR chains resemble the BCR lambda chain?
The lambda chain is light, so -> α and γ
Explain the Loop mechanism of VDJ recombination:
RAG1/2 bring RSS sequences of gene segments together and forms a loop -> cleavage at the RSS sequence -> forms a hairpin
Artemis is messy and cuts the hairpin forming overhangs that are filled by repair enzymes generating palindromic nucleotides (sequence and reverse sequence)
Exonuclease don’t like overhangs -> they cut some nucleotides off (trimming) -> gap is filled by RAG1/2
How does the VDJ recombination process contribute to variability?
Due to cleavage and hairpin formation of the gene segments, followed by random addition of nucleotides by RAG1/2
-In heavy chains (not in light chains) of BCRs and both TCR chains, Terminal deoxynucleotidyl transferase Tdt inserts non-template (N) nucleotides next to palindromic (P) nucleotides (or just between the segments)
SUMMARY for possibilities of variability:
-gene segment arrengment
-for BCR: Each heavy chain can pair with a kappa or lambda?? (2 Chromosomes - 2 chances)
-Palindromic nucleotides -> by Artemis
-Exonuclease trimming
-for heavy chains of BCR and both TCR: Nontemplated nucleotides by Tdt
In CLASS QUIZ 2:
Which enzyme is responsible for generating P nucleotides from hairpin cleavage?
-Artemis opens the hairpin and inserts for P nucleotides
(Terminal deoxynucleotidyl transferase Tdt for Nontemplated nucleotides -> just random nucleotides without a template
In which process does the frameshift check occur?
2 chromosome copies the heavy chains and light chain
-Allelic exclusion: we do not want to express two receptors so 1 chromosome is turned OFF the other is turned ON
-> heavy chains first: for TCR β first, for BCR the heavy chain
-> if two attempts fail -> apoptosis
What is the stepwise formation of B/T-Cell receptors?
-In B-cells, the heavy chain is made first and is paired with the surrogate light chain first (SLC) forming a pre-BCR -> if it works (TEST if there is no frameshift, stability test f.e.) the machinery starts again for light chain recombination
-In T-cells, the heavy β chain is formed first and paired with pre-Tα (helps stabilize it) -> if it passes the clustering process and forms a functional CD3 complex -> the α-chain recombination starts
What is the meaning of double negative and double positive in the development of T cells mean?
In Corticomedullary junctions, the Cortex, and Subcapsular Cortex: Double negative
-Double negative (DN) means no CD4 and no CD8 yet
-> Recombination of TCR gene segments yields αβ or a γδ T cell
(during the process the T cells are tested to self-peptide binding - bc random gene arrangement can lead to self-peptide recognition)
-After recombination the cell double positive for CD4 and CD8
In medulla: double positive
-Positive/negative tolerance shifts the cell either to CD4 or CD8
How is the formation of αβ or a γδ TCR ensured?
-With allelic exclusion -> shutting off one chromosome so that only one TCR is expressed - similar to BCR
First shot formation of β and see if it works, if not 2nd shot, if it does -> pre TCR formation with fake α, followed by alpha gen recombination and formation of alpha and check, if it works alpha will replace surrogate pre TCR-α, if not 2nd shot
Which co-receptors (CD4, CD8) are formed during the development phase in the thymus?
Both are formed first, and the cell is tested for correct function at this stage -> Double Positive
-> So this is the positive selection, TEST of FUNCTION -> does it interact with any MHC? Does it interact with a peptide? (hopefully not self -> checked in NEGATIVE selection)
Binding affinity in the selection process:
Positive selection: TCR binding MHC with low affinity to self-MHC are restricted
95% fail here -> don’t receive surviving signal -> apoptosis
Negative selection: TCRs binding with high affinity to self-MHC/peptide is restricted
Why do we want binding to self-MHC at all??? regardless of low or high affinity?? Why are low-affinity self-binding restricted, shouldnt that be good?
