Antigen receptors (BCR, TCR)

Question 1

Constant region in antibodies

Answer 1

 μ encodes for IgM
 δ encodes for IgD
 γ encodes for IgG
 α encodes for IgA
 ε encodes for IgE

Question 2

What does VDJ stand for?

Answer 2

V = variable
D = diversity
J = joining

V(D)J recombination is the unique mechanism of genetic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It involves somatic recombination, and results in the highly diverse repertoire of antibodies/immunoglobulins (Igs) and T cell receptors (TCRs) found on B cells and T cells, respectively.

Heavy chain have VDJ
Light chain only have VJ

Most T cell receptors are composed of an alpha chain and a beta chain. The T cell receptor genes are similar to immunoglobulin genes in that they too contain multiple V, D and J gene segments in their beta chains (and V and J gene segments in their alpha chains) that are rearranged during the development of the lymphocyte to provide that cell with a unique antigen receptor. The T cell receptor in this sense is the topological equivalent to an antigen-binding fragment of the antibody, both being part of the immunoglobulin superfamily.

Question 3

What is the consequence of VDJ recombination failure?

Answer 3

Severe combines immunodeficiency: (SCID)

If there is a defect in the VDJ recombination process, it can lead to the development of SCID, which is a very severe disorder that usually results in death of the affected individual in early childhood

One cause of SCID is mutations in the RAG enzyme, which leads to improper function, which leads to generation of very few T cells and no B-cells in the affected individual

This decreases the individuals ability to fight of potential pathogens significantly, as the adaptive immune system is virtually absent, and patients with such deficiency usually die in early childhood (Ommen syndrome)

Question 4

TCR

Answer 4

The TCR is composed of two different protein chains (that is, it is a heterodimer). In humans, in 95% of T cells the TCR consists of an alpha (α) and beta (β) chain, whereas in 5% of T cells the TCR consists of gamma and delta (γ/δ) chains.

The TCR is a disulfide-linked membrane-anchored heterodimeric protein normally consisting of the highly variable alpha (α) and beta (β) chains expressed as part of a complex with the invariant CD3 chain molecules. T cells expressing this receptor are referred to as α:β (or αβ) T cells, though a minority of T cells express an alternate receptor, formed by variable gamma (γ) and delta (δ) chains, referred as γδ T cells

Outer variable region and inner constant region. The variable domain of both the TCR α-chain and β-chain each have three hypervariable or complementarity determining regions (CDRs), whereas the variable region of the β-chain has an additional area of hypervariability (HV4) that does not normally contact antigen and, therefore, is not considered a CDR.

Heterodimer that only exists as a transmembrane antigen receptor. It is not secreted since T cells function by direct cell contact

The TCR alpha chain is generated by VJ recombination, whereas the beta chain is generated by VDJ recombination (both involving a somewhat random joining of gene segments to generate the complete TCR chain)

TCR binds the processed peptide (linear epitope)

NO somatic hypermutation in TCR or T-cells in general!

beta (HC)
alpha (LC)

Question 5

BCR

Answer 5

The B-cell receptor is composed of two parts:
i) A membrane-bound immunoglobulin molecule of one isotype (IgD, IgM, IgA, IgG, or IgE - Native BCR have IgM or IgD). With the exception of the presence of an integral membrane domain, these are identical to their secreted forms.

ii) Signal transduction moiety: A heterodimer called Ig-α/Ig-β (CD79), bound together by disulfide bridges. Each member of the dimer spans the plasma membrane and has a cytoplasmic tail bearing an immunoreceptor tyrosine-based activation motif (ITAM). CD19 + CD 21

BCR bind the whole molecule (conformation epitope)

VDJ recombination: Heavy chain first (D+J -> DJ + V). Light chain only have VJ

Somatic hypermutation occur only in T-cells

Question 6

What are the coreceptors for TCR?

Answer 6

The signal from the T-cell complex is enhanced by simultaneous binding of the MHC molecules by a specific co-receptor.

On helper T cells and regulatory T cells, this co-receptor is CD4 that is specific for MHC class II.
On cytotoxic T cells, this co-receptor is CD8 that is specific for MHC class I.

They also need CD3 for signal transduction.
Cd3 is a 2x heterodimer)+ Also another zeta chain is required (homodimer).

Question 7

Light chains are either…… or……

Answer 7

Kappa or lambda

Question 8

What is the name of the molecule that links antibodies together? E.g. in IgM and IgA

Answer 8

J chain

Question 9

Papain cleave AB into?

Answer 9

FAB and FC region

Question 10

CDR stands for?

Answer 10

Complementarity determining region

In T cell receptor recognizing: two polymorphic residues (CDR1-CDR2) of the MHC molecule and one residue (CDR3) of the peptide antigen.

Question 11

Which enzyme are important in recombination?

Answer 11

Rearrangement is mediated by RAG1/RAG2 enzyme complex (recombinases)

RAG is only expressed in developing lymphocytes

RAG complex recognizes RSS (recombination signal sequence) and cuts one strand of DNA

1) RAG enzyme recognize RSS signal and produce nick.
2) hairpin formation.
3) LIGATION BY ADDING ADDITIONAL NUCLEOTIDES done by Artemis endonuclease
+ Terminal deoxynucleotidyl transferase (TdT).

** not sure about this: Artemis endonuclease nicks the dna strand at VJ region and adds palindromic sequence?
and TdT adds nucleotides and joins the regions.

*These enzymes are responsible for another mechanism generating even more diversity as they are able to add palindromic sequences to the gene strand (P nucleotides)

Question 12

Which other molecules that TCR, CD3, MHC 2 and CD4 is required for T-cell activation? Which IL is synthesized and what does it do?

Is there a negative signaling molecule complex?

Answer 12

B7 (on APC) and CD28 (on T-cell)

IL2 by the T-cell. Autocrine mediator -> T-cell proliferation

B7 on (APC) and CTLA4 on T-cell

Question 13

When activated, what does CD8+ T-cells release?

Answer 13

perforin, granzyme and/or FasL

Question 14

What does B and T cell receptors recognize?

Answer 14

B cell receptors recognize conformational epitopes, whereas T cells recognize linear epitopes presented on MHC class molecule.

Question 15

Allotype, idiotype, isotype… immunology just has names for everything.

Answer 15

not bothering.

Question 16

Lamba chain chromosome, kappa chain chromosome, heavy chain chromosome.

Answer 16

kappa= 2
lamba= 22
heavy chain= 14

Question 17

immunoglobulin recombination-

Answer 17

First step: (DJ recomb): In the developing B cell, the first recombination event to occur is between one D and one J gene segment of the heavy chain locus. Any DNA between these two gene segments is deleted.

Second step (VDJ formation): This D-J recombination is followed by the joining of one V gene segment, from a region upstream of the newly formed DJ complex, forming a rearranged VDJ gene segment. All other gene segments between V and D segments are now deleted from the cell’s genome.

Primary transcript (unspliced RNA) is generated containing the VDJ region of the heavy chain and both the constant mu and delta chains (Cμ and Cδ). (i.e. the primary transcript contains the segments: V-D-J-Cμ-Cδ).

The kappa (κ) and lambda (λ) chains of the immunoglobulin light chain loci rearrange in a very similar way, except the light chains lack a D segment. In other words, the first step of recombination for the light chains involves the joining of the V and J chains to give a VJ complex before the addition of the constant chain gene during primary transcription. Translation of the spliced mRNA for either the kappa or lambda chains results in formation of the Ig κ or Ig λ light chain protein.

Question 18

Gamma-delta TCR genes

Answer 18

the locus encoding the δ chain is located within the α-chain locus

Question 19

allelic exclusion

Answer 19

As we know, all somatic cells express 2 sets of chromosomes, one from each parent
(paternal and maternal set). This means that each B cell contains both the maternal and
paternal genes encoding for heavy and light chains, however, only one are expressed
in individual B cells, which means that either the maternal or paternal heavy/light
chain gene is silences (called allelic exclusion)

Question 20

Omenn syndrome

Answer 20

RAG mutation -> defect VDJ -> absence B-cell and decreased T-cells

Question 21

SCID

Answer 21

Nonsense mutation in RAG gene -> NO VDJ -> Lack of B and T-cells ->