Antigen recognition and lymphocyte development

Question 1

What are the components of the B cell receptor complex?

Answer 1

• An antibody specific to the antigen in its native form (the BCR proper)
• Igα non-covalently bound to Igβ, associated with the antibody for signal transduction

Question 2

What forms of antigen does the BCR recognize?

Answer 2

• Proteins
• Polysacharides
• Lipids
• Nucleic acids
• Small chemicals

All in their conformational (native) and linear (processed) epitopes

Question 3

What are the components of the T cell receptor complex?

Answer 3

• αβ TCR
• CD3
• ζ protein

Question 4

What forms of antigen does the TCR recognize?

Answer 4

Mainly peptides in linear conformation presented on MHC molecules by APCs

Question 5

What distinguishes BCRs from TCRs?

Answer 5

• The type of protein involved (immunoglobulin vs. αβ protein)
• The types of antigen recognized (BCR is far more diverse)
• The TCR has no effector functions, while the constant region of the BCR Ig does

Question 6

How are monoclonal antibodies produced?

Answer 6

• B cells from an animal immunized with an antigen are fused in vitro with myeloma cells, which can replicate indefinitely in tissue culture. This is a hybridoma
• Hybridomas are screened for the type of antibody they produce. Those with the same clonality are purified, giving monoclonal antibodies

Question 7

What is the structure of the αβ TCR protein?

Answer 7

• A transmembrane α chain with a variable and a constant region, linked by a disulfide bridge to
• A transmembrane β chain with a variable and a constant region

Question 8

What is the role of CD3 and ζ in the TCR complex?

Answer 8

They are noncovalently associated with the TCR heterodimer and transduce signals from it

Question 9

What are the general phases of lymphocyte development?

Answer 9

• Commitment of progenitor cells to the B lymphoid or T lymphoid lineage
• Proliferation of progenitors and immature committed cells, providing a large pool of cells
• The sequential and ordered rearrangement of antigen receptor genes and the expression of antigen receptor proteins
• Selction events that preserve cells that have produced functional antigen receptor proteins and eliminate potentially dangerous cells that strongly recognize self antigens
• Differentiation of B and T cells into functionally and phenotypically distinct subpopulations

Question 10

What is the order of antigen receptor gene rearrangement processes?

Answer 10

• First: rejoining of Ig H / TCR β chains
• Then: rejoining of Ig L / TCR α chains

Question 11

Where is the Ig H chain locus located?

Answer 11

Chromosome 14

Question 12

Where is the Ig κ chain locus located?

Answer 12

Chromosome 2

Question 13

Where is the Ig λ chain locus located?

Answer 13

Chromosome 22

Question 14

Where is the TCR β chain locus located?

Answer 14

Chromosome 7

Question 15

Where is the TCR α chain locus located?

Answer 15

Chromosome 14

Question 16

Which isotype of antibody is the one first expressed by B lymphocyte precursors?

Answer 16

IgM (μ chain)

Question 17

What are the component enzymes that make up the V(D)J recombinase?

Answer 17

RAG-1 and RAG-2

Question 18

The V(D)J recombinase used for B cell antigen gene rearrangement is the same as the one used in T cells. How does the cell prevent recombination of the incorrect gene locus (i.e. how do B cells prevent rearrangement of the TCR genes, and vice versa)?

Answer 18

Lineage-specific transcription factors guide the function of the recombinase

Question 19

Define

Combinatorial diversity

Answer 19

The diversity produced by the random recombination of different V, (D,) and J gene segments in different clones of lymphocyte

Question 20

Define

Junctional diversity

Answer 20

The diversity produced by the random addition and deletion of nucleotides at the junctions between V, D, and J segments

Question 21

Which type of diversity, combinatorial or junctional, produces more variety in antigen receptors?

Answer 21

Junctional diversity. This is since combanitorial diversity is limited to the number of available V, D, and J segments

Question 22

How is junctional diversity created?

Answer 22

• Exonucleases may remove nucleotides from V, D, and J gene segments at the sites of recombination
• A lymphocyte specific enzyme, terminal deoxyribonucleotide transferase (TdT), catalyzes the random addition of nucleotides at the junctions between V and D segments and D and J segments, forming N regions
• During V(D)J rejoining, two broken strands of the DNA form hairpin loops, which are cut and filled back in with new nucleotides. This process creates more diversity

Question 23

What are the sequential forms of B cell precursors?

Answer 23

HSC → pro-B → pre-B → immature B → mature B

Question 24

What are the features of pro-B cells?

Answer 24

• They do not express any BCRs
• They do not have recombined DNA yet
• Found in the bone marrow
• Undergo recombination of the Ig H locus

Question 25

What are the features of pre-B cells?

Answer 25

• Express a pre-BCR (Ig without light chains)
• Found in the bone marrow
• Undergo recombination of a Ig L locus

Question 26

What is the role of pre-BCR in B cell development?

Answer 26

• Prevents recombination of the other chromosome’s Ig H locus (allelic exclusion)
• Promotes cell survival

Question 27

What are the features of immature B cells?

Answer 27

• Express a complete BCR (IgM)
• The response to an antigen is negative selection or receptor editing

Question 28

How do immature B cells complete maturation?

Answer 28

When they leave the bone marrow (and hence become mature), they express both IgM and IgD

Question 29

How are B cells selected?

Answer 29

• Positive selection based solely on the expression of a complete BCR, regardless of what it recognizes
• Negative selection of immature B cells if they bind to an antigen in the bone marrow with high affinity

Question 30

How does negative selection of B cells occur?

Answer 30

• Deletion: the cells die by apoptosis
• VDJ recombinase is re-expressed, leading to additional light chain VJ recombination, generating a different light chain with different specificity

Question 31

What are the sequential forms of T cell precursors?

Answer 31

HSC → pro-T → pre-T → double positive T → single positive (immature) T → mature T

Question 32

What are the features of pro-T cells?

Answer 32

• They are negative for both CD4 and CD8
• They do not express any TCRs
• Found in the thymus
• Undergo recombination of the β chain
• If recombination of both loci fails, the cell dies

Question 33

What are the features of pre-T cells?

Answer 33

• They are negative for both CD4 and CD8
• They express a pre-TCR (β chain)
• Found in the thymus
• Undergo recombination of the α chain

Question 34

What is the role of pre-TCR in T cell development?

Answer 34

• Prevents recombination of the other chromosome’s β locus (allelic exclusion)
• Promotes cell survival

Question 35

What are the features of double positive T cells?

Answer 35

• Express a full TCR
• Are positive for CD4
• Are positive for CD8

Question 36

How are T cells selected?

Answer 36

• Positive selection for double-positive T cells that recognize a self MHC molecule in the thymus with low or moderate affinity. Cells that do not recognize an MHC molecule die by apoptosis
• Negative selection of double-positive T cells that recognize a self MHC molecule in the thymus with high affinity. They die by apoptosis

Question 37

How do double-positive T cells become single-positive?

Answer 37

• T cells that recognize class I MHC–peptide complexes retain CD8 and lose CD4
• T cells that recognize class II MHC–peptide complexes retain CD4 and lose CD8