Antigen recognition and lymphocyte development Flashcards
What are the components of the B cell receptor complex?
- 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
What forms of antigen does the BCR recognize?
- Proteins
- Polysacharides
- Lipids
- Nucleic acids
- Small chemicals
All in their conformational (native) and linear (processed) epitopes
What are the components of the T cell receptor complex?
- αβ TCR
- CD3
- ζ protein
What forms of antigen does the TCR recognize?
Mainly peptides in linear conformation presented on MHC molecules by APCs
What distinguishes BCRs from TCRs?
- 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
How are monoclonal antibodies produced?
- 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
What is the structure of the αβ TCR protein?
- 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
What is the role of CD3 and ζ in the TCR complex?
They are noncovalently associated with the TCR heterodimer and transduce signals from it
What are the general phases of lymphocyte development?
- 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
What is the order of antigen receptor gene rearrangement processes?
- First: rejoining of Ig H / TCR β chains
- Then: rejoining of Ig L / TCR α chains
Where is the Ig H chain locus located?
Chromosome 14
Where is the Ig κ chain locus located?
Chromosome 2
Where is the Ig λ chain locus located?
Chromosome 22
Where is the TCR β chain locus located?
Chromosome 7
Where is the TCR α chain locus located?
Chromosome 14
Which isotype of antibody is the one first expressed by B lymphocyte precursors?
IgM (μ chain)
What are the component enzymes that make up the V(D)J recombinase?
RAG-1 and RAG-2
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)?
Lineage-specific transcription factors guide the function of the recombinase
Define
Combinatorial diversity
The diversity produced by the random recombination of different V, (D,) and J gene segments in different clones of lymphocyte
Define
Junctional diversity
The diversity produced by the random addition and deletion of nucleotides at the junctions between V, D, and J segments
Which type of diversity, combinatorial or junctional, produces more variety in antigen receptors?
Junctional diversity. This is since combanitorial diversity is limited to the number of available V, D, and J segments
How is junctional diversity created?
- 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
What are the sequential forms of B cell precursors?
HSC → pro-B → pre-B → immature B → mature B
What are the features of pro-B cells?
- 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
What are the features of pre-B cells?
- Express a pre-BCR (Ig without light chains)
- Found in the bone marrow
- Undergo recombination of a Ig L locus
What is the role of pre-BCR in B cell development?
- Prevents recombination of the other chromosome’s Ig H locus (allelic exclusion)
- Promotes cell survival
What are the features of immature B cells?
- Express a complete BCR (IgM)
- The response to an antigen is negative selection or receptor editing
How do immature B cells complete maturation?
When they leave the bone marrow (and hence become mature), they express both IgM and IgD
How are B cells selected?
- 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
How does negative selection of B cells occur?
- 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
What are the sequential forms of T cell precursors?
HSC → pro-T → pre-T → double positive T → single positive (immature) T → mature T
What are the features of pro-T cells?
- 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
What are the features of pre-T cells?
- 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
What is the role of pre-TCR in T cell development?
- Prevents recombination of the other chromosome’s β locus (allelic exclusion)
- Promotes cell survival
What are the features of double positive T cells?
- Express a full TCR
- Are positive for CD4
- Are positive for CD8
How are T cells selected?
- 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
How do double-positive T cells become single-positive?
- 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
