LECTURE 3 (Selection and development of T and B lymphocytes) Flashcards
How is each cell of the lymphoid lineage clinically identified?
By the characteristic surface molecules that it possesses
What does the mature, naive B lymphocyte in its mature ready-to-respond form express?
2 isotopes of antibody or immunoglobulin called IgM and IgD within its surface membrane
What does the mature, naive T cell express?
A single genetically related molecule called the T-CELL RECEPTOR (TCR) on its surface
What is the antigen receptor of the B lymphocyte/membrane bound immunoglobulin?
A 4-chain glycoprotein molecule that serves as the basic monomeric unit for each of the distinct antibody molecules destined to circulate freely in the serum
Describe the antigen receptor of the B-lymphocyte
- 2 identical halves (each composed of a heavy chain and a light chain)
- 2 halves held together by disulphide bonds into a shape resembling a “Y”
- Some flexibility is permitted between the halves by DISULPHIDE BONDS forming a HINGE REGION
- Although 2 isotopes of membrane immunoglobulin (IgM and IgD) are coexpressed on the surface, only ONE IDIOTYPE is expressed per cell
What is an Idiotype?
The unique structure of the antigen binding site
What is the antigen receptor of the T lymphocyte?
2 glycoprotein chains, a-beta and alpha chain that are similar in length
Describe the antigen receptor of the T lymphocyte
- On the CARBOXY-TERMINUS of the chains, a cytoplasmic tail extends through the membrane for anchorage
- On the N-TERMINAL END of the molecule, an antigen-binding site is formed between the 2 chains
[whose 3D shape will accommodate the binding of a small antigenic peptide complexed to an MHC molecule presented on the surface of an antigen-presenting cell] - Groove forms IDIOTYPE of TCR
- No hinge region -> quite rigid
What happens when a lymphocyte binds to an antigen complementary to its idiotype?
A cascade of messages transferred through its signal transduction complex will culminate in INTRACYTOPLASMIC PHOSPHORYLATION events leading to activation of the cell
What are the B cell and T cell signal transduction complex?
B cell = CD19, CD21 & CD81
T cell = Multichain structure called CD3
How is the generation of diverse antigen receptors accomplished?
By a complex and unique set of rearrangements of DNA segments that takes place during the maturation of lymphoid cells
It has been discovered that individuals inherit a large number of different segments of DNA which may be recombined and alternatively spliced to create unique amino acid sequences in the _____________ _______ of the chains that compose their antigen recognition sites
N-terminal ends
An analogous random selection is made during the formation of the beta-chain of the ______
TCR
VDJ rearrangements in DNA produce the diversity of which chain variable domains?
Heavy chain
VJ rearrangements in DNA produce the diversity of which chain variable domains?
Light chain
Which enzymes are responsible for gene rearrangements/Recombination?
Enzymes encoded by the genes RAG1 and RAG2 -> The RAG1 and RAG2 gene products are 2 proteins found within the recombinase
What is Recombinase?
A protein complex that includes a repair mechanism as well as DNA-modifying enzymes
What happens while heavy chain gene segments are undergoing recombination?
1) The enzyme terminal DEOXYRIBONUCLEOTIDYL TRANSFERASE (Tdt) randomly inserts bases (without a template on the complementary strand) at the junctions of V, D and J segments (N-nucleotide addition) -> The random addition of the nucleotide generates junctional diversity
2) When the light chains are rearranged later, Tdt is not active, though it is active during the rearrangement of all gene segments in the formation of the TCR
3) This generates even more diversity than the random combination of V, D and J segments alone
Which gene segments do heavy chains have?
- V (Variable)
- D (Diversity)
- J (Joining)
Which gene segments do light chains have?
- V (Variable)
- J (Joining)
What happens in VDJ gene recombination?
1) Genes have RECOMBINATION SIGNAL SEQUENCE motifs that bind specifically to RAG1+RAG2
2) Individual gene segments are selected at random from each gene locus
3) RAG protein complexes bring together the gene segments to be RECOMBINED and cleave the DNA at exactly the junction of the gene segment and its RSS motif
4) Cleavage creates a hairpin of DNA at the ends of gene segments and DOUBLE STRANDED BREAKS at the end of the RSS motifs
5) DNA-dependent protein kinase (DNA-PK), Artemis and DNA Ligase are incorporated to a large complex with the RAG proteins
6) RSS ends are joined forming a signal joint to create a closed circuit of DNA which plays no further role in the recombination process -> DNA segments are cleaved and TERMINAL DEOXYNUCLEOTIDYLTRANSFERASE (TdT) add additional nucleotides to the DNA strand
Why must somatic recombination take place?
Immunoglobulin genes are composed of separated segments of DNA that must be joined to make a functional gene
