T-CellDGRD Flashcards
What are the events in lymphocyte development?
→- Commitment
→ Proliferation
→ Selection
→Differentiation into distinct functional effector subpopulations
What are the stem cell factors important in Tcell development?
→c-KIT
Which stem cells give rise to B and T cells?
→Multipotent HSCs
What are the stages of maturation of Tcells?
→stem cell →pro-lymphocytes →pre-lymphocyte →immature lymphocyte →mature lymphocyte
What are the major events in the stem cell and pro-lymphocyte stages?
→growth factor mediated commitment
→proliferation
→initiation of antigen receptor gene rearrangement
Describe the journey of Tcells through development before proliferation
→Notch signals by the thymic stroma
→Induction of GATA3
→Commitment to the T cell lineage
→Intense proliferation
What do notch signals induce?
→commitment
Why does the thymus not increase in size?
→cells die off
Describe the T-cell journey through development
→Tcell precursor rearranges its Tcell receptor genes in the thymus
→immature T cells that recognise self MHC receive signals for survival
→those that interact strongly with self antigen are removed from the repertoire
→mature Tcells encounter foreign antigens in the peripheral lympjoid organs and are actiavted
→activated Tcells proliferation and eliminate infection
What happens after 1 week after arrival of precursors into the thymus?
→progenitors commit to the T cell lineage
→Express early markers of the T cell lineage (CD2 and Thy1)
What are double negative Tcells?
→double negatives
What do thymocytes do at the DN stage?
→TCR locus
What two types of cells do DN cells divide into?
→CD3+ alpha, beta
→CD3+ gamma, delta which goes into periphery
What do Tcells express in the periphery if successful?
→TCR
Describe the structure of TCR
→heterodimer
→two transmembrane polypeptide chains
→covalently linked to each other by disulphide bonds
What are the two types of TCRs?
→alpha-beta
→gamma-delta
Describe a chain of a TCR
→Ig-like N terminal variable domain (V)
→one Ig-like constant domain (C),
→a hydrophobic transmembrane region
→a short signalling cytoplasmic region
What do the V regions of both chains contain?
→short stretches of amino acid sequence that is highly variable between receptors
What do V regions contain?
→complementary determining regions
What forms the peptide-MHC binding site?
→3 CDRs of the alpha chain
→ 3 of the beta chain
Compare the TCR and Ig system
TCR →alpha and beta components →one V domain and C domain in each chain →six CDRs → there is no change in cellular activation
Ig
→heavy and light chains
→heavy chain has one V domain, three or four C domains
→light chain has one V domain and one C domain
→six CDRs
→changes after cellular activation
What do the C regions have and do?
→cysteines residues
→bring the chains together
What residues bind to CD3 in the transmembrane region?
→charged residues
→also bind to zeta chain to form TCR
What do CD3 and zeta allow in TCR function?
→transduction of signals upon MHC-peptide binding
What conformation of antigens do T cells recognise?
→linear peptides on MHC
What is MHC?
→Major Histocompatibility Complex →MHC class I and MHC class II
What do MHC1 complexes display?
→antigens derived from pathogens that replicate inside the cell, such as viruses
What do MHC2 display?
→peptides from pathogens and antigens that are present outside the cell taken up by endocytic vesicles of phagocytic cell
Describe the MHC structure
Extracellular peptide binding cleft
Ig-like domain
Cytoplasmic tail
What does it mean that MHC is polymorphic?
→multiple variants of each gene within the population
What does it mean that MHC is polygenic?
→contains several different MHC class I and class II genes
What is the difference in the peptides that MHC and MHC2 can bind?
→MHC class II bids to longer peptides than class I
What does it mean that MHC interactions are low off-rate?
→once peptide is there, it needs to stay there to activate MHC
What is the difference between beta 1 and beta 2?
→Beta 1 has more variability
Describe the secondary structure of MHC
→2 alpha helices
→a beta sheet
Which cells express MHC1?
→all cells but erythrocyte
Which cells express MHC2?
→antigen presenting cells
Describe the process of antigen presentation in MHC2 complexes
→microbial proteins enter acidic intracellular vesicles, called endosomes or phagosomes, which may fuse with lysosomes
→the proteins are broken down by proteolytic enzymes, generating many peptides of varying lengths
→APCs constantly synthesize class II MHC molecules in the endoplasmic reticulum (ER).
→Each newly syn-thesized class II molecule carries with it an attached protein called the invariant chain, which contains a sequence (called the class II invariant chain peptide [CLIP]) that binds tightly to the peptide-binding cleft of the class II molecule.
→the cleft of the newly synthesized class II molecule is occupied.
→ class II molecule begins its transport to the cell surface in an exocytic vesicle, which then fuses with the endosomal vesicle containing peptides derived from ingested extracellular proteins. The same endosomal vesicle contains a classII–like protein called DM, whose function is to remove CLIP from the class II MHC molecule.
→After removal of CLIP, the cleft of the class II molecule becomeavailable to accept peptides.
What happens if the MHC2 molecule does not find a peptide?
→the empty molecule is unstable and is degraded by proteases in the endosomes
Describe the MHC1 formation process
→viral, tumour, and mutated genes are targeted for proteolysis
→ These proteins are unfolded, covalently tagged with multiple copies of a small peptide ubiquitin
→Some classes of proteasomes efficiently cleave cytosolic proteins into peptides with the size and sequence properties typical of class I MHC–binding peptides
→TAP binds peptidesfrom the cytoplasm and actively pumps them across the ER membrane into the interior of the ER
→Newly synthesized class I MHC molecules are loosely attached to the interior face of the TAP molecule
→Thus, as peptides enter the ER, they can be captured by the class I molecules →is able to resist proteolysis by endosomal proteases
What keeps MHC2 molecules from binding peptides?
→CLIP
What is the site of peptide loading in MHC1 and MHC2 pathway?
→MHC1= ER MHC2= specialised vesicular compartment
How is the variable chain encoded?
→germline
What is one way a TCR is different from Ig?
→not secreted
Describe TCR gene in germline configuration before rearrangement
→gene segments are arranged in a similar pattern to immunoglobulin gene segments
→rearranged by the same enzymes Rag 1 and Rag 2
Which enzymes arrange gene segments in the TCR germline?
→Rag1 and Rag2
Where do T-cell receptors concentrate diversity?
→third hypervariable region CDR3
How are gamma and delta T-cell receptors generated?
→by gene rearrangement
Is recombination by Rag1 and Rag2 antigen- dependent?
→process is
antigen-independent
Describe TCR-alpha chain genes
→do not have D gene segments
When are TCR-alpha chains rearranged?
→rearranged only after the TCRβ chain gene locus has been rearranged
What is TdT in junctional diversity ?
→terminal deoxynucleotidyl transferase
→addition (or removal) of nucleotides
Where are progenitor cells found in the thymus?
→medulla
Where do checkpoints in T-cell development occur?
→cortex
What is allelic exclusion?
→ensures that only one TCRβ chain gene is expressed
→β-selection
→Signalling through the pre-TCR suppresses expression of the RAG genes
→So, no more rearrangement at this stage
What are the successful signalling of a pre-TCR?
→Halts further b chain rearrangements
→Induces expression of CD4 and CD8
→Initiates alpha chain rearrangement
→downregulation of Rag 1&2
What does alpha chain rearrangement coincide with?
→the transition of T cells to the CD4 CD8 DP
Describe the VDJ rearrangement
→D-beta and J-beta
→V beta and DJ-beta
→surface expression of beta chain with surrogate alpha
→V-alpha and J-alpha rearrangement
