11. T Lymphocyte Development Flashcards
what is lymphopoiesis
the production of new lymphocytes in central lymphoid organs
In the thymus what helps differentiate T-lymphocytes
thymic hormones
what are thymocytes
immature lymphocyte in the thymus gland
what happens in different regions of the thymus (e.g. cortex vs medulla)
different stages of T cell development
cortex = positive selection
thymus = negative selection
match locations of T cells in the thymus to their development stage
outer cortex = immature proliferating thymocytes
inner cortex = mature thymocytes undergoing selection
what makes up the thymic stroma
cortical epithelial cells and medullary epithelial cells
what are the key players in T-cell precursor apoptosis in the thymus
macrophages in the thymus cortex and medulla - delete faulty T cells
successive stages in thymocyte development are marked by status of ______ changes in ______
TCR genes and TCR expression
Cell-surface molecule expression (CD3, 4 and 8)
what are the 2 distinct lineages of thymocytes
yd T-cells
aB T-cells: develop into CD4 and CD8 (most common)
stages in T-cell development and where they take place in the thymus
- Double negative (CD3- CD4- CD8-): sub capsular zone of cortex
- Double positive (CD3+ CD4+ CD8+): inner cortex
- Single positive: medulla
how many double negatives occur in T cell development
4
If a T cell does or does not proceed past DN2, what lineage does it become a part of
does not = yd T-cells (~5%)
does = aB T-cells
when is notch signalling on vs. off
in the bone marrow (CLP) = OFF
Early T-cell precursor = ON
B-selection = OFF
what is notch signalling essential for
T cell lineage commitment and early phases of thymocyte differentiation up to DN3
when does B-chain rearrangement happen in T cell development
Double negative 3 - VDJ recombination and B-selection happen
why is notch signalling turned off for common lymphoid progenitors in the bone marrow
allows cells that stay in the bone marrow to develop into B cells
characteristics of Double-Negatives - the earliest T cell precursors
- have not yet rearranged TCR loci
- don’t express CD4 or CD8
- don’t express CD3
3 markers of Double-Negatives - how we know the T cell is at this stage
- c-KIT: receptor for stem cell growth factor
- CD44: adhesion molecule
- CD25: alpha chain of the IL2 receptor
*by DN4 the cell lacks these markers
what is the T-cell known as at each of the 4 double-negative stages
DN1: early T-cell precursor
DN2: Pro-T cell
DN3: small pre-T cell
DN4: large pre-T cell
when does CD3 expression appear
between DN2 and DN3
what makes the pre-T cell receptor
new B chain
Pre-Ta (surrogate chain)
CD3
which proteins are important in T-cell commitment
notch and IL-7R
important signalling proteins in T cell development and when they are active
Notch: DN1 to DN4
CD3: DN2 to single positive
IL-7R: DN1 to DN4, single positive
what are the checkpoints of T-cell development
- TCRB rearrangement (pre-TCR checkpoint) - at DN3
- TCRa rearrangement (positive and negative selections) - at DP
why is the pre-TCR important
- productive TCR B-chain rearrangement
- signals for proliferation and maturation
- suppresses further B chain rearrangement (allelic exclusion)
- signals for TCR a chain rearrangement
- induces development of CD4+CD8+ (DP)
what is allelic exclusion
recombination shut down to prevent further recombination of the B-chain - prevents the thymocyte from producing more than one functional B-chain
why are cells considered double-positive
both CD4 and CD8 are expressed
what does expression of CD4 and CD8 initiate
rearrangement of a-chain locus in double-positive cells
B-chain rearrangement happens before a-chain rearrangement. what are the specific stages at which these happen?
B-chain rearrangement: DN3
a-chain rearrangement: Double positive
what are cTECs and mTECs
cTECs = cortical epithelial cells, express MHC II , roll in positive selection
mTECs = medullary epithelial cells, present antigens, roll in negative selection
why do we have 4 attempts at B-chain rearrangement
because 2 clusters of each D and J gene segments for the B-chain
what are the stages of gene rearrangement in aB-T cells
- Germaine gene configuration
- DB and JB rearrangement
- VB and DJB rearrangement - B chain produced
- surface expression of B-chain with surrogate a-chain, B-rearrangement stops, CD4/8 induction causes a-transcription to start
- Va and Ja rearrangement
there are many rounds of a-chain rearrangement until…
positive selection or death
what happens after an aB TCR becomes expressed on the T-cell surface
cells undergo positive and negative selection
those that fail either selection undergo apoptosis
those that pass selection lose either CD4 or CD8 and become single positives
when do thymocytes interact with cortical epithelial cells
in the double positive stage
characteristics of positive selection of T-cells
- occurs in the thyme cortex (inner)
- T-cells that are able to bind to self-MHC molecules in the thymus go onto mature
- results in MHC restriction
characteristics of negative selection of T-cells
- occurs in the cortex and medulla
- removes T cells whose TCR has high affinity for self-MHC with self-antigen
- cells die by apoptosis in the thymus
- results in self tolerance
what will happen in T-cell selection based on T-cell receptor affinity for self peptides:self MHC
too little = no selection
just right = positive selection
too much = negative selection
the role of cTECs in positive selection
DP tested by APCs or cTECs for the affinity of their T-cell receptor for the MHC molecules presenting antigen
the role of mTECs in negative selection
SP in the medulla that bind to MHC molecules presenting self antigens of mTECs or dendritic cells die by apoptosis
what are the 3 possible fates of developing thymocytes in the thymic cortex
- cTEC receptor doesn’t bind peptide = death by neglect (apoptosis)
- cTEC receptor has moderate binding to peptide = positive selection - maturation to SP
- cTEC or APC has high binding to peptide = negative selection (apoptosis)
different ways positive selection coordinates expression of CD4 and CD8 (+potential effector functions)
- normal MHC II expression = both CD8 and CD4 mature
- MHC II-negative mutant = CD8 matures
- mutant with MHC II trans gene expressed in cortical thymic epithelium = both CD8 and CD4 mature
- mutant with MHC II trans gene expressed that cannot interact with CD4 = CD8 matures
what is AIRE
- autoimmune regulator which enables presentation of tissue-specific self-antigens
important in central tolerance - makes sure that developing T cells are exposed to a wide variety of the body’s proteins to ensure they are not self-reactive
where does negative selection occur
the thymic corticomedullary junction and medulla
what induces thymic emigration of T-cells
signalling through the sphingosine 1-phosphate receptor
characteristics of the pre-T cell receptor
- inhibition of B chain gene recombination
- proliferation of pre-T cells
- stimulation of a-chain recombination
- expression of CD4 and CD8
- shut off of pre-Ta transcription
thymocytes at different developmental stages are found in distinct parts of the thymus…
DN1: cortico-medullary junction
DN2: cortex
DN3: sub scapular region
DN4: sub scapular region
DP: inner cortex
SP: medulla
