IMMUNO 10 _TCR, T CELL MATURATION

thymocyte

immature T cell

what is the production of lymphocytes in central lymph organs called?

lymphopoesis

what are early T cells called in each stage:

bone marrow

blood stream

thymus

bone marrow- common lymphoid proginator

blood stream- immature T cell precursor

thymus- thymocytes

how many lobes does thymus have?

what are layers of thymus?

2

cortex and medulla

what is the outer layer of thymus called?

capsule

in the thymus cortex what cells are present?

few macrophages, immature thymocytes, cortical epithelial cells

in the thymus medulla what cells are present?

more mature thyrocytes, machrophages, dendritic cells, some B cells, medullary epithelial cells

what two cell types make thymic stroma?

cortical and medullary epithelial cells

what percent of T cells die in the thymus before ever maturing?

98%

what clears debris of all the apoptosis in the thymus?

macrophages

self tolerance is important why?

without you get autoimmunity

two lineages of T cells

aB and yd

what are the phases a T cell goes through to get from DN (double negative)→ SP (single positive) and what layers of thymus?

subscapular cortex

DN1

DN2

DN3

DN4

inner cortex

DP (large then small)

medulla

SP

what type of cell during DN1?

ETP (early T cell precursor/early thymic progenator)

what occurs at DN2?

irreversible commitment to T cell lineage

where do yd branch off from aB?

between DN2 and DN3

what protein at DN2 signals for commitment to T cell lineage? what releases this protein?

notch

thymic epithelial cells

what stage does notch turn on vs off?

turns on in early DN stage before DN3. commitment occurs in DN2

what percent of thyrocytes make it to the stage of SP?

5%

what percent of thyrocytes become T cells?

2%

what three markers are on DN thyrocytes?

1. c-KIT- receptor for stem cell growth

2. CD44- adhesion- homing to thymus

3. CD25- alpha chain of IL2 receptor

other term for each DN phase?

DN1

DN2

DN3

DN4

DN1- ETP

DN2- Pro-T

DN3- Small Pre-T

DN4- Large Pre-T

in which DN phase is VDJ recombination and RAG1:2 initiated?

DN3 (small pre-T)

in small pre-T (DN3) what are the components of its pre-TCR?

VDJ recombination B chain, and surrogate a chain to test the B chain before committing to making a full a chain

CD3 expression appears when?

between DN2 and DN3- it is needed to test the signalling of the B chain

where does B chain VDJ start? where is it tested with surrogate a chain?

DN2

DN3

if B chain arrangement passes test in DN3, what happens in DN4?

proliferation into DP (expressing CD8 and CD4)

in what phase is the a chain made?

DP

without notch signaling what happens?

thyrocytes fail to adapt the T cell lineage (could be a B or NK cell instead)

CD3 appears in the DN→DP transition what does it do?

signals that the pre-TCR is working, and also eventually that the TCR is working

IL-7R is expressed in early DN (DN1-DN3) and does what?

tells the T cell to stay alive and proliferate until TCR is formed then TCR takes over that job

RAG 1:2 are expressed in DN2-DN3, what do they do?

encodes enz (VDJ/lymphoid specific recombinase) that cuts to join the V(D)J in recombination, turned off after sucessful recombination

pre TCR can signal without ligand (T/F)

T- constituitive signaling

what is allelic exclusion? do a or B chains have allelic exclusion?

VDJ recombination genes in B chain have 2 clusters x 2 alleles so maximum 4 total attempts to make a good B chain

where is teh B chain checkpoint?

Pre TCR (DN3)

expression of ______ in DP stage initiates a chain rearrrangement to form complete aB TCR (low level)

CD4 and CD8

to recap: where would you find DNs, DPs, SPs in the thymus?

DN- subscapular cortex

DP- deep cortex

SP - medulla

how do thyrocytes enter thymus?

venules (blood vessels)

cTECs (cortical thymus epithelial cells) express MHC and MHCII and are involved in ________selection

positive

mTECs (medullary thymus epithelial cells) have periphreal antigens and are involved in ________ selection

negative

what are the clusters and alleles that B chain can use to do 4 recombination attempts?

DB1, JB1

DB2, JB2

(note also CB1 and CB2)

see how the clusters are only made of DBJ how odd why isnt V there we dunno i dunno

in DN2 vs DN3 where is the B chain?

DN2- intracellular

DN3- expressed on cell surface

if B chain is sucessful with pTa chain in DN3 then what happens in DN4?

proliferation

the a chain does not express allelic exclusion, what does this mean?

it has many attempts over and over it can try to make a chain in VJ recombination

what happens in DP?

a chain moves to surface to form functional TCR with B chain

a has series of J and V segments where as B has _____ of VDJ segments

clusters

the a chain VJ recombination process where each rearrangement can replace previous one on same allele continues until:

thymocte sucessfully forms abTCR that can recognixe selfMHC for positive selection

OR
it fails until apoptosis (each recombination keeps eating at DNA)

how many chromoseomes B chain vs a chain?

B- 7

a- 14

what is the “endgame” of a vs B chain

B- ensure each T cell expresses only one B chain (monoclonal)

a- allow more chances for sucess under intesnse positive slection

checkpoint 1

checkpoint 2

before moving on to SP (only 5% make it)

checkpoint 1 - DN3-DN4 where TCRB selection and test with pTa

checkpoint 2 - DP where TCRa rearrangement and positive selection

positive selection occurs in the thymic _____ and keeps only T cells that can ________ and the others die by neglect

overall effect: __________

cortex

bind self MHC

MHC restriction

negative selection occurs in the thymic _________ and removes T chells whose TCR can bind _____ too strong these cells die by apoposis

overall effect: _________

cortext and medulla

self MHC

self-tolerance

outcome of each TCR-MHC binding

no/weak-

moderate-

strong-

no/weak- die by neglect

moderate- positively selected for good mhc restriction

strong- negative selection to apoptosis

DP:

positive selection occurs in the cortex and selects for TCR that recognize MHC

negative selection also occurs in the cortex and eliminates TCR that bind too tight to MHC

what cells do this testing?

SP:

in medulla negative selection occurs again: cells are killed if they react to self peptide-MHC

what cells do this testing?

cTECs

mTECs

MHC recognition by T cells in the ______ determines if they are going to keep CD4 or CD8

DP- cortex

how can thymus test the T cells against self antigens from all around the body?

AIRE

(enables tissue specific expression of genes from across the body in tissues outside the thymus)

what establishes central tolerance and prevents autoimmunity?

negative sleection in the thymic medulla (SP)- using AIRE

after maturation, CD4 and CD8 T cells upregulate _______ receptor which senses S1P gradients in blood and lymph to guide thymic emigration into circulation

S1PR1

S1P is found in high concentrations in blood and lymph, and S1PR1 expression senses and guides __________ out of the thymus

mature T cells

how long do T cells spend in the medulla before S1PR1 upregulation?

7-14 days