T-cell development & Central tolerance

discrimination of self & non-self antigens

but difficult as not much differences

not 100 % perfect

tolerance break = auto-immunity

thymus lies above the heart & made of several lobules (each containing cortex & medulla)

separated in upper part (cortex) & peripheral part (medulla)

cortex: cortical epithelial cells, immature thymocytes —>densly packed

medulla: medullary epithelial cells, DC, macrophages, mature thymocytes —> less dense

site of cell degradation & cell aggregates of eosinophils (hassall’s corpuscle)

1. CD4- CD8- double negative

2. CD4+ CD8+ double positive

3. single positive CD4+ or CD8+

—>stages correlate with rearrangement

divided into 4 substages

DN3 important: ß-rearrangement & first expression of TCR as a pre-TCR (substitute for α-chain & finished ß-chain) —> quality control to check completely folded ß-chain

pre-TCR gives a proliferative signal

CD44 low & CD25+

1. rearrangement D to Jß

2. rearrangement V to DJß (race αß or γδ)

   —>V to Jγ and V to DJδ rearrangement

V to Jα rearrangement

pTα is a substitute for the α-chain

CD3 coreceptor transmits proliferative signal (ligand-independent signaling)

arrest of ß-chain rearrangement

pTα is a substitute for the α-chain

CD3 coreceptor transmits proliferative signal (ligand-independent signaling)

arrest of ß-chain rearrangement

repeated rearrangements can rescue nonproductive VαJα joins at the double positive stage

rearrangements continue until RAG shuts down

similar to light chain

sometimes 2 productive α-chains —> not possible on ß-chain

rearrangement continues until a productive rearrangement leads to positive selection or cell dies

almost simultaneously —> who will be first pre-TCR or γδ

γδ T-cells CD4 & CD8- —>no MHC restriction/ no negative or positive selection

timing tightly controlled —> clone with 1 specificity should expand -> inhibit second ß-chain from another allele after productive rearrangement

allelic exclusion

RAG expression tightly controlled

thymocytes come from bone marrow progenitors

bone marrow progenitor marker, immaturity marker lost with time

ligand = stem cell factor

function = signaling

ends in DN3

surrogate α-chain in double negative & double positive stage

gives survival signal to pre-TCR

expressed upon DN3

function = signaling

co-receptor, both in double positive stage

either CD4 or CD8 in single positive stage

TCR signaling kinase always expressed

adhesion molecule to APC for selection

starts in DN4 until end

lymphoid-specific recombination tightly controlled for rearrangement & allelic exclusion

N-nucleotide addition during rearrangement

expressed until rearrangement done

stage in αß T-cell development in the thymus correlates with the program of gene rearrangement & expression of cell-surface proteins, signaling proteins & TF

mediated in the cortex by cortical thymic epithelial cells (constitutive MHCII expression)

select positively thymocytes recognizing host self MHC molecules in association with a self-peptide —> survival of peripheral naive T-cells

ignore thymocytes without recognition

depletion of auto-reactive T-cells wanted but in positive selection first selected for TCR binding to MHC

survival signal to all thymocytes docking host seld MHC with self-peptide

if no MHC molecule is recognized no survival signal

—>dying cell cleared by macrophages

random process —> major fraction of generated T-cells die

mediated in the medulla by medullary thymic epithelial cells (mTEC) & DC

select negatively thymocytes recognizing host self MHC molecules in association with a self peptide with a too high avidity —> remove auto reactive

nTregs are the cells with the highest avidity that are still selected

avidity = as surface protein & no monomer used instead of affinity

high avidity self-reactive T-cells deleted

not possible to express al self antigens in the thymus

specific TF in mTEC

most genes are expressed but some lack

AIRE driven expression but central tolerance not perfect

for example insulin not expressed

second TF: FEZF2

TCR- transgenic mice express TCR specific for an OVA peptide presented by MHCII

OT-II with weak avidity & DO11.10 with strong avidity

OVA-transgenic mice: ovalbumine under the control of the tyrosinase promotor expressed in the skin but also mTECs —> crossing of the mice

high avidity: in WT no ovalbumine in the system -> 4 subsets

in Tyr-OVA thymocytes should be killed during negative selection after full rearrangement —> should not see CD4+ cells with high avidity

αß TCR DO11.10 should be killed due to high avidity (higher than the one of nTreg)

but no change in terms of percentage —> similar to WT

single positive TCR selected but express other TCR —> allelic exclusion doesn’t work completely —> 100% negative selection

low avidity: decrease in CD8+ T-cells —>not possible produce MHCII restricted cells (allelic exclusion works depending on where) —> TCR still expressed in selected CD4+ T-cells

low avidity TCR escapes central tolerance -> auto immune cells die

no negative selection

after single positive stage they leave the thymus & mature —> colonize peripheral tissue

low avidity self-reactive TCR escape negative selection

Clonal deletion, receptor editing, anergy induction & clonal ignorance

no self-reaction —> migrates to periphery & mutation of B-cell (IgD & IgM) ->normal maturation if no Ag encountered

multivalent self-molecule: clonal deletion or receptor editing (apoptosis/ or generation of non-autoreactive mature B-cell —> light chain rearrangement

soluble self molecule: migrates to periphery —> anergic B-cell (somehow not killed)

low avidity non-ceoss linking self molecule —> migrates to periphery & maturation

strong ligation of IgM by slef-antigen

arrest of B-cell development & contiued light-chain rearrangement

low cell surface IgM —> new receptor specificty is expressed

if still self-reactive apoptosis or no longer self-reactive (migrates to periphery & maturation)