tolerance

role of thymus

site of lineage commitment, repertoire selection and functional maturation
ME provides instruction and cell-cell contact

TRB rearrangement

1. rag1 recognises RSS flanking each segment

2. recruits rag2

3. DSBs repaired by NHEJ

4. ligation facilitated by ligase and TdT

role of Pre TCR

facilitates signalling - proliferation, differentiation and allelic exclusion
TCRa rearrangement

role of IL7 signalling

controls TCRB rearrangement - induces rag

role of notch signalling

regulates lineage decisions
functions to prevent B cell development

study - role of TECs

KO TEC = no T cells subsets
KO TEC and IL7 = no TCR rearrangement
KO TEC and DP = non functional SP cells

cTECs

have thymoproteasome - distinct antigen presentation to generate peptides with lower affinity to promote positive selection
CD83 antagonises Ub of MHCII for longer presentation

mTECs

negative selection
have AIRE

cDC2

clonal deletion
migratory
express CCR2
acquire mTEC derived antigens for CP

pDCs

clonal deletion
may play role in food antigen tolerance
promote differentiation of Tregs
low co stimulatory category - weak T cell activation

positive selection

positive selection picks beneficial peptides
T cells with potential to react to self MHC are selected
mediated by cTECs

models of transition from DP to SP

instructional model
stochastic model
strength of signal

instructional model

engagement of TCR:MHC instructs downregulation of other co receptor and commits to CD4 or CD8

stochastic model

DP thymocytes randomly downregulate a co receptor prior to interaction with pMHC and only appropriate combinations survive

strength of signal model

TCR-CD4 interactions are longer at lower intensity - less Lck present
CD8 interactions are shorter at higher intensity

nTregs

self reactive T cells with only intermediate-high level of signalling are stimulated to express foxp3 and become Tregs

thymus and immunoaging

thymus involutes with age
decreased naive T cell output
disorganised ME
increased memory T cell skew

immunosenescence

aged B cells - more T cell independent Ig production, lower affinity Ab production
aged NK cells - increased number, impaired cytotoxicity
thymocytes - reduced number of ETP and decline in ability to produce DP cells

receptor editing in B cells

light chain can have multiple attempts at recombination at both loci
only has V and J genes
progress to deletion if no recombinations are successful

study - membrane autoantigen deletion

Ab reactive to mouse kappa chain
under general promoter = no B cells in spleen or lymph nodes
under liver promoter = B cells in spleen but not LN

anergy

T3 phenotype
hyporesponsiveness
decreased IgM, CD21 and increased CD23

study - anergy

BCR for sHEL in sHEL mouse
B cells in spleen = anergic phenotype
knock in kappa chain induces receptor editing and and rescue from anergy

BAFF

determines threshold for tolerance in B cell repertoire
limiting resource for B cells

competition for BAFF

no BAFF = transitional B cells die
normal BAFF = autoreactive B cells dont get enough survival signal
high BAFF = autoreactive B cells enter mature repertoire

study - BAFF overexpression

mouse expressing sHEL and sHEL reactive BCR = decrease in B cells
express BAFF = increase B cells and decrease anergy

split tolerance

CD4 T cell help required to license full B cell responses

study - split tolerance

mouse with BCR against self Tgm2
B cells in periphery - decreased lambda expression, no receptor editing
KO Tgm2 = B cell phenotype doesn’t change, not anergic
add T cell antigen fused to Tgm2 = T cells become autoreactive and rescue IgG

need for peripheral tolerance of T cells

no receptor editing
PTM of self peptides
innocuous foreign antigens not expressed in thymus

PTM of self peptides

arginine modification = methylation or citrullination
iodination = thyroglobulin
proteolytic processing = insulin
neopeptides = hybrid fusion peptides

discovery of Tregs

thymectomy on day 3 neonatal mice - progressive wasting inflammatory disease
day 14 - no autoantibody
Tregs with high CD25 emerge post 3 days of life

foxp3

lineage specifying
coincides with CD25 expression
required throughout life to limit inflammation

pTregs in the gut

CD103 DCs induce foxp3 through TGFB
upregulate a4b7 integrin via RA - homes to gut
different enhancer than nTregs

pTreg deficiency

Th2 autoimmune activation in mucosa

cell contact dependent methods of Treg action

downmodulation of co stimulation by CTLA4
decreased MHC on APC
modulation of cytokines
membrane bound TGFB
direct killing

CTLA4

reduces ability of APCs to stimulate Tconv
competes for CD80/86
no ITAMs
also maintains low CD80/86 on APC

membrane bound TGFB

latent complex bound LAP
Tconv cells have TGFBR which activates through conformational change
recruits signalling molecules
decreases IL2 production
can induce foxp3

cell contact independent Treg mechanisms

cytokines
CD25 mediated cytokine deprivation
ATP

immune privilege

production/activation of immunosuppressive cytokines
FasL and TRAIL

other mechanisms of peripheral tolerance

quiescence
ignorance
exhaustion
senescence
peripheral deletion