Lecture 7 - Adaptive recognition and immunological tolerance

Question 1

T cell generation: how is their diversity formed and what happens when they recognise an epitope in a dangerous situation?

Answer 1

Somatic recombination in a small number of receptor genes during lymphocyte development results in a large numbers of naïve circulating lymphocytes, but each with a different specificity

It is expanded - the majority respond to combat the threat while others generate memory, ready to respond faster and better if the same threat is encountered again

Question 2

Multiple tolerance mechanisms: what is it and why is it needed?

Answer 2

Employing a strategy of multiple layers of checkpoints, deleting the most dangerous ‘reactivities’ and then controlling, or in-effect tuning-down, others to retain the breadth of recognition that may be needed

Self-reactivity is a normal component of any healthy immune system, but it is kept in check by various mechanisms of regulation

Question 3

Tolerance mechanism: what does it aim to do?

Answer 3

• limit the production of self-reactive T and B cell clones during lymphocyte development
• prevent unwanted destructive responses by any clones that enter the circulating pool

Question 4

Central tolerance

Answer 4

Central Tolerance:
Primary lymphoid organs (bone marrow for B cells, thymus for T cells)
Removal of highly self-reactive clones during lymphocyte development
Deletion, (and conversion to tTreg))

Central tolerance is not an absolute process
ie some self-reactive cells will enter the periphery

Question 5

Peripheral tolerance

Answer 5

Peripheral Tolerance:
Peripheral organs & tissues, secondary lymphoid organs (LNs, spleen)
multiple mechanisms limit reactivity against self & harmless antigens in the periphery
suppression by tTreg
ignorance (eg sequestered inaccessible or low affinity antigen)
deletion (activation induced cell death; immune privilege)
anergy (functional unresponsiveness)
induction of iTreg (functional deviation)
lack of T cell help for B cells

Question 6

TCR: what does it need to be active?

Answer 6

Make contacts with MHC and peptide (‘self-restriction’)

Question 7

Lineage commitment: what is it?

Answer 7

CD4 and CD8 co-receptors bind to invariant
sites on MHC-II and MHC-I respectively
(‘lineage commitment’)

What determines if a DP cell that survives positive selection
develops as a
SP CD4+ (TH) or SP CD8+ (CTL) cell?

(ie what determines lineage commitment?)

Lineage commitment requires silencing of expression of one of the co-receptors (CD4 or CD8) and initiation of a gene expression program characteristic of T helper (expression of cytokines) or cytotoxic T cells (genes for targeting cell killing).

Positive selection initiates lineage commitment, during which MHC reactivity informs whether a DP thymocyte becomes a CD8+SP or a CD4+SP.

Adapted from Ashby, KM andHogquist, KA, July 2023. Nature Reviews Immunology, 23: 697
https://doi-org.manchester.idm.oclc.org/10.1038/s41577-023-00927-0

slide 39…?

Question 8

TCR germline gene loci: what are the different gene loci and how do they develop diversity?

Answer 8

The TCR α locus
VJ = V exon
CDR1 & 2 provided by the V gene segment, CDR3 spans join

The TCR β locus
Two gene arrangements; D to J and V to DJ
VDJ = V exon
V, D and J all contribute to CDR3

Question 9

What aspect of TCR is diversity focussed on?

Answer 9

TCR diversity is focused on CDR3, which contacts the antigenic peptide

Question 10

Common leukocyte progenitos: where do they develop and where do T cells develop?

Answer 10

CLP progenitors originate in the bone marrow, but commit to the T cell lineage and develop into T cells in the Thymus

Question 11

T cell forms

Answer 11

alpha beta (95%) delta gamma

Question 12

T cell development

Answer 12

Antigen independent quality checkpoint

DN - DP - PS - NS - SP

PS - bind to MHC (Y - survival signal N - death by neglect (or editing))

NS - Self reactivity removed

Question 13

αβ versus γδ lineage choice

Question 14

The thymus and its framework of epithelial cells

Answer 14

steps in T cell development do not all happen in one place within the thymus but involve movement of cells around different ‘environmental niches’ suitable for providing signals to drive each different stage.

Medulla - paler, less cells

Question 15

What is functionally different between the medulla and cortex compartments that means that signalling from the same TCR, binding to the same MHC molecules gives +ve selection in one region and –ve selection in the other?

Answer 15

Localisation - Same MHC molecules, but the Repertoire of Peptides Presented Differs in the Thymic Cortex and Medulla

Cortex
Cortical Thymus Epithelial Cells (cTEC)
Constitutive MHC I and II expression
MHC I peptides: Thymo-proteasome
MHC II peptides: Cathepsin L
Thymus specifc serine protease (TSSP) [endosomal/lysosomal]
Constitutive macroautophagy [delivers self ag to MHCII]

Medulla
MedullaryThymus Epithelial Cells (mTEC) & Dendritic cells (DC)
Constitutive MHC I and II expression
MHC I peptides: Housekeeping-proteasome & Immuno-proteasomes
MHC II peptides: Cathepsin S
Macroautophagy (mTEC)
Co-stimulation

Question 16

How does the thymus have knowledge of non-thymus antigens to direct negative selection?

Answer 16

Sources of Self Antigen for Tolerance
Induction in the Thymus

Medulla
mTEC - inefficient at uptake and presentation of exogenous antigens
Express AIRE (AutoImmune REgulator)
- interacts with many proteins involved with transcription, promoting lengthening of ‘stalled’ transcripts
- [Genetic deficiency of AIRE results in an autoimmune syndrome, APECED]
Express Fezf2
- a Zn finger transcription factor with an AIRE independent and non-redundant role
→ Promiscuous Expression of genes not normally expressed in the thymus, TRAs
Macroautophagy

DC - subsets sample antigen from mTECs and from the periphery via the blood
Uptake of TRAs shed from mTECs and mTEC cell debris
Uptake of blood-bourne antigens
Cross-presentation of ingested antigens
Immigrant DC from the periphery

‘Mimetic Cells’
Small numbers of mTECs express lineage defining transcription factors and co-express lineage related TRAs
eg express genes that define ciliated cells in the lung

Question 17

What determines if a clone survives to exit into the circulation, or is deleted during negative selection? (or develops into a CD4+ T reg?)

Answer 17

No selection - death by neglect

Too highly - death bc dangerous

Intermediate affinity - we like

High affinity to self reactive - use as Tregs

Question 18

Newly generated SP thermocytes

Answer 18

Newly generated SP thymocytes mature further to gain functional competency before exiting
the thymus

SP thymocytes - approx 4 days further maturation

Thymocytes exit via blood vessels [not lymphatics]
- Up regulation of the shingosine 1 phosphate receptor (S1P1) promotes exit of mature thymocytes toward high concentration of S1P in blood

Recent thymic emigrants (RTE) are not as mature as circulating peripheral naïve T cells (do not proliferate or secrete cytokines in response to antigens)

RTE express the lymph node homing receptors CD62L and CCR7
→ localisation to peripheral lymphoid organs

Further mature within secondary lymphoid tissue.