2. Topics we will understand today
◦ Early Development
◦ Repertoire selection Processes
◦ Late Development
◦ Activation
◦ Differentiation
◦ Cell Death
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3. I AM RASHMI
PhD Student,
Allergy and Immunology Unit,
Dept.of Pediatrics,
PGIMER, Chandigarh
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‘All T cells show adaptive immune response’
4. Immune response
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◦ Gamma delta T cells may be considered a component of adaptive immunity in that they rearrange TCR
genes to produce junctional diversity and can develop a memory phenotype
◦ Activated NKT cells produce several cytokines with the capacity to jump-start and modulate an adaptive
immune response
5. Importance of adaptive immune response
◦ Innate immune
response can plateau
the pathogenic infection.
◦ However, complete
clearance requires
adaptive immune
response
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6. T cell development and activation
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T-cell precursor rearranges its T-
cell receptor genes in the thymus
Immature T cells that recognize
self MHC receive signals for
survival. Those that interact
strongly with self antigen are
removed from the repertoire
Mature T cells encounter foreign
antigens in the peripheral
lymphoid organs and are activated
Activated T cells proliferate and
eliminate infection
T-cell progenitors develop in the
bone marrow and migrate to
thymus
Positive and negative selection in
the thymus
Matute T cells migrate to the
peripheral lymphoid organs
Activated T cells migrate to sites
of infection
8. T cell precursor migration from Bone marrow to thymus
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1o lymphoid organ 1o lymphoid organ
Chemo attractants:
Thymosin
Thymotaxin
Thymopoietin
Thymic factors
T cell
precursor
9. Each of the several million T and B cells circulating in
the body expresses a novel and unique antigen
receptor
○Cells entering the thymus are not yet committed to become a
lymphocyte and express no antigen-specific receptors
○Cells leaving the thymus are functional and express unique TCRs
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T cell development
10. T cell commitment
◦ Progenitor T cells migrate from
bone marrow to thymus
◦ T cells can be grown in vitro in
absence of thymic fragments
▫ Grown on bone marrow stem
cells with Notch protein
(transcription factor)
■ Notch protein is key in
determining T-lineage
specification
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11. T cell development
Vertebrates generate 2
broad categories of T
cells:
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○TCR-αβ – dominant participant in adaptive
immune response
○TCR-γδ – protect barrier tissues from outside
infection
-First cells to arise during fetal development, drop off after
12. T cell development
Begins with arrival of small numbers of
lymphoid precursors migrating from
blood to thymus
○When they do arrive in thymus, T-cell
precursors don’t express signature surface
markers (CD3, CD4, and CD8)
○Do not yet express RAG-1 or RAG-2 that
are necessary for gene rearrangement
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13. 13
Thymic cells
Thymosin
Thymotaxin
Thymopoietin
Thymic factors
RAG1 & RAG2
CD4
CD8
TCR
Various TCR gene rearrangements
takes place at this point
MHC II
MHC I
+ selection
- selection
Thymocytes failing to recognize
both MHC molecules through
respective CDs, undergo
APOPTOSIS
FAS
FAS L
Self-antigen
Thymocytes recognizing self
antigen with high affinity undergo
APOPTOSIS
Stronger signal
between these two
decides the fate of
Single positive cell
14. Let’s just revise what we have learnt till now!!
◦ Progenitor T cells migrate to thymus
▫ At about 8th or 9th week of gestation in humans
◦ T cell maturation involves rearrangements
of the germ-line TCR genes
◦ In thymus, thymocytes proliferate and
differentiate
▫ Early Thymocyte Development
▫ Positive and Negative Selection
▫ Lineage Commitment
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18. You have Fluorescein-labeled anti-CD4 and phycoerythrin- labeled
anti-CD8. You use these antibodies to stain thymocytes and lymph-
node cells from normal mice and from RAG-1 knockout mice. In the
forms below, draw the FACS plots that you would expect.
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24. Selection process in thymus
Positive selection
○Survival of only T cells whose TCRs recognize self-MHC molecules
○Ensure MHC restriction
—
Negative selection
○Eliminates T cells that react too strongly with self MHC or MHC with
self-peptides
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25. Thymocytes “learn” MHC restriction in thymus
ž
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• Selection process results in development of
T cells that might respond to certain MHC
molecules
• This prevents supernumerary wandering
lymphocyte generation and also
‘economical’
26. Death by neglect
ž
T cell development is expensive for host
○98% of all thymocytes do not mature, die by
apoptosis within thymus
○However, the benefits of having functional T cells
outweigh the costs
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30. Do we delete Thymocytes reactive to tissue-specific
antigen?
Thymus has an extraordinary capacity to express and present proteins
from all over the body.
Medullary epithelial cells express a unique protein, AIRE (Autoimmune
regulator), that allows cells to express, process, and present proteins
that are ordinarily only found in specific organs.
AIRE is part of transcriptional complex that facilitates expression of
tissue-specific genes by regulating not only translation, but also
chromatin packing
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Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED) syndrome
Thymus: Cosmopolitan city of body
32. What stages of T-cell development (DN1, DN2, DN3,
DN4, DP, CD4 SP, or CD8 SP) would be affected in
mice with the following genetic modifications? Justify
your answers.
a. Mice that do not express MHC Class II.
b. Mice that do not express AIRE.
c. Mice that do not express the TCR-alpha chain.
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33. Exit from thymus and final maturation
ž
ž
Mature T cells that survive the selection
process leave the thymus
○Recent thymic emigrants
RTE are not as functionally mature as most naïve T cells in the periphery: they do not proliferate or
secrete cytokines as vigorously in response to T-cell receptor stimulation.
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34. Treg Cells
◦ Shown to inhibit proliferation of other T
cells in vitro CD4+CD25+
◦ Shown to inhibit development of
autoimmune diseases
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36. Cell Death and T cell populations
Apoptosis plays critical role
◦ —
Deletion of potentially autoreactive
thymocytes
◦ —
Deletion of T cell populations after activation
◦ Fas and FasL pathway to induce self death
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54. 54
Anergy
Lack of reaction by the body's defense mechanisms to foreign
substances, and consists of a direct induction of peripheral
lymphocyte tolerance
56. Q/A
◦ Which of the following conditions would lead to T-
cell anergy?
▫ A naïve T-cell interaction with a dendritic cell in
the presence of CTLA-4 Ig.
▫ A naïve T cell stimulated with antibodies that
bind both the TCR and CD28.
▫ A naïve T cell stimulated with antibodies that
bind only the TCR.
▫ A naïve T cell stimulated with antibodies that
bind only CD28.
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57. 57
Antigen Co-stimulation No co-stimulation
Infectious agent Protective immunity Recurrent infection
Innocuous substance Allergy No Allergy
Grafted organ Rejection Acceptance
Self organ Autoimmunity Self tolerance
Tumor Tumor immunity Cancer
Regulation of Co-stimulation is critical
60. 60
• Your lab acquires mice that do not have the GATA-3
gene (GATA-3 knockout mice). You discover that this
mouse has a di cult time clearing helminth (worm)
infections. Why might this be?
Q/A
65. Immunological Exhaustion
◦ Loss of CD8 T cell function
◦ Occurs with chronic infection or
cancer
◦ CD8 T cell exhaustion correlates
with higher antigen, duration of
infection, loss of CD4 help
◦ Blockade of inhibitory receptors can
rescue CD8 T cells
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Transporter associated with antigen processing (TAP) protein complex belongs to the ATP-binding-cassette transporter family
C-kit- receptor for stem cell growth factor
CD44-adhesion molecule
CD25- alpha chain of IL-2 Receptor
CCR7 directs t cells to lymph nodes
Assembly of this complex results in intracellular signals that induce a variety of processes, including the maturation to the DP stage
cortical thymic epithelial cells
Central tolerance
HEV specialized post-capillary venous swellings characterized by plump endothelial cells as opposed to the usual thinner endothelial cells found in regular venules.
Present in all secondary lymphoid organs
This tight binding stops further movement of the T cell which can then move between HEV cells into the lymph node by a process termed 'diapedesis’
Grail: Gene related to Anergy in lymphocytes
Cbl: negatively regulated activation signals; defect in cbl display lower activation threshold in patients
Clonal anergy: low-affinity ligand in the presence of costimulation
Tr1: subset of adaptive CD4+ T cells that promote immune tolerance and control excessive and/or inappropriate inflammation mediated by effector T cells and APCs
CD25: Also expressed by myeloid dendritic cells
Adaptive immune tolerance: Raised against Stress; Liver usually remains in relatively immune-compromised because it has to process many things
T-T: The unresponsive state induced (T-T Cell presentation in Table 1) had clonal anergy properties in that IL-2 production was inhibited, the state persisted in the absence of antigen, and it was reversible upon stimulation with IL-2.
Can talk about adverse effects of overstimulation like MAS.