T-Cell Maturation, Activation, and

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Chapter 10 T-Cell Maturation, Activation,
and Differentiation
MHC
CD4/CD8
TCR
gde
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Dec 7 12, 2006
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???????? 1. ???? positive selection ? negative
selection? ????????? 2. T ????
costimulatory signals ??????? 3. ???? clonal
anergy? 4. Superantigen ???????? 5. T ??????
subpopulations? 6. T ?????
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Two-step Selection Process of Thymocytes
Positive Selection permits the survival of only
those T cells whose TCRs recognize
self-MHC molecules. ? generation of a
self-MHC-restricted
repertoire of T cells Negative Selection
eliminates T cells that react too strongly
with self-MHC or with self-MHC plus
self-peptides. ? generation of a T-cell
repertoire
that is self-tolerant Thymic stromal
cells, which express high levels of class I
and class II MHC molecules, play a role in
this process.
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Thymic Stromal Cells Play Essential Roles in
Positive and Negative Selection
Stromal cells (expressing Notch ligand) produce
regulatory factors and express high levels of
class I and class II MHC molecules
maturation

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Development of ab T Cells in the Mouse
receptor for stem-cell growth factor
adhesion molecule involved in homing
CD3-, CD4-, CD8-
recombination-activating gene (RAG-1/2)
a chain of IL-2R
(double negative CD4-CD8-)
H
L
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positive selection negative selection
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Pre-TCR (pre-Ta / TCRb) Activates Signal
Transduction Pathways
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Thymic Selection of the T-Cell Repertoire
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Positive Selection Ensures MHC Restriction
(double positive cells)
class I
class II
death by neglect !
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Negative Selection Ensures Self-tolerance
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Thymus only selects for T cells whose TCRs
recognize Ag presented on target cells with the
haplotype of the thymus
irradiated
infect with LCMV
chimeric mouse
(source for cytotoxic T cells)
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Positive Selection Requires Binding of
Thymocytes to Class I or Class II MHC Molecules
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CD8 T cells only mature in transgenics with H-2k
corresponding to the MHC restriction of the TCR
transgene
? Interaction between TCR and self-MHC on
immature thymocytes is required for
positive selection.
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Negative Selection of Thymocytes Requires Both
Self-peptide Self-MHC
? Interaction between TCR and MHC
peptide on immature thymocytes is
required for negative selection. ?
H-Y-reactive thymocytes were self-reactive
in the male mice and were eliminated.
?
?
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A Paradox in Positive and Negative Selection
The T cells which recognize self-MHC
after positive selection ? survival
? after negative
selection ? death What is left
???
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Study of Thymic Selection in Vitro
consisting of CD4-CD8- thymocytes
gestational age of day 16
? Study the development of T cells in vitro
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Effect of Peptides in Thymic Selection

• TAP deficient cells (described in Chapter 8 p.
  211)
• RMA-S cell line is an antigen
  processing-defective tumor cell line. It
• expresses only 5 of the normal levels of
  class I MHC on the membrane,
• although the cells synthesize normal levels
  of class I a chains and b2M.
• Addition of pre-digested peptides to RMA-S
  cells restores the expression
• of class I MHC molecules on the membrane.

TAP -/- mouse
very few CD8 cells
When a diverse peptide mixture is added to the
culture, the CD8 T-cell restoration is greater
than when a single peptide is added.
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dependent on TCR-peptide-class I interactions
recognizes a specific MHC- LCMV peptide
unable to form peptide-MHC complexes unless
peptide is added
avidity hypothesis (quantitative) vs differential-
signaling hypothesis (qualitative)
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Models for the Role of CD4/CD8 Coreceptors in DP
? SP (double positive to single positive) T Cells
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TH -Cell Activation
The central event in the generation of both
humoral and cell-mediated immune responses is the
activation and clonal expansion of TH cells.
Interaction of a TH cell with Ag initiates a
cascade of biochemical events that induces the
resting TH cell to enter the cell cycle,
proliferating and differentiating into effector
cells or memory cells.
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TCR Engagement Initiates Multiple Signaling
Pathways Overview of Common Themes in Signal
Transduction (Chapter 1 p.11)
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Multiple Signaling Pathways Are Initiated by TCR
Engagement
ITAM immunoreceptor tyrosine-based activation
motif
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Activation of the small G protein, Ras
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Immediate genes, expressed within 30 min of Ag
recognition, encode a number of transcription
factors.
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Early genes, expressed within 1 2 hr of Ag
recognition, encode cytokines cytokine
receptors.
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Late genes, expressed more than 2 days after Ag
recognition, encode various adhesion molecules.
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Co-stimulatory Signals Are Required for Full
T-cell Activation Naïve T cells require 2
distinct signals for activation and proliferation
into effector cells Signal 1 the initial
signal, is generated by
interaction of an antigenic peptide
with the TCR-CD3 complex. Signal 2
a subsequent Ag-nonspecific co-
stimulatory signal, is provided by
interactions between CD28 on the T
cell and members of the B7 family
on the APC.
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TH-cell Activation Requires a Co-stimulatory
Signal Provided by APCs
CD28 is expressed by both resting and activated
T cells.
-
B7-1 (CD80)
B7 (B7-1 and B7-2) are constitutively expressed
on dendritic cells, and induced on activated
macrophages and activated B cells.
B7-2 (CD86)
(CD152)
CTLA-4 is expressed on activated T cells.
CD28 CTLA-4 act antagonistically.
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Clonal Anergy Ensues if a Co-stimulatory Signal
Is Absent Clonal Anergy (Unresponsiveness)
- inability of cells to proliferate in
response to a peptide-MHC
complex - If a resting TH cell
receives the TCR-mediated signal
(signal 1) in the absence of a suitable
co-stimulatory signal (signal 2),
the TH cell become anergic. - In the
presence of both signal 1 and signal 2, clonal
expansion results.
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induction of B7 is inhibited.
CD28
no signal 2
no signal 2
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The Resulting Anergic T Cells Cannot Respond to
Normal APCs
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signal 1
signal 2
signal 1
signal 2
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Differences in the Properties of 3 Kinds of
Professional APCs
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Activation of a TH Cell Up-regulates Expression
of IL-2 and IL-2 Receptor, Leading to the Entry
of the T Cell into the Cell Cycle
The co-stimulatory signal increases the half-life
of the IL-2 mRNA ? IL-2 100 x ?
entry into cell cycle
effector T ? TH T C
memory cells
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Effector TH cells short-lived TH1 subset
- secretes IL-2, IFN-g, and TNF-b
- responsible for cell-mediated
functions, such
as delayed-type hypersensitivity and
the activation of CTL TH2
subset - secretes IL-4, IL-5, IL-6, and IL-10
- helper for B-cell
activation Memory TH cells long-lived -
Less stringent requirements for activation due to
the expression of high levels of
numerous adhesion molecules Regulatory T cells
(Treg) CD4CD25 cells that inhibit
the proliferation of other T-cell
populations in vitro.
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Superantigen-mediated Crosslinkage of TCR and
Class II MHC Molecules
Exogenous superantigens exotoxins secreted by
G() bacteria, e.g., staphylococcal
enterotoxin A B (SEA, SEB) Endogenous
superantigens cell-membrane proteins
encoded by certain viruses that infect
mammalian cells, e.g., mouse mammary tumor
viral (MMTV) protein

• Viral or bacterial proteins that bind
  simultaneously
• to particular Vb sequences of TCR and to the a
• chain of a class II MHC molecule induce poly-
• clonal T-cell activation proliferation

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Programmed Cell Death Is an Essential Homeostatic
Mechanism
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normal
apoptotic thymocyte thymocyte
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AICD activation-induced cell death
TCR-mediated negative selection
Two Pathways to T-cell Apoptosis
Fas-associated protein with death domain
apoptosis- inducing factor
apoptotic protease-activating factor 1