Chapter 10

T-Cell Maturation, Activation, and Differentiation

Dec 7 & 12, 2006

MHC

CD4/CD8TCR

你需要學習的課題：

1. 什麼叫做 positive selection 及 negative selection ？ 它們產生什麼結果？

2. T 細胞需要 costimulatory signals 才得完全活化。

3. 什麼叫做 clonal anergy ？

4. Superantigen 有何性質及作用？

5. T 細胞分化及其 subpopulations 。

6. T 細胞凋亡。

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.

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

m

aturation

receptor for stem-cell growth factor

adhesion moleculeinvolved in homing

chain of IL-2R

(double negative; CD4-CD8-)

recombination-activating gene(RAG-1/2)

Development of T Cells in the Mouse

[CD3-, CD4-, CD8-]

H

L

< 5%

positive selection

negative selection

Pre-TCR (pre-T/ TCR) Activates Signal Transduction Pathways

Thymic Selection of the T-Cell Repertoire

Positive Selection Ensures MHC Restriction

death by neglect !

(double positive cells)

class Iclass II

Negative SelectionEnsures Self-tolerance

chimeric mouse

irradiated

(source for cytotoxic T cells)

Thymus only selects for T cells whose TCRs recognize Ag presented on target cells with the haplotype of the thymus

infect with LCMV

Positive Selection Requires Binding of Thymocytesto Class I or Class II MHC Molecules

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.

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.

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 ???

Study of Thymic Selection in Vitro

gestational age of day 16

consisting of CD4-CD8- thymocytes

↓

Study the development

of T cellsin vitro

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 chains and 2M.

- Addition of pre-digested peptides to RMA-S cells restores the expression of class I MHC molecules on the membrane.

Effect of Peptides in Thymic Selection

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.

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)

Models for the Role of CD4/CD8 Coreceptors in DP → SP (double positive to single positive) T Cells

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.

TH -Cell Activation

TCR Engagement Initiates Multiple Signaling Pathways

Overview of Common Themes in Signal Transduction (Chapter 1 p.11)

(hydrophobic)

Multiple Signaling Pathways Are Initiated by TCR Engagement

ITAM: immunoreceptor tyrosine-based activation motif

Activation of the small G protein, Ras

Immediate genes, expressed within 30 min of Ag recognition, encode a number of transcription factors.

Early genes, expressed within 1 – 2 hr of Ag recognition, encode cytokines & cytokine receptors.

Late genes, expressed more than 2 days after Ag recognition, encode various adhesion molecules.

Co-stimulatory Signals Are Required forFull 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.

TH-cell Activation Requires a Co-stimulatory Signal Provided by APCs

CD28 is expressed by both resting and activated T cells.

B7 (B7-1 and B7-2) are constitutively expressed on dendritic cells, and induced on activated macrophages and activated B cells.

CTLA-4 is expressedon activated T cells.

B7-1 (CD80)

B7-2 (CD86)(CD152)

CD28 & CTLA-4 act antagonistically.

+

-

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.

CD28CD28 no signal 2no signal 2

no signal 2no signal 2

inductionof B7 is inhibited.

The Resulting Anergic T Cells Cannot Respond to Normal APCs

signal 1signal 1

signal 2signal 2

signal 1signal 1

signal 2signal 2

Differences in the Properties of 3 Kinds of Professional APCs

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 ↑

effector T → TH & T Cmemory cells

entry into cell cycle

Effector TH cells: short-lived

TH1 subset: - secretes IL-2, IFN-, and TNF- - 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): CD4+CD25+ cells that inhibit

the proliferation of other T-cell populations in vitro.

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 Vsequences of TCR and to the chain of a class II MHC molecule – induce poly- clonal T-cell activation & proliferation

Programmed Cell Death Is an Essential Homeostatic Mechanism

normal apoptoticthymocyte thymocyte

Two Pathways to T-cell Apoptosis

Fas-associated protein with death domain

AICD : activation-inducedcell death

TCR-mediated negative selection:

apoptosis-inducing factor

apoptotic protease-activating factor 1