Chapter 10 T-Cell Maturation, Activation, and Differentiation Dec 7 & 12, 2006 MHC CD4/CD8 TCR 

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 maturation

receptor for stem-cell growth factor adhesion molecule involved 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 I class II

Negative Selection Ensures 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 Thymocytes to Class I or Class II MHC Molecules

CD8 + T cells only mature in transgenics with H-2 k 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 cells in 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  2 M. - 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 T H cells. Interaction of a T H cell with Ag initiates a cascade of biochemical events that induces the resting T H cell to enter the cell cycle, proliferating and differentiating into effector cells or memory cells. T H -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 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.

T H -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 expressed on 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 T H cell receives the TCR-mediated signal (signal 1) in the absence of a suitable co-stimulatory signal (signal 2), the T H cell become anergic. - In the presence of both signal 1 and signal 2, clonal expansion results.

CD28 no signal 2 induction of B7 is inhibited.

The Resulting Anergic T Cells Cannot Respond to Normal APCs

signal 1 signal 2 signal 1 signal 2

Differences in the Properties of 3 Kinds of Professional APCs

Activation of a T H 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 x ↑ effector T → T H & T C memory cells entry into cell cycle

Effector T H cells: short-lived T H 1 subset: - secretes IL-2, IFN- , and TNF-  - responsible for cell-mediated functions, such as delayed-type hypersensitivity and the activation of CTL T H 2 subset: - secretes IL-4, IL-5, IL-6, and IL-10 - helper for B-cell activation Memory T H cells: long-lived - Less stringent requirements for activation due to the expression of high levels of numerous adhesion molecules Regulatory T cells (T reg ): 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 V  sequences 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 apoptotic thymocyte

Two Pathways to T-cell Apoptosis Fas-associated protein with death domain AICD : activation- induced cell death TCR- mediated negative selection: apoptosis- inducing factor apoptotic protease- activating factor 1