T Cell Maturation and Activation

T cells are generated in the bone marrow and educated in the thymus.

The generation of a mature αβ T cell in the mouse

Vα-Jα

Pre-T cell

In the thymus

•< 5% of thymocytes develop into double-negative CD3+γδ T cells

Figure 7-13 part 2 of 2

Time course of appearance of γδ and αβthymocytes during mouse fetal development

After the double negative (DN) thymocytesexpress a productive rearranged TCR β chain, the newly synthesized b chains will associate with a 33-kDa glycoprotein called pre-Tαchain together with CD3 to form the pre-T cell receptor (pre-TCR).

Although the ligand to pre-TCR is unknown, pre-TCR will deliver several important signals.

Structure and activity of pre-T cell receptor (pre-TCR)

• Before the double negative pre-T cells advance to next stage, they begin proliferatebefore the TCR-α chain rearrangement take places.

• The proliferation phase prior to the rearrangement of α-chain increases the diversity of the T cell repertoire by generating a clone of cells expressing a single TCR−βchain with a different α chain.

T Cell Maturation undergo 2 Selection Processes in the Thymus:

Positive selection: permits the survival of only those T cells whose TCRs are capable of recognizing self-MHC molecule. MHC restriction

Negative selection: eliminates T cells that react too strongly with self-MHC plus self-peptide.

Self tolerance

~98% of thymocytes die by apoptosis, it is expensive for the host. However, both processes are necessary to achieve MHC restriction and self-tolerance.

Zinkernagel et al.

Thymus is Important in Selection of T Cell Repertoire

Thymus express MHC H-2b

(Containing LCMV specific cytotoxic T cells)

T cells only recognize targets expressing H-2b: the MHC haplotype of the thymus determines their MHC restriction.

What are cells important for positive and negative selections in the thymus?

Thymic stromal cells include:• Epithelial cells• Dendritic cells• Macrophages

They all express class I MHC molecules and high levels of class II MHC molecules to help the selection processes.

Selections occur in the cortical region of the thymus

Only those cells whose αβ TCR recognizes a self-MHC molecule are selected for survival

Only those cells whose αβ TCR has an intermediate affinity for self-MHC molecule are selected for survival

What are essential elements for positive and negative selections?

+

Class I MHC molecules are required for positive selection of CD8 T cells and class II MHC molecules are required for CD4 T cells

The TCR-MHC Interaction is Required for Positive Selection

Most thymocytesexpress the TCR transgene

CD8 thymocytes fail to mature because lack of interaction with MHC molecules

Evidence for Negative Selection: self antigen and MHC are required(CD8)

Most thymocytesexpress the TCR transgene

Thymocytesreactive to self-antigen are deleted during thymicselection

Hypotheses to explain the MHC-dependent positive and negative selection

• Avidity hypothesis: the outcome of MHC-peptide binding by TCR depends on the strength of the signal delivered by the receptor on binding, and this will, in turn, depend on both the affinity of the TCR for MHC-peptide complex and the densityof the complex on thymic epithelial cells.

• Differential-signaling hypothesis: the signals leading to positive and negative selection are different. The qualityrather than quantity determines different outcome.

• Both hypotheses have supporting evidence, it is possible that no single mechanism accounts for all the outcomes in the thymus.

Fetal thymic lobes are excised and place in culture. At this stage, thymus contains mainly CD4-CD8- thymocytes. It is a good model to study the details of thymic selections

leads to positive selection

leads to negative selection

No peptide on MHC class I molecules

The avidity of the TCR-MHC interaction was varied by the use of different concentrations of peptide.

Evidence Supports the Avidity Hypothesis

How a double positive precursor become CD4+ or CD8+ cells?

• Instructive model: the interaction between TCR, CD8, CD4, and MHC class I or class II instruct a cell to differentiate into either CD8+ or CD4+ cells.

• Stochastic model: CD4 or CD8 expression is switched off randomly.

• So far, it is not possible to choose one model over the other.

Summary:Summary:• The surface expression of pre-TCR is important for

pre-T cell development. Pre-TCR will deliver signals to stop further TCR β chain rearrangement, begin proliferation and CD4/CD8 expression, and possibly start TCR α chain rearrangement.

• T cell maturation occurs in the thymus where MHC restriction and self tolerance are achieved. (bypositive and negative selections)

• MHC molecules and the interaction between thymocytes and thymic stromal cells are important for the development of mature T cells.

T Cell Activation

T cells are activated in the secondary lymphoid tissues

Distribution of antigen-presenting cells in the lymph node

Strongest activators of naïve T cells

Naïve T cells encounter antigen during recirculation

The initiation of TCR signalingAPC

SLP-76

Ca2+

PKC

ZAP-70 phosphosylatesadaptor proteins that recruit components of several signal pathways

Immediate early gene expression after T helper cell activation

Early gene expression after T helper cell activation

Late gene expression after T helper cell activation

Two signals are necessary for full T cell activation:

• Signal 1: generated by interaction of MHC-peptide with the TCR-CD3 complex

• Signal 2: generated by interaction of CD28 on the T cells and members of the B7 family on the APC, it is also called co-stimulatory signal

Lack of co-stimulatory signal results in clonal anergy

•Anergy: cells are alive but not functional

Lack of co-stimulatory signal results in clonal anergy

Lack of co-stimulatory signal results in clonal anergy

Co-stimulatory signals are required for full T cell activation

Expressed on activated T cells

Expressed on dendritic cells activated macrophages andactivated B cells

Expressed on resting andactivated T cells

CTLA-4, an inhibitory receptor for B7 molecules, is up-regulated after T cell activation

Comparison of different antigen-presenting cells

Figure 8-18 part 2 of 2Comparison of different antigen-presenting cells

Superantigens: proteins that bind simultaneously to TCR Vβ chain and α chain of MHC class II. Superantigens induce T cell activation and proliferation.

Superantigens induce T-cell activation by binding the TCR and MHC II simultaneously

Exogenous superantigens are mainly exotoxins secreted by gram-positive bacteria

Endogenous Superantigens: cell-membrane proteins encoded by certain viruses that infect mammalian cells.

• The activation by superantigens is polyclonal.

• The massive activation by superantigens results in overproduction of T helper cell cytokines, leading to systemic toxicity.

Activation of T cells generate effector and memory T cells

• Naïve T cells: cells never meet antigens before. They can only be activated by dendritic cells.

• Effector cells: short-lived cells with special functions such as cytokine secretion and B-cell help and cytotoxickilling activity. Effector cells are derived from naïve or memory cells after antigen activation. TH1 and TH2 subsets.

• Memory cells: long-lived resting cells that are derived from naïve and effector cells. They respond faster and stronger to a subsequent challenge with the same antigen.

• CD4+CD25+ regulatory T cells: cells that can inhibit the proliferation of other T cell population in vitro and also inhibit the development of experimental autoimmune systems.

Termination of T cell activation

Two pathwaysLead to apoptosisIn T cells

Activation-inducedcell death (AICD)

Failure of apoptosis causes defective lymphocyte homeostasisCanale-Smith symdrome:fas+/- , elevated lymphocyte numbers,Hyper-gammaglobulinemia,Autoantibodies against RBC and platelets

lpr/lpr mice:Fas mutant gld/gld mice: FasL mutant

Blood samples

Summary:Summary:• Two signals are required for full T cell activation, one

from TCR-CD3 interactions with MHC-peptide complex, the other from CD28 interactions with B7 family proteins. Thus, CD28 and B7 family are called “co-stimulatory molecules”.

• Lack of co-stimulatory signals in T cells results in clonal anergy or sometimes apoptosis.

• What are superantigens?

• Fas-mediated and activation-induced cell death are used to remove activated T cells and maintain T cells homeostasis.