T cell Development

Basic Principles of T Cell Development

• Each cell randomly rearranges a specific TCR.• The presence of the pMHC epitope, stage of

development and “costimulation” determine the cell’s fate.

• Functional Immunity & Tolerance result from the sum of a large population of many sequential pMHC epitope specific cellular responses.

• Following development into mature, antigen-responsive T cells, these T cells emerge from the thymus and migrate to secondary lymphoid tissues, where they interact with antigen, antigen-presenting cells, and other lymphocytes:  

• The importance of the thymus in T cell development is demonstrated by inherited immune deficiencies: people that do not have a thymus (DiGeorge’s syndrome, aka Thymic Aplasia) do not develop functional T cells.

• DiGeorge’s syndrome results from a developmental defect – the failure of the third and fourth pharyngeal pouches to develop, which results not just in thymic defects, but also in absent parathyroids and in aortic arch defects.

• Thymectomy early in life reduces the ability to produce T cells.

• Thymectomy later in life does not markedly impair T cell number.

• In fact, the thymus decreases in size with age.

• However, the thymus can still produce new T cells up to middle-age, especially in situations where there is loss of T cells (HIV/AIDS).  

• While in the thymus, immature T cells, or thymocytes, undergo several changes that allow them to develop into mature T cells, ready for contact with antigen.

• Thymocytes interact with thymic epithelial cells and various other cells while in the thymus.

• The thymus is composed of several lobes, each of which has cortical and medullary regions:

• The cortex contains immature thymocytes in close contact with thymic epithelial cells.

• Medullary areas contain more mature thymocytes, epithelial cells, and dendritic cells and macrophages

• During thymic differentiation, the great majority of thymocytes die by apoptosis, and are ingested by macrophages.

• Only a small minority of these T cell progenitors make it out as mature T cells

• Thymic development occurs in two phases:

1) production of T cell receptors for antigen, by rearrangement of the TCR genes

2) selection of T cells that can interact effectively with self-MHC

 

• Changes in the expression of cell-surface molecules accompany the thymic differentiation of T cells:

– entering thymocytes are TCR, CD3, CD4, and CD8-negative

– as thymocytes mature, and undergo rearrangement of their TCR genes to generate a functional TCR, they begin to express CD3, CD4, and CD8

– mature T cells ready to go to the periphery are TCR/CD3+, and either CD4 or CD8 positive

First phase of thymic development: rearrangement of TCR genes to

produce a functional TCR

• Progenitor T cells enter the thymus (sub-capsular region of the outer cortex).

• These cells do not have rearranged TCR genes and lack expression of characteristic T cell surface molecules.

• Interaction with thymic stromal cells induces these progenitor T cells to proliferate.

• These immature thymocytes do not yet express CD4 or CD8, molecules that are expressed by mature T cells: double-negative thymocytes.

  

• There are two types of T cell receptors: gd and ab

 

• ab TCR T cells are the most abundant, by far:

 

(or g & d chain)

Unlike B cells, in which the genes that encode the BCR rearrange in a set order, the TCR b, g, and d genes start to rearrange at about the same time.

 If a productive g or d rearrangement occurs first, the T cell is committed to that lineage, and stops further rearrangement of the b TCR gene.

 

However, if b is rearranged first, then the T cell continues to proliferate, and undergoes further rearrangements.

This results either in rearranged a TCR gene, yielding an ab TCR lineage cell, or rearranging g and d genes, resulting in a gd TCR cell.

 

Rearrangements that lead to an ab T cell begin the rearrangement of the b TCR gene.

The first step is D-J joining, followed by VDJ rearrangement.

Expression of b chain stops further b chain rearrangements.

 

• b chain is then expressed on the surface of the thymocyte in association with a surrogate a chain (pTa).

 • Following this, there is rearrangement of the a TCR

gene, resulting in a functional a chain, and in the expression of surface TCR, in association with other T cell-associated cell surface molecules.  

• During this process, a cell that makes an unproductive a chain rearrangement can try again until gets a good a chain, or it exhausts its possibilities:

• Thymocytes that have a functional b rearrangement, and express ab or b + the surrogate a chain (pTa) are induced to express both CD4 and CD8 simultaneously – these are called double-positive cells.

 • Immature T cells that do not undergo a productive

rearrangement die by apoptosis.

Second phase of thymic development: selection of T cells that can interact with

self MHC and antigen

• This applies only to ab TCR-bearing cells (>95% of T cells).

• gd T cells are not restricted to interactions with MHC class I or class II molecules

• This phase of T cell development consists of two steps:

– positive selection (TCR that can interact with self-MHC)

– negative selection (eliminate self-reactive cells that are stimulated by MHC + self)

 

Transgenic TCRs to Self AntigensClonal Deletion in Thymus

• Double TCR transgenic mice (both TCR a and b) using a TCR from a CTL clone specific for male antigen (H-Y/H-2Db).

• Female mice – most mature T cells expressed transgene TCR– normal proportion of CD4+ and CD8+– 30% of CD8+ cells specific for male H-Y.

• Male mice– large decrease (90%) in thymocytes (including

CD4+8+)– down regulation of CD8+ among peripheral T cells– No H-Y reactive cells found; no autoimmunity

P. Kisielow, H. Blüthmann, U. D. Staerz, M. Steinmetz, & H. von Boehmer. Nature 333:742-746 (1988)

C57BL/6 control male

HY TCR transgenic Female

HY TCR transgenicmale

Expression of TCR Tg in females with H-2Db

results in development of CD8+T (increased CD8/CD4)

Expression of TCR tg and autoantigen in males results in deletion of DP thymocytes and blocks development of Tg+ CD8 SP

Harold von Boehmer HY TCR transgenic mice

In Rag-/-HY TCR transgenics all thymocytes express the TCR tg. More dramatic skewing to

CD8 in females and deletion in males

Female HY TCR tg

CD4

0.6%;

5.8%;

1.2x106 101x106

35x106 21x1062.8%;

91%;

75%; 26x106

11%; 2x106 5%; 1.5x106

9%; 2.7x106 CD4

CD8

Male HY TCR tg

CD4/CD8 lineage commitment

• Positive selection refers to the selection of thymocytes that are able to bind to, and interact with, self-MHC molecules

• In positive selection developing thymocytes continue to live if they bind MHC well enough to receive a signal through their TCR.

• This signal is mediated by the interactions of these cells with MHC-expressing thymic cortical epithelial cells.

• The ~95% of thymocytes that do not receive this signal undergo apoptosis.

Positive Selection

Positive selection takes place in the cortex of the thymus lobules:

• These CD4+ CD8+ TCR+ thymocytes interact with thymic epithelial cells that express both MHC class I and MHC class II molecules, complexed with self-peptides.

• Thymocytes that bind MHC survive; those that don’t bind to self-MHC die.

 • TCR a chain rearrangements can continue during positive

selection, allowing cells to explore alternative a chains for MHC binding.

• Once a T cell is positively selected, TCR rearrangement stops.

 

 

Topi KO per MHC di classe II e transgenici per MHC classe II espressa esclusivamente a livello della regione corticale del timo sotto il controllo di un promotore specifico per i cheratinociti

Selezione positiva di tutte le cellule T che riconoscono complessi MHC + peptide anche quelle fortemente autoreattive

In assenza di presentazione dell’Ag a livello della midollare si ha selezione positiva incontrastata

• Negative selection refers to the elimination of those thymocytes that bind to self-MHC molecules + self with high affinity.

• In negative selection developing thymocytes die if they bind MHC + self peptides too well (strongly enough so that they would be activated by this interaction, via signaling through their TCR).

Negative Selection

• Thymocytes undergo negative selection in the medullary region:

• There, they interact with antigen-presenting cells (dendritic cells, macrophages) that express self-antigens + MHC class I or MHC class II molecules.

• Thymocytes that bind to self + MHC too strongly are eliminated as possibly self-reactive cells, and undergo apoptosis.

• If self-reactive T cells were allowed to exit the thymus, such cells would mediate autoimmune disease.

Thymic Selection

Effects of peptides acting as agonists, antagonists or neither on thymic selection

Receptor editing