Dr.Fadwa Al-Ghalib
Immunology
T-cell maturation and selection
2nd year
T-Cell production and maturation
THYMUS:
 The Thymus is a lymphoid organ in the upper anterior thorax , above the heart.
 It contains a multilobed organ which are divided into outer region The Cortex and
the inner Medulla.
Anatomical Organization of the Thymus
• The epithelial nurse cells are in the outer cortex
•
The Cortex contains epithelial cells and many immature thymocytes with a
few macrophages.
•
The corticomedullary junction contains interdigitating dendretic cells
(IDC) and macrophages
•
The Medulla contains epithelial cells, mature thymocytes, lots of
macrophages and dendritic cells.
•
Epithelial cells and IDCs and macrophages express MHC class II
molecules which are crucial to T-cell development and selection.
•
Hassall’s corpusculs are found in the thymic medulla. Their function is
unknown.Appear to contain degenerating epithelial cells.
Maturation of T cells
Maturation of thymocytes into mature T cells occur in distinct stages:
 Changes in the status of the T-cell receptor genes
 Expression of the T- cell receptor protein
 Production of other T-cell surface glycoproteins (essential for receptor’s full
function, such as CD4,CD8 and CD3 complex).
Phenotypic Variations of T-cell during maturation
After T cells enter the thymus they begin to proliferate and differentiate into what will be
the T cells
1- Stage 1 ( early thymocytes), Double –negative cells:
 These are the cells which enter the subcapsular region of the thymus, arriving as
pre-T cells from the bone marrow. (progenitor cells)
 In this region, progenitor cells interact with thymic stromal cells, they get
signaled to proliferate, within a week they express certain T-cell specific
glycoproteins, such as: CD44(adhesion molecule), CD25 (receptor for IL-2).
 At this stage, they do not express TCR complex or the co-receptors CD4 &CD8.
(therefore CD4-, CD8- are = double negative).
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Dr.Fadwa Al-Ghalib
Immunology
T-cell maturation and selection
2nd year
 These cells develop into large, actively proliferating, self-renewing lymphoblasts
which generate the thymocyte population.
 They are 3% of the cells in the thymus.
2- Stage II (Intermediate) thymocytes, Double-positive Cells:
 expressing CD1+, CD44-,CD25 At this stage, the cells develop the CD3:TCR complex, CD4 and CD8.
 Therefore the cells are CD3:pTa:b+,CD4+,CD8+.
 Genes encoding the TCR  chain are rearranged in this stage
 They are about 80% of cells in the thymus.
3- Stage III (mature) thymocytes.
Show major phenotypic changes;
 Loss of CD1
 cell surface CD3 is associated with high density  TCR
 mature T cells express either CD4 or CD8
There are at least two pathways of T-cell differentiation in the thymus:
1. Less than 1% of mature thymic lymphocytes express the  TCR
2. Most of thymic lymphocytes differentiate into βTCR cells which account >95%
of T- lymphocytes found in the secondary lymphoid tissue and in the circulation.
T-Cell surface diversity (β & )
 T cells express either β (95% of T cells) or  TCR for their whole life
span.
 The earliest T cells seen during fetal development express  TCR.
 It is believed that if  and  are productively rearranged first, the cell will
probably become a  T cell.
 If β is productively rearranged first and expressed on the membrane with
surrogate  chain (pT), the cell will usually go on to rearrange a chain
gene segments and become an β T cell.
 Less than 5% of peripheral T cells have  TCRs
 but they are the major type of T cells in skin, intestinal epithelium and
pulmonary epithelium
 TCR
These T cells having  TCR are distributed in two populations.
1.
One population, the intraepidermal lymphocytes, are found in the skin and are
CD4- 8- (double-negative).
2. The other population, the intraepithelial lymphocytes, are found in the intestinal
epithelium and are CD8+.
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Dr.Fadwa Al-Ghalib
Immunology
T-cell maturation and selection
2nd year
The  TCRs are characterized by limited diversity. Some of these cells can bind
antigens directly, without the mediation of MHC molecules.
The development of T cells in the Thymus
1)---Subcapsular region:
Thymocytes enter from blood to the subcapsular region. Are known as double-negative
thymocytes. (they don’t express the Ag receptor: CD3 complex, CD4 &CD8 co-receptors.
These cells proliferate, rearrange their β,, and  T-cell receptor gene. production of 
cells & cells expressing the pre-T-cell receptor. Finally producing double- positive cells
(DP) for CD4 & CD8.
2)---Cortex (Positive selection).
• DP thymocytes rearrange their -chain genes.
• Express an :βT-cell receptor & CD3 complex, become sensitive to interaction
with self-peptide:self:MHC complexes.
• DP –cells undergo positive selection in thymus cortex via contact with cortical
epithelial cells.
What is the role of Thymic Cortical Cells in positive selection?
• Is to ask the thymocytes if they can recognize/bind self Ag-MHC?.
Either:
A- if the cell cannot recognize the MHC molecules on the surface of the thymic cortical
epithelial cells, it “fails” positive selection and dies.
B- if it recognizes self-MHC, it “ passes” positive selection.
Positive Selection:
•
•
•
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DP cells die within 3-4 days unless they recognize pMHC-I or pMHC-II on
cortical epithelial cells in the cortical region.
Thymocytes surviving positive selection express only CD4 or CD8 in addition to
CD3 and TCR.
The developing double- positive cell will become a single positive cell and
retains either CD4 or CD8.
Retention of a co-receptor is based on which class of MHC molecules is
recognized by the TCR.
3)---Cortico-medullary junction (Negative Selection)
• DP cells undergo negative selection for self-reactivity.
• Occurrs at the cortico-medullary junction.
• Nearly mature thymocytes encounter a high density of dendritic cells.
a- Medullary thymocytes that interact strongly with self peptides on MHC-I or
MHC-II of APC (macrophages & dendritic cells) undergo apoptosis.
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Dr.Fadwa Al-Ghalib
Immunology
T-cell maturation and selection
2nd year
b- If a developing CD4+, CD8+ T cell does not recognize any peptide/MHC
complex it too will be induced to undergo apoptosis
This is called : Negative Selection
4)---Medulla:
Thymocytes that survive both positive and negative selection leave the thymus in the
blood as mature single positive CD4 or CD8 T cells and enter the circulation
 Only 1-2% of the cells in the thymus pass both selection procedures and are allowed to
continue maturation
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Dr.Fadwa Al-Ghalib
Immunology
T-cell maturation and selection
2nd year
Positive Selection:
About being able to recognize self-MHC
o Occurs in the cortex of thymus.
o Requires CD3: :β+, CD4+/ CD8+ thymocytes.
o They are bound by cortical epithelial cells
( have both Class I and Class II MHC molecules on their surface).
 [If a peptide:MHC complex is bound within 3-4days of the thymocytes
expressing a functional receptor, then a positive signal is delivered to the
thymocytes, which continues its maturation].
Negative selection
• occurs when double positive T cells bind to bone-marrow derived APC
(macrophages and dendritic cells) expressing Class I or Class II MHC plus self
peptides with a high enough affinity to receive an apoptosis signal.
•
If the T cell receptor of CD4+, CD8+ T lymphocytes reacts STRONGLY with
the peptide/MHC complex,  T cell will be negatively selected and induced to
undergo programmed cell death (apoptosis).
•
If a developing CD4+, CD8+ T cell does not recognize any peptide/MHC
complex it too will be induced to undergo apoptosis
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Dr.Fadwa Al-Ghalib
•
•
Immunology
T-cell maturation and selection
2nd year
Note that selection occurs on self peptides in the thymus; MHC presents self
peptides in the absence of pathogen.
Only 1-2% of the cells in the thymus pass both selection procedures and are
allowed to continue maturation
Cortico thymocytes are less mature than medullary thymocytes, it means that cortico
cells migrate to, and mature in medulla
Most mature cells leave the thymus via postcapillary venules located at the corticomedullary junction, However other routes of exit may exist including lymphatic vessels.
4-Entry to the circulation
The Net site for the T-cell selection animation is :
http://www.bio.davidson.edu/courses/Immunology/Flash/Main.html
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