Download T Cell Development in the Thymus David Straus

T Cell Development in the
Thymus
David Straus
Lecture objectives
•
T cell life cycle: development in the thymus, circulation as mature,
naïve cells, activation in response to antigen, differentiation into
memory cells. Change in composition of the T cell pool with age.
•
Generation of diverse antigen receptors on T cells through sequential
rearrangement of ß and a gene segments.
•
Shaping of the repertoire through positive and negative selection.
•
Relation of DiGeorge, Omenn and Autoimmune Polyglandular
syndromes to thymic development.
•
Why matching of HLA types is important for restoration of immune
function following bone marrow transplantation.
•
Generation of distinct T cell lineages: CD4, CD8, T reg as well as g/d
T cells.
Adaptive immunity relies on having a huge pool of lymphocytes,
each with unique antigen recognition capabilities. These clones can
be expanded and go on to generate a memory population.
death
memory
Antigen receptors on T cells only
recognize antigen when it is processed
and bound to MHC molecules on the
surface of other cells.
This allows T cells to specifically target
their immune function to cells which
have encountered pathogens.
A variety of distinct T cell lineages provide immune functions
gd T cells
Treg cells
gd
Treg
cytokines
?
T
T cell precursors migrate from the bone marrow to the thymus where
they mature. Mature T cells exit the thymus, enter the circulation, and travel through
secondary lymphoid organs.
Figure 1-11
Mature T cells circulate through
the blood and lymphatics.
Processed antigen, bound to
MHC on antigen presenting
cells, is encountered in the
lymph nodes where T cell
activation takes place. Activated
T cells provide effector function
either in the lymph nodes, or
after migration to the site of
infection.
The pool of circulating T cells is composed of thymus-derived
naïve T cells, and memory T cells generated following
stimulation with antigen. The size of the pool is regulated by
competition for cytokine and receptor signals.
Thymus
proliferation
Circulating T cell pool
Cellular organization of the thymus
Epithelial cells form a
dense network surrounding
the developing thymocytes
DiGeorge Syndrome
• Genetic disease associated with chromosomal
rearrangements which effect development– in severe
form thymus fails to develop.
• T cells cannot develop
• SCID (Severe Combined Immunodeficiency Disease)
- no T or B cell function
Thymic atrophy alters composition of the
T cell pool
•
Thymus is most active in young children and
atrophies markedly with age. Thymic output is
substantially diminished by age 50.
•
T cell pool in older adults relies on homeostatic
proliferation of existing T cells which favors
memory T cells.
Memory T cells have a greater capacity for homeostatic proliferation, and so
contribute a greater fraction of the T cell pool as thymic output is lost. This
results in a restriction of the T cell repertoire in older adults
Child
How is the T cell pool restored following
lymphoablative conditions (e.g. chemotherapy,
acquired immunodeficiency)?
-In children, restoration of the T cell pool primarily
by thymic production.
-In older adults, restoration accomplished primarily
by expansion of remaining lymphocytes - a limited
repertoire.
Thymocyte development
progenitor
initial repertoire
selected repertoire
multiple lineages
A pool of T cells with diverse antigen specificity is generated by
rearrangement of gene segments coding for the T cell antigen
receptor (TCR)
a
TCR
b
Rearrangement of gene segments encoding the T cell receptor b
subunit occurs prior to alpha gene rearrangement
Following successful ß gene rearrangement, ß is expressed on the
cell surface with pre-T alpha (pTa) as part of the Pre-TCR. This
blocks further ß gene rearrangement (allelic exclusion).
Pre-TCR expression promotes a gene rearrangement. When a
is successfully rearranged and expressed, it replaces pTa, and
forms the mature ab TCR on the cell surface.
Rearrangement summary
•
TCR b gene rearrangement occurs first – successful b
rearrangement leads to cell surface expression with a surrogate a chain (pTa).
•
The resulting pre-TCR signals cause b rearrangement to stop (allelic
exclusion) and promotes further development - proliferation and the
start of a chain rearrangement.
•
With successful a chain rearrangement, pTa is replaced and the
mature ab TCR is expressed on the cell surface.
Omenn Syndrome
1. Recombinase enzyme, encoded by RAG genes,
mediates gene rearrangements necessary for
antigen receptor expression during development.
2. Omenn syndrome is a result of inherited RAG
deficiency which causes a block in development.
3. Patients have a SCID (Severe Combined
Immunodeficiency Syndrome) phenotype. RAG
function is required for both T and B cell
development.
Question:
An infant has recurrent infections, including infections
with ‘opportunistic’ pathogens. This is consistent with Severe
Combined Immunodeficiency (SCID).
You propose either Omenn syndrome, or DiGeorge syndrome as a
possible cause.
How could you tell the difference between these two?
Flow cytometry can be used to determine the representation of
specific cell populations (such as T and B cells) in complex mixtures
(such as peripheral blood).
Figure A-17 part 1 of 2
Antibodies with an attached fluorophore will emit light at a characteristic
wavelength when stimulated with the appropriate light.
Flow cytometry allows the
rapid determination of the
amount of antibody
labeling, and the type of
fluorescence(color of
fluorophore), associated
with individual cells.
The DNA rearrangements that take place during
lymphoid development can generate chromosomal
translocations which give rise to malignancies.
Thymocyte development
progenitor
Initial repertoire
selected repertoire
Positive selection
1. T cell function demands that the antigen receptor
recognize MHC- antigen complex. Receptors which
cannot bind MHC are not useful.
2. Thymocyte development requires that newly expressed
ab TCR engage MHC on thymic epithelium well
enough to send a survival signal.
3. Failure of newly expressed ab TCR to engage MHC
results in thymocyte death.
Positive selection: Recognition of MHC-peptide on thymic epithelial cells provides a
survival signal for developing thymocytes. Rearrangement of a-chain gene stops after a
positive selection signal is generated.
(This is also the stage of CD4 and CD8 lineage commitment which will be discussed later)
Bone marrow transplantation demonstrates the importance of HLA (MHC)
recognition in positive selection and T cell function. Immune function can be
restored to immunodeficient patients (e.g those with an inherited deficiency, or
treated with radiation and chemotherapy) by bone marrow transplants.
Transfer of hematopoietic stem cells in bone marrow provides progenitors for
entire immune system including T & B cells and antigen presenting cells.
Restoration of immune function following bone marrow transplantation requires matching
of donor and recipient HLA types. T cell development requires recognition of HLA in the
thymus, and T cell activation requires recognition of HLA in the periphery.
Figure 5-11
Question:
A patient receives a bone marrow transplant following
ablative chemotherapy. However immune function
does not appear to be restored. A re-test of the donor
shows that in fact very few HLA alleles are shared
between donor and recipient.
What would an analysis of peripheral blood show in
terms of T and B lymphocytes numbers?
Negative selection
1. To avoid autoimmunity, cells are removed which
express receptors that bind strongly to MHC-self
antigen.
2. Thymocytes expressing an ab TCRs which engage
MHC-self antigen with high affinity undergo
apoptosis (induced to die).
3. Negative selection of developing thymocytes is
known as “central tolerance” – the basic mechanism
for inducing tolerance to self. (Mechanisms for
inducing tolerance in circulating, mature, T cells are
known as “peripheral tolerance”).
Thymic maturation: moderate strength TCR signals are
required for continued development
No selection - death
Positive
selection Maturation to
CD4+ or
CD8+
Thymic
stromal
cell
CD4+8+
thymocyte
Negative
selection death
Positive and negative selection are mediated by different types of cells
in the thymus.
Figure 5-13
A thymocyte which is initially positively selected, may subsequently
undergo negative selection.
Autoimmune Polyglandular Syndrome (APS)
demonstrates the importance of negative
selection in the thymus
•
Removal of self-reactive T cells during thymic development
depends upon the expression of a large variety of self-antigens
in the thymus.
•
Individuals with APS have an autosomal recessive mutation
which effects activity of a transciption factor controlling
expression of 200 -1200 genes in the thymic medulla.
Autoimmune responses against organ-specific antigens
develop with age. Endocrine glands appear to be particularly
targeted.
•
Anderson, et al paper discusses mouse model for APS.
Development of functionally distinct T cell lineages
ab TCR
CD4+8+
TCRCD4-8-
T reg (ab TCR, CD4+)
ab TCR, CD4+
gd TCR
CD4-8-
ab TCR, CD8+
Mature T cells express either CD4 or CD8. These cell surface
proteins are “co-receptors” which help the TCR bind either to MHC
class I or to MHC class II. Expression of CD4 is associated with
helper T cells, and CD8 is associated with cytolytic T cells.
CD4 and CD8 lineages are
generated during
thymocyte development.
TCR+ thymocytes initially
express both CD4 and
CD8. During positive
selection expression
becomes limited to either
CD4 or CD8
The choice of lineage
corresponds to the ability
of the ab TCR on an
individual thymocyte to
recognize either MHC
class I or MHC class II.
CD4
CD8
T reg cell lineage
1. T reg cells block autoimmune activity of other T cells
through a variety of mechanisms.
2. T regs develop in thymus as ab TCR, CD4+ cells with
high affinity for self-antigen/MHC. It remains unclear
how Treg development is distinguished from negative
selection. Treg lineage T cells express the FoxP3
transcription factor and IL-2 receptor a
3. Defective FoxP3 alleles are associated with IPEX
(Immune dysfunction, polyendocrinopathy/enteropathy,
X-linked): male infants exhibit diarrhea, dermatitis,
diabetes & thyroiditis.
g/d T cell lineage
1 g/d T cells express an alternate form of the TCR.
2 g/d and a/b T cell development is exclusionary.
3 g/d T cells represent a few % of total mature T
cells and they have a relatively restricted
repertoire.
4. Resident in epithelial sites - skin and mucosa, as
well as in circulation.
The g/d TCR has a very similar structure to the a/b TCR. It is
generated by rearrangement of gene segments and the antigenbinding subunits pair with CD3 subunits for expression on the cell
surface.
ab and gd T cells develop from the same precursor population.
Successful rearrangement and expression of gd receptor genes leads
to commitment to that lineage and blocks ab T cell development.
Thymocyte development summary
progenitor
Initial repertoire
TCR a/b, TCR g/d
MHC-selected repertoire
TCR a/b, CD4+
TCR a/b, CD8+
T reg
Lecture objectives
•
T cell life cycle: development in the thymus, circulation as mature,
naïve cells, activation in response to antigen, differentiation into
memory cells.
•
Generation of diverse antigen receptors on T cells through
sequential rearrangement of ß and a gene segments.
•
Shaping of the repertoire through positive and negative selection.
•
Relation of DiGeorge, Omenn and Autoimmune Polyglandular
syndromes to thymic development.
•
Why matching of HLA types is important for restoration of immune
function following bone marrow transplantation.
•
Generation of distinct T cell lineages: CD4, CD8, T reg and g/d T
cells.