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Flashcards in T-Cell Maturation & Differentiation Deck (44):
1

Describe the thymus as an organ. When does it develop. What cells "home" to the thymus? What cells will develop here?

The thymus is an epithelial-lymphoid organ that develops very early during embryonic
development. Hematopoietic stem cells (HSC) home to the thymus and develop into: T helper (CD4), cytotoxic (CD8), natural killer T (NKT) and T regulatory (Treg) cells.

2

How does the epithelial component of the thymus develop? When during gestation?

The epithelial component of the thymus derives bilaterally from epithelium of the third
pharyngeal pouch at fourth week of gestation.

3

What happens in regards to the thymus between the 4-7th week of gestation?

What happens during the 12th week?

Between 4-7th week of gestation, the primordial
thymic glands lose connections with the pharynx and migrate to the final position in the
mediastinum forming a single bilobate gland.

7th week- colonization by HSC
12th week- begin to produce T cells at 12-13th week, mature T cells egress the thymus at 13-14th week
p 2

4

The normal human thymus develops early on in fetal development;

When are glands colonized by HSC?

When does thymus begin to produce T cells?

What happens at the end of 13th week and beginning of 14th week?

What is the the end result?

the glands are colonized
by hematopoietic stem cells at 7-8 weeks of gestational age.

The thymus begins to produce T cells around 12-13 week of gestation.

Mature T cells egress the thymus and colonize peripheral lymphoid organs at the end the 13
and the beginning of the 14 week of gestation.

Thus, by the time the baby is born, the peripheral T cell repertoire is established to the
point that thymectomy does not cause immediate immune deficiency.

5

What causes DiGeorge Syndrome (DGS)/ Velo-Cardio-Facial Syndrome (VCSF)?


Describe the disease. What might symptoms include? Why recurrent infections occur?

How may it be treated?

DiGeorge syndrome is caused by a large deletion in chromosome 22 which is caused by an error
in recombination at meiosis.

- It is a rare congenital (i.e. present at birth) disease syndrome whose symptoms vary greatly between individuals but commonly include a history of heart defects, characteristic facial
features and recurrent infection due to absence of the thymus and T cells.

The absence of the functional thymus results in complete T cell deficiency and severe
immunodeficiency.

Transplantation of an allogeneic thymus graft into patients with DGS rescues T cell
deficiency.

6

What are the implications for a Mutation in the FOXN1 gene:

What results?

What type of treatment might help?

The FOXN1 gene (on chromosome 17) encodes a transcription factor that is essential for the functional maturation of thymic epithelial cell progenitors.

In these patients, the thymus gland fails to form in utero because epithelial progenitor cells fail to undergo functional differentiation and instead form cyst-like structures with immature morphology.

The immature epithelial cells fail to recruit hematopoietic stem cells into the organ.

Thus, the functional maturation of thymic epithelial cells is required for the development
of a normal thymic architecture, which is essential for the production of various thymicdependent T cells subsets and the initial establishment of the peripheral T cell pool in
animals and humans.

- Thymic implant recently been shown to restore T cell immune response in patients with
Foxn1 mutations

7

Describe the cellular composition of the thymus glands.

What does the thymic stroma include?

See p 4
capsule: fibroblasts
cortex: developing T cells
medulla- mature T cells
cortical-medullary junction

Thymic stroma: this includes the predominant thymic epithelial cells (TEC) and fibroblasts.

a. Fibroblasts: found in the thymic capsule and septa.
b. Thymic epithelial cells (TEC): provide three critical functions for the development of T
cells.

8

Thymic epithelial cells (TEC):

From where are they derived?

Provide three critical functions for the development of T
cells. Describe.

All TEC are derived from
endoderm.

TEC produce cytokines such as IL1, IL6, IL7, and SCF (stem cell factor), TSLP
(thymic stroma lymphopoietin) that are requires for growth and differentiation of
various immature T cells.

TEC also express cell surface molecules such as ligands Delta-like-4 (DL-4) and DL-1 for the notch receptor. Signal of notch receptors expressed on progenitor cells is required for T cell lineage commitment.

TEC expression of MHC classes I, and II/self antigen complexes controls the
selection of maturing T cells.

Expression of peripheral tissue antigens: for example insulin

9

Describe macrophages and dendritic cells.

Where do they mature?

Where are they located?

Function?

these cells mature from the bone marrow and migrate into the thymus. These cells are scattered in the cortex and medulla; however, they highly populate the cortical-medullary junction. They function: in antigen presentation; deletion of
autoreactive T cells (negative selection) and phagocytosis of apoptotic thymocytes.

10

Describe thymocytes:

How are thymocytes derived?

Describe the different types of T cells the thymus is responsible for?

The predominant lymphoid cells in the thymus. In postnatal animals,
thymocytes are derived from progenitor cells of the bone marrow, the hematopoietic stem cells (HSC). After arriving in the thymus, bone marrow HSC progress through tightly
regulated steps to develop into mature T cells. The thymus is responsible for the
development of four functionally different T cells: CD4 T helper, CD8 T cytotoxic,
regulatory T cells (Treg) and natural killer T cells (NKT).

11

How can flow cytomettric analysis be used in regards to T cells? Describe process.

Flow cytometric analysis is a powerful tool to identify thymocyte subsets: cells stained with anti-CD4 and CD8 antibodies are separated into 4 distinct populations: DP, double positive (CD4 and CD8 positive); DN, double negative (either CD4 or CD8 is expressed).

12

Draw a flow chart for the early stages of human T cell development.

p 6

The thymus is populated by blood born progenitor cells that are derived from the bone marrow
hematopoietic stem cells (HSC). These cells express the unique cell surface marker CD34 and
have the capacity to develop into T cells as well as B cells, dendritic cells, and NK cells.

Upon entry into
the thymus, the lineage potential of the CD34pos cells is restricted to only the T lineage.

13

Why can cord blood be a good source for HSC CD34pos cells for transplant

Because the migration of CD34pos cells is initiated at 7-8 week of gestation and is highly active in the
neonatal period, cord blood is good source for HSC CD34pos cells for transplant.

14

How does T cell production by thymus change with age.

The production of T cells by the thymus is declines with age; This age-associated decline in the production of T cells is responsible for the decline in immune response in the elderly.

Numbers given p 7

15

What are the four developmental events in T cells. (List)

- T lineage commitment
- Proliferation and differentiation
- Selection: positive and negative
- Maturation

16

When is a T cell committed to T lineage?

See chart p 8

Rearrangement of gamma, delta beta

Pre-T with CD34 and CD1A pos

17

Discuss the significance of the Notch receptor signal.

The Notch receptor signal is essential for T cell lineage commitment of the CD34pos
HSC. Signal through the notch receptor terminates the potential to commit to B and myeloid lineages (monocytes and DC). The cells have potential to become T or NK cells
(T/NK). Persisting Notch signaling terminates NK development.

18

What happens in T cell development after the notch signal by Notch ligand DL4 and DL1?

What factors/enzymes will be needed?

cells commit to T lineage,
express CD1A and begin to rearrange TCRgamma,delta and beta genes. Cells at this stage, pre-T, can
develop into either TCRgamma/delta or TCRalpha/beta T cells. Te rearrangement of TCR genes requires the recombinant activation gene -1 (RAG1) and RAG2. IL7 is also needed for this developmental stage.

19

After the notch signal by Notch ligand DL4 and DL1 cells commit to T lineage,
express CD1A and begin to rearrange TCRgamma,delta and beta genes. Cells at this stage, pre-T, can
develop into either TCRgamma/delta or TCRalpha/beta T cells.

Describe what happens in the next stage.

In the next stage, cells express CD4 and become immature single positive (CD4ISP). These cells express the preTalpha (pTalpha); the expression of pTalpha also induces the expression of CD3.

The pT-alpha together with CD3 and TCR-beta forms the preTCR complex. Signaling via pre-TCR generate a weak activation of the extracellular signal related kinase (ERK)
which terminate rearrangement of TCR-gamma and TCR-delta genes.

20

What composes the pre-TCR complex?

What does expression of pre-TCR complex allow for?

The pT-alpha together with CD3 and TCR-beta forms the preTCR complex.

The expression of the preTCR allows the ISP cells to undergo the Beta
selection process

21

The expression of the preTCR allows the ISP cells to undergo the Beta
selection process. Describe the Beta selection process.

What two things could happen at the end of this process?

This process selects cells with productive and functional rearranged
TCR-beta genes signaling via the preTCR. Cells that do not from a functional preTCR die by apoptosis. Cells that survive the Beta selection step proliferate and expand.

This enormous expansion of cells at this stage is responsible for generating the large number of thymocytes with TCR-alpha/beta in the thymus. Expression of CD3 is up-regulated at this stage.

22

Cells that survive the Beta selection step proliferate and expand.

What happens to ISP cells that are selected to progress?

ISP cells that are selected progress to express CD8 and thus are termed CD4 CD8 double
positive cells (DP); these cells begin to rearrange TCR-alpha genes. Because the TCR-delta locus
is within the TCR-alpha genes, rearrangement of the TCR V-alpha genes will delete the delta locus.
There is no allelic exclusion in TCRV-alpha gene rearrangements. It is possible that there are two different rearranged V-alpha chains, each is associated with a common V-alpha.

Subsequent positive selection will ensure that each T cell only has a single functional specificity

23

How is it determines whether a T cell becomes CD4 or CD8?

Depend on the signal generated via the interaction of the co-receptor CD4 or CD8 surface
molecules, DP cells down modulate one of the co-receptors and mature into either single
CD4 or CD8 T cells.

p 10

24

Describe the positive selection of TCR-alpha/beta T cells: Selection of T cells that recognize antigens presented by MHC molecules.

The DP thymocytes with functional TCRalpha-beta/CD3 complex must recognize self-peptides
in the MHC I and II complex presented by cortical TEC.

- The interaction of TCRalpha-beta/CD3 complex with self peptides and MHC antigens must be of low affinity; DP cells with low affinity TCR for self peptides are rescued from apoptosis and proliferate (1 or 2 rounds of replications).

These cells shut down expression of
recombination-activating gene (RAG); no further rearrangement can take place if RAG
proteins are not present.

Cells express TCRalpha-beta that do not recognize self peptides undergo apoptosis.

Positive selection skews the selected TCRalpha-beta repertoire toward self peptides and the
potential of generating autoreactive T cell clones.

25

What happens with patients that receive bone marrow transplants?

Patients who receive bone marrow transplant generate T cells that recognize foreign antigens in the context of their own MHC antigens and not donor MHC, indicating that
developing donor T cells occurs on TEC of the patient recipient.

26

Describe negative selection. (What is it/what does it result in?

Deletion of mature T cells that bind strongly to MHC/antigens
(autoreative T cells); negative selection results in the generation of central tolerance.

27

Negative Selection: Deletion of mature T cells that bind strongly to MHC/antigens
(autoreative T cells); negative selection results in the generation of central tolerance.

Describe this process. Where does it primarily occur?

DP cells with TCRalpha-beta/CD3 complex with high affinity for self-peptide and MHC are instructed to undergo apoptosis; this process is necessary to eliminate autoreactive T cells and to establish the central tolerance.

Negative selection occurs predominantly in the cortical-medullary region where a high density of thymic DC cells is present. The DC cells are also responsible for phagocytosis
of apoptotic cells.

28

What is AIRE and what does it do?

TEC also participate in clonal deletion of T cells that are reactive to organ specific
antigens. The Auto Immune Regulator Element gene (AIRE) encodes a transcription factor that induces expression of a battery of peripheral-tissue antigens by thymic
medullary epithelial cells; thus, AIRE promotes central tolerance of thymocytes by
inducing negative selection, contributing to the prevention of organ specific
autoimmunity.

29

Describe disease associated with mutations of AIRE.

AIRE mutations are responsible for the
development of autoimmune polyendocrinopathy candidiasis ectodermal dystrophy (APECED) or Autoimmune polyendocrinopathy syndrome type 1 (APS1). The affected organs include: adrenal, thyroid, parathyroid, and pancreas.

30

What happens to DP cells that survive both the positive and negative selection processes?

Where do mature CD4 and CD8 cells reside?

DP cells that survive both the positive and negative selection processes will commit to either
single CD4Pos or CD8Pos cells.

- Selection with CD4 co-receptor and MHC-II will generate mature CD4 cells.
- Selection with CD8 co-receptor and MHC-I will generate mature CD8 cells
- Mature CD4 and CD8 cells are predominantly resided in the medulla of the thymus.
- Mature CD4 and CD8 cells egress the thymus via blood vessels in the septa in the
cortical-medullary junction.

31

Describe the development of TCR gamma/delta cells.

develop from the ISP cells, the exact mechanism for the lineage choice
between TCRalpha/beta and TCRgamma/delta is not well understood. It is generally accepted that the expression
of pT-alpha and signaling of this receptor skews the cells toward TCRgamma/delta lineage.

32

Describe TCR gamma/delta cells. How prevalent are they?

Where are they? What type? MHC I or MHC II?

- Only present at low level, they present 1-5% of spleen and lymph node T cells
- Are predominantly CD4 and CD8 negative

- The TCRgamma/delta receptor bind antigens directly, no antigen presentation by MHC I or MHC-II is required

- TCRgamma/delta repertoire is very narrow, there are two dominant TCRgamma/delta T cells:

33

Describe the cells that express delta1.

What tissues do they populate? What do they recognize?

Cells that express delta1 with various gamma genes: these are the first gamma/delta T cells that emerge from fetal thymus.

They eventually populate epithelia tissues such as the
intestine and skin. The TCRdelta1 T cells recognize stressed cells and lipid antigens presented by CD1B or CD1C molecules.

34

What are the majority of circulatory gamma/delta T cells?

TCR gamma 9 delta2- 80% of all gamma/delta T cells

35

Describe CD1 proteins.

CD1 proteins transmembrane proteins that are distantly related to MHC molecules; however these proteins lack polymorphism.

36

Describe the four human CD1 proteins.

There are four human CD1 proteins: CD1A, CD1B, CD1C and CD1D, each is encoded by different genes:

- CD1B and CD1C are noncovalently associated with B2m (CD1B and C particularly bind to bacterial glycolipid antigens.)

- CD1B, CD1C and CD1D are known to bind to glycolipid antigens; CD1B and C particularly bind to bacterial glycolipid antigens.

37

What are the TCR gamma delta functions?

TCRdelta1 cells: lyse stressed/ transformed epithelial cells

TCRgamma9delta2: recognize non-peptide pyrophosphomonoester antigens (phosphor antigens) found on mycobacterium and malaria parasite. These cells can also kill intra- and extracellular M. tuberculosis, produce IFNgamma that affects cytotoxicity of NK and NKT cells and the generation of TH1 cells.

38

Describe the development of Natural Killer cells.

Where do they develop? From what?

Which cells develop into NKT cells?

What types?

-NKT cells develop in the thymus from the CD4Pos CD8Pos DP thymocytes.

- DP that recognize glycolipid antigens presented by CD1D positive cortical thymocytes develop into NKT cells

- NKT are either CD4Pos or CD4NegCD8Neg

39

What do NKT express?

What tissues do they populate?

NKT express both the NK maker CD56 and the T cell maker TCRalphaB/CD3 complex

- Mature NKT egress the thymus and populate the liver, spleen, BM, and lymph nodes.

- Human NKT population constitutes approximately 0.02- 0.2% of the peripheral blood T cell compartment.

40

What are the functions of NKT cells? What happens when they are triggered?

NKT cells rapidly produce both Th1 (IFN-gamma) and Th2 cytokines (IL4, IL5, IL10) upon triggering; thus they play a role in immunoregulation.

41

How were Regulatory T cells (Treg) discovered? Describe the observations that led to their discovery in mice.

- Mice subjected to thymectomy between day 2 and day 4 post natal develop organ-specific autoimmune diseases
- The autoimmune diseases can be prevented by infusion of syngeneic T cells obtained from adult thymus or spleen
- It was later identified that the CD4Pos CD25Pos T cells are capable of suppressing the autoreactive T cells.

From these observations, it was concluded that the thymus generate CD4Pos CD25pos suppressor cells with specificity for autoreactive T cells that may escape negative selection during the development of T cells. This mode of suppression of autoreactivity is referred to as dominant tolerance in contrast with central tolerance in which the autoreactive T cells are deleted.

42

Describe Treg development in humans.

Where do they develop (from which cells) and how early?

Develop in the thymus from CD3Pos CD4Pos T cells as early as 14 weeks of gestational age.

Treg represent 5-10% of mature CD4pos thymocytes and 10% of peripheral blood CD4PosT cells.

43

Describe the Treg that develop in the periphery.

How are they genearated here?

What types are there?

Treg can be generated from peripheral mature CD4pos T cells by TGF-B

T regulatory type 1 (Tr1)

T helper 3 (Th3)

44

Describe Treg and human diseases.

What is IPEX?
What do patients with IPEX present with clinically?
What gene is responsible for IPEX?

Immune dysregulation, polyendocrinopathy, enteropathy, and X-linked inheritance (IPEX) is a clinical syndrome that presents with multisystem autoimmune disease. Clinically, patients with IPEX manifests most commonly with diarrhea, insulin-dependent diabetes mellitus, thyroid disorders, and eczema. FOXP3, the gene responsible for IPEX, maps to chromosome Xp11.23-Xq13.3 and encodes a transcription factor. Because patients with IPEX lack Treg, it was determined that expression of FOXP3 in CD4Pos T cells is required for the development of Treg in the thymus.