Lecture 10, T Cell Maturation Flashcards by Dave Ed

What 4 types of T cells can a HSC (originally from the marrow) develop into in the thymus?

T helper (CD4)
T cytotoxic (CD8)
Natural killer T (NKT)
T regulatory (Treg)

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The epithelial component of the thymus develops from the ________ pharyngeal pouch bilaterally at the ________ week of gestation.

- 4th

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Thymic cells begin to migrate during the ________ week of gestation, and they fuse at the midline (superior to the heart) by the ________ week of gestation.
What types of cells fuse?

6th
7th
HSCs (colonize during 7th-8th week)

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The thymus begins to produce T cells during the _______ week of gestation, and mature T cells begin to egress the thymus at the end of the _______ week of gestation.

12th-13th

- 13th

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What 2 mutations provided evidence that the thymus is an organ of thymopoeisis?

Large deletion that causes DeGeorge Syndrome (AKA velo-cardial-facial syndrome- VCFS)
Nonsense mutation in FOXN1 gene

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What are the effects of DeGeorge Syndrome?

How could you surgically treat it?

Hypoparathyroidism, heart defects, athymia: T cells are rarely detectable
Thymus implants give rise to higher T cell number and restore immune response

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What are the effects of a FOXN1 gene mutation?

FOXN1 can’t work as a TS factor

- No hair (affects keratinization), no thymus (*requires keratin 5 for thymic medulla and keratin 8 for thymic cortex)

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What 2 cell types make up the thymic stroma?

Fibroblasts (capsule, septa w/bv’s)

- Epithelial cells (cortical, medually, Hassall’s bodies)

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Besides fibroblasts and epithelial cells, what other 3 major cell types are present in the thymus?

DCs
Macrophages
HSCs (and T cells are different stages)

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Are thymic epithelial cells developed from endo, meso, or ectoderm?

Endoderm

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*What TS factor is essential for the development of thymic epithelial cells (TECs)?

FOXN1

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*What important ligand do thymic epithelial cells express?

What does this ligand bind?

Delta-like 1, 4

- Notch receptor

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What MHC do thymic epithelial cells express?

I (HLA A, B, C) and II (HLA DR)

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Why are developing T cells interacting w/thymic epithelial cells?

TECs are expressing peripheral tissue ag, such as insulin (testing for selection)

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In which parts of the thymus would you find DCs/macrophages?

Where are they concentrated?

Cortex and medulla

- Concentrated in cortico-medullary junction

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*What are the 2 primary functions of macrophages/DCs in the thymus?

Ag presentation and phagocytosis of apoptotic thymocytes

- Deletion of autoreactive T cells: negative selection

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About what % of T cells in the thymus are double positive (DP)?
CD4+ only?
CD8+ only?
Double negative (DN)?

~ 80%
~ 10%
~ 5%
~ 5%

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Name the 4 major events of T cell development (just read).

T lineage commitment: restricted of lineage choices
Proliferation: Expansion of committed cells
Differentiation: gaining of new surface markers (helps us track and ID cells)
Maturation: selection and gaining of immune functions

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*Describe the CD markers on a HSC that will eventually become a mature T cell (step 1).
What receptor does it express that binds Delta-like 1,4 ligand of TEC?

CD34+, CD1a-, CD7-
- Notch

(at this point can still become B cells, monocytes, DCs)

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*What cell does the HSC become on its way to becoming a mature T cell (step 2)?
What CD markers does it express?

T/NK cell

CD34+ (starts declining), CD1a-, CD7+

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*What cell does the T/NK cell become on its way to becoming a mature T cell (step 3)?
What CD markers does it express?

Pre-T cell

CD34 (low/neg), CD1a+

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*What cell does the pre-T cell become on its way to becoming a mature T cell (step 4)?
What CD markers does it express?

ISP (immature single-positive T cell)

CD34-, CD1a+, CD4+, CD3 (low)

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*What cell does the ISP become on its way to becoming a mature T cell (step 5)?
What CD markers does it express?

Double positive T cell

CD1a+, CD4+, CD8+

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*What cell does the DP T cell become on its way to becoming a mature T cell (step 6)?
What CD markers does it express?

Single positive T cell

CD1a-, CD4 or 8+

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At which stage in T cell development is the T cell committed to T cell lineage?

Pre-T cell (started *via Notch binding delta-like 1,4 of TEC during HSC stage) - Becomes CD1a+

At which stage in T cell development does positive and negative selection occur?

DP T cell (rearrangement of TCRalpha)

What gene rearranges during the pre T cell stage?

Beta (via RAG) | - Also gamma, delta

During which stage is the surrogate alpha chain created (pTalpha)? What does pTalpha combine with?

- Immature single positive (ISP) T cell - Combination of the rearranged TCRbeta chain w/ pTalpha forms the pre-TCR. (generally thought that this skews T cells to alpha-beta vs. gamma-delta)

MHC expressed by ____________________ dictates down modulation of either the CD4 or CD8 co-receptors.

Thymic cortical epithelial cells

What is the defect in Bare Lymphocyte Syndrome (BLS)? | What types of cells are affected?

Defect in MHC II expression; the development of CD4+ T cells is affected

Explain the general basis of negative selection. What stage of T cell does it? *What cell(s) induce it?

When TCR on a DP cell binds antigen/MHC with high affinity, it is signaled to undergo apoptosis. - Induced by DCs/macrophages, + TECs as well.

GAD67, Insulin, and IA-2 are e.g.'s of genes that are induced by the TS factor known as AIRE, found in what type of thymic cells?

Medullay TECs. (not found in DCs...) | - Used to negatively select T cells (w/high affinity for these specific organ ag's)

A mutation in AIRE would lead to what dz? | Major sx?

Autoimmune Polyendocrinopathy Candiasis Ectodermal Dystrophy (APECED). - Fungal infections to tongue, toes

Positive selection occurs in the thymic ________, negative selection occurs in the thymic ________.

- Cortex | - Medulla

Where do HEVs enter the thymus?

Cortico-medullay junction | - T cell progenitors go from medulla, out to cortex, then back to medulla and can leave if successful

Which stage in T cell development is the Precursor for both TCRalpha/beta and TCRgamma-delta T cells? - What dictates lineage choice?

Immature single-positive (ISPs) - Successful beta selection (This process selects cells with productive and functional rearranged TCR-beta genes signaling via the preTCR. Cells that do not form 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.)

Are the CD4 and CD8 positive or negative in gamma-delta T cells?

Usually both CD4- and CD8-

What markers do natural killer T cells (NKTs) express?

Both markers for T cells (TCRalpha-beta) and NK cells (CD56+) - Either CD4+ or CD4- + CD8-

*What 2 TS factors are essential for the development of Tregs?

TSLP and FOXP3 (NOT FOXN1 of thymic dz)

Why would we care about Th3 cells?

Oral tolerance induction | - E.g. allergies to peanuts

A mutation in FOXP3 would cause someone to not have any Tregs. What condition would this lead to?

Immune dysregulation, polyendocrinopathy, and X-linked inheritance (IPEX)

What types of cells are found in Hassall's bodies (of the thymus)?

Concentric organization of epithelial cells.

Of which T cell stage does expression of RAG genes first occur for rearrangement of TCR beta (+ delta, gamma) chain?

Pre-T cell (CD34-/low, CD1a+)

Of which T cell stage does expression of RAG genes occur for rearrangement of TCR alpha chain?

DP T cell (CD4+, CD8+, CD1a+)

Of which T cell stage does expression of precursor for both TCRalpha-beta and TCR-gamma-delta T cells occur, as well as the expression of preT-alpha and preTCR-beta selection?

ISP T cell (CD34-, CD1a+, CD4+, CD3low)

Of which T cell stage does positive and negative selection occur?

DP T cell (CD4+, CD8+, CD1a+)

During TCR gene rearrangement, what causes deletion of the TCR delta locus? During what stage does this occur?

RAG expression is re-expressed and rearrangement of TCRV𝛂 genes are initiated. The rearrangement of V𝛂 causes deletion of TCRδ locus - DP stage

Explain allelic exclusion w/r/t TCRV-alpha.

There is no allelic exclusion in TCRV𝛂 rearrangement. It is possible that there are two different rearranged V𝛂 chains, each is associated with a common Vβ. Subsequent positive selection will ensure that each T cell only has a single functional specificity, although two different 𝛂 chains are expressed

DP CD4+CD8+ thymocytes that recognize CD1d/glycolipids expressed on cortical thymocytes develop into _________ cells.

NKT

Where are NK T cells typically found?

Liver, spleen, BM and lymph nodes

What surface marker presence indicates an HSC has become an NK/T cell?

CD7+

What surface marker presence indicates an HSC has become an ISP cell?

CD4+ | express preT-alpha, which induces some expression of CD3, to join and form the pre-TCR

How are CD1 proteins similar to MHC?

Distantly related to MHC molecules, however they lack polymorphism.

What are some of the names of the 2 types of gamma-delta T cells? Which predominates?

- TCRδ1 | - TCRγ9δ2 (80% of circulating gamma-delta T cells)

What are the main functions of TCRδ1 gamma-delta T cells?

- TCRδ1: Cytotoxic activity, recognized stressed cells and lipid Ag; lyse stressed-epithelial cells

What are the main functions of TCRγ9δ2 gamma-delta T cells? | Also, what important cytokine to they produce?

- TCRγ9δ2: Anti-microbial immuno-surveillance; recognize intracellular bacteria (also extracellular- TB); phosphor antigens. Immune regulation. - Produce IFN-gamma that affects cytotoxicity of NK and NKT cells and the generation of Th1 cells.

NKT cells develop in the thymus from the CD4(_) CD8(_) thymocytes.

CD4+ and CD8+

Mature NKTs egress the thymus and populate the ________, _________, ________, and ________.

Liver, spleen, BM, and lymph nodes. (0.02- 0.2% of the peripheral blood T cell compartment)

Discuss the general pattern of cytokine release from NKTs.

Rapidly produce both Th1 (IFN-gamma) and Th2 cytokines (IL-4, IL-5, IL-10) upon triggering. Thus, they play a role in immunoregulation.

*Mice subjected to thymectomy between day 2 and 4 post natal develop organ-specific autoimmune dz's. The autoimmune dz's can be prevented by infusion of syngeneic T cells obtained from adult thymus or spleen. What discovery did this lead to? What is this phenomenon referred to as?

It was concluded that the thymus generate 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.

These "suppressor cells" turned out to be Tregs, which are positive for what 2 major CD markers?

CD4+, CD25+

``` *During T cell devo: What CD becomes present as an HSC? NK/T cell? Pre-T cell? ISP T cell? ```

34, 7, 1a, 4