9 - Development of T Cell Mediated Immunity

Question 1

Where do T cells develop and mature

Answer 1

• T cell precursors travel from the bone marrow to develop in the thymus
• Mature T cells leave the thymus and travel to secondary lymphoid tissues

Question 2

Secondary lymphoid tissues

Answer 2

• Lymph nodes
• Spleen
• GALT

Question 3

Types of cell in thymus (from cortex to medulla)

Answer 3

• Cortical epithelial cell
• Thymocyte (bone marrow origin)
• Medullary epithelial cell
• Dendritic cell (bone marrow origin)
• Macrophage (bone marrow origin)

Question 4

Two classes of TCR

Answer 4

αβ and γδ

Question 5

What is stages in development of thymocytes marked by

Answer 5

Changes in cell surface molecules

Question 6

Stages of development of thymocytes

Answer 6

1. Double negative thymocytes (CD3-4-8-)
2. Large active double postive (CD3+4+8+)
3. Small resting double positive (CD3+4+8+)
4. Small resting single positive (CD4+8- OR CD4-8+)

Question 7

Recombination of TCR α

Answer 7

• Coming together of 2 segments
• Joining of Vα (Variable segement) to Jα (Joining segment)

Question 8

Recombination of TCRβ

Answer 8

• Coming together of 3 segments
• Joining of Vβ (variable segment), Dβ (diversity segment), and Jβ (joining segment)

Question 9

Recombination signal sequences (RSS)

Answer 9

Flank TCR gene segments

Question 10

Which recombination in TCRs occurs first

Answer 10

• Beta chain D-J recombination occurs first, followed by V-DJ recombination
• Alpha chains do not have a diversity segment and thus recombination is limited to V-J

Question 11

V(D)J recombinase

Answer 11

• Recombination-activating genes RAG-1 and RAG-2 encode the RAG proteins that initiate V(D)J recombination
• Complex is referred to as V(D)J recombinase
• V(D)J recombinase introduces double-stranded breaks in germline DNA at the border between a RSS and a coding segmen

Question 12

RSS

Answer 12

Recombination signal sequences that flank TCR gene fragments

Question 13

Severe combined immune deficiency (SCID)

Answer 13

• Occurs to do insufficient TCR diversity (recombination introduces diversity)
• Absent or low T cells
• Absent or non-functional B cells
• High mortality in 1st year of life
• Possible cure by bone marrow transplant

Question 14

TCR V region rearrangement

Answer 14

• Occurs by V(D)J recombination
• RAG has endonuclease activity and makes single stranded cuts in the DNA between each coding
  segment and its RSS
• This creates a 3’-OH group which then reacts with a phosphodiester bond on the opposite DNA strand to generate a hairpin, leaving a double stranded blunt end
  break
• At the coding ends, the enzyme Artemis opens the hairpin
  and yields either two flush-ended DNA strands or a single strand extension
• Cut end is then modified by enzyme terminal deoxynucleotidyl transferase (TdT) and exonuclease which randomly add or remove nucleotides
• The two coding ends are ligated by DNA ligase IV

Question 15

Which chain is arranged first

Answer 15

• Functional β chain must be arranged first, before an ⍺ chain is generated
• Each thymocyte has 4 attempts at making a functional β chain

Question 16

4 attempts at making a functional β chain

Answer 16

• 2 attempts on each allele
• First try with Cβ1 fragment, then with Cβ2

Question 17

Pre T cell receptor (pre-TCR)

Answer 17

• Consists of the TCRβ chain and the pre-TCR alpha (pT⍺) chain
• Associated with with signal transducing CD3 molecules
• Cells with productive TCRβ rearrangements and CD3 are rescued from apoptosis
• The pre-TCR induces expansion and differentiation of these cells such that they become TCRαβ bearing CD4+8+ thymocytes

Question 18

How many attempts to make a single productive alpha chain

Answer 18

Many, not just a single attempt

Question 19

What does a common double negative T cell progenitor give rise to

Answer 19

αβ and γδ T cells

Question 20

Thymocyte development

Answer 20

• Develops into o ⍺:β or γ:δ T cell
• First rearrangement is either β chain or γ:δ (rearrangement happens at same time, first successful rearrangement determines cell lineage)
• Gene rearrangement stops at this point (cells proliferate and express both CD4 and CD8 - double
  positive thymocyte)
• Rearrangement of ⍺ chain can now proceed (competitive γ:δ rearrangements can continue
• End result is committed ⍺:β or γ:δ T cell
• ⍺:β T cell is double positive CD4+ CD8+
• TCR is tested in a selection process that occurs in association with medullary epithelial cells MEC

Question 21

Early development of ⍺:β T cells in thymus

Answer 21

Progenitor cells –> Proliferation –> double negative T cells commit to T lineage –> rearrange β genes –> proliferating double negative pre T cells –> immature double positive cells –> rearrange ⍺ genes –> mature double positive cells

Question 22

Where to TCRs concentrate diversity

Answer 22

In third hypervariable region

Question 23

Central tolerance

Answer 23

• Autoreactive cells are removed in the thymus
• Double positive CD4+ and CD8+ thymocytes undergo further selection to develop into fully mature CD4+ or CD8+ T cells
• Positive selection occurs in the thymic cortex

Question 24

What molecules do cortical epitheial cells express

Answer 24

MHC Class I and MHC Class II molecules expressing self peptides

Question 25

What determines whether a thymocyte will become CD4+ or CD8+ T cell

Answer 25

Positive selection (e.g. if receptor binds self peptide: self MHC class 1, it’ll become a CD8 T cell)

Question 26

The autoimmune regulator gene AIRE

Answer 26

• Part of a complex that regulates the expression of tissue restricted antigens (TRAs) in medullary thymic epithelial cells (MTECs).
• Peptides derived from these antigens are displayed on the MTEC and recognised by immature antigen-specific T cells, leading to the deletion of many self-reactive T cells.

Question 27

What happens in absence of AIRE

Answer 27

• Immature self reactive T cells are not eliminated
• Leads to autoimmunity

Question 28

What is negative selection mediated by

Answer 28

APCs

Question 29

Negative selection

Answer 29

• Single positive CD4+ or CD8+ cells
  move from cortex to medulla
• Come in contact with DCs and macrophages at the corticomedullary junction

Question 30

What is negative selection dependent on

Answer 30

• Strength of TCR/MHC binding
• Strong/tight binding = apoptosis is induced
• Low/moderate binding = survival

Question 31

Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED)

Answer 31

• Caused by a number of different mutations in the gene that encodes autoimmune regulator
  (AIRE)
• Characterised by damage to endocrine organs and chronic mucocutaneous Candidiasis