9 - Development of T Cell Mediated Immunity Flashcards
Where do T cells develop and mature
- 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
Secondary lymphoid tissues
- Lymph nodes
- Spleen
- GALT
Types of cell in thymus (from cortex to medulla)
- Cortical epithelial cell
- Thymocyte (bone marrow origin)
- Medullary epithelial cell
- Dendritic cell (bone marrow origin)
- Macrophage (bone marrow origin)
Two classes of TCR
αβ and γδ
What is stages in development of thymocytes marked by
Changes in cell surface molecules
Stages of development of thymocytes
- Double negative thymocytes (CD3-4-8-)
- Large active double postive (CD3+4+8+)
- Small resting double positive (CD3+4+8+)
- Small resting single positive (CD4+8- OR CD4-8+)
Recombination of TCR α
- Coming together of 2 segments
- Joining of Vα (Variable segement) to Jα (Joining segment)
Recombination of TCRβ
- Coming together of 3 segments
- Joining of Vβ (variable segment), Dβ (diversity segment), and Jβ (joining segment)
Recombination signal sequences (RSS)
Flank TCR gene segments
Which recombination in TCRs occurs first
- 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
V(D)J recombinase
- 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
RSS
Recombination signal sequences that flank TCR gene fragments
Severe combined immune deficiency (SCID)
- 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
TCR V region rearrangement
- 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
Which chain is arranged first
- Functional β chain must be arranged first, before an ⍺ chain is generated
- Each thymocyte has 4 attempts at making a functional β chain
4 attempts at making a functional β chain
- 2 attempts on each allele
- First try with Cβ1 fragment, then with Cβ2
Pre T cell receptor (pre-TCR)
- 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
How many attempts to make a single productive alpha chain
Many, not just a single attempt
What does a common double negative T cell progenitor give rise to
αβ and γδ T cells
Thymocyte development
- 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
Early development of ⍺:β T cells in thymus
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
Where to TCRs concentrate diversity
In third hypervariable region
Central tolerance
- 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
What molecules do cortical epitheial cells express
MHC Class I and MHC Class II molecules expressing self peptides
What determines whether a thymocyte will become CD4+ or CD8+ T cell
Positive selection (e.g. if receptor binds self peptide: self MHC class 1, it’ll become a CD8 T cell)
The autoimmune regulator gene AIRE
- 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.
What happens in absence of AIRE
- Immature self reactive T cells are not eliminated
- Leads to autoimmunity
What is negative selection mediated by
APCs
Negative selection
- Single positive CD4+ or CD8+ cells
move from cortex to medulla - Come in contact with DCs and macrophages at the corticomedullary junction
What is negative selection dependent on
- Strength of TCR/MHC binding
- Strong/tight binding = apoptosis is induced
- Low/moderate binding = survival
Autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED)
- 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
