Immunology #8 - Adaptive Immunity, T-Cell Development/differentiation

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Lymphoid progenitor cells

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Give rise to B cells, NK cells, and progenitor/precursor T cells

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IL-7

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Progenitor cells originating in the bone marrow differentiate to naïve mature T cells, under the influence of IL-7

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TCR complex (Receptor)

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- T-cell receptors (TCRs) are composed of two transmembrane polypeptide chains classified either as alpha/beta (α/β), or gamma/delta (γ/δ)
- α/β T cells = more than 90%
- Each heterodimeric TCR is expressed on the cell surface with 5 invariant polypeptides = CD3

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CD3

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- Links the antigen binding receptor of the T cell with signaling pathways

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Structure of T Cell Receptor

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- A variable and a constant region
- TCRα variable regions are composed of a “V” and a ”J” segment (VJ)
- TCRβ regions are composed of “V”, “D”, and “J” segments (VDJ)

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Diversity in T-Cells

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1. Multiple copies of germline V, D, and J gene segments
2. Random selection and combination of V, (D) and J gene segments
3. Junctional diversity generated by the addition or deletion of bases
4. Random assortment of TCRα and TCRβ chains.

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T cell differentiation

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- Thymic cortex
- CD2, pre-TCR-CD3 complex, CD4, CD8, TCR-CD3 complex, are expressed on the cell surface

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CD2

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- 1st molecule to be expressed continues to be expressed throughout the life of the cell
- It is considered to be a “pan marker”

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CD3

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- Continues to be expressed throughout the life of the cell

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pre-TCR

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- Only transiently expressed
- Critical in that the preTCR complex can only form on the developing thymocyte if the TCR beta chain variable

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CD4 and CD8

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- Both CD4 and CD8 are expressed on the thymocyte (double positive T cell)
- Lineage selection occurs and the cells become either CD4+ or CD8+, not both (single positive)

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Somatic Recombination

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- Initiated by recombinases, nucleoprotein products of the RAG-1 and RAG-2 genes (recombination-activating genes)
- During differentiation of the T cell the DNA in the locus is cut and recombined with the variable regions in the alpha chain and beta chain

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Allelic exclusion

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- Somatic recombination is “inhibited”
- VJ or VDJ recombination is successful on one of the alleles (e.g, father‟s allele), then recombination will not occur on the other allele (mother‟s allele)
- If the recombination is unsuccessful on both alleles, the cell dies

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Tolerance Induction

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- T cells whose receptors are self-reactive = auto-immunity
- The random selection of V, D and J gene segments again leads to the generation of T cells expressing TCRs that may be autoreactive
- These T cells must be deleted during the screening process in the thymus - this is done by selecting T cells to recognize self-MHC in the thymus

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Selection/Screening Processes in Thymic Cortex

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- Double positive (CD4+, CD8+) thymocytes are selected to live or die depending on the interactive avidity of their TCR with self-antigen/MHC on thymic epithelial cells
- Interactive avidity dependent on: intrinsic affinity of the TCR for self antigen/MHC complex; density of TCRs;the density of self antigen/MHC complexes on the thymic epithelium; and density of antagonistic peptide complexes

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Negative Selection

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- If recognition exceeds a pre-determined threshold, thymocytes are clonally eliminated or functionally inactivated by an active process termed negative selection

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Lineage Selection

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- Silencing of either the CD4 or the CD8 gene occurs in each individual T cell
- Lineages are classified by their expression of CD4 or CD8 (reflective of subsequent biological activity)

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Thymic Medulla

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- Selection conditions in the medulla are different from those in the cortex
- More finely-tuned than in the cortex, with multiple high avidity interactions, including TCR, CD4 (CD8) and class II (class I) MHC all important

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AIRE Protein

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- A transcription factor encoded by the autoimmune regulator (AIRE) gene, which promotes expression of peripheral tissue antigens in thymic epithelial cells and is essential for deletion (negative selection) of T cells specific for these antigens
- Mutations in AIRE lead to the autoimmune polyendocrine syndrome type 1

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APECED syndrome

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- Autoimmune polyglandular syndrome type 1 (APS1)
- Mutation in AIRE
- >29 mutations have been identified
- Manifests as chronic mucocutaneous candidiasis, hypoparathyroidism, autoimmune adrenal insufficiency

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CD4+ Thp and CD8+pCTL

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- These two subsets of T cells are naïve T cells that will undergo antigen-induced differentiation in a secondary lymphoid organ following initial encounter with antigenic peptide-MHC

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High endothelial venules (HEV)

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- Migration of naive lymphocytes from the blood into lymph nodes occurs at specialized post capillary venules
- Glycocalyx on luminal surface that traps molecules

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Extravasation of naive lymphocytes into tissues at HEV requires

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1. L-selectin on lymphocytes with its ligand on endothelial cells to induce lymphocyte rolling
2. Activation of integrins, (e.g.LFA-1) increasing their adhesiveness and allowing stable binding to HEV
3. Lymphocyte secretion of matrix metalloproteinases that proteolytically degrade collagen to generate
4. Transendothelial migration of lymphocytes into the tissue

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SUMMARY

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1. Expression of CD2
2. Expression of PRE-TCR-CD3 complex ( Requires somatic recombination of beta chain )
3. Expression of both CD4 and CD8 on the same cell
4. Somatic recombination of the alpha chain
5. Expression of TCR-CD3 complex
6. SELECTION -SCREENING (positive, negative, death by neglect)
7. Lineage Selection
8. Cross into medulla
9. Negative Selection
10. Exit to periphery
11. Effector cells that exit include CD4+Thp, CD8+pCTL

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Natural T Regulatory cells (nTregs)

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- CD4+, CD25+, and FOXP3+ arise as a separate lineage in the thymus and exit to the periphery
- IMPORTANT in controlling self reactive T cells
- Transcription factor FOXP3 (forkhead box p3) is necessary for development and function

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Mutations in FOXP3

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- Mutations in FOXP3 develop a multi- organ autoimmune inflammatory disease “IPEX” (immune dysregulation, polyendocrinopathy, enteropathy, X-linked) syndrome
- Presents as watery diarrhea, eczema, and Type I diabetes (or other endocrine pathology)
- Bone marrow transplantation or large doses of immunosuppression are required