T-Cell Development - MIC 660 Fall 2024 PDF

T-Cell Development Carolina Francelin Rovarotto [email protected] MIC 660 Fall 2024 SL229  T cells T cells are a big part of the adaptive arm of the immune system – cellular immune response T cells possess a diverse repertoire...

T-Cell Development Carolina Francelin Rovarotto [email protected] MIC 660 Fall 2024 SL229  T cells T cells are a big part of the adaptive arm of the immune system – cellular immune response T cells possess a diverse repertoire of receptors - TCRs Activated by MHC antigen presentation Post-activation, they differentiate into effector T cells  Where do T cells develop? Before recognition of antigens via MHC presentation occurs, lymphatic progenitors must differentiate into naïve T cells. This process occurs in the thymus. Ideal outcome: Properly developed naïve T cells, no self-reactivity.  Thymus A supportive environment for T cell training and development The thymus has 2 lobes, and each lobe contains lobules with a cortex and a medulla Cortex: site of differentiation, somatic recombination, positive selection Medulla: site of negative selection, promotion of central tolerance  Thymus Cells of the thymus T cells develop with the helping hands of these players: Thymic epithelial cells (TECs) – activate thymocyte development through direct interactions & cytokine production Macrophages – clean up defective thymocytes along the way Dendritic cells – present self antigens to promote tolerance  T cell development Origin and developmental steps of T cells + locations in body Hematopoietic stem cell Bone marrow Lymphoid progenitor cell Bone marrow, trafficking to thymus Double-negative thymocyte Thymus – cortex Double-positive thymocyte Thymus – cortex Single-positive T cell Thymus – medulla and onward  T cell development Origin and developmental steps of T cells + surface markers Hematopoietic stem cell CD34+, CD38low, c-kit (CD117)+ Lymphoid progenitor cell CD34+, CD38+, c-kit (CD117)+, IL-7Rlow Double-negative thymocyte Multiple steps after Notch1 signaling Double-positive thymocyte CD4+, CD8+ Single-positive T cell either CD4 or CD8+  T cell development Notch signaling is key for T cell differentiation Notch is a signaling pathway in cell proliferation and differentiation that requires proteolytic cleavage of the receptor. The ligand for Notch1 is present on the surface of TECs. The result is the activation of gene expression required for T-cell differentiation.  T cell development overview Double-Positive Single-Positive Double-Negative Single-Positive, Naïve T cell  T cell development Double-negative lymphocytes have several stages along the way Relates to cell-surface markers, but also roughly translates to stages of development: DN1: Beginning of somatic recombination DN2: Committed to T-cell DN3: Finished 𝜷-chain development DN4: Active 𝜶-chain recombination recombination  T cell development TCR genes rearrangement Somatic recombination There are two temporal checkpoints, with 3 possible outcomes: 1. Functional 𝜸𝜹 T-cell receptor 2. Functional 𝜷 chain 3. Functional 𝜶𝜷 T-cell receptor  T cell development TCR genes rearrangement Checkpoint #1 Recombination occurs at 𝜷, 𝜸, and 𝜹 loci The race is on (𝜸𝜹 vs. 𝜷) - winner If 𝜷 prevails, its functionality is is carried forward tested with a pre-T 𝜶 chain. If this pT𝜶 + 𝜷 receptor works, becomes a double-positive Pre- T cell. If neither recombination is successful => apoptosis  T cell development TCR genes rearrangement Checkpoint #1 𝜷 chain rearrangements occur on both copies of chromosome 7, and each have a possibility of two variable domains (based on After a functional 𝜷 chain is D and J segments): 4 total determined, recombination of this locus is ‘locked in’ with chromatin remodeling: allelic exclusion  T cell development TCR genes rearrangement Checkpoint #2 Somatic recombination resumes at the 𝜶, 𝜸, and 𝜹 loci. Another race – either the 𝜸𝜹 T-cell becomes the fate, or 𝜶 receptor is completed and successfully forms an 𝜶𝜷 T-cell If 𝜶𝜷 T cell, it will also possess receptor. both kinds of co-receptors (Double-positive)  T cell development TCR genes rearrangement Checkpoint #2 𝜶-chain recombination is an iterative process until a productive rearrangement occurs. Successful 𝜶-chain recombination precludes further 𝜹-chain recombination (the 𝜹 locus is excised).  T cell development TCR genes rearrangement Checkpoint #2 𝜶-chain recombination is an iterative process until a productive rearrangement occurs. Successful 𝜶-chain recombination precludes further 𝜹-chain recombination (the 𝜹 locus is excised).  T cell development TCR genes rearrangement Productive vs. unproductive rearrangement During all recombination events, there’s a check for a functioning protein. A frameshift during recombination can result in a non-functional protein: “unproductive rearrangement” Recombination continues exhaustively until either outcome.  T cell development overview Double Positive Single Positive Double Negative Single Positive, Naïve T cell  T cell development Functional TCR selection Selection begins in the cortex If successful,thymocyte will move to the medulla for negative selection First, must ensure that the 𝜶𝜷 T- cell receptor works and can recognize self-MHC. There are only self-antigens in the thymus!  T cell development functional TCR selection Three possible outcomes: Death by neglect – No recognition of MHC = useless Positive selection – recognition of MHC below “high” threshold – pass! Will undergo lineage commitment. Negative selection – binds too tightly, indicates self-recognition = also useless  T cell development CD4 or CD8 Lineage commitment occurs upon positive selection Will it bind MHC I or MHC II? As the cell moves from double-positive to single- ? positive, it will express CD4 only or CD8 only. How does it decide? Two theories: instructive model or kinetic signaling model ?  T cell development CD4 or CD8 Lineage commitment occurs upon positive selection Will it bind MHC I or MHC II?  T cell development CD4 or CD8 Lineage commitment occurs upon positive selection Will it bind MHC I or MHC II?  T cell development overview Double Positive Single Positive Double Negative Single Positive, Naïve T cell  T cell development auto-reactive TCR negative selection Selection ends in the medulla If successful, will become a mature T lymphocyte. Dendritic cells and mTECs mediate negative selection. AIRE: autoimmune regulator, allows the cell to express tissue-specific antigens at a high rate  T cell development recap Double Positive Single Positive Double Negative Single Positive, Naïve T cell  Diversity revisited TCRs can be 𝜶𝜷 or 𝜸𝜹𝜶-chain:. 47 V x 50 J segments = 2350 𝜷-chain: 48 V x 2 D x 13 J segments = combinations Random 𝜶𝜷 assembly = 2.9 x 106 1248 combinations possibilities! Don’t forget about junctional diversity!