T cell development, maturation, activation and differentiation

This presentation is a part of Anna University B.Tech Biotechnology Semester 6 BT6602 Immunology syllabus Unit 2

1. T-cell Maturation, Activation, & Differentiation BT6602 Unit 2 Dr K.Geetha Associate Professor Department of Biotechnology Kamaraj College of Engg & Tech Madurai

2. Overview Early Development T-Cell Repertoire Selection Processes Late Development Activation Differentiation Cell Death

3. Progenitor T cells migrate to thymus At about 8 th or 9 th week of gestation in humans T cell maturation involves rearrangements of the germ-line TCR genes In thymus, thymocytes proliferate and differentiate

4. The development of T cells

5. Figure 7-8 part 1 of 2 The cellular organization of the human Thymus

6. Figure 7-9 The cellular organization of the human Thymus

7. Selection process in thymus Positive selection Survival of only T cells whose TCRs recognize self-MHC molecules Negative selection Eliminates T cells that react too strongly with self MHC or MHC with self-peptides

8. T-cell Development Begins with arrival of small numbers of lymphoid precursors migrating from blood to thymus When they do arrive in thymus, T-cell precursors don’t express signature surface markers (CD3, CD4, and CD8) – Double Negatives (DN) Do not express RAG-1 or RAG-2 that are necessary for gene rearrangement

9. Figure 7-12 Changes in cell surface molecules throughout T-cell maturation in the Thymus

10. T-cell Development During 3 week development, differentiating T cells pass through stages of development based on surface phenotypes

11. DN= double negative Cell-Surface Molecules c-Kit : Receptor for stem cell growth factor CD44 : Adhesion Molecule CD25 : Alpha-Chain of the IL-2 receptor

12. TCR is now in the Double Positive (DP) State

13. Positive Selection Results in MHC restriction Mechanism: Immature thymocytes cluster with MHC molecules on the cortical cells of the thymus If TCR interacts with MHC  protective signal results that prevents apoptosis. If TCR does not interact with MHC  no protective signal and apoptosis occurs. Result? Only reactive thymocytes survive.

14. Negative Selection Ensures self-tolerance Weeds out High affinity thymocytes Mechanism: APC’s bearing MHC’s interact with thymocytes If avidity is too strong  thymocyte undergoes apoptosis. Details unknown… Result? Only self-tolerant thymocytes survive. Avidity (functional affinity) is the accumulated strength of multiple affinities.

15. T cell development is expensive for host 98% of all thymocytes do not mature, die by apoptosis within thymus

16. T cell Activation Initiated by TCR-CD3 complex with processed antigen on MHC molecule CD8 + cells with Class I CD4 + cells with Class II Initiates cascade of biochemical events Inducing resting T cell to enter cell cycle, proliferate, differentiate into memory and effector T cells

17. T cell Activation Cascade of biochemical events leading to gene expression: Interaction of signal and molecule (example: TCR + MHC and antigen) Generation of “second messenger” that diffuses to other areas of cell Protein kinases and protein phosphatases are activated or inhibitied Signals are amplified by enzyme cascades

18. T cell Activation Gene products after activation Immediate genes (1/2 hour of recognition) Transcription factors (c-Myc, NFAT, NF- κ B) Early genes (1-2 hours from recognition) IL-2, IL2R, IL-6, IFN- γ Late genes (more than 2 days later) Encode adhesion molecules

19. Signal-transduction pathways associated with T-cell activation. (a) Phospholipase C (PLC) is activated by phosphorylation. Active PLC hydrolyzes a phospholipid component of the plasma membrane to generate the second messengers, DAG and IP 3 . (b) Protein kinase C (PKC) is activated by DAG and Ca 2 . Among the numerous effects of PKC is phosphorylation of IkB, a cytoplasmic protein that binds the transcription factor NF-kB and prevents it from entering the nucleus. Phosphorylation of IkB releases NF-kB, which then translocates into the nucleus. (c) Ca 2 -dependent activation of calcineurin. Calcineurin is a Ca 2 /calmodulin dependent phosphatase. IP 3 mediates the release of Ca 2 from the endoplasmic reticulum. Ca 2 binds the protein calmodulin, which then associates with and activates the Ca 2 /calmodulin-dependent phosphatase calcineurin. Active calcineurin removes a phosphate group from NFAT, which allows this transcription factor to translocate into the nucleus.

20. Activation of the small G protein, Ras. Signals from the T-cell receptor result in activation of Ras via the action of specific guanine nucleotide exchange factors (GEFs) that catalyze the exchange of GDP for GTP. Active Ras causes a cascade of reactions that result in the increased production of the transcription factor Fos. Following their phosphorylation, Fos and Jun dimerize to yield the transcription factor AP-1. Note that all these pathways have important effects other than the specific examples shown in the figure.

21. Chokingly complex- what to remember Importance of CD 3 Phosphorylation activates proteins Cascade G proteins, 2 nd messengers Gene activation Ongoing proliferation- IL2 and its receptor.

22. “Signal 1 and 2”- TCR activation isn’t the whole story TCR activation is necessary, but not sufficient, to produce activation. It is called “signal 1”. The T cell also needs “signal 2”- CD28-B7 interaction. Its absence produces clonal anergy T H -cell recognition of an antigenic peptide–MHC complex sometimes results in a state of nonresponsiveness called clonal anergy, marked by the inability of cells to proliferate in response to a peptide-MHC complex

23. Second Signal

24. Superantigens (SAgs) are a class of antigens that cause non-specific activation of T-cells resulting in polyclonal T cell activation and massive cytokine release. SAgs are produced by some pathogenic viruses and bacteria most likely as a defense mechanism against the immune system.

25. T-Cell Differentiation CD4+ and CD8+ cells leave thymus and enter circulation in G phase Naïve cells (condensed chromatin, little cytoplasm) About twice as many CD4+ Naïve cell recognized MHC-antigen complex Initiated primary response After 48 hours, enlarges into blast cell and undergoes repeated rounds of cell division Differentiate into: Effector cells – cytokine secretion, B-cell help Memory cells – long lived, respond with heightened activity (secondary response)

26. T reg Cells Shown to inhibit proliferation of other T cells in vitro CD4+CD25+ Shown to inhibit development of autoimmune diseases. maintain tolerance to self-antigens.

27. Cell Death and T Cell Populations Apoptosis plays critical role Deletion of potentially autoreactive thymocytes Deletion of T cell populations after activation Fas and FasL pathway to induce self death

28. Cell Death! How does apoptosis occur during thymocyte selection?? Specialized Protease called “Caspases” Every cell produces these proteases which are maintained in an inactive form Must get activated to undergo apoptosis

29. 2 Pathways Death Signal!!! (Apoptosis Inducing Factor) BID: BH3 Interacting Domain Apaf-1: Apoptotic Protease Activating Factor 1

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