TODAY

1. TODAY T LYMPHOCYTE DEVELOPMENT

2. The TCR is a heterodimer composed of an a and a b chain. Each chain contains a variable (V) and constant (C) region. THE T CELL RECEPTOR (TCR) The V and C regions are similar in structure to the V and C regions of Antibodies. Likewise, the V regions contribute most to antigen recognition. In contrast to Antibodies, TCR are anchored in the plasma membrane and are not secreted.

3. Both are composed of two chains that combine to form variable and constant domain. Both have flexible hinge regions. In contrast to an antibody, the TCR is directly and permanently anchored in the plasma membrane. Thus, the C-terminus has no effector functions, it is needed for signaling. The TCR compared with a free Ig molecule.

4. B CELL AND TCR-MEDIATED ANTIGEN RECOGNITION 1. TCR recognize peptides presented by MHC molecules. B cell receptor recognizes antigens without a requirement for presentation. 2. Recognition is mediated by variable domains. 3. TCR does not perform effector functions.

5. ANATOMY OF AN MHC-PEPTIDE-TCR COMPLEX TCR 1. Antigenic peptide contacts the 1 and 2 domains of the MHC I molecule V 2. Both the a and b chains of the TCR interact with the peptide. Ag Ag V Ag 1 1 2 2 3. Additional TCR residues interact with the MHC molecule. 2 2 3 3 MHC I MHC I

6. APC present antigens to T cell on major histocompatibility complex (MHC) class I and class II molecules. APC APC MHC II MHC I CD8 CD4 TCR TCR CD4 CD8 TCR TCR Th : CD4 Tc : CD8 T Ag are linear molecules in the context of MHC

7. THE TCR INTERACTS WITH THE PEPTIDE AND THE MHC antigen CD4 and CD8 interact with invariant residues on MHC class II and I. antigen antigen CD4 and CD8 do NOT interact with the antigenic peptide. Thus, the specificity of the response in not determined by the identity of the antigen. It is determined by the nature of the cell presenting the antigen.

8. CD4/CD8 Distinctions fine-tune the adaptive response. Extracellular microbes: captured and presented on class II by APCs (macrophages, B cells). CD4 T cells help B cells produce antibodies and macrophages ingest and destroy. Thus, CD4 T cells activate best defenses against extracellular microbes.

9. CD4/CD8 Distinctions fine-tune the adaptive response. Intracellular microbes: antigens from cytoplasmic microbes are captured and presented on class I by all nucleated cells. CD8 T cells directly eliminate infected cells. Thus, CD8 T cells activate best defenses against intracellular microbes.

10. The end result: a naïve CD4 or CD8 T cell that recognizes a 3 dimensional surface comprised of both peptide and MHC. What a T cell recognizes in an ‘antigen’ is a unique combination of MHC haplotype and peptide with the recognition assisted by either CD4 or CD8.

11. FEATURES OF TCR-MEDIATED ANTIGEN RECOGNITION 1. TCR recognize peptides presented by MHC molecules. 2. Each clone is specific for a single antigen. 3. Antigen recognition is mediated by specific domains of the TCR. 4. Signaling triggers T lymphocyte activation.

12. During T cell development alpha chain V, J and C genes recombine to form the T cell alpha gene that characterizes that T cell. The beta chain gene is formed from different V, D, J, and C gene recombination. TCR proteins on different T cells arise from gene rearrangement of multiple germ line genes (just like BCR gene rearrangement).

13. Recombinase rearranges V and J gene segments to form an alpha chain. ALLELIC EXCLUSION PATERNAL MATERNAL Va Ja Ca Va Ja Ca Each individual inherits maternal AND paternal alpha chain genes. If the recombination reaction is productive, recombinase is shut off at the other alpha chain locus. Prevents individual cells from expressing more than one TCR. The same event occurs during BCR rearrangement.

14. 1 2 T cell progenitors (thymocytes) migrate from bone marrow to the thymus. TCR genes rearrange in the thymus to produce T cells, which then migrate to peripheral lymphoid organs to find antigen TCR rearrangement occurs in the thymus.

15. T LYMPHOCYTE MATURATION IN THE THYMUS THYMIC EPITHELIAL CELL MHC-peptide TCR T cells that bind MHC-peptide complexes survive (positive selection). TCR Positive selection 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. Positive selection ensures that T cells CAPABLE of interacting with peptide-MHC complexes remain alive.

16. T LYMPHOCYTE MATURATION IN THE THYMUS THYMIC EPITHELIAL CELL MHC-peptide TCR CD4 T cells that bind Class II MHC-peptide complexes preserve expression of CD4 and lose CD8. CD8 TCR CD4 TCR 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. Thus, CD4 T cells develop on the basis of the ability of their TCR to interact with MHC class II molecules.

17. T LYMPHOCYTE MATURATION IN THE THYMUS THYMIC EPITHELIAL CELL TCR CD8 MHC-peptide CD4 TCR T cells that bind Class I MHC-peptide complexes preserve expression of CD8 and lose CD4. TCR CD8 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. Thus, CD8 T cells develop on the basis of the ability of their TCR to interact with MHC class I molecules.

18. T LYMPHOCYTE MATURATION IN THE THYMUS THYMIC EPITHELIAL CELL MHC-peptide T cells that do not recognize MHC molecules die by apoptosis (failure of positive selection). CD4 TCR CD8 TCR 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. This type of selection weeds out useless TCRs that are INCAPABLE of detecting MHC-antigen complex.

19. T LYMPHOCYTE MATURATION IN THE THYMUS T cells that strongly bind MHC-peptide complexes die by apoptosis (negative selection). TCR 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. THYMIC EPITHELIAL CELL MHC-peptide This type of selection deletes TCRs that detect self-antigens in complex with MHC molecules (removes autoreactive lymphocytes).

20. thymocyte maturation occurs in stages. http://www.ag.uidaho.edu/mmbb/kgustin/mmbb409509/Lectures.html

21. Why this complicated system of positive and negative selection? • Positive selection gives MHC restriction but why? Ensures that CD8+ T cells are specific for complexes of MHC class I with peptide and that CD4+ T cells are specific for MHC class II/peptide complexes. 2. Negative selection removes self-reactive cells.

22. Is antigen-presentation all that is needed? What happens next? The naive CD4 and CD8 T cells migrate from thymus to peripheral lymphoid organs to look for antigen presented on MHC class I or II, then they clonally expand.

23. The second signal comes from the interaction of APC B7 with T cell CD28. Two signals are required to activate a naïve T cell.

24. T CELL SIGNALING - COSTIMULATORS 1. “Resting” APCs do not express costimulators, even though they may present peptide antigens to T cells. 2. Naïve T cells that encounter antigens in the absence of costimulators become anergic. 3. Microbes or innate immune cytokines stimulate expression of costimulators on APCs. 4. Costimulators are recognized by their receptors on T cells and provide the second signal necessary for T cell activation.

25. Not only does 1 signal not activate a naïve T cell, it makes it anergic (unreactive). Even if it goes to an APC that can deliver the second signal, it cannot be activated. Naïve cell: 1 signal No killing or activation Anergy: A state of immune unresponsiveness. Induced when the T cell's antigen receptor is stimulated in the absence of a second signal.

26. Simultaneous detection of a foreign antigen and a second signal activates the T cell and drives subsequent immune responses. Resting APCs express little or no co-stimulators and display self-antigens that are present in the tissue. As a result the T cell becomes anergic and is no longer capable of mediating immune responses. Thus, anergy serves as an important safeguard against autoimmunity in the periphery. Why?

27. Two signals are required to activate a T cell but only one for the activated (mature) cell to function. Naïve CD8 cell: 2 signals: activated Activated CD8 cell (CTL): 1 signal kills.