1. Asilmi 08 - T CELL DEVELOPMENT TODAY T LYMPHOCYTE DEVELOPMENT

2. Asilmi 08 - T CELL DEVELOPMENT THE T CELL RECEPTOR (TCR) Each chain contains a variable (V) and constant (C) region. 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. The TCR is a heterodimer composed of an  and a  chain.

3. Asilmi 08 - T CELL DEVELOPMENT The TCR compared with a free Ig molecule. 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.

4. Asilmi 08 - T CELL DEVELOPMENT 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. Asilmi 08 - T CELL DEVELOPMENT Ag 22 11 22 33 MHC I ANATOMY OF AN MHC-PEPTIDE-TCR COMPLEX 1. Antigenic peptide contacts the  1 and  2 domains of the MHC I molecule Ag 22 11 22 33 VV VV MHC I TCR 2. Both the  and  chains of the TCR interact with the peptide. 3. Additional TCR residues interact with the MHC molecule.

6. Asilmi 08 - T CELL DEVELOPMENT APC present antigens to T cell on major histocompatibility complex (MHC) class I and class II molecules. T Ag are linear molecules in the context of MHC Tc : CD8 Th : CD4 TCR APC CD8 CD4 TCR CD8 MHC I TCR MHC II CD4

7. Asilmi 08 - T CELL DEVELOPMENT THE TCR INTERACTS WITH THE PEPTIDE AND THE MHC CD4 and CD8 do NOT interact with the antigenic peptide. antigen CD4 and CD8 interact with invariant residues on MHC class II and I. antigen 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. Asilmi 08 - T CELL DEVELOPMENT 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. Asilmi 08 - T CELL DEVELOPMENT 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. Asilmi 08 - T CELL DEVELOPMENT 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. The end result: a naïve CD4 or CD8 T cell that recognizes a 3 dimensional surface comprised of both peptide and MHC.

11. Asilmi 08 - T CELL DEVELOPMENT FEATURES OF TCR-MEDIATED ANTIGEN RECOGNITION 4. Signaling triggers T lymphocyte activation. 3. Antigen recognition is mediated by specific domains of the TCR. 2. Each clone is specific for a single antigen. 1. TCR recognize peptides presented by MHC molecules.

12. Asilmi 08 - T CELL DEVELOPMENT TCR proteins on different T cells arise from gene rearrangement of multiple germ line genes (just like BCR gene rearrangement). The beta chain gene is formed from different V, D, J, and C gene recombination. During T cell development alpha chain V, J and C genes recombine to form the T cell alpha gene that characterizes that T cell.

13. Asilmi 08 - T CELL DEVELOPMENT ALLELIC EXCLUSION MATERNAL VV CC JJ PATERNAL VV CC JJ Each individual inherits maternal AND paternal alpha chain genes. Recombinase rearranges V and J gene segments to form an alpha chain. 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. Asilmi 08 - T CELL DEVELOPMENT TCR rearrangement occurs in the thymus. T cell progenitors (thymocytes) migrate from bone marrow to the thymus. 1 TCR genes rearrange in the thymus to produce T cells, which then migrate to peripheral lymphoid organs to find antigen 2

15. Asilmi 08 - T CELL DEVELOPMENT T LYMPHOCYTE MATURATION IN THE THYMUS 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. THYMIC EPITHELIAL CELL MHC-peptide T cells that bind MHC-peptide complexes survive (positive selection). TCR Positive selection TCR Positive selection ensures that T cells CAPABLE of interacting with peptide-MHC complexes remain alive.

16. Asilmi 08 - T CELL DEVELOPMENT CD4 TCR CD8 TCR 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. THYMIC EPITHELIAL CELL MHC-peptide T cells that bind Class II MHC-peptide complexes preserve expression of CD4 and lose CD8. CD4 TCR Thus, CD4 T cells develop on the basis of the ability of their TCR to interact with MHC class II molecules. T LYMPHOCYTE MATURATION IN THE THYMUS

17. Asilmi 08 - T CELL DEVELOPMENT CD8 CD4 TCR 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. THYMIC EPITHELIAL CELL MHC-peptide CD8TCR T cells that bind Class I MHC-peptide complexes preserve expression of CD8 and lose CD4. Thus, CD8 T cells develop on the basis of the ability of their TCR to interact with MHC class I molecules. T LYMPHOCYTE MATURATION IN THE THYMUS

18. Asilmi 08 - T CELL DEVELOPMENT TCR CD4 CD8 T cells that do not recognize MHC molecules die by apoptosis (failure of positive selection). 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 weeds out useless TCRs that are INCAPABLE of detecting MHC-antigen complex. T LYMPHOCYTE MATURATION IN THE THYMUS

19. Asilmi 08 - T CELL DEVELOPMENT THYMIC EPITHELIAL CELL MHC-peptide TCR T cells that strongly bind MHC-peptide complexes die by apoptosis (negative selection). T LYMPHOCYTE MATURATION IN THE THYMUS 6. Negative and positive selection sorts out useful T lymphocytes from useless, or potentially dangerous T lymphocytes. This type of selection deletes TCRs that detect self-antigens in complex with MHC molecules (removes autoreactive lymphocytes).

20. Asilmi 08 - T CELL DEVELOPMENT thymocyte maturation occurs in stages. http://www.ag.uidaho.edu/mmbb/kgustin/mmbb409509/Lectures.html

21. Asilmi 08 - T CELL DEVELOPMENT Why this complicated system of positive and negative selection? 1.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. Asilmi 08 - T CELL DEVELOPMENT 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. Is antigen-presentation all that is needed? What happens next?

23. Asilmi 08 - T CELL DEVELOPMENT Two signals are required to activate a naïve T cell. The second signal comes from the interaction of APC B7 with T cell CD28.

24. Asilmi 08 - T CELL DEVELOPMENT 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. T CELL SIGNALING - COSTIMULATORS

25. Asilmi 08 - T CELL DEVELOPMENT Not only does 1 signal not activate a naïve T cell, it makes it anergic (unreactive). 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. Even if it goes to an APC that can deliver the second signal, it cannot be activated.

26. Asilmi 08 - T CELL DEVELOPMENT Why? 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.

27. Asilmi 08 - T CELL DEVELOPMENT 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.