1. DIFFERENTIATION AND MATURATION OF T CELLS IN THE THYMUS

2. REGULATED T-CELL DIFFERENTIATION a a pre T cell pro T cell immature T cell NO ANTIGEN RECOGNIZING RECEPTOR SIGNALING RECEPTOR ANTIGEN RECOGNIZING RECEPTOR preT-  CD4+CD8+ TCR Epithelial cell APC

3. T- CELL DEVELOPMENT NK cell No rearrangement Lymphoid precursor Pro-T  -rearrangement T Pre-T  -rearrangement Pre-T  Selection clonal deletion T T T Mature-T Mature-B c-kit/CD44 H rearrangementSurrogate LL rearrangement Selection clonal deletion B B B B  Pro-B Pre-B RAG-1/RAG-2

4. 1.Generation of NK cells – no TCR 2. Differentiation of γδ and αβ TCR carrying T cells 3. Selection of αβ TCR – positive selection – negative selection 4. Differentiation of CD4+ and CD8+ T cell lineages EVENTS OF T CELL DIFFERENTIATION IN THE THYMUS Early pre-T Pre-Tα-chain Lck signal β rearrangement γδ T-cell No selection αβ NKT-cell αβCD4+ αβCD8+ CD4+CD8+ IL-7-dependent proliferation Pro-T unsuccesful β-chain unsuccesful α-chain no positive selection negative selection α rearrangement Late pre-T CD4+CD8+

5. 1.The primary T cell pool is biased to MHC-specificity (V genes) 1-2% for one allotype 2.Focusing the T cell pool to self MHC recognition (+) 3.Elimination of useless and self agressive clones (-) 4.CENTRAL TOLERANCE 5.Focusing the T cell repertoire for recognition of non self same TCR repertoire 6.CD4+ and CD8+ T cell use the same TCR repertoire 7.Individualized T cell repertoire available in the periphery 8.CD4 and CD8 co-stimulatory molecules are involved in positive selection αβTCR CD4+ CD8+ SELECTION OF T LYMPHOCYTES IN THE THYMUS UNDER THE CAPSULE CORTEX CORTEX/ MEDULLA IL-7-dependent proliferation β+preTα CD4-CD8- DN CD4+CD8+ DP MEDULLA TCRαβ TCR(-) sMHC+sP sMHC+fP fMHC+fP  selection – selection  – AICD NO  PERIPHERAL TOLERANCE AICD – Activation Induced Apoptosis

6. CD4+CD8+ POSITIVE SELECTION OF DOUBLE POSITIVE (DP) T CELLS ALSO DIRECTS CD4 AND CD8 SINGLE POSITIVE (SP) T CELL COMMITMENT MHC-II + peptide complexes recruit CD4 Thymic epithelial cell MHC-I + peptide complexes recruit CD8 BARE LYMPHOCYTE SYNDROME (BLS) Lack of MHC class I – no CD8+ cellsLack of MHC class II – no CD4+ cells POSITIVE SELECTION FOR 3 – 4 DAYS, SUCCESSIVE α-GENE REARRANGEMENTS

7. POSITIVE SELECTION – Thymic education (no instruction for specificity) Low avidity interaction of MHC - self peptide - TCR Thymic epithelial cells Self peptide composition and concentration (foreign peptides are not present) Low peptide dose induces positive selection – special ligands 80-90% of DN (CD4-CD8-) T cells is NOT positively selected PASSIVE CELL DEATH BY NEGLECTION NEGATIVE SELECTION – Central self tolerance High avidity of MHC - self peptide - TCR interaction Ubiquitous and abundant self antigens are present in the thymus High peptide dose induces negative selection Any thymic antigen presenting cell: epithelial cells, bone marrow-derived macrophages, dendritic cells THE GENERATION OF SELF MHC + FOREIGN PEPTIDE SPECIFIC T CELLS REQUIRES WEAK INTERACTION WITH SELF MHC + SELF PEPTIDE SELF RESTRICTED AND TOLERANT PERIPHERAL T CELL REPERTOIRE PHYSIOLOGICAL TRESHOLD NOT COMPLETE SELECTION OF THE T CELL REPERTOIRE – CENTRAL TOLERANCE

8. Homozygote Heterozygote HOMEOSTASIS OF POSITIVE AND NEGATIVE SELECTION IN THE DEVELOPMENT OF THE AVAILABLE T LYMPHOCYTE REPERTOIRE Number of MHC molecules Ratio of positive selection Ratio of negative selection increases with the number of MHC genes

10. a a Activated T-cell Mature naive T-cell Memory T-cell T-CELL DIFFERENTIATION IN THE PERIPHERY Ag CD4 TCR APC CD8 TCR APC CD4 TCR APC CD8 TCR APC CD4 TCR APC CD8 TCR APC Ag

11. Normal tissue cells do not express MHC class II NO SIGNAL 1. for CD4+ Th activation Normal tissue cells do not express co-stimulatory molecules and do not produce T cell differentiating cytokines NO SIGNAL 2. for CD4+ Th activation Migration of naive T lymphocytes to normal tissues is limited Antigen presenting cells are not activated in normal tissues NO SIGNAL 3. for CD4+ Th activation PERIPHERAL TISSUES TOLERIZE THEMSELVES PERIPHERAL TOLERANCE IMMUNE RESPONSES ARE NOT INITIATED IN THE PERIPHERY

12. ANERGY – Functional unresponsiveness, no IL-2 secretion SIGNAL 1 Recognition of auto-antigen on tissue cell SIGNAL 2 No B7 and CD40 expression, no co-stimulation Tissue resident professional APC are not activated SIGNAL 3 Innate immunity is not activated No inflammation CLONAL DELETION – Activation induced cell death Requires persistant high antigen dose Fas – FasL interaction SUPPRESSION – Activity of other cells Cytokine-mediated balance Effector functions are inhibited by regulatory T cells CLONAL IGNORANCE No contact with the immune system Immunologically privileged sites Central nervous system, eye No recognition in the periphery MECHANISMS OF PERIPHERAL TOLERANCE