DIFFERENTIATION AND MATURATION OF T CELLS IN THE THYMUS

1. DIFFERENTIATION AND MATURATION OF T CELLS IN THE THYMUS

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

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

4. EVENTS OF T CELL DIFFERENTIATION IN THE THYMUS Pro-T IL-7-dependent proliferation Early pre-T Pre-Tα-chain Lck signal β rearrangement unsuccesful β-chain γδ T-cell No selection Late pre-T CD4+CD8+ α rearrangement CD4+CD8+ αβ NKT-cell unsuccesful α-chain no positive selection negative selection αβCD4+ αβCD8+ • 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

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

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

7. SELECTION OF THE T CELL REPERTOIRE – CENTRAL TOLERANCE 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

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

10. a a APC APC APC APC APC APC CD8TCR CD8TCR CD8TCR CD4 TCR CD4 TCR CD4 TCR Ag Ag Ag T-CELL DIFFERENTIATION IN THE PERIPHERY Memory T-cell Activated T-cell Mature naiveT-cell

11. PERIPHERAL TOLERANCE IMMUNE RESPONSES ARE NOT INITIATED IN THE PERIPHERY 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

12. MECHANISMS OF PERIPHERAL TOLERANCE ANERGY – Functional unresponsiveness, no IL-2 secretion SIGNAL 1Recognition of auto-antigen on tissue cell SIGNAL 2 No B7 and CD40 expression, no co-stimulation Tissue resident professional APC are not activatedSIGNAL 3Innate 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