DIFFERENTIATION AND MATURATION OF T CELLS IN THE THYMUS

DC mackrophage

MYELOID PRECURSOR

BONE MARROW

HSC HEMATOPOIETIC STEM CELL

mast

DC monocyte neutrophilmast

neutrophil

TISSUES

BLOOD

B-cell T-cell

T-cellNK-cell

THYMUS

B-cell

LYMPHOIDPRECURSOR

LYMPHOID TISSUES

BLOOD

T- CELL DEVELOPMENT

NK cell

Pro-T

-rearrangementT

Pre-T

-rearrangement

Pre-T

Selectionclonal deletion

TT

TMature-T

Lymphoid precursor

Mature-B

c-kit/CD44

H rearrangement

Surrogate L

L rearrangement

Selectionclonal deletion

B

B

B

B

Pro-B

Pre-B

RAG-1/RAG-2

REGULATED T-CEREGULATED T-CELLL DIFFERENTIATIONL DIFFERENTIATION

pre T cellpro T cell

immature T cell

NO ANTIGEN RECOGNIZING RECEPTOR

SIGNALING RECEPTOR

ANTIGEN RECOGNIZING RECEPTOR

preT-CD4+CD8+

TCR

Epithelial cellAPC

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-cellNo selection

αβ NKT-cell

αβCD4+ αβCD8+

CD4+CD8+

IL-7-dependent proliferation

Pro-T

unsuccesful β-chain

unsuccesful α-chainno positive selectionnegative selection

α rearrangement

Late pre-TCD4+CD8+

++

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 clones4. Elimination of self agressive

clones (-)5. CENTRAL TOLERANCE6. Focusing The T cell repertoire

for recognition of non self7. Individualized T cell repertoire

is available in the periphery 8. CD4 and CD8 co-stimulatory

molecules are involved in positive selection

αβTCR αβTCRCD4+ CD8+

SELECTION OF T LYMPHOCYTES IN THE THYMUS

UNDER THE CAPSULE

CORTEX

CORTEX/MEDULLA

IL-7-dependent proliferation

β+preTαCD4-CD8-

DN

CD4+CD8+DP

MEDULLA

TCRαβ

TCR(-) sMHC+sP sMHC+fP fMHC+fP

selection

– selection

–AICD

NO

PERIPHERAL TOLERANCE

AICD – Activation Induced Apoptosis

CD4+CD8+ 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+ cells Lack of MHC class II – no CD4+ cells

POSITIVE SELECTION FOR 3 – 4 DAYS, SUCCESSIVE α-GENE REARRANGEMENTS

POSITIVE SELECTION – Thymic education (no instruction for specificity)Low avidity interaction of MHC - self peptide - TCR Thymic epithelial cellsSelf peptide composition and concentration (foreign peptides are not present)Low peptide dose induces positive selection – special ligands80-90% of DN (CD4-CD8-) T cells is NOT positively selected PASSIVE CELL DEATH BY NEGLECTION

NEGATIVE SELECTION – Central self toleranceHigh avidity of MHC - self peptide - TCR interactionUbiquitous and abundant self antigens are present in the thymusHigh peptide dose induces negative selectionAny 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 REPERTOIREPHYSIOLOGICAL TRESHOLD

SELECTION OF THE T CELL REPERTOIRE – CENTRAL TOLERANCE

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

Activated T-cellMature naive T-cell

Memory T-cell

T-CELL DIFFERENTIATION IN THE PERIPHERY

Ag

Ag

CD4 TCR

APC

CD8 TCR

APCCD4 TCR

APC

CD8 TCR

APC

CD4 TCR

APC

CD8 TCR

APC

Ag

EFFECTOR T CELLS

Cytotoxic T cells Th1 cells Th2 T cells

Virus infected cellMacrophage

containing bacteriaB cell presenting specific antigen

CytotoxinsPerforin

Granzyme

CytokinesFasLIFNγ

TNF-βTNF-α

ActivationIFNγ

GM-CSFTNF-αCD40LFasL

CytokinesIL-3

GM-CSFIL-10TGFβ

Eotaxin

ActivationIL-4IL-5

CD40L

CytokinesIL-3

TNF-βIL-2

EFFECTOR T LYMPHOCYTES

Effector T cells interact with and act on antigen presenting cells

Effector T cells secrete cytokines and cytotoxins

B-cell T-cell

Appearance of antigen Soluble, molecules on the surface of any cell, particles

Cells carrying self MHC-antigen derived peptide complexes

Nature of the antigén Natíve protein, carbohydrate, lipids, metals,any structure

Processed protein fragments = peptides

Ligand Conformational determinantsequential determinant

MHC-peptide complex

Antigen recognizing receptor on the cell surface

variable BCRligand (antigen) specificbivalent

variable TCR MHC + peptide pecificmonovalent

Soluble antigen recognizing receptor

antibody -

Collaboration of other cells - Antigen processing and presenting cells APC

Antigen processing, presentation

- Intracellular enzymatic degradation, peptide transportation

Result of full activation Production of effector molecule (antibody= soluble BCR)

Activation of new genesproduction of lymphokines

Possibililties of cell activation FULLDifferentiation to plasma cell, antibody productionPARTIAL funcional anergyAPOPTOSIS

FULLLimphokine productionPARTIAL functional anergyproduction of certain lymphokinesAPOPTOSIS

Co-receptors CD19, CD21, CD22 CD4, CD8, CD28/CTLA4, CD2, CD38