T Cell Ontogeny

Takes place in the thymus

Progenitor cells from the BM arrive at cortico-medullary junction

Migrate to subcapsular region, begin to differentiate

1. Acquire Ag-specificity

2. Make CD4 vs CD8 decision (lineage commitment)

3. Learn to recognize “self”

+ selection to recognize self MHC

- selection to delete very strong MHC affinity (auto-reactive cells)

Most developing thymocytes fail these selective processes

First TCR gene to rearrange is TCR, at day 13 in gestation

and genes rearrange at day 14

starts rearranging at day 16-17

However, the majority of thymocytes early in gestation express /on the cell surface

For an T cell

chain pairs with pre-TCR, gp33This is a pseudo receptor, which signals a successful rearrangement occurs

Cell must also express CD3 for TCR to be surface associated

Subsequently chain genes rearrange, and is expressed to pair with

Thymocyte differentiation is complex

Cells acquire CD4 and CD8 (Double Positive)They then lose either CD4 or CD8 to become single positive

This happens around the cortico-medullary junction

Thymocyte Maturation Steps

Positive selectionTCR must be able to react with self MHCIf cell does not bind MHC, it dies

Negative selectionTCR must not bind MHC too stronglyIf signal too strong, cell dies

Lineage CommitmentCD4 Vs CD8 decision

Instructive theoryBinding to MHC triggers commitment

If TCR binds MHC I, cell becomes CD8 SPIf TCR binds MHC II, cell becomes CD4 SP

Selective theoryCD4 or CD8 randomly inactivated

If cell retains CD4 and TCR can bind Class II, cell survivesIf cell retains CD8 and TCR can bind Class I, cell survives

CD

4

CD8

Asymmetric Lineage CommitmentDifferential requirements for CD4 and CD8 commitment

CD4

CD8

CD8

CD4

Pronase digestion

Incubate

CD4 CD8

CD

4

CD8

Normal

CD

4

CD8

Class I KO

CD

4

CD8

Class II KO

CD4 CD8

CD4

CD4 CD8

CD8 commitmentRequires MHC I

CD4 commitmentDoes not requireClass II

CD4 cells dieIn Class II KOBecause they fail+ selection

Asymmetric Lineage Commitment

MHC Restriction

CTLs from %T Cells killed (Targets):

infected mouse H-2k H-2k/v* H-2d H-2d/v*

H-2k 0 80 0 0

H-2d 0 0 0 80

The ability to distinguish self + Ag

Positive Selection

Evidence for + Selection

TCR V 17a is expressed at high levels on CD4 cells in certain strains (H-2q and H-2s)Haplotype: % periph T cells V17a+

Mousestra :in K IA IE D 4CD 8CDSWR Q Q - Q 19 5

10B .Q Q Q - Q 14 4SJL S S - S 16 757C B B - B 3 610B B B - B 4 7

ATx BM Thymus % V17acellsmous :e source: source: CD4 CD8

-H 2b -H 2b -H 2b 3.5 4.9

-H 2b -H 2b -H 2q 13.6 5.8

-H 2q -H 2q -H 2b 4.0 4.2

-H 2q -H 2q -H 2q 20.7 5.1

The thymus determines level of + selection Bone marrow source is irrelevant

+ selection occurs on thymic epithelium

Allelic Exclusion of TCR

o,βo = unrearranged locus (germline)

α+,β+ = productively rearranged locus

α-,β- = abortively rearranged locus

+/ο +/− +/+TcR-β (10) 0 9 1

α + /α ο α + /α − α + /α +

TcR-α (31) 2 21 8

For TCR the most significant factor in allelic exclusion is abortive rearrangement

TCR shows abortive rearrangement, but also may have 2 productively rearranged alleles

In this case, see selective chain pairing of with a single chain protein

Results in a single TCR specificity at the cell surface

DNA for TCR and TCR loci in T cell lines analyzed for rearrangement

Transgenic mice constructed with rearranged TCR show no changes in endogenous loci

TCR transgenics do demonstrate continued rearrangement of endogenous loci

Allelic exclusion of TCR is somewhat “leaky”

Delayed Type Hypersensitivity (DTH)

Localized inflammatory reaction:Large influx of inflammatory cells (M)Delayed onsetTissue damageMediated by TH1 cells

First noted in TB (tuberculin reaction)

Sensitization phase1-2 weeks following first Ag exposureTH1 cells proliferate due to Ag/APCsOccasionally see CD8 cells responding

Effector phaseSeen during subsequent Ag exposureCytokines secreted

APCs recruited and activatedHuge inflammatory responsePeaks at 48-72 hours post exposure

Granulomas may develop, displace normal tissue

Ags that induce DTH

Intracellular bacteriaVarious mycobacterium speciesListeria monocytogenes

Intracellular fungi

Pneumocystis cariniiCandida albicans

Intracellular parasites

Leishmania sp.

VirusesHerpes simplexVariola (smallpox)Varicella (measles)

Contact allergensPoison oak, poison ivy, etc.