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

For an  T cell  chain pairs with pre-TCR , gp33 This 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 selection TCR must be able to react with self MHC If cell does not bind MHC, it dies Negative selection TCR must not bind MHC too strongly If signal too strong, cell dies Lineage Commitment CD4 Vs CD8 decision Instructive theory Binding to MHC triggers commitment If TCR binds MHC I, cell becomes CD8 SP If TCR binds MHC II, cell becomes CD4 SP Selective theory CD4 or CD8 randomly inactivated If cell retains CD4 and TCR can bind Class II, cell survives If cell retains CD8 and TCR can bind Class I, cell survives

CD4 CD8 Asymmetric Lineage Commitment Differential requirements for CD4 and CD8 commitment CD4 CD8 CD4 Pronase digestion Incubate CD4CD8

CD4 CD8 Normal CD4 CD8 Class I KO CD4 CD8 Class II KO CD4CD8 CD4 CD8 CD8 commitment Requires MHC I CD4 commitment Does not require Class II CD4 cells die In Class II KO Because they fail + selection

Asymmetric Lineage Commitment

MHC Restriction 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-2 q and H-2 s )

The thymus determines level of + selection Bone marrow source is irrelevant + selection occurs on thymic epithelium

Allelic Exclusion of TCR 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 onset Tissue damage Mediated by TH1 cells First noted in TB (tuberculin reaction) Sensitization phase 1-2 weeks following first Ag exposure TH1 cells proliferate due to Ag/APCs Occasionally see CD8 cells responding Effector phase Seen during subsequent Ag exposure Cytokines secreted APCs recruited and activated Huge inflammatory response Peaks at hours post exposure Granulomas may develop, displace normal tissue

Ags that induce DTH Intracellular bacteria Various mycobacterium species Listeria monocytogenes Intracellular fungi Pneumocystis carinii Candida albicans Intracellular parasites Leishmania sp. Viruses Herpes simplex Variola (smallpox) Varicella (measles) Contact allergens Poison oak, poison ivy, etc.