T Cell Development

Question 1

Thymus

Answer 1

• major but not exclusive site for the maturation of T cells
• site of TCR gene rearrangements
• provides the appropriate microenvironment for the maturation of precursors and their selection (pos and neg) based on their TCR specificities
• site of ordered stages of maturation that have been phenotypically identified and functionally committed

Question 2

Thymus 2

Answer 2

• immunodeficiency associated with defects in T cell function caused by- neonatal thymectomy, DiGeorge’s Syndrome, natural mutations
• only about 3% of the thymocytes ever leave the thymus, a site of maturation, cell proliferation, and cell death by apoptosis
• very active in fetal and early life, it later undergoes involution but can be reactivated as a site of thymocyte development in older individual

Question 3

embryonic thymic development

Answer 3

• thymus formed from the fusion of 3rd pair of pharyngeal pouch-endoderm-and cleft- ectoderm
• mesenchyme surrounding the epithelial components condense to form the capsule. The mesenchyme also penetrates the thymic rudiment to form the trabeculae
• hematopoietic precursors seed the thymic rudiment beginning at 8 weeks gestation

Question 4

cellular organization of the thymus

Answer 4

• cortex and medulla
• capsule and trabeculae
• cortico-medulllary junction
• medulla has hassalls corpuscles

Question 5

immigration and emigration from thymus

Answer 5

• T cell precursors (prothymocytes) derived from fetal liver and bone marrow seed the thymus
• thymic anlage produces chemotactic factors that attract T cell progenitors
• cells that seed the thymus enter at the cortico-medullary junction via blood vessels
• thymocytes that survive the rigors of selection leave the thymus from venules in the medullla-also controlled by chemokines and sphingosine 1-phosphate receptors

Question 6

T cell and flow cytometry

Answer 6

-T cells go from double neg to double pos to CD4 or CD8 pos

Question 7

Double negative cells

Answer 7

• 2-5% of total thymocytes
• contain least mature cells, considerable cell division
• 2/3 are triple neg based on TCR expression
• TCR beta, gamma, delta rearrangement occurs at this stage
• 1/3 are mature TCRdelta/gamma and TCR alpha//beta cells

Question 8

double positive cell

Answer 8

80-85% of total thymocytes

• TCRalpha rearrangement here
• most have rearranged TCRalpha and beta genes and express low levels of TCR
• small subset have high levels of TCR (most maturem positively selected cells)
• small subset is actively dividing (earliest DP cells
• most apoptosis occurs here, very sensitive to apoptosis inducing agents, especially glucocorticoids

Question 9

Single positive cells

Answer 9

• 10-15% of total thymocytes
• most are mature cells with high levels of CD3 and TCRbeta
• CD4:CD8 approx 2:1
• most SP cells are functionally mature and are destined to leave the thymus
• small subsets of SP are immature (ISP)-have low CD3 and TCRbeta-transitional cells that are on the way from DN to DP

Question 10

Checkpoint 1

Answer 10

• assesses whether TCRbeta chain functionally rearranged (like heavy chain on B cell)-need TCRbeta and pTalpha and maybe CD3 on pre-TCR cell, Beta checkpoint slightly beforehand
• consequences of passing checkpoint:
• allelic exclusion at the TCRbeta locus, proliferation, expression of CD4 and 8, no TCRgamma transcription, initiation of alpha rearrangement
• what stim?- no known ligand, preTCR can mediate signal without ectodomains of pTa and TCRb
• inhibited by no RAG, no CD3, no TCRbeta, no pTa

Question 11

Checkpoint 2

Answer 11

• cell now has mature TCRalpha and is heading toward single positive
• assesses whether TCRa is functionally rearranged, TCR is self MHC restricted, whether TCR is auto reactive
• consequences of passing: maturation of thymocyte to functionally competent SP cell, establishes a self MHC restricted, non-autoractive TCR repertoire with appropriately matched co-receptors and functional potential

Question 12

how is specificity of abTCR assessed?

Answer 12

-requires peptide/MHC molecules interactions to induce a signal

Question 13

if no interaction

Answer 13

-dies

Question 14

if interaction too strong

Answer 14

-dies

Question 15

ligands for pos/neg selection

Answer 15

• ligands are peptide?MHC complexes expressed on stromal and hematopoietic cells. peptides are derived from endogenous sources in the thymus or serum components
• critical to delete thymocytes expressing TCRs that would react with BM-derived APCs in the periphery-need to present self/endogenous peptides to shape repertoire
• Aire TF produces peptides/antigens from other specific locations to check T cells-no aire=autoimmunity

Question 16

avidity model

Answer 16

• self peptide and self MHC lead to maturation or death
• avidity determines strength of signal delivered
• too small- death by neglect
• too strong-negative selection
• just right is pos

Question 17

positive selection

Answer 17

• cortical epithelial cells promote pos selection
• key experiment- when class II MHC were missing from cortex (only in medulla) no selection for CD4 cells occurred. CD4 developed in wt and cortex MHC cells

Question 18

negative selection

Answer 18

• can occur in cortex or medulla, depending on where high acidity interaction occurs
• normally concentrated at cortico-medullary junction and in medulla
• see increased autoreactive CD4 cells if medulla lacks class II MHC.
• tissue specific AI develops if there is an aire mutation
• induced by:thymic DCs, cortical epithelial cells, medullary epithelial cells,
• thymic macrophages can’t induce neg selection-they eat apoptotic cells.