T cell development

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T cell development

• Penny Morel
• 2/29/08
• 4-0343, morel_at_pitt.edu

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Two fundamental features characterize mature T
cell antigen recognition

• MHC restriction T cells recognize foreign
  peptides bound in the groove of self MHC class I
  or class II molecules.
• Self tolerance T cells cannot recognize self
  peptides bound in the groove of self MHC class I
  or class II molecules.

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Two critical requirements for mature T cell
development

• T cell antigen receptor (TCR) Without
  rearrangement and expression of a functional TCR
  no T cells will develop. e.g. SCID
• Thymus The thymus is the organ in which TCR
  rearrangement and selection takes place. In
  DiGeorge syndrome the thymus fails to develop and
  the individual can produce B cells but no T cells
  develop.

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Thymus

• Located in mediastinum
• Has cortex and medulla Cortex from third
  branchial cleft of the ectoderm medulla from
  third pharyngeal pouch of the endoderm
• T cell precursors enter thymus from BM -
  Thymocytes.
• Most thymocytes die in thymus - only 2 emerge as
  mature Tcells

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Thymocyte developmental stages are defined
by expression of CD4 and CD8 DN DP SP
CD4
CD8
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Successful rearrangement of the beta chain leads
to expression of the pre-TCR on the cell surface.
Nat Rev, Immunol 5571, 2005
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No ligand has been identified for the pre-TCR,
But this paper shows that when TCR ? and pre T?
are expressed they localize to the lipid raft and
initiate signaling. SCIET - Scid-derived
thymocyte cell line SCB.29 - with transfected TCR
? chain MCD - disrupts rafts PP2 - inhibits lck
phosphorylation
Nature 406524, 2000
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Stages of T cell selection and
important signaling molecules
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Signal through pre TCR stimulates

• Rearrangement of ? chain stops
• Thymocytes proliferate
• CD4 and CD8 expression is induced
• TCR ? gene transcription starts

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Figure 7-21 part 3 of 3
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Features of TCR rearrangement

• V-D-J (b) and V-J (a,g,d) rearrangements
• N region diversity (N, P)
• D regions can be read in all three frames
• Successive rearrangements are possible
• 1016 potential ab TCR, 1018 gd TCR
• No somatic mutation
• No allelic exclusion at TCRa locus (30 of T
  cells have 2 a rearrangements 5-10 T cells in
  periphery express 2 TCR)

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Positive Selection of ab TCR

• TCR rearrangement is a random process - many TCR
  generated that cannot recognize self MHC
• TCR rearrangement signals expression of CD4 and
  CD8 -double positive thymocytes
• Cells expressing TCR interact with thymic
  epithelial cells of the cortex - these cells
  express high levels of self MHC class I and class
  II molecules that contain peptides

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MHC expression is required for ve selection
J. Exp Med. 1861149, 1997
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Cortical thymic epithelium mediates positive
selection

• Bone marrow chimeric experiments
• Animals are irradiated with high doses of
  radiation - kills the bone marrow, but the thymic
  epithelium is radioresistant.
• The immune system is reconstituted by the
  injection of bone marrow from healthy animal

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Mechanism of positive selection

• If TCR, CD4, CD8 thymocytes do not bind to MHC
  expressed on thymic epithelium they will die
• Positive selection is a rescue from death
• 50 x 106 thymocytes die each day because they
  fail positive selection

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CD4 and CD8 expression

• Choice of which coreceptor to express is part of
  positive selection
• Instructive model CD4 interacts with MHC class
  II, delivers signal to T cell, CD8 expression is
  turned off
• Stochastic/selection model double positive
  thymocytes randomly decrease expression of CD4 or
  CD8 and only those cells with coreceptor
  appropriate for their TCR will survive

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Negative Selection

• Occurs at cortico medullary junction
• Occurs by clonal deletion
• Cells express high levels of TCR and have
  downregulated either CD4 or CD8
• Interact with bone marrow-derived APC (dendritic
  cells or macrophages) that present self antigens

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Is negative selection effective?

• Yes - if protein is highly expressed in thymus
• Transcription factor called autoimmune regulator
  (AIRE) turns on expression of self proteins such
  as insulin in the thymus
• Mutations in AIRE are the cause of
  APS1(autoimmune polyendocrinopathy syndrome type
  1) - a rare autoimmune disease

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Diverse tissue antigens are expresssed by
medullary thymic epithelial cells
Gene array analysis
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Medullary Epithelial Cells express antigens from
other tissues - controlled by AIRE (autoimmune
regulator)
Science 2981395, 2002
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AIRE KO mice develop autoimmunity Science
2981395, 2002
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Some T cells interacting with self antigens in
the thymus develop into regulatory T cells
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Figure 13-14
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Role of TSLP in Treg development

• Thymic stromal lymphopoetin (TSLP) expressed in
  Hassalls corpuscles of thymus - Nature 4361181,
  2005
• TSLP induces DC maturation
• TSLP-induced DC stimulates the production of
  CD25 Tregs

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How does the same interaction induce both
positive and negative selection?

• Avidity Hypothesis - threshold binding for MHC by
  the TCR required to signal positive selection is
  lower than the threshold required for negative
  selection
• Differential signaling hypothesis - nature of
  signal delivered by TCR differs antagonist vs
  agonist peptides

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Avidity Hypothesis
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