The Development of T Lymphocytes

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Chapter 5

• Understand how the thymus is the site of
  development for T cells
• How do cells commit to a lineage during T cell
  development
• How do we eliminate self-reactive T cells without
  eliminating the ability to recognize self-MHC

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The Development of T Lymphocytes
Two types of TCR 1. aß T cells CD4
CD8 2. ?d T cells
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T cells develop in the thymus
T cells (T-lymphocytes) thymus-dependent
lymphocytes
The thymus is a primary lymphoid organ because it
is only involved in development, not fighting
infection.
The thymus contains
a) Thymocytes (immature T cells) b) Thymic stroma
(epithelial cells)
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The Thymus
2 areas of the thymus
Cortex outer, close-packed consists of
ectodermal cells can contain thymocytes and
macrophages
Medulla inner, less dense consists of
endodermal cells contains thymocytes, dendritic
cells, and macrophages
Thymic anlage The combination of the
ectodermal and endodermal cells, colonized by
progenitor cells from the bone marrow.
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Thymus facts

• Fully developed at birth and increases in size
  until puberty
• Most active in the young
• Degrades after puberty (involution), being
  replaced with fat tissue
• Even after involution (30 yrs. old) or a
  thymectomy immunity by T cells is not impaired
  significantly
• Mature T cell repertoire is long-lived and
  self-renewing

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T cell markers
T Cell receptor aß or ?d chains bind peptide
antigens
CD3 complexes of CD3?e, CD3de, and 2 ? chains
for signaling to the interior of the cell
CD4 or CD8 receptors that bind MHC molecules
Progenitor cells entering the thymus have NO TCR,
CD3, or CD4/CD8 surface receptors immature
thymocytes called double-negative thymocytes
After ß chain rearrangement, T cells express both
CD4 and CD8 surface receptors immature
thymocytes called double-positive thymocytes
Mature T cells express either CD4 or CD8 surface
receptors thymocytes called single-positive
thymocytes
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TCR gene rearrangement
Order of TCR gene rearrangements controls the
distribution of T cell lineages 95 aß T
cells 5 ?d T cells
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T cell lineages
Pre T cell ? (pT?) - a place-holder
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aß TCR rearrangement

• ß chain
• rearranges first
• has variable (V), diversity (D), and joining (J)
  gene segments
• Can attempt gene rearrangements on both
  chromosomes or by a second rearrangement on the
  same chromosome
• Tandem DJ and V segments
• 80 of T cells make successful rearrangements
• Successful rearrangement leads to expression of
  CD4 and CD8
• a chain
• rearranges second
• has several variable (V) and joining (J) gene
  segments
• Can undergo several gene rearrangements
• Both a chain loci can rearrange leading to the
  potential for 2 different a chains and 2
  different TCRs on a single cell

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Gene rearrangement . . . In pictures
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ß chain rearrangement
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a chain rearrangement
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?d T cells

• Express rearranged TCRs, generate immunologic
  memory and induce dendritic cell maturation
• Effector functions similar to ?? T cells making
  them part of the Adaptive immune system
• HOWEVER, they have limitied TCR gene usage, TCRs
  act as pattern recognition receptors, and respond
  fast
• Might be a conserved, primitive form of immunity
  that bridges INNATE with ADAPTIVE

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Proposed scheme to use ?? T cells for
immunotherapy
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T cell generation in mice
The first ?d T cells express receptors based on
the first V segment nearest the D segments
(epidermis- yellow). Later the majority of ?d T
cells express receptors with the other V segment
(reproductive tract - red). After birth the aß T
cells dominate.
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Positive and Negative Selection of T Cells
Where cortex, cortico-medullary junction
(thymus) Who double positive, ab T cells What
is being selected the TCR

• Purpose
• Positive Selection Select TCRs that recognize
  self MHCs
• -In periphery, T cells recognize foreign
  peptide that is presented by self MHCs
• -T cells must recognize (bind to) self MHCs to
  be activated by
• foreign peptide
• Negative Selection Eliminate TCRs that recognize
  self
• -T cells recognizing self MHC containing self
  peptides are
• potentially autoreactive

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Positive Selection (PS)

• TCR physically binds to MHC on APC
• Select T cells that can recognize self MHCs
• Primary thymic repertoire has bias toward
  general MHCs
• -select those TCR that recognize inherited self
  MHCs
• (6 MHC I and minimally 6 MHC II)
• Only 2 of a given thymic repertoire can
  recognize self
• MHCs PS process of stimulation those T
  cell to mature

POINT Thymic T cells undergoing positive
selection are pre-programmed to die unless they
receive a signal to live and mature (98 DIE)

• How are thymocytes (T cells in the thymus)
  positively selected?
• thymic cortical epithelial cells express MHC I
  and MHC II on their surface
• MHC molecules not stable in absence of peptide
  all contain self peptides
• ab chains of TCR test all MHC complexes for
  ones they recognize (bind to)
• -if bound with 3-4 days signal to live if
  not, DIE by default
• -120,000 self peptides presented by 12
  different MHC molecules most involved
• in positive selection

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Bone Marrow Transplant Share HLA Allotypes

• Bone marrow transplant destroy
• recipient bone marrow cells including
• hematopoietic stem cells
• Reconstitute all blood cells including
• lymphocytes with donor cells
• T cells selected on self (recipient)
• MHC (thymus)
• APCs developing in bone marrow are
• donor
• Recipient T cells can not recognize
• donor APCs, unless donor and

• recipient have some common (share) HLA (MHC)
  molecules
• Minimum share one common MCH I molecule and one
  common MHC II molecule

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Transition from Double Positive to Single Positive

• Choice of whether to become a CD4 T cell
• or CD8 T cell is determined during
• positive selection
• Single positive T cells express either CD4
• or CD8, but NOT both
• Developing T cell interacts with MHC I on
• thymic stromal epithelial cells CD8
• Developing T cell interacts with MHC II on
• thymic epithelial cells CD4
• Bare lymphocyte syndromes
• -patients lacking MHC I have only CD4
• cells
• -patients lacking MHC II have only CD8
• cells

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a Chain Gene Rearrangement During Positive
Selection

• a chains of the TCR can continue to rearrange to
  enhance chances of passing
• positive selection (reminiscent of light chain
  rearrangement BCR)

• Permits development of
• single T cell with two
• different a chains
• Probability of both chains
• passing positive selection
• is extremely remote
• Result only one functional
• TCR per T cell

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Negative Selection

• Select cells for elimination that express
• TCRs that strongly recognize self MHC
• presenting self peptides

Where cortico-medullary junction Who is
selecting macrophages and dendritic cells How
strong binding of the TCR sends signal to DIE
(apoptosis)
Two individuals of completely different
haplotypes have non-overlapping T cell
repertoires the T cell repertoire is highly
personalized.
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Mature, Naïve T Cells Meet Ag in Secondary
Lymphoid Tissue

• Only 1-2 of immature T cells survive selection
  in the thymus
• Surviving cells leave, enter the periphery and
  circulate through secondary
• lymphoid tissue
• Naïve T cells are long lived and can circulate
  for years in the absence of
• Ag (antigen)
• Meet Ag in the T cell area of the secondary
  lymphoid tissue

Ag stimulates naïve T cells
Effector T cells
CD8 Cytotoxic T cells
CD4 Helper T cells
CD4 Helper T cells
TH1
TH2
Thymus Leave
Leave Stay

• Twice as many CD4 T cells in the circulation
  than CD8 T cells

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Alloreactive T Cells

• 5-10 of T cell repertoire reacts strongly to
  allogenic
• cells (non-self MHC)
• Allograft rejection (kidney)
• T cell repertoire biased to recognize MHC
  molecules in
• general
• Elimination of those T cells that recognize self
  MHCs
• (negative selection) leaves an increased
  proportion of
• T cells recognizing non-self MHC (allogenic)

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12 Different MHC Isotypes is Optimal

• Double MHC isotype number double those passing
• positive selection
• Double number of isotype number geometrically
• increase those deleted by negative selection
• Too many MHC isotypes will significantly limit
  the T
• cell repertoire
• 12 isotypes appear optimal

Note Skip Fig. 5.15
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Development of T Cell Tumors
T cell tumors represent different stages of T
cell development (like B cell tumors)
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T Cell Development in the Thymus
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Stages of ab T Cell Development