TCR and T lymphocyte development in thymus

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• TCR and T lymphocyte development in thymus

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Students should know

• Structure of TCRs
• CDRs
• Differences of recognized antigen between TCR and
  immunoglobulins (Ig)
• TCR germ line configuration and rearrangements
• TCR specificity
• Clonality of T cells
• Differences between TCR a and b chains
• T cell receptor gene rearrangement and lineage
  commitment
• preTCR
• Positive selection
• Negative selection
• Changes in CD4/CD8 expression during T cell
  maturation in thymus.
• The order and location of T cell selection
• Cell types involved in T cell selection
• Why is it important to match MHC molecules
  between donors and recipients for bone marrow
  transplantation (for donor-derived T cells to be
  functional in recipients)?

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Figure 3-4
If you dont make antigen receptors (as in SCID
patients),
Oral Thrush
you can not make T and B cells and are
susceptible even to opportunistic pathogens (e.g.
C. albicans)
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• T lymphocytes
• T cell origin (bone marrow) and events associated
  with maturation in the thymus (cytokinesIL1, 2,
  3, 6, 7, GM-CSF thymic hormones)
• (1) Stem cells (multipotent) migrate to thymus
  and move from cortex to medulla while interacting
  with MHC Class II-bearing nurse, epithelial and
  interdigitating cells.
• (2) Maturation progression
• (a) Early thymocyte (CD4-8-, T cell receptor
  (TCR) gene rearrangements)
• (b) Common thymocyte (CD48, T cell receptor
  gene rearrangements low TCR and CD3 surface
  expression)
• (c) Mature thymocyte (CD4 or CD8 subsets high
  TCR and CD3 surface expression somatic
  recombination of TCR genes)
• (d) Positive and negative selection occurs and
  most self-reactive T cells eliminated.
• (e) All T cells positive for TCR, CD2, 3, and 28

CORE
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T cell antigen receptor (genetics, structure,
accessory proteins and signal transduction)(1) A
ntigen-specific TCR dimers ab (90-95 of all T
cells) or gd V, D, J, and C (constant) genes
for b and g chains V, J, and C genes for a and
d chains.(2) CD3 complex e2z2gd associated with
TCR has a signal transduction role.(3) TCR/CD3
overall stoichemistry (ab)e2z2gd
CORE
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Similarity between TCR and Ig

• Both
• Bind antigen
• Have variable region
• Constant region
• Each binding site is a heterodimer (composed of 2
  different chains)
• TCRs act only as receptors
• Igs act as receptors and effector molecules
  (soluble antigen-binding molecules)

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Figure 3-6
TCR complex
CD3 chains transmit signals
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Figure 3-7
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Origin, generation and differentiation of T cells

• T cell progenitors migrate from bone marrow and
  seed thymus. T cell progenitors undergo
  differentiation to CD4, CD8 and NKT cells in
  thymus. Mature CD4 and CD8 T cells circulate
  between blood and lymphoid tissues until they
  meet antigens presented on dendritic cells in
  lymphoid tissues. T cells further undergo
  maturation to become functional memory or
  effector T cells in LT

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Figure 5-2
Thymic involution Human thymus is fully
developed before birth and increases in size
until puberty. It then progressively shrinks
during adult life. Most thymectized adults have
no problem in T cell immunity because they have
enough memory T cells in the periphery, and these
T cells are long-lived.
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Figure 5-3 part 1 of 2
DN (CD4-CD8-) and DP (CD4CD8) Immature
thymocytes are here
Differentiation
More mature SP (CD4CD8-or CD8CD4-) thymocytes
are here
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Figure 5-3 part 2 of 2
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TCR genes undergo DNA rearrangement in thymus
No Ds in Va gene DJ first then VDJ in b gene
rearrangement
Un-rearranged
rearranged
expression
rearranged
Un-rearranged
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TCR gene rearrangement generates the TCR
repertoire
Pre-TCR complex stops further gene rearrangement
at b locus, and induces thymocyte
proliferation Finally TCR DP cells are made
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Two chances for productive (correct reading
frame) rearrangement b chain
Successful rearrangement at one b copy blocks
that at the other chromosome.
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Multiple chances for productive (correct reading
frame) rearrangement in a chain
Successful rearrangement at one a copy does not
block that at the other. Therefore, many T cells
express two different a chains.
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Lineage commitment to ab or gd T cells

• Successful gene rearrangement in g and d before b
  ? gd T
• Successful gene rearrangement in b before g or d
  ? pTab T (not committed yet). This signals to
  halt rearrangement of the b, g and d-chain genes
  and to enter a phase of proliferation.
• Further rearrangement in a, g and d. Lineage
  commitment now depends on whether a functional
  ab or gd T-cell receptor is made first.
• More ab T cells are made than gd T cells

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Figure 3-8 part 2 of 2
Most gd T cells do not express CD4 or CD8. They
are thought to be First line of defense? Bridge
between innate and adaptive responses?
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Figure 3-8 part 2 of 2

• gd T cells recognize a limited set of unusual
  antigens
• Small microbial compound (E)-4-hydroxy-3-methyl-bu
  t-2-enyl pyrophosphate (HMB-PP, an essential
  metabolite in most pathogenic bacteria including
  Mycobacterium tuberculosis and malaria parasites,
  but is absent from the human host).
• Host MHC class 1b T10/22, MICA, MICB (structure
  similar to MHC I expressed by transformed or
  stressed host cells)
• Nonprotein alkylamines (derived from microbes and
  plants)
• Bacterial products Mycobacterial HSP (heat shock
  protein), superantigens (SEA)
• Host heat shock proteins
• Do not need antigen processing and presentation
  on MHC molecules.

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CD8 binds MHC class ICD4 binds MHC class II
Most mature T cells are either CD4 or CD8. CD8
T cells kill cells infected with intracellular
pathogens or tumor cells while CD4 T cells
regulate (activate or suppress) other immune
cells function (e.g.B cells and mac).
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Figure 3-10
The structures of CD4 and CD8
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CD4CD8 DP cells To be CD4 or CD8?
Interaction of DP cells with AgMHC I ? CD8 T
cells Interaction of DP cells with AgMHC II ?
CD4 T cells Thus, the antigen-specificity of
TCRs determines the fate.
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To survive in thymus, T cells need to bind self
MHC (but not too strongly).
Positive selection (DP stage) Negative
selection (SP stage)
Self MHCs shape the TCR repertoire. Individuals
with different MHCs will have different TCR
repertoire. Most DP thymocytes dont survive to
become SP cells.
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Positive selection selects T cells that recognize
peptides on self MHCThis is to assure that
mature T cells can respond to antigen-presented
on self MHC.-Self MHC I and II harboring self
peptides on thymic epithelial cells recognize and
activate TCRs on some DP thymocytes.-DP
thymocytes should receive this signal within 3-4
days to survive.Otherwise they undergo
apoptosis.
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Negative selection eliminates T cells with TCRs
that bind too strongly to self antigen/MHC
complex.This is to assure that T cells dont
react against self antigens. In other words,
autoreactive cells are removed by this
process.Dendritic cells and macrophages in
cortico-medullary junction mediate it. Negative
selection cannot eliminate T cells whose
receptors are specific for self peptides that are
expressed outside of thymus (These cells enter
circulation, but soon to be rendered anergic or
unresponsive by other mechanims).
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Step 1 Selected people for the CBS
show(selected useful T cells by epithelial
cells)
Is this a positive or negative selection?
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Step 2 Selected persons are eliminated (eliminat
ed harmful T cells by thymic dendritic cells)
Is this a positive or negative selection?
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The number of MHC molecules and size of T cell
repertoire
deletion rate by negative selection
As the number (N) of MHC molecules increases, the
proportion of T cells that are positively
selected ( of the cells that survive) goes up
arithmetically (N times), while that of
negatively selected ( of deleted cells) goes
up geometrically (N2 times). N number of MHC
isotypes a person expresses Therefore, the magic
N to result in maximum T cell repertoire is
around 13.
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Figure 5-10
Bone marrow transplantation therapy in leukemia
patients
What happens if there is a complete mismatch in
MHC I/II TYPE? See the next slide.
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Figure 5-11
What you learned at school should work in the
real world !
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What happens if you do not have the
thymus? DiGeorges syndrome No or few T cells
due to very small or no thymus Symptoms similar
to SCID patients Thymic involution Thymus
degenerates with age. If you are older than 60,
your thymus is too small to produce T cells.
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Generation of naïve T cells in thymus
T cell progenitors
TCR gene rearrangement
TCRab
100
TCRgd
not selected by MHC I/II
Selections for T cells that are MHC-restricted
and not self reactive
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Blood
CD4 or CD8 TCRab T cells
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Generation of T cell clones clonality
A
G TCR in germ line configuration A, B, C
rearranged TCRs with different specificities
A
A
A
Ag For TCR A
A
G
G
B
B
C
C
C
Ag For TCR B
C
Secondary Lymphoid tissues. Ag-dependent
expansion of clones.
Thymus Selection for The T cells with good TCR
Thymus TCR recombination
C
Stem cells
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Figure 5-18
Summary of T cell development in thymus
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Figure 5-19
Summary of T cell development in thymus
Genes