Chapter 9 T-Cell Receptor

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Chapter 9 T-Cell Receptor
Interaction of ab TCR with class I MHC-peptide
Dec 5, 2006
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• ????????
• T ??????????? ??????
• ab TCR ? gd TCR??????????
• TCR ???????
• T ?????,??TCR ?????? coreceptors ?

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T Cells Recognize Ag Only When Presented on the
Membrane of a Cell by a Self-MHC Molecule
This work was published in 1974. Zinkernagel and
Doherty were awarded the Nobel prize In 1996.
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• Identification of T-cell Receptors
• Clonotypic mAb
• how?
• 2. Gene cloning by subtractive hybridization

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Identification and Cloning of the TCR Genes
Hedrick and Davis,
1984 Well-thought-out assumptions of TCR
Genes 1. mRNAs are associated with
membrane-bound polyribosomes rather than
with free cytoplasmic ribosomes. 2. mRNAs are
only expressed in T, but not in B cells.
subtractive hybridization (98 of the genes
expressed in
T and B cells are the same) 3.
DNA is rearranged in mature T cells.
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Production and Identification of a cDNA Clone
Encoding the TCR
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subtractive hybridization
2 of total cDNA 3 x 2 0.06
each of the 6 T-cell clones showed different
Southern blot patterns.
, and clones 3, 4, 5, 6, 7, 8, 9, 10
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The cDNA clone 1 identified by the
Southern- blot analysis shown in the previous
slide has all the hallmarks of a putative TCR
gene 1. It represents a gene sequence that
rearranges. 2. It is expressed as a
membrane-bound protein. 3. It is expressed only
in T cells. The cDNA clone 1 is the b chain
of the TCR. Later, cDNA clones were identified
encoding thea chain, the?chain, and finally the d
chain.
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ab and gd T-cell Receptors Structure and
Roles
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Structural Similarity between mIgM and TCR
Fab
or gd T-cell receptor

• resembling an
• Fab fragment ?

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TCRab TCRgd

The d-chain gene segments are located between
the Va and Ja segments.
a, g genes V, J, C gene segments b, d genes
V, D, J, C gene segments
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Comparison of the gd TCR and ab TCR
of CD3 1-10
90-99 T cells
elbow angle
? Contribute to differences in signaling
mechanism and in how the molecules
interact with coreceptors.
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(majority)
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• gd T cells
• In humans, the predominant receptor expressed
• on circulating gd cells recognizes a
  microbial
• phospholipid Ag, 3-formyl-1-butyl
  pyrophosphate,
• found on M. tuberculosis and other bacteria
  and
• parasites (similat to pattern recognition
  receptor?)
• This specificity for frequently encountered
  pathogens
• led to speculation that gd cells may function
  as an
• arm of the innate immune response, allowing
  rapid
• reactivity to certain Ags without the need
  for a
• processing step.

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• The specificity of circulating gd cells in the
• mouse and of other species studied does not
• parallel that of humans, suggesting that the
  gd
• response may be directed against pathogens
• commonly encountered by a given species.
• Since gd cells can secrete a spectrum of
• chemokines and cytokines, they may play a
• regulatory role in recruiting other cells to
  the
• site of invasion by pathogens.

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Ligands Recognized by gd T Cells - gd T
cells appear to bind directly to Ags
without requiring Ag processing and
presentation by MHC. - Some gd T cells may
uniquely respond to heat-shock proteins
and may have evolved to eliminate damaged
cells as well as microbial invaders.
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Organization and Rearrangement of TCR Genes
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Germ-line Organization of the Mouse TCR a-, b-,
g-, and d-chain Gene Segments
d
a productive rearrangement of a-chain
gene segments deletes Cd
between Va and Ja
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Gene Rearrangements That Yield a Functional Gene
Encoding the ab TCR
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• The C region of TCR is much simpler than
• the C region of Ig genes
• TCR is expressed only in a membrane-bound
• form thus, no differential RNA processing is
• required to produce membrane and secreted
  form.
• TCRa has only a single C gene segment and
• TCRb has two C gene segments.
• No known functional differences exist in C
• regions.

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• Although B cells and T cells use very similar
• mechanisms for V-region gene rearrangements,
• the Ig genes are not rearranged in T cells
  and the
• TCR genes are not rearranged in B cells.
• The recombinase enzyme system is differently
• regulated in B and T cell lineage, so that
  only
• rearrangement of the correct receptor DNA
  occurs.
• Chromatin is also uniquely re-configured in B
• cells and T cells to allow the recombinase
  access
• to Ig and TCR genes, respectively.

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Domains and CDRs of ab-TCR
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Comparison of Mechanisms for Generating
Diversity in TCR Genes and Ig Genes
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The Location of One-turn (12-bp) and Two-turn
(23-bp) Recognition Signal Sequences (RSS) in
TCR b- and d-chain DNA Differs from That in Ig
H-chain DNA

or V-D-D-D-J in humans ? generate considerable
additional
diversity in TCR genes.
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N-addition occurs in all the TCR genes.

• Although each junctional region in a TCR gene
  encodes only 10 to 20 a.a.,
• enormous diversity can be generated in these
  regions.
• The combined effects of P- and N- addition plus
  joining flexibility are
• estimated to be 1013 possible a.a. sequences
  in the TCR CDR3 region.

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abTCR 3.0 x 103 x 4.6 x 102 1.4 x 106
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T-cell Receptor Complex TCR-CD3
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T-Cell Receptor Complex TCR-CD3
CD3 ge de zz (90) or zh
(10)
or ??
immunoreceptor tyrosine-based activation motif
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T-Cell Accessory Membrane Molecules
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Accessory Molecules Which Strengthen the
Interaction between T Cells and APC
(T cell) (APC)
(costimulatory)
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CD4 and CD8 Coreceptors Bind to Conserved Regions
of MHC Class II or I Molecules
sometimes aa homodimer
55-kDa
30-38 kDa each chain
CD4
CD8
class I
class II
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Interaction of CD8 Coreceptor with TCR and Class
I MHC Molecule
a1
a2
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Interaction of CD4 Coreceptor with TCR and Class
II MHC Molecule
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Dissociation Constants (Kd) for Various
Biological Systems
(10-4 to 10-7)
(10-6 to 10-10)
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Interactions between TCR/Peptide-MHC and
Accessory Molecules/Ligands
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Two-point Contact - extracellular portion of CD4
MHC
intracellular CD4 p56lck - z
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Three-dimensional Structures of TCR-peptide-MHC
Complexes
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Interaction between TCR and HLA-A2 with Bound
HTLV-I Tax Peptide
HTLV-1 tax peptide
HLA-A2
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MHC Molecule Viewed from Above
HV loops of TCR-Vb
Peptide
HV loops of TCR-Va
HLA-A2
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Ternary Complex of TCR Bound to H-2Kb and Peptide
TCR CDR1 CDR2 of Vb CDR3 of
Vb peptide H-2Kb
CDR1 CDR2 of Va CDR3 of Va
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Comparison of the Interactions between ab TCR
and MHC-peptides

1. The angles at which the TCR molecule sits on the
   class I and class II MHC-peptide are different.
2. More number of contact residues between TCR and
   class II

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Alloreactivity of T Cells - a puzzling finding
T cell
recognition 1. self-MHC foreign peptides
2. allo-MHC foreign peptides

self-MHC restriction
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Alloreactivity of T Cells - a puzzling finding
T cell
recognition 1. self-MHC foreign peptides
2. allo-MHC foreign peptides
however, 3.
allo-MHC allo-peptides
why? one explanation cross-reactivity
of 1 and 3
self-MHC restriction
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Models for Alloreactivity of T Cells