Transcript gd T cells

Chapter 9
T-Cell Receptor
Interaction of ab TCR with class I MHC-peptide
Dec 5, 2006
你需要學習的課題：
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T 細胞受體是如何發現的? 有什麼特性?
ab TCR 與 gd TCR在辨認上有什麼不同?
TCR 的基因及分子。
T 細胞辨識時，除了TCR 之外還有哪些
coreceptors ?
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.
Identification of T-cell Receptors:
1. Clonotypic mAb
how?
2. Gene cloning by subtractive hybridization
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.
Production and
Identification
of a cDNA Clone
Encoding the TCR
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
The cDNA clone 1 identified by the Southernblot 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 theα
chain, theγchain, and finally the δ chain.
ab and gd T-cell Receptors:
Structure and Roles
Structural Similarity between mIgM and TCR
Fab
or gd T-cell receptor
 resembling an
Fab fragment ?
TCRab
*
TCRgd
* The δ-chain gene segments are located between the Vα and
Jα segments.
a, g genes : V, J, C gene segments
b, d genes : V, D, J, C gene segments
Comparison of the gd TCR and ab TCR
% of CD3+
T cells
1-10 %
90-99 %
elbow
angle
→ Contribute to differences
in signaling mechanism
and in how the molecules
interact with coreceptors.
(majority)
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.
- 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.
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.
Organization and Rearrangement
of TCR Genes
Germ-line Organization of the Mouse
TCR a-, b-, g-, and d-chain Gene Segments
d
between Va and Ja
: a productive
rearrangement
of a-chain
gene segments
deletes Cd
Gene Rearrangements That Yield a
Functional Gene Encoding the ab TCR
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.
- 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.
Domains and CDRs of ab-TCR
Comparison of Mechanisms for Generating
Diversity in TCR Genes and Ig Genes
The Location of One-turn (12-bp) and Two-turn (23-bp)
Recognition Signal Sequences (RSS) in TCR b- and dchain DNA Differs from That in Ig H-chain DNA
or V-D-D-D-J in humans  generate considerable additional
diversity in TCR genes.
N-addition
occurs in
all the
TCR genes.
1+41+42+43+44+45+46
=5461
- 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.
abTCR:
3.0 x 103 x
4.6 x 102
= 1.4 x 106
T-cell Receptor Complex:
TCR-CD3
T-Cell Receptor Complex: TCR-CD3
CD3 :
ge + de +  (90%)
or 
or  (10%)
immunoreceptor
tyrosine-based
activation
motif
T-Cell Accessory Membrane Molecules
Accessory Molecules Which Strengthen the
Interaction between T Cells and APC
(T cell)
(APC)
(costimulatory)
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
class I
CD8
CD4
class II
Interaction of CD8 Coreceptor with
TCR and Class I MHC Molecule
a1
a2
Interaction of CD4 Coreceptor with
TCR and Class II MHC Molecule
Dissociation Constants (Kd) for
Various Biological Systems
(10-4 to 10-7)
(10-6 to 10-10)
Interactions between TCR/Peptide-MHC
and Accessory Molecules/Ligands
Two-point Contact - extracellular portion of CD4 : MHC
intracellular CD4 : p56lck - 
Three-dimensional Structures of
TCR-peptide-MHC Complexes
Interaction between TCR and HLA-A2
with Bound HTLV-I Tax Peptide
HTLV-1 tax
peptide
HLA-A2
MHC Molecule Viewed from Above
HV loops
of TCR-Vb
Peptide
HV loops
of TCR-Va
HLA-A2
Ternary Complex of TCR Bound
to H-2Kb and Peptide
TCR
CDR1 & CDR2 of Va
CDR3 of Va
CDR1 & CDR2 of Vb
CDR3 of Vb
peptide
H-2Kb
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
Alloreactivity of T Cells
- a puzzling finding
T cell recognition
1. self-MHC + foreign peptides
+
2. allo-MHC + foreign peptides
–
self-MHC
restriction
Alloreactivity of T Cells
- a puzzling finding
T cell recognition
1. self-MHC + foreign peptides
+
2. allo-MHC + foreign peptides
–
self-MHC
restriction
however,
3. allo-MHC ± allo-peptides
+++
one explanation : cross-reactivity of 1 and 3
why?
Models for Alloreactivity of T Cells