How T cells recognize antigen:
The T Cell Receptor (TCR)
Lecture 11, MCB 150
Laurent Coscoy
1
Identifying the TCR:
Why was it so hard to do?
•By the early 1980s, much about T cell function
was known, but the receptor genes had not been
identified
•Recall that Ig was purified as soluble myeloma
protein
•There is no soluble form of T Cell Receptor
2
Monoclonal antibody approach
(Allison and colleagues 1982)
3
Liver cells
B cell lymphoma
Hedrick & Davis strategy: Subtractive cDNA cloning
T cell clones
Probed with cDNA clone 1
Probed with cDNA clone 2
4
TCR: An
ab
heterodimer
5
A second type of TCR
• Hetero dimer of g and d chains (like a and b
chains)
• Expressed on a rare subset of T cells with
fascinating properties (more later)
ab T cells
CD4 T cells
CD8 T cells
gd T cells
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Comparison of TCR and BCR (Ig)
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T cell receptor loci
Analogous to B cell receptor loci
8
Generation of the T cell receptor
9
TCR structure
10
TCR:
Peptide/MHC I
Interaction
Binding groove
Peptide
Va
CDR3’s
Vb
Variable domains
Cb
Ca
Constant domains
11
Peptide Binding Grooves of MHC Molecules
Class I
Class II
12
Soluble peptide/MHC complexes: a
probe to detect specific T cells
MHC I
Made by genetic
engineering; does
not exist in nature
Ig HC Fc Region
13
HTLV-I and tropical spastic paraparesis
HTLV-I:
- human retrovirus, persistent infection
- 20 millions people are infected worldwide
- encodes the oncogene Tax
TSP:
- progressive inflamatory disease of the CNS
- pathogenesis not fully understood
14
Following
Antigen-Specific T
Cells
CD8
15
CD4 and CD8:
Co-receptors for
the TCR
16
What’s with these “CD” names?
• “CD” stands for cluster of differentiation
• Many labs generated monoclonal antibodies against cellsurface proteins --> naming of antibodies and their targets
was a mess
• Conference was called to “clean up” the naming business
• Target of monoclonal ab directed against the same surface
protein or complex was given a number, as in CD3, CD4,
etc.
17
Co-receptors bind to non-polymorphic
regions of MHC I (CD8) or II (CD4)
18
19
CD4 and CD8 mark mutually
exclusive subsets of mature T cells
Anti-CD4 & Anti-CD8
Anti-CD4
Anti-B220
Flow cytometry of spleen
Anti-CD3
Anti-CD8
Anti-gd TCR
20
Contrast how B and T cells see antigen
B Cell
T Cell
APC
21
TCR affinity is quite low
22
Mysteries in T cell antigen recognition
• About 105 copies of each MHC molecule on
cell surface
• Frequency of any particular Peptide:MHC
complex may be as low as 0.01%
• How does the TCR sort through all those
incorrect complexes and find ones with
which to trigger?
23
Co-receptor and adhesion
molecules improve avidity
CD2
T cell
TCR
LFA-3
MHC
target cell
CD4 or CD8
LFA-1
ICAM-1
24
Immunological Synapse:
A model
Red: adhesion
Green: Antigen
T cell
T cell
APC
APC
T cell
APC
25
Immunological “synapse”
Peptide-MHC complexes
Adhesion molecules (ICAM-1)
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The synapse in real-time
B-Cell
(Peptide-pulsed
)
QuickTime™ and a Vid eo d eco mp re ss or a re nee de d to s ee th is picture.
T-Cell
8 minutes in 20 seconds of video
27
The synapse in real-time
QuickTime™ and a Vid eo d eco mp re ss or a re nee de d to s ee th is picture.
28
TCR Signal Transduction
Why do we care?
It helps us understand T cell function
It gives us molecular targets for immunotherapy
It is an amazing example of biological complexity
29
Consequences of T cell activation
Proliferation
Cell cycle entry and cell division
Clonal expansion
Differentiation
Secretion of cytokines (helper cells)
Activation of killer functions (cytotoxic cells)
Acquisition of effector function
Memory
Death
Important for down-regulation of immune response
How is TCR:peptide:MHC binding linked to these events?
TCR signaling induces changes in gene expression
30
Example of T cell activation
TCR binds peptide:MHC
Induces expression of genes
T cells divides many times creating
more T cells with the same TCR
T cells differentiate and
acquire new characteristics
31
Signal Transduction
Transmission of external signals across the
membrane and conversion of these signals into
intracellular biochemical events
Peptide:MHC
TCR
?
Changes in gene expression
32
TCR signaling: Big Picture
RECEPTOR
?
?
?
?
?
?
?
?
?
Alterations in
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Gene expression
How does the TCR transduce signals with such a little tail?
34
TCR associates with the CD3 Complex
Analogous to the Iga
and Igb proteins that
associate with BCR on
B cells
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