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How T cells recognize antigen:The T Cell Receptor (TCR)

Lecture 11, MCB 150Laurent Coscoy

Identifying the TCR:Why was it so hard to do?

•By the early 1980s, much about T cell functionwas known, but the receptor genes had not beenidentified

•Recall that Ig was purified as soluble myelomaprotein

•There is no soluble form of T Cell Receptor

Monoclonal antibody approach

(Allison and colleagues 1982)

Probed with cDNA clone 1

Probed with cDNA clone 2

T cell clones

Live

r cel

lsB

cell

lym

phom

a

Hedrick & Davis strategy: Subtractive cDNA cloning

TCR: Anαβ

heterodimer

A second type of TCR

• Hetero dimer of γ and δ chains (like α and βchains)

• Expressed on a rare subset of T cells withfascinating properties (more later)

αβ T cells γδ T cells

CD4 T cells CD8 T cells

2

Comparison of TCR and BCR (Ig) T cell receptor lociAnalogous to B cell receptor loci

Generation of the T cell receptor TCR structure

TCR:Peptide/MHC I

Interaction

Constant domains

Variable domains

CDR3’s

Peptide

Binding groove

C C

V

V

Class I Class II

Peptide Binding Grooves of MHC Molecules

3

Soluble peptide/MHC complexes: aprobe to detect specific T cells

Ig HC Fc Region

MHC I

Made by geneticengineering; doesnot exist in nature

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

FollowingAntigen-Specific T

Cells

CD8

CD4 and CD8:Co-receptors for

the TCR

What’s with these “CD” names?

• “CD” stands for cluster of differentiation

• Many labs generated monoclonal antibodies against cell-surface proteins --> naming of antibodies and their targetswas a mess

• Conference was called to “clean up” the naming business

• Target of monoclonal ab directed against the same surfaceprotein or complex was given a number, as in CD3, CD4,etc.

Co-receptors bind to non-polymorphicregions of MHC I (CD8) or II (CD4)

4

CD4 and CD8 mark mutuallyexclusive subsets of mature T cells

Anti-CD8

Ant

i-CD

4

Anti-CD3

Ant

i-B22

0

Flow cytometry of spleen

Anti- TCR

Ant

i-CD

4 &

Ant

i-CD

8

Contrast how B and T cells see antigen

B Cell

T Cell APC

TCR affinity is quite low

Mysteries in T cell antigen recognition

• About 105 copies of each MHC molecule oncell surface

• Frequency of any particular Peptide:MHCcomplex may be as low as 0.01%

• How does the TCR sort through all thoseincorrect complexes and find ones withwhich to trigger? �

Co-receptor and adhesionmolecules improve avidity

CD2 LFA-3

LFA-1 ICAM-1

TCR

CD4 or CD8

MHCT cell target cell

5

Immunological Synapse: A model

T cell T cellT cell

APC APC APC

Red: adhesionGreen: Antigen

Immunological “synapse”

Peptide-MHC complexes

Adhesion molecules (ICAM-1)

The synapse in real-time

T-Cell

B-Cell(Peptide-pulsed )

8 minutes in 20 seconds of video

The synapse in real-time

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

Consequences of T cell activation

How is TCR:peptide:MHC binding linked to these events?

ProliferationCell cycle entry and cell divisionClonal expansion

DifferentiationSecretion of cytokines (helper cells)Activation of killer functions (cytotoxic cells)Acquisition of effector functionMemory

DeathImportant for down-regulation of immune response

TCR signaling induces changes in gene expression

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Example of T cell activation

TCR binds peptide:MHC

Induces expression of genes

T cells divides many times creatingmore T cells with the same TCR

T cells differentiate andacquire new characteristics

Signal Transduction

Transmission of external signals across themembrane and conversion of these signals intointracellular biochemical events

?

TCR

Peptide:MHC

Changes in gene expression

RECEPTOR

?

? ?

?? ?

Alterations inGene expression

TCR signaling: Big Picture

???

How does the TCR transduce signals with such a little tail?

TCR associates with the CD3 Complex

Analogous to the Igαand Igβ proteins thatassociate with BCR onB cells