Chapter 9 T-Cell Receptor

Dec 5, 2006

Interaction of TCR with class I MHC-peptide

你需要學習的課題：

1. T 細胞受體是如何發現的 ? 有什麼特性 ?

2. TCR 與 TCR 在辨認上有什麼不同 ?

3. TCR 的基因及分子。4. T 細胞辨識時，除了 TCR 之外還有哪些 corec

eptors ?

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 Dohertywere 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 6T-cell clones showed different Southern blot patterns.

, and clones 3, 4, 5, 6, 7, 8, 9, 10

The cDNA clone 1 identified by the Southern-blot analysis shown in the previous slide has allthe 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 chain of the TCR. Later, cDNA clones were identified encoding theα chain, theγchain, and finally the δ chain.

and T-cell Receptors: Structure and Roles

Structural Similarity between mIgM and TCR

resembling an Fab fragment ?

Fab

or T-cell receptor

* The δ-chain gene segments are located between the Vα and Jα segments.

*

, genes : V, J, C gene segments

, genes : V, D, J, C gene segments

TCR

TCR

Comparison of the TCR and TCR

elbow angle

→ Contribute to differences in signaling mechanism and in how the molecules interact with coreceptors.

% of CD3+ 1-10 % 90-99 % T cells

(majority)

T cells

- In humans, the predominant receptor expressed on circulating 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 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 cells in the mouse and of other species studied does not parallel that of humans, suggesting that the response may be directed against pathogens commonly encountered by a given species.

- Since 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 T Cells: - T cells appear to bind directly to Ags without requiring Ag processing and presentation by MHC.

- Some 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 -, -, -, and -chain Gene Segments

between V and J : a productive

rearrangement of -chain gene segments deletes C

Gene Rearrangements That Yield aFunctional Gene Encoding the TC

R

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.

- TCR has only a single C gene segment and TCRhas 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 -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 - and -ch

ain DNA Differs from That in Ig H-chain DNA

or V-D-D-D-J in humans generate considerable additional diversity in TCR genes.

1+41+42+43+44+45+46

=5461N-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.

TCR:

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

or

immunoreceptortyrosine-based activation motif

CD3 :

or

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

55-kDa30-38 kDa each chain

CD8 CD4class I

class II

sometimes homodimer

Interaction of CD8 Coreceptor with TCR and Class I MHC Molecule

1

2

Interaction of CD4 Coreceptor with TCR and Class II MHC Molecule

Dissociation Constants (Kd) for Various Biological Systems

(10-6 to 10-10)

(10-4 to 10-7)

Interactions between TCR/Peptide-MHCand 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-V

HV loops of TCR-V

Peptide

HLA-A2

TCR

CDR1 & CDR2 of V

CDR3 of V

peptide

H-2Kb

CDR1 & CDR2 of V

CDR3 of V

Ternary Complex of TCR Bound to H-2Kb and Peptide

Comparison of the Interactions between 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 –

however,

3. allo-MHC ± allo-peptides +++ why?

one explanation : cross-reactivity of 1 and 3

self-MHC restriction

Models for Alloreactivity of T Cells