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Topic 6
The T Cell AntigenReceptor Complex
©Dr. Colin R.A. Hewitt
crah1@le.ac.uk
• Each clone of T cells expresses a single TcR specificity
• How the TcR was discovered
• The similarities and differences between TcR and antibodies
• The structure and organisation of the TcR genes
• Somatic recombination in TcR genes
• Generation of diversity in TcR
• Structure function relationship of TcR
• Why TcR do not undergo somatic mutation
What you should know by the end of this lecture
Discovery of the T cell antigen receptor (TcR)
Polyclonal T cellsfrom an immunised
strain A mouse
Monoclonal (cloned) T cells
Grow and clone a single antigen-specific T cell in-vitro with antigen, IL-2 and antigen presenting cells
In vitro “clonal selection” means each daughter cell has the same antigen specificity as the parent cell
Most molecules present on the monoclonal T cells will be identical to the polyclonal T cells EXCEPT for the antigen
combining site of the T cell antigen receptor
Making anti- clonotypic TcR antibodies
T cell clonefrom a strain A mouse Naïve strain A mouse
The strain A mouse will not make antibodies to the hundreds of different molecules associated with strain A T cells due to self tolerance
BUTThe naïve mouse has never raised T cells with the specificity of the T cell clone,
SOthe only antigen in the immunisation that the A strain mouse has never seen will be
the antigen receptor of the monoclonal T cells
Make monoclonal antibodies by hybridisation of the spleen cellswith a myeloma cell line
Anti-TcR Abs that recognise only one clone of T cells are CLONOTYPICHypothesise that anti-clonotype Abs recognise the antigen receptor
Screen the supernatant of each cloned hybridoma against a panel of T cell clones of different specificity
(i.e.cells with subtly different antigen-binding structures)
Making anti- clonotypic TcR antibodies
YY Y
Y
YYYYYYYYMonoclonal antibodies
T cell clones
Clone used for immunisation
Lyse cells and add anti-clonotype Abthat binds to unique T cell structures
YYYYCapture anti-clonotype Ab-Agcomplex on insoluble supportIMMUNOPRECIPITATIONWash away unbound protein
Elute Ag from Ab and analyse the clonotypically-expresssed proteins biochemically
Principal component was a heterodimeric 90kDa proteincomposed of a 40kDa and a 50kDa molecule ( and chains)
Several other molecules were co-immunoprecipitated.
Discovery of the T cell antigen receptor (TcR)
Y YYYY YYYY
YYYY
Structure of the TcR polypeptides
Cyanogen bromide digestion of the and proteins
Biochemical analysis of digestion products
T cell clone A T cell clone B T cell clone C
Polypeptides contain a variable, clone-dependent pattern of digestion fragments and a fragment common to all TcR
Intact TcR chain polypeptides
C C CV VV
Cloning of the TcR genes
BT
The experimental strategy
•The majority of genes expressed by T and B lymphocytes will be similar
•Genes that greatly differ in their expression are most likely to be directly related to the specialised function of each cell
•Subtract the genes expressed by B cells from the genes expressed by T cells leaving only the genes directly related to T cell function
AAAAA
AAAAA
Isolate non-hybridising material specific to T cells
T cell singlestranded cDNA
Cloning of TcR genes by subtractive hybridisation
BTAAAAAAAAAA
mRNA
Discard hybridsAAAAA
Digest unhybridised B cell mRNA
Clone andsequence T cell-specific genes
Hybridise the cDNA and mRNA shared
between T and B cellsAAAAA
Analysis of T cell-specific genes
GERMLINEDNAV D J C
32P
V DJ CREARRANGEDDNA
Restrictionenzyme sites
Of the T cell-specific genes cloned, which cDNA encoded the TcR?
Assumptions made after the analysis of Ig genes:
TcR genes rearrange from germline configuration
Find two restriction sites that flank the TcR region
Ig gene probes can be used as TcR genes will be homologous to Ig genes
Cut the T cell cDNA and placental (i.e. germline) DNAand Southern blot the fragments
32P
Placenta B T
Size ofdigestedgenomic
DNA
Gel electrophoresis followed by Southern blot using a TcR probe
The TcR genes rearrange, but are not immunoglobulin genes
Rearrangedallele
The T cell antigen receptor
V V
C CCarbohydrates
Hinge
Monovalent
Resembles an Ig Fab fragment
Fab
VHVL
Fc
CL
CH
VLVH
CH CL
CH CH
CHCH
No alternative constant regions
Transmembrane region
Never secreted
Domain structure: Ig gene superfamily
Heterodimeric, chains are disuphide-bonded
Cytoplasmic tail
Very short intracytoplasmic tail++
+Positively charged amino acids in the
TM region
Antigencombining site
Antigen combining site made of juxtaposed V and V regions
30,000 identical specificity TcR per cell
CH CL VH VLof Ig C C V Vof the TcR
View structures
• Unlike MHC molecules TcR are highly variable in the individual
• Diversity focused on small changes in the charge & shape presented at the end of the T cell receptor.
• TcR diversity to the peptide antigens that bind to MHC molecules
• Mechanisms of diversity closely related to T cell development
• Random aspects of TcR construction ensures maximum diversity
• Mechanisms of diversity generation similar to immunoglobulin genes
T cell antigen receptor diversity
Generation of diversity in the TcR
COMBINATORIAL DIVERSITYMultiple germline segments
In the human TcR
Variable (V) segments: ~70, 52Diversity (D) segments: 0, 2Joining (J) segments: 61, 13
The need to pair and chains to form a binding sitedoubles the potential for diversity
JUNCTIONAL DIVERSITYAddition of non-template encoded (N) and palindromic (P) nucleotides at
imprecise joints made between V-D-J elements
SOMATIC MUTATION IS NOT USED TO GENERATE DIVERSITY IN TcR
TcR
Organisation of TcR genes
L & Vx70-80 C
TcR
D1 J1 x 6 C1 D2 J2 x 7 C2
TcR genes segmented into V, (D), J & C elements(VARIABLE, DIVERSITY, JOINING & CONSTANT)
Closely resemble Ig genes (~IgL and ~IgH)
This example shows the mouse TcR locus
J x 61
L & Vx52
TcR gene rearrangement bySOMATIC RECOMBINATION
Spliced TcR mRNA
Germline TcR
Vn J CV2 V1
Rearranged TcR1° transcript
Rearrangement very similar to the IgL chains
TcR gene rearrangement RESCUE PATHWAY
There is only a 1:3 chance of the join between the V and J region being in frame
Vn J CV2 V1Vn+1
chain tries for a second time to make a productive join using new V and J elements
Productivelyrearranged TcR
1° transcript
Rearranged TcR 1° transcript
Spliced TcR mRNA
L & Vx52 D1 J C1 D2 J C2
Germline TcR
TcR gene rearrangementSOMATIC RECOMBINATION
D-J Joining
V-DJ joining
C-VDJ joining
D1 J C1 D2 J C2
Germline TcR
D-J Joining
V-DJ joining
V
TcR gene rearrangement RESCUE PATHWAYThere is a 1:3 chance of productive D-J rearrangement and a 1:3 chance of
productive D-J rearrangement(i.e only a 1:9 chance of a productive chain rearrangement)
2nd chance atV-DJ joining
Need to remove non productiverearrangement
Use (DJC)2 elements
V, D, J flanking sequences
V 7 23 9
Sequencing upstream and downstream of V, D and J elements revealed conserved sequences of 7, 23, 9 and 12 nucleotides.
J7129
D7129 7 12 9
V 7 23 9 J7239
Recombination signal sequences (RSS)
12-23 RULE – A gene segment flanked by a 23mer RSS can only be linked to a segment flanked by a 12mer RSS
V 7 23 9
D7129 7 12 9
J7239
HEPTAMER - Always contiguous with coding sequence
NONAMER - Separated fromthe heptamer by a 12 or 23
nucleotide spacer
V 7 23 9
D7129 7 12 9
J7239
√ √
23-mer = two turns 12-mer = one turn
Molecular explanation of the 12-23 rule
Intervening DNAof any length23
V 97
12
DJ79
23-mer
12-mer
Loop of intervening
DNA is excised
• Heptamers and nonamers
align back-to-back
• The shape generated by the
RSS’s acts as a target for
recombinases
7
9
97
V1 V2 V3 V4
V8V7
V6V5
V9 D J
V1 D J
V2
V3
V4
V8
V7
V6
V5
V9
• An appropriate shape can not be formed if two 23-mer flanked elements
attempted to join (i.e. the 12-23 rule)
Molecular explanation of the 12-23 rule
V D J712
9
723
9
7 12 97239
V D J
Imprecise and random events that occur when the DNA breaks and rejoins allows new nucleotides to be inserted or lost from the sequence at
and around the coding joint.
Junctional diversity
Mini-circle of DNA is permanently lost from the
genome
Signal jointCoding joint
V1 V2 V3 V4 V9 D J
Looping out works if all V genes are in the same transcriptional orientation
V1 V2 V3 V9 D J
Non-deletional recombination
D J7129V47239
V1 7 23 9 D7129 J
How does recombination occur when a V gene is in opposite orientation to the DJ region?
V4
D J7129V47239V4 and DJ in opposite transcriptional orientations
DJ
712
9V47239
1.
DJ
712
9
V47239
3.
DJ7
129
V47239
2.
D J7129
V472394.
Non-deletional recombination
D J7129
V47239
1.
D J
V4
7129
7239
3.
V to DJ ligation - coding joint formation
D J7129
V47239
2.
Heptamer ligation - signal joint formation
D JV47 12 97239
Fully recombined VDJ regions in same transcriptional orientationNo DNA is deleted
4.
V 7 23 9
D7 12 9J
V 7 23 9
7 23 9
7 12 9D7129 J
7 23 9
7 12 9
V
DJRecombination activating gene products, (RAG1 & RAG 2) and ‘high mobility group proteins’ bind to the RSS
The two RAG1/RAG 2 complexes bind to each other and bring the V region adjacent to the DJ region
• The recombinase complex makes single stranded nicks in the DNA, the ends of each broken strand.
• The nicks are ‘sealed’ to form a hairpin structure at the end of the V and D regions and a flush double strand break at the ends of the heptamers.
• The recombinase complex remains associated with the break
Steps of TcR gene recombination
V
DJ
7 23 9
7 12 9
A number of other proteins, (Ku70:Ku80, XRCC4 and DNA dependent protein kinases) bind to the hairpins and the heptamer ends.
V D J
The hairpins at the end of the V and D regions are opened, and exonucleases and transferases remove or add random nucleotides to the gap between the V and D region
V D J 72
39
71
29
DNA ligase IV joins the ends of the V and D region to form the coding joint and the two heptamers to form the signal joint.
Steps of TcR gene recombination
7D 12 9J
Junctional diversity: P nucleotide additions
7V 23 9
D7 12 9J
V 7 23 9TC CACAGTGAG GTGTCAC
AT GTGACACTA CACTGTG
The recombinase complex makes single stranded nicks at random sites close to the
ends of the V and D region DNA.
7D 12 9J
7V 23 9CACAGTGGTGTCAC
GTGACACCACTGTG
TCAG
ATTADJ
V TCAG
ATTA
UU
The 2nd strand is cleaved and hairpins form between the complimentary bases at ends of the V and D
region.
V2V3
V4
V8
V7V6
V5
V9
7 23 9CACAGTGGTGTCAC
7 12 9GTGACACCACTGTG
V TCAG U
DJ ATTA U
Heptamers are ligated by DNA ligase IV
V and D regions juxtaposed
V TCAG U D JAT
TA
U
V TCAG U D JAT
TA
U Endonuclease cleaves single strand at random sites in V and D segment
V TC~GAAG D JAT
TA~TAThe nucleotides that flip out, become part of the complementary DNA strand
Generation of the palindromic sequence
In terms of G to C and T to A pairing, the ‘new’ nucleotides are palindromic.The nucleotides GA and TA were not in the genomic sequence and
introduce diversity of sequence at the V to D join.
V TCAG U D JAT
TA
U
Regions to be joined are juxtaposed
The nicked strand ‘flips’ out
Junctional Diversity – N nucleotide additions
V TC~GAAG D JAT
TA~TA
Terminal deoxynucleotidyl transferase (TdT) adds nucleotides randomly to the P nucleotide ends of the single-stranded V and D segment DNA
CACTCCTTA
TTCTTGCAA
V TC~GAAG D JAT
TA~TA
CACACCTTA
TTCTTGCAA Complementary bases anneal
V D JDNA polymerases fill in the gaps with complementary nucleotides and DNA ligase IV joins the strands
TC~GAAG
ATTA~TA
CACACCTTA
TTCTTGCAA
D JTA~TAExonucleases nibble back free endsV TC~GACACACCTTA
TTCTTGCAA
V TCDTA
GTT AT AT
AG C
V D JTCGACGTTATATAGCTGCAATATA
Junctional Diversity
TTTTTTTTTTTTTTT
Germline-encoded nucleotides
Palindromic (P) nucleotides - not in the germline
Non-template (N) encoded nucleotides - not in the germline
Creates an essentially random sequence between the V region, D region and J region in beta chains and the V region and J region in alpha chains.
How does somatic recombination work?
1. How is an infinite diversity of specificity generated from finite
amounts of DNA?
Combinatorial diversity and junctional diversity
2. How do V region find J regions and why don’t they join to C regions?
12-23 rule
3. How does the DNA break and rejoin?
Imprecisely, with the random removal and addition of nucleotides to
generate sequence diversity.
Why do V regions not join to J or C regions?
IF the elements of the TcR did not assemble in the correct order, diversity of specificity would be severely compromised
Full potential of the beta chain for diversity needs V-D-J-C joining - in the correct order
Were V-J joins allowed in the beta chain, diversity would be reduced due to loss of the imprecise join between the V and D regions
DIVERSITY
2x
DIVERSITY
1x
V D J C
V-DJoin
D-Jjoin
TcR chain
V-JJoin
TcR chain
Location of junctional diversity
Amino acid No.of TcR chain
Var
iabi
lity
CDR1CDR2
CDR3
CDR = Complemantarity determining region
Location of junctional diversity in TcR
TcRV monomer TcR chain
213
21 3
CDR’s
MHC class I and TcR V/V MHC class II TcR /
The trimolecular complex
V and V of TcR recognisinga peptide from MHC class I
ribbon plot
TcR recognising a peptidefrom MHC class II
ribbon plot
V and V of TcR recognisinga peptide from MHC class I wire
plot showing amino acid sidechains
TcR recognising a peptidefrom MHC class II wire plot showing
amino acid sidechains
Turn through 90º
TcR contact and anchor residue side chainsinteract with side chains of TcR
Hypervariable loops - CDRs
/3 /3/2/2 /2 /2
The most variable loops of the TcR - the CDR3 interact with the most variable part of the MHC-peptide complex CDR’s 1 and 2 interact largely with the MHC
molecule
View structures
T cell co-receptor molecules
CD8
MHC Class I MHC Class II
3 2
TcR TcR
CD4
Lck PTK Lck PTK
CD4 and CD8 can increase the sensitivity of T cells to peptide antigen MHCcomplexes by ~100 fold
MHC class II
CD8 and CD4 contact points on MHCclass I and class II
CD8 binding site
MHC class I
CD8 binding site
TcR-CD3 complex
TcR
CD3CD3
The intracytoplasmic regionof the TcR chain is too short
to transduce a signal
Signalling is initiated by aggregation of TcR by MHC-peptide complexes on APC
The CD3or (zeta)chainsare required for cell surfaceexpression of the TcR-CD3
complex and signallingthrough the TcR
Transduction of signals by the TcR
The cytoplasmic domains of the CD3 complex contain 10 Immunoreceptor Tyrosine -based Activation Motifs (ITAMS) - 2 tyrosine residues separated
by 9-12 amino acids - YXX[L/V]X6-9YXX[L/V]
CD3
ITAMs
As with B cell receptors, immunoreceptor tyrosine-based activation motifs (ITAMs) are involved in the transmission of the signals from the receptor
and require clustering of TcR/CD3 and the CD4 or CD8 co-receptors
Kinase domain Unique regionSH3 domainSH2 domain
Enzyme domain thatphosphorylates tyrosine
residues (to give phosphotyrosine)
Phosphotyrosinereceptor domain
Adaptor protein
recruitment domain
ITAM binding domain
• Phosphorylation is rapid and requires no protein synthesis or degradation to change the biochemical activity of a target protein
• It is reversible via the action of phosphatases that remove phosphate
• Phosphorylation changes the properties of a protein, by changing its conformation• Changes in conformation can activate or inhibit a biochemical activity, or create a
binding site for other proteins
Phosphorylation by Src kinases
Kinase domain Unique regionSH3 domain
Regulation of Src kinases
SH2 domain
Activating tyrosine residueInhibitory tyrosine residue
Phosphorylation of ‘Activating Tyrosine’ stimulates kinase activity
Kinase domain Unique regionSH3 domainSH2 domain
Phosphorylation of ‘Inhibitory Tyrosine’ inhibits kinase activityby blocking access to the Activating Tyrosine Residue
LckFyn
Zap-70
Receptor associated kinases accumulate under the membrane in close proximity to the cytoplasmic domains of the TcR -
CD3 complex
CD4MHC IIMHC II
CD45
As the T cell antigen receptor binds the MHC-peptide antigen, the
phosphatase CD45 activates kinases such as Fyn
This mechanism of activation is similar to the used to activate Syk in
B cells
P
Early T cell activation
The tyrosine kinase ZAP-70 binds to the phosphorylated ITAMs of
CD3 - further activation requires ligation of the co-receptor, CD4
Zap-70
Fyn phosphorylates the ITAMs of CD3, , and ITAMS
LckFyn
CD4CD45
P
MHC II T cell activation
LckFyn
P
MHC II
Zap-70
T cell activation
Binding of CD4 co-receptor to MHC class II brings Lck into the complex,
which then phosphorylates and activates ZAP-70
Tyrosine rich cell membrane associated Linker of Activation in T cells (LAT) and SLP-76 associate with cholesterol-rich lipid rafts
LATSLP-76
Activated ZAP-70 phosphorylates LAT & SLP-76
P PP P
ZAP-70 phosphorylates LAT and SLP-76
T cell activation
LckFyn
P
MHC II
Zap-70
LATSLP-76P PP P
SLP76 binds Tec kinases and activates phospholipase C- (PLC-)
Tec Tec
PLC- cleaves phosphotidylinositol bisphosphate (PIP2) to yield diacylglycerol (DAG) and inositol trisphosphate (IP3)
Activated ZAP-70 phosphorylates Guanine-nucleotide exchange factors (GEFS) that in turn activate the small GTP binding protein Ras
Ras activates the MAPkinase cascade
Transmission of signals from the cellsurface to the nucleus
• T cell-specific parts of the signalling cascade are associated with receptors unique to T cells - TcR, CD3 etc.
• Subsequent signals that transmit signals to the nucleus are common to many different types of cell.
• The ultimate goal is to activate the transcription of genes, the products of which mediate host defence, proliferation, differentiation etc.
Once the T cell-specific parts of the cascade are complete, signalling tothe nucleus continues via three common signalling pathways via:
1.The mitogen-activated protein kinase (MAP kinase) pathway2.An increase in intracellular calcium ion concentration mediated by IP3
3.The activation of Protein Kinase C mediated by DAG
Almost identical to transmission in B cells
• MAP Kinase cascadeSmall G-protein-activated MAP kinases found in all multicellular animals - activation of MAP kinases ultimately leads to phosphorylation of transcription factors from the AP-1 family such as Fos and Jun.
• Increases in intracellular calcium via IP3
IP3, produced by PLC-, binds to calcium channels in the ER and releases intracellular stores of Ca++ into the cytosol. Increased intracellular [Ca++] activate a phospatase, calcineurin, which in turn activates the transcription factor NFAT.
• Activation of Protein Kinase C family members via DAGDAG stays associated with the membrane and recruits protein kinase C family members. The PKC, serine/threonine protein kinases, ultimately activate the transcription factor NFB
The activated transcription factors AP-1, NFAT and NFB induce B cell proliferation, differentiation and effector mechanisms
Simplified scheme linking antigen recognition with transcription of T cell-specific genes
ElementImmunoglobulin TcR
Variable segments
Diversity segments
D segments inall 3 frames
Joining segments
Joints with N & Pnucleotides
No. of V gene pairs
Junctional diversity
Total diversity
H
40
27
Yes
6
22360 3640
~1013 ~1013
~1016** ~1016
59
0
-
9
(1)*
52 ~70
2 0
Yes -
13 61
2 1
* Only half of human chains have N & P regions**No of distinct receptors increased further by somatic hypermutation
Estimate of the number of human TcR and IgExcluding somatic hypermutation
Self Antigen
Foreign antigen
APC
YT
Antigen presentation
YB
T cell help
YT
Anergy or deletionof anti-self cells
YB
NoT cell help
Affinity maturation due to somatic
mutation
Antibody
Why do TcR not undergo somatic mutation?
YB
Occasional B cellthat somatically mutatesto become self reactive
Why do TcR not undergo somatic mutation?
YB
Affinity maturation due to somatic mutation
YBY
B
YB
YB
YB
YB
Occasional B cellthat somatically mutatesto become self reactive
T cell that doesn’t mutatecan not help the
self reactive B cell
YT
XNo T cell help
YT
T cell that mutates can may help the self reactive
B cell
T cell help
Autoantibody production
The lack of somatic mutation in TcR helpsto prevent autoimmunity
If TcR did undergo somatic mutation:
TcR interacts with entire top surface of MHC-peptide antigen complex
Somatic mutation in the TcR could mutate amino acids that interact with the MHC molecule causing a complete loss of peptide-MHC recognition
If TcR did undergo somatic mutation:
TcR-MHC interaction is one of many between the T cell and APC
On-off rate of TcR determines rate of ‘firing’ to give qualitatively different outcomes
Must be of relatively low affinity as cells with high affinity TcR are deleted to prevent self reactivity.
If TcR underwent affinity maturation, they would be deleted
Y`
`
Y` `
Toxin bindingblocked
Preventstoxicity
Why do B cell receptors need to mutate?
Neutralisation ofbacterial toxins
Ab-Ag interaction must be of high affinity to capture and neutralise toxins in
extracellular fluids
There is a powerful selective advantage to B cells that can somatically mutate
their receptors to increase affinity
SOMATIC MUTATION
An alternative TcR:
Discovered as Ig-homologous, rearranging genes in non TcR T cells
The locus is located between the V and J regions
V to J rearrangement deletes D, Jand CTcR cells can not express TcR
Few V regions, but considerable junctional diversity as chain can use 2 D regions
3x D 3x J 1x C
Human locus
3x J C1 2x J C212x V1
V V VVV J C
T cells
Distinct lineage of cells with unknown functions
1-5% of peripheral blood T cells
In the gut and epidermis of mice, most T cells express TcR
Ligands of TcR are unknown
Possibly recognise:Antigens without involvement of MHC antigens - CD1
Class IB genes
• The TcR was discovered using clonotypic antibodies
• Antibodies and TcR share many similarities, but there are significant differences in structure and function
• The structure and organisation of the TcR genes is similar to the Ig genes
• Somatic recombination in TcR genes is similar to that in Ig genes
• The molecular mechanisms that account for the diversity of TcR include combinatorial and junctional diversity
• TcR do not somatically mutate
• The highly variable CDR loops map to the distal end of the TcR
• The most variable part of the TcR interacts with the peptide
Summary
Recommended