TCR-MHC INTERACTIONS

Nathalie LabrecqueGuy-Bernier Research CentreMaisonneuve-Rosemont Hospitalnathalie.labrecque@umontreal.ca252-3552

September 27, 2006

TCR/CD3

CD8

V V

C CCD3CD3 CD3CD3

MHC class I

peptide

APC

T cell

How MHC class I and class II molecules bind peptides?

Davis et al. 1998. Annu. Rev. Immunol. 16:523-544

MHC-peptide structure

-the TCR and locus are composed of multiple gene segments

-the variable portion of the TCR and chains is encoded by different gene segments

-random juxtaposition of V, D and J gene segments can generate a big number of different TCRs

TCR diversity

-random association of V, D and J-flexible joining of gene segments-N nucleotide addition- and association-no somatic hypermutation

1015 TCR

LOCALIZATION OF DIVERSITY

CDR1-CDR2: région VCDR3:junction V-J ou V-D-J

CDR3 is hypervariable

Davis et al. 1998. Annu. Rev. Immunol. 16:523-544

Hennecke and Wiley. 2001. Cell 104:1-4

Davis et al. 1998. Annu. Rev. Immunol. 16:523-544

Peptide-MHC surfaces are not flat

What are the constrainsts that fixed TCR-MHC-peptide orientation?

1. Conserved contacts between conserved portions of the TCR and MHC:NO

2. Shape of the TCR and/or peptide-MHC-binding surfaces limits the number of docking orientations:YES

-CDR1 eand CDR2 interact with MHC molecules (helices)

-CDR3 interacts with the peptide

-interaction always in the same orientation-45 to 70 degrees angle related to peptide-V see N-ter of the peptide-V see C-ter of the peptide

TRI-MOLECULAR COMPLEX CHARACTERISTICS

Consistent features of TCR-MHC-peptide complex

1. Peptide contributes a smaller proportion of the buried surface area and a smaller number of contact than the MHC surface-peptide binding interface:21-34%-peptide proportion of contact:26-47%

2. Conformational flexibility-one or more of the CDR loops adopt different conformation-CDR3 loops show the greatest conformational changes

Consequence of TCR affinity for its ligand

During T cell development

Mature T lymphocytes

Goldrath and Bevan 1999. Nature 402:255-262

T cell responses correlate very well with the binding characteristics of their TCRs

•Higher affinity variants elicit more robust T cell responses

•Increasing dissociation rates correlates with a decrease in agonistic activity

•Antagonist peptides: differed only slightly in affinity with the weakest agonist but dissociation rate differed by 10-fold or more (increased)

Physico-chemical characteristics of TCR-MHC-peptide interaction

-On-rate ~1000 à 200 000 (binding speed)

-Off-rate ~ 0.5-0.01s-1 ou t½ ~12-30s (dissociation speed)

-KD ~1-50M (affinity)

Correlation between the speed of dissociation and biological effect

1. Specificity

2. Sensibility

3. Context discrimination

PARADOX

-TCR-MHC interaction has a weak affinity-affinity ~ 10 M-half-life ~10s

-restricted numbers of ligands (~100) are displayed at the surface of antigen presenting cells

-T cell activation requires a long interaction with antigen presenting cells (>2h)

TCR engagement: challenge for activation

-small size of the TCR

-T cell surface is covered abundantly with big glycoproteins (LFA-1, CD48 ,CD45…)

-weak affinity of the TCR for its ligand

-half-life of the TCR-MHC complex is short (10s)

-small number of specific peptide-MHC complex

-T cells are moving

-T cells must interact for long period with antigen presenting cells

• Oligomerisation• Membrane microdomains (rafts)• Serial engagement• Immunological synapse• Binding in two-steps of the TCR to its ligand

(sampling of MHC-peptide complexes at the surface of APCs)• Self-recognition sensitizes T cells

Proposed models to reconcile T cell sensitivity to Ag

Serial engagement

Tiré de Dustin and Cooper 2000. Nature Immunology 1:23-29

(Valittuti et al. 1995. Nature 375:148-151)

« SUPRAMOLECULAR ACTIVATION COMPLEX »

Monks et al. 1998. Nature 395:82-86

Immunological synapse

IMMUNOLOGICAL SYNAPSE

Malissen 1999. 285:207-208

La petite taille du RCT est requise pour une activation efficace des lymphocytes T

Choudhuri K et al. 2005. Nature 436, 578-582

La petite taille du RCT permet d’exclure les grosses molécules du centre de la synapse, incluant la

phosphatase CD45

Choudhuri K et al. 2005. Nature 436, 578-582

L’exclusion de la phosphatase CD45 des RCT engagés permet l’activation des lymphocytes T

Choudhuri K et al. 2005. Nature 436, 578-582

Two-steps binding of the TCR to its ligand

- MHC residues ( helices) affect association (guide the TCR to its ligand)-allow conformational change of the CDR3 loops-peptide residues affect the dissociation or stability of the tri-molecular complex

Tiré de Wu et al. 2002. Nature 418,552-556

key and lock

-initial association via CDR1 et CDR2 (on rate)-induces fitting of the CDR3 loops on the peptide (off rate)-stabilize the interaction

allow an efficient scanning of the surface of Ag presenting cell to detect foreign peptide (rare and very similar to self) in a very sensitive manner

TCR-self-peptide-MHC interaction regulates T cell homeostasis -naive T lymphocyte survival-homeostatic proliferation (lymphopenia)

« TCR tickling » -partial phosphorylation of (p21) -ZAP-70 asociation to the TCR

TCR/CD3

CD8

V V

C CCD3CD3 CD3CD3

MHC class I

peptide

APC

T cell

soi

1. lck phosphoryles CD3 chains2. recrutes ZAP-70 to the TCR/CD3 complex3. ZAP phosphoryles different adaptors4. adaptors propagate the signal to the main 3 signaling pathways

-ras-map kinase-PLC1 (calcineurin, PKC)-PI-3K

Lack of TCR-MHC-self peptide interaction abolished T cell reactivity to foreign Ag

Tiré de Stefanova et al. 2002. Nature 420, 429-434

Role of the TCR-MHC-self peptide interaction

1. MHC restriction2. Naive T cell survival3. Homeostatic proliferation4. Increased T cell sensitivity to foreign peptides

IMMUNOLOGICAL SYNAPSE

facilitated by

1. Two steps binding (scanning)2. Small size of the TCR (exclusion of CD45)3. Increased sensitivity via self interactions4. Rafts; colocalization of signaling molecules5. Serial engagement ???

Allow for sensitive and specific T cell responses