TCRpMHC Interaction

Whereas in humoral immunity antibodies identify anti-genic molecules as distinct entities, in the cellular response TCRs recognize antigenic peptide fragments only when presented by an appropriate MHC molecule. A fundamental

Handbook of Cell Signaling, Volume l

Figure 1 Schematic representation of the components in a class I TCR/pMHC/CD8/CD3 signaling complex. The heavy chain consists of the a^a3 domains, to which the light chain P2-microglobulin (P2m) is noncovalently attached. The peptide-MHC (pMHC) complex is anchored to the plasma membrane of the antigen-presenting cell via its a3 domain while the a^2 super-domain binds the peptide (□). The CDR loops of the ap TCRs recognize the pMHC complex, while the coreceptor CD8 binds simultaneously to the a3 domain either as an aa homodimer or an ap heterodimer. The signal from the pMHC complex (if any) is then transmitted through the T-cell plasma membrane by the CD3 signaling modules. Phosphorylation of the CD3Z chain by the ZAP70 kinase (not shown) is an early step in this signal transduction cascade.

Figure 1 Schematic representation of the components in a class I TCR/pMHC/CD8/CD3 signaling complex. The heavy chain consists of the a^a3 domains, to which the light chain P2-microglobulin (P2m) is noncovalently attached. The peptide-MHC (pMHC) complex is anchored to the plasma membrane of the antigen-presenting cell via its a3 domain while the a^2 super-domain binds the peptide (□). The CDR loops of the ap TCRs recognize the pMHC complex, while the coreceptor CD8 binds simultaneously to the a3 domain either as an aa homodimer or an ap heterodimer. The signal from the pMHC complex (if any) is then transmitted through the T-cell plasma membrane by the CD3 signaling modules. Phosphorylation of the CD3Z chain by the ZAP70 kinase (not shown) is an early step in this signal transduction cascade.

difference between antibody/antigen and TCR/pMHC recognition is that the specificity of the former is dependent on high affinity (Kd is nanomolar) for the free antigen, whereas in the latter low affinities predominate (Kd is ~0.1-500 ^M); thus, specificity must be ensured by a different mechanism. Possible mechanisms are outlined in the following sections.

Orientation of the TCR in TCR/pMHC Complexes

The seven independent TCR/pMHC complexes determined to date (reviewed in references [10] to [12]; Table 1) confirm that the TCR heterodimer is oriented approximately diagonally relative to the long axis of the MHC peptide-binding groove [13,14]. The Va domain is located above the N-terminal half of the peptide, while the VP domain can contact the C-terminal portion of the peptide (Fig. 2). The fluctuation in the TCR orientation has been described generally as diagonal [13,14] and, in one case, orthogonal [15], but it appears that the TCR orientation, or twist, on MHC class I and class II shows a relatively restricted spread of about 35° (Fig. 3). However, the TCR deviates not only in its twist, but also in its roll and tilt, which can be gleaned from the angle of the pseudo two-fold axis between the TCR Va and VP domains and the MHC P-sheet floor (Fig. 3). In addition, the TCRs can differ in their aP chain pairings, such that the pseudo- Va/VP two-fold angle can also contribute to the variation in TCR orientation on the pMHC. As a result of the various TCR orientations, the buried surface for the TCR/pMHC complex can vary extensively between 1240 and 1930 A2, with the peptide contributing a relatively restricted range of 21 to 34% to the pMHC side of that interface. Va can contribute from 37 to 74% (average 57%) and VP from 26 to 63% (average 43%) of the TCR buried

Figure 2 Similar structural architecture of class I and class II TCR/pMHC complexes. The Ca traces of the TCRs [34,51] are shown on top with the colored CDR loops contacting the pMHC at the complex interfaces. The Va and Vp domains are positioned over the N-terminal and C-terminal halves of the peptide, respectively. The peptides are drawn as red ball-and-stick representations and have fixed termini in class I MHC but can extend out of the binding groove in class II MHC. The CDR loops are colored as follows: CDR1a (residues 24-31): dark blue, CDR2a (48-55): magenta, CDR3a (93-104): green, CDR1P (26-31): cyan, CDR2P (48-55): pink, CDR3P (95-107): yellow, and HV4 (69-74): orange.

Figure 2 Similar structural architecture of class I and class II TCR/pMHC complexes. The Ca traces of the TCRs [34,51] are shown on top with the colored CDR loops contacting the pMHC at the complex interfaces. The Va and Vp domains are positioned over the N-terminal and C-terminal halves of the peptide, respectively. The peptides are drawn as red ball-and-stick representations and have fixed termini in class I MHC but can extend out of the binding groove in class II MHC. The CDR loops are colored as follows: CDR1a (residues 24-31): dark blue, CDR2a (48-55): magenta, CDR3a (93-104): green, CDR1P (26-31): cyan, CDR2P (48-55): pink, CDR3P (95-107): yellow, and HV4 (69-74): orange.

surface. This bias in chain usage has also been noted for antibodies, where VH usually provides a larger contribution to the antibody-antigen interface [16].

Peptide Recognition by the TCR CDR Loops

The suggestion that only a few up-pointing peptide side chains contribute to the specificity of the TCR/pMHC interaction [17] was confirmed by TCR/pMHC crystal structures. In class I, these interactions are dominated by the peptide residues that extend or bulge most out of the groove and, hence, represent functional hotspots [18] in the TCR/pMHC interface. For nonamer and octamer peptides, these represent residues P5, P7, and P8 and P4, P6, and P7, respectively. For class II peptides, the key side-chain contributions are more uniformly dispersed (P1, P2, P3, P5, P8). On the other hand, the contribution of the peptide backbone to TCR interaction is very modest for both class I and class II, where none to only a handful of contacts are made. The only exception so far is for the HLA-A2/Tax complex, where the large P4-P5 bulge includes a glycine at P4 that enables the TCR to access the peptide backbone [14,19].

Analysis of the number of contacts reveals that CDR1^ and CDR2P often make minimal contact with the pMHC

Table 1 Overview of TCR/pMHC Complex Structures (1996-2002)

Complex

PDB ID

Peptide activity

Constructs and expression systems

Ref.

2C/H-2Kb/dEV8

2C/H-2Kb/SIYR

2C/H-2Kbm3/dEV8

scBM3.3/H-2Kb/pBM1

B7/HLA-A2/Tax

A6/HLA-A2/Tax

A6/HLA-A2/TaxP6A

A6/HLA-A2/TaxV7R

A6/HLA-A2/TaxY8A

KB5-C20/H-2Kb/pKB1

scD10/I-Ak/CA

HA1.7/HLA-DR1/HA

HA1.7/HLA-DR4/HA

2ckb

1g6r

1jtr

1fo0

1bd2 1ao7 1qrn 1qse 1qsf 1kj2

1d9k

1fyt

1j8h

Weak agonist

Superagonista Weak agonist Agonist

Strong agonist^ Strong agonist^ Weak antagonist Weak agonist Weak antagonist Agonist

Agonist Agonist Agonist

D. melanogaster, acidic/basic leucine zipper for specific TCR chain pairing

Myeloma cells for TCR, E. coli for

MHC (refolded from inclusion bodies) E. coli, refolded from inclusion bodies E. coli, refolded from inclusion bodies

Myeloma cells for TCR, E. coli for MHC (refolded from inclusion bodies)

E. coli for TCR, refolded from inclusion bodies; CHO cells for MHC; peptide covalently connected to the MHC

E. coli for TCR, refolded from inclusion bodies; D. melanogaster for MHC; peptide covalently connected to the TCR

39 30

14 21 21 21 12

15 51

aThe nomenclature superagonist or strong agonist is equivalent in these instances. Class I and class II complexes are separated by the horizontal line; sc: single-chain Fv fragment of the TCR. (Adapted from Rudolph, M. G. and Wilson, I. A., Curr. Opin. Immunol., 14, 52-65, 2002.)

compared to CDR30. In Va, CDR2a tends to have fewer contacts with the pMHC than CDR3a, although an exception is found in the allogeneic BM3.3 complex, where CDR3a has almost no contacts (see above). However, in most cases, peptide contacts are made primarily through the central CDR3 loops, which also exhibit the greatest degree of genetic variability. In contrast, the majority of conserved MHC contacts are mediated by the CDR1 and CDR2 loops [20], particularly in Va.

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