The importance of Science's classic TCR-pMHC docking mode

Written by Tang Xiaotang | Recognition of peptide-major histocompatibility complexes (pMHCs) by Zyme T cell receptor (TCR) is the initial step to mediate T cell immunity, and TCR-pMHC is also the most important in biology One of the diverse forms of receptor-ligand interaction [1].

So far, almost all TCR-pMHC complexes show highly conservative docking polarity.
The TCRa chain is located on the MHC a2 or MHCII b1 helix, while the TCRb is docked on the MHC I and MHCII a1 helix [1].

However, whether this docking polarity is driven by the highly conserved amino acid complementation of TCR and MHC, or driven by the signal restriction associated with core receptors, is a long-standing issue of debate.

TCR-pMHC binding mode map [2] On June 4, 2021, the Nicole Gruta team of Monash University led an online research paper entitled Canonical T cell receptor docking on peptide–MHC is essential for T cell signaling on Science.
Using the recognition model of pMHCI by CD8+ T cells including "180° reversal" TCR (Reverse docking), it is proved that this highly conservative TCR-pMHCI docking polarity is driven by the structural limitation of TCR signal transduction.

Researchers have previously described two natural TCRs that recognize the epitope of H-2b-NP366 by 180° reversal: TRBV17+ TCR (B17.
R1 and NP2-B17), using H-2Db-NP366 tetramer (including important Single amino acid substitution), to study whether reversed TCR docking is a general feature of TRBV17+ H-2Db-NP366 specific TCR.

The results showed that the single amino acid substitution tetramer (H-2DbAb89E-NP366) completely abolished the binding of the tetramer to B17.
R1 at high TCR expression levels without affecting the binding of B13.
C1 TCR.

Next, the researchers used H-2bWT-NP366 and H-2bA89E-NP366 tetramers for comparative staining and found that although the two tetramers can detect a similar number of TRBV13+ cells, the mutant A89E tetramer only detects TRBV17+ cells.
Of about 48%.

In contrast, these two tetramers stained TRBV13+ and TRBV17+ T cells in mice infected with influenza A virus (IAV).

Therefore, although TRBV17+ TCRs that bind H-2Db-NP366 in the opposite direction are widespread, they are not recruited into the immune response after IAV immunization.

So what factors play a decisive role in the recruitment of TCR? The researchers analyzed the TRBV17+ H-2Db-NP366-specific TCRab sequence of uninfected and infected wild mice and found that the natural TRBV17+ TCRab is diverse.
In contrast, each immune combination is characterized by only one or two clones.
Dominant, so the lower proportion of TRBV17+ in H-2Db-NP366 is because most clones do not respond to IAV.

Further structural and biophysical analysis of TRBV17+ TCR revealed that active TCRs (such as B17.
C1 TCR) bind similarly to H-2bWT-NP366 and H-2bA89E-NP366 tetramers, but B17.
R2 TCRs that were not recruited The binding to H-2bA89E-NP366 tetramer was significantly reduced, indicating that the docking polarity of TCR-pMHCI was reversed.

SPR measurement of TCR affinity results showed that B17.
R2 TCR is higher, so TCR recruitment is related to docking polarity rather than affinity.

Subsequently, the researchers analyzed the crystal structure, B17.
R2 TCR adopted the opposite docking mode to H-2Db-NP366, the TCR chain (CDR3a loop) had a smaller effect, and the b chain (FWB, CDR2b, CDR3b) had a greater effect.

What’s important is that regardless of the affinity of TCR-H-2Db-NP366, only TCRs in the classic docking mode can support immune recruitment.
This is in the case of retrovirus-transduced "retroviral" mice expressing TCRs of different polarities and affinities.
It was confirmed in the model that after adoptive transfer of T cells with different TCRs, IAV immunization, compared with the classic B13.
C1, the reverse polarity B17.
R1 was not effectively recruited and expanded.

Similarly, the high-affinity reverse polarity B17.
R2 was not effectively recruited and amplified.

This result indicates that the topology of TCR-pMHCI docking, rather than affinity, is the main determinant of immune recruitment in vivo.

In addition, the reversed TCR-pMHCI docking does not hinder the formation of multimeric TCR-CD3 structure, but affects the localization of CD3 and CD8.

Finally, the researchers proved that it is the CD8 core receptor that inhibits the signal transmission of the reverse polarity TCR.
When the polarity of TCR-pMHCI is reversed, the combination of Lck and CD8 prevents Lck from effectively localizing to CD3.

Detect the signal after stimulation with wild-type CD8ab (CD8WT), no CD8 (CD8NULL), mutant CD8 abolished LcK (CD8CxC) expression classic B13.
C1 TCR or high affinity reverse polarity B17.
R2 TCR Transduction, the results show that compared with B13.
C1 TCR, when B17.
R2 TCR is co-expressed with CD8WT, the pERK signal is weak and the level of IL-2 cannot be detected, indicating that the signal transduction ability is impaired.

When CD8CxC cells were stimulated, the conduction signal levels of B13.
C1 TCR+ and B17.
R2 TCR+ cells were similar, indicating that LcK transmission also contributed.

TCR-pMHC docking model In general, this study shows that the signal mediated by the classic TCR-pMHCI docking is enhanced through the binding of CD8, and the transmission of Lck through CD8 is greatly enhanced.

The reversed polarity TCR cannot be recruited by the immune response, which has nothing to do with the binding affinity of TCR-pMHCI, but is related to the docking polarity and the co-localization of key signal molecules.

The core receptor-Lck association plays a role in preventing the signal transmission of non-classical TCR-pMHC recognition.
Such negative regulation helps to limit the occurrence of functional TCR cross-reactions and also limits the number of TCRs.

The CD8-LCK interaction model Science issued concurrent reviews, confirming the significance of this discovery and raising new questions about how to accurately understand the role of co-receptors and LcK in initiating TCR signals, promoting docking bond formation and limiting the direction of TCR-pMHC The connection between them is the key to be solved next.
In addition, understanding the details of the mechanism of TCR activation and signal transduction will also help rationally design antigen receptors for immunotherapy.

Original address: https://doi.
org/10.
1126/science.
abe9124 Platemaker: 11 References 1.
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Rossjohn et al.
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2.
Buckle, AM, & Borg, NA Integrating Experiment and Theory to Understand TCR-pMHC Dynamics.
Frontiers in Immunology, 9(2898).
(2018).
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