Functional Implications

• The proposed structure involves no direct TCR-CD4 contact. This supports previous data indicating that TCR and CD4 interact with the pMHC complex independently
• The membrane proximal sub-units of TCR (Cα and Cβ) and CD4 (D4) are far apart (70A). This gives enough room for CD3 to be located in-between the TCR and CD4 on the cell membrane, with the CD3εγ and CD3εδ ectodomains located inside the arch created by the TCR-pHC-CD4 structure. This also suggests a possible mechanism for relaying the signal via the CD3 molecules: If the angle of the TCR relative to the cell membrane is larger in the unbound state, then this suggests that a full interaction of CD4-pMHC-TCR forces the TCR to get closer to the membrane (this can be visualised by imagining the TCR being literally “pulled” towards the membrane when the pMHC-CD4 interaction occurs), making the angle smaller, and thus physically displacing the CD3 ectodomains. This will result in the dissociation of CD3 associated ITAMS, continuing down the signalling pathway.
  
  
  
  
  Dimerization
  
  
  Dimerization of the studied complex is known to be involved in TCR signalling, though it is not certain how this occurs. The presented structure allows certain conclusions to be drawn regarding how this might occur.

• CD4 molecules are known to dimerize in crystals, through their D4 domains, and it was regarded as plausible that this dimerization might be a step in T-cell activation. This hypothesis is contradicted by the structure proposed in this paper. As shown in figure X (retrieved from the paper), given the proposed structure, a dimer of CD4 molecules which are both part of such a TCR-pMHC-CD4 complex would result in a structure in which the CD4 transmembrane region is significantly out of the plane defined by the two TCR transmembrane regions. Hence, such a complex is highly unlikely to occur on the T-cell membrane.

The shape of a hypothetical ternary complex dimer with interaction at the CD4. Retrieved from Yin Y. et al. 2012

• Similarly, some HLA-DR molecules are also known to dimerize in crystals, and even some MHC class II dimers have been observed on cells, leading to the hypothesis that this dimerization might be significant to TCR signalling. Again, the structure proposed in this paper contradicts this hypothesis, as the putative dimerization site on HLA-DR coincides with the CD4 binding site, making such dimerization impossible in a TCR-MHC-CD4 complex
• Finally, another proposed hypothesis is that dimerization of two TCR molecules occurs and relays the signal, aided by cross-linking via CD4. One of the two TCRs is bound to an agonist-pMHC, while the other is bound to a self-pMHC. This is contradicted by recent evidence that TCR dimerization can occur independently of CD4, and that mutations that inhibit dimerization also inhibit T-cell activation. A slightly modified model is proposed where two TCR molecules dimerize via interactions on the outside of the arch generated by the TCR-pMHC-CD4 complex (see figure). CD4 is entirely uninvolved in this process.

The shape of a hypothetical ternary complex dimer with interaction at the TCR. Retrieved from Yin Y. et al. 2012