Structural basis of assembly of the human TCR-CD3 complex.

The αβ T cell receptor (TCR), in association with the CD3γ/e, δ/e, and ζ/ζ signaling hexamer, is the primary determinant for T cell development, activation and immune responses to foreign antigens. The assembly mechanism of the TCR-CD3 complex remains unknown. Here we report a cryo-EM structure of human TCRα/β in complex with the CD3 hexamer at 3.7 A resolution. The structure contains the complete extracellular domains (ECDs) and all the transmembrane (TM) helices of TCR-CD3. The octameric TCR-CD3 complex is assembled with the 1:1:1:1 stoichiometry of TCRα/β:CD3γ/e:CD3δ/e’:CD3ζ/ζ’. Assembly of the ECDs of TCR-CD3 is mediated by the constant domains and connecting peptides of TCRα/β that packs against CD3γ/e:CD3δ/e’, forming a trimer-like structure proximal to the plasma membrane. The TM segment of the CD3 complex adopts a barrel-like structure formed by interaction of the two TM helices of CD3ζ/ζ’ with those of CD3γ/e and CD3δ/e’. Insertion of the TM helices of TCRα/β into the barrel-like structure via both hydrophobic and ionic interactions results in TM assembly of the TCR-CD3 complex. Together, our data reveal the structural basis for TCR-CD3 complex assembly, providing clues to TCR triggering and laying a foundation for rational design of immunotherapies targeting the complex.