Thymus-derived lymphocytes protect mammalian hosts against virus-or cancer-related cellular alterations through immune surveillance, eliminating diseased cells. In this process, T cell receptors (TCRs) mediate both recognition and T cell activation via their dimeric ␣␤, CD3⑀␥, CD3⑀␦, and CD3 subunits using an unknown structural mechanism. Here, site-specific binding topology of anti-CD3 monoclonal antibodies (mAbs) and dynamic TCR quaternary change provide key clues. Agonist mAbs footprint to the membrane distal CD3⑀ lobe that they approach diagonally, adjacent to the lever-like C␤ FG loop that facilitates antigen (pMHC)-triggered activation. In contrast, a non-agonist mAb binds to the cleft between CD3⑀ and CD3␥ in a perpendicular mode and is stimulatory only subsequent to an external tangential but not a normal force (ϳ50 piconewtons) applied via optical tweezers. Specific pMHC but not irrelevant pMHC activates a T cell upon application of a similar force. These findings suggest that the TCR is an anisotropic mechanosensor, converting mechanical energy into a biochemical signal upon specific pMHC ligation during immune surveillance. Activating anti-CD3 mAbs mimic this force via their intrinsic binding mode. A common TCR quaternary change rather than conformational alterations can better facilitate structural signal initiation, given the vast array of TCRs and their specific pMHC ligands. The T cell receptor (TCR)3 is a multimeric transmembrane complex composed of a disulfide-linked antigen binding clonotypic heterodimer (␣␤ or ␥␦) in non-covalent association with the signal-transducing CD3 subunits (CD3⑀␥, CD3⑀␦, and CD3) (reviewed in Ref. 1). TCR signaling via CD3 dimers evokes T cell lineage commitment and repertoire selection during development, maintains the peripheral T cell pool, and further differentiates naïve T cells into effector or memory cell populations upon immune stimulation (2-5). The interaction between an Fab-like ␣␤ TCR heterodimer and an antigenic peptide bound to a major histocompatibility complex molecule (pMHC) initiates a cascade of downstream signaling events via the immunoreceptor tyrosine-based activation motif elements in the cytoplasmic tails of the associated CD3 subunits (6 -9). The length of these CD3 cytoplasmic tails is substantial, relative to those of the TCR ␣ and ␤ chains (6, 7).How recognition of pMHC by a weakly interacting (ϳ1-100 M K d ) clonotypic heterodimer on the T cell surface evokes intracellular signaling via the adjacent CD3 components remains undefined (1). Solution structures of CD3⑀␥ and CD3⑀␦ heterodimers reveal a unique side-to-side hydrophobic interface with conjoined ␤-sheets involving the G-strands of the two Ig-like ectodomains of the pair (10, 11). The squat and rigid CD3 connecting segments contrast sharply with the long and flexible TCR ␣ and ␤ connecting peptides linking their respective constant domains to the transmembrane segments.To investigate the basis of signal transduction involving the ectodomain components within the TCR membrane complex,...

Invariant CD3 subunit dimers (CD3␥, CD3␦, and CD3) are the signaling components of the ␣␤ T cell receptor (TCR). The recently solved structure of murine CD3␥ revealed a unique side-to-side interface and central ␤-sheets conjoined between the two C2-set Ig-like ectodomains, with the pairing of the parallel G strands implying a potential concerted piston-type movement for signal transduction. Although CD3␥ and CD3␦ each dimerize with CD3, there are differential CD3 subunit requirements for receptor assembly and signaling among T lineage subpopulations, presumably mandated by structural differences. Here we present the solution structure of the heterodimeric CD3␦ complex. Whereas the CD3 subunit conformation is virtually identical to that in CD3␥, the CD3␦ ectodomain adopts a C1-set Ig fold, with a narrower GFC front face ␤-sheet that is more parallel to the ABED back face than those ␤-sheets in CD3 and CD3␥. The dimer interface between CD3␦ and CD3 is highly conserved among species and of similar character to that in CD3␥. Glycosylation sites in CD3␦ are arranged such that the glycans may point away from the membrane, consistent with a model of TCR assembly that allows the CD3␦ chain to be in close contact with the TCR ␣-chain. This and many other structural and biological features provide a basis for modeling putative TCR͞CD3 extracellular domain associations. The fact that the two clusters of transmembrane helices, namely, the three CD3-CD3␥-TCR␤ segments and the five CD3-CD3␦-TCR␣-CD3-CD3 segments, are presumably centered beneath the G strand-paired CD3 heterodimers has important implications for TCR signaling.single-chain C1-Ig fold ͉ immunoreceptor tyrosine-based activation motif ͉ NMR structure ͉ T cell development T he ␣␤ T cell receptor (TCR) is a multimeric complex composed of an antigen-binding ␣␤ clonotypic heterodimer and the signal-transducing invariant CD3 subunit dimers CD3␥, CD3␦, and CD3 (1-8). Thus, the ␣␤ TCR complex consists of eight polypeptides (5,8,9). Sequence determination and biochemical analyses suggest that each CD3, CD3␥, and CD3␦ subunit contains an extracellular Ig-like domain, a membrane-proximal stalk region, a transmembrane (TM) helix, and a cytoplasmic tail. The interaction between an ␣␤ TCR heterodimer and a specific antigenic peptide bound to an MHC molecule (pMHC) initiates a cascade of downstream signaling events via the immunoreceptor tyrosinebased activation motifs (ITAMs) in the cytoplasmic tails of the associated CD3 subunits (10-12). The various CD3 chains interact differentially with intracellular adaptors and signaling molecules, inducing distinct patterns of cellular protein tyrosine phosphorylation upon activation (11,(13)(14)(15)(16).How recognition of pMHC by a clonotypic ␣␤ heterodimer on the T cell surface evokes intracellular signaling via the adjacent CD3 components remains unknown. However, the solution structure of a heterodimeric murine CD3␥ complex revealed a unique side-to-side hydrophobic interface with conjoined ␤-sheets between the two Ig-like ectodomai...

Mechanotransduction is a basis for receptor signaling in many biological systems. Recent data based upon optical tweezer experiments suggest that the TCR is an anisotropic mechanosensor, converting mechanical energy into biochemical signals upon specific peptide-MHC complex (pMHC) ligation. Tangential force applied along the pseudo-twofold symmetry axis of the TCR complex post-ligation results in the αβ heterodimer exerting torque on the CD3 heterodimers as a consequence of molecular movement at the T cell–APC interface. Accompanying TCR quaternary change likely fosters signaling via the lipid bilayer predicated on the magnitude and direction of the TCR–pMHC force. TCR glycans may modulate quaternary change, thereby altering signaling outcome as might the redox state of the CxxC motifs located proximal to the TM segments in the heterodimeric CD3 subunits. Predicted alterations in TCR TM segments and surrounding lipid will convert ectodomain ligation into the earliest intracellular signaling events.