Piecing together the family portrait of TCR-CD3 complexes

The pre-T-cell receptor (TCR)-, αβTCR-, and γδTCR-CD3 complexes are members of a family of modular biosensors that are responsible for driving T-cell development, activation, and effector functions. They inform essential checkpoint decisions by relaying key information from their ligand-binding modules (TCRs) to their signaling modules (CD3γε + CD3δε and CD3ζζ) and on to the intracellular signaling apparatus. Their actions shape the T-cell repertoire, as well as T-cell-mediated immunity; yet, the mechanisms that underlie their activity remain an enigma. As with any molecular machine, understanding how they function depends upon understanding how their parts fit and work together. In the 30 years since the initial biochemical and genetic characterizations of the αβTCR, the structure and function of the individual components of these family members have been extensively characterized. Cumulatively, this information has allowed us to piece together a portrait of the αβTCR-CD3 complex and outline the form of the remaining family members. Here we review the known structural and functional characteristics of the components of these TCR-CD3 complex family members. We then discuss how these data have informed our understanding of the architecture of the αβTCR-CD3 complex as well as their implications for the other family members. The intent is to provide a framework for considering: (i) how these thematically similar complexes diverge to execute their specific functions and (ii) how our knowledge of the form and function of these distinct family members can cross-inform our understanding of the other family members.