Kruger, Travis

Abstract Previously we and others observed that the signaling module of the TCR, the CD3 complex, undergoes a conformational change at the level of its cytoplasmic tails when T cells are stimulated. At that time, we interpreted this data as proof that antigen recognition modifies the conformation of CD3 as part of the receptor triggering mechanism. In absence of specific structural information, the induced conformation has been referred to as "open CD3". Here we present new evidence that thymocytes display detectable amounts of "open CD3" even in the absence of antigenic stimulation. However, treatment of thymocytes with stimulatory anti-CD3 antibodies significantly increases the amount of "open CD3". We conclude that ligand binding causes many CD3 molecules to adopt the "open CD3" conformation. We propose that CD3 is expressed in an equilibrium between two distinguishable conformations, which may reflect "on" and "off" states that are perturbed by TCR engagement. The accumulation of a certain level of "open CD3" might trigger T cell activation. Research support: Diana Gil, Travis Kruger and Javier de la Cruz were supported with start-up funds from the Mayo Foundation.

Abstract The CD3ε cytoplasmic tail contains a conserved proline-rich sequence (PRS) that influences TCR–CD3 expression and signaling. Although the PRS can bind the SH3.1 domain of the cytosolic adapter Nck, whether the PRS is constitutively available for Nck binding or instead represents a cryptic motif that is exposed via conformational change upon TCR–CD3 engagement (CD3Δc) is currently unresolved. Furthermore, the extent to which a cis-acting CD3ε basic amino acid-rich stretch (BRS), with its unique phosphoinositide-binding capability, might impact PRS accessibility is not clear. In this study, we found that freshly harvested primary thymocytes expressed low to moderate basal levels of Nck-accessible PRS (“open-CD3”), although most TCR–CD3 complexes were inaccessible to Nck (“closed-CD3”). Ag presentation in vivo induced open-CD3, accounting for half of the basal level found in thymocytes from MHC+ mice. Additional stimulation with either anti-CD3 Abs or peptide–MHC ligands further elevated open-CD3 above basal levels, consistent with a model wherein antigenic engagement induces maximum PRS exposure. We also found that the open-CD3 conformation induced by APCs outlasted the time of ligand occupancy, marking receptors that had been engaged. Finally, CD3ε BRS–phosphoinositide interactions played no role in either adoption of the initial closed-CD3 conformation or induction of open-CD3 by Ab stimulation. Thus, a basal level of open-CD3 is succeeded by a higher, induced level upon TCR–CD3 engagement, involving CD3Δc and prolonged accessibility of the CD3ε PRS to Nck.