T Cell Activation through Isolated Tight Contacts.

T cells engage antigen-presenting cells in search for cognate antigens via dynamic cell protrusions before forming a tight immune synapse. The spatiotemporal events that may lead to rapid TCR triggering and signal amplification in microvilli-driven isolated contacts, and in subsequent, more uniform contacts, remain poorly understood. Here, we combined interference-reflectance microscopy and single-molecule localization microscopy in live cells to resolve TCR-dependent signaling at tight cell contacts. We show that early contacts are sufficient for robust TCR triggering and ZAP-70 recruitment. With cell spreading, TCR activation and ZAP-70 recruitment increase and shift to the edges of the growing tight contacts. CD45 segregates from TCR at tight contacts and is enriched at high local curvature membrane. Surprisingly, cortical actin and LFA localized at contact regions of intermediate tightness. Our results show in molecular detail the roles of early and tight T cell contacts in T cell activation, as both sensing and decision-making entities.