Rac Activation by the T-Cell Receptor Inhibits T Cell Migration

Background: T cell migration is essential for immune responses and inflammation. Activation of the T-cell receptor (TCR) triggers a migration stop signal to facilitate interaction with antigen-presenting cells and cell retention at inflammatory sites, but the mechanisms responsible for this effect are not known. Methodology/Principal Findings: Migrating T cells are polarized with a lamellipodium at the front and uropod at the rear. Here we show that transient TCR activation induces prolonged inhibition of T-cell migration. TCR pre-activation leads to cells with multiple lamellipodia and lacking a uropod even after removal of the TCR signal. A similar phenotype is induced by expression of constitutively active Rac1, and TCR signaling activates Rac1. TCR signaling acts via Rac to reduce phosphorylation of ezrin/radixin/moesin proteins, which are required for uropod formation, and to increase stathmin phosphorylation, which regulates microtubule stability. T cell polarity and migration is partially restored by inhibiting Rac or by expressing constitutively active moesin. Conclusions/Significance: We propose that transient TCR signaling induces sustained inhibition of T cell migration via Rac1, increased stathmin phosphorylation and reduced ERM phosphorylation which act together to inhibit T-cell migratory polarity.