The Lifeact-EGFP mouse is a translationally controlled fluorescent reporter of T cell activation

It has become increasingly evident that T cell functions are subject to translational control in addition to transcriptional regulation. Live imaging of CD8+ T cells isolated from the Lifeact-EGFP mouse reveals that they exhibit a gain in fluorescence intensity following engagement of cognate tumour target cells. The GFP signal increase is governed by Erk-dependent distal TCR signalling and its magnitude correlates with IFN-γ and TNF-α production, hallmarks of T cell activation. Enhanced fluorescence is due to increased translation of Lifeact-EGFP protein, without an associated increase in messenger RNA. Activation-induced gains in fluorescence are also observed in naïve and CD4+ T cells from the Lifeact-EGFP reporter, and are readily detected by both flow cytometry and live cell microscopy. This unique, translationally controlled reporter of effector T cell activation simultaneously enables tracking of cell morphology, F-actin dynamics and activation state in individual migrating T cells. It is a valuable addition to the limited number of reporters of T cell dynamics and activation, and opens the door to studies of translational activity and heterogeneities in functional T cell responses in situ.