Capturing resting T cells: the perils of PLL

Full understanding of lymphocyte activation will require thorough characterization of the ‘resting’ state and how it changes. Surfaces coated with the cationic homopolymer poly-l-lysine (PLL) are widely used for total internal reflection fluorescence (TIRF) imaging of the organization of surface proteins on resting lymphocytes1,2,3,4,5 because PLL is assumed to be inert. Here we found that PLL initiated T cell signaling and profoundly altered the activity of membrane proteins such as the T cell antigen receptor (TCR). Therefore, the emerging idea that receptors and signaling proteins cluster by default1,2,3,4,5, which has been based mostly on studies of lymphocytes interacting with PLL-coated surfaces, needs reconsideration.