T cell receptor (TCR) activation orchestrates an immune response by translating extracellular antigen recognition into intracellular signalling cascades. The spatial organisation of phosphorylated TCR and the signalling proteins recruited to it occur on a nanoscale and determine cellular outcomes ( 1). However, understanding the spatial organisation of these proteins is hindered by imaging limitations, including protein labelling that can withstand long imaging without bleaching and microscopes that can maintain precision over long acquisitions.
To overcome these limitations, we developed a novel Single Molecule Localisation Microscopy technique, Protein-PAINT ( 2). This technique utilises the transient interactions between Src Homology 2 (SH2) domain-containing proteins and phosphotyrosine (pTyr) sites on receptors, to directly image TCR phosphorylation. By combining Protein-PAINT with our custom-built 3D-stabilised Total Internal Reflection Fluorescence (TIRF) microscope ( 3), we can now measure the single-molecule binding kinetics of the SH2-domain to pTyr sites in situ. Our results show that these binding kinetics are consistent with in vitro SPR measurements and the measured on-rate in situ is proportional to the number of binding sites.
Using our novel approach to measure single-molecule binding kinetics, we can precisely quantify the location and phosphorylation levels of individual receptors. This enables us to characterise the interplay between TCR phosphorylation, spatial organisation, and the intracellular signalling cascade that drives T cell activation.