An antigen-specific T cell binding agent is provided, where a multivalent ‘spheromer’ system utilizes a scaffold of a self-assembling polypeptide nanoparticle, for example using selfassembling ferritin polypeptides. The system is compatible with current pMHC reagents, including both MHC-I and MHC-II molecules, and streptavidin reagents that allow ease-of-use. The spheromer assembly pipeline provides a consistent reagent across multiple batches of synthesis with ease of production. The defined geometry of the scaffold allows precise site-directed conjugation of pMHC, leading to a homogenous reagent. The spheromer binds cognate TCRs with a significantly higher avidity than a tetrameric reagent.