Brain-resident regulatory T cells (brain Tregs) are potent anti-inflammatory cells capable of driving neuro-protection and neuro-repair across multiple inflammatory and degenerative conditions. The cells have a high therapeutic potential, with the key unknown limiting the clinical exploitation of brain Tregs being the unknown antigenic targets recognised by these cells. The technology and knowledge-base required for identifying the target antigens for T cell receptors (TCRs) are being rapidly developed, although few studies look at the self-reactive Treg population. Recent advances by the applicant groups have enabled these approaches to be scale-able, allowing for the first time the ambition to identify the antigens of an entire T cell repertoire to be imaginable. The small size of the murine brain Treg population, among the smallest tissue T cell population, combined with the proposed iterative multiplexed approaches, make the repertoire-level identification aim feasible for the first time. We can further validate homologous antigen targets in human patients, using cerebral spinal fluid samples and cutting-edge antigen- led screening. This project will give key insights to the nature of the Treg TCR repertoire, potentiate brain Treg exploitation, and unlock repertoire- scale TCR target identification for the broader field.