INTRODUCTION: Cardiac allograft vasculopathy (CAV) is a critical predictor of the long-term success of heart transplantation and once it is established, progression to graft dysfunction and loss is inevitable, despite adherence to immunosuppression and medications that ameliorate cardiac risk factors. Regulatory T cells (Tregs) are key for maintaining immune balance in the periphery. Studies investigating adoptive transfer of ex vivo expanded Tregs isolated from blood have been shown to be feasible and safe with good evidence for Tregs reducing CAV lesions in animal models of transplantation. Here, we describe the protocol for the ATT-Heart Study which is a phase I clinical trial investigating autologous thymus-derived Treg cell therapy in nine paediatric heart transplant recipients. METHODS AND ANALYSIS: Patients will be recruited from the heart transplant waiting list at Great Ormond Street Hospital. Individualised autologous thymus-derived and expanded Tregs (TR006) will be injected into patients 3-6 months after transplant and follow-up will be conducted as per the post-transplant standard of care protocol with no wean of standard of care immunosuppression. Primary endpoint includes occurrence of Dose-limiting Toxicities in patients receiving TR006. Further data from blood tests, endomyocardial biopsy tissue, coronary imaging and clinical follow-up will be collected. ETHICS AND DISSEMINATION: This article is based on the ATT-Heart study Protocol (V.1.1; dated 19 December 2024). The ATT-Heart trial has received a favourable ethical opinion from the Health Research Authority and South-Central Oxford A Research Ethics Committee (IRAS Number: 1008875/REC reference: 24/SC/0333). Clinical trials authorisation approval from UK Medicines and Healthcare products Regulatory Agency has also been received. The clinical trial will be conducted in accordance with the principles of Good Clinical Practice and following the guidelines set as part of the Research Governance Framework for Health and Social Care and all applicable necessary local policies. It is intended that the findings of the clinical trial will be presented at national/international conferences and using social media and through patient groups for dissemination among their members. The results will also be published in international peer-reviewed journals. TRIAL REGISTRATION NUMBER: ISRCTN15374803.