New precise medical treatments for leukemia and cancer are emerging in the last decade. They are based on immune system cells, T lymphocytes, as key elements of modern immunotherapy. The primary organ for T lymphocyte differentiation and selection is the thymus where adaptive immune diversity is created. It provides a stromal microenvironment for close interactions of T cells with molecules expressed by epithelial, endothelial and dendritic cells. However, thymus fitness deteriorates with age and it is the first organ whose function starts to decline already at puberty. It is also very susceptible to damage caused by irradiation, corticosteroids or chemotherapy so thymus function renewal is the major focus of the proposed project. Reconstitution of thymus function in vitro from epithelial stem cells in terms of maturation, differentiation, and development of immunocompetent T lymphocytes will be performed. The objectives are based on our recent discovery of a new stem cell population from the postnatal human thymus which is able to produce thymus epithelial cells (TEC) and organoids in vitro. Besides this general goal technological, immunological and material science aspects of T lymphocyte development, thymus cultures for ex vivo drug testing will be addressed in the course of the project. These objectives are addressed by our interdisciplinary team of scientists aimed at new unmet clinical needs of large patient groups identified as leukemia patients and the population with iatrogenic or aging based decreased thymic function.