Thymic dendritic cells and T cells develop simultaneously in the thymus from a common precursor population

DENDRITIC cells, a minor cell population in lymphoid tissues, are specialized for presentation of antigenic peptides to T lymphocytes 1 . Thymic dendritic cells are involved in the deletion of self-reactive T lymphocytes 2,3 . Although all dendritic cells are ultimately of bone-marrow origin 4–7 , it has not been clear whether thymic dendritic cells are produced in the adult thymus from a precursor cell or whether they migrate there preformed from the periphery. Recently we isolated from adult mouse thymus a population of early T precursors that could still form B lymphocytes, but not erythroid or myeloid cells, when transferred intravenously 8,9 . Here we show that these thymic lymphoid precursor cells, as well as bone-marrow haematopoietic stem cells, are able to form both dendritic cells and T-cell progeny when transferred into an irradiated thymus. Such linked development may ensure that developing T cells are negatively selected predominantly by self antigens presented on newly formed thymic dendritic cells.