A missing link in thymic dendritic cell development

Negative selection in the thymus prevents the generation of self-reactive T cells through the deletion of thymocytes with high affinity for self-antigens. Within the thymus, self-antigens are presented by thymic epithelial cells and DCs. Both cell types can mediate negative selection, although the relative contribution of each cell type remains elusive. Similar to DCs of other lymphoid organs, thymic DCs come in different flavors. Over the past years, various lines of evidence have emerged that either favor a common origin for some thymic DCs and thymocytes or, conversely, indicate the existence of separate intrathymic T lineage and DC precursors. In this issue of the European Journal of Immunology, a study reports the identification of an intrathymic DC precursor that is likely to be unrelated to the earliest physiological T-cell progenitors. Thus this intrathymic DC precursor may constitute a "missing link" between extrathymic DC precursor-types, which are able to generate DCs in secondary lymphoid organs and intrathymic DCs, and supports the notion that intrathymic DCs and thymocytes arise from different precursors.