CD34+CD38-lin- cord blood cells develop into dendritic cells in human thymic stromal monolayers and thymic nodules.

Thymic dendritic cells (DCs) appear to have distinct biologic and functional properties compared with DCs in other tissues. Currently, little is known about human thymic DCs because they have been difficult to isolate and culture in vitro. Here, we report that human thymic stroma can support the development of primitive human hemopoietic stem cells into mature DCs without cytokine or serum supplementation. Coculture of CD34+CD38-lineage (lin)- and CD34+CD38+lin- umbilical cord blood cells with thymic stromal monolayers induced 43 +/- 17-fold and 32 +/- 16-fold expansions, respectively, of umbilical cord blood progenitors and also generated large numbers of cells with the morphologic, phenotypic, and functional characteristics of mature DCs. These cells expressed class I and class II MHC, CD1a, CD2, CD4, CD11c, CD40, CD45, CD80, CD83, and CD86 and were potent stimulators of allogeneic T cell activation. Primitive hemopoietic progenitors also developed into mature DCs in a novel tissue culture system of thymic nodules wherein thymic epithelial cells and fibroblasts were grown in nodular aggregates in vitro. These results demonstrate that human thymic stroma efficiently supports the development of CD34+CD38-lin- cord blood cells into mature DCs. In addition, the culture conditions described in this report are useful systems for studying the ontogeny of human DCs in thymic microenvironments.