Origin of Thy-1⁺ dendritic epidermal cells of adult mice from fetal thymic precursors

THE skin of mice contains dendritic epidermal cells carrying the Thy-1 antigen (Thy-1⁺ dEC) which express antigen receptors composed of the T-cell antigen receptor (TCR) γ- and δ-chains^1-5. Although the role of the thymus in the generation of most T cells is well established⁶, the involvement of the thymus in the generation of Thy-1⁺ dEC is not clear. Because bone marrow cells can give rise to Thy-1⁺ dEC in chimaeric mice⁷ and Thy-1⁺ dEC are detected in the skin of athymic nude nice⁸, it has been proposed that Thy-1⁺ dEC arise continuously from bone marrow precursors by a thymus-independent mechanism⁹. But it has recently been determined that Thy-1⁺ dEC in nude mice do not express TCR at the cell surface, and that the γ- and δ-chain genes are in germ-line configuration¹⁰, leaving the role of the thymus in the generation of Thy-1⁺ dEC uncertain. Most Thy-1⁺ dEC in all normal mouse strains examined express TCR containing the Vγ3 gene product¹¹. This V gene segment is expressed on the first wave of TCR-expressing cells to emerge during fetal development, and in adult mice is detectable only on cells in the epidermis¹². In addition to use of this 'fetal' Vγ segment^1,2,13,14, other features of the Thy-1⁺ dEC TCR genes, including absence or minimal presence of non-germ-line-encoded nucleotides at the junctions and use of a single D element in the rearranged δ-chain gene^15,16 are typical of rearrangements found in fetal, and not adult, thymus^17,18. Here we demonstrate that precursors that are present only in the fetal thymus give rise to Thy-1⁺ dEC in the skin of adult mice.