The epidermis of normal mice contains two distinct populations of dendritic cells derived from the bone marrow, Ia+ Langerhans cells and Ia - cells that express the Thy-1 alloantigen1-5. The Thy-1-bearing dendritic epidermal cells (Thy-1+ dEC) have a surface phenotype similar to that of very early T-lineage cells6-8, produce IL-2-like growth factors10 and exhibit cytotoxicity which is not restricted by the major histocompatibility complex (MHC)10,11. The relationship of Thy-1+ dEC to the T-cell lineage is unclear. Most T lymphocytes bear a receptor for antigen composed of an α chain and a β chain12-17 associated with a nonpoly-morphic complex termed CD3 (T3)18-21. A minor population carries a receptor in which CD3 is associated with a γ/δ complex21-25. We have analysed clones of Thy-1+ dEC for rearrangement and expression of the genes for the α-, β- and γ-chains of the T-cell receptor (TCR). They do not express α or β but do carry a γ/δ complex. Activation of the cells with Con A is associated with a rapid decrease in the steady-state level of γ-chain RNA. Because Thy-1+ dEC resemble early stage T lymphocytes, down-regulation of TCR expression may reflect a necessary event during T cell differentiation.
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