ASSESSMENT OF CD4 EXPRESSION BY EARLY-T PRECURSOR CELLS AND BY DENDRITIC CELLS IN THE HUMAN THYMUS

The adult mouse thymus contains a minute population of early lymphoid precursor cells that express moderate levels of CD4. We searched for a corresponding population of early T precursors in the infant human th ymus, by first depleting the majority of more mature thymocytes, then using immunofluorescence and flow cytometry to analyze cells bearing a range of early T lineage markers. No discrete population of early T p recursors expressing CD4 was observed, in contrast to the murine thymu s. Most putative very early human thymocytes were CD4(-)8(-)3(-)1(-)2( lo)44(+)34(+)7(hi) class I MHC(hi) class II MHC(-). However, a distinc t population of human thymic dendritic cells expressing high levels of CD4 was isolated. These were CD4(hi)8(-)3(-)1(-)2(-)44(+)34(-)7(-) cl ass I MHC(hi) class II MHC(hi), and lacked markers of B cells, NK cell s, or myeloid cells. They were large cells that exhibited dendritic mo rphology after brief periods of culture, and they were efficient stimu lators of allogeneic T cells. The biologic implications of CD4 express ion by thymic dendritic cells are discussed.