The embryonic thymus is seeded by invading hemopoietic precursor cells that differentiate intrathymically into T lymphocytes. We have recently reported that avian thymic epithelial cells secrete chemotactic peptides, which provoke oriented migration of hemopoietic precursor cells in vitro. The established rat thymic epithelial cell line IT-45 R1 produced a polypeptide that resolves as a single band in the region of 11 kDa on NaDodSO4/polyacrylamide gels. This molecule, which we have named thymotaxin, induced a chemotactic response in a subpopulation of hemopoietic cells from juvenile rat bone marrow. Responding cells were generated by short-term coculture of rat bone marrow hemopoietic cells with mouse bone marrow stroma in a steroid-free medium. Cells selected in a chemotactic chamber have a lymphoid or blast cell morphology. The phenotype of the responding cells is Thy-1+, CD4- [corrected] and CD8-. In contrast, CD8 T-lymphocyte differentiation antigen was expressed after coculture with embryonic thymic monolayers, suggesting that the responding cells correspond to the precursors colonizing the thymus
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