Induction of limited growth and differentiation of early thymic precursor cells by thymic epithelial cell lines.

The early thymic precursor population of adult mice (low CD4 precursor) has the potential to produce T cells, B cells and dendritic cells if transferred into the appropriate inductive environment of an irradiated recipient. To assess its developmental potential in vitro, this population was isolated and cultured, alone and with various stromal cell lines. Cultured alone, these precursor cells all died rapidly. Co-culture with 3T3 fibroblasts gave good survival but no growth. Co-culture with thymic cortical epithelial cell lines induced significant proliferation after an initial 50% cell loss. However, the supernatant of these cortical epithelial cell lines caused only limited proliferation after very extensive cell death. Examination of the surface phenotype of the cultures on the cortical epithelial layer showed some changes which were compatible with very early steps of thymocyte development, but none of the features of more developed T cells were seen. A proportion of the proliferating cells developed some of the surface markers and morphology of dendritic cells. Immature myeloid cells also grew in these cultures; these appeared to derive from a small number of myeloid progenitors, possibly contaminants within the preparation, and their outgrowth required only soluble factors released by the cortical epithelial cells.