The mechanisms that regulate thymocyte proliferation and differentiation within the thymus are still poorly understood. As a novel approach to analyzing these problems, we have used a retroviral vector to insert oncogenes into fetal thymic cells, and construct a library of thymic cell lines. Infection with a double promoter recombinant retrovirus containing the v-Ha-ras and v-myc oncogenes resulted in the apparent immortalization of a range of cell lines derived from 13- and 14-day murine fetal thymus, and expressing both v-Ha-ras and v-myc mRNA at high levels. Cell lines were established from untreated thymic lobes (group 1), deoxyguanosine-treated lobes (group 2), and in the presence of IL-2 (group 3). Cell lines with a wide range of surface phenotypes were established. Group 1 were adherent cell lines expressing many antigens, including Mac-1, FcR, and Ia. Group 2 were also adherent cells but expressed very few Ag. Group 3 were IL-2-dependent lymphoid-like non-adherent cells, with a null or early thymocyte-like phenotype. None demonstrated full rearrangement and expression of TCR alpha-, beta-, or gamma-genes, although cell lines in group 2 expressed short beta and those in group 3, short beta and gamma gene transcripts. This library of immortalized cell lines appears to represent several types of stromal cell and early thymocyte precursors present in 13- and 14-day fetal thymus. These cell lines should prove invaluable in reconstructing the environment of the intact fetal thymic lobe, to study the cellular interactions that appear to govern thymocyte proliferation, development, and selection of Ag reactivity.

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