Isolation of cell lines possessing functional and serological properties resembling those of thymocyte precursors

The immortalization of thymic nurse cells by SV40 virus

Thymic nurse cells (TNCs) are stromal elements that contain between 20 and 200 T cells within their cytoplasm. Because of this unique feature they are believed to play a role in thymocyte development. Unfortunately, it has been difficult to obtain pure TNCs in quantities sufficient for extensive evaluation of their thymic function. As a result, only a limited amount of information is available that characterizes TNCs or the T cell population(s) found within their cytoplasm. We have now used SV40 to infect and immortalize TNCs from C57BL/6 mice. SV40-transformed TNCs were found to specifically bind and internalize cells from an immature thymocyte line isolated in our laboratory. These results describe a method of obtaining pure populations of TNCs for future studies of their thymic function, and suggest that binding to specific subpopulations of lymphoblasts may be necessary for internalization.

Prothymocyte seeding in the thymus

A major gap in our understanding of T lymphocyte development is the process of stem cell differentiation into T lymphocyte precursors. An important question is whether bone marrow-derived stem cells become committed to T lymphoid lineage within the bone marrow, or whether this occurs once cells have entered the microenvironment of the thymus. Attempts to identify a haemapoietic precursor of thymocytes in mice, a "prothymocyte", have involved cell transfer experiments involving isolated and selected populations of bone marrow stem cells, as well as transformed or continuous cell lines representing early stage in mouse T cell development. Current information on the properties of stem cells which can seed the thymus is reviewed in this paper, and the possibility that progenitor T cells may be identified by their expression of receptor(s) which localise them into the thymus is considered.