Cloning of a Thymic Stromal Cell Capable of Protecting Thymocytes from Apoptosis

A clone of thymic stromal cells, namely 2BH4, was established by primary culture, cellular transfection and limiting dilution. Morphological analysis by transmission electron microscopy revealed that these cells grow as multilayers, producing a well-defined basement membrane to which they attach and frequently form structures similar to hemidesmosomes. The adjoining cells are connected by intercellular junctions, as tight junctions, intermediate junctions, and desmo-some-like junctions, as well as interdigitations. Their cytoplasm contains microtubules, strands of actin filaments, and scarce intermediate filaments. Fluorescence microscopy revealed that 2BH4 cells stain with anti-cytokeratin antibodies, the majority of them giving a faint reaction. In addition, they express Thy-1.1, LFA-1, ICAM-1, and the gp23 epithelial antigen, and synthesize laminin. They have a doubling time of 16 hr and are able to bind thymocytes. Thymocytes cultured in the presence of 2BH4 cells are partially protected from both spontaneous and PMA- or dexamethasone-induced apoptosis. This protection is conferred neither by soluble factors normally produced by the 2BH4 cells nor by the sole contact with fixed 2BH4 cells. Rather, thymocytes must interact with metabolically active 2BH4 cells in order to receive the protective signal(s).