Differentiation of rat thymic myoid progenitor cell line established by coculture with human T-lymphotropic virus type-I producing human T cells.

A thymus-derived myoid precursor cell line (ST1), which differentiates to myoid cells in the growth arrest condition, was established by the cocultivation of F344 rat thymic cells with human T-lymphotropic virus type-I (HTLV-I)-producing human lymphoid cells. No integration of HTLV-I was detected in ST1 cells by Southern blot hybridization. In a differentiation culture condition such as confluent culture or serum starvation, ST1 cells began to fuse, creating multinuclear giant cells, with the induced expression of MyoD1 and various muscle-specific antigens, including alpha-sarcomeric actin, skeletal muscle myosin, myoglobin, desmin, and acetylcholine receptor. Ultrastructural investigation revealed that differentiated ST1B cells created aggregates of thick and thin filaments with Z-band-like composition, then formed sarcomeric structures and tubular honeycomb arrays. Finally, these cells spontaneously contracted with a frequency of 0.5-2.0 Hz and synchronized with adjoining cells. Transplantation of ST1B cells into nude mice produced a small tumor nodule, showing clear differentiation to skeletal muscle cells. ST1B cells did not indicate any colony-forming activities in soft agar, demonstrating that ST1B cells retain some of the physiologically normal phenotypes. This rare cell line is promising for use in various physiological and pathological investigations including functional research of thymic myoid cells and the pathological role in autoimmune diseases, as well as animal model experiments of cell therapy related to muscular degenerative disorders or regeneration of injured muscles.