Subpopulations of hyman thymus cells differing in their capacity to form stable E rosettes and in their immunologic reactivity

The majority of human thymus cells from young donors form stable E rosettes with sheep red blood cells (SRBC) that do not disintegrate after prolonged incubation at 37°C. With advancing age the proportion of thymus cells forming such rosettes decreases gradually. The thymus of a patient receiving prednisone treatment was found to contain only a few cells that formed stable E rosettes. The minor population of thymus cells that fails to form stable E rosettes (non rosetting or NR cells) was isolated and tested for its cell surface markers and immunologic reactivity in vitro. Most of the NR cells were capable of forming regular E rosettes with SRBC at room temperature. Like the majority of human thymus cells they were sensitive to the cytotoxic effect of normal rabbit serum. Lymphocytes with B ell markers constituted less than 0.2% of the original thymus cell suspensions, but about 1 to 3% of the NR population. Thymus cells from donors over the age of 36 and from a prednisone treated child responded in vitro to stimulation with either phytohemagglutinin (PHA) or concanavalin A (Con A). Unfractionated thymus cells from children up to the age of 14 failed to react to either PHA or Con A, but their NR population responded vigorously to both lectins. In contrast to unfractionated thymus cell suspensions from children, the NR fraction showed a significant reactivity in mixed lymphocyte cultures with mitomycin C treated allogeneic lymphocytes. It is concluded that like the thymus of other species, the human thymus contains a minor population of cortisone resistant cells endowed with many of the immunologic properties characteristic for peripheral T lymphocytes.