Ultrastructural characteristics of the non-immune rosette-forming cell.

Non-immune rosette formation, where a non-sensitized lymphocyte is surrounded by three or more sheep red blood cells, is an in vitro technique which is thought to be specific in the identification of thymus-dependent (T) lymphocytes. Using the electron microscope and tissue culture techniques, we have studied the ultrastructure of the rosette-forming lymphocyte which has been stimulated by various cellular mitogens. Non-sensitized but stimulated lymphocytes, which form rosettes, have a morphology similar to that of previously identified T cells, adding further credence to the concept that non-sensitized rosette-forming cells represent a population of T cells. Changes in the binding pattern of the sheep red cell membrane to the lymphocyte have been identified which may represent an early phase of cell 'activation'. This marker of activation offers a potential method of studying membrane responses in attempts to localize cellular defects involving the T lymphocyte. In addition, preliminary studies using fluorescein-labelled sheep red blood cell fragments suggest specific attachment of labelled membrane to the T lymphocyte, allowing rapid identification of T cells in smears or tissue preparations.