Cytological events in allo-stimulated lymphocytes triggered by exposure to stimulatory alloantigens. III. Changes in the areal density of cytoplasmic vacuoles containing endocytized Thorotrast.

H-2b lymphocytes were sensitized against H-2d alloantigens by a lymphocyte culture reaction (MLR) and incubated with H-2d mastocytoma cells. Thirty minutes before incubation with the mastocytoma cells Thorotrast was added to the lymphocyte culture medium. The interaction between lymphoid cells and mastocytoma cells was stopped by fixation with glutaraldehyde. The areal densities of the cytoplasmic vacuoles were examined by electron microscopy. Two populations of lymphocytes were observed, small lymphocytes with heterochromatic nuclei and larger lymphocytes (lymphoblasts) with euchromatic nuclei. Only the lymphoblasts showed changes following interaction with target cells. The areal density of Thorotrast containing vacuoles in sensitized lymphoblasts increased during the three hour observation period, but the percentage of vacuoles containing Thorotrast in the total area of cytoplasmic vacuoles decreased in the first thirty minutes of interaction with target cells. This observation may indicate greater formation of vacuoles from the Golgi apparatus compared with the formation of vacuoles from the cytoplasmic membrane at the onset of interaction with target cells. In the multivesicular bodies (MVB) the Thorotrast was first observed inside the small vesicles and later on it was also observed in the matrix of the MVB. After three hours of interaction Thorotrast was observed laying "free" in the matrix of phagosomes along with myelin figures.