Humoral immunostimulation. III. Requirements for divalent antibody and cellular aggregation

The intracellular incorporation of radioactive nucleoside into mouse L cells was stimulated by rabbit anti-L cell IgG antibody and by its divalent pepsin F(ab')2 fragment but not by univalent pepsin Fab' or papain Fab fragments. Divalent antibody also caused the cells to grow in an aggregated pattern and undergo capping, but univalent antibody did not. Prevention of aggregation of antibody-treated cells by growth in agar abrogated immunostimulation. Univalent antibody fragments partially blocked stimulation of cells by divalent antibody and binding of 125I-labeled divalent antibody to cells. These results suggest that immunostimulation of cell growth is dependent upon divalent antibody and that aggregation of cells is an associated event.