Lymphocyte capping induced by polycationized ferritin

In order to better understand the mechanism of lymphocyte surface receptor redistribution induced by externally added ligands, polycationized ferritin (PCF), a nonconventional ligand, was tested using both fluorescence and electron microscopy for its ability to cause patching and capping of anionic molecules on the surface of both transformed and normal mouse lymphocytes. Binding of PCF at 0°C for 1 hour induces the appearance of patches; subsequent incubation at 37° for 30–60 minutes causes the formation of a cap structure with the lymphoid cells tested (T‐lymphoma cells and splenic lymphocytes). Using various experimental treatments (e.g., sodium azide, cytochalasin B and D, colchicine, prefixation, and cold temperatures), PCF‐induced capping has been found to be temperature sensitive, and to require metabolic energy and an intact cytoskeletal system. In addition, using double immunofluorescence techniques which involve rhodamine‐labeled PCF and fluorescein‐conjugated heavy meromyosin, it has been observed that the formation of the PCF‐induced cap coincides with an accumulation of intracellular actin directly beneath the cap structure. Furthermore, agents such as dibutyryl cyclic AMP and theophylline, which cause an increase in intracellular cyclic AMP, have been shown to stimulate PCF‐associated capping. This study suggests that increasing levels of intracellular cyclic AMP may activate, directly or indirectly, membrane‐associated contractile elements required for the aggregation of membrane proteins into patches and caps.