Ultrastructure of Effector–Target Cell Interaction in Secondary Cell‐Mediated Lympholysis

The ultrastructure of the secondary cell‐mediated lympholysis (CML) reaction and the effects on interacting lymphocytes of colchicine, cytochalasin B, and effector cell‐Specific antisera were examined using transmission and scanning electron microscopy. Surface labelling of cytotoxic secondary effector cells with cationized ferritin allowed them to be distinguished from unlabelled target lymphocytes. Effector target interactions were characterized by intercellular junctions involving extensive areas of membrane opposition and interdigitation and extension of pseudopod‐like processes by the effector cell. The abolition of such interactions when effector populations were pretreated with anti‐Ly2 sera plus complement demonstrated target cell destruction in secondary CML to be dependent on the activity of restimulated cytotoxic T lymphocytes. Cytochalasin B and colchicine dramatically decreased the numbers of Specific effector‐target cell interactions observed. Although the data presented do not allow the possible activity of soluble lytic factors associated with the effector cell surface to be ruled out, they suggest that target cell lysis in the secondary CML system examined results From immune‐specific binding of alloantigen‐sensitized effectors to targets and osmotic effects which follow localized disruption of the target cell membrane.