The mechanism of K-cell (antibody-dependent) mediated cytotoxicity. III. The ultrastructure of K cell projections and their possible role in target cell killing.

The cytotoxic interaction between lymphoid K cells from normal rat spleen and antibody-coated P815 mastocytoma cells has been studied in conditions under which the number of cytolytic events occurring at the time of observation was at a maximum. Electron micrographs of material fixed during the first 15 min after contact between the target and effector cells had been initiated by centrifugation showed that the K cells produce long projections which push deeply into the P815 cells, causing infoldings of the plasma membrane and distortion of the nucleus. The plasma membranes of the effector and target cells, and the nuclear membrane, remain intact. Subsequently the target cells undergo violent cytoplasmic blebbing (zeiosis) which is the first stage of cell lysis. The evidence for the hypothesis that projections from lymphoid K cells develop as a result of contact between receptors on the K cell surface and antibody bound to the target cell, and that the projections are involved in the cytotoxic mechanism is discussed.