The Natural Killer Cell‐Like Lytic Activity Expressed by Cytolytic T Lymphocytes is Associated with the Expression of a Novel Function‐Associated Molecule

Experiments were performed to analyse the natural killer cell (NK)‐like cytotoxicity frequently expressed by human antigen‐specific cytolytic T lymphocytes (CTL). To this end, several monoclonal antibodies (MoAbs) previously shown to identify a novel function‐associated molecule (FAM) involved in human NK function were utilized. Flow cytometry revealed that these MoAbs reacted with the majority of human NK, but only with a subpopulation of CTL isolated from primary mixed lymphocyte cultures. Preincubation of CTL with the MoAbs inhibited the NK‐like lysis of K562 targets. Experiments with anti‐CD3 MoAb demonstrated that neither the NK‐like cytotoxicity of CTL nor the lytic activity of NK were mediated by the CD3 complex. Expression of the novel FAM was found to develop in T‐cell cultures at the time that NK‐like cytotoxicity was observed. Repeated in vitro antigenic stimulation of CTL was shown to result in loss of NK‐like cytotoxicity, as well as loss of the FAM on the CTL surface. Thus, NK‐like cytotoxicity displayed by antigen‐specific CTL appears to be mediated by a novel FAM that is identical to that structure found on NK.