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.