The use of VEP13 monoclonal antibody for definition of natural killer cells: Spontaneous killer cells directed against fresh human leukaemia cells carry the VEP13 antigen

VEP13, an IgM monoclonal antibody (MoAb), produced against human large granular lymphocytes, is able to deplete natural killer (NK) cell activity in complement-dependent lysis. Here we report that VEP13 also reacts with the majority of interferon (IFN) activated NK cells. By contrast cytotoxic activity of unstimulated monocytes and cytotoxic T cells directed against allogeneic lymphocytes were unaffected by VEP13 plus complement treatment. Thus among the major types of cytotoxic cells VEP13 selectively reacts with NK cells and hence can be employed to identify these cells. We therefore used VEP13 in complement-dependent lysis and FACS separation to analyse NK cells involved in enhanced killing of fresh leukaemia cells. Spontaneous cell-mediated lysis of human leukaemia cells was enhanced in two ways: (a) effector cells were pre-treated with β-IFN and (b) leukaemia cells were pre-treated with a pulse of actinomycin D. In complement-dependent lysis VEP13 removed all NK cell activity of IFN activated PBM against untreated and against ActD pre-treated leukaemia cells. FACS separation of VEP13 positive cells further supported this finding, in that all activity of IFN activated NK cells against actinomycin D pre-treated targets was found in the VEP13 positive fraction. Thus enhanced killing of fresh human leukaemia cells appears to be mediated VEP13 positive NK cells which are distinct from cytotoxic T cells and cytotoxic monocytes.