Human 'null' cells mediating antibody-dependent cellular cytotoxicity express T lymphocyte differentiation antigens

There is considerable controversy regarding the cell lineage of killer (K) or effector cells in ADCC isolated from the 'null' cell subpopulation (ER^-sIg^-) in human blood. Null cells were examined for the presence of T cell antigens by using a fluorescein-conjugated anti-T cell xenoantiserum (AMT). AMT⁺ and AMT^- cells were then physically separated with a fluorescence-activated cell sorter (FACS) for comparing lytic activity against IgG antibody-coated tissue culture target cells. By three criteria, the K-cell-enriched fraction of ER^-sIg^- lymphocytes clearly possessed the same T cell differentiation antigens expressed on ER⁺sIg^- T cells from blood and thymus. First, by direct membrane immunofluorescence studies, 66% of ER^-sIg^- cells reacted with AMT antiserum. Less than 0.1% of these cells expressed ER receptors, less than 0.5% expressed sIgM, and none contained cytoplasmic IgM. Second, adsorption of AMT with ER^-sIg^- lymphocytes completely removed fluorescence reactivity with ER⁺sIg^- T cells. Third, immunoprecipitation analysis with AMT demonstrated the same PAGE profiles of 25,000- and 16,000-dalton antigens isolated from solubilized radiolabeled surface membrane of ER^-sIg^- (null) cells from blood and ER⁺sIg^- lymphocytes from blood and from thymus. The antigens were not detected on B cell sources. The FACS was used to separate ER^-sIg^- cells incubated with AMT into the 30% most reactive and 30% least reactive fractions. These two subpopulations were compared wth unsorted ER^-sIg^- cells and with PBL for effector activity in ADCC. The results obtained by using cells from three individuals demonstrated that: 1) unsorted ER^-sIg^- cells had more ADCC activity compared with the starting population of unfractionated PBL, 2) unsorted ER^-sIg^- cells stained with AMT and trypsin treated had ADCC activity comparable to that of unstained cells, indicating that AMT labeling did not block ADCC function, and 3) ADCC activity of ER^-sIg^- cells was associated almost entirely with the subpopulation expressing T cell antigens detectable with AMT. These results thus demonstrate that K cells capable of lysing IgG-coated, nucleated target cells in this experimental system express T cell differentiation antigen but lack detectable ER receptors. 'Null' cells (ER^-sIg^-) from peripheral blood therefore have T cell characteristics, since they express the same differentiation antigens found on T cells defined by ER receptors. Furthermore, K cell functional activity in an ADCC assay resides in the null cell fraction expressing these T cell antigens.