John R . Ortaldo , and Ronald

In contrast to the several antigenic markers identified on murine natural killer (NK) 1 cells (1-3), human NK cells have so far been characterized only by surface properties and receptors (4, 5) that do not allow discrimination from many other peripheral blood mononuclear cells. Detection of human NK cells is therefore largely based on the measurement of in vitro cytolytic reactivity against certain target cells (5). Cytotoxicity assays, however, are impractical for large-scale studies of cell subpopulations, and interpretation of the results of these assays may be complicated by the spontaneous cytolytic and cytostatic activities of other cell types, such as monocytes (6), granulocytes (7), and polyclonally activated T cells (8). Recently, evidence has been presented on the morphological association of human NK cells with large granular lymphocytes (LGL), which have a high cytoplasmic: nuclear ratio and azurophilic granules in their cytoplasm (9-12) (Fig. 1A). The central role of LGL in NK activity has been suggested by several observations: (a) LGL were enriched in target cell-adherent, NK cell-enriched populations (9-11), (b) the number of LGL binding to target K562 cells correlated with the levels of cytotoxic activity among normal donors (10), and (c) NK activity and LGL peaked in the same fractions obtained by discontinuous density gradient centrifugation of human peripheral blood lymphocytes (12). Because the morphology of LGL is a potentially useful marker for human NK cells, we have initiated a series of studies to characterize LGL and to further evaluate their cytolytic activity. In the experiments reported here, we have used highly enriched populations of LGL, obtained by density gradient centrifugation (12). Evidence is presented that both spontaneous and interferon (IFN)-boosted NK and antibodydependent cellular cytotoxic (ADCC) activities are confined to LGL-enriched fractions. Almost all LGL were found to express receptors for the Fc portion ofIgG (FcvR) and about 50% formed low affinity rosettes with sheep erythrocytes (E). More than 90% purity of LGL was achieved by enriching FcvR-positive cells or by depleting high affinity E rosette-forming cells from the LGL-containing Percoll fractions.