Human natural killer (NK) cells: requirements for cell proliferation and expansion of phenotypically novel subpopulations.

Previous studies established that the high density (resting) natural killer (NK) cell subset (R-NK) of peripheral blood NK cells is unresponsive to interleukin-2 (IL-2) but can be induced to proliferate when cultured with gamma-irradiated malignant melanoma (MM-170) cells or mitomycin-C-treated activated T cells in the presence of an IL-2-conditioned medium (IL-2-CM). This study has examined additional requirements of this activation process. The induction of proliferation was dependent on cell to cell contact with metabolically active stimulator cells, although no evidence was obtained that stimulation was effected by soluble factors produced by the stimulator cells. Compared with IL-2-CM, rIL-2 was an inefficient costimulator for the induction of NK cell proliferation, suggesting that factors in IL-2-CM were required in addition to IL-2, but rIL-2 was as efficient as IL-2-CM in maintaining the proliferation of activated NK cells. Under optimum culture conditions, NK growth of up to 3200-fold occurred during a proliferation cycle of 18 days. Phenotypic analysis of the culture-generated quiescent NK cells revealed novel heterogeneity in CD16 (Fc gamma RIII) and CD56 (N-CAM) expression. Some NK cells lacked expression of both CD16 and CD56 (as identified using currently available monoclonal antibodies), while other NK cells showed differential CD16 epitope expression. Since quiescent NK cells can be obtained in large numbers and high purity, they will be a convenient source of NK cells to study the molecular processes involved in initiating NK cell proliferation.