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The proliferative response of murine natural killer (NK) cells to a T cell growth factor, interleukin 2 (IL 2) was investigated by using the NK-enriched, low-density fractions (F.1) and NK-depleted, T cell-enriched, and high-density fractions (F.3) of normal spleen cells after Percoll-discontinuous density gradient centrifugation. When F.1 and F.3 cells were cultivated with IL 2-containing medium without addition of any other factors, like lectins, feeder layer, or macrophage products, F.1 but not F.3 cells showed an IL 2-dependent proliferative response. Growing populations showed a potent NK activity and consisted largely of cells with a morphology of large granular lymphocyte (LGL) and a surface marker profile characteristic for murine NK cells. By limiting dilution in the liquid culture and colony formation in the soft agar medium, F.1 but not F.3 cells showed a high frequency of clonal replication in IL 2. Almost all growing colonies in either medium showed a typical morphology of LGL. Further, F.1 cells treated with anti-asialo GM1 antibody and C, from which NK cells and cytolytic activity were almost completely abrogated and for which T cells were enriched, conversely formed a few IL 2-dependent colonies. From the colonies in soft agar, 11 clones, all of which showed a typical morphology (LGL), a typical surface profile with the exception of lacking IgG-Fc receptors for NK cells (Thy-1 +/-, Ly-5+, ASGM1+, Lyt-1-, Lyt-2-, Fc gamma R-), and a cytotoxic activity of activated nature, were established and maintained for a period of longer than 1 yr in the medium containing IL 2 alone. These results indicate that a major population in the naive spleens of normal mice that respond directly to IL 2 and clonally replicate without other stimulating factors may be NK cells.
Suzuki et al. (Tue,) studied this question.