| Summary: | The heterogeneity of cells capable exerting spontaneous cytotoxicity in vitro was explored using antisera to several genetically determined surface markers on mouse lymphocytes. Four phenotypes of cells derived either from fresh or cultured murine lymphoid tissue were found to exert natural killer (NK) activity in vitro. One affector cell subset, termed NKI cells, had the serological phenotype of Thy-1-, Lyt-2-, Qa5+, and lysed measles virus persistently infected target cells (HeLa- Ms) but not P815 mastocytoma cells. It corresponds with the NK cells described in most systems in which lymphoma targets are commonly used. A second subset, with the same target cell specificity, termed NKT is a thymus-independent cell with the phenotype Thy-1+, Lyt-2-, Qa-5+, Ly- 5+. A third subset of NK cells, termed T killer (TK) cells deriving from cultures of conventional but not nude mouse spleens, mediated spontaneous cytotoxicity of P815 mastocytoma cells, but not of virus- infected targets. It has a phenotype of Thy-1+, Lyt-2+, Qa-5-, Ly-5+, apparently identical with that of conventional, antigen-specific cytotoxic T lymphocytes. The fourth phenotype of NK cells, termed NKM, derived primarily from cultures of bone marrow, is cytotoxic for HeLa- measles but not P815, and expresses only Ly-5+ among the various markers tested. Beige mice possess normal TK and NKM activities, but had normal NKI, NKT as well as NKM activity. All NK cell subsets express the Ly-5 surface marker. The existence of four phenotypically distinct NK effector cells was strengthened by studies on selective regulation of their activity by two different biological factors. Interferon (IFN) augmented NK activity of primarily one of the subsets examined, the NKI cell; the activity of IFN on NKT cells could not be directly tested, but IFN was without positive effect on TK or NKM cells. In contrast, partially purified IFN-free interleuken 2 (IL-2) augmented the activities of both the TK and NKT subsets, but not of NKI or NKM cell. IL-2 was active in augmenting NK activity in spleen cells obtained from both conventional and nu/nu mice, but was without effect on spleens of nu/nu mice depleted of Thy-1+ cells. These and other data suggest that IL-2 acts primarily, if not exclusively, on THy-1+ cells. These results strengthen the view that natural cytotoxicity in vitro can be mediated by several distinct cell populations under different genetic and regulatory control and indicate the importance of defining and delineating the cell lineages of each and the role of the independent subsets in resistance to virus infections and tumors in vivo.
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