Using cloned lines with the morphology of large granular lymphocytes (LGL) from BALB/c mice, we studied the exact requirements for proliferation and their functional characteristics, as well as their regulation. Although these cloned LGL lines were interleukin 2 (IL-2) dependent for growth, experiments using human recombinant IL-2 (rIL-2), known to be active on murine cells, indicated that IL-2 was a necessary but not sufficient factor. Coexistance of normal macrophages in addition to rIL-2 was found to support continuous proliferation of cloned LGL in vitro. This role of macrophages could be replaced by partially purified IL-1 derived from macrophage-conditioned medium. An IL-2 binding assay using 125I-rIL-2 suggested that the role of normal macrophages was to selectively induce and/or maintain high affinity IL-2 receptors (IL-2R) (Kd, 0.2-0.5 nM) without affecting low affinity ones (Kd, 10-30 nM). Functional studies indicated that most of the LGL clones killed various combinations of representative groups of natural killer (NK)-susceptible target cells, including leukemic cells (YAC-1, RL male 1), virus-infected cells (HeLa-measles, HeLa-herpes simplex virus), and normal bone marrow cells (BMC), whereas none of them affected any of NK-resistant target cells, including uninfected HeLa cells. Some of these clones also suppressed in vitro hematopoiesis. Such characteristic cytotoxic spectra, as well as serological phenotypes (Thy-1+, Lyt-1-2-, asialo GM1-positive, T200+, TdT-, Fc receptor-positive) indicated that these LGL clones exactly represent endogenous NK cells, rather than a variety of anomalous killer cells generated in various culture conditions. Although there was significant heterogeneity of cytotoxic spectrum among LGL clones, no clonotypic distribution of specificities was observed. Normal macrophages were found to modulate the functional expression of LGL clones. They augmented the cytotoxic potential of the clones against leukemic and virus-infected targets, but suppressed intrinsic reactivity against normal BMC. Similarly, LGL clones maintained with macrophages showed much less suppressive effect on in vitro hematopoiesis. The present observations on the interaction of cloned LGL and normal macrophages provide a basic explanation for the mechanisms by which the immediate responsiveness to IL-2 of the NK effector system, without exogenous stimulation, and the functional selectivity toward abnormal rather than normal cells, are actively maintained in vivo.

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