The mammalian host defence system can be divided broadly into adaptive and non-adaptive immunity. Adaptive immunity is acquired and is mediated by B and T lymphocytes. Non-adaptive immunity is mediated in part by a small subclass of heterogeneous peripheral blood mononuclear cells. This population, termed null cells, consists of haematopoietic precursors and cells mediating natural killer (NK) activity and antibody-dependent cellular cytotoxicity (ADCC). NK cells are a class of bon-adherent, non-phagocytic, rapidly cytotoxic lymphocytes which can efficiently lyse a wide variety of tumour cells1, virally infected cells2 and immature cell types of normal origin3. Despite the broad range of targets, only a limited number of specificities are thought to be involved in target-cell recognition4-6. Morphologically, NK cells are large granular lymphocytes7,8, but they have been shown to exhibit cell-surface markers characteristic of both T cells9-11 and monocytes12, raising doubt over their lineage. The recent cloning of the β-chain of the T-cell antigen receptor13,14 has now allowed us to investigate whether some NK celle are T-cell-related. We have examined rearrangement and expression of the β-chain of the T-cell receptor in cloned murine NK cell lines and fresh murine NK cell populations, and our results support the hypothesis that a subpopulation of NK cells is related to T cells and provide a basis for examining whether some NK activity is mediated by a small number of T-cell receptors.
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