AbstractIntraepithelial lymphoid cells (IEL) obtained from the mucosa of mouse small intestine were tested for natural killer (NK) activity in a 20‐h51Cr‐release assay against YAC‐1 and RL♂ 1 tumor cells. It was found that IEL possess a strong NK activity, higher than spleen cells, whereas lymphocytes from Peyer's patches (PPL) did not show any NK activity. The cytotoxic activity of IEL could be boosted by interferon (IFN‐β) treatment, but no NK activity could be induced in PPL byin vitroIFN‐β treatment. The characteristics of the NK effector cells present in the mouse intestine strictly resemble those of NK cells in the spleen. In fact, intestinal NK activity is not affected by depletion of adherent cells and only partially reduced by anti‐Thy‐1.2 antibodies plus complement. Moreover, the age dependency of NK cytotoxicity is identical for IEL and spleen. Finally, NK‐insensitive target cells a
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