EFFECT OF IRRADIATION ON HEPATIC NATURAL-KILLER-CELLS

The rat liver contains a population of natural killer cells consisting of two morphologically and functionally different subsets, a low-dens ity and a high-density fraction. In this work we describe the influenc e of low-dose radiation on hepatic natural killer activity. The effect on the cytotoxicity against YAC-1 lymphoma and CC531 colon adenocarci noma tumor cells was measured in chromium-51 assays, and morphological changes were analyzed by means of electron microscopy. The low densit y natural killer fraction showed increased cytotoxicity against YAC-1 which was associated with an increased binding of natural killer cells to the YAC-1 tumor cells shortly after irradiation, These phenomena w ere paralleled by an increased number of multivesicular bodies and cyt oplasmic granules with an electron-lucent halo. In contrast, the other hepatic natural killer cell fraction, the high-density natural killer cells, did not show increased cytotoxicity, binding or morphological alterations. The radiation-stimulated lysis of YAC-1 cells was also ob served when in vivo irradiated cells were isolated and tested immediat ely for in vitro lysis of YAC-1 cells. Sixteen hours after in vitro or in vivo irradiation, the cytotoxicity of hepatic natural killer cells against YAC-1 was no longer enhanced. The cytolysis of and binding of hepatic natural killer cells to CC531 colon adenocarcinoma cells was not stimulated by in vitro irradiation. From these experiments, we con clude that low-dose radiation stimulates the cytotoxicity of hepatic l ow density natural killer cells against YAC-1 lymphoma cells immediate ly after irradiation as the result of enhanced binding of the cells to the tumor cells, in addition to the synthesis of new cytotoxic granul es.