Ultrastructural characteristics of lymphokine-activated killer cells of the rat in comparison with natural killer cells

Lymphokine-activated killer (LAK) cells with a broad spectrum of tumor cell killing have been reported to be related to natural killer (NK) cells morphologically and phenotypically. We here examine the ultrastructural characteristics of LAK cells of the rat, comparing them to those of normal and OK-432-activated NK cells. Results show that, five days after the culturing of spleen lymphocytes with human recombinant interleukin-2, there were induced LAK cells, which were large granular lymphocytes and had a cytotoxic capacity against NK-resistant P-815 tumor cells. They were larger in size than NK cells and richer in cell organelles such as ribosomes, rough endoplasmic reticulum, a Golgi apparatus, granules and vesicles. The granules of LAK cells were shown to be related to multivesicular bodies as those of NK cells; they included multivesicular bodies, fully dense granules and intermediate forms between them. The average numbers and sizes of the granules and the proportion of multivesicular bodies and intermediate forms among the total granules were greater in LAK cells than in NK cells. The density of the small vesicles packed in multivesicular bodies and intermediate forms was much higher in LAK cells. At the contacting surface of the LAK cells bound to the target cells, exocytosis of multivesicular bodies was shown to occur. We recognized here two populations of LAK cells with different types of vesicles, one containing rod-cored vesicles and the other a new type of vesicles termed "demilune-cored vesicles". The latter vesicles were the same in size as the rod-cored ones and contained a dense core located eccentrically. Between these two populations of LAK cells, there was no difference concerning the profile of the dense granules. The present study indicates that, although LAK and NK cells share several ultrastructural features, the former show markedly enriched cell organelles, which indicate an accelerated metabolism of the cell for continuous proliferation.