Brooks, C.G., Flannery, G.R., Webb, P.J. and Baldwin, R.W.
Cancer Research Laboratories, University of Nottingham, U.K.

Immunology 41: 673-680, 1980.
 

Cell fractionation techniques were used to identify the cells in spleen responsible for natural killing of a syngeneic sarcoma cell in short-term (6 h and 18 h) and long-term (72 h) cytotoxicity assays. Cytotoxicity was quantified precisely using a method previously derived from consideration of natural cytotoxicity as an enzyme-substrate reaction, and by analysing results in terms of lytic units. Killing in all three assays displayed 'single-hit' kinetics implying that a single effector cell was sufficient to lyse a single target cell. The fractionation studies, using glass adherence, carbonyl iron, nylon wool, EA and EAC monolayers and congenitally athymic individuals, revealed two populations of cytotoxic cells. In the 6 h assay most of the activity was due to cells with similar characteristics to the NK cells previously defined using leukaemic targets, but in the 18 h and 72 h assays macrophages played an important role. The activity exerted by the macrophages was cell lysis and not cytostasis. No evidence that the macrophages acted by releasing factors which stimulated NK cells could be found.