Studies on the microcytotoxicity test. I. Evidence that the effects of normal lymphoid cells on tumour cell growth in microtest plates may be caused by non‐immunological modifications of the culture medium

Lymph node cells from normal Wistar rats were found to affect the growth/survival of syngeneic chemically‐induced solid tumour cells in the microcytotoxicity test. Whether inhibition or enhancement of tumour cell numbers occurred depended on the particular tumour cell type and, in some cases, on the particular in vitro subline used. Fractionation of LNC on nylon wool columns revealed that the two effects were mediated by distinct subpopulations of lymphoid cells: column‐retained cells showed predominantly an inhibitory effect and column‐eluted cells predominantly an enhancing effect. When column‐retained and column‐eluted cells were cultured under the conditions of the microcytotoxicity test but in the absence of tumour cells, the resulting cell‐free supernatants caused inhibition and enhancement respectively of tumour cell growth. The inhibitory activity was maximal within one hour of lymphocyte culture, while the enhancing effect developed slowly during incubation. Furthermore, the tumour cells themselves were found to produce growth‐enhancing activity. It is proposed that interaction between these various supernatant activities accounts at least in part for the non‐specific effects of normal lymphoid cells in the microcytotoxicity test.