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The radiosensitivity of human lymphocytes was investigated by the method of colony formation in the absence of interleukin-2 (IL2) and feeder cells, both of which enhance growth of T-cell colonies. The shape of the survival curve and the radiosensitivity was shown to depend upon the ability of lymphocytes to produce IL2: the survival curve for lymphocytes that were the most competent producers of IL2 is the closest to linearity; the lymphocytes that were poor producers show biphasic survival curves. The radiosensitivity of the lymphocytes from the first group is less than that of the latter, when the comparison is based on the first part of the biphasic survival curve. This is more easily seen when cultures are irradiated 24 h after stimulation by phytohemagglutinin (the time of the peak IL2 production) than when cultures are irradiated 2 h before stimulation. This study demonstrates that growth conditions influence the response of lymphocytes to irradiation and that optimal growth conditions result in a linear survival curve.

An originally virus-induced, non-producer tumour system has been studied in relation to humoral and cellular cytotoxic responses to transplantation and other immunization techniques. In all experimental groups cytotoxic lymphocytes (CTL) were observed either directly or after mixed culture of lymphocytes and tumour cells (MLTC). Except for C'-dependent cytotoxic antibodies in mice immunized by irradiated cells, no antibody-mediated cytotoxicity was observed. In 2 protocols (transplantation and immunization by mitomycin-treated cells) CTL in vitro were not protective. In a third protocol (immunization by irradiated cells) CTL afforded partial protection and other factors appeared to be involved. The best in vivo protection was induced by immunization consequent on early surgical removal of a small number of transplanted tumour cells. This study provides lines of evidence for the effectiveness of protection supplied by CTL in well-defined conditions. Comparison with other modes of immunization indicated that these conditions were related to the quantity and to the characteristics of antigen involved.