Modulation of the growth of pulmonary tumour colonies in mice after single or fractionated low-level irradiations with X-rays

A number of epidemiological and experimental data indicate that exposures to low doses of low-LET ionizing radiation may trigger the activity of natural anti-tumour immune mechanisms and inhibit tumour growth. Natural killer (NK) cells and activated macrophages play important roles in the anti-tumour defence of the host. In view of this, the aim of the present study was to correlate the tumour-inhibitory effect of low doses of X-rays with the activities of NK cells and macrophages. BALB/c mice were whole-body irradiated with single or fractionated doses of 0.1, 0.2,or 1.0 Gy X-rays and then intravenously injected with L1 sarcoma cells; 14 days later, tumour colonies on the lungs' surface were counted. Cytotoxic activities of NK cells and macrophages were estimated using the Cr-51-release and [H-3]thymidine-uptake assays, respectively. The anti-asialo GM, antibody and carrageenan (CGN) were intraperitoneally injected to block the NK cell- and macrophage-mediated activities in vivo, respectively. Single and fractionated whole-body irradiation (WBI) of mice with 0.1 or 0.2 Gy X-rays led to reduction in the number of the pulmonary tumour colonies accompanied by the enhanced cytotoxic activities of both NK lymphocytes and macrophages. Treatment of mice with anti-asialo GM, antibody or CGN abrogated the tumour-inhibitory effects of the low-level exposures to X-rays. The obtained data suggest that suppression of the development of pulmonary tumour colonies by single or fractionated irradiations of mice with the low doses of X-rays may result from stimulation of the natural anti-tumour defence reactions mediated by NK cells and/or cytotoxic macrophages.